Hair drying and / or styling devices and methods

Abstract

A hair drying / styling device comprises a tubular arm 14 coupled to a handle portion 12. The tubular arm comprises a curved multilayer heater which has a plurality of layers (62, 63, 66, 68, fig. 3a; l

Description

Field of the Invention The present invention relates to hair drying and / or styling devices and to their method of manufacture and use. Such styling and / or drying of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to styling and / or drying devices comprising one or more low thermal mass heaters. Background to the Invention Heated hair styling tools use heat to increase the temperature of hair to a desired styling temperature. For example, a hair straightener having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling / drying. The term “wet” as used herein should be interpreted broadly, to encompass not only hair wetted by water, but also hair wetted by liquids other than water. For example, hair may be wetted by styling aids, solvent-based colourants, etc., with which the invention may be used to dry and / or style the hair. Also, when stylers are used in the styling / drying of such wet hair, the styler is sometimes referred to as a “wet-to-style” (WtS) styler. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a hair glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair. Similarly, a heated brush or hair dryer can also be used to style hair by heating air which in turn heats the hair to a temperature suitable for styling. The hair is typically styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance (often such plates are manufactured from metal substrates). These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. Disadvantageously, users of such current devices may bum their skin when using the device (e.g. their fingers, hands, ears or neck), because the device is substantially heated before the device is used for styling. Also, whilst the user waits for the device to heat, the user may inadvertently leave the device on a surface which could become damaged if the surface contacts the heated substrate (e.g. flooring, such as carpets, could become damaged). Moreover, current heating technology is poor at heating complex styling surfaces, such as the surfaces of stylers which are curved (or otherwise shaped) in a uniform manner (e.g. curlers / rollers), particularly when the styler is loaded with hair being styled (and / or dried), and therefore current curling devices are relatively inefficient styling devices. Additionally, current heating technologies also typically heat the entirety of the heater surface, thereby causing inefficiencies in terms of excessive energy use, given that much of the energy provided to such stylers is lost to the surroundings rather than being used for styling hair. In an attempt to reduce energy wastage, current temperature control technology uses a limited number of sensors to reduce the cost of the styling device (e.g. thermistors / thermocouples, etc.) to measure the temperature of the hair styling heater(s). These sensors are often placed a distance away from the surface of the hair being styled, and are often separated from the hair by high thermal mass materials such as aluminium, ceramic or other high thermal mass materials, which form the heater’s heating surface. Consequently, the reaction time and accuracy of the temperature sensor is reduced, and as a result it is difficult to sense or predict hair temperature accurately. There have been recent developments by the Applicant and other companies in developing hair styling appliances that use heaters having a lower thermal mass that can therefore heat up and cool down much more quickly. Such low thermal mass heaters are therefore more responsive to loading with hair and are easier to control to dynamically vary the temperature with time, and hence control the hair temperature and moisture content of the hair being styled. However, there is a need for further improvements in hair drying and / or styling devices. The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary of the Invention Disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device; a tubular arm coupled to the handle portion, the tubular arm comprising a curved heater for heating hair; wherein the curved heater is a multilayer heater comprising: a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted on the tubular arm so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer. The curved heater may comprise a first flexible portion and a rigid portion, wherein the rigid portion is mounted within the tubular arm and the first flexible portion extends over at least a first portion of the outer surface of the tubular arm. The curved heater may comprise a second flexible portion that extends over a second different portion of the outer surface of the tubular arm. The first flexible portion may extend over a first half of the outer surface of the tubular arm and the second flexible portion may extend over a second half of the outer surface of the tubular arm. The or each flexible portion may comprise heater electrodes that heat the curved heater when current is applied to the heater. The rigid portion may comprise drive and control circuitry for controlling application of current to the heater electrodes The device may further comprise a second curved heater comprising a third flexible portion and a second rigid portion, wherein the second rigid portion may be mounted within the tubular arm and the third flexible portion may extend over a second portion of the outer surface of the tubular arm. The tubular arm may comprise a support for supporting the flexible substrates. The support may be formed from one of a liquid crystal polymer or glass filled nylon. The device may further comprise a means for cooling the device. The means for cooling the device may comprise a heat sink and / or heat pipe and / or a thermoelectric cooler. Also disclosed is a hair drying and / or styling device comprising: a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other at their proximal ends and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first head portion comprises an inner surface that convexly faces towards a concave inner surface of the second head portion, wherein the inner surfaces of the first and second head portions each comprise a curved heater that heats hair that is sandwiched between the first and second head portions when the arms are in the closed configuration. The device may further comprise means for cooling an edge of the concave inner surface. The edge of the concave inner surface may be porous. The means for cooling may comprise a fan, and the device may further comprise means for dispensing a styling fluid via the fan to the porous edge of the concave inner surface. The means for cooling may be disposed in the first and / or second arm. The means for cooling may comprise a heat sink and / or heat pipe and / or a thermoelectric cooler. Power may be supplied to the means for heating hair in response to a means for detecting motion of the device determining that the device is in motion. The edge of the concave inner surface may be configured to determine the temperature of the hair sandwiched between the first head portion and the second head portion. Also disclosed is a hair drying and / or styling device comprising: a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first and second head portions each includes an inner surface that faces towards the inner surface of the other head portion and an outer surface that faces away from the inner surface of the other head portion; wherein the inner surfaces of the first and second head portions each comprise a first heating portion for heating hair that is sandwiched between the first and second head portions when the arms are in the closed configuration; and wherein the outer surfaces of the first and second head portions each comprise a second heating portion for heating hair that is wrapped around the outer surfaces of the first and second head portions. The outer surfaces of the first and second head portions may be configured to sense the temperature of hair that is wrapped around the outer surfaces of the first and second head portions. The device may be configured to operate in a first mode in use, in which the first heating portion is powered, and the second heating portion is not powered. The device may be configured to operate in a second mode in use, in which the first heating portion is not powered, and the second heating portion is powered. The device may further comprise a means for locking the first and second arms in the closed configuration. The device may further comprise a sensor for sensing whether the first and second arms are in the open configuration or the closed configuration. The device may further comprise means for cooling the first and second heating portions. The means for cooling may comprise a heat sink and / or heat pipe and / or a thermoelectric cooler. Also disclosed is a hair drying and / or styling device comprising: a first elongate arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second elongate arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first head portion of the first elongate arm and the second head portion of the second elongate arm extend along a curved path. The first and second head portions may include respectively opposing first hair contacting surfaces. The first and second head portions may include respectively opposing second hair contacting surfaces. The first hair contacting surface and the second hair contacting surface of each head portion may be arranged successively along the length of the respective arm. The first hair contacting surface of each head portion may be flat and the second hair contacting surface may be curved. At least one of the first hair contacting surfaces may be heated. The first head portion and the second head portion may have a length between 90 mm and 150 mm. Also disclosed is a hair styling device comprising: a handle portion for holding the device; first and second elongate roller portions each rotatably coupled to the handle portion and each comprising an outer curved surface having a plurality of lobes and troughs arranged circumferentially around the respective roller portion; wherein the first and second roller portions are arranged to rotate in opposite directions about their longitudinal axis and are positioned so that during rotation of the first and second roller portions, the lobes of the first roller portion engage with the troughs of the second roller portion and the lobes of the second roller portion engage with the troughs of the first roller portion; wherein during use, hair is sandwiched between the outer surfaces of the first and second roller portions and is styled as the first and second roller portions rotate; wherein the outer curved surface of at least one of the first and second roller portions comprises a curved heater for heating hair as it passes between the outer surfaces of the first and second roller portions. The device may comprise a motor disposed within the handle portion, wherein the motor may be configured to drive rotation of the first and second rollers The motor may be configured to increase or decrease the rate of rotation of the first and second rollers in response to an input. The first and second roller portions may each comprise three lobes and three troughs. At least one of the first and second roller portions may comprise a position sensor configured to determine the position of the first and second roller portion. The device may further comprise a means for controlling the temperature of the curved heater in response to the rate of rotation of the first and second rollers. Also disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device; first and second elongate head portions coupled to the handle portion and moveable between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first and second elongate head portions; wherein the first elongate head portion comprises a first elongate concave portion, a first elongate convex portion and a second elongate concave portion arranged successively in a direction perpendicular to a longitudinal axis of the first elongate head portion; wherein the second elongate head portion comprises first and second elongate concave portions; wherein the first and second head portions are arranged so that when in said closed configuration, the first and second elongate concave portions of the second elongate head portion respectively nest with the first and second concave portions of the first elongate head portion; wherein each of the first and second elongate head portions comprises heating means for heating the first and second concave portions of the respective elongate head portion, whereby hair that passes between the first and second head portions during use is heated and styled by the concave portions. The handle portion may further comprise means for cooling, wherein the means for cooling may be configured to cool the first and second concave portions of the respective elongate head portion. The means for cooling may comprise one of a speed-adjustable fan, heat sink, and / or heat pipe and / or a thermoelectric cooler. The heating means may be configured to sense the temperature of hair contacting the heating means. When the heating means determines that hair reaches a preconfigured or preselected styling temperature, the device may be configured to provide a signal. Also disclosed is a hot roller system comprising: one or more hot rollers having a curved outer surface on which hair can be wound; wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered; a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers; wherein the handle portion is configured to provide electrical power to the electric heater of a hot roller when the handle portion is coupled to the hot roller. The hot roller may comprise: a curved outer surface on which hair can be wound, the curved outer surface having a curved electric heater that heats hair wound on the hot roller when powered; and a coupling portion for coupling the hot roller to a handle portion of the system through which electrical power for heating the curved heater is provided. The hot roller may further comprise a plurality of bristles, said bristles being extendable from and retractable into a main body of the hot roller by an actuating means. The actuating means may be configured to extend and retract the bristles via a rotary-type mechanism. The actuating means may be configured to extend and retract the bristles via a radial-type mechanism. The actuating means may be configured to extend and retract the bristles via an axial-type mechanism. The handle may comprise a motor configured to engage with and to rotate the hot roller. The handle may comprise a ratcheting means for disengaging the application of rotational forces to the hot roller in response to a predetermined torque being reached. Also disclosed is a hot roller system comprising: one or more hot rollers having a curved outer surface on which hair can be wound; wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered; a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers; wherein the handle portion is configured to rotate the coupling portion of a hot roller when coupled to the handle portion to facilitate winding of user hair onto the hot roller. Also disclosed is a hot roller for curling hair comprising: a curved housing around which hair can be wound an elongate sheet attached at one end to said curved housing and having an unravelled state in which hair can be placed on the sheet and a ravelled state in which the elongate sheet and hair sandwiched between the elongate sheet and the curved housing are wound around the curved housing; wherein the elongate sheet and / or the curved housing comprises a curved heater for heating the hair when wound between the curved housing and the elongate sheet. A distal end of the elongate sheet may comprise a hair clip, said hair clip comprising a first releasable securing portion and second releasable securing portion, said securing portions mutually configured to mate to secure hair sandwiched therebetween. The curved housing may bear a plurality of elongate notches for tensioning hair wound between the curved housing and the elongate sheet. The curved housing may comprise means for winding the elongate sheet from the unravelled state to the ravelled state. Also disclosed is a hot roller for curling hair comprising: an elongate sheet comprising: a biasing means for biasing the elongate sheet in a ravelled state, a proximal end having a rigid portion for connecting to a power supply, and a distal end having a plurality of bristles for securing the elongate sheet to hair to be curled, wherein a side of the elongate sheet which contacts the hair to be styled comprises a curved heater for heating hair wound thereagainst. The hot roller may further comprise a clip for releasably securing the elongate sheet to the hair being curled. The clip may be formed from tensile plastic. The clip may magnetically secure the elongate sheet to the hair being curled. Power may be supplied to the hot roller via a battery placed inside a cavity formed by the roller in its ravelled state, or via a separate external power supply. Also disclosed is a hot roller system for curling hair comprising: a tubular clip moveable between an open configuration in which hair can be inserted into the clip and a closed configuration in which the clip grips the inserted hair; an actuator for moving the tubular clip between the open and closed configurations; a curved heater mounted to the tubular clip; and a controller for controlling the actuator and for controlling the heater to heat hair that is gripped by and wound around an outer surface of the tubular clip. The system may further comprise a handle insertable into a cavity formed by the tubular clip, wherein the handle may comprise a motor which may be configured to rotate the tubular clip to facilitate the winding of hair onto the outer surface of the tubular clip. Also disclosed is a hair drying and / or styling device comprising: a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first and second head portions each includes an inner surface that faces towards the inner surface of the other head portion and an outer surface that faces away from the inner surface of the other head portion; wherein the inner surfaces of the first and second head portions each comprise a first heating portion for heating hair that is sandwiched between the first and second head portions when the arms are in the closed configuration; and wherein at least one heating portion comprises a plurality of perforations therethrough. The plurality of perforations may be arranged along a length and width of the at least one heating portion. The perforations furthest from the proximal end of the at least one heating portion may have a reduced size relative to perforations arranged closest to the proximal end of the at least one heating portion. The perforations closest to a peripheral edge of the at least one heating portion may be smaller than perforations closest to a central portion of the at least one heating portion. The device may further comprise means for conveying fluid to exit / enter the device via the plurality of perforations. The heating portion may be configured to sense the temperature of the heating portion, and to increase or decrease temperature of the heating portion upon comparing the sensed temperature with a desired temperature. Also disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device; a head portion coupled to the handle portion, wherein the head portion comprises a heater having a surface for heating hair; wherein the heater forms an outer hair contacting surface of the head portion and wherein the heater comprises a plurality of perforations disposed on the hair contacting surface. The device may further comprise a plurality of bristles which are configured to protrude via a subset of the plurality of perforations disposed on the hair contacting surface. The device may further comprise a plurality of bristles which are configured to protrude via each of the plurality of perforations disposed on the hair contacting surface. The device may further comprise means for conveying fluid to exit / enter the device via the plurality of perforations. The head portion may be tubular. The head portion may have a diameter between 22 mm and 40 mm. The head portion may be paddle shaped. The paddle shaped head portion may have a length between 90 mm to 120 mm, and wherein the paddle shaped head portion may have a width between 30 mm to 50 mm. Also disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device; a head portion coupled to the handle portion, wherein the head portion comprises a hair contacting surface, wherein the head portion comprises a heater disposed within a cavity formed inside the head portion; wherein the heater is configured to provide heat to the hair contacting surface of the head portion, and wherein the head portion comprises a plurality of perforations disposed on the hair contacting surface. The device may further comprise means for conveying fluid to exit / enter the device via the plurality of perforations. The head portion may be tubular, and may have a diameter of between 22 mm to 40 mm. Also disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device; a tubular head portion coupled to the handle portion, wherein the tubular head portion comprises at least one slot that extends longitudinally along a length of the tubular head portion and at least one curved heater for heating hair; wherein the at least one heater forms an outer hair contacting surface of the tubular head portion; and wherein the handle portion comprises a fan configured to drive fluid to exit or enter the tubular head portion through the at least one slot. The at least one slot may extend substantially parallel to a longitudinal axis of the head portion. The at least one slot may extend longitudinally along a length of the head portion at an angle to a longitudinal axis of the head portion. The handle portion may comprise a fluid dispenser for dispensing a hair styling product. The longitudinal slot may be configured to output fluid over a curved outer surface of the head portion whereby, during use, hair is caused to be wrapped around the outer surface of the head portion due to a Coanda effect of the fluid flowing over the curved outer surface of the head portion. Also disclosed is a hair drying and / or styling device comprising: a handle portion for holding the device, the handle portion comprising a fan for generating a stream of air; a head portion coupled to the handle portion to receive the stream of air from the fan, the head portion having a proximal end and a distal end; wherein the head portion comprises a longitudinal slot extending along the head portion from the proximal end to the distal end of the head portion; wherein the fan is configured to drive the stream of air to exit the head portion through the longitudinal slot; and wherein the longitudinal slot is divided longitudinally into first and second portions by a longitudinal dividing portion disposed within, and extending along, the longitudinal slot. The longitudinal slot may taper towards the distal end of the head portion. The longitudinal slot may comprise a plurality of bristles which extend therefrom. The handle portion may comprise a motor configured to vibrate the longitudinal slot in use. The handle portion may comprise a fluid dispenser for dispensing a hair styling product into air exiting the device. The longitudinal dividing portion may extend out of the longitudinal slot. The longitudinal slot may extend in a direction that is substantially parallel to a longitudinal axis of the head portion. The device may comprise a single longitudinal slot. The longitudinal slot may comprise a nozzle which projects perpendicularly therefrom and which may be configured to concentrate the air being driven by the fan. The device may further comprise an air heater for heating the air stream before it is driven through the longitudinal slot. Also disclosed is a hair drying and / or styling device comprising: a handle for holding the device, the handle comprising a fan for generating a stream of air; a tubular head coupled to the handle to receive the stream of air from the fan, the tubular head having a proximal end and a distal end; wherein the tubular head comprises: a plurality of bristles disposed on an upper portion of the tubular head; a plurality of longitudinally extending slots disposed on a lower portion of the tubular head; and a heater mounted on an outer surface of the upper portion of the tubular head; wherein the fan is configured to drive air to exit the device via the plurality of longitudinal slots. The bristles may project from a longitudinal slot formed in the tubular head. The device may further comprise an air heater for heating the stream of air from the fan. The device may further comprise means for heating the plurality of bristles. The means for heating the plurality of bristles may comprise portions of the heater that extend over a surface of the bristles. The device may further comprise a sensor for sensing the temperature of the lower portion of the tubular head. The speed of the fan may be increased when the sensor senses that the temperature of the lower portion of the tubular head exceeds a threshold temperature, and wherein the speed of the fan may be decreased when the sensor senses that the temperature of the lower portion of the tubular head is below a threshold. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1a shows an overview of an exemplary hair styling device; Figure 1b shows a hair styling device in use; Figure 2 is a block diagram illustrating the main electronic components of the hair styling device shown in Figure 1; Figure 3a is an exploded view of a heater forming part of the hair styling device shown in Figure 1; Figure 3b is an assembled partially transparent view of the heater shown in Figure 3a; Figure 4a schematically illustrates the heating zones on the heating surface of the heater shown in Figure 3; Figure 4b schematically illustrates an alternative arrangement of heating zones; Figure 5 schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes; Figure 6a illustrates the way in which the heating zones may be formed on a tubular substrate for use in a curling tong or the like; Figure 6b illustrates the way in which the heating zones may be arranged on a curved substrate which may be used on a heated brush; Figure 7 illustrates a tress of hair that partly overlaps with zones Z2 and Z4 of a heater; Figure 8 illustrates a cross-sectional view of a further example of a low thermal mass heater that has curved edges and a supporting substrate onto which the heater is attached with an adhesive or via a diffusion bonding process (e.g. by melting them together); Figure 9 is a partially exploded cross-sectional and perspective view of the different layers that form the heater shown in Figure 8; Figure 10 is a plan view illustrating the form of a heat spreading layer forming part of the heater illustrated in Figure 8; Figure 11 illustrates a main heating element layer forming part of the heater shown in Figure 8; Figure 12 is a simplified block diagram illustrating the way in which the heater electrodes of the heater shown in Figure 8 are used to heat the heater and to sense the temperature of the heating zones; Figure 13 shows a perspective view of another exemplary hair styling device; Figures 14a, 14b and 14c are plan views of layers which respectively form a heater; Figures 15a and 15b are partially exploded cross-sectional and perspective view of the different layers shown in Figures 14a, 14b and 14c; Figures 16a and 16b are schematic plan views of heating zones of a hair styling heater; Figures 17a and 17b show a hair styling heater configured in a barrel shape; Figures 17c and 17d show the heater of Figures 17a and 17b having a support member; Figures 18a and 18b show a first method of manufacture of the heater of Figure 17a; Figures 18c and 18d show a second method of manufacture of the heater of Figure 17a; Figures 19a and 19b show a third method of manufacture of the heater of Figure 17a. Figure 20a shows an overview of another exemplary hair styling device in its open configuration; Figure 20b shows the hair styling device of Figure 20a in in its closed configuration; Figure 20c shows the hair styling device of Figure 20a in a front-on profile view and showing hair styling “cool zones”; Figure 21a shows an overview of another exemplary hair styling device in its open configuration; Figure 21b shows the hair styling device of Figure 21a in its closed configuration; Figures 22a, 22b and 22c show in an exploded perspective view of the layers which form a hair styling heater for the hair styling device illustrated in Figure 21a; Figure 23a shows a perspective overview of another exemplary hair styling device in its open configuration; Figure 23b shows a profile view of the hair styling device illustrated in Figure 23a in its open configuration; Figure 23c shows a profile view of the hair styling device illustrated in Figure 23a in its closed configuration; Figure 24a shows a perspective view of another exemplary hair styling device; Figure 24b shows an end view of the hair styling device illustrated in Figure 24a; Figure 25a shows a perspective overview of another exemplary hair styling device in its open configuration; Figure 25b shows a perspective overview of the hair styling device of Figure 25a in its closed configuration; Figure 26 shows an end view the hair styling device of Figure 25a in its closed configuration; Figure 27a shows a perspective overview of another exemplary hair styling device; Figure 27b shows in a schematic cross-sectional view the hair styling device of Figure 27a with bristles retracted; Figure 27c shows in a schematic cross-sectional view of the hair styling device of Figure 27a with bristles deployed; Figure 28 shows the hair styling device of Figure 27a in use; Figure 29a shows a perspective view of another exemplary hair styling device in an unravelled configuration; Figure 