Hair styling apparatus

Abstract

A hair drying / styling appliance 1 comprising a heater 6, a case 102 and a cooling component configured to cool the case. The heater is a multilayer heater comprising a hair contacting surface, capable

Description

Field of the Invention The present invention relates to an apparatus and method for improved styling of hair. In particular, it relates to an appliance for and method of applying heat and active cooling to hair when styling, which can improve the effectiveness of the styling, for example improve the curl compression. Background to the Invention Heated hair styling tools use heat to increase the temperature of hair to a desired styling temperature. For example, a hair straightener having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a hair glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair. Similarly, a heated brush or hair dryer can also be used to style hair by heating air which in turn heats the hair to a temperature suitable for styling. The hair is typically styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been recent development by the applicant and other companies in developing hair styling appliances that use heaters having a lower thermal mass that can therefore heat up and cool down much more quickly. Such low thermal mass heaters are therefore more responsive and are easier to dynamically vary the temperature with time. Such low thermal mass heaters are also much more space-efficient and lighter within the styler, which facilitates scope for introducing further components which can provide further functionality for styling. 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. In the following, any examples and embodiments not falling within the scope of the claims do not form part of the invention and are provided for illustrative purposes only. According to a first aspect, there is provided a hair drying and / or styling appliance comprising: a heater for providing heat for drying and / or styling hair; a case; and a cooling component; wherein the heater is a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30 pm and 2 mm; and wherein the cooling component is configured actively to cool the case during styling. The active cooling of the case during styling can improve the efficacy of the styling; for example, this can assist in improving curl compression. The multilayer heater can minimize heat leakage into the case. The multilayer heater typically occupies less space within the case than conventional heaters, which facilitates sufficient space for active cooling components to be provided. In preferable implementations, the case is arranged at least partially surrounding the heater. In some implementations, the case may be arranged to support the heater. The heater may be arranged on a first face of the appliance and the case may be located on at least one further face of the appliance. This can facilitate the heating and then cooling of hair, in particular in a desired style (for example, heating and then cooling a tress of hair wrapped around the appliance to form a curl). In some implementations, the appliance may comprise an arm, and the heater and the case may both be provided on the arm; preferably the heater may be arranged on a first surface of the arm and the case located on at least one further face of the appliance (for example, the case located on at least one further face of the arm). Preferably, the cooling component may be configured to cool the case via heat exchange. This can provide efficient cooling. In some implementations, the cooling component may comprise a fluid within a conduit adjacent an inner surface of the case. The fluid may be liquid or gas. In some implementations, the cooling component may comprise a fan device for moving gas, preferably wherein the gas is air. In some implementations, the appliance may comprise two arms joined by a shoulder, and wherein the fan may be provided in the shoulder and configured for moving gas along one or both the arms. The fan device may preferably be configured for moving gas within an inner cavity of the case. In some implementations, the case may comprise apertures arranged such that air can flow from the inner cavity of the case to outside of the case. This can ensure good airflow. The apertures may be formed as slots, preferably wherein the slots are arranged parallel to the length of the appliance, more preferably parallel to the length of an arm of the appliance. In some implementations, the appliance may further comprise ribs protruding from a surface of the case, preferably protruding radially inwards from an inner surface of the case. The ribs can direct the flow of air, and / or provide further surface area for cooling. This can aid in increasing the efficiency of the cooling. The ribs may be arranged such that they run along a direction parallel to the length of the appliance, preferably the length of an arm of the appliance. This can direct air flow along the length of the appliance. In some implementations, the fluid (i.e. the fluid within the conduit adjacent an inner surface of the case) may be a liquid, preferably a coolant liquid. The cooling component may comprise a thermoelectric cooling element. In some implementations, the appliance may further comprise at least one support within the case to support the heater, preferably wherein the support is arranged to stabilize the heater relative to an or the inner cavity of the case. This can facilitate support of the heater whilst facilitating space (for example a cavity) for the cooling component. In some preferable implementations, the heater may comprise a plurality of