Hair drying and / or styling device

Abstract

A hair drying and / or styling device comprises a handle portion (12, fig.13) and a head portion (14, fig.13) coupled to the handle portion. A plurality of bristles 108 project from an outer hair contac

Description

Field of the Invention The present invention relates to a hair drying and / or styling device and to its 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 having hair styling bristles. Background to the Invention Heated hair styling tools use heat to increase the temperature of hair to a desired styling temperature. For example, a heated plate / brush can 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. 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 styler over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair. Current heating technology is poor at heating complex styling surfaces, such as surfaces 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 these 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. The above problems are particularly pronounced in the context of stylers which bear bristles, such as styling combs / styling brushes (or the like). Typically, bristles can be moulded into the heater carrier (the part of the styler into which the hair styling heater is provided); made as a separate part and inserted into the styling head; or made as a bristle array and inserted into the styling head. However, as bristles are a complex shape and are required in large numbers, manufacturers often provide bristles which are not heated for simplicity of design. In the case where the manufacturer does incorporate heated bristles into the styler, the bristles are often provided using a metallic substrate and in-built into the main body of the styling head. Accordingly, their temperature is uncontrolled, they are unresponsive to changes in load and they also add significant complexity to the manufacture of the styling head. Moreover, current bristles provide no information / feedback on their state of operation to a user of the styler. There have been recent developments by the Applicant and other companies in developing hair styling appliances that use heaters having properties for use with complex styling surfaces and which have a lower thermal mass, which 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, particularly in the context of hair styling devices bearing hair drying and / or styling bristles (e.g. styling brushes, styling combs and the like). The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary of the Invention The present invention is set out in the appended independent claims. Optional features are set out in the appended dependent claims. According to an example, a hair drying and / or styling device is provided comprising: a handle portion for holding the device; a head portion coupled to the handle portion; and a plurality of bristles that project from an outer hair contacting surface of the head portion; wherein the head portion comprises a heater for heating the hair contacting surface of the head portion; wherein the heater comprises a substrate carrying a plurality of heater tracks, wherein the substrate comprises a plurality of perforations that define a plurality of strip portions of the substrate, each of which includes at least one heater track and is configured to wrap over a portion of one of said bristles. A portion of each bristle covered by the heater may be configured to provide heat non-contiguously over each bristle. The plurality of bristles may comprise a grooved receiving portion for receiving the heater. The heater may be provided with pairs of foldable tabs which respectively abut against the side walls formed between pairs of bristles of the plurality of bristles. The plurality of bristles may be provided in at least one row along a longitudinal axis of the head portion. Optionally, the head portion may be tubular, flat, or paddle shaped. The heater may be configured to provide heat to at least one of a plurality of heating zones. One or more of said bristles may be formed of a heat insulative material. According to another example, a hair drying and / or styling device is provided comprising: a handle portion for holding the device; a head portion coupled to the handle portion; and a plurality of bristles that project from an outer hair contacting surface of the head portion; wherein the head portion comprises a heater for heating the hair contacting surface of the head portion; wherein the heater comprises a substrate carrying a plurality of heater tracks, wherein the substrate comprises a first plurality of perforations that define a first plurality of foldable portions, each first foldable portion comprising at least one heater track, wherein the plurality of bristles are configured to protrude through the perforations of the substrate and cause each foldable portion to abut against a portion of a respective bristle. The heater may comprise a second plurality of perforations that define a second plurality of foldable portions, each second foldable portion comprising at least one heater track, wherein the plurality of bristles may be configured to protrude through the second plurality of perforations of the substrate and cause each second foldable portion to abut against a second portion of a respective bristle. Each bristle may comprise a thermally insulative tip. The plurality of bristles may comprise a grooved receiving portion for receiving the first foldable portion of the heater. The plurality of bristles may be provided in at least one row along a longitudinal axis of the head portion. The head portion may be tubular, flat or paddle shaped. The