Hair styling apparatus and method

Abstract

A hair drying and / or styling appliance 1 comprising a heater 6a, 6b with independently operable heating zones (642, fig.3b), a processor (29, fig.2), and an illumination display 8a, 8b adjacent the he

Description

Field of the Invention The present invention relates to a hair drying and / or styling appliance for and method of outputting in real time feedback relating to styler values and / or a status of a styler or styler component. In particular, the invention relates to providing information relating to individual zones of a heater using an illumination display located adjacent to that heater. 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 therefore more responsive, heat up faster, and have further capabilities. The inventors have realized that this means that users will be unaccustomed to the capabilities of such products and may misuse the product or at least not use the product to its full potential. The present invention seeks to provide feedback to a user as to the status of the device, including their usage of it. 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 of the invention, there is provided a hair drying and / or styling appliance comprising: a heater for providing heat for drying and / or styling hair, wherein the heater comprises independently operable heating zones; a processor; and an illumination display located adjacent to at least one side of the heater and comprising independently operable illumination portions; wherein the processor is configured to receive styler values and control illumination of the independently operable illumination portions in dependence on the styler values. The illumination display can advantageously provide real time and intuitive feedback to a user regarding the styler values. Preferably, individual portions of the illumination display are configured to portray information relating to at least one corresponding heating zone. In some preferable implementations, the processor is configured to determine thermal load of the heating zones in dependence on the styler values and the individual portions of the illumination display are configured to portray information relating to the thermal load of the at least one corresponding heating zone, preferably wherein thermal load comprises thermal load of hair applied to the heating zone. In preferable implementations, the processor is configured to determine temperature of the heating zones in dependence on the styler values and the individual portions of the illumination display are configured to portray information relating to the temperature of the at least one corresponding heating zone. In some implementations, the illumination display may comprise at least one illuminated strip comprising a series of the independently operable illumination portions arranged parallel and adjacent to the at least one side of the heater, preferably at least two sides of the heater, and / or preferably adjacent the length of the heater. The portions of the display may be located adjacent to heating zones and configured to display information relating to those same heating zones. This can assist in displaying clear and intuitive information regarding the heating zones. In some implementations, the styler values may relate to progression of a styler process, preferably wherein the styler process comprises: heating up; cooling down; loading firmware; and / or loading firmware updates. The illumination display may typically be configured to display information relating to more than one of the styler values and / or styler processes (as appropriate). Preferably, the control of illumination of the illumination display comprises control of colour and / or pattern and / or animation of the illumination. The illumination display may typically comprise a series of illumination sources (such as LEDs) which can be illuminated in different arrangements to form different colours and / or patterns. Different arrangements may be implemented in a sequence overtime such that an amination of the illumination can be output. The appliance may further comprise a light sensor, and the processor may be configured to control illumination of the illumination display in dependence on detection of a light level below a threshold value. The illumination of the illumination display may be asymmetric, for example different patterns may be displayed on different sides of the heater. This can assist with determination of the position of the styler. Preferably, the illumination display comprises a plurality of LEDs, preferably a plurality of LEDs of different colours. The LEDs may typically comprise a series of RGB LEDs and / or white LEDs. Preferably, the appliance further comprises a control display, more preferably wherein the control display is configured to allow a user to control settings of the styler. The control display may typically be a separate component to the illumination display. The control display may also comprise illumination elements, such as an illumination display. In some implementations, the appliance may further comprise an inertial measurement unit (IMU), wherein the processor may be further configured to receive styler values from the IMU and to control illumination of the illumination display in dependence on the orientation and / or speed of the styler. In some implementations, the processor may be configured to determine from the styler values from the IMU that the styler is idle and to control the illumination of the illumination display to portray that the styler is idle, preferably wherein the illumination comprises an alternative colour scheme. In some implementations, the appliance may be configured to operate in a styling mode and a training