independently controllable heating zones, and preferably the cooling component may comprise a plurality of independently controllable cooling zones. A combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness may preferably be less than 15 W / m.K and greater than 0.1 W / m.K. According to a further aspect, there is provided a method of operating a hair drying and / or styling appliance comprising a heater configured to heat hair for styling, the heater being arranged in a case, wherein the method comprises heating the heater while simultaneously cooling the case. The cooling the case may comprise operating an active heat exchange mechanism, preferably provided within an interior of the appliance. In example implementations of the method, the active heat exchange mechanism may comprise at least one of: a fan for facilitating flow of a gas, preferably air; a liquid cooling system; and a thermoelectric cooling system. The method may further comprise directly cooling the hair by facilitating flow of a gas, preferably air, out of the case. The hair drying and / or styling appliance may preferably be the hair drying and / or styling appliance as described above. According to a further aspect, there is provided a computer program product comprising computer implementable instructions for causing a programmable device to carry out the method. The invention extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. 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 1 a shows an overview of an exemplary hair styling device; Figure 1b shows a hairstyling 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 13a shows a side view of a hair styling device comprising an active cooling system; Figure 13b shows a perspective view of the hair styling device of Figure 13a; Figure 14 illustrates an exemplary use of a hairstyling device; Figure 15 shows an internal view of the hair styling device of Figure 13a and 13b; Figure 16a shows a perspective view of a further exemplary implementation of a hair styling device comprising an active cooling system; Figure 16b shows an internal view of the hair styling device of Figure 16a; Figure 17a shows a perspective view of a yet further exemplary implementation of a hair styling device comprising an active cooling system; Figure 17b shows an internal view of the hair styling device of Figure 17a; and Figure 18 shows an internal view of a yet further exemplary implementation of a hair styling device comprising an active cooling system. Detailed Description of Preferred Embodiments Overview of Hair Styling Device Figure 1 a 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 1b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. One or more user interfaces 11 are 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 so that the hair styler 1 can be used to straighten the user’s hair. The hair styling device 1 shown in Figure 1 can also be used to curl the hair by turning the device 1 through approximately 180 degrees or more after clamping the hair between the arms 4a, 4b and before moving the device 1 along the tress of hair 40. Hair has a relatively high thermal mass and when in contact with the heating surface of the heater 6 the hair absorbs a significant amount of the heat energy. The heaters 6 must quickly supply the lost heat energy back to the heating surface otherwise the temperature of the heating surface will drop and potentially impact on the quality of the thermal styling. If the temperature of the heaters 6 fall below that required to raise the hair temperature above the glass transition temperature of the hair, the hair will not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo significant damage. As such, the device 1 must be able to control the temperature so that the heating surface of the heaters 6 remains within a particular temperature range. Furthermore, it must 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 fora 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 (not shown). 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 Safe Extra Low Voltage (SELV) operation (less than 42.4 Volts) and 0.8mm to 2.0mm in the case of AC operation) and with very low thermal mass. The upper surface of the layer 62 provides the hair contacting surface of the heater 6, although a non-stick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair over the heating surface if the layer 62 does not itself have such non-stick properties. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 into a rigid support 68 which forms a base. These layers may be mounted onto the rigid support after the layers themselves have been bonded together or they may be bonded one at a time (or multiple at a time) onto the rigid support 68. If a flexible heater is desired, then there is no need for the rigid support 68 or if a support is used, this may be a non-rigid support. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 6, and instead, the heaters 6 directly heat the user’s hair. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down much more quickly than prior art stylers. In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. 