heater may be configured to provide heat to at least one of a plurality of heating zones. One or more of the bristles may be formed of a thermally conductive material. One or more of the bristles may be formed from a thermally insulative material and a thermally conductive heat spreading layer may be provided around at least a portion of the bristle. According to another example, a hair drying and / or styling device is provided comprising: a handle portion for holding the device; a head portion coupled to the handle portion; and a plurality of bristles that project from an outer hair contacting surface of the head portion; wherein the head portion comprises a heater for heating the hair contacting surface of the head portion; wherein the heater comprises a substrate carrying a plurality of heater tracks, and wherein a resistive track is embedded within a plurality of bristles for heating the plurality of bristles. The plurality of bristles may be formed from one of the following materials: plastics containing ceramic particles; crystalline polymers; or amorphous polymers. The ceramic particles may comprise at least one of: aluminium nitride; boron nitride; aluminium oxide; and / or sulphur dioxide. The embedded resistive track may be formed flat by laser cutting; chemical etching; thick film printing; or stamping. A first portion of the embedded resistive track within each of the plurality of bristles may be thinner than a second portion of the resistive track that connect between adjacent first portions. The embedded resistive track may be formed from a metallic wire. The plurality of bristles may be provided in at least one row along a longitudinal axis of the head portion. The head portion may be tubular, flat or paddle shaped. 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 1 b 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 having an interchangeable head; Figure 14a shows a perspective view of the assembly process of a hair styling head having insulative bristles; Figure 14b and its inset show a plan view of a flexible heater having a plurality of perforations; Figure 14c shows a perspective close-up of a portion of the styling head illustrated in Figure 14a; Figure 15a shows a schematic assembly process of another hair styling head having conductive bristles; Figure 15b shows a perspective the flexible heater of the styling head illustrated in Figure 15c shows a perspective close-up of a portion of the styling head illustrated in Figure 15a; Figure 15d and its inset show a plan view of electrode tracks which form two zones around the bristles of the styling head shown in Figure 15a; Figure 16a shows a schematic assembly process of another hair styling head having insulative bristles; Figure 16b shows a perspective close-up cutaway of a portion of the styling head illustrated in Figure 16a; Figure 16c shows a perspective close-up of a portion of the styling head illustrated in Figure 15a with the flexible heater omitted; Figure 17a shows a cutaway plan view of another hair styling head having bristles comprising an embedded resistive track; and Figure 17b shows a schematic close up of a portion of the resistive track illustrated in Figure 17a. Overview of 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 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 1 b, 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. 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 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, energising 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 and so 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 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. 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 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 low voltage operation (less than about 40 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. 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 that 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. 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 is 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. 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. 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. 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 bum 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 within the heater, 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: G.IV2 T — T -I- -_____ 1 max 1 Tar ' r 16t. k where, TMax = maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone; TTar = target operational temperature or average temperature (°C) of individual heating zones; q = power density (Wm2) 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 k = 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 of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of the heater materials and without causing the temperature of the unloaded part of a heating zone 642 to exceed the maximum temperature (LTmax) that could cause burning of relatively small bundles / strands of hair that come in contact with such overheated regions of the heating zone. Specifically, the required divisions along the length can be determined from: nL > t. k. (TMax TTar) Where, L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface); nL = number of zonal divisions along the length of the heater plate; TMax = 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; TTar = target operational temperature or average temperature (°C) of individual heating zones; k = 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 q = 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 (q) 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 (k) (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 (TTar) 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’. 