mode, wherein the heater is set to a higher temperature in the styling mode than in the training mode, and wherein illumination of the illumination display is controlled according to different colour schemes in the styling mode and the training mode. The appliance may further comprise a haptics unit for outputting haptic feedback and / or a speaker for outputting audio feedback. In preferable implementations, the heater may have a heat up rate greater than 30 °C per second. In some preferable implementations, the heater may be 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 30pm and 2mm; and wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 15 W / m.K and greater than 0.1 W / m.K. According to a further aspect of the invention, there is provided a method of operating a hair drying and / or styling appliance comprising a heater comprising independently operable heating zones and an illumination display located adjacent the heater and comprising independently operable illumination portions; wherein the method comprises: receiving styler values; processing the styler values to determine at least one styler status; and controlling illumination of the independently operable illumination portions to portray information relating to the styler status. This can provide a method of displaying information relating to the styler status in a real time and intuitive manner. The at least one styler status may comprise a series of statuses of the independently operable heating zones and the controlling the illumination may comprise controlling illumination of individual portions of the illuminated display to portray information relating to the status of at least one corresponding heating zone. In some preferable implementations, the series of statuses may comprise thermal loads of the heating zones and controlling the illumination may comprise controlling illumination of individual portions of the illuminated display to portray information relating to the thermal load of at least one corresponding heating zone, preferably wherein thermal load comprises thermal load of hair applied to the heating zone. In some preferable implementations, the series of statuses may comprise temperatures of the heating zones and controlling the illumination may comprise controlling illumination of individual portions of the illuminated display to portray information relating to the temperature of at least one corresponding heating zone. The status may comprise progress of a styler process, preferably wherein the styler process comprises: heating up; cooling down; loading firmware; and / or loading firmware updates. In some implementations, the controlling of the illumination may be performed according to a different colour scheme in dependence on a mode of the styler, preferably wherein the mode may comprise at least one of: styling mode, training mode, low light mode and idle mode. The method may be implemented in the hair drying and / or styling appliance as described above. According to a further aspect of the invention, there is provided a hair drying and / or styling appliance comprising: a heater for providing heat for drying and / or styling hair, wherein the heater comprises a plurality of independently controllable heating zones arranged along a length of the appliance; a processor; and an illumination strip located adjacent to at least one side of the heater and comprising a plurality of independently controllable illumination portions corresponding to the plurality of heating zones; wherein the processor is configured to control illumination of the independently controllable illumination portions and to control heating of the independently controllable heating zones. Preferably, each independently controllable illumination portion is positioned adjacent a corresponding one or more of the independently controllable heating zones. In some preferable implementations, each independently controllable illumination portion may be aligned with a corresponding one or more of the independently controllable heating zones. In some implementations, the processor may be configured, in a first mode, to control the plurality of independently controllable illumination portions in dependence upon a temperature ora loading of the corresponding independently controllable heating zones. According to a further aspect of the invention, there is provided a computer program product comprising computer implementable instructions for causing a programmable device to carry out the method as described. 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. The terms ‘heating zone’ and ‘heater zone’ may be used interchangeably. 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 illustrates a schematic side view of a hair styler comprising an illuminated display and illuminated strips; Figure 14 illustrates a schematic top view of an arm of a hair styler comprising a heater with individual heating zones and illuminated strips either side of the heater; Figure 15 illustrates a schematic top view of an alternative arrangement of an arm of a hair styler comprising a heater with individual heating zones and illuminated strips either side of the heater; Figure 16 illustrates a schematic top view of a yet further alternative arrangement of an arm of a hair styler comprising a heater with individual heating zones and illuminated strips either side of the heater; Figure 17 is a block diagram illustrating the main electronic components of the hair styling device shown in Figures 13 to 16; Figure 18a illustrates a hair styler in heating mode; Figure 18b illustrates a hair styler once heated; Figure 19 illustrates a tress of hair across heating zones of a heater and the corresponding illumination if the illuminated strips; Figure 20a illustrates