29b shows a perspective view of the hair styling device of Figure 29a in a ravelled configuration; Figure 29c shows a portion of hair styling device of Figure 29a for securing the device to a user’s hair; Figure 29d shows the portion of hair styling device of Figure 29c in its secured configuration; Figure 30 shows a profile view of the hair styling device illustrated in Figure 29a; Figure 31a shows a perspective view of another exemplary hair styling device in an unravelled configuration; Figure 31b shows a perspective view of the hair styling device of Figure 31a in a ravelled configuration; Figure 31c shows a perspective view of the hair styling device of Figure 31a in use; Figure 32a shows a perspective overview of another exemplary hair styling device in its unsecured configuration; Figure 32b shows a perspective overview of the hair styling device of Figure 32a its secured configuration; Figures 32c and 32d each show cross-sectional views of the hair styling device of Figure 32a and showing directions of respective biasing forces which may be applied to the device; Figure 33a shows a schematic plan view of a first repeating unit pattern for a perforated hair styling heater; Figure 33b shows a schematic plan view of a second repeating unit pattern for a perforated hair styling heater; Figure 34 shows a schematic plan view of the relationship between a tress of hair and a hair styling heater; Figures 35a and 35b graphically show a three-stage hair heating / styling procedure; Figure 36a shows the interaction of a tress of hair sandwiched between two hair styling heaters; Figure 36b schematically shows a barrel heater in plan view; Figure 37 shows in plan view a perforated hair styling heater; Figure 38 shows in plan view an alternative perforated hair styling heater; Figure 39a shows a perspective view a hair styling apparatus having perforated heaters; Figure 39b shows in an exploded perspective view the three layers which form a perforated hair styling heater; Figure 40a shows in plan view a perforated hair styling heater having a symmetric arrangement of perforations; Figure 40b shows in plan view an alternative perforated hair styling heater having another symmetric arrangement of perforations; Figure 41a shows in perspective view another exemplary hair styling device having a perforated tubular heater; Figure 41b shows in a disassembled plan view and in an assembled perspective view the perforated tubular heater of Figure 41a; Figure 42a shows in perspective view another having a tubular heater bearing bristles; Figure 42b shows in perspective view another having a paddle-shaped heater bearing bristles; Figure 43a shows in perspective view another exemplary exemplary exemplary hair hair hair styling styling styling device device device having a tubular heater comprising perforations and bearing bristles; Figure 43b shows in a disassembled plan view and in an assembled perspective view the perforated tubular heater of Figure 43a; Figure 43c shows a perspective view the perforated tubular heater of Figure 43a; Figure 43d shows a perspective view of an alternative perforated tubular heater to that shown in Figure 43c; Figure 44a shows in perspective view another exemplary hair styling device having a paddle-shaped heater comprising perforations and bearing bristles; Figure 44b shows in plan view the paddle-shaped heater comprising perforations and bearing bristles illustrated in Figure 44a; Figure 45a shows in plan view a hair styling device bearing a tubular heater having elongate perforations; Figure 45b shows in a disassembled plan view the heater used by the hair styling device illustrated in Figure 45a, Figure 45c shows a close-up plan view of the heater illustrated in Figure 45b; Figure 46a shows in a schematic profile view the hair styling device illustrated in Figure 45a; Figure 46b shows the head-end of a hair styling device in a plan view bearing a tubular heater having elongate perforations offset with respect to the device’s longitudinal axis, Figure 46c shows in a schematic profile view the hair styling device illustrated in Figure 46b; Figure 47a shows in a schematic plan view a zonal hair styling device having two zones; Figure 47b shows in a schematic plan view a zonal hair styling device having four zones; Figure 48a shows a plan view of a hair styling apparatus having an elongate tapered outlet; Figure 48b shows an alternative plan view of the hair styling apparatus having an head comprising an elongate tapered outlet illustrated in Figure 48a; Figure 48c shows a cross-sectional view of the head of the hair styling apparatus along the line A-A' of Figure 48b; Figure 49a shows a perspective view of an alternative hair styling apparatus having an elongate tapered outlet; Figure 49b shows a cross-sectional view of the head of the hair styling apparatus along the line A-A' of Figure 49a; Figure 50a shows in perspective view a head of a hair styler comprising bristles and having two zones; Figure 50b shows in a schematic profile view the head of a hair styler having two zones; Figure 50c shows a plan view of the head illustrated in Figure 50a; Figure 51a shows a schematic sectional view of the head illustrated in Figure 50a; Figure 51 b shows a side view of the head illustrated in Figure 50a; Figure 51 c shows a schematic sectional view of the head illustrated in Figure 51 d; Figure 51 d shows a side view of an alternative to the head illustrated in Figure 50a; Figure 52a shows in perspective view a head of a hair styler comprising bristles and having two zones; Figure 52b shows in a schematic profile view the head of a hair styler having two zones; Figure 52c shows a plan view of the head illustrated in Figure 50a; Figure 53a shows a plan view of a heater having two different perforations; and Figure 53b shows a perspective view of the heater illustrated in Figure 53a. Overview of a Hair Styling Device Figure 1a illustrates a hand held (portable) hair styler 1. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled at proximal ends thereof to a shoulder (or hinge) 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. A user interface 11 is provided to allow the user to set user defined parameters and for the device to output information to the user. For example, a desired operating temperature may be set via the user interface 11. The user interface 11 may have a dial, button or touch display for allowing the user to input information to the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. In this embodiment, the user interface 11 also comprises a control button or switch 14 to enable the user to turn the device 1 on or off; and an indicator light 15 to show whether the power is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11 (additionally or instead, the printed circuit board may be provided external to the device 1, e.g. be incorporated into a plug and / or incorporated into a PSU located on the cable between the plug and the device 1). In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains or a DC power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. In use, the device 1 is turned on, enabling power to flow through the heaters 6 to cause them to heat up. The user then opens the first and second arms 4a, 4b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 40 (which may be clumped) is introduced between the arms 4a, 4b, transversely across the heaters 6a, 6b. The user then closes the arms 4a, 4b so that the length of hair 40 is held between the first and second arms 4a, 4b and then the user pulls the hair through the closed arms (as illustrated in Figure 1b). The outer (hair contacting) surface of the heaters 6 is flat in this embodiment so that the hair styler 1 can be used to straighten the user’s hair. The hair styling device 1 shown in Figure 1 can also be used to curl the hair by turning the device 1 through approximately 180 degrees or more after clamping the hair between the arms 4a, 4b and before moving the device 1 along the tress of hair 40. Hair has a relatively high thermal mass and when in contact with the heating surface of the heater 6 the hair absorbs a significant amount of the heat energy. The heaters 6 must quickly supply the lost heat energy back to the heating surface otherwise the temperature of the heating surface will drop and potentially impact on the quality of the thermal styling. If the temperature of the heaters 6 fall below that required to raise the hair temperature above the glass transition temperature of the hair, the hair will not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo significant damage. As such, the device 1 should be able to control the temperature so that the heating surface of the heaters 6 remains within a particular temperature range. Furthermore, it should maintain the temperature range both when hair is frequently and quickly loaded and unloaded onto the heating surface, and when hair is held on the heating surface for a prolonged period of time. Control Circuitry Figure 2 is a simplified block diagram of control circuitry 15 that controls the operation of the hair styler device 1 shown in Figure 1. As shown, the control circuitry 15 comprises a power supply 21 that, in this embodiment, derives power from a battery power source. A mains power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 1. Alternatively, the power supply 21 may derive power from an AC mains supply input. In this example, power is provided to the heaters 6 for heating the users hair. The power supplied to the heaters 6 is controlled by a controller 28 having a microprocessor 29. The power supplied to the heaters 6 is controlled by drive circuitry 23 (which may include one or more power semiconductor switching devices (triacs)) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery, to the heaters 6 in accordance with instructions from the microprocessor 29. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The temperature measurement circuitry 25 may be temperature sensors such as thermistors or they may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 6, which resistance depends on the temperature of the heater electrode. Figure 2 also shows that the user interface 11 is coupled to the microprocessor 29, for example to provide one or more user controls and / or output indications such as a visual indication or an audible alert or haptic feedback, for example. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 6 or if they are moving the device 1 too quickly along the hair tress 40. Finally, the control circuitry includes communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g. on a mobile telephone). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. Heaters The heaters 6a, 6b are low thermal mass heaters and can therefore heat up and cool down quickly. Figures 3a and 3b show an exemplary embodiment of such heaters 6a, 6b, which comprise a stack of thin layers. Referring in particular to Figure 3a, the heaters 6a, 6b include an upper dielectric (electrically insulating) layer 62, an electrode layer 63 that has a plurality of separate heater electrodes 64, and a lower dielectric layer 66 which electrically insulates the heater electrodes 64 from other components mounted behind the heater 6a, 6b. The three layers 62, 63 and 66 are bonded together either through an adhesive layer (pressure set or thermoset), or through diffusion bonding (thermoforming) of the contacting materials (e.g. melting them together) and define a heater 6 that is very thin (the three layers have an overall thickness of between 30pm to 1000pm in the case of Safe Extra Low Voltage (SELV) operation (less than 42.4 Volts) and 0.8mm to 2.0mm in the case of AC operation) and with very low thermal mass. The upper surface of the layer 62 provides the hair contacting surface of the heater 6, although a non-stick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair over the heating surface if the layer 62 does not itself have such non-stick properties. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 into a rigid support 68 which forms a base. These layers may be mounted onto the rigid support after the layers themselves have been bonded together or they may be bonded one at a time (or multiple at a time) onto the rigid support 68. If a flexible heater is desired, as discussed in greater detail below, then there is no need for the rigid support 68 or if a support is used, this may be a non-rigid support. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 6, and instead, the heaters 6 directly heat the user’s hair. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down much more quickly than prior art stylers. In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. It may be generally understood that this is a nonlimiting arrangement and other serpentine layouts that snake across and back across the width of the heater 6, folding once or more, may be used. Alternatively, non-serpentine arrangements of heater electrodes, such as interleaving fingers or spirals of heater electrodes, could be used. The ends of each of the heater electrodes 64 are electrically connected through the lower dielectric layer 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry 23 that may be mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies electrical power to the individual heater electrodes 64 to control the heat generated by each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 3a and 3b as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction). Each of the heater electrodes 64 thus creates an individual heating zone 642 on the hair contacting surface of the heater 6, which spans the width (which we shall refer to as the x-direction) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction) of the heater 6. Figures 4a and 4b show schematic views of different arrangements of such heating zones 642. Figure 4a shows an arrangement corresponding to that of Figures 3a and 3b, in which the heating zones 642-1 to 642-10 are arranged along the y-direction only. Figure 4b shows an alternative arrangement, in which heating zones 642-1 to 642-16 are arranged in both the x- and y-directions. Such an arrangement of heating zones 642 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 3a side by side in the width (x-) direction, such that the heater electrodes 64 snake back and forth in a serpentine arrangement between positions adjacent to an outer edge of the heater 6 and a position adjacent to the mid-point of the heater 6 in the x-direction in their respective half of the heater 6. The heaters 6 may be separated in this way into any number of heating zones 642 and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 4b shows two zones along the x-direction, a greater number of zones in the x-direction could also be provided, with the heater electrodes and their electrical connections being designed in a suitable manner to fit such an arrangement. The heating zones 642 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6 such as those shown in Figure 3. It will be appreciated that the arrangement of the heater electrodes 64 may comprise any suitable layout to enable the desired arrangement of heating zones 642. The heating zones illustrated in Figure 4 are all the same size. Of course, different sized heating zones 642 may be provided, as illustrated in Figure 5, which shows a heater 6 having seven different sized heating zones (labelled Z1 to Z7). The way in which the heater electrodes 64 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here. The heating zones 642 described above form part of a heater having a flat hair contacting surface. The heater is not limited to flat hair contacting surfaces and can be configured for use a tubular form (as illustrated in Figure 6a) for example for use in a hair curler device or in a curved form (as illustrated in Figure 6b) for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. The temperature of each heating zone 642 is independently controllable. Each heating zone 642 can be set to a target temperature. The target temperature of each heating zone 642 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 642 which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone 642 can be determined by determining the resistance of the corresponding heater electrode 64. The microprocessor 28 controls the heating in order to reduce the difference between the actual temperature of the heating zone 642 and the target temperature for that heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 6 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 6 up into plural heating zones. These heating zones can be equally and / or unequally sized and can be arranged regularly and / or irregularly across the width and length of the heater. However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair (which is assumed to be the realistic worst case scenario during operation) and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone in its entirety. That applied power will bring the average temperature of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will be above the average temperature of the heating zone. This temperature increase may be sufficient to cause the unloaded part to overheat. At the same time the loaded part of the heating zone will be below the average temperature causing a reduction in heat transfer and reduced styling performance. This situation is illustrated in Figure 7, which shows a tress of hair 40 overlying heating zones Z2, Z3 and Z4, with heating zone Z3 being fully loaded with hair and heating zones Z2 and Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 64 for each heating zone back to the drive circuitry 23 as well as the number of control switches in the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature difference between the loaded and unloaded halves of a heating zone, a maximum size of the heating zones can be defined which depends on the maximum power density to hair that can be extracted from the heating zone and the material characteristics and thicknesses of the layers forming the heating zone. Specifically, if it is assumed that only one half of a heating zone 642 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone 642 can be defined with the equation below: where, = maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone; = target operational temperature or average temperature (°C) of individual heating zones; 4 = power density (Wm-2) required to heat hair passing over the surface to the desired temperature for styling; W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and fe = the thickness averaged thermal conductivity of the layers that constitute the heating zone. If it is assumed that the thickness averaged thermal conductivity of the constituent layers of a heating zone 642 and the total thickness of the layers that form the heating zone 642 are known and fixed (for any given device), then the above equation can be used to determine the required zone width (W) and hence a number of divisions along the length of the heater that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to maintain and / or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that should be positioned along the length the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of heater materials and without causing the temperature of the unloaded part of a heating zone 642 to exceed a maximum differential temperature (LTmax) that could cause burning of relatively small bundles / strands of hair that come in contact with the overheated regions of the heating zone. Specifically, the maximum area can be determined from: Where, L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface); = number of zonal divisions along the length of the heater plate; = maximum permitted temperature (°C) on the surface of the heater (which would occur in the unloaded half (worst case) of an individual heating zone) needed to avoid damage to hair or the heater; = target operational temperature or average temperature (°C) of individual heating zones; fe = the thickness averaged thermal conductivity (Wm'1oC'1) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and 4 = power density (Wm-2) required to heat hair passing over the surface to the desired temperature for styling. For a hair styling device, the inventors have found the following suitable ranges for these parameters: - Power density required for styling ($) is greater than 40,000 W / m2 and less than 100,000 W / m2 - The average thermal conductivity of the layers forming the heating zone (fc) (averaged through the depth of the various layers) is between 80 and 200 W / m.K. - The maximum permitted temperature of a heating zone to manage (ideally avoid) hair damage is less than 250°C, more preferably less than 220°C and most preferably less than 200°C. - The total thickness of the layers (t) which make up the heater is less than 300pm but no less than 75pm due to manufacturing limitations. - The target operational temperature of the heater (Trsr) is between 150°C and 230°C. Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1.7 cm. Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 4a). If multiple rows of heating zones 642 are provided along the length of the heater (such as is shown in Figure 4b), then each row of heating zones 642 should meet the limits defined above if the above described overheating problem is to be avoided. Alternative Heater Arrangement An alternative flexible heater 6’ is illustrated in Figure 8, which shows on the left hand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 8, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive or diffusion bonding (thermoforming) of the underlying materials to the upper surface of the rigid substrate 68’, or by over-moulding in which the carrier is injection moulded over the back of the flexible heater within the mould. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 8 also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example thermistors for sensing the temperature of the heating zones 642 of the heater 6’ or fuses that can electrically isolate the heater electrode of each zone in case of the zone overheating. Figure 8 also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 3 that controls the heating of the different heating zones 642 of the heater 6’. As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm. The different layers forming part of the heater 6’ are shown in exploded cross-sectional and perspective views in Figure 9. A description of each layer is given below. Low Friction Coating 81 (Optional) This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 3 pm) to reduce the thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant. This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. This is needed because the coating is prone to cracking when flexed, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape. Alternatively, this coating may itself comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm). Heat Spreading Layer 82 (optional) This is also an optional layer and, when provided, helps to spread the heat within each heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. This layer may be formed from copper or from another suitable material. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics 15 would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 -from the perspective of the control electronics 15, the heating zone 642 would be at the “correct” temperature despite having hot and cold regions. Each heating zone 642 would have its own heat spreader, which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption, reduce warm up time and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 10 illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive material over the corresponding heating zone that substantially does not touch neighbouring heat spreaders to reduce heat spreading from one heating zone to an adjacent heating zone. The heat spreaders 91 may be separated from each other by a solid material having a thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91, it is difficult for heat from one heating zone 642 to pass into neighbouring heating zones 642. The solid material (dielectric and / or scratch resistant low frictions material(s)) that is provided in the gap between adjacent heat spreaders 91 may be provided by a PVD DLC, bond film, coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. Alternatively other suitable methods may be used to form substantially or fully physically separated individual heat spreaders 91 and any suitable method may be used to provide solid material in the gaps between adjacent heat spreaders 91, such as masking and vapour deposition, etc. This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality. Polyimide Separator layer 83 The polyimide separator layer 83 provides electrical isolation between the hair contacting surface of the heater 6’ (which may be the upper surface of this layer 83 if the optional layers 81 and 82 are not provided) and the main heater electrode layer. This layer 83 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642. Main Heater Electrode &Sensing Layer 84 This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 11 illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source. The resistive material may be stainless steel, nickel alloy, copper, or formed from another appropriate resistive material. Each heater electrode 64 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 11 are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. Other processes such as printing, thick film printing, physical vapour deposition and the like could be used to form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at suitable vias 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flow through each heater electrode 64. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 64 may be physically separate from all other heater electrodes 64 in which case, each end of each heater electrode 64 would be connected separately back to the drive and control board 78. As schematically illustrated in Figure 11, the end of each heater electrode 64 that is connected to the switch is provided at the side of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the heater which is curved over the upper surface 74 of the rigid support substrate 68’). The inventors have found that this arrangement helps heat generated in the heater electrodes 64 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with the curved edge portion, then the hair will still be heated as this curved edge portion is heated. The conductive material used in the layer 84 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 64 depends upon its temperature - and so the temperature of the heating zone 642 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 64. Figure 12 is a schematic view of the way in which the heater electrodes 64 may be connected together and to the drive circuitry 23 and the power supply 21. As shown in Figure 12, each heater electrode 64 is connected at one end to the power supply 21 and at the other end to a respective switch (in this case a MOSFET switch) 95-1 to 95-20. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently. When the temperature of a selected heating zone 642 is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, by measuring the voltage dropped across the resistor R (using the operational amplifier 97), the microprocessor 29 can determine the resistance of the selected heater electrode 64 and hence can determine the temperature of the corresponding heating zone 642. If the determined temperature is above the desired temperature for that heating zone 642, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone 642 is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642. Polyimide Separator (Optional) 85 When an auxiliary heater electrode layer is provided, this layer is required to provide the required electrical separation between that auxiliary heater electrode layer and the main heater electrode layer 84 described above. This polyimide layer 85 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W / mK. Other dielectric materials could be used instead of polyimide. Auxiliary Heater Electrode Layer (Optional) 86 Some embodiments of the heater 6’ may benefit from the presence of an additional heating element layer 86. This additional layer 86 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 21, for example the main power source could be a power supply and the power source for the auxiliary heater electrode layer 86 could be one or more batteries. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively still, the conductors on this auxiliary layer 86 could be used for temperature sensing, in which case, the heater electrodes 64 in the main layer 84 may only be used for heating. The heater electrodes on the auxiliary layer 86 will typically have the same form as the heater electrodes 64 used in the main heater electrode layer 84 - so that they will define the same heating zones 642 as the heating zones 642 defined by the heater electrodes 64 on the main heater electrode layer 84. The path taken by the heater electrodes on the auxiliary layer 86 do not need to follow the same path as the corresponding heater electrodes 64 formed on the main heater electrode layer 84. For example, whilst the main part of each heater electrode 64 on the main heater electrode layer 84 (ignoring the edge part of each heater electrode 64) serpentines in the longitudinal direction of the heater 6’ in Figure 11, the corresponding heater electrodes of the auxiliary heater electrode layer 86 could be arranged to serpentine in the width direction of the heater 6’. Such an arrangement may help to spread the heat flow within the heating zone 642 particularly if the heating zone 642 is only partially loaded with hair. Polyimide backing 87 This layer encapsulates the bottom heating layer (either the main or the auxiliary heating layer) so as not to allow it’s accidental exposure and to prevent moisture ingress. This backing layer 87 electrically separates the bottom heating layer from any surface mounted components that are mounted in the surface mounting layer 88 (discussed below) on the bottom of the heater 6’. If desired, this dielectric layer 87 can be made thicker than the upper dielectric layers to provide enhanced structural integrity of the flexible part of the heater system. As with the other dielectric layers, this backing layer 87 does not need to be a polyimide layer and other dielectric materials could be used. Rear Side Surface Mount Components (Optional) 88 This layer is used to mount components on to the rear of the flexible heater 6. These components may be temperature sensors (e.g. thermistors) or other components involved in providing fusing functionality for the heater (e.g. solder links). This layer may be produced using standard chemical etching methods from the PCB manufacturing process or other suitable methods such as those described above. Additional surface mount components would be added later. High Temperature Adhesive 89 The function of this layer is to enabling bonding of the flexible heater 6’ to the rigid substrate 68 (shown in Figure 8) that forms the final shape of the overall heater. Various types of adhesive could be used such as a pressure activated adhesive (PAA) or a heat activated adhesive (HAA). It could also be a thermoplastic film which sets after heat and pressure have been applied in a forming tool. Stylers Having Curved Heaters Barrel Curler As noted above, hair styling heaters in accordance with the present disclosure may adopt a curved (non-linear) configuration (as shown in Figure 6). In this regard, reference is now made to Figure 13 which schematically illustrates a handheld (portable) hair styler 10 having such a curved heater 16 that extends around the entire outer circumference of the styler 10. The hair styler 10 includes an arm 14 which extends from a handle 12. The arm 14 bears a curved zoned heater 16 at its distal end formed as described above. In the illustrated example, the curved zoned heater 16 may adopt a substantially round cross-sectional profile, e.g., a round barrel, and hence such a heater may be suitable for curling hair. However, it will be appreciated that curved heaters adopting different profiles (e.g. elliptical / tapered barrel curlers) may be used instead. The arm 14 bearing the heater 16 may be detachable from the handle 12 of the device 10, and hence multiple sizes of heater, e.g., round barrel heaters having diameters in the range of about 22 mm to 40 mm may be used interchangeably, or, instead, different profiles of heater could be used interchangeably, e.g. elliptical or tapered barrel curlers, to achieve different hairstyles. During use of styler 10, a tress of hair is wrapped around the arm 14 so that the hair to be styled is in contact with outer heating surface of the curved heater 16. At this stage, the curved heater 16 is not heated so the user will not bum themselves when loading their hair on the styler. Once the hair is in place, the user depresses a control button 24 which causes power to be supplied to loaded zones of the heater 6 (i.e., zones of the heater 6 where the tress of hair is wrapped). Because the curved heater 16 is a low thermal mass heater, the heater 16 heats up quickly. Therefore, as the tress of hair is held against the heated surface, the tress of hair is heated by conductive heating to a suitable temperature to facilitate styling / curling. To aid rapid heating of the device 10 to temperatures suitable for styling, supercapacitors may be used to boost the power available to the user at the start of the styling procedure (which may be particularly advantageous where the device 10 operates in a cordless manner). The style may be set by allowing the heated zones to cool before the hair is unwrapped from the styler 10. In some embodiments, only the zones of the curved heater that are loaded with hair may be heated (for example, by the heater being configured to detect loaded zones based, e.g., on the resistance of heater electrodes that are used to heat the heaters, or via pressure sensor) which again reduces the likelihood of the user burning themselves during use. A user interface 21 is provided to allow the user to set user defined parameters and for the device to output information to the user. For example, a desired operating temperature may be set via the user interface 21 or may be selected from pre-set temperatures stored at the device’s control circuitry 15. The control circuitry 15 may control the temperature of the loaded zones of the heater by an algorithm that detects the temperature of the hair, allowing the device to heat the hair to a target temperature. Once the hair is at the target temperature, the control circuitry 15 stops heating the zones and hence the device cools, setting the curl / style. The user interface 21 may have a dial, button or touch display for allowing the user to input information to the device 10 and the user interface 21 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user, e.g. indicating that the curl is ready as the heater has cooled to a pre-determined temperature suitable for setting the curl in the hair. In this embodiment, the user interface 21 may also comprise an indicator light to show whether the styler’s power is on. To further assist with the cooling of hair being styled, and therefore the speed at which curling can be achieved, once the control circuitry 15 stops heating the zones, the device 10 may be actively cooled by incorporation of a fan or a thermoelectric cooler, e.g. a Peltier cooler (not illustrated) into the body of the device 10, e.g. within the handle 12 or the arm 14. Such active cooling could be initiated automatically by the device 10, e.g. upon expiry of a predetermined timer which runs once heat ceases being supplied to the heater, or upon power being supplied to heater dropping below a predetermined threshold. Additionally or instead, such active cooling could be initiated by a manual user input (e.g. the user depresses / releases a control button provided on the device’s housing to initiate active cooling). Whilst the hair curler shown in Figure 13 has a “head” (i.e. the arm 14 bearing the heater 16) that is fixed to the handle 12, this need not necessarily be the case. For example, the arm 14 may be configured for attachment / detachment to / from the handle 12, and hence heaters having different curved profiles may be used with the handle 12 (e.g. differently sized barrels for forming larger / smaller curls during styling). Heater Designs for Stylers Having Curved Heaters The heater design has been described generally above, and a specific example of heater design that can be used with the heater 16 illustrated in Figure 13 will now be described with reference to Figures 14a, 14b and 14c. Figure 14a illustrates layer 1 of the heater stack up comprising a flexible insulative substrate carrying on the front side the heater electrode layer 84' (which corresponds to the main heater electrode and sensing layer 84 described above with reference to Figure 9) and vias that connect through the insulative layer to the individual heater electrodes on the heater electrode layer 84’. Each heating zone is made from a serpentine heater electrode (as before) that is connected to the control power drive circuitry via a common terminal (ground or positive) and a switching terminal. The heater electrode design is dependent on the number of zones and the required resistance of each zone (which in turn depends on the power requirements for the heater). The heater electrodes share terminals to reduce the number of connections to the control system and the control system only needs to control one switching terminal per heating zone. As in the embodiment described above, the heater electrodes are preferably switched on the ground side meaning that the heater electrodes will share a positive terminal, as this reduces the requirements on the switching MOSFETs. As illustrated in Figure 14a, the orientation of the serpentine heater electrodes is not the same in all heater zones as this helps to reduce heat spreading between adjacent zones (although it will be appreciated that the alternative heater electrode arrangements described above may be used instead). Figure 14b illustrates layer 2 of the heater stack up comprising a flexible insulative substrate carrying on the front side a heat spreading layer 82’ (which corresponds to the heat spreading layer 82 described above with reference to Figure 9) and power connection traces on the rear side of the insulative substrate that are used to connect the heater electrodes to the control system. A heat spreader may be provided per-zone (as illustrated) or a heat spreader may be provided for a given group of heating zones, or a single heat spreader may be provided over the entire arm 14. The heat spreading layer 82’ may be provided at the hair contacting surface of the heater 16 (and hence be between the heater electrode layer 84’ and the hair being styled) or it may be provided below the heater electrode layer 84' as set out in more detail below. Figure 14c illustrates layer 3 of the heater stack up. This layer includes a rigid PCB portion 78' which provides a secure point of connection for making the electrical connections between the control system and the heater electrodes. The control electronics may also be mounted on this rigid PCB portion 78’. The three layers shown in Figures 14a, 14b and 14c can be bonded securely to one another using an adhesive or using diffusion bonding or any other suitable method. The resulting heater stack will have a rigid PCB end portion connected to a flexible portion that carries the heater electrodes for the different heater zones. An exploded view of how these three layers are bonded together is illustrated in Figures 15a (front view) and 15b (back view). In this embodiment, the rigid PCB portion 78' carries the above-described drive and control electronics illustrated in Figure 3 that control the operation of the styler 10 (e.g. the heating of the different heating zones of the curved heater 16). Figures 16a and 16b respectively schematically illustrate four heater electrodes (corresponding to four zones) carried on layer 1 and the corresponding heat spreaders 82’ carried on layer 2 forming part of the heater shown in Figure 14. As shown in Figure 16a, the heater electrodes of zones 1 and 2 have serpentine tracks which are formed substantially perpendicularly to those of zones 3 and 4, and all four zones share a common terminal 84-1 (ground or positive) shown by the circle in the middle of the figure and each zone has a switching terminal 84-2 (ground or positive) at the other end of the corresponding heater electrode. Each of the four zones has a corresponding heat spreader 82’ on layer 2 to facilitate the even distribution of heat across that zone’s surface (as well as to assist in the even cooling of that zone once the power supply to that zone is interrupted and / or if active cooling of the zone using a fan or a thermoelectric cooler, e.g. a Peltier cooler, is started). The via 82-1 for connecting to the common terminal of the heater electrodes and several of the vias 82-2 that connect to the switching terminals of the heater electrodes are illustrated and labelled in Figure 16b, and these are electrically connected to the rigid PCB layer 78' via the power connections shown on the back of layer 2 in Figure 15a, 15b. Figure 17a is a cross-sectional view and Figure 17b is a perspective view of the heater stack shown in Figure 14 once assembled and mounted around the outer surface of the arm 14. As shown, the rigid portion 78’ of the heater stack is provided on the inside of the arm 14 whilst the flexible part 82’,84’ of the heater stack is wrapped around the entire outer surface of the arm 14. Thus, the length of the flexible part of the heater stack is 2tt times the radius of the arm 14 for a circular arm 14. To provide a robust barrel curler using the above-described curved heater 16, a support 90 for the heater 16 may be provided as illustrated in profile and in perspective in Figures 17c and 17d respectively. The support 90 may be manufactured from a material having a low thermal conductivity, such as liquid crystal polymers, glass filled nylon, or other alternative materials having a low thermal conductivity. The flexible heater 16 may be secured to the support 90 using an adhesive that can survive high temperatures; via diffusion bonding between the support 90 and flexible portion of the heater 16; by tensioning the flexible portion of the heater over the barrel support and using the rigid PCB part of the heater 16' to maintain the tension; or by any other suitable method. In addition to the above described “single wrap” method of constructing the barrel curler 16, alternative methods of manufacture may be used (in the examples which follow, the support 90 described above has been omitted for clarity). For example, referring to Figure 18a, the rigid PCB portion 78' may be located centrally of two flexible portions 82'a, 84'a, 82'b, 84'b each of which carries the heater electrodes for the different zones and each of which wraps around half of the barrel forming the ‘“S’ Setup” barrel curler illustrated in Figure 18b. In another alternative, two heaters may be layered on top of one another as illustrated in Figures 18c and 18d to form a two-sided heater. Figure 18c illustrates one of the two heaters, having a rigid portion 78’a and a flexible portion 82’a, 84’a as before. As illustrated by the arrow, the edge of the flexible portion 82’a, 84’a that is not coupled to the rigid portion 78a’ is bent back and connected to the far edge of the rigid portion 78a’ to form a hemispherical heater. This process is repeated for a second heater. The two hemispherical portions are then brought together and secured to form the “layered” heater illustrated in Figure 18d. Instead of the two heaters being layered, the heaters may instead be formed in a side by side manner, as illustrated in Figures 19a and 19b to form a two-sided heater having an alternative configuration. As shown in Figure 19a, first and second rigid PCBs 78'a, 78'b are connected to one another longitudinally, and a first flexible portion that is connected to the first rigid PCB 78'a is bent over and secured to the edge of the second rigid PCB 78'b (as illustrated by the dotted arrow). Similarly, the second flexible portion that is connected to the second rigid portion 78'b is bent under and secured to the edge of the first rigid PCB 78'a (as illustrated by the dotted arrow), thereby forming the “connected” heater shown in Figure 19b. Several examples of barrel curlers having flexible heaters have been described above. However, alternative stylers to curlers can be formed using the flexible heater described herein. One Stroke Curler For example, the flexible heaters 16 described herein can be used in a so-called “one stroke curler” 30 that is illustrated in Figure 20a. Figure 20a shows the styler 30 in its open position. The styler 30 has two arms 34a, 34b analogous to those of the styler described above with reference to Figure 1a (and common features, e.g. the styler’s control / drive circuitry, will not be described again). Arm 34a comprises a flexible heater 36a which curves from a straight edge of a U-shaped portion projecting perpendicular to the arm 34a (the convex portion of arm 34a shown in Figure 20a), over the curved part of the U-shaped portion, then returning to the arm 34a via the other perpendicular straight edge of the U-shaped portion. A recess (the concave portion of arm 34b shown in Figure 20a), which corresponds to the profile of the heater 36a, is provided in the other arm 34b and bears a flexible heater 36b at the bottom of the recess (the heaters 36a, 36b may have one or more heating zones, as described above). In use, the user places a tress of hair to be styled between the arms 34a, 34b and then closes the styler 30 as illustrated in Figure 20b (the tress of hair being omitted for clarity), thereby sandwiching the hair in the “U” shaped recess between the styler’s heaters 36a, 36b. The heaters 34a, 34b then heat the hair being styled as the user pulls the hair through the styler 30. Once heated, the hair is then cooled while in tension over the curve defined by the radius of the arm portions shown generally at ‘R’ in Figure 20c. Cooling of the hair being styled may be done actively, e.g. by a fan, heat pipe, heatsink, or a thermoelectric cooler, e.g. a Peltier cooler (or similar) disposed within the body of the styler 30 and, beneficially, as the flexible heaters as described herein do not transfer as much heat to the styler’s casework as conventional heaters, a “cool zone” may be formed more readily by the styler 30 (shown generally at ‘C’ in Figure 20c). The cool zone can include a hair temperature sensor to feedback temperature information about the hair as the curl is being set. If the fan is low voltage / isolated from mains voltage, the hair temperature sensor could be placed close to the device’s surface, maximising sensitivity / responsiveness and hence maximising performance of the device in use. The cool zone may be a solid, porous or perforated surface to allow a heat transfer fluid to pass through it. Such fluids may be air, or may be “wetline” (hair styling products which may help to tame and / or smooth and / or add volume and texture to hair being styled, and which may in addition or instead comprise heat protection agents to help protect the hair when styling) stored in the main body of the styler 30 and conveyed from a reservoir in the body of the styler to the cool zone as needed (e.g. by a user depressing a button provided on the body of the styler or automatically, e.g. in response to the device sensing hair has been loaded thereinto). By using temperature sensing at the heaters 36a, 36b and / or detecting that the device’s arms 34a, 34b are closed, and hence hair styling is being performed, the heaters 36a, 36b may heat the heating zones when the device 30 is closed and in motion, advantageously avoiding the hair overheating when stationary on the heated plates 36a, 36b. To facilitate motion detection, motion sensors such as gyroscopes, accelerometers (or other suitable sensors) could be used by styler 30 and incorporated into the device’s control electronics. Similar control (where the arms are to be closed and movement is to be detected before heating is enabled) may be performed in other embodiments. 2 in 1 Straightener / Curler Another styler which may benefit from use of the curved flexible heaters disclosed herein is shown in Figures 21a and 21b which illustrate a so-called “2 in 1 Straightener / Curler” 40. This styler is a straightener (e.g. the styler as described with reference to Figure 1a) and a curling wand (e.g. the styler as described with reference to Figure 13) in a single device. The styler 40 has two arms 44a, 44b and Figure 21a shows the “2 in 1 Straightener / Curler” styler 40 with the arms in the open position ready to receive a tress of hair, and Figure 21b shows the styler 40 with the arms in the closed position. The arms 44a, 44b are coupled to one another at the styler’s proximal end (i.e., the end of the styler 40 which the user grips when styling hair). Heaters 46a and 46b are provided at a head end of each arm 44a, 44b (towards the arms’ distal end). The heaters 46a and 46b comprise a respective flat heating portion that come together when the arms are in the closed configuration (like the flat heater portions in the styler shown in Figure 1a) that can be used to perform straightening of the hair being styled via sandwiching the hair between the arms 44a, 44b. Each heater 46 also includes a flexible heater portion that wraps around the arm’s curved outer casework. When the arms are in the closed configuration, these curved heater portions together provide a tubular surface suitable for forming curls in the hair being styled when wrapped around the tubular outer surface. The user may select in which mode the heaters 46 are to operate (curved portions powered on for a curling mode, or flat portions powered on for a straightening mode) depending on which styling operation is being performed by the user. The mode may be chosen via a switch, or by automatically sensing the temperature of the different zones when hair is loaded onto the styler 40. Alternatively, the respective flat / curved surface of the heaters could also be activated by an open / close sensor, or a switch activated by a hinge lock (e.g. when the device is locked in the closed position at the hinge between the arms 44a, 44b the styler 40 functions as a curler as described above with respect to the curler 30), or by other similar sensors. To assist in setting curls when the styler 40 is used in curling mode, a small fan (or another cooling device such as a thermoelectric cooler, e.g. a Peltier cooler) may be incorporated into the main body of the styler 40 that is activated by the styler’s control electronics when the device detects that the user’s hair has reached a desired temperature and power is removed from the heaters 46a, 46b. This cooling device may also be used (e.g. activated via a control button) when the styler 40 operates in the straightening mode, as some stylists like to use flat heaters when styling curls - by rotating the tress of hair over the non-heated outer surface of the head end of the arm. The heaters 46 may be formed as one large heater per arm as illustrated in Figure 22a, with the curved section of the heater 46c being wrapped over an articulated stiff heater carrier (e.g. as described above with reference to the curler 30). An optional heat spreader layer 46-1 and the serpentine electrode track layer 46-2 (both on top of polyimide) are shown to have ten respective zones in the flat styling section of the heater 46f (i.e. the two leftmost columns of the heater 46) and in the curling section of the heater 46c (i.e. the remaining two columns of the heater 46), but the number of heating zones across the length and width of the heater 46 can be varied as needed. Instead of one large heater per arm, a sub-heater per arm could be provided for each of the flat portion 46f and curved portion 46c of the overall heater 46, as shown in Figures 22b and 22c respectively. Curved Head Straightener A further alternative styler 100 having a curved styling head is illustrated in Figures 23a to 23c. The styler 100 has two arms 104a, 104b which are analogous to those of the styler described above with reference to Figure 1a (and common features of that styler, e.g. the control / drive circuitry, will not be described again). The arms 104a, 104b are coupled to one another at (or towards) the styler’s proximal end (i.e., the end of the styler 100 which the user grips when styling hair), and are adapted for movement between an open position for receiving a tress of hair to be styled and a closed position which sandwiches the tress of hair between the arms 104a, 104b. In this alternative, the styler 100 has a zoned curved heater 106a, 106b (formed as described above) per respective arm 104a, 104b, as is shown in the styler’s open configuration in Figures 23a and 23b). The arms 104a, 104b, and the heaters 106 mounted on them, curve downwards and curve to the left in the orientation illustrated in Figure 23. It will be appreciated that whilst the heaters 106a, 106b could be manufactured to any size as needed, heaters having a length of around 90-150 mm are particularly beneficial. Styler 100 therefore represents an alternative to current stylers which are typically flat (and which often have a heater plate length of around 90 mm). Also, the design of current stylers limits the amount of hair that can be styled in a single section due to the plate’s length and the curvature of the human head. The first curved heater 106a of styler 100 comprises a straight flat portion, closer to the styler’s proximal end. The second curved heater 106b also has a straight flat portion that is positioned and sized to match the straight flat portion of the first curved heater 106a, so that when the styler 100 is placed in its closed configuration for styling (as shown in Figure 23c), the two straight flat portions of the two curved heaters 106 mutually oppose one another. The two curved heaters 106 also include a curved end portion that abuts against, and is more distal than, the corresponding straight flat portion. The configuration of styler 100 therefore facilitates a greater styling area around the styler’s head because the styler’s heater plates 106a, 106b curve with the shape of the human head. Moreover, by using such curved heaters, faster styling times for a full head of hair can be realised by being able to style a larger hair tress in the same amount of time that a traditional styler can style a smaller hair tress (and therefore styler 100 requires less energy for styling a full head of hair relative to its traditional styler counterparts). As noted above, curved heaters can be more readily formed (e.g. via injection moulding) compared to metal extrusion procedures used in the manufacture of metal plate based heaters used in traditional styling devices. Accordingly, styler’s having unique styling properties, such as the styler 100, can be more readily manufactured. Roller Crimper Another alternative styler having curved heaters will now be described with reference to Figures 24a and 24b, which show a “roller crimper” styler 200. The main body of the styler 200 comprises common features with the styler illustrated in Figure 1a, e.g. the control / drive circuitry, power supply, etc., which will not be described again. Instead of flat or curved arms, as described with reference to the other stylers above, styler 200 comprises two crimping arms 204a, 204b (shown in Figure 24a), which project from a handle portion 202, and which are mutually rotatable along a respective longitudinal axis of the styler 200. Each of the arms 204a, 204b may be rotatably coupled to the handle portion 202 (e.g. via one or more bearings), such that the arms 204a, 204b may be rotated manually by a user when styling hair. Alternatively, the arms 204a, 204b may instead be rotatably driven by one or more motors housed within the handle portion 202 in response to a user input (e.g. the speed of rotation of arms 204a, 204b may be governed by the user depressing / toggling a switch provided on the handle portion 202, thereby increasing / decreasing the arms’ rate of rotation). The crimping arms 204a, 204b each have a “triple-barrel” configuration, which allows the user to style hair to have waves / crimps / curls along the tress of hair being styled by the styler 200. Each triple-barrel comprises a plurality of lobes 208a, 208b and a plurality of troughs 210a, 210b arranged circumferentially about the barrel’s roller portion, and are positioned so that during rotation of the first and second arms 204, 204b, the lobes 208a of the first arm 204a engage with the troughs 210b of the second arm 204b and the lobes 208b of the second arm 204b engage with the troughs 210a of the first arm 204a. In the current example, each arm comprises three lobes and three troughs, although it will be appreciated that more / fewer lobes and troughs may be disposed on the arms as needed. A curved zoned heater 206a, 206b is provided over the outer surface of each respective arm 204a, 204b and may be affixed to these arms via, for example, diffusion bonding. The user then inserts the tip of the tress of hair to be styled between the arms 204a, 204b and pulls the hair through the arms 204a, 204b in the manner shown with reference to Figure 24b, thereby sandwiching the tress of hair being styled between the arms 204a, 204b. Alternatively, the arms 204a, 204b may be hinged and may be opened by the user if the user wishes to style the tress of hair from its roots. Depending on the pull speed applied by the user when styling the tress of hair, the heaters disposed on each arm style the hair to a greater / lesser extent depending on the style desired by the user. Each of the arms 204a, 204b may be provided with a position sensor coupled to the styler’s control / drive circuitry, thereby enabling the styler 200 to sense the rotation of each of the arms as the user styles the tress of hair, and hence control the temperature of the heaters 206a, 206b disposed on the styler’s arms 204a, 204b depending on the user’s pull speed (e.g. increase / decrease the temperature depending on the speed of rotation of the arms). Such a styler 200 may beneficially assist users who have little skill in styling hair having curls, crimps and or waves, because curved heaters according to the present invention heat the hair more evenly and respond to load more rapidly relative to traditional stylers (a traditional styler would also require several ceramic or cartridge heaters, which would disadvantageously increase the weight of the styler and hence be uncomfortable (heavy) to hold during use). Triple Barrel Waver Another example of a styler which may be used to form waves in a tress of hair is described with reference to Figures 25a and 25b, such as styler 300 may be referred to as a “triple barrel waver” styler. The main body of the styler 300 comprises common features with the styler illustrated in Figure 1a, e.g. the control / drive circuitry, power supply, etc., which will not be described again here. Styler 300 comprises two thermally insulative arms 304a, 304b which are coupled to and project distally from a handle portion 302. The first arm 304a comprises two projections which are configured for mutual alignment with a second arm 304b, which adopts a wave-like profile, when the styler 300 is placed in its closed configuration by the user depressing an opening / closing means 308 (as shown by the transition between Figure 25a (open) and Figure 25b (closed)). Specifically, the first arm 304a comprises first and second elongate concave portions, onto each of which a curved zoned heater 304a-1, 304a-2 is provided. The second arm 304b comprises an elongate concave portion, a first elongate convex portion and a second elongate concave portion, each arranged successively in a direction perpendicular to a longitudinal axis of the arms 304a, 304b, and a respective curved zoned heater 304b-1, 304b-2 and 304b-3 is provided on each of these portions. In use, the first and second arms 304a, 304b are arranged so that when in their closed configuration, the first and second elongate concave portions of the first arm 304a respectively nest with the first