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’. 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 100 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 inherently rigid, 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. 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 regions to the cold. 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 and reduce warm up time. 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. 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 that is provided in the gap between adjacent heat spreaders 91 may be provided by a 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. 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 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. 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 the central 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 would be produced using standard chemical etching methods from the PCB manufacturing process. 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 Figures 6a and 6b). 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 (tubular) 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 bearing bristles, to achieve different hairstyles (as set out in more detail below). 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 predetermined 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, device 10 may be actively cooled by incorporation of a fan (not illustrated) into the body of the device 10, e.g. within the handle 12 or the arm 14. Once the user has heated the hair to the desired / preset styling temperature, and upon releasing control button 24, the device control circuitry 15 may trigger the fan to activate, causing air to circulate at a rapid rate via the heater 16 and hence reduce the heater’s temperature faster than relying on ambient cooling conditions. 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), as explained in more detail below. Stylers Having Bristles 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). 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 and which can be configured to heat hair styling heads comprising bristles. Heater Designs for Stylers Having Bristles Flexible Heater Wrapped over Insulative Bristles As described above with reference to Figure 13, the head 14 of styler 10 can be detached from the Styler’s handle 12. In this way, alternative styling heads can be provided depending on the style that a user wishes to achieve. One such styling head will now be described with reference to Figures 14a and 14b, which show an alternative styling head 104 having a low thermal mass heater 106 (as described above) wrapped over an arm 100 (which may sometimes be referred to as a heater carrier or simply as a carrier) bearing a plurality of triangular shaped insulative bristles 108. Other features of styler 10, such as the handle and associated control circuitry, are not shown in Figure 14 for simplicity. The heater 106 of the styling head 104 is shown leftmost in Figure 14a in perspective view and in Figure 14b in plan view. The heater 106 comprises a plurality of perforations 110 formed therethrough, which facilitates the wrapping of the heater 106 over the plurality of bristles 108 during the assembly of the styling head 104. In this example, the bristles 108 are thermally insulative (e.g., manufactured from a polymer) and are arranged in a row, in this example, parallel to a longitudinal axis of the styling head 104. However, it will be appreciated that more than one row of bristles may be provided around the circumference of the styling head 104, with a corresponding number of perforations 110 (and hence a corresponding number of the strip-like portions 111) being provided in the heater of such a styling head. Moreover, whilst the styling head 104 illustrated in Figures 14a and 14b generally adopts a barrel shape, it will be appreciated that alternative shapes of styling head may be provided, e.g. flat or paddle shaped. The bristles 108 may be in-moulded onto the arm 100 of the styler head 104, or added to the styler’s head as a separately manufactured piece before the heater 106 is overlayed onto the arm 100 of the styling head and the bristles 108. As shown in Figure 14a, the heater 106 is wrapped around the arm 100 and over bristles 108 of the styler head 104 and may, for example, be secured to the arm 100 of the styler head 104 via diffusion bonding, heat bonding, physical vapour deposition, screen printing, adhesive (pressure set or thermoset), or another coating process. By wrapping the heater 106 over the bristles 108 in this way, both sides of the bristles 108 receive a portion of the heater 106 for heating hair during the styling process and hence both the ‘front’ and ‘back’ of each triangular shaped bristle 108 can be used to style / dry hair by the user. As described above, heater 106 may be manufactured with one or more controllable heating zones (which are electrically are connected via a common terminal (ground or positive) and a switching terminal (ground or positive)), and hence heat may be delivered via the heater 106 and over bristles 108 over certain portions of the styling head 104 depending on the style the user wishes to achieve. As shown in more detail in the inset illustrated in Figure 14b, the heater 106 is formed from a serpentine heater electrode tracks 109 (as described in more detail above) that are arranged over the substrate of the heater 106. Clearly, no heater tracks 109 are provided where the perforations 110 are provided within the substrate