a hair styler while firmware updates are loading; Figure 20b illustrates a hair styler once firmware updates have been installed; Figure 21a illustrates a hair styler while firmware updates are loading according to an alternative implementation; Figure 21b illustrates a hair styler once firmware updates have been installed according to the alternative implementation; Figure 22a illustrates a left side of the styler in a low light mode; Figure 22b illustrates a right side of the styler of Figure 22a in a low light mode; and Figure 23 shows a process flow illustrating how feedback is determined in dependence on styler values. Detailed Description of Preferred Embodiments 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 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 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 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 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, 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 a tubular form (as illustrated in Figure 6a) for example for use in a hair curler device or in a curved form (as illustrated in Figure 6b) for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. The temperature of each heating zone 642 is independently controllable. Each heating zone 642 can be set to a target temperature. The target temperature of each heating zone 642 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 642 which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone 642 can be determined by determining the resistance of the corresponding heater electrode 64. The microprocessor 28 controls the heating in order to reduce the difference between the actual temperature of the heating zone 642 and the target temperature for that heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length 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 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. 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 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 whilst 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 control switches of 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: LT m,<=K{LT where, ^Tmax = maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone AT’ioad = initial temperature difference that is created between the hair loaded and unloaded halves of a heating zone upon loading with hair AP = 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 LTioad, &.P, A, a, and t If it is assumed that the combined thermal diffusivity 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 area (A) and hence a 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 (&Tmax) 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 ATmax ( (K t) / Pdmax) Where, ATmra = 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 For a hair styling device, the inventors have found the following suitable ranges for these parameters: 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 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 t - 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. 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. 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 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. 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. 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 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 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 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. Feedback components Figure 13 illustrates a particular embodiment of the hair styler 1, which is configured to provide feedback to a user. In particular, users will be unaccustomed to using a styler 1 comprising multiple heating zones 642 or styling technology have such rapid heat-up and cool-down times. This can lead to user’s misusing the product, which may result in undesirable styling performance or longer styling times (possibly leading to damage to hair). It can therefore be advantageous to provide ‘feedback’ to a user regarding their use of the styler 1, in order to provide real-time updates on a status of the styler (e.g. temperatures) and potentially guide their use of the styler 1. This may be achieved by a combination of visual, audio and / or haptic feedback. Visual feedback components typically include the user interface 11. In the particular embodiment illustrated in Figure 13, an LED display 10 is provided on one of the arms 4a, 4b (this is illustrated on arm 4b and on a surface external to the heater 6b, for example opposite to the heater 6b, so that it can be easily viewed by the user). Although the LED display 10 may be monochromatic, it preferably comprises an array of RGB (red, green, blue) LEDs. The LEDs can be operated independently and in different configurations, and so can therefore provide displays of different patterns and - if RGB LEDs are used - different colours. This includes ‘animation’ displays, in which LEDs are illuminated in sequence overtime. Additionally, as also illustrated in Figures 13 and 14, LED strips 8a, 8b are provided adjacent each heater 6a, 6b respectively. Again, while the LED strips 8a, 8b may be monochromatic, preferably they comprise an array of RGB (red, green, blue) LEDs. The LED strips 8a, 8b can be operated independently and in different configurations, and so can therefore provide displays of different colours and patterns, including animated displays. As can be seen from Figures 14, 15 and 16, which show a plan view of the heater 6a on an arm 4a of the hair styler, the LED strips 8a, 8b are divided into independently operable sections 8a-1 to 8a-20. The sections 8a-1 to 8a-20 correspond in location to the independently operable heating zones 642. For example, in