and second concave portions of the second arm 304b. Hence, these portions mutually engage the curved heaters of the first arm and second arm when the tress of hair is sandwiched therebetween, as well as an additional curved zoned heater 304b-2 which does not engage with the heaters of the first arm 304a but which engages with the tress of hair being styled as described in more detail below (alternatively, a single curved zoned heater could be provided across the same surfaces of the second arm 304b). Specifically, as illustrated in profile in Figure 26a (which shows the triple barrel waver shown in Figure 25b in transverse cross-section), a tress of hair T has been sandwiched between the arms 304a, 304b of the styler 300 by a user. Accordingly, the tress of hair T takes a waved-path as it passes through the styler’s arms 304a, 304b, and hence the hair is heated from both sides by curved heaters disposed on the first and second arms 304a, 304b. Current barrel wavers instead rely on three barrels to heat the hair (and hence there are areas of the barrels which although they are heated never make contact with the tress of hair being styled, which is both wasteful in terms of energy use given the higher thermal mass, as well as more awkward to handle and style given the exposed barrel heaters). Beneficially, with the device illustrated in Figures 25 and 26, as all hair contact surfaces can be heated by the styler 300 and there are no exposed areas of heater which do not contribute to hair styling, the tress of hair being styled will reach the target temperature with less heat being lost to the environment relative to current devices. Optionally, the styler’s main body may comprise a fan (or a thermoelectric cooler, e.g. a Peltier cooler) which can cool the styler’s heaters in response to a user command (e.g. depressing a button on the body of the styler) or depending on a pre-set temperature being reached as sensed by the styler’s heaters (the device may signal this to the user, e.g. via lights, haptics, etc.). Such rapid cooling (which may be referred to as a “cool-shot”) forms tighter and longer lasting curls relative to current stylers. The degree of the fan’s speed may be altered, such that a more rapid fan speed may be selected to result in tighter curls, whereas a lower fan speed may be selected if the user wishes to style looser curls. Advantageously, as the styler 300 does not rely on heavy metal barrel heaters which are slow to heat (and which have a high thermal mass), users may obtain hairstyles more rapidly, using a lighter styler which uses less energy to achieve the hairstyle. Hot Rollers A further alternative use of the curved zoned heaters described herein will now be discussed in the context of hot rollers. Current hot rollers can create curls with significant root lift and different hair curl factor (i.e. the ratio of hair length when curled relative to hair length when straight), as described in more detail below. The hair’s curl factor is dependent on the time the hot roller is left in the hair being styled (which is typically in the order of 10 to 30 minutes). Over this time period, the hair cools with the hot roller in tension, and thus the temperature at which the roller is removed determines the level of curl. Hot rollers are typically heated by a heating element on a plate / hub, and then the user rolls the heated rollers onto the hair - this requires a high degree of skill to do correctly, and also risks the user burning their skin due to the high temperature of the hot rollers. Moreover, achieving a uniform, consistent tension in the hair tress being styled by the hot roller is critical for achieving the best hairstyle, because uniform tension achieves a consistent form within the hair tress which is set when the hair cools as the hot roller decreases in temperature. To generate tension in the hair tress, the hot roller typically has small bristles for gripping the hair. However, when removing the roller, the curl must be unravelled and the roller’s bristles can get tangled with the hair, which lowers the quality of the resulting style. Some current hot rollers incorporate “claw grip clips” which may be used to secure and maintain the hair tension in the roller, but these types of clip leave crimping marks on the hair which is also detrimental to the hairstyle. Examples of hot rollers which do not suffer from these drawbacks, and which use the curved heaters described above, will now be described. By using a curved heater as described above, hot rollers according to the present invention can be placed in the hair at room temperature (and hence prevent a user burning themselves with a pre-heated roller), and then once in position rapidly heated to drastically reduce the time it takes to generate the curls. Power can be delivered to the heater electrodes on such hot rollers via one or more batteries disposed within the hot roller, or via a separate handle which is inserted into the hot roller’s cavity as part of a hot roller system. Once the hot rollers are inserted into the hair, either a button on each hot roller is depressed to turn on the hot rollers if powered by batteries, or the handle is inserted into or connected to each hot roller, and power is supplied to the heater electrodes via the handle. Once heated, the hot roller heats the hair to the glass transition temperature within a relatively short time frame and remains powered to hold that temperature for a duration much shorter than current hot roller products. Once the power is removed from the heater electrodes, the roller’s curved heater then cools quickly, along with the hair, setting the curl in place. The user may then remove the hot rollers, and hence achieve a faster style than with traditional hot rollers. A first example of a hot roller using a curved heater is illustrated with reference to Figures 27a, 27b and 27c, which show a hot roller 400 having a plurality of bristles 406 which are configured to retract into the main body of the roller 400 once the roller has been used for styling. Specifically, hot roller 400 comprises a curved heater 404 having perforations through which a plurality of bristles 406 may be extended / retracted via a mechanism of an actuating means 408. The bristles may be heated to improve heat transfer to the hair tress or they may not be heated to reduce the manufacturing complexity / cost of the roller 400. It will be appreciated that there are several different possible configurations for implementing extension / retraction of the bristles 406 into / out of the styler device 400, as detailed below. For example, the bristles 406 may be retractable using an actuating means 408 such as a rotary-type mechanism. Figure 27b illustrates the bristles in their retracted state, and these bristles 406 are configured to extend from the actuating means 408 through openings in the heater 404 in response to rotating an element of the hot roller 400, such as its insertable handle (the bristles 406 are shown in their extended state in Figure 27c). In such a scenario, the bristles 406 may be arranged on sets of rails that allow the rotation of entire sets of bristles 406 in response to rotating the insertable handle when it is engaged with the hot roller. Alternatively, the bristles 406 may be retractable using a radial-type mechanism wherein bristles of the roller 400 extend from the actuating means through openings in the heater 404 in response to rotating an element of the hot roller 400, such as its insertable handle. In such a scenario, the bristles 406 may also be arranged on sets of rails that allow the radial movement of entire sets of bristles 406 in response to rotating the insertable handle when it is engaged with the hot roller. Alternatively, the bristles 406 may be retractable using an axial-type mechanism wherein bristles 406 of the roller 400 extend from the actuating means through openings in the heater 404 in response to pushing / pulling an element of the roller 400, such as its insertable handle. In such a scenario, the bristles 406 may also be arranged on sets of rails that allow the axial movement of entire sets of bristles 406 in response to the pushing / pulling of the insertable handle. In each of the hot rollers described above, the mechanism of the actuating means may be actuated by twisting a part of the handle’s endcap, or pushing a button disposed on the housing of the handle / hot roller which causes the mechanism to actuate. The mechanism could also be motorised by a motor or servo, to reduce the required mechanical input from the user. Once the bristles 406 have been extended, the user places one or more rollers 400 into their hair as needed. In the example shown in Figure 28, the user has inserted three rollers 400-1, 400-2, 400-3 into their hair for styling. The user may maintain the roller’s tension including a twist in the tress of hair being styled in the axis which is perpendicular to the scalp, and hence the roller does not unravel. To assist the user to roll the hair, the insertable handle 402 of the device can be inserted into the roller 400 and then either rotated by the user to roll the hair up to the scalp, or the handle 402 could be motorised, rotating the body of the hot roller 400 onto the tress of hair to give a specified tension in the tress of hair. The handle 402 could utilise a ratcheting mechanism to prevent the user applying too much tension to the hair being styled (this could be for a motor-operated or manually-operated handle), e.g. by disengaging the handle’s ability to apply rotation force to the roller once a predetermined torque has been reached. By use of the actuating means 408 to retract the bristles 406 once the hot roller has cooled in-situ, a user is able to slide the roller 400 out of the styled hair without disrupting the style (e.g. caused by tangling of the hair with the bristles). Beneficially, by providing one of the bristle retraction mechanisms as described above, the bristles of the roller 400 may be stored away when not in use (and which also beneficially allows the user to clean the roller’s surface more easily than when bristles protrude from the hot roller). That in turn reduces the amount of space that the device takes up during storage. In addition, the ability to be able to retract the bristles, allows the user, if they wish, to style the hair using the hot roller without the bristles being in an extended position. An alternative hot roller 500 which makes use of the curved heaters discussed above will now be described with reference to Figures 29a and 29b, which respectively show the hot roller 500 in its unravelled and ravelled configurations. This type of roller 500 may be referred to as a “blanket roller”. Specifically, the hot roller 500 comprises a main roller body 502 which may have notches 504 that help to maintain hair tension in use, a rollable / flexible heater sheet 506 connected to the main body 502 and which can be rolled / curved over the main body 502 in use, and a root clip 508 connectable to the other end of the heater sheet 506 for connecting to the roots of the tress of hair being styled. To style the hair, the roots of the hair tress to be styled are affixed to the heater sheet 506 using the root clip 508 whilst the styler 500 is in its unravelled configuration (as shown in Figure 29a). As illustrated in more detail in Figures 29c and 29d, the hair clip 508 is formed from a first portion 508-1 which is connected to the heater 506, and which is configured for mutual connection to the releasable hair retaining second portion 508-2. Whilst a toothed mechanism is illustrated in Figure 29c for connecting the first and second portions 508-1, 508-2 together, alternative mechanisms may be used (e.g. a magnetic connection, or the like). Additionally, the first portion 508-1 is symmetrical about its longitudinal axis, and hence the releasable hair retaining portion 508-2 can be connected to the first portion 508-1 from either side in use. Once the two portions 508 have been mated, as shown in Figure 29d, the hair clip 508 is clamped to the roots of the hair tress to be styled and isolates tension applied to the hair by the roller 500 from being applied to the user’s scalp. The tress of hair is then laid along the rollable heater sheet 506 and the tip of the hair tress to be styled is inserted into the main body 502 of the styler 500 as illustrated in Figure 30 at position T, with optional elongate notches 504 disposed on the main body 502 providing tension to the tress of hair as the main body 502 is rolled along the hair tress. The tress of hair and the rollable heater sheet 506 are then rolled onto the main body 502 towards the hair’s roots. This may be done manually by the user, via a spring-loaded mechanism disposed within the main body 502, or the main body 502 may be driven by a motor-ratchet system provided in a separate handle which is insertable into the main body 502 of the hot roller 500, and then held in place via use of a hair clip, a magnetic clip which engages with magnets disposed on the root clip 508, or by a similar retaining means. According to the illustrated example, the heater sheet 506 could be made in the same manner as the curved heaters described above (although, alternatively, a Kapton heater could be used instead). In another alternative example, part of the heater sheet 506 could be manufactured as a heater with the remainder of the sheet being made from a heat spreading layer (e.g. a metallic foil). To further reduce the manufacturing complexity of the styler 500, instead of the entire heater sheet carrying heater electrodes, the sheet 506 could instead be manufactured from a purely heat spreading layer (e.g. a metallic substrate, such as copper), with the main body 502 being manufactured as a hot roller with a curved heater mounted around the outside of the main body 502. To provide heat to the hair, the main body 502 could be provided with a battery which may be turned on by the user once the roller has been rolled into its ravelled state (as shown in Figure 29b), or a separate handle could be provided and inserted into the main body 502, onto which a power connection is supplied, to provide power to the heater 506 (as described above with respect to the hot roller 400). Once heated to a given temperature suitable for styling, the power supply is interrupted and the heater 506 is allowed to cool, thereby setting the curl in place. Subsequently, the hair clip is removed and the roller 500 may be removed from the hair. Another alternative hot roller 600 to the hot roller 500 discussed above will now be described with reference to Figures 31a and 31b, which respectively show the alternative hot roller 600 in its unravelled and ravelled configurations. The principal difference between the hot roller 500 and the hot roller 600, is that hot roller 600 does not comprise a main body 502 like that used in the hot roller 500. Instead, the heater sheet 606 comprises an integral biasing means which biases the heater sheet 606 into its ravelled configuration (as illustrated in Figure 31b). For example, the heater sheet 606 could be manufactured with internal stresses that cause it to roll into a cylinder (i.e. into a first configuration in which the sheet 606 is biased), wherein the stresses are achieved by using rolled metal struts or a material with sufficient elastic properties stretched over the internal surface of the heater sheet 606, forcing the heater to roll up into a first configuration (i.e. as shown in Figure 31b). Then, when a user wishes to style a tress of hair, the user may apply a force to the sheet 606 which is greater than the force which biases the sheet 606 into its first configuration, and hence the user may flatten the heater sheet 606 into its unravelled configuration (i.e. as shown in Figure 31a). Once the sheet 606 is placed in its unravelled configuration, the user may then place the tress of hair to be styled onto the flattened heater sheet 606, and then allow the sheet 606 to roll into it a cylinder through the sheet’s internal tension, thereby returning the heater sheet 606 to its ravelled configuration which is suitable for styling. The root end of the heater sheet 606 may comprise bristles which assist the user to locate the root end of the heater sheet 606 into the roots of the tress of hair to be styled, and a separate hair clip (which may be made from tensile plastic, or which may be magnetic) may be provided to further secure the roots of the tress of hair to be styled to the heater sheet 606. The tip end of the sheet 606 comprises a power connector 604 for connection to a separate power supply handle 612 to supply power to heater sheet 606 when needed. Alternatively, a battery could be connected to the power connector 604 and placed inside the main body of the roller 600 to provide power instead of the power supply handle 612. Once the user allows the heater sheet 606 to return to its first, ravelled, configuration, the user may then secure the sheet 606 in the hair in the first configuration by sliding a clip 610 over the ravelled sheet 606, as shown in Figure 31b. The clip 610 may be made from tensile plastic, or may instead secure the hair and the heater sheet 606 magnetically. Since the sheet 606 is biased towards the ravelled configuration, less skill is required of the user to use the hot roller 600 due to the “self-rolling” action of the roller 600, particularly when compared to conventional devices which instead require a user to manually roll the hair into the hot roller. Once the hot roller 600 has been secured in the hair to be styled, a power supply handle 612 may then be connected to the power connector 604 as shown in Figure 31c and power may then be supplied to bring the heater sheet 606 to a temperature suitable for styling the hair tress. Alternatively, a battery may be connected to the power connector 604 and then placed inside the roller when in its ravelled configuration. Once the styling temperature has been achieved, the user removes the handle 612 (or battery) and waits for the styler 600 to cool such that the curl is set in place. Once the heater sheet 606 has cooled sufficiently, the user removes the clips and then removes the styler 600 from the styled hair. Another alternative hot roller 700 to the hot rollers discussed above will now be described with reference to Figures 32a to 32d. Figure 32a illustrates a main portion 706 of the hot roller 700, wherein the main portion 706 functions as a clip for retaining a tress of hair to be styled T. In a first example, a curved heater (as described above), or a Kapton heater, could be provided on the outer surface of the main portion 706 (i.e. the surface which contacts the tress of hair to be styled) to provide heat to hair being styled. As before, power may be supplied to the heater on the main portion 706 via a battery or a power supply handle, which is connectable to a power supply port (not shown) disposed on the main portion 706, in a manner as described above e.g. with reference to hot roller 600. In a second example, the main portion 706 could instead be manufactured from sheet metal, and hence the main portion 706 may function as a heat spreading layer (thereby reducing the complexity of the hot roller relative to the first example described above). The main portion 706 according to this second example may be made via sheet metal bending, extrusion or (metal) injection moulding. A separate handle may again be used to provide power to the main portion 706, and hence to spread heat to the hair tress affixed to the main portion 706. As shown in Figure 32, the main portion 706 comprises an opening for receiving a tress of hair to be styled T. The tip of the hair tress T is inserted into the opening of the main body 706 by a user, and then the hot roller 700 is rolled manually by the user or via a motor insertable into the main portion 706 (e.g. via a separate handle, or via a battery-powered motor) to generate tension in the tress of hair T. As the roller 700 does not use bristles to create tension with respect to the hair tress T, the opening of the main body for receiving the hair tress may instead be configured to provide tension in the hair tress T. For instance, an inherent bias / tension may be configured across the main body 706 by means of a mechanical actuator, as shown in Figure 32c, in which a force F is applied by the actuator when the actuator is in its off state, which in turn creates a pinching force at the main body’s opening for receiving a tress of hair. Likewise, and as illustrated in Figure 32d, when the actuator is set to its on state, the force F is removed and hence the pinch force at the main body’s opening is also removed. Accordingly, when the actuator is in the on state, the user may insert a tress of hair to be styled into the roller 700, and then place that actuator in the off state, thereby gripping the tress of hair inserted into the roller, such that the user can then pull the tress of hair to be styled and wind it over the roller. The actuator may be actuated by a button / switch disposed at the periphery of the hot roller 700 (not illustrated). It will be appreciated that the actuator could adopt the opposite configuration to that described above, and hence in an alternative arrangement the actuator when in its on position could provide the force F which causes pinching at the main body’s opening for receiving hair, and when in its off position the force F is removed thereby removing the pinching force at the main body’s opening. Once the tress of hair has been inserted into the opening and the device rolled into a styling position, a separate hair clip 710 may then be slid over the main body 706 and the hair tress, thereby holding the tress's tension before heat is provided to style the hair. Beneficially, as the hot roller 700 design is bristleless, the hot roller 700 may be removed from the styled hair without unfurling the curl (the user merely needs to remove the hair clip 710 and place the actuator in a position which removes the hair pinching force at the main body’s opening), thereby improving the achieved shape and style relative to current devices. Stylers Having Perforated Heaters As noted in the introduction above, current heater technology (e.g., conductive heaters, heat exchangers, IR lamps) require a thermally conductive (usually metal) surface to transfer heat from the heat source to the hair being styled. This arrangement causes several restrictions when manufacturing stylers which incorporate styling bristles (e.g. stylers such as heat brushes / combs, or the like, where the bristles of the brush project out of a hole in the heater’s surface) and / or stylers incorporating actively cooled heaters (i.e. heaters comprising holes via which fluid, such as wetline, may be supplied) and / or stylers having perforations through which hot air is supplied to increase the drying rate of a hair tress being styled. For instance, air / bristle hole size (and placement thereof) is problematic in respect of current heater technology, because manufacturing techniques associated with the heater’s metal surface are inherently complicated and therefore often expensive. Also, most current metallic heaters have a thermally conductive part (which is typically large) which has a high thermal mass, and hence such heaters are slow to heat up and respond to heavier loads (such as wet hair). Moreover, conventional heat sources also struggle to react to uneven loads on thermally conductive surfaces, which can be especially prevalent on stylers having brush heads. For instance, if one area of the brush is used by the hair stylist to dry the hair being styled, that area of the styler will cool down and reduce in styling performance, whilst unloaded portions of the styler will heat up relatively more significantly, and hence uneven styling conditions can arise. Some current devices, in an attempt to replenish the heat evenly across the head of the styler, are required to use several heat sources. However, this approach increases the weight, complexity, and cost of the styler significantly. Also, a heater having a perforated layout (i.e. comprising holes) does not allow for traditional ceramic heaters to conductively heat a plate or a substantially cylindrical head uniformly, because the gaps between the holes create thermal bottlenecks. It will be appreciated that uniform heating is important for a fast-drying rate and for optimum styling performance. The inventors therefore propose using the flexible heaters described herein, which have been perforated using PCB manufacturing techniques, e.g. by drilling, laser cutting, or punching the perforations, to create a heater which heats (and cools) uniformly without the risk of forming thermal bottlenecks. A general overview of such a perforated heater will now be provided, as well as an overview of the hardware which may be used with stylers having such a perforated heater. Detailed specific embodiments of stylers having such heaters will then be described. The design of a perforated heater must be carefully considered. If the perforations are too large or there are too many perforations in the available area, then the remaining conductive heaters may not be able to provide sufficient heat for styling the hair. Similarly, if the perforations are too small or there are not enough of them, then convective heat transfer with respect to the hair may not be sufficient (e.g. upon drying, heating or cooling). The analysis below illustrates how the size and number of perforations may be determined to achieve an efficient drying and styling of the user’s hair. Perforation Quantification To determine the number of perforations for a given heater, an upper and lower bound for the porosity (i.e. the size and number of the holes) of the heater is calculated. These bounds vary depending on the application of the heater (e.g. the size of the product with which the heater is used for styling). In the analysis of the “hole count” which follows, the following assumptions are made: • Heat is generated uniformly across the conductive track area (temperature variation in y is negligible) and provides exactly enough power for hair heating needs without losses. • The hair section thickness is far smaller than the width and length of the heater and is assumed to heat up uniformly (temperature variation in z is negligible). • All heat energy generated by the heater is absorbed by the hair tress being styled (100% conductive efficiency) • All heat energy delivered by the air flow is absorbed by the hair tress (100% convective efficiency) • The heater is a Separated Extra Low Voltage, SELV, component (not mains powered - although the model below can easily be adapted for a mains powered heater). • The rate of water removal is approximated by a linear function with thermal power. • On a barrelled product, half of the heater circumference is loaded with hair and the thickness of the hair section is taken to be 1 mm. • In a styler-like product (e.g. the heaters are substantially flat in profile), both plates are fully covered by the hair section and the thickness of a hair section within a product is assumed to be 2 mm. • The hair tress exits the contact area of the heater at the target temperature. The following properties of hair are assumed: • Properties of hair are taken as: o Density / ¼ = 1100 kg m-3 o Specific heat capacity (dry) CPh = 1602 J kg-1 K~] • Hair pull speed u = 45 mm s’1 • Fan characteristics: o Max fan pressure: 4kPa o Fan flow rate: 500L min-1 • Water content to = 30% relative to the mass of hair The following dimensions are referred to: • Length of contact area 1 (mm) • Width of contact area (mm) • Heating stage width (mm) • Hole diameter D (mm) • Dimensions of hole spacing q, (mm) • Total Area (mm2) • Total Open Area (mm2) • Thickness of hair tress (mm) • Hair pull speed (mm x-1) • Hair contact time t (s) • Porosity (ratio of open area to total area) f The following convective parameters are referred to: • Maximum air pressure p(9) ( / ¾) • Pressure Loss Factor A' • System Flow Rate Q. (L mm-1) • Air Temperature q (°C) • Air density at air temperature .