of the heater 106. In this example, two serpentine tracks 109-1, 109-2 extend from each end of the strip like portion 111 of the heater substrate which wraps over the bristles 108. The two serpentine tracks 109-2,109-2 do not overlap and a small gap G is left in the middle of the strip like portions 111 of the substrate at a location corresponding to the tip of the bristle 108, so that the tip of the bristle 108 (which may come into contact with the user) is not heated. As a result, the tip of the bristle 108 is kept cool because of the non-contiguous heating over the bristle’s faces, thereby avoiding the bristles 108 burning a user’s skin (e.g. their scalp / neck) during the styling process. In another example, however, this gap G may be omitted and hence the tip of the bristle 108 may be heated as well (which may be useful for stylers which are, for example, to be used by hair styling professionals). Referring now to Figure 14c, a closeup view of the heater 106 having been adhered over bristles 108 is illustrated, wherein the tips T of the bristles 108 are not heated (as noted above). In this example, guide rails 112 are provided at each edge of each bristle 108. These guide rails 112 are useful to guide or align the strip like portions 111 of the heater substrate over the bristles 108 during the assembly process. In addition to the leading ‘front’ and ‘rear’ faces 108f, 108r of the bristles 108 (the front face 108f of the bristles 108 is visible in Figure 14c, with the Tear’ faces 108r of the bristles 108 not being visible but facing in the opposite direction to the front faces) having heating portions 109-1, 109-2, the heater 106 may also be provided with foldable tabs (not illustrated) that have heating tracks and which project along the side walls of the bristles 108 (i.e. the portions of the bristles 108 indicated at P in Figure 14c). These foldable tabs can be adhered to the bristles 108 via diffusion bonding, heat bonding, physical vapour deposition, screen printing, adhesive (pressure set or thermoset), or another coating process, and beneficially their inclusion also provides heating on these side walls during styling. The way in which such foldable tabs may be manufactured is described in greater detail in subsequent examples. Flexible Heater Wrapped over Conductive Bristles Instead of a heater being wrapped over thermally insulative bristles to form a styling head in the manner detailed above, an alternative styling head 204 will now be described with reference to Figure 15, that uses thermally conductive bristles. The styling head 204 also has a low thermal mass heater 206 (like those described above) wrapped over the arm 200 of the styling head 204, and, as mentioned above the styling head 204 bears a plurality of thermally conductive bristles 208 (which may be triangularly shaped in the manner illustrated) which conduct heat from the heater 206 to the hair being styled (and hence the entire perimeter of the bristle 208 can provide heat to the hair being styled). The thermally conductive bristles 208 may have a high thermal mass which beneficially facilitates the rapid transfer of heat to hair being styled. Corresponding features of styler 10, such as the handle and associated control circuitry, will not be described again for simplicity. The heater 206 of the styling head 204 is shown leftmost in Figure 15a in plan view and rightmost in perspective view once wrapped over the styling head’s arm 204. As can be seen from Figure 15a, the heater 206 comprises a plurality of perforations 210 formed through the heater 206, through which the plurality of bristles 208 are received during the assembly of the styling head 204. In this example, as noted above, the bristles 208 are thermally conductive (e.g., manufactured from a metallic substrate) and are arranged in a row parallel to a longitudinal axis of the styling head 204. However, it will be appreciated that more than one row of bristles 208 may be provided on the styling head 204, with a corresponding number of perforations being provided in the heater 206 of such a styling head. Moreover, whilst the styling head 204 illustrated in Figure 15a generally adopts a barrel shape, it will be appreciated that alternative shapes of styling head may be provided, e.g. flat or paddle shaped. The bristles 208 may be added to the styler’s head as a separately manufactured piece before the heater 206 is overlayed onto the styling head’s arm 200. As shown in Figure 15a, the heater 206 is wrapped around the arm 200 and bristles 208 of the styler head 204 and may, for example, be secured to the arm 200 of the styler head 204 via diffusion bonding, heat bonding, physical vapour deposition, screen printing, adhesive (pressure set or thermoset), or another coating process. The heater’s perforations 210 are shaped such that the perforated portions 210 of the heater 206 form ‘tabs’ 21 Ot (as shown rightmost in Figure 15a) that extend up a leading face of the respective bristle 208, once the heater 206 has been wrapped over the bristles 208 of the styling head 204. These tabs 21 Ot of the heater 206 can be seen more clearly in Figure 15b in which the styling arm 200 has been omitted for clarity. As shown in Figure 15a, the tabs 21 Ot are configured to abut against a substantive portion (e.g. more than half) of one face 208f of the bristle 208 (although less than half of the bristle can be covered as needed). It will be appreciated that, similar to the bristles 108 described above, bristles 208 may comprise ‘front’ and ‘rear’ faces 208f, 208r (i.e. the front face 208f of the bristles 208 is shown in Figure 15b, with the ‘rear’ faces 208r