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 in 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 forthat heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater6 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 and the other half is not loaded with hair (which is assumed to be the realistic worst case scenario during operation), 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 loaded part 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. This situation is illustrated in Figure 7, which shows a tress of hair 40 overlying heating zones Z2, Z3 and Z4, with heating zone Z3 being fully loaded with hair and heating zones Z2 and Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 64 for each heating zone back to the drive circuitry 23 as well as the number of control switches in the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature difference between the loaded and unloaded halves of a heating zone, a maximum size of the heating zones can be defined which depends on the maximum power that can be applied to the heating zone and the material characteristics 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 difference that occurs between the hair loaded and unloaded halves of a heating zone 642 can be defined with the equation below: ^Tmax=K{^TioadAP , a, t}^TioadAP.AIa. t where, ^Tmax = maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone ATiOad = initial temperature difference that is created between the hair loaded and unloaded halves of a heating zone upon loading with hair AT = increase in power supplied to the heating zone A = surface area of the heating zone a = combined thermal diffusivity of the layers that constitute the heating zone t = total thickness of the layers that constitute the heating zone K = a constant whose value is also dependent on the values of ATtoad, AT, A, a, and t In particular, if it is assumed that only one half of a heating zone 467 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone 467 can be defined with the equation below: q.W2 T — T -L -____ 1 max 1 Tar T 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 (Wm 2) required to heat hair passing over the surface to the desired temperature for styling; W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and k = the thickness averaged thermal conductivity of the layers that constitute the heating zone. If it is assumed that the combined thermal diffusivity and / or 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 equations can be used to determine the area (A) and / or required zone width (W) and hence a number of divisions along the length and / or length and width of each heating zone that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to retain 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 must be positioned across the length and width of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of heater materials and without causing the differential temperature between a loaded part and an unloaded part of a heating zone 642 from exceeding a maximum differential temperature {LTmax) that could cause burning of relatively small bundles / strands of hair that come in contact with the overheated regions of the heating zone. Specifically, the maximum area can be determined from: Amax = Mk LTmax *( (K * t) / Pdrraz ) Where, Mmux = maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone (in Kelvin) to avoid damage to the hair k = the combined lateral thermal conductivity of the layers that constitute the heating zone (in W / m.K) t = total thickness of the layers that constitute the heating zone (in mm) Pdmax = the maximum power density of the heating zone (in W / cm2) Mk = a constant Specifically, the required divisions along the length can be determined from: nL > 4. t.k.(TMax TTar) q A 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 (Wnr1°C’1) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and q = power density (Wm2) 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. Maximum power density (Pdmax) is greater than 0.8 W / cm2 and less than 100 W / cm2 and preferably greater than 2 W / cm2 and most preferably greater than 8 W / cm2 and less than 15 W / cm2. - 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 combined thermal conductivity of the layers forming the heating zone (k) (in a plane perpendicular to the thickness of the heating zone) is between 15 and 100 W / m.K. In comparison the combined thermal conductivity of the multilayer heater across the entire heater and measured from end to end along a straight line passing along the longest dimension of the heater 6 is between 0.1 and 15 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 maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone to avoid hair damage is preferably less than 20 Kelvin, more preferably less than 10 Kelvin and most preferably less than 5 Kelvin. - The total thickness of the layers (t) which make up the heater is less than 300 pm but no less than 75 pm 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 maximum area of the heating zones is about 20 cm2. The inventors have found that a heater zone area of between 4 cm2 and 7 cm2 provides a good compromise between not having too many heating zones whilst avoiding the risks of burning the user’s hair. Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1.7 cm. Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 4a). If multiple rows of heating zones 642 are provided along the length of the heater (such as is shown in Figure 4b), then each row of heating zones 642 should meet the limits defined above if the above-described overheating problem is to be avoided. Alternative Heater Arrangement An alternative flexible heater 6’ is illustrated in Figure 8, which shows on the left hand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 8, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive or diffusion bonding (thermoforming) of the underlying materials to the upper surface of the rigid substrate 68’, or by over-moulding in which the carrier is injection moulded over the back of the flexible heater within the mould. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 8 also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example thermistors for sensing the temperature of the heating zones 642 of the heater 6’ or fuses that can electrically isolate the heater electrode of each zone in case of the zone overheating. Figure 8 also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 3 that controls the heating of the different heating zones 642 of the heater 6’. As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm. The different layers forming part of the heater 6' are shown in exploded cross-sectional and perspective views in Figure 9. A description of each layer is given below. Low Friction Coating 81 (optional) This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 3 pm) to reduce the thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant. This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. This is needed because the coating is prone to cracking when flexed, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape. Alternatively, this coating may itself comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm). Heat Spreading Layer 82 (optional) This is also an optional layer and, when provided, helps to spread the heat within each heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics 15 would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 - from the perspective of the control electronics 15, the heating zone 642 would be at the “correct” temperature despite having hot and cold regions. Each heating zone 642 would have its own heat spreader, which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption, reduce warm up time and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 10 illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive material over the corresponding heating zone. The heat spreaders 91 may be separated from each other by a solid material having a thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91, it is difficult for heat from one heating zone 642 to pass into neighbouring heating zones 642. The solid material (dielectric and / or scratch resistant low frictions material(s)) that is provided in the gap between adjacent heat spreaders 91 may be provided by a Physical Vapor Deposition Diamond-Like Carbon (PVD DLC), bond film, coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. Alternatively other suitable methods may be used to provide solid material in the gap between adjacent heat spreaders, such as masking and vapour deposition, etc. This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality. Polyimide Separator layer 83 The polyimide separator layer 83 provides electrical isolation between the hair contacting surface of the heater 6’ (which may be the upper surface of this layer 83 if the optional layers 81 and 82 are not provided) and the main heater electrode layer. This layer 83 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642. Main Heater Electrode &Sensing Layer 84 This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 11 illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source. Each heater electrode 64 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 11 are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. Other processes such as printing, thick film printing, physical vapour deposition and the like could be used to form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at suitable vias 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flow through each heater electrode 64. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 64 may be physically separate from all other heater electrodes 64 in which case, each end of each heater electrode 64 would be connected separately back to the drive and control board 78. As schematically illustrated in Figure 11, the end of each heater electrode 64 that is connected to the switch is provided at the side of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the heater which is curved over the upper surface 74 of the rigid support substrate 68’). The inventors have found that this arrangement helps heat generated in the heater electrodes 64 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with the curved edge portion, then the hair will still be heated as this curved edge portion is heated. The conductive material used in the layer 84 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 64 depends upon its temperature - and so the temperature of the heating zone 642 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 64. Figure 12 is a schematic view of the way in which the heater electrodes 64 may be connected together and to the drive circuitry 23 and the power supply 21. As shown in Figure 12, each heater electrode 64 is connected at one end to the power supply 21 and at the other end to a respective switch (in this case a MOSFET switch) 95-1 to 95-20. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently. When the temperature of a selected heating zone 642 is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, by measuring the voltage dropped across the resistor R (using the operational amplifier 97), the microprocessor 29 can determine the resistance of the selected heater electrode 64 and hence can determine the temperature of the corresponding heating zone 642. If the determined temperature is above the desired temperature for that heating zone 642, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone 642 is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642. Polyimide Separator (Optional) 85 When an auxiliary heater electrode layer is provided, this layer is required to provide the required electrical separation between that auxiliary heater electrode layer and the main heater electrode layer 84 described above. This polyimide layer 85 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W / mK. Other dielectric materials could be used instead of polyimide. Auxiliary Heater Electrode Layer (Optional) 86 Some embodiments of the heater 6’ may benefit from the presence of an additional heating element layer 86. This additional layer 86 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 21, for example the main power source could be a power supply and the power source for the auxiliary heater electrode layer 86 could be one or more batteries. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively still, the conductors on this auxiliary layer 86 could be used for temperature sensing, in which case, the heater electrodes 64 in the main layer 84 may only be used for heating. The heater electrodes on the auxiliary layer 86 will typically have the same form as the heater electrodes 64 used in the main heater electrode layer 84 - so that they will define the same heating zones 642 as the heating zones 642 defined by the heater electrodes 64 on the main heater electrode layer 84. The path taken by the heater electrodes on the auxiliary layer 86 does 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 its accidental exposure and to prevent moisture ingress. This backing layer 87 electrically separates the bottom heating layer from any surface mounted components that are mounted in the surface mounting layer 88 (discussed below) on the bottom of the heater 6’. If desired, this dielectric layer 87 can be made thicker than the upper dielectric layers to provide enhanced structural integrity of the flexible part of the heater system. As with the other dielectric layers, this backing layer 87 does not need to be a polyimide layer and other dielectric materials could be used. Rear Side Surface Mount Components (Optional) 88 This layer is used to mount components on to the rear of the flexible heater 6. These components may be temperature sensors (e.g. thermistors) or other components involved in providing fusing functionality for the heater (e.g. solder links). This layer may be produced using standard chemical etching methods from the PCB manufacturing process. Additional surface mount components would be added later. High Temperature Adhesive 89 The function of this layer is to enable 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. Hair Cooling When heat-styling hair, the process of cooling the hair after it has been heated can be very important for improved styling. If the hair is cooled whilst being held in a curled style, then it has been found that the curl compression (the tightness of the curl that remains) is improved. For all styling types, not just curling, hair typically may also hold the style better if cooled in a desired style. For example, users can use a hair straightening appliance, such as the styler 1, to curl hair by wrapping the hair around one arm 4a of the styler 1 such that it wraps around and passes over the heater 6a - and then running the styler 1 over the tress of hair. This is illustrated in Figure 14, which shows a user curling a tress of hair 40 using the styler 1. The tress of hair 40 is wrapped around the styler 1 such that it touches the case 102a and 102b of each arm and, when closed, will also touch the heater 6a and 6b of each arm. The hair can then be curled by passing the styler 1 over the tress 40 in this position, which means it will pass between the heaters 6a, 6b and over the case 102a, 102b. If the case 102a, 102b of the styler 1 cools the hair 40, then a tighter (and typically longer-lasting) curl can be achieved as the hair is cooled while in a curled position. This is referred to as improved curl compression. For hair stylers with conventional heaters (i.e. ceramic heating plate), heat typically leaks into the surrounding components, such as the casing, due to the nature of its heat generation and materials. This means the case is typically already somewhat heated while the user curls the hair, and typically the temperature of the hair increases the temperature of the case even further. This can lead to a sub-optimal curl. The conventional heater technology also fills the space inside the caseworks of the styler, making it difficult to include any cooling components without making the styler too large. By contrast, the very low thermal mass heaters 6, as described above, firstly lead to a much lower level of heat leakage into the surround casing in comparison to conventional heating technology. Secondly, the very low thermal mass heaters 6 occupy less space within the casework, which facilitates active cooling components to be provided as well. This can be useful as, although the heater 6 will heat the casing to a lesser degree than a conventional ceramic heater might, the heat of the hair itself can create an increase in the temperature of the casing. Active cooling components can therefore be provided within the styler 1 to facilitate cooling of the hair in addition to heating. Fan cooling Figures 13a and 13b show a schematic side view and perspective view, respectively, of an embodiment of the styler 1 comprising active cooling components. In particular, the two arms 4a and 4b, each comprising a heater, 6a and 6b respectively, and a case, 102a and 102b respectively, are connected by and moveable relative to a shoulder 2’, within which is provided a fan cooling mechanism. The small fan causes air to be circulated within the case 120a, 102b, thereby actively cooling the case via a heat exchange. The fan causes air to flow from the shoulder 2’ along at least one, but typically both, of the arms 4a, 4b of the styler 1. In some implementations, the airflows