¾ (¾ m-s) • Specific heat capacity of air ( / ) • Thrust 7 (A1) And the following conductive parameters are referred to: • Heat flux 4 [Wm-2] • Heater Depth averaged thermal conductivity [Wnr1K’1] • Specific heat capacity of hair c (J • Specific heat capacity of water • Heat of Vaporisation of water c ( / • Target Temperature (X) • Track Tem perature (°c) • Ambient Temperature Te (“C) The calculations in the following may be in respect of: • with reference to Figure 33a, a heater 56a having a square arrangement of holes (having diameter D - the basic repeating unit is illustrated for clarity), whereby each of the holes H across the surface of the heater 56 are placed generally squarely relative to neighbouring holes (by the distance C1C2); or • with reference to Figure 33b, a heater 56b having a square staggered arrangement of holes (having diameter D), as illustrated in Figure 33b (the basic repeating unit is illustrated for clarity), which has the same arrangement as that of the square arrangement, with an additional hole H placed in the middle thereof (at a distance of (Ci / 2) + (C2 / 2) apart from the four holes at the periphery of the basic unit). The minimum bound is calculated as follows. The most powerful, high RPM fans used in hair styling devices can deliver pressure up to 4kPa. Let the maximum operating pressure of the fan at a given flow rate be »(<?). If the perforated substrate (i.e. the heater) is treated as a thin porous plate, the loss factor, A', and pressure drop, AP, can be determined via the porosity, f, where: op era fetal For a generic arrangement of holes where « is a numeric constant dependent on spacing and arrangement (e.g. square / square staggered): c C, The loss factor is related to the porosity f, and the pressure drop is related to the loss factor via: i / g Where p is the fluid density, Q is the volumetric flow rate and A is the open area at the outlet. The combined general boundary condition for determining the minimum acceptable porosity for a given heater is hence: Consideration will now be given to the calculations in respect of the maximum bound for the perforations of a perforated heater. Specifically, the maximum bound is determined by the heat energy (conduction and convection, assuming 100% efficiency) that is required to heat the tress of hair being styled to the target temperature T_target in one pass. With reference to Figure 34, consider that the hair in contact with the styler completely covers the entirety of a single-sided heater surface (the temperature distribution in the y- and z-directions are assumed to be constant), and hence the heater plate 56 covers the entirety of a portion of the tress of hair being styled. The temperature distribution into the hair, and the hair’s associated water content with time x is assumed to be a piecewise linear function over three stages (as illustrated in Figures 35a and 35b, which respectively illustrate temperature over time, and heat flux over time): 1) Heating of hair and water content to 100°C; 2) Vaporisation of water content from hair; and 3) Heating of hair to target temperature The required heat flux to heat the volume of hair and water for stages 1 to 3 can be calculated as: + J(Tawi -To) x ft             r w - 100) (1) The mass flow rate of the water can be directly linked to the mass flow of the hair by the water content The mass flow rate of hair per unit area can be derived beforehand to simplify the equations into known parameters: Simplifying the above equations, the required heat flux for each stage is determined as: + (2) . / ¾ ¾ 4 1?n. "1 =----------\ (3) k               — 100)        (4) A boundary condition to the problem can be set to determine the minimum required width of each heating stage to deliver the required power to the hair. This assumes 100% efficiency: all the power from the heater and the convective air goes to the hair and water content: = £h^ 4¾ is inversely proportional to the width w of the plate section for each stage from equations (2), (3) and (4), so the width of each plate section w2, ^) is given by: The minimum required total width is hence given by: = X i Thus, a bound can be derived that will eventually give a minimum conductive area per total area requirement and hence a maximum bound for the porosity: (6) The minimum required width to raise the temperature of the hair to the target temperature in one pass should be less than the total width of the contact area. Turning now to the conductive power of the heater for stages 1 to 3, the maximum heat flux that a heater stack up (i.e. the fully assembled heater) can deliver is dependent on the maximum electrode track operating temperature 7^^ and the material stack up of the heater (an example of a heater stack up is given below). Fourier’s Law determines the maximum heat flux: (7) Over the contact area, the porosity of the heater reduces the effective heat flux that is delivered: = (1 - 7) k (8) This is the upper bound for the maximum heat flux that the heater can deliver. It is worth noting at this stage that the maximum heat flux reduces as the target temperature increases. The following table provides an example of a heater stack up where polyimide is layered onto the heater electrode (which may be made from steel), and a copper heat spreader layer is layered onto the polyimide layer: Layer Material Thermal Conductivity fc(Wm-1K'1) Thickness ti (pm) 1 Copper 398 5 2 Polyimide 0.12 25 Total Heater Thickness dxheater (pm) 30 Depth Averaged Thermal Conductivity^' (Wm-1K'1) 0.144 As regards the air convective power of the heater for stages 1 to 3, the maximum heat flux that the air can deliver is dependent on the mass flow rate across each stage width temperature of the air and open area across each stage width =^cvJTa~Ta) (9) The mass flow rate is assumed to be uniform across the contact area and so should be scaled according to the width of the section: Therefore, equation (9) becomes: = (1°) This is the upper bound for the maximum heat flux that the air can deliver (100% efficiency). Referring now in more detail to the three stages illustrated in Figures 35a and 35b, in Stage 1 (generally shown at “1” in each figure) the hair and water content is raised from the ambient temperature To to the boiling point of water (100°C). Equation (5) described above determines the minimum width for stage 1 1¾ = 7------;;----- Substituting in equations (2), (8) and (10): Viz, =------------------—,    --------------------- (111 -1   (1_ / 5 J             ^-p„(? The only unknown in equation (11) is the average temperature in stage 1. This can be calculated by: T f i f x . ioa+To — | Fix) =----- (12) Therefore, can be determined from a setup of known input parameters: (13) In Stage 2 (generally shown at “2” in each of Figures 35a and 35b), the water content is removed from the hair, at constant temperature, by supplying the heat of vaporisation for the given mass of water. From equation (3): For the removal of water from the hair, it is known experimentally that the air thrust T has a significant impact. We can calculate the mass of water removed per pass due to thrust and subtract it from the mass that must be heated for vaporisation. Like Stage 1, substituting this into equation (5): = _______; 2 <"_ f s r (j 1 (14) Experimentally, it has been shown that the air thrust T is proportional to the average mass of water removed after 6 passes per total water mass F by the relation: The thrust is the product of the air mass flow rate and the air velocity: T = = aaMq = 77 (16) The average mass of water removed after 6 passes can be related to the mass of water in the hair (assuming a linear relationship for water removed per pass): Substituting into equation (15): And rearranging and dividing by time: = (17) Further reduction to known variables can be applied: = ¢0¾ = Substituting this and equations (16) and (17) into the numerator of equation (14): (mw: — ?W)cPv = [1 — 0.1517(paQ2(4ef )-1)8,3]¾^ p ■               -                                         -■                                        ■ _            _ [I - 0.15 l / Cp^GV )-1)0¾ £0 / ¾ I1¾ A J, L W -? Therefore, the minimum width to evaporate the water content from the hair with thrust adjustment is: w2 [l-0AS>'17(pap2(.Ag / ) (18) In the final stage (stage 3, shown generally at “3” in Figures 35a and 35b), only the hair is heated to the target temperature The heat flux required is given by equation (3): / ¾ It £ & = ------- loo) w3 1 n Similar to stage 1 and 2, substituting this into equation (5): puu - loo) '       '       ■'           ' fl w = ------------------------------- (1 - n k fc* ~ nar) + ^jpaQc„. (T„ - To) As stage 1, the average temperature at stage 2 can be calculated via equation (12): 1 f          100 + 4^2 = — TW rfx =--- w3J '          2 Therefore, the required width of stage 3 is: (19) To summarise the maximum porosity calculation, the contact width of the hair with the perforated heater must be greater than the minimum required width for heating hair to the required temperature in one pass as per equation (6): Where: [1 - ) (1 - n k ~ Ttar) + wi-fPaQ^.(Ta - To) v ^tester 7 u / ¾¾ bOJ __ ‘ ?? X X / f 'p __ T5 1 C1 - f) 1 t ~ The above-described calculations may then be used when calculating the size and number of perforations of a heater having a flat plate or of a heater having a curved surface. For instance, as illustrated in Figure 36a, in the case of a styler having a flat plate set up (e.g. in the manner of the styler described above with reference to Figure 1a), there is a heater plate 56a, 56b either side of the tress of hair, and so the thickness of the hair tress is doubled. The width of the contact area between the hair and the plate is hence: = In the case of a styler having a substantially curved surface, such as the barrel styler 10 described above with reference to Figure 13, the barrel acts as a single sided brush on the tress of hair being styled. It is assumed, as shown in Figure 36b, that the width of the contact area is taken as half of the barrel circumference: Perforated Heater Overview Examples of heaters having perforations (the numbers of which may be calculated in accordance with the detailed description above) will now be described, firstly with reference to Figure 37, which illustrates in overview a perforated flexible heater electrode 64p. The perforated heater electrode 64p is formed of a serpentine track of resistive material, whose geometry (track width, thickness, length) and material is specified to achieve the desired resistance and peak power requirements for the relevant power source (i.e. mains or battery-powered). The resistive material may be stainless steel, nickel alloy, or copper, or formed from another appropriate material. Similar to the heaters described above, the serpentine heating track may define a number of heating zones, which are electrically connected via a common terminal (ground or positive) and a switching terminal (ground or positive). The inset shown in Figure 37 illustrates two such heating zones side by side, with a common terminal 64p-c and a respective switching terminal 64p-s1 and 64p-s2. By sharing terminals in this way, the total number of connections may be reduced, and the control system only requires one switching terminal per heating zone (thereby simplifying the manufacture of the styler). As illustrated in Figure 37, the perforations (which in this example are substantially circumferential) form a lattice structure of holes across the heater electrode 64p, though it will be appreciated that alternative perforations (of any shape) may be formed across the surface of the electrode 64p, such as the holes which are formed substantially elliptically with respect to the electrode 64p' illustrated in Figure 38 (which in the illustrated example has three zones, although more or fewer zones may be provided as needed). It will be appreciated that the perforated flexible electrodes 64p, 64p' may be used instead of the electrode 64 described above with reference to Figures 3a and 9, and hence that the perforated flexible electrodes 64p, 64p' may be used as part of a perforated heater in stylers having flat or curved styling surfaces (common components of such stylers will not be repeated here). Beneficially, when such electrodes 64p, 64p' are used as part of a curved surface, the tracks may remain perpendicular to maximise the coverage of the track area per total surface area. Hair stylers using the perforated flexible electrodes 64p, 64p' may also comprise one or more wetline product dispensers for dispensing fluids such as water or styling products onto the user’s hair via the heater’s perforations. Wetline hair products (such as sprays, serums, oils, etc.) are either water-based or oil-based and may be applied before or after styling dependent on the use case. Wetline dispensing stylers may be configured to dispense fluid from the dispenser (which may be disposed, for example, within the body of the styler) onto the user’s hair during the drying or styling process, to provide improved control of the moisture level of the hair, with the wetline product being delivered by atomisation, nebulisation or by pumping. Alternatively, a wetline product dispenser could be provided separately from the hair styling device, for example as a separate diffuser (e.g. a desktop-based diffuser). By using wetline during a hair styling process when the heater is on, the hair drying rate, as well as the style of the hair, can be more rapidly and precisely controlled by the user. In addition to the wetline dispenser, stylers having perforated heaters may also (or instead) comprise a fan (e.g. disposed within the body of the device) to convey hot or cold air, or steam, via the heater’s holes to facilitate hair styling as desired by the user (a thermoelectric cooler, e.g. a Peltier cooler, may be used for cooling instead of a fan). Beneficially, during a hair styling procedure in which the hair styling heater is off (so the heater can function as a hair temperature sensor), the fan / wetline dispenser can be activated to quench the hair and set a style, which may be of particular use to the user when curling hair using a barrel / brush hair styler. The perforated heater may be configured to measure the approximate air temperature of the hot air exiting from each zone (or zones) by monitoring power requirements during heating. For instance, the convective heat generated at the perforated heater will cause thermal drift in the perforated heater, and thus lowers the power requirement for the perforated heater to reach its target temperature. If this power drop is measured by the styler’s control circuitry, the air temperature could be estimated in each heater zone (or zones) when the flow rate over the heater’s surface is known (e.g. from the flow rate of the fan disposed within the body of the styler, or as determined by a sensor disposed within the device). If the perforated heater is not being heated, it may function purely as a temperature sensor and can be used to accurately measure the temperature of the air (and the hair). Moreover, zonal air temperature measurements (i.e. measurements of one or more zones across the heater’s surface) could be used to monitor and control the air heater (the fan disposed in the styler’s housing). If a basic air heater is used, its power consumption can be minimised by reducing the power to the air heater if the air temperature is sensed as being too hot. Such power minimisation is beneficial for efficiency, because the air heater is the most power consuming component in many wet-to-style (WtS) products. If a multi-zone air heater is used (i.e. an air heater which is configured to direct streams of air to one or more zones), the styler’s perforated heater could be used as a sensor to monitor and control the duty cycle of the air heater via the separate channels. This air temperature monitoring approach could be used by the styler’s control circuitry to control zonal air venting / wetline dispersion at the air outlets of the relevant zone(s), thereby facilitating more efficient hair sty I ing / dry ing. For instance, where the fan’s speed is controlled using air temperature monitoring, if the air becomes too hot and the temperature spikes, the fan speed and flow rate would increase to reduce the average temperature of the air being heated. An overview and the general uses of perforated heaters in hair stylers has been described above. Detailed examples of hair stylers comprising such heaters will now be provided. Styler Having Flat Perforated Heaters A styler 800 of the type illustrated with reference to Figure 39a having the perforated heaters detailed above will now be described. As will be appreciated, this styler 800 is a variant of the styler 1 described in detail above with reference to Figure 1a, and hence a description of corresponding components will not be repeated again here. Styler 800 comprises two arms 804a, 804b, which are hingedly movable relative to one another (as illustrated generally at 802 in Figure 39a). Each arm 804a, 804b is configured to receive a respective perforated heater 806a, 806b. The heaters 806a, 806b generally correspond to the heater described above with reference to Figure 9, except that layer 82 (the heat spreader layer), layer 84 (the perforated main heater electrode and sensing layer) and layer 87 (the backing layer) have been substituted for a perforated heat spreader layer 882, a perforated main heater electrode and sensing layer 884, and a heater substrate layer 887 as shown in Figure 39b. The heater substrate layer 887 comprises holes which mutually align with the perforations present on each of the heat spreader layer 882 electrode layer 884 (and on the other layers which form the heater in accordance with the description provided above with reference to Figure 9), thereby allowing for the transfer of fluid therethrough via the mutually aligned holes. Whilst the holes illustrated in the heat spreader layer 882 and in the electrode layer 884 are shown to be substantially elliptical (i.e. “slot-like”) in shape in Figure 39b, this need not necessarily be the case. For instance, holes adopting a substantially circular shape may be used instead. As each arm 804a, 804b of the styler 800 shown in Figure 39a comprises heaters having perforations, air (e.g. blown by a fan disposed within the styler) can pass through the hair being styled (wetline may also be dispensed into the airflow, as needed) - the hole / slot pattern on each arm can also be different for optimal heating / styling. For instance, as airflow is provided from both sides of the styler 800 in Figure 39a (such flow being generally perpendicular to the longitudinal axis of the styler 800), the styler 800 is configured to provide airflow via the perforations at an angle (or via separate channels) to allow for airflow in each direction towards the tress of hair being styled. Alternatively, perforations may only be provided in the heater of one of the two arms 804a, 804b. In this instance, the perforations provided in the electrode layer 884 may be configured to be larger than the mutually aligned perforations provided on the heat spreader layer 882 to reduce the thermal contact between the heating substrate 884 and the supporting substrate 887, thereby minimising heat loss to the device’s casework. As the styler 800 typically operates at relatively low pressures, the airflow exiting the heater 806 will typically be imbalanced and hence more air will exit from the distal end of the heater (i.e., the end of the heater which is furthest from the styler’s handle) in comparison to the proximal end of the heater. To balance the airflow, the exit holes may be varied across the heater parallel to the length of the styler 800, with more / larger holes at the proximal end of the heater (i.e. the end of the heater closest to the styler’s handle may have a more dense arrangement of larger perforations) and fewer / smaller holes at the distal end of the styler (i.e. the end of the heater furthest away from the styler’s handle may have a less dense arrangement of smaller perforations), as illustrated with reference to Figures 40a and 40b. Specifically, in Figure 40a, perforations are formed in the heater 806a / 806b in a generally symmetric manner either side of the heater’s centreline (indicated with a dotted line, and which corresponds to the styler’s longitudinal axis). The perforations generally decrease in size in a proximal-to-distal direction across the heater’s longitudinal axis, such that the heater’s largest and most numerous holes are at its proximal end (i.e. the end of the heater closest to the user’s hand) whilst the smallest and fewest holes are at its distal end (i.e. the end of the heater furthest from the user’s hand). By using this arrangement, the airflow flowing out of the heater 806a, 806b is better balanced as a consequence of increasing the pressure along the heater’s longitudinal axis, thereby causing more air to exit at the proximal end of the heater 8106a, 8106b in comparison to the case where the perforations are all the same size, thereby balancing the airflow along the length of the heater 806a, 806b. In an alternative arrangement of perforations, illustrated with reference to Figure 40b, larger holes could be placed along the centreline (illustrated with a dotted line) of the heater to bias air exit here at these locations. This alternative arrangement would allow more of the air to be in contact with the tress of hair being styled, as the path for air to escape the head of the styler is longer. Once the hair being styled has reached a certain temperature, and hence it can be inferred that the hair being styled by the styler 800 has been dried due to the hair’s associated moisture level, the styler’s user interface may indicate this to the user via an indicator light, display, sound generator or haptic feedback generator. Moreover, the temperature of the heater plates 806a, 806b can be controlled via a moisture sensing or hair temperature sensing algorithm, to steadily increase the plate temperature as the heater plate dries. The styler’s fan speed could also be controlled in this way. Accordingly, by dynamically controlling the temperature of the styler’s heating plates 806a, 806b, the styler 800 is configured for providing a maximum dry rate without causing thermal damage to the hair being styled, as well as facilitating an automatic transition to styled hair from wet without the use of a “shineshot” mode. Specifically, “shineshot” is a mode in which the styler would not produce any airflow and which the heating plate temperature is set to operate at around 185°C. By dynamically controlling the temperature of the styler’s heating plates, the user can avoid the use of the “shineshot” mode, and hence more typical operating parameters can be used to facilitate the automatic transition to styled hair from wet (e.g. by use of a “normal” operating mode, where the styler produces an airflow and the heating plate temperature is around 120°C). Exemplary parameters of the styler 800 are given in the table below, which illustrate that the minimum and maximum porosity of the heater plates 806a, 806b to remove the hair’s water content and heat the hair to 120°C in one pass at the given plate size and flow rate is (independent of hole shape), around 18% and around 36% respectively (using the calculations described above): Heater plate 806, hair being styled having 30% water content Hole Type Plate Dimensions (mm) Flow Rate (L / mi n) No. of holes Lower Bound Diameter / Width (mm) Upper Bound Diameter / Width (mm) f, Minimum Porosity Maxi mum Porosi ty Small Holes 90x42.5 500 22 x 12 1.87 2.62 18.6% 35.4% Large Holes 90x42.5 500 11x6 4.18 5.98 18.5% 35.5% Styler Having a “Barrel” Perforated Heater An alternative styler 900 will now be described with reference to Figure 41a. As will be appreciated, this styler 900 is a variant of the stylers 10 and 800 described in detail above with reference to Figure 1a and Figure 39a respectively, and hence a description of corresponding components will not be repeated again here. In the example illustrated in Figure 41a, the styler 900 comprises a handle 902 bearing a barrel shaped heater 906 having perforations which allow for air or wetline to be sucked or blown through the holes into / onto the hair during styling. The styler’s heater 906 may comprise one or more controllable heating zones (as described in detail above), to facilitate the curling of hair during styling. The heater 906 is shown in its “flattened” plan format leftmost in Figure 41b, before it is wrapped over the insulative housing of the styler 900 and bonded thereto (e.g., via diffusion bonding) during manufacture of the styler 900 (as schematically shown rightmost in Figure 41b). As the heater 906 adopts a generally cylindrical profile once bonded to the main body of the styler 900, the heater’s support body should not comprise any compound curved surfaces. Whilst in this example the perforations are shown as adopting a generally circular profile, the perforations could take any shape (e.g. slots, elliptical, etc.). Moreover, the heater 906 may instead adopt an ovular profile, an elliptical profile, a paddle profile, a tapered barrel profile, a flat profile or a curved profile, as may be needed to facilitate a user to create certain hairstyles. Also, the barrel’s diameter could be within a range of around 26mm to 38mm (with some specific diameters being 26mm, 32mm, 38mm), but it will be appreciated that barrels having larger or smaller diameters may be used as needed for assisting the user to achieve a particular style. In use, the styler’s user presses a button provided on the main body of the styler 900 (not illustrated) when they are ready to style, heating the zones that are loaded with hair to a set temperature. To aid the heat up of the device, supercapacitors could be used to boost the power available at the start of the style (which is especially beneficial for cordless devices). This temperature could be controlled by an algorithm that detects the temperature of the hair (via the heater), allowing the styler 900 to heat the hair to a target temperature. Once the hair is sensed as being at the target temperature, the styler 900 will stop heating the loaded zones and start cooling the device. The cooling of the hair is driven by air moving technology that delivers cool fluid onto the hair by either sucking or blowing (e.g. fan(s) / pump(s) disposed within the body of the styler 900). The fluid could be cool ambient air and / or a wetline suspension in air. The wetline could be preheated by the heater 906 to allow for easier dispersion into the airflow. Such cooling of the hair assists in setting the style, and hence stylers of this type may be considered as perforated styling tools having a cooling capability. Once the style has set (i.e. styler 900 measures that the hair being styled has cooled to a target temperature), the user could be notified that the style is ready via lights, noise or haptics conveyed by a user interface integrated onto the styler’s handle 902, and then the user may remove the styler 900 from the styled tress of hair. Exemplarily parameters of the styler 900 are given in the table below, which illustrate the minimum and maximum porosity of the heater 906 to remove the hair’s water content and heat the hair to ~140°C in one pass at the given barrel size and flow rate (independent of hole shape, using the calculations described above): Heater 906, dry hair Barrel Diameter (mm) Barrel Lengt h (mm) Flow Rate (L / mi n) No. of holes Lower Bound Diameter / Width Upper Bound Diameter / Width f, Minimum Porosity Maximu m Porosity (mm) (mm) 26 115 200 22x6 3.5 4.95 13.0% 25.8% 32 115 200 22x6 3.69 6.86 11.8% 39.6% 38 115 200 22x6 3.87 8.38 10.9% 49.2% It will be appreciated from the table above that: when the barrel diameter is 26mm, the maximum porosity is around 26%; when the barrel diameter is 32mm, the maximum porosity is around 40%; and that when the barrel diameter is 38mm, the maximum porosity is around 49%. Hence, the maximum porosity increases with barrel size as the contact area with the hair increases. Styler Having a Perforated Heater for Receiving Bristles An alternative styler 1000 will now be described with reference to Figure 42a. As will be appreciated, this styler 1000 is a variant of the styler 900 described in detail above with reference to Figure 41a, and hence a description of corresponding components will not be repeated again here. In the example illustrated in Figure 42a, the styler 1000 comprises handle 1002 from which protrudes a barrel shaped heater 1006 at the styler’s head end, the heater 1006 having perforations which allow for bristles to be provided therethrough, so that styler 1000 may function as a “hot brush / hot comb” (it will be appreciated that alternative barrel head shapes, such as elliptical or tapered barrels, could also be used instead of the cylindrical barrel depicted). Specifically, the styler’s heater 1006 may comprise one or more controllable heating zones (as described in detail above), to facilitate the styling of hair during styling. The heater 1006 adopts substantially the same “flattened” plan format as that of the heater shown with reference to Figure 41b, before it is wrapped over the insulative housing of the styler 1000 and bonded thereto (e.g., via diffusion bonding) during manufacture of the styler 1000. In this example the styler’s main body comprises bristles 1008 over which the heater 1000 is wrapped during bonding of the heater 1006 to the main body of the styler 1000. As the perforations in this example are substantially circular in shape, the bristles 1008 provided on the main body of the styler 1000 are also substantially circular in shape, and are configured on the styler 1000 for mutual alignment with corresponding perforations over which the heater 1006 is wrapped. In the illustrated example, as the heater adopts a generally cylindrical profile once bonded to the main body of the styler 1000, the styler’s body which supports the perforated heater should not comprise any compound curved surfaces. The styler’s 1000 barrel head may have a diameter between a range of about 22 to about 40 mm (with some specific diameters being 26mm, 32mm, 38mm), although alternative larger / smaller diameters may be used as needed. Exemplarily parameters of the styler 1000 are given in the table below, which illustrate the maximum porosity of the heater 1006 to remove the hair’s water content and heat the hair to 185°C in one pass at the given barrel size and flow rate (independent of hole shape, using the calculations described above). Note that the minimum porosity for a hot brush styler is determined by mechanical bristle strength, a minimum number of bristles which prevent a user from burning their scalp in use, and manufacturing techniques (such as injection moulding): Heater 1006 Barrel Diameter (mm) Barrel Length (mm) Target Temp. (°C) No. of holes Lower Bound Diameter / Width (mm) Upper Bound Diameter / Width (mm) f, Maximum Porosity 26 115 185 22 x 6 1 4.95 25.8% 32 115 185 22 x 6 1 6.86 39.6% 38 115 185 22 x 6 1 8.38 49.2% It will be appreciated from the table above that: when the heater’s barrel Diameter is 26mm, the maximum porosity is around 26%; when the heater’s barrel diameter is 32mm, the maximum porosity is around 40%; and that when the heater’s barrel diameter is 38mm, the maximum porosity is around 49%. Hence, the maximum porosity increases with heater’s barrel size as the contact area with the hair increases. Whilst the styler 1000 described with reference to Figure 42a comprises perforations adopting a generally circular profile, the perforations could take any shape (e.g. slots, elliptical, etc.). For instance, the perforated heater may instead adopt a paddle shape, as illustrated with reference to Figure 42b, which shows a styler 1100 having handle 1102, from which protrudes a paddle-shaped heater 1106 comprising generally elliptical perforations, through which substantially elliptical bristles 1108 may pass from the body of the styler 1100 via the paddleshaped perforated heater 1106 when the heater 1106 is wrapped over the bristles 1108 and bonded to the main body of the styler 1100 (e.g. via diffusion bonding). The styler’s 1100 paddle head may have dimensions of around 30 to 50 mm by 90 to 120 mm, although alternative paddle shapes formed from alternative larger / smaller ranges may be