of the bristles 208 facing in the opposite direction). Moreover, whilst in this example one ‘tab’ 21 Ot is provided per bristle 208, and hence the front face 208f or the rear face 208r is heated, in an alternative example two tabs 21 Ot could be provided such that both faces 208f, 208r can be heated by providing another perforation in the heater 206 prior to bonding to the styling head’s arm 200. In this “dual-tab” scenario, assuming each tab is to be about the same size, then the length of each individual tab 21 Ot will be about half the distance between the front and back faces 208f and 208r at the surface of the arm 200. Referring to Figure 15c, which shows in closeup the bristles 208 of the arm 200 of the styler with the heater 206 having been om itted for clarity, the tips T of the bristles 208 have, in this example, been coated with a thermally insulative material to avoid a user burning their skin (e.g. the scalp / neck) during a styling procedure. The thermally insulative material may take the form of a plastic insert (or another thermally insulative insert) which is adhered over the bristle 208 e.g. by over-moulding an insulative material over the tips of the bristles 208; or by a high-temperature dip coating of the tips of the bristles 208. Moreover, in this example illustrated in Figure 15c, a guide rail 212 for receiving a respective tab has been provided via a depression which is formed along a portion of the face 208f of each bristle 208 (e.g. a grooved receiving portion). By providing the guide rail 212, the heater substrate 206 can be readily aligned with respect to the arm 200 of the styling head 204 during the manufacturing process. As described above, the heater 206 may be manufactured with one or more controllable heating zones (which are electrically are connected via a common terminal (ground or positive) and a switching terminal (ground or positive)), and hence heat may be delivered via the heater 206 to the bristles 208 over certain portions of the styling head 204 depending on the style the user wishes to achieve. To provide control of the heating of bristles 208 of the styling head 204, two zones may be provided around each bristle 208 as illustrated in Figure 15d. Specifically, the inset of Figure 15d shows a first zone 206-A and a second zone 206-B (shown enclosed within dotted lines) provided over the surface of the heater 206. The heating of the second zone 206-B, which generally corresponds to a ‘tab’ portion 21 Ot shown in Figure 15b, can therefore be controlled independently of the heating of the first zone 206-A and hence the heating provided over each conductive bristle 208 can be adapted (independently controlled) as needed by the stylist in use. Alternative Flexible Heater Wrapped over Insulative Bristles In the example described above, the heater 206 and associated tabs 21 Ot were wrapped over conductive bristles. In the alternative styling head 304 described below with reference to Figures 16a and 16b, a heater 306 and associated tabs 31 Ot are wrapped over a plurality of triangular shaped insulative bristles 308 bearing a heat spreading layer 314. Other features of styler 10, such as the handle and associated control circuitry, are not shown in Figure 16 for simplicity. In more detail, the alternative styling head 304 has a low thermal mass heater 306 (as described earlier) wrapped over an arm 300 bearing a plurality of thermally insulative bristles 308, which may be made from a polymer. To conduct heat from the heater 306 to the hair being styled, a heat spreading layer 314 is provided around the perimeter of the base of each bristle 308 as illustrated with reference to the two rightmost bristles in close-up in Figure 16b. Accordingly, the perimeter of the bristle 308 in this example can provide heat to the hair being styled via the heat spreading layer 314. The heat spreading layer could be applied to the bristles 308 by electroplating, spray coating (arc or flame), or physical vapour deposition (PVD)A / acuum Metallisation of an appropriate substrate onto the bristles 308 and over the tabs of the heater 306. The tips T of the bristles 308 may be left uncovered by the heat spreading layer 314 to avoid the tips becoming too hot and potentially burning the skin near the hair being styled. In another example, however, the tips may instead be covered by the heat spreading layer 314, and hence the tip of the bristle may also be heated (which may be useful for stylers which are, for example, to be used by hair styling professionals). In the illustrated example, the bristles 308 are arranged in a row along a longitudinal axis of the styling head 304. However, it will also be appreciated that more than one row of bristles 308 may be provided on the styling head 304, with a corresponding number of perforations being provided in the heater 306 of such a styling head 304. It will also be appreciated that, similar to the bristles 108, 208 described above, bristles 308 may comprise ‘front’ and ‘rear’ faces 308f, 308r (i.e. the front face 308f of the bristles 208 is shown in Figures 16a, 16b and 16c, with the ‘rear’ faces 308r of the bristles 308 facing in the opposite direction). Moreover, whilst the styling head 304 illustrated in Figure 16a generally adopts a barrel shape, it will be appreciated that alternative shapes of styling head may be provided, e.g. flat or paddle shaped. The bristles 308 may be added to the styler’s head 304 as a separately manufactured piece before the heater 306 is overlayed onto the styling head’s arm 300, or may be moulded onto the styling head’s arm 300 before being overlayed by the heater 306. As shown in Figure 16a, the