Figure 14, LED section 8a-1 corresponds to and is physically aligned with heating zone 642-1, LED section 8a-2 corresponds to and is physically aligned with heating zone 642-2, and so on. Each LED section 8a-1 to 8a-20 is also provided adjacent its corresponding heating zone 642-1 to 642-20. As the LEDs within the LED strips 8a, 8b can be illuminated independently, the sections 8a-1 to 8a-20 can be illuminated independently in order to provide information relating to the operation of specific zones 642. In addition, the sections of the LED strip 8a-1 to 8a-20 may relay information relating to general status update of the styler, unrelated to the heating zones, for example the progress of software updates. Figure 14 illustrates a first implementation of the LED strips 8a, divided into a first strip comprising sections 8a-1 to 8a-10, which runs along one side of the heater, and a second strip comprising sections 8a-11 to 8a-20, which runs along the other side of the heater. In this example, the heater is comprised of twenty heating zones, 642-1 to 642-20 which are arranged in two rows of ten, such that the twenty sections of the LED strips 8a-1 to 8a-20 are located adjacent to their corresponding heating zone 642-1 to 642-20. Figure 15 shows a slightly altered implementation in which two further LED sections 8a-21 and 8a-22 are located at the distal end of the heater 8a. These may, for example, correspond to their adjacent heating zones - for example, heating zone 642-1 may be represented by both LED sections 8a-1 and 8a-21 and heating zone 642-11 may be represented by LED sections 8a-11 and 8a-22. Alternatively, the end sections 8a-21 and 8a-22 may be used to display information about the heater 6a generally, the strips of heating zones 642 and / or the styler 1 generally. Figure 16 shows a yet further implementation in which the heater 6a comprises only one row of heating zones 642-1 to 642-10 along the length of the heater 6a. In this instance, the LED strips 8a again run along either side of the heater 6a and the LED sections either side of each heating zone 642-1 to 642-10 will both represent that zone. As illustrated in Figure 16, this means that heating zone 642-1 is represented by LED sections 8a-1 and 8a-11, heating zone 642-2 is represented by LED sections 8a-2 and 8a-12, and so on. Figure 16 also illustrates an implementation in which the heating zones 642-1 to 642-10 are not all the same size and shape, in particular the width (in the length wise direction of the heater 6a) of the heating zones 642-1 to 642-10 varies. The width of the LED sections 8a-1 to 8a-20 corresponds to the width of their respective heating zone 642-1 to 642-10 and so also varies. In alternative implementations (not illustrated), sections of the LED strips 8 may represent more than one heating zone 642. For example, sections of LED strips 8 may represent two or more heating zones 642 sequentially arranged along the length of the heater 6a. Figure 17 is a block diagram of the main components of this embodiment of the hair styling device 1 (corresponding to that shown in Figure 2), illustrating that the LED strips 8 and the LED display 10 form part of the user interface 11 which is coupled to the microprocessor 29. The microprocessor 29 is configured to implement 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 microprocessor 29 is further configured to implement one or more algorithms (stored in the memory 30) for illuminating the LED strips 8 and the LED display 10 in dependence on desired operating temperatures of the heaters 6 and / or sensed temperatures of the heaters 6 obtained from the temperature measurement circuitry 25. In particular, the desired operating temperatures and sensed temperatures of the heaters 6 includes desired and sensed temperatures for each heating zone 642 of each heater 6, and the illumination of the LED strips 8 and / or the LED display 10 can also be controlled according to the sensed and / or desired temperature for the corresponding zone 642. Additionally, the microprocessor 29 may be further configured to implement one or more algorithms (stored in the memory 30) for illuminating the LED strips 8 and the LED display 10 in dependence on status updates sent via the communications circuitry 27. Particular processes and / or algorithms may be sent to the memory 30 and microprocessor 29 from an external source (such as an application on a mobile device) via the communications circuitry. This may be in dependence on selections made via the user interface 11. In some preferable implementations, the hairstyling device further comprises an optional Inertial Measurement Unit (IMU) 12. The IMU 12 typically comprises an accelerometer, a gyroscope, and in some instances also a magnetometer. This is configured to determine measurements indicative of movements (e.g. speed and orientation) of the styler 1, and so is configured to determine information relating to the manner in which the user is moving - and therefore using -the styler 1. In some implementations, the styler 1 may comprise an optional light sensor 18, such as an ambient light sensor - the information from which may be used to control the brightness of the light strips 8 (for example reducing their brightness as the ambient light intensity reduces). Additionally, the hair styling device may optionally further comprise a haptics unit 14 and a speaker 16, both of which are coupled to the microprocessor 29. The haptics unit 14 typically comprises any kind of devices capable of exerting forces on a user to create tactile sensations, thereby providing haptic feedback. The speaker 16 is typically configured to output audio sounds to a user, thereby providing audio feedback. The microprocessor 29 is configured to