used, as needed. In use, the styler’s user presses a button provided on the main body of the styler 1110 (not illustrated) when they are ready to style, heating the zones that are loaded with hair to a set temperature. To aid the heat up of the device, supercapacitors could be used to boost the power available at the start of the style (this is especially beneficial for cordless devices). The bristles 1108 could be fixed on the main body of the styler 1100, or could be retractable (e.g. into the main body of the styler 1100 via a bristle retraction means) to reduce the tension and tangling which may be caused by bristles 1108. The bristles 1108 could also comprise heater electrodes to increase the hair contact area with the heater 1106. To achieve this, the bristles 1108 may be connected to the main heater substrate via folded tabs which connect to the heater’s main heater electrode and sensing layer (not shown in Figure 42a). As the bristles 1108 would not operate at as hot a temperature as those of a traditional hot brush, the bristles 1108 could be made of a softer plastic than current devices, and hence be more comfortable in use. Moreover, the bristles 1108 could be flexible allowing for a “tangle-teaser” hot brush (via lower hair tension forces generated relative to a conventional fixed-bristle hot brush, as bristles 1108 flex to better-cope with hair entanglement). The temperature of the hair being styled could be controlled by an algorithm that detects the temperature of the hair (via the perforated heater 1106), allowing the styler 1100 to heat the hair to a target temperature. Once the hair is sensed as being at the target temperature, the styler 1100 will stop heating the loaded zones and start cooling the device. Such cooling may be achieved by air being driven from a fan located e.g. within the body of the styler 1100, and this may beneficially rapidly cool heater 1106 due to the low thermal mass properties of the heather 1106. Accordingly, as the styler 1100 has this cooling capability, hair styles may be more rapidly set and have improved style longevity. Once the style has set (e.g. the styler’s heater 1106 measures that the hair being styled has cooled to a target temperature), the user could be notified that the style is ready via lights, noise or haptics conveyed by a user interface integrated onto the styler’s handle, before the styler is removed from the styled tress of hair. Alternative Styler Having a Perforated Heater for Receiving Bristles An alternative styler 1200 will now be described with reference to Figure 43a. As will be appreciated, this styler 1200 is a variant of the styler 900 described in detail above with reference to Figure 41a, and hence a description of corresponding components will not be repeated again here. In the example illustrated in Figure 43a, the styler 1200 comprises a handle 1202, from which projects a barrel shaped heater 1206 at the styler’s head end. The heater 120 comprises perforations which allow for bristles 1208 to be provided therethrough, so that styler 1200 may function as a “wet-to-curl” product having curling functionality. Alternative barrel head shapes, such as elliptical or tapered barrels, could also be used instead. In another alternative, the styler 1200 may instead be used as a “hot brush” in combination with cold air (e.g. delivered by a fan in the main body of the styler 1200) to quench the hair and deliver optimal styling results. The styler’s heater 1206 may comprise one or more controllable heating zones (as described in detail above), to facilitate the styling of hair. However, unlike the heater 906 of the styler 900 illustrated in Figure 41a, the number of perforations provided on the heater 1206 are greater than the number of bristles 1208, and hence some perforations 1208b are configured to receive bristles 1208 which protrude from the main body of the styler 1200 when the heater 1206 is wrapped over the styler’s main heater support body (as shown in Figure 43c), whilst other perforations 1208p do not receive bristles 1208, and hence fluid flow can be provided via the unoccupied perforations 1208p. Figure 43b illustrates leftmost in a plan view (with the main body of the styler 1200 being omitted for clarity) heater 1206. In this example, the heater 1206 comprises six columns of perforations 1208b, 1208p, with perforations 1208b being mutually offset with respect to perforations 1208p, and the ratio of perforations to be left unoccupied by bristles (i.e. perforations 1208p) relative to occupied by bristles i.e. perforations 1208b) is 2:1 (although it will be appreciated that an alternative number of perforations 1208b, 1208p and / or alternative ratios thereof may be provided as needed). As shown in the rightmost part of Figure 43b, the heater 1206 is wrapped over the insulative housing of the styler 1200m and bonded thereto (e.g., via diffusion bonding) during manufacture of the styler 1200. Figure 43c shows a perspective view of the assembled heater 1206 bearing bristles 1208 and comprising open perforations 1208p. Perforations 1208b are occupied by bristles 1208 once the heater 1206 is assembled via the mutual configuration of bristles 1208 and perforations 1208b (in Figure 43a, all of perforations 1208b are illustrated as being occupied by bristles 1208), whereas the other perforations 1208p remain “open” and are not occupied by bristles, and hence fluid can be transferred via the styler 1200 through these holes when the device is in use. As the perforations 1208b, 1208p in this example are substantially circular in shape, the bristles 1208 provided therethrough are also substantially circular in shape and are configured for mutual alignment with corresponding perforations 1208b as the heater 1206 is wrapped thereover (in some alternative examples, the diameter of the bristles 1208 may be smaller than the diameter of the open perforations 1208p, although it will be appreciated that these diameters may be substantially the same). In the illustrated example, the heater 1206 adopts a generally cylindrical profile once bonded to the main body of the styler 1200, and so the styler’s body should not comprise any compound curved surfaces. The styler’s 1200 barrel head may have a diameter between a range of about 22 to 40 mm, although barrels having different ranges of sizes may be used as needed. Whilst in this example the perforations are shown as adopting a generally circular profile, the perforations could take any shape (e.g. slots, elliptical, etc.). In use, the styler’s user presses a button provided on the main body of the styler 1200 (not illustrated) when they are ready to style, heating the zones that are loaded with hair to a set temperature. To aid the heat up of the device, supercapacitors could be used to boost the power available at the start of the style (which is especially beneficial for cordless devices). The bristles 1208 could be fixed on the main body of the styler 1200, or could be retractable (e.g. into the main body of the styler 1200 by a bristle retraction means (not illustrated)) to reduce the tension and tangling which may be caused by the bristles 1208 when the styler 1200 is removed from the styled hair. The bristles 1208 could also comprise heater electrodes which are electrically connectable to the heater 1206 to increase the hair’s contact area with the heater 1206. To achieve this, the bristles 1208 may be connected to the main heater substrate via folded tabs which connect to the heater’s main heater electrode and sensing layer (not shown in Figure 43). The temperature of the hair being styled could be controlled by an algorithm that detects the temperature of the hair, allowing the styler 1200 to heat the hair to a target temperature. Once the hair is sensed as being at the target temperature (e.g. via the heater 1206), the styler 1200 will stop heating the loaded zones and start cooling the device. This active cooling of the styler 1200 may be driven by air from a fan located e.g. within the handle 1202 of the styler 1200, and this may beneficially rapidly cool heater 1206 due to the low thermal mass properties of the heater 1206. Additionally or alternatively, styling products (wetline) may also be applied to the hair being styled, for example by atomisation, nebulisation or pumping into the air flow, to further assist a user to achieve a desired style / drying (e.g. via a wetline dispenser housed within the handle 1202 of the styler 1200). Accordingly, heater 1206 may be configured for providing a maximum dry rate without causing thermal damage to the hair being styled, as well as facilitating an automatic transition to styled hair from wet without the use of a “shineshot” mode (where the styler’s normal operating mode is airflow on with a heating plate temperature of around 120°C, whereas a "shineshot” mode would not have any airflow, and the heating plate temperature is increased to around 185°C). Active cooling of the hair in the manner described above may be described as a “coolshot”, which beneficially sets curls in the styled hair to a greater degree than conventional hot curlers which lack this functionality. Once the style has set (i.e. styler 1200 measures that the hair being styled has cooled to a target temperature as sensed by the heater 1206), the user could be notified that the style is ready via lights, noise or haptics conveyed by a user interface integrated onto the styler’s handle, before the styler 1200 is removed from the tress of styled hair. Exemplarily parameters of the styler 1200 are given in the table below, which illustrate the maximum porosity of the heater 1206 to remove the hair’s water content and heat the hair to 120°C in one pass at the given barrel size and flow rate (independent of hole shape, using the calculations described above). Note that the minimum porosity increases with decreasing barrel size to suit air pressure requirements (assumed max. pressure drop 4kPa), and that the maximum porosity increases with barrel size as the contact area with the hair increases: Heater 1206, hair being styled having 30% water content Bareli Diameter (mm) Bareli Lengt h (mm) Flow Rate (L / mi n) No. of holes Lower Bound Diameter / Width (mm) Upper Bound Diameter / Width (mm) / , Minimum Porosity Maximu m Porosity 26 115 500 22x6 3.50 4.50 13.0% 28.5% 32 115 500 22x6 3.69 5.28 11.8% 32.0% 38 115 500 22x6 3.87 6.10 10.9% 35.6% It will be appreciated from the table above that: when the heater’s barrel Diameter is 26mm, the minimum porosity is around 13% whilst the maximum porosity is around 29%; when the heater’s barrel diameter is 32mm, the minimum porosity is around 11% whilst the maximum porosity is around 32%; and that when the heater’s barrel diameter is 38mm, the minimum porosity is around 10% whilst the maximum porosity is around 36%. Hence, the maximum porosity increases with barrel size as the contact area with the hair increases. In a further alternative, the styling head 1206 illustrated in Figure 43a may be configured for attachment to / detachment from the styler’s handle, and hence alternative head designs may be used with the styler 1200 depending on the hair style the user wishes to achieve. For instance, styling heads comprising alternative barrel shapes (e.g. ovular), barrel diameters, and / or bristle densities may be attached to / detached from the styler 1200. Once such alternative styling head for the styler 1200 illustrated in Figure 43a will now be described with reference to Figure 43d. In this alternative, the barrel head 1204 (i.e. the portion of the styler which comes into contact with hair during hair styling) is manufactured from a metallic substrate (such as aluminium, titanium alloy or magnesium alloy, in a diameter between about 22mm and about 40mm). A perforated, zoned resistive heater 1206' is provided circumferentially inside of the metal head 1204, and is connected to a rigid PCB 1210 in the manner described with reference to the heaters described above. The heater 1206' can be diffusion bonded onto the metal substrate, thereby maximising the thermal contact between the two components. The heater 1206' may be zoned around the inner circumference of the barrel 1204, and along its width, thereby maximising the control of the heater’s temperature and ensuring a constant temperature on the surface of the head 1204. As the styling head in this example comprises an "internal” heater 1206' (as opposed to the heaters described above which make direct contact with the hair), the number of zones that are required are reduced, and the peak power requirements of the heater 1206' are lowered, reducing the need for a sizeable power supply unit. All of these advantages in turn may further beneficially reduce the cost of the styler. Further Alternative Styler Having a Perforated Heater for Receiving Bristles An alternative styler 1300 will now be described with reference to Figure 44a. As will be appreciated, this styler 1300 is a variant of the styler 1200 described in detail above with reference to Figure 42b, and hence a description of corresponding components will not be repeated again here. According to this example, the styler’s heater 1306, which projects from the handle 1302, may adopt a paddle shape (instead of the above discussed barrel shape of the styler 1200) at the device’s head end 1304. As shown in more detail in Figure 44b, which illustrates in plan view heater 1306, the heater 1306 comprises generally elliptical perforations 1308b, through which substantially elliptical bristles 1308 may pass when the heater 1306 is wrapped over the bristles 1308 and bonded to the main heater support body of the styler 1300 (e.g. via diffusion bonding). Figure 44b illustrates further perforations 1308p which do not receive any bristles 1308 when the paddle head 1304 is assembled, and hence these perforations 1308p are configured to remain “open” (and hence fluid can transfer via the styler 1300 through these holes when the device is in use as detailed below). The styler’s 1300 paddle head 1304 on which the heater 1306 is disposed may have dimensions of around 120 x 60 mm, although alternative sizes may be provisioned (as may alternative perforation layouts / shape of the perforations on the heater 1306), and the styler’s heater 1306 may comprise one or more controllable heating zones (as described in detail above) to facilitate the styling of hair. Whilst in this example the perforations 1308b are shown as adopting a generally elliptical profile, the perforations could take any shape (e.g. slots, circular, etc.), with corresponding mutually aligned bristles 1308 adopting a configuration suitable to be received by those perforations 1308b. Moreover, the “open” perforations 1308p may also adopt alternative configurations to those illustrated in Figures 44a and 44b, e.g. slots, elliptical, etc. Such alternative configurations may be used to improve the hair drying rate, reduce damage to the tress of hair being styled, improve the shine of the styled hair, and / or to improve the softness of the styled hair, etc. In use, the styler’s user presses a button provided on the main body of the styler 1300 (not illustrated) when they are ready to style, heating the zones that are loaded with hair to a set temperature. To aid the heat up of the device, supercapacitors could be used to boost the power available at the start of the style (this is especially beneficial for cordless devices). The bristles 1308 could be fixed on the main body of the styler 1300, or could be retractable (e.g. into the main heater support body of the styler 1300 via a bristle retraction means (not illustrated)) to reduce the tension and tangling which may be caused by bristles 1308 when removing the styler 1300 from the styled hair. The bristles 1308 could also comprise heater electrodes to increase hair contact area with the heater 1306. To achieve this, the bristles 1308 may be connected to the main heater substrate via folded tabs which connect to the heater’s main heater electrode and sensing layer (not shown in Figure 44a). The temperature of the hair being styled could be controlled by an algorithm that detects the temperature of the hair, allowing the styler 1300 to heat the hair to a target temperature. Once the hair is sensed as being at the target temperature (e.g. by the heater 1306), the styler 1300 will stop heating the loaded zones and start cooling the device. Such active cooling of the styler 1300 may be driven by air from a fan located e.g. within the body of the styler 1300, e.g. within handle 1302, and this may beneficially rapidly cool heater 1306 due to the low thermal mass properties of the heater 1306. Alternatively, or in addition, styling products (wetline) may also be applied to the hair being styled, for example by atomisation, nebulisation or pumping into the air flow, to further assist a user to achieve a desired style / drying (e.g. via a means for providing wetline to the hair provisioned within the styler’s handle 1302). Accordingly, heater 1306 may be configured for providing a maximum dry rate without causing thermal damage to the hair being styled, as well as facilitating an automatic transition to styled hair from wet without the use of a “shineshot” mode as discussed above. Once the style has set (i.e. styler 1300 measures that the hair being styled has cooled to a target temperature), the user could be notified that the style is ready via lights, noise or haptics conveyed by a user interface integrated onto the styler’s handle before removing the styler 1300 from the hair. Exemplary parameters of the styler 1300 are given in the table below (diameter 2 being taken on the styler’s longitudinal axis, with diameter 1 being taken perpendicular thereto), which illustrate the maximum porosity of the heater 1306 to remove the hair’s water content and heat the hair to 120°C in one pass at the given paddle size and flow rate (independent of hole shape, using the calculations described above): Heater 1306, hair being styled having 30% water content Paddle Diameter Paddle Diamet Flow Rate No. of Lower Bound Upper Bound f, 1 (mm) er 2 (mm) (L / mi n) holes Diameter / Width (mm) Diameter / Width (mm) Minimum Porosity Maximum Porosity 55 120 500 17x6 3.35 4.25 17.6% 27.5% “Autocurler” Hair Styler Having a Perforated Heater Another hair styler 1400 will now be described with reference to Figure 45a. This styler 1400 comprises a handle 1402 in which the styler’s control electronics are housed, in addition to an adjustable-speed fan (and / or a wetline dispenser), as described in detail with respect to the stylers above. The styler 1400 further comprises a barrel 1404 projecting from the handle 1402, the barrel bearing a curved perforated zoned heater 1406. The heater 1406, shown in a simplified plan view in Figure 45b, comprises a rigid PCB (shown hatched leftmost in Figure 45b) which provides a point of connection (electrically and mechanically) for the flexible PCB heater layers 1406 to the device’s control electronics which are carried on the rigid PCB layer (such drive and control electronics being illustrated in Figure 3 and described above, and which control the operation of the styler 1400 (e.g. the heating of the different heating zones of the curved heater 1406, the fan speed, wetline dispensing, etc.)). A close-up of part of the heater 1406 is shown in Figure 45c, which illustrates the heater tracks 1406 being disposed either side of elongate perforations 1408p. The barrel 1404 itself may be formed according to any of the barrel heater examples described above, e.g. with reference to Figures 18 and 19, except that in this example the heater 1406 comprises a plurality of elongate perforations 1408p (which are substantially elliptical in profile), which are formed along the styler’s longitudinal axis in a proximal-to-distal manner. In the illustrated example, five such longitudinal perforations 1408p are illustrated, though it will be appreciated that more or fewer perforations 1408p may be used as needed. The heater barrel’s support body comprises slots which mutually align with the perforations 1408p, such that fluid (e.g. air / wetline) can be conveyed therethrough as desired by the user during hair styling. The fan, operating together with the perforations 1408p disposed on the heater barrel, leverage the Coanda effect in a manner consistent with known devices, such as Dyson’s Airwrap™ device. In summary, the Coanda effect relates to the tendency of a stream of fluid (such as air being driven by a fan) emerging from an orifice (such as the perforations 1408p) to follow a curved (or even flat) surface (such as the barrel-shaped heater 1406) and to entrain fluid from the surroundings so that a region of lower pressure develops. As a result of this effect, when the styler’s fan is running, the local area of low pressure formed by air flowing about the barrel 1406 (as shown with reference to the dotted arrows in Figure 46a), draws a tress of hair to be styled around the barrel “automatically”, and hence such a styler 1400 can be considered as an “auto curler” by using the Coanda effect. A significant advantage of the described “auto curler” styler 1400 relative to known devices is that the styler’s heater barrel 1406 heats the tress of hair which is wrapped therearound during the styling procedure, and hence the curls generated by the styler are tighter and longer-lasting relative to devices which do not comprise the curved heater 1406. Whilst longitudinal perforations 1408p have been described with respect to styler 1400 above, the Coanda effect may be leveraged by way of perforations which do not adopt this configuration. For example, and referring now to Figure 46b, styler 1500 is an alternative to styler 1400 (a discussion of shared components will not be repeated here), by virtue of the styler’s perforations 1508p being formed at an angle which is offset relative to the styler’s longitudinal axis (the styler 1500 is shown in perspective view in Figure 46c). Hence, the perforations 1508p can be considered to be “skewed” or “angled” with respect to the longitudinal axis in a proximal-to-distal direction as opposed to the perforations 1408p of the alternative styler 1400 (and hence these perforations 1508p are extend longitudinally offset along the heater’s surface). Stylers using a Zonal Air Heater Further types of stylers will now be described, which operate using a multi-zoned air heater. In overview, stylers operating according to this principle may be configured as dual-zoned, as illustrated schematically in Figure 47a, which shows two hemi-spherical controllable heating zones, or instead the styler may be quad-zoned, as illustrated schematically in Figure 47b, which shows four controllable heating zones which mutually form a cylindrical profile. In both figures, an air heater is schematically illustrated in the centre of the respective zones, such that air can be conveyed via one or more outlets formed in each of the respective zones. Using this principle, styling heads can be formed (as described in detail below) that have multiple sections which allow for hot air and cold air to be used at the same time, as may be needed depending on the style the user wishes to achieve. Accordingly, the air heater can be zonal to allow heating of the entirety of the air during a heating or drying stage, and then zonal heating of the air in a styling stage. “Blast Dryer” An example of a styler using an air heater will now be described with reference to Figure 48a, which shows a styler 1600 which is formed from a handle 1602 and a detachable head portion 1604. The handle 1602 may comprise a sheath over which the head portion 1604 is provided. The styler’s handle 1602 houses the styler’s drive electronics (as described above with reference to Figure 3) and the styler’s air heater (and a battery, if the device is intended to operate without a mains power supply). User-operable buttons / switches may be disposed on the handle 1602, which may control the supply of power to the device 1600, as well as the temperature / speed at which the styler’s air heater is operable (not illustrated in Figure 48a). The styler’s head portion 1604 comprises a longitudinal air outlet 1606 which projects out of the head portion 1604, as illustrated in Figure 48b. In one example, the head portion 1604 may be between 60 mm to 100 mm in length, and the outlet 1606 may have a width which tapers between 3 mm to 1 mm. This outlet may be referred to as a "bulb” given the outlet’s bulbous profile. The cross-section indicated as A-A’ in Figure 48b is illustrated in Figured 48c, which shows how two parallel streams of air exit out of the head 1604 around the bulbous projection 1606 (as indicated with arrows) - note that the handle’s 1602 sheath over which the head portion 1604 is provided is schematically illustrated as a solid circle. The air outlet 1606 tapers to its narrowest point in at the head’s distal end (i.e. the end of the head 1604 which is furthest away from the styler’s handle 1602). By tapering in this way, a uniform pressure and velocity gradient is achieved for air exiting via the outlet 1606, thereby facilitating a uniform temperature distribution at distances away from the outlet 1606 (and hence an “air blade” is formed by air exiting the outlet 1606). Using such a tapered-outlet 1606 also enables the styler 1600 to operate at relatively high-pressures, thereby maximising the efficiency of the styler’s air heater, as well as assisting in the projection of heat generated by the air heater over a significant distance from the styler’s outlet 1606. Accordingly, this styler 1600 is suitable, amongst other things, for blow-drying a user’s hair. In an alternative, the longitudinal outlet 1606 may comprise bristles to assist in forming tension with the tress of hair being dried / styled (e.g. during blow-drying). Additionally, or alternatively, the entirety of the longitudinal outlet 1606 may be configured to vibrate along the handle’s longitudinal axis (e.g. by connecting the outlet 1606 to a user-operable vibration motor disposed e.g. within the handle 1602), and hence the styler 1600 may function as a “tangle teaser” using air to achieve detangling. Further additionally or alternatively, the handle portion 1602 may comprise a fluid dispenser for dispensing a hair styling product (e.g. wetline) into the stream of air conveyed via the longitudinal outlet 1606. A further alternative styler 1700 will now be described with reference to Figure 49a, which is common with the styler 1600 of Figure 48a, except that the styler’s head portion 1704 is different (a description of common components will not be repeated here). Specifically, the styler’s head portion 1704 also comprises a longitudinal air outlet 1706 which projects out of head portion 1704, but this outlet 1706 projects to a greater extent away from the head portion 1704 relative to the corresponding outlet 1606 provided on head portion 1604 of styler 1600. In one example, the head portion 1704 may be between 60 mm to 100 mm in length, and the outlet 1706 may have a width which tapers between 6 mm to 2 mm. The head portion 1704 of this styler may therefore be considered more “nozzle-like” than the head portion 1604 of styler 1600, and due to the additional length of the outlet 1706, air exiting the outlet 1706 has a longer distance to travel relative to air which exits the outlet 1606 of the alternative styler 1600. The cross-section indicated as A-A’ in Figure 49a is illustrated in Figured 49b, which shows how two parallel streams of air exit out of the head 1704 around the bulbous projection 1706 (as indicated with arrows) - note that the handle’s 1702 sheath over which the head portion 1704 is provided is schematically illustrated as a solid circle. Accordingly, as the air exiting from this styler’s outlet 1706 has a relatively longer path to travel, the exiting air is concentrated to a greater degree by the outlet’s nozzle 1706, thereby delivering a smooth and unform blade of air out of the styler’s head portion 1704. The nozzle 1706 straightens to guide the air via the outlet in a thin blade profile, which creates a “paint brush”-like jet of air, facilitating particularly precise styling of hair. Further additionally or alternatively, the handle portion 1702 may comprise a fluid dispenser for dispensing a hair styling product (e.g. wetline) into the stream of air conveyed via the longitudinal outlet 1706. Root Lift Comb A styler 1800 which comprises dual-zone heating / cooling functionality will now be described with reference to Figures 50a, 50b and 50c. Figure 50a shows the styling head 1802 of the styler 1800, and this styler 1800 shares common components with the stylers 1600, 1700 described with reference to Figures 48 and 49, e.g. the sheathed handle, the drive circuitry, etc., which will not be described again here. The principal difference between the aforementioned stylers and styler 1800 are with respect to the design and functionality of the styler’s head portion 1802. Specifically, head portion 1802 comprises a tapered outlet 1804 which vents fluid out of the styler’s head portion 1802 via bristles 1806, in addition to via six longitudinal perforations 1808 (although more or fewer perforations may be used as needed) which are disposed hemispherically opposite to the outlet 1804. The head portion comprises a curved heater 1810 disposed over part of the circumference of the barrel head 1802, as discussed in further detail below. Figure 50b shows, in a simplified schematic view, two separate hemispherical zones Z1, Z2, respectively formed about each hemisphere of the circumference of the main barrel portion 1802 illustrated in Figure 50a. As will become apparent from the following description, the first zone Z1 is for providing heat to hair being styled via bristles 1806, whilst the second zone Z2 is for providing a means for cooling the heated hair, and hence to facilitate the setting of the hairstyle. The heater 1810 is configured for providing heat to the hair loaded onto the styler in the first zone Z1, and is configured to detect the temperature of the hair loaded onto the styler in the second zone Z2. In this example, the bristles 1806 may be electrically connected to the heater 1810 via folded tabs, and hence the bristles 1806 may also apply heat directly to the tress