heater 306 is wrapped over the arm 300 and the bristles 308 of the styler head 304 and may, for example, be secured to the arm 300 of the styler head 304 via diffusion bonding or adhesive. The heater’s perforations 310 are shaped such that portions of the heater 306 form a tab 31 Ot once the heater 306 has been wrapped around the arm 300 and over the bristles 308 of the styling head 304, as can be seen in close-up in Figure 16b. In this example, tabs 31 Ot are configured to cover a substantive portion (e.g. more than half) of one face 308f of the bristle 308 (although less than half of the bristle 308 can be covered as needed). Moreover, whilst in this example one tab 31 Ot is provided per bristle 308, in an alternative example and as described before, two tabs could be provided - one on each face 31 Of, 31 Or of the bristle 308 by providing another perforation 310 in the heater 306 prior to bonding to the styling head’s arm. Referring to Figure 16c, an optional guide rail 312 is provided for receiving the tabs 31 Ot of the heater 306. The guide rail 312 is formed via a depression which is provided along the face of each bristle 308 into which the tab portions 31 Ot of the heater 306 are received (hence the guide rail 312 can be considered as a grooved receiving portion), thereby assisting the alignment of the heater substrate 306 onto the arm 300 of the styling head 304 during the manufacturing process (the tabs 31 Ot of the heater 306 and the heat spreader layer 314 have been omitted for clarity in Figure 16c). As described above, heater 306 may be manufactured with one or more controllable heating zones (which are electrically connected via a common terminal (ground or positive) and a switching terminal (ground or positive)), and hence heat may be delivered via the heater 306 to individual the bristles 308 or over certain portions of the styling head 304 depending on the style the user wishes to achieve. To provide control of the heating of bristles 308 of the styling head 304, two zones may be provided around each bristle 308 in the manner described above with reference to Figure 15c, and will not be repeated here. Accordingly, the heating provided over each bristle 308 via its corresponding heat spreading layer 314 can be adapted as needed by the stylist in use. Embedded Resistive Bristle Heaters In the examples described above, a flexible heater was wrapped around the arm and over thermally insulative or thermally conductive bristles and adhered thereto to provide heating to hair being styled (either indirectly in the case of thermally insulative bristles, or directly in the case of thermally conductive bristles). An alternative styling head 404 will now be described below with reference to Figures 17a and 17b, which show a styling head 404 having a flexible heater 406, and bristles 408 having an embedded resistive track 405. Corresponding features of the styler 10, such as the handle and associated control circuitry, will of course be provided but will not be described again for simplicity. Specifically, Figure 17a is a cutaway view through the barrel of the styling head 404 showing a low thermal mass heater 406 (as described earlier) wrapped over an arm 400 bearing a plurality of bristles 408 each having a portion of a resistive track 405 embedded therein which heats the bristles 408 which in turn heats the hair being styled. That is a single resistive track is provided that heats multiple bristles in the row of bristles 408. The track 405 may be electrically connected via the heater 406 to the styler’s power supply, or the track may instead be directly connected to the styler’s power supply (i.e. independently of connection to the heater 406). The resistive track 405 may be electrically connected to the heater 406 within the product or may be powered independently of the heater 406. The material forming the bristles 408 that could be over-moulded onto the resistive track 405 is preferably electrically insulative and thermally conductive, with a high temperature resistance capability. Examples of suitable materials for forming the bristles 408 which embed the track 405 include: plastics containing ceramic particulates (such as aluminium nitride, boron nitride, aluminium oxide, sulphur dioxide, etc.); crystalline polymers; and amorphous polymers such as high temperature epoxy resins. Accordingly, the perimeter of the bristle 408 can provide heat to the hair being styled upon current being supplied to the embedded resistive track 405. The resistive track 405 can be formed from a metallic substrate via laser cutting, chemical etching, or stamping. Alternatively, the track 405 could be manufactured from a metallic wire that is formed to a desired shape (e.g. depending on the number and size of bristles 408 being used on a given styling head). The track 405 could also be formed from a flat substrate via thick film printing, or the like. Of course, other manufacturing techniques may be used as needed to form the resistive track 405. As noted above, the resistive track 405 may be made from a wire or may be made from a flat material. In the latter case, and as illustrated with reference to Figure 17b, to bias power dissipation in the bristle region 408r proximate to the bristles 408, the track 405 can be made thinner in the bristle region 408r relative to the connecting portion between two bristle regions. Specifically, Figure 17b shows that the thickness feof the track 405 in the region 405r between bristles 408 is greater than the thickness ti of the track 405 in the bristle region 408r (i.e., the “connecting portion” 405r between two bristle regions 408r). This is because, at