wholly within the case 102a, 102b while in other implementations the airflows out of the case 102a, 102b. Figure 15 shows a schematic perspective view of one arm 4a of the styler, in which the end of the arm 4a has been removed to show the inner components within the case 102a below the thin heater 6a. Immediately below the heater 6a is a carrier 100 for the heater, which supports the heater 6a. Below and connected to the carrier 100 is a structure arranged to support the heater 6a and carrier 100, while still allowing air to flow within the case 102a. As such, the structure has an open structure which comprises floats 104, 106 which are in contact with the case 102a to keep the structure securely in place. At least one float 106 is provided which extends parallel to the length of the arm 4a, such that it extends towards the case 102a and engages with a corresponding feature (e.g. a tab) in the case 102a at the end of the arm 4a. This holds the heater 6a and the carrier 100 in position within the arm 4a. At least one further float 104 extends radially outwards (for example, in a direction perpendicular to the surface of the heater 6a) to the case 102a. The at least one further float 104 is typically formed of silicone, which can facilitate some small movement relative to the case 102a. This arrangement can stabilize the structure relative to the case 102a in both directions, while still allowing for air flow. The cavity 108 in which the air can flow is adjacent to the majority of the surface area of the case 102a. Figures 16a and 16b show an exemplary implementation of the fan cooling arrangement, in which air from the fan is used to cool the case via a heat exchange cooling mechanism. Figure 16a shows a schematic perspective view of the whole styler 1, with the ends of the casing removed such that the inner components within the casing 102a, 102b can be seen. Figure 16b shows a magnified view of just the open end of one arm 4a, showing the inner components within the casing 102a. As described above, this arrangement comprises the carrier 100, and support structure comprising floats 104 and 106. In addition to this, the casing 102a comprises inwardly protruding ribs 110 that are arranged on the inner surface of the casing 102a. The ribs 110 run parallel to the length of the arm 4a and act to guide the flow of air from the fan, located within the shoulder 2’, along the length of the case 102a. The ribs 110 can help to create a smooth flow of air within the cavity 108. This can improve the efficiency with which the airflows over the surface of the case 102a and removes heat. The ribs 110 also increase the surface area of the case 102a over which the air can flow, cooling the case 102a via a heat exchange cooling effect. Figures 17a and 17b show an alternative cooling mechanism, in which the air from the fan is used to cool the hair directly. Figure 17a shows a schematic perspective view of the styler 1, with heaters 6a, 6b on each arm 4a, 4b and a fan located within the shoulder 2’. Figure 17b shows an enlarged perspective view of an arm with the end removed such that the internal components can be seen within the casing 102a. As for the previous implementation, the internal components comprise the carrier 100 below the heater 6a, and the support structure comprising floats 104 and 106. In this implementation, the case 102a comprises apertures 112, which facilitate airflow from the cavity 108 within the case 102a to outside of the case 102a. This air flow can therefore directly cool hair located on the external surface of the case 102a. The apertures 112 as illustrated are formed as long slots running parallel to the length of the arm 4a of the styler 1. However, other arrangements and configurations could also be used. The extent of the cooling may be changed by altering the flow throughput of the fan. This may be altered in dependence on the temperature of the heater 6 - which may be the desired setpoint temperature, a measured temperature (as measured by the temperature measurement circuitry 25) and / or may be in dependence on the thermal load of the heater 6. Liquid cooling In an alternative implementation, cooling could be performed using a heat exchange cooling mechanism using a fluid. In such an implementation, a conduit containing a fluid is provided adjacent to the case 102 such that it can be cooled by the liquid via a heat-exchange mechanism. The conduit is fluidically sealed to prevent fluid from leaking out of the styler 1 or onto the internal components. The fluid may be water or may be a specialized coolant fluid. The conduit may be configured such that it covers a continuous spread across the inner surface of the case 102, or it may be arranged as a series of conduits passing over the inner surface of the case 102 (these may be connected, effectively forming a long conduit snaking across the inner surface), or indeed an alternative arrangement. There may additionally (and optionally) be provided a pump and / or agitator for enhancing the flow of the fluid through the conduit(s). As for airflow cooling systems, the cooling effect may be altered by increasing the throughput of fluid through the conduit, for example by changing the settings of a pump and / or agitator. This may be performed in dependence on the setpoint temperature of the heater, the measured temperature and / or the thermal load on the heater. Thermoelectric cooling In a yet further implementation, the cooling of the case 102 can be implemented using a thermoelectric cooling system. This is illustrated in Figure 18, which shows a schematic perspective view of an arm 4a of the styler with the end removed so that