of hair being styled. The arrangement about the outlet 1804 is shown in plan view in Figure 50c, which better illustrates the tapered nature of the outlet 1804 (described above with reference to outlet shown in Figure 48a for styler 1600). The outlet 1804 is configured to provide two parallel streams of heated air from the first heating zone Z1, the zone Z1 being formed about a hemisphere of the head portion 1802 as described above, either side of bristles 1806. On the opposite hemisphere to the first zone Z1, a means for cooling hair is provided at the second zone Z2 (i.e. the cooling zone). In this example, the cooling means is a stream of cool air which is provided by the longitudinal slots 1808. Specifically, cooling may be achieved by activating a fan disposed within the styler’s handle to provide cool air via the longitudinal perforations 1808 in response to heater 1810 detecting that hair loaded in the second zone Z2 exceeds a predetermined (or preprogrammed) temperature, thereby forming a “cool cloud” of air vented out of the underside to increase cooling power (as illustrated in Figure 51b, with hot air projecting out via the bristles 1806 and cool air being vented out of the perforations 1808 at the base of the barrel head 1802 using the fan). An alternative means for cooling hair is illustrated with reference to Figures 51c and 51 d, in which the head 1802 is instead provisioned with a heat exchanger for cooling the second zone Z2. By using a heat exchanger within the head’s second zone Z2, instead of using the above described “cool cloud”, a “cool case” portion is created in the second zone Z2 against which hot hair can be quenched. Whilst a heat exchanger is relatively passive at cooling, the likelihood of hair being blown in an undesired manner is reduced when compared to active cooling by use of a fan. Use of styler 1800 will now be described. Firstly, the user inserts the bristles 1806 of the styler 1800 into the roots of the tress of hair to be styled. Then, the heater is activated and the hair is heated past its glass transition temperature whilst in contact with the heated side (first zone) of the styler 1800. The user then twists the bristles 1806 in position, allowing the heated hair to run through the bristles 1806, which keeps tension in the hair. As the heated tress of hair is rotated over the barrel head’s 1802 heating zone (first zone Z1), the heated hair encounters the cooling surface (second zone Z2) and cools into shape, quenching the style, and thus creating significant root lift. The diameter and length of the barrel (which may be sized as needed, e.g. between 26 mm to 38 mm in diameter and between 90 mm to 120 mm in length) determines the extent of the root lift. The user then repeats this procedure until the desired hairstyle has been achieved. An alternative styler 1900 to styler 1800 discussed above will now be described with reference to Figures 52a, 52b and 52c. Specifically, and unlike styler 1800 described above, styler 1900 does not rely on an air heater disposed within the body of the styler 1900 (and hence the tapered longitudinal slot used in styler 1800 is dispensed with in the styler 1900). Instead, heating of the hair is solely provided by curved perforated heater 1910. The heater may be overlayed over bristles 1906 to heat them indirectly, or may be connected to bristles 1906 (e.g. via foldable tabs which project from the heater’s electrodes) to heat the bristles 1906 directly in the first zone Z1. The heater electrode(s) mounted on the portion of the heater 1910 disposed at the second zone Z2 is not used for heating and instead is used as a temperature sensor, the signals from which are used to control the cooling power delivered, via perforations 1908, by a fan disposed in the styler’s handle. Figure 52b illustrates each of the heating zone (first zone Z1) and the cooling zone (second zone Z2) and their relative hemispherical separation over the circumference of the barrel head 1902. To provide heating of the styler's bristles, as well as the sensing of the temperature of the cooling zone, a curved perforated heater 1910 is disposed over the bristles 1906 and the head portion 1902. A plan view of this heater 1910 is shown in Figure 52c, with perforations 1906p being mutually configured to wrap over bristles 1906 formed in the head portion 1902. As noted above, tabs may be provided on the heater electrode which connect to the bristles 1906 to provide heating to the tress of hair being styled. The user then uses the styler 1900 in the manner described above for styler 1800 to achieve a desired hairstyle. Figure 53a illustrates in greater detail the heater 1910 used in the styler 1900 described above. As shown, the heater 1910 comprises a number of perforations, 1906p which correspond to, and which are configured to mutually align with, the number of bristles 1906 provided in the heating zone of the styler 1900. Similarly, the heater 1910 comprises a number of elongate perforations 1908 which correspond to, and which are configured to mutually align with, the elongate slots provided on the barrel head’s cooling zone of the styler 1900. Figure 53b illustrates the heater 1910 once the heater has been rolled over the barrel head 1902 (with the head being omitted for clarity). As noted above, foldable tabs may be provided on the heater 1910 (not illustrated) such that the styler’s bristles may be heated directly. Modifications and Alternatives Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. Electrical power may be provided to the hair styling devices described above by means of a power supply located at an end of the device, e.g. via a power supply cord (not illustrated). The power supply may be an AC mains power supply. However, in alternative embodiments, the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains power supply or from a DC supply via a charging lead), thereby enabling the above described devices to operate as a cordless products, as needed. The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 6. However, it could alternatively be implemented in any form of hair styling device, such as (but not limited to) crimpers, curlers or heated brushes. The heaters 6 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. The heaters described above may also be used in hair dryers or in combination devices that use conductive heating and air to dry and style the user’s hair (such as those described in the applicant’s earlier PCT application WO 2021 / 019239). In embodiments where air is used, the heaters 6 may be perforated so that air passes through the heater and is warmed by the heater as the air passes through. In the above embodiments, Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switches were used to control powering and sensing of the heater electrodes. As those skilled in the art will appreciate, other switches could be used instead. For example, Field Effect Transistors (FETs) could be used, such as Gallium Nitride FETs or bipolar junction transistors (BJTs). In the above embodiments, a DC power source was used to provide electrical power for heating the heater electrodes 64. This DC power source will typically be one or more batteries, although DC supplies that derive their power from a mains power AC signal may be used. Thicker or more dielectric layers are typically used between the heater electrodes 64 and the hair contacting surface of the hair styler when AC power is used to heat the heaters. In some of the above described embodiments, a heat spreading layer was provided above the heating electrodes to help spread the heat within each heating zone. In other embodiments, the heat spreading layer may be provided below the heater electrode layer as the heat spreaders can still perform their function of spreading the heat within an individual zone regardless of whether it is above or below the heater electrode layer. However, positioning the heat spreader layer above the heating electrode layer (i.e. closer to the hair contacting surface) can be advantageous as this layer can provide a scratch resistance function to the heater 6. In the above embodiments that have a heat spreading layer, the individual heat spreaders were formed as islands that do not touch neighbouring heat spreaders, to minimise the ability of heat to transfer from one heating zone to a neighbouring heating zone. This helps signal to noise for sensing and the independent control of the different heating zones. In some embodiments, it may be desirable to provide an electrical connection between neighbouring heat spreaders - for example to ground the heat spreading layer. In this case the individual heat spreaders may have some conducting material connecting them with at least some of their neighbouring heat spreaders. Even though an electrical connection is provided between adjacent heat spreading elements, as long as the connection is relatively small (for example less than 1 / 1 Oth of the length / width of the heat spreader), there will still be, in effect, a thermal break or decoupling between neighbouring heat spreaders. In one possible implementation, each heat spreader may be electrically connected to the vias 82-1, 82-2 that couple to the common terminal of the heater electrodes. This will prevent the build-up of unwanted static in the heat spreading layer, as the connection to the electronics can then be made by connecting to the heat spreader(s) closest to the edge of the flexible heater. However, since there is minimal physical connection between the heat spreaders they can still perform the desired function of spreading the heat within the respective heating zones whilst minimizing the spread of heat from one zone to an adjacent zone or zones. In the above-described examples the hair styling device 10 may comprise a single heater 6, or may alternatively comprise two or more heaters 6. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. The expressions “to dry hair”, “drying hair” or “decrease a moisture level of hair” and the like, as used in the present disclosure, can refer both to the removal of “unbound” water that exists on the outside of hair when wet, or the removal of “bound” water, which exists inside individual hairs, and which can be interacted with when heat styling hair. The “bound” water need not necessarily be removed when drying hair, although removal of some bound water may occur during a drying or styling process. Various other modifications will be apparent to those skilled in the art and will not be described in further detail here. This application also includes the following numbered clauses that summarise some of the aspects described above: 1. A hair drying and / or styling device comprising: a first elongate arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second elongate arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first head portion of the first elongate arm and the second head portion of the second elongate arm extend along a curved path. 2. The device according to clause 1, wherein the first and second head portions include respectively opposing first hair contacting surfaces. 3. The device according to clause 2, wherein the first and second head portions include respectively opposing second hair contacting surfaces. 4. The device according to clause 3, wherein the first hair contacting surface and the second hair contacting surface of each head portion are arranged successively along the length of the respective arm. 5. The device according to clause 3 or clause 4, wherein the first hair contacting surface of each head portion is flat and the second hair contacting surface is curved. 6. The device according to any of clauses 1 to 5, wherein at least one of the first hair contacting surfaces is heated. 7. The device according to any of clauses 1 to 6, wherein the first head portion and the second head portion have a length between 90 mm and 150 mm. 8. A hair styling device comprising: a handle portion for holding the device; first and second elongate roller portions each rotatably coupled to the handle portion and each comprising an outer curved surface having a plurality of lobes and troughs arranged circumferentially around the respective roller portion; wherein the first and second roller portions are arranged to rotate in opposite directions about their longitudinal axis and are positioned so that during rotation of the first and second roller portions, the lobes of the first roller portion engage with the troughs of the second roller portion and the lobes of the second roller portion engage with the troughs of the first roller portion; wherein during use, hair is sandwiched between the outer surfaces of the first and second roller portions and is styled as the first and second roller portions rotate; wherein the outer curved surface of at least one of the first and second roller portions comprises a curved heater for heating hair as it passes between the outer surfaces of the first and second roller portions. 9. The device according to clause 8, wherein the device comprises a motor disposed within the handle portion, wherein the motor is configured to drive rotation of the first and second rollers 10. The device according to clause 9, wherein the motor is configured to increase or decrease the rate of rotation of the first and second rollers in response to an input. 11. The device according to any of clauses 8 to 10, wherein the first and second roller portions each comprise three lobes and three troughs. 12. The device according to any of clauses 8 to 11, wherein at least one of the first and second roller portions comprises a position sensor configured to determine the position of the first and second roller portion. 13. The device according to clause 12, wherein the device further comprises a means for controlling the temperature of the curved heater in response to the rate of rotation of the first and second rollers. 14. A hair drying and / or styling device comprising: a handle portion for holding the device; first and second elongate head portions coupled to the handle portion and moveable between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first and second elongate head portions; wherein the first elongate head portion comprises a first elongate concave portion, a first elongate convex portion and a second elongate concave portion arranged successively in a direction perpendicular to a longitudinal axis of the first elongate head portion; wherein the second elongate head portion comprises first and second elongate concave portions; wherein the first and second head portions are arranged so that when in said closed configuration, the first and second elongate concave portions of the second elongate head portion respectively nest with the first and second concave portions of the first elongate head portion; wherein each of the first and second elongate head portions comprises heating means for heating the first and second concave portions of the respective elongate head portion, whereby hair that passes between the first and second head portions during use is heated and styled by the concave portions. 15. The device according to clause 14, wherein the handle portion further comprises means for cooling, wherein the means for cooling is configured to cool the first and second concave portions of the respective elongate head portion. 16. The device according to clause 15, wherein the means for cooling comprises one of a speed-adjustable fan, heat sink, and / or heat pipe and / or a thermoelectric cooler. 17. The device according to any of clauses 14 to 16, wherein the heating means is configured to sense the temperature of hair contacting the heating means. 18. The device according to clause 17, wherein when the heating means determines that hair reaches a preconfigured or preselected styling temperature, the device is configured to provide a signal. 19. A hot roller system comprising: one or more hot rollers having a curved outer surface on which hair can be wound; wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered; a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers; wherein the handle portion is configured to provide electrical power to the electric heater of a hot roller when the handle portion is coupled to the hot roller. 20. A hot roller for use in the system of clause 19, the hot roller comprising: a curved outer surface on which hair can be wound, the curved outer surface having a curved electric heater that heats hair wound on the hot roller when powered; and a coupling portion for coupling the hot roller to a handle portion of the system through which electrical power for heating the curved heater is provided. 21. The system of clause 19 or clause 20, wherein the hot roller further comprises a plurality of bristles, said bristles being extendable from and retractable into a main body of the hot roller by an actuating means. 22. The system of clause 21, wherein the actuating means is configured to extend and retract the bristles via a rotary-type mechanism. 23. The system of clause 21, wherein the actuating means is configured to extend and retract the bristles via a radial-type mechanism. 24. The system of clause 21, wherein the actuating means is configured to extend and retract the bristles via an axial-type mechanism. 25. The system of any of clauses 19 to 24, wherein the handle comprises a motor configured to engage with and to rotate the hot roller. 26. The system of any of clauses 19 to 25, wherein the handle comprises a ratcheting means for disengaging the application of rotational forces to the hot roller in response to a predetermined torque being reached. 27. A hot roller system comprising: one or more hot rollers having a curved outer surface on which hair can be wound; wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered; a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers; wherein the handle portion is configured to rotate the coupling portion of a hot roller when coupled to the handle portion to facilitate winding of user hair onto the hot roller. 28. A hot roller for curling hair comprising: a curved housing around which hair can be wound an elongate sheet attached at one end to said curved housing and having an unravelled state in which hair can be placed on the sheet and a ravelled state in which the elongate sheet and hair sandwiched between the elongate sheet and the curved housing are wound around the curved housing; wherein the elongate sheet and / or the curved housing comprises a curved heater for heating the hair when wound between the curved housing and the elongate sheet. 29. The hot roller according to clause 28, wherein a distal end of the elongate sheet comprises a hair clip, said hair clip comprising a first releasable securing portion and second releasable securing portion, said securing portions mutually configured to mate to secure hair sandwiched therebetween. 30. The hot roller according to clause 28 or 29, wherein the curved housing bears a plurality of elongate notches for tensioning hair wound between the curved housing and the elongate sheet. 31. The hot roller according to any of clauses 28 to 30, wherein the curved housing comprises means for winding the elongate sheet from the unravelled state to the ravelled state. 32. A hot roller for curling hair comprising: an elongate sheet comprising: a biasing means for biasing the elongate sheet in a ravelled state, a proximal end having a rigid portion for connecting to a power supply, and a distal end having a plurality of bristles for securing the elongate sheet to hair to be curled, wherein a side of the elongate sheet which contacts the hair to be styled comprises a curved heater for heating hair wound thereagainst. 33. The hot roller according to clause 32, further comprising a clip for releasably securing the elongate sheet to the hair being curled. 34. The hot roller according to clause 33, wherein the clip is formed from tensile plastic. 35. The hot roller according to clause 33, wherein the clip magnetically secures the elongate sheet to the hair being curled. 36. The hot roller according to any of clauses 32 to 35, wherein power is supplied to the hot roller via a battery placed inside a cavity formed by the roller in its ravelled state, or via a separate external power supply. 37. A hot roller system for curling hair comprising: a tubular clip moveable between an open configuration in which hair can be inserted into the clip and a closed configuration in which the clip grips the inserted hair; an actuator for moving the tubular clip between the open and closed configurations; a curved heater mounted to the tubular clip; and a controller for controlling the actuator and for controlling the heater to heat hair that is gripped by and wound around an outer surface of the tubular clip. 38. The system according to clause 37, wherein the system further comprises a handle insertable into a cavity formed by the tubular clip, wherein the handle comprises a motor which is configured to rotate the tubular clip to facilitate the winding of hair onto the outer surface of the tubular clip. 39. A hair drying and / or styling device comprising: a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair; a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair; wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion; wherein the first and second head portions each includes an inner surface that faces towards the inner surface of the other head portion and an outer surface that faces away from the inner surface of the other head portion; wherein the inner surfaces of the first and second head portions each comprise a first heating portion for heating hair that is sandwiched between the first and second head portions when the arms are in the closed configuration; and wherein at least one heating portion comprises a plurality of perforations therethrough. 40. The device according to clause 39, wherein the plurality of perforations are arranged along a length and width of the at least one heating portion. 41. The device according to clause 40, wherein perforations furthest from the proximal end of the at least one heating portion have a reduced size relative to perforations arranged closest to the proximal end of the at least one heating portion. 42. The device according to clause 41, wherein perforations closest to a peripheral edge of the at least one heating portion are smaller than perforations closest to a central portion of the at least one heating portion. 43. The device according to any of clauses 39 to 42, wherein the device further comprises means for conveying fluid to exit / enter the device via the plurality of perforations. 44. The device according to any of clauses 39 to 43, wherein the heating portion is configured to sense the temperature of the heating portion, and to increase or decrease temperature of the heating portion upon comparing the sensed temperature with a desired temperature. 45. A hair drying and / or styling device comprising: a handle portion for holding the device; a head portion coupled to the handle portion, wherein the head portion comprises a heater having a surface for heating hair; wherein the heater forms an outer hair contacting surface of the head portion and wherein the heater comprises a plurality of perforations disposed on the hair contacting surface. 46. The device according to clause 45, comprising a plurality of bristles which are configured to protrude via a subset of the plurality of perforations disposed on the hair contacting surface. 47. The device according to clause 45, comprising a plurality of bristles which are configured to protrude via each of the plurality of perforations disposed on the hair contacting surface. 48. The device according to clause 46, further comprising means for conveying fluid to exit / enter the device via the plurality of perforations. 49. The device according to any of clauses 45 to 48, wherein the head portion is tubular. 50. The device according to clause 49, wherein the head portion has a diameter between 22 mm and 40 mm. 51. The device according to any of clauses 45 to 48, wherein the head portion is paddle shaped. 52. The device according to clause 51, wherein the paddle shaped head portion has a length between 90 mm to 120 mm, and wherein the paddle shaped head portion has a width between 30 mm to 50 mm. 53. A hair drying and / or styling device comprising: a handle portion for holding the device; a head portion coupled to the handle portion, wherein the head portion comprises a hair contacting surface, wherein the head portion comprises a heater disposed within a cavity formed inside the head portion; wherein the heater is configured to provide heat to the hair contacting surface of the head portion, and wherein the head portion comprises a plurality of perforations disposed on the hair contacting surface. 54. The device according to clause 53, wherein the device further comprises means for conveying fluid to exit / enter the device via the plurality of perforations. 55. The device according to clause 53 or clause 54, wherein the head portion is tubular, and has a diameter of between 22 mm to 40 mm. 56. A hair drying and / or styling device comprising: a handle portion for holding the device; a tubular head portion coupled to the handle portion, wherein the tubular head portion comprises at least one slot that extends longitudinally along a length of the tubular head portion and at least one curved heater for heating hair; wherein the at least one heater forms an outer hair contacting surface of the tubular head portion; and wherein the handle portion comprises a fan configured to drive fluid to exit or enter the tubular head portion through the at least one slot. 57. The device according to clause 56, wherein the at least one slot extends substantially parallel to a longitudinal axis of the head portion. 58. The device according to clause 56, wherein the at least one slot extends longitudinally along a length of the head portion at an angle to a longitudinal axis of the head portion. 59. The device according to any of clauses 56 to 58, wherein the handle portion comprises a fluid dispenser for dispensing a hair styling product. 60. The device according to any of clauses 56 to 59, wherein the longitudinal slot is configured to output fluid over a curved outer surface of the head portion whereby, during use, hair is caused to be wrapped around the outer surface of the head portion due to a Coanda effect of the fluid flowing over the curved outer surface of the head portion. 61. A hair drying and / or styling device comprising: a handle portion for holding the device, the handle portion comprising a fan for generating a stream of air; a head portion coupled to the handle portion to receive the stream of air from the fan, the head portion having a proximal end and a distal end; wherein the head portion comprises a longitudinal slot extending along the head portion from the proximal end to the distal end of the head portion; wherein the fan is configured to drive the stream of air to exit the head portion through the longitudinal slot; and wherein the longitudinal slot is divided longitudinally into first and second portions by a longitudinal dividing portion disposed within, and extending along, the longitudinal slot. 62. The device according to clause 61, wherein the longitudinal slot tapers towards the distal end of the head portion. 63. The device according to clause 61, wherein the longitudinal slot comprises a plurality of bristles which extend therefrom. 64. The device according to any of clauses 61 to 63, wherein the handle portion comprises a motor configured to vibrate the longitudinal slot in use. 65. The device according to any of clauses 61 to 64, wherein the handle portion comprises a fluid dispenser for dispensing a hair styling product into air exiting the device. 66. The device according to any of clauses 61 to 65, wherein the longitudinal dividing portion extends out of the longitudinal slot. 67. The device according to any of clauses 61 to 66, wherein the longitudinal slot extends in a direction that is substantially parallel to a longitudinal axis of the head portion. 68. The device according to any of clauses 61 to 67, wherein the device comprises a single longitudinal slot. 69. The device according to any of clauses 61 to 68, wherein the longitudinal slot comprises a nozzle which projects perpendicularly therefrom and which is configured to concentrate the air being driven by the fan. 70. The device according to any of clauses 61 to 69, further comprising an air heater for heating the air stream before it is driven through the longitudinal slot. 71. A hair drying and / or styling device comprising: a handle for holding the device, the handle comprising a fan for generating a stream of air; a tubular head coupled to the handle to receive the stream of air from the fan, the tubular head having a proximal end and a distal end; wherein the tubular head comprises: a plurality of bristles disposed on an upper portion of the tubular head; a plurality of longitudinally extending slots disposed on a lower portion of the tubular head; and a heater mounted on an outer surface of the upper portion of the tubular head; wherein the fan is configured to drive air to exit the device via the plurality of longitudinal slots. 72. The device according to clause 71, wherein the bristles project from a longitudinal slot formed in the tubular head. 73. The device according to clause 71 or clause 72, further comprising an air heater for heating the stream of air from the fan. 74. The device according to any of clauses 71 to 73, further comprising means for heating the plurality of bristles. 75. The device according to clause 74, wherein the means for heating the plurality of bristles comprises portions of the heater that extend over a surface of the bristles. 76. The device according to any of clauses 71 to 74, further comprising a sensor for sensing the temperature of the lower portion of the tubular head. 77. The device according to clause 76, wherein the speed of the fan is increased when the sensor senses that the temperature of the lower portion of the tubular head exceeds a threshold temperature, and wherein the speed of the fan is decreased when the sensor senses that the temperature of the lower portion of the tubular head is below a threshold.