narrower parts of the track 405, resistance increases and for a continuous track without branching, current remains constant. Hence, power dissipation (and hence heating) is higher at narrower parts 408r of the track 405 which beneficially ensures that the bristle heating is as efficient as possible as heat is mainly dissipated in regions 408r that are in contact with hair being styled. In the illustrated example, the bristles 408 are arranged in a row along a longitudinal axis of the styling head 404 and a single resistive track 405 is provided for each the bristles in the row. However, it will be appreciated that more than one row of bristles 408 may be provided, wherein each row may have its own embedded track 405 for heating the bristles in the row. Alternatively, a single resistive track may be provided to heat the bristles in multiple rows. Similarly, multiple resistive tracks may be provided to heat the bristles in each row, with each resistive track heating multiple bristles. Moreover, whilst the styling head 404 illustrated in Figure 17a generally adopts a barrel shape, it will be appreciated that alternative shapes of styling head 404 may be provided, e.g. flat or paddle shaped. The bristles 408 may be added to the styler’s head 404 as a separately manufactured piece (e.g. as a bristle array piece) before the heater 406 is overlayed onto the styling head’s arm 400, or may be moulded onto the styling head’s arm 400 before being overlayed by the heater 406. As described above, the heater 406 may be manufactured with one or more controllable heating zones (which are electrically connected via a common terminal (ground or positive) and a switching terminal (ground or positive)), and hence heat may be delivered via the heater 406 to the bristles 408 over certain portions of the styling head 404, depending on the style the user wishes to achieve. In addition to providing heat for styling hair, the embedded track 405 may be used additionally or instead as a sensor. For example, the track 405 could be configured to sense hair tension and to measure hair loading. In the case of sensing hair tension, the embedded resistive track 405 may be used as a strain gauge - as hair is pulled through the device’s styling head 404, the bristles 408 may be configured to bend by increasing amounts under increasing hair tension. The device may then monitor the resistance of the embedded track 405 within the bristles. The change in resistance can then be measured, and hence hair tension sensed. In the case of measuring hair loading, such measurement may be achieved in combination with the heating function of the embedded resistive track 405. For instance, as the hair is loaded, the temperature of the bristles 408 drop as heat is transferred to the hair being styled. The power draw can be monitored to give a proxy measurement of the hair load applied to the styling head 404. By using the track 405 as a sensor in these ways, a control system could be provisioned in the styler for use in combination with a bristle vibration motor (not illustrated) to detangle hair sensed as being strained, or generally for feedback to the user (e.g. via lights, sounds and / or haptics). Moreover, the track 405 could be adapted for use as a temperature sensor to sense through the depth of the hair tress being styled, and to monitor and control the heater system in response to the detected temperature relative to a desired (or preprogrammed) temperature to achieve a desired style. For example, the track 405 may be formed of a material whose resistance changes with temperature. Hence by sensing the resistance of the track 405, the controller can determine the temperature of the user’s hair being styled and take appropriate action. 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. In the above-described example with reference to Figure 15 the bristles 208 were described as being thermally conductive, whilst in the above-described example with reference to Figure 16 the bristles 308 were described as being thermally insulative. However, for an alternative styler it will be appreciated the bristles along a given row in that styler’s styling head may alternate between being thermally conductive and thermally insulative (or instead the bristles may be a mixture of thermally conductive / insulative). Additionally or instead, in the case where more than one row of bristles are provided on a styling head, each row may alternate between being between being a thermally conductive row of bristles and thermally insulative row of bristles (or each row may comprise a mixture of thermally conductive / insulative bristles). Bristles in the above-described examples have been described as being substantially triangularly shaped. However, it will be appreciated that the bristles may instead adopt a different shape. For instance, the bristles may instead be oval shaped or round shaped (or polygonally shaped). For these alternatives in the context of the example described with reference to Figure 14, the heater substrate may still wrap over these alternative bristle shapes similar to the manner illustrated in Figure 14a. Moreover, a styling head may comprise a row of bristles having a mixture of bristle shapes. Additionally or instead, rows of bristles having an alternate bristle shape (or a mixture of alternative bristle shapes) may be provided on a styling head to assist the user when styling. 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. The power supply may be provided via a Separated Extra Low Voltage (SELV) system. 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 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.