the internal components can be seen. As for the previous implementations, the inner components comprise a carrier 100 below the heater 6a, and a support structure comprising floats 104 and 106. In addition, on the inner surface of the case 102 is provided a thermoelectric cooling system 114. This is configured to act as a solid-state heat exchanger to cool the case 102 via thermoelectric cooling. As illustrated in Figure 18, the thermoelectric cooling system 114 may be arranged such that it spans across the available inner surface area of the case 102. However, alternative arrangements of the thermoelectric cooler may be used, for example in which the cooling system is arranged in a pattern configuration. Thermoelectric cooling systems offer the advantage of good control over the cooling (i.e. the cooling temperature). The cooling can therefore be controlled, for example, in dependence on the temperature of the heater 6 (based on the desired operating temperature, thermal load and / or on measurements made by the temperature measurement circuitry 25). Additionally, as thermoelectric coolers can be made to very small sizes, an array of thermoelectric coolers may be provided within the case 102. Each cooler of the array may be arranged to correspond to one heating zone 642 of the heater 6a, or to a number of heating zones 642 of the heater 6a. This can allow the cooling to be defined in dependence on the loads of individual heating zones 642. For example, a lower cooling temperature may be used when it is determined that adjacent heating zones 642 have been loaded with hair. 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. 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 hairstyling device 10 may comprise a single heater6, or may alternatively comprise two or more heaters 6. In some implementations of the styler 1 in which the case 102a, 102b comprises apertures 112 (as illustrated in Figures 17a and 17b), a styling product can be distributed through the apertures 112 while a user is styling their hair. Such a styling product may for example be a hair spray, which could be used to help set a style, or a conditioning spray, etc. In such an implementation, the styler 1 may comprise a reservoir for holding the product and / or may be configured to receive a cartridge containing a hair product. The styler 1 may then further be configured to disperse the product into a mist. Furthermore, the cooling mechanism has be described in reference to a hair straightening appliance comprising two arms; however, active cooling mechanisms such as those described may also be implemented in other hair drying and styling appliances. For example, a wandshaped curling iron may comprise heating and cooling portions. The cooling may be implemented in dependence on the heating of the heaters; for example, there may be particular combinations of heating temperature and cooling temperature may be used. In some implementations, a user may be able to choose this combination. Additionally or alternatively, the combinations may be predefined for each heating temperature. Additionally, the very thin nature of the heaters can facilitate the heaters and the cooling regions being provided very close to one another and / or in some implementations the heater itself being cooled by the cooling system. As the heaters have very low thermal mass, they can cool down very quickly, and then can undergo active cooling, which is transmitted across to the hair. This could facilitate heating and cooling the hair with a same or adjacent portion of the styler. In the above embodiments that have a heat spreading layer, the individual heat spreaders were formed as islands that do not touch neighbouring heat spreaders, in order to minimise the ability of heat to transfer from one heating zone to a neighbouring heating zone. This helps signal to noise for sensing and the independent control of the different heating zones. In some embodiments, it may be desirable to provide an electrical connection between neighbouring heat spreaders - for example to ground the heat spreading layer. In this case the individual heat spreaders may have some conducting material connecting them with at least some of their neighbouring heatspreaders. Even though an electrical connection is provided between adjacent heat spreading elements, as long as the connection is relatively small (for example less than 1 / 1 Oth of the length / width of the heat spreader), there will still be, in effect, a thermal break or decoupling between neighbouring heat spreaders. In one possible implementation, each heat spreader may be electrically connected to the vias that couple to the common terminal of the heater electrodes. This will prevent the build-up of unwanted static in the heat spreading layer and may also obviate the need for a bus bar - as the connection to the electronics can then be made by connecting to the heat spreader(s) closest to the edge of the flexible heater. However, since there is minimal physical connection between the heat spreaders, they can still perform the desired function of spreading the heat within the respective heating zones whilst minimizing the spread of heat from one zone to an adjacent zone or zones. 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 5 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. 10 Various other modifications will be apparent to those skilled in the art and will not be described in further detail here. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims. 15 No doubt many other effective alternatives will occur to the skilled person. It will 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.