Claims

1. A hair drying and / or styling device comprising:a handle portion for holding the device;a tubular arm coupled to the handle portion, the tubular arm comprising a curved heater for heating hair;wherein the curved heater is a multilayer heater comprising:a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted on the tubular arm so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes:a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; andat least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer.

2. The device according to claim 1, wherein the curved heater comprises a first flexible portion and a rigid portion, wherein the rigid portion is mounted within the tubular arm and the first flexible portion extends over at least a first portion of the outer surface of the tubular arm.

3. The device according to claim 2, wherein the curved heater comprises a second flexible portion that extends over a second different portion of the outer surface of the tubular arm.

4. The device according to claim 3, wherein the first flexible portion extends over a first half of the outer surface of the tubular arm and the second flexible portion extends over a second half of the outer surface of the tubular arm.

5. The device according to any of claims 2 to 4, wherein the or each flexible portion comprises heater electrodes that heat the curved heater when current is applied to the heater.

6. The device according to claim 5, wherein the rigid portion comprises drive and control circuitry for controlling application of current to the heater electrodes7. The device according to any preceding claim, further comprising a second curved heater comprising a third flexible portion and a second rigid portion, wherein the second rigid portion is mounted within the tubular arm and the third flexible portion extends over a second portion of the outer surface of the tubular arm.

8. The device according to any preceding claim, wherein the tubular arm comprises a support for supporting the flexible substrates.

9. The device according to claim 8, wherein the support is formed from one of a liquid crystal polymer or glass filled nylon.

10. The device according to any preceding claim, wherein the device further comprises a means for cooling the device.

11. The device according to claim 10, wherein the means for cooling the device comprises a heat sink and / or heat pipe and / or a thermoelectric cooler.

12. A hair drying and / or styling device comprising:a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair;a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair;wherein the first and second arms are coupled to each other at their proximal ends and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion;wherein the first head portion comprises an inner surface that convexly faces towards a concave inner surface of the second head portion,wherein the inner surfaces of the first and second head portions each comprise a curved heater that heats hair that is sandwiched between the first and second head portions when the arms are in the closed configuration.

13. The device according to claim 12, further comprising means for cooling an edge of the concave inner surface.

14. The device according to claim 13, wherein the edge of the concave inner surface is porous.

15. The device according to claim 14, wherein the means for cooling comprises a fan, and wherein the device further comprises means for dispensing a styling fluid via the fan to the porous edge of the concave inner surface.

16. The device according to any of claims 13 or 15, wherein the means for cooling is disposed in the first and / or second arm.

17. The device according to any of claims 13 to 16, wherein the means for cooling comprises a heat sink and / or heat pipe and / or a thermoelectric cooler.

18. The device according to any of claims 12 to 17, wherein power is supplied to the means for heating hair in response to a means for detecting motion of the device determining that the device is in motion.

19. The device according to any of claims 12 to 18, wherein the edge of the concave inner surface is configured to determine the temperature of the hair sandwiched between the first head portion and the second head portion.

20. A hair drying and / or styling device comprising:a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair;a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair;wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion;wherein the first and second head portions each includes an inner surface that faces towards the inner surface of the other head portion and an outer surface that faces away from the inner surface of the other head portion;wherein the inner surfaces of the first and second head portions each comprise a first heating portion for heating hair that is sandwiched between the first and second head portions when the arms are in the closed configuration; andwherein the outer surfaces of the first and second head portions each comprise a second heating portion for heating hair that is wrapped around the outer surfaces of the first and second head portions.

21. The device according to claim 20, wherein the outer surfaces of the first and second head portions are configured to sense the temperature of hair that is wrapped around the outer surfaces of the first and second head portions.

22. The device according to claim 21, wherein the device is configured to operate in a first mode in use, in which the first heating portion is powered, and the second heating portion is not powered.

23. The device according to claim 20, wherein the device is configured to operate in a second mode in use, in which the first heating portion is not powered, and the second heating portion is powered.

24. The device according to any of claims 20 to 23, wherein the device further comprises a means for locking the first and second arms in the closed configuration.

25. The device according to any of claims 20 to 24, wherein the device further comprises a sensor for sensing whether the first and second arms are in the open configuration or the closed configuration.

26. The device according to any of claims 20 to 25, wherein the device further comprises means for cooling the first and second heating portions.

27. The device according to claim 26, wherein the means for cooling comprises a heat sink and / or heat pipe and / or a thermoelectric cooler.

28. A hair drying and / or styling device comprising:a first elongate arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair;a second elongate arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair;wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion;wherein the first head portion of the first elongate arm and the second head portion of the second elongate arm extend along a curved path.a handle portion for holding the device;first and second elongate roller portions each rotatably coupled to the handle portion and each comprising an outer curved surface having a plurality of lobes and troughs arranged circumferentially around the respective roller portion;wherein the first and second roller portions are arranged to rotate in opposite directions about their longitudinal axis and are positioned so that during rotation of the first and second roller portions, the lobes of the first roller portion engage with the troughs of the second roller portion and the lobes of the second roller portion engage with the troughs of the first roller portion;wherein during use, hair is sandwiched between the outer surfaces of the first and second roller portions and is styled as the first and second roller portions rotate;wherein the outer curved surface of at least one of the first and second roller portions comprises a curved heater for heating hair as it passes between the outer surfaces of the first and second roller portions.

30. A hair drying and / or styling device comprising:a handle portion for holding the device;first and second elongate head portions coupled to the handle portion and moveable between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first and second elongate head portions;wherein the first elongate head portion comprises a first elongate concave portion, a first elongate convex portion and a second elongate concave portion arranged successively in a direction perpendicular to a longitudinal axis of the first elongate head portion;wherein the second elongate head portion comprises first and second elongate concave portions;wherein the first and second head portions are arranged so that when in said closed configuration, the first and second elongate concave portions of thesecond elongate head portion respectively nest with the first and second concave portions of the first elongate head portion;wherein each of the first and second elongate head portions comprises heating means for heating the first and second concave portions of the respective elongate head portion, whereby hair that passes between the first and second head portions during use is heated and styled by the concave portions.

31. A hot roller system comprising:one or more hot rollers having a curved outer surface on which hair can be wound;wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered;a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers;wherein the handle portion is configured to provide electrical power to the electric heater of a hot roller when the handle portion is coupled to the hot roller.

32. A hot roller system comprising:one or more hot rollers having a curved outer surface on which hair can be wound;wherein the curved outer surface of each hot roller comprises a curved electric heater that heats hair wound on the hot roller when powered;a handle portion being configured to couple to and decouple from a coupling portion of each of said one or more hot rollers;wherein the handle portion is configured to rotate the coupling portion of a hot roller when coupled to the handle portion to facilitate winding of user hair onto the hot roller.

33. A hot roller for curling hair comprising:a curved housing around which hair can be woundan elongate sheet attached at one end to said curved housing and having an unravelled state in which hair can be placed on the sheet and a ravelled state in which the elongate sheet and hair sandwiched between the elongate sheet and the curved housing are wound around the curved housing;wherein the elongate sheet and / or the curved housing comprises a curved heater for heating the hair when wound between the curved housing and the elongate sheet.

34. A hot roller for curling hair comprising:an elongate sheet comprising:a biasing means for biasing the elongate sheet in a ravelled state,a proximal end having a rigid portion for connecting to a power supply, anda distal end having a plurality of bristles for securing the elongate sheet to hair to be curled,wherein a side of the elongate sheet which contacts the hair to be styled comprises a curved heater for heating hair wound thereagainst.

35. A hot roller system for curling hair comprising:a tubular clip moveable between an open configuration in which hair can be inserted into the clip and a closed configuration in which the clip grips the inserted hair;an actuator for moving the tubular clip between the open and closed configurations;a curved heater mounted to the tubular clip; anda controller for controlling the actuator and for controlling the heater to heat hair that is gripped by and wound around an outer surface of the tubular clip.

36. A hair drying and / or styling device comprising:a first arm having a proximal end and a distal end having a first head portion for engaging hair to dry and / or style the hair;a second arm having a proximal end and a distal end having a second head portion for engaging hair to dry and / or style the hair;wherein the first and second arms are coupled to each other and are adapted for movement between an open configuration for receiving a length of hair therebetween and a closed configuration for sandwiching hair between the first head portion and the second head portion;wherein the first and second head portions each includes an inner surface that faces towards the inner surface of the other head portion and an outer surface that faces away from the inner surface of the other head portion;wherein the inner surfaces of the first and second head portions each comprise a first heating portion for heating hair that is sandwiched between the first and second head portions when the arms are in the closed configuration; andwherein at least one heating portion comprises a plurality of perforations therethrough.

37. A hair drying and / or styling device comprising:a handle portion for holding the device;a head portion coupled to the handle portion,wherein the head portion comprises a heater having a surface for heating hair;wherein the heater forms an outer hair contacting surface of the head portion and wherein the heater comprises a plurality of perforations disposed on the hair contacting surface.

38. A hair drying and / or styling device comprising:a handle portion for holding the device;a head portion coupled to the handle portion,wherein the head portion comprises a hair contacting surface,wherein the head portion comprises a heater disposed within a cavity formed inside the head portion;wherein the heater is configured to provide heat to the hair contacting surface of the head portion, andwherein the head portion comprises a plurality of perforations disposed on the hair contacting surface.

39. A hair drying and / or styling device comprising:a handle portion for holding the device;a tubular head portion coupled to the handle portion,wherein the tubular head portion comprises at least one slot that extends longitudinally along a length of the tubular head portion and at least one curved heater for heating hair;wherein the at least one heater forms an outer hair contacting surface of the tubular head portion; andwherein the handle portion comprises a fan configured to drive fluid to exit or enter the tubular head portion through the at least one slot.

40. A hair drying and / or styling device comprising:a handle portion for holding the device, the handle portion comprising a fan for generating a stream of air;a head portion coupled to the handle portion to receive the stream of air from the fan, the head portion having a proximal end and a distal end;wherein the head portion comprises a longitudinal slot extending along the head portion from the proximal end to the distal end of the head portion;wherein the fan is configured to drive the stream of air to exit the head portion through the longitudinal slot; andwherein the longitudinal slot is divided longitudinally into first and second portions by a longitudinal dividing portion disposed within, and extending along, the longitudinal slot.

41. A hair drying and / or styling device comprising:a handle for holding the device, the handle comprising a fan for generating a stream of air;a tubular head coupled to the handle to receive the stream of air from the5 fan, the tubular head having a proximal end and a distal end;wherein the tubular head comprises:a plurality of bristles disposed on an upper portion of the tubular head;a plurality of longitudinally extending slots disposed on a lower10 portion of the tubular head; anda heater mounted on an outer surface of the upper portion of the tubular head;wherein the fan is configured to drive air to exit the device via the plurality of longitudinal slots.15s

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