Claims

1. A hair drying and / or styling device comprising:a handle portion for holding the device;a head portion coupled to the handle portion; anda plurality of bristles that project from an outer hair contacting surface of the head portion;wherein the head portion comprises a heater for heating the hair contacting surface of the head portion;wherein the heater comprises a substrate carrying a plurality of heater tracks, wherein the substrate comprises a plurality of perforations that define a plurality of strip portions of the substrate, each of which includes at least one heater track and is configured to wrap over a portion of one of said bristles.

2. The device according to claim 1, wherein the portion of each bristle covered by the heater is configured to provide heat non-contiguously over each bristle.

3. The device according to claim 1 or claim 2, wherein the plurality of bristles comprise a grooved receiving portion for receiving the heater.

4. The device according to any preceding claim, wherein the heater is provided with pairs of foldable tabs which respectively abut against the side walls formed between pairs of bristles of the plurality of bristles.

5. The device according to any preceding claim, wherein the plurality of bristles is provided in at least one row along a longitudinal axis of the head portion.

6. The device according to any preceding claim, wherein the head portion is tubular.

7. The device according to any of claims 1 to 4, wherein the head portion is flat or paddle shaped.

8. The device according to any preceding claim, wherein the heater is configured to provide heat to at least one of a plurality of heating zones.

9. The device according to any preceding claim, wherein one or more of said bristles is formed of a heat insulative material.

10. A hair drying and / or styling device comprising:a handle portion for holding the device;a head portion coupled to the handle portion; anda plurality of bristles that project from an outer hair contacting surface of the head portion;wherein the head portion comprises a heater for heating the hair contacting surface of the head portion;wherein the heater comprises a substrate carrying a plurality of heater tracks, wherein the substrate comprises a first plurality of perforations that define a first plurality of foldable portions, each first foldable portion comprising at least one heater track,wherein the plurality of bristles are configured to protrude through the perforations of the substrate and cause each foldable portion to abut against a portion of a respective bristle.

11. The device according to claim 10, wherein the heater comprises a second plurality of perforations that define a second plurality of foldable portions, each second foldable portion comprising at least one heater track,wherein the plurality of bristles are configured to protrude through the second plurality of perforations of the substrate and cause each second foldable portion to abut against a second portion of a respective bristle.

12. The device according to claim 10 or claim 11, wherein each bristle comprises a thermally insulative tip.

13. The device according to any of claims 10 to 12, wherein the plurality of bristles comprise a grooved receiving portion for receiving the first foldable portion of the heater.

14. The device according to any of claims 10 to 13, wherein the plurality of bristles are provided in at least one row along a longitudinal axis of the head portion.

15. The device according to any of claims 10 to 14, wherein the head portion is tubular.

16. The device according to any of claims 10 to 14, wherein the head portion is flat or paddle shaped.

17. The device according to any of claims 10 to 16, wherein the heater is configured to provide heat to at least one of a plurality of heating zones.

18. The device according to any of claims 10 to 17, wherein one or more of the bristles is formed of a thermally conductive material.

19. The device according to any of claims 10 to 18, wherein one or more of the bristles is formed from a thermally insulative material and wherein a thermally conductive heat spreading layer is provided around at least a portion of the bristle.

20. A hair drying and / or styling device comprising:a handle portion for holding the device;a head portion coupled to the handle portion; anda plurality of bristles that project from an outer hair contacting surface of the head portion;wherein the head portion comprises a heater for heating the hair contacting surface of the head portion;wherein the heater comprises a substrate carrying a plurality of heater tracks, andwherein a resistive track is embedded within a plurality of bristles for heating the plurality of bristles.

21. The device according to claim 20, wherein the plurality of bristles are formed from one of the following materials: plastics containing ceramic particles; crystalline polymers; or amorphous polymers.

22. The device according to claim 21, wherein the ceramic particles comprise at least one of: aluminium nitride; boron nitride; aluminium oxide; and / or sulphur dioxide.

23. The device according to any of claims 19 to 22, wherein the embedded resistive track is formed flat by laser cutting; chemical etching; thick film printing; or stamping.

24. The device according to claim 23, wherein a first portion of the embedded resistive track within each of the plurality of bristles is thinner than a second portion of the resistive track that connect between adjacent first portions.

25. The device according to any of claims 20 to 22, wherein the embedded resistive track is formed from a metallic wire.

26. The device according to any of claims 20 to 25, wherein the plurality of bristles is provided in at least one row along a longitudinal axis of the head portion.

27. The device according to any of claims 20 to 26, wherein the head portion is tubular.

28. The device according to any of claims 20 to 26, wherein the head portion is flat or paddle shaped.

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