Claims

1. A hair drying and / or styling appliance comprising:a heater for providing heat for drying and / or styling hair;a case; anda cooling component;wherein the heater is a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes:a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; andat least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer;wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30 pm and 2 mm; andwherein the cooling component is configured actively to cool the case during styling.

2. The appliance of claim 1, wherein the case is arranged at least partially surrounding the heater.

3. The appliance of claim 1 or 2, wherein the case is arranged to support the heater.

4. The appliance of any preceding claim, wherein the heater is arranged on a first face of the appliance and the case is located on at least one further face of the appliance.

5. The appliance of any preceding claim, wherein the appliance comprises an arm, and the heater and the case are both provided on the arm; preferably wherein the heater is arranged on a first surface of the arm and the case is located on at least one further face of the appliance.

6. The appliance of any preceding claim, wherein the cooling component is configured to cool the case via heat exchange.

7. The appliance of any preceding claim, wherein the cooling component comprises a fluid within a conduit adjacent an inner surface of the case.

8. The appliance of any preceding claim, wherein the cooling component comprises a fan device for moving gas, preferably wherein the gas is air.

9. The appliance of claim 8, wherein the appliance comprises two arms joined by a shoulder, and wherein the fan is provided in the shoulder and configured for moving gas along one or both the arms.

10. The appliance of claim 8 or 9, wherein the fan device is configured for moving gas within an inner cavity of the case.

11. The appliance of claim 10, wherein the case comprises apertures arranged such that air can flow from the inner cavity of the case to outside of the case.

12. The appliance of claim 11, wherein the apertures are formed as slots, preferably wherein the slots are arranged parallel to the length of the appliance, more preferably parallel to the length of an arm of the appliance.

13. The appliance of any of claims 8 to 12, further comprising ribs protruding from a surface of the case, preferably protruding radially inwards from an inner surface of the case.

14. The appliance of claim 13, wherein the ribs are arranged such that they run along a direction parallel to the length of the appliance, preferably the length of an arm of the appliance.

15. The appliance of claim 7, wherein the fluid is a liquid, preferably a coolant liquid.

16. The appliance of any preceding claim, wherein the cooling component comprises a thermoelectric cooling element.

17. The appliance of any preceding claim, further comprising at least one support within the case to support the heater, preferably wherein the support is arranged to stabilize the heater relative to an or the inner cavity of the case.

18. The appliance of any preceding claim, wherein the heater comprises a plurality of independently controllable heating zones, and preferably wherein the cooling component comprises a plurality of independently controllable cooling zones.

19. The appliance of any preceding claim, wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness is less than 15 W / m.Kand greater than 0.1 W / m.K.

20. A method of operating a hair drying and / or styling appliance comprising a heater configured to heat hair for styling, the heater being arranged in a case, wherein the method comprises heating the heater while simultaneously cooling the case.

21. The method of claim 20, wherein the cooling the case comprises operating an active heat 5 exchange mechanism, preferably provided within an interior of the appliance.

22. The method of claim 21, wherein the active heat exchange mechanism comprises at least one of: a fan for facilitating flow of a gas, preferably air; a liquid cooling system; and a thermoelectric cooling system.

23. The method of any of claims 20 to 22, further comprising directly cooling the hair by facilitating 10 flow of a gas, preferably air, out of the case.

24. The method of any of claims 20 to 23, wherein the hair drying and / or styling appliance is the hair drying and / or styling appliance of any of claims 1 to 19.

25. A computer program product comprising computer implementable instructions for causing a programmable device to carry out the method of any of claims 20 to 24.15Application No: GB2411003.3 Examiner: Ben MilesClaims searched: 1-25Date of search: 8 January 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X Y X: 1-11, 13-15, 17, 19-25;Y: 12, 18 US 6354305 Bl (JANOUCH et al.) See whole document especially column 6 lines 26-32 and 60-62, column 7 lines 1-15 and 36-50 and the figures (especially Figure 1). X,Y X: 1-11, 15, 17, 19-25; Y: 12, 18 US 2014 / 0338691 Al (WEATHERLY et al.) See whole document especially paragraphs [0095], [0104-0106], [0114], [0115] and [0134] alongside Figures 12 and 13. X Y X: 1-6, 8, 10, 11, 16, 17, 19-25; Y: 12, 18 KR 101424122 Bl (JIN) See whole document especially paragraphs [0007], [0022], [0025-0027] and [0036-0038] alongside Figures 1 and 2. X,Y X: 1-6, 16, 17, 19-22, 24, 25; Y: 18 WO 2023 / 132391 Al (KIM et al.) See whole document especially the machine translated description and figures (especially Figure la). 12 CN 220275114 U (LI) See whole document especially the machine translated section titled 'Reference signs' and Figures 1, 5 and 6. Y 18 GB 2610606 A (SMITH et al.) See whole document especially page 3 lines 7-16 alongside the figures.Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB. EP, WO &US patent documents classified in the following areas of the UKCX :Worldwide search of patent documents classified in the following areas of the IPCA45D___________________________________________________The following online and other databases have been used in the preparation of this search reportSEARCH-PATENTInternational Classification:Subclass Subgroup Valid From A45D 0002 / 00 01 / 01 / 2006 A45D 0006 / 20 01 / 01 / 2006

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