implement processes (typically stored in the memory 30) for outputting visual feedback (via the LED strips 8a, 8b and / or the LED display 10), haptic feedback (via the haptics unit 14) and / or audio feedback (via the speaker 16) in dependence on movement data from the IMU 12 and / or temperature data from the temperature measurement circuitry 25. Exemplary feedback processes The components described above can provide visual feedback to a user, providing intuitive and fast feedback. This can minimize the impact of user error by removing ambiguity of plate temperature and modifying user behaviour. This can be of particular benefit as the user will typically be unaccustomed to the capabilities of the kind of multi-zone very low thermal mass heaters 6a, 6b described in this application. Particular examples of processes by which the styler 1 might provide feedback will now be described in turn. Heating up As illustrated in Figures 18a and 18b, the LED strips 8a, 8b change colour as the heaters 6a, 6b heat up. This may, by way of example, comprise the LED strips 8a, 8b moving through different colours from blue to red to indicate the progression of the heating. In some implementations, this may pass from blue to green to yellow to orange to red as the temperature rises to indicate to the user how the heating is progressing. By way of example, Figure 18a illustrates the styler at a point in time during the heating process when the LED strips 8a, 8b display a first colour and Figure 18b illustrates a point when the heaters 6a, 6b have reached the desired temperature and the LED strips 8a, 8b display a second colour. When the heaters 6a, 6b reach the desired temperature the LED strips 8a, 8b typically flash (turning repeatedly on and off in quick succession). This can alert the user that the heaters 6a, 6b are at the desired temperature and so are ready to be used. The LED strips 8a, 8b can also indicate when the heaters 6a, 6b are cooling down, utilising the heat-up colour scheme in reverse. In some implementations this is implemented with a low-frequency pulsing to highlight the styler 1 is powered down. In some implementations, regions of the LED strips 8a, 8b light up corresponding to particular heating zones 642 on the heaters 6a, 6b. This can be useful to provide multizonal heater visualisation to the user, so that they can see the relative temperatures of the different heating zones 642. Styling When the user first loads hair onto the heaters 6a, 6b, it can be determined how much hair is loaded into the styler. These features can be determined at least from power measurements to the heaters 6 and the drive circuitry 23 in combination with or just based on temperature measurements from the temperature measurement circuitry 25, which can indicate the thermal load of the hair applied to the heaters 6. In particular, the thermal load on each of the heating zones 642 can be determined individually and this thermal load indicates an amount of hair that has been loaded onto each heating zone 642. As shown in Figure 19, the load on each of the individual heating zones 642 can be displayed via illumination of the corresponding sections of the LED strips 8a. A lock of hair 40 is positioned across the heater 6a, falling across seven of the heating zones (642-4, 642-5, 642-6, 642-13, 642-14, 642-15, 642-16). The seven LED sections (8a-4, 8a-5, 8a-6, 8a-13, 8a-14, 8a-15, Sa-16) corresponding to the loaded seven heating zones (642-4, 642-5, 642-6, 642-13, 642-14, 642-15, 642-16) are illuminated in a manner to indicate that they are loaded, in particular illuminated to a brightness and / or colour different to the rest of the LED strips 8a. Additionally, as illustrated in the exemplary implementation of Figure 19, they may be illuminated at different brightness and / or colour to one another, dependent on the load (amount of hair) placed on each one. Based on this illumination, the user may be prompted to distribute the hair more evenly over the surface of the heater 6a which can lead to better and more consistent styling results. As a particular example, in addition to determining that hair has been placed on the heating zones, it can also be determined how wet the hair is, and what type of hair is loaded onto the plates. The optimum quantity of hair to be placed onto the heaters 6’ will be dependent on the hair type and how wet the hair is, and so the styler can provide feedback to a user how to improve their technique based on the determined optimum quantity (which can prevent overloading of the hair onto the styler 1). In one example, if it is determined that too much hair is loaded, the LED strips 8a, 8b can indicate this by changing colour (perhaps yet further) or by changing animation (for example flashing). In addition to the above, positive feedback can also be implemented to encourage users when they are styling by reinforcing beneficial habits. For example, if a user moves the styler 1 over the hair at a correct speed, positive feedback can be output via any combination of the haptics unit 14, speaker 16, LED strips 8a, 8b and LED display 10. In some embodiments, the styler 1 may be configured to output visual feedback, via the LED strips 8a, 8b and LED display 10, relating to the speed and / or orientation of the styler as determined from the IMU 12. In an example implementation, once the hair is loaded onto the styler 1, and the user begins to style the hair, the IMU 12 determines the speed with which the user is moving the styler 1 and the direction in which the user is rotating the styler 1. If the user moves the styler 1 too fast over the hair, this can cause the hair to not be effectively styled; this may then cause the user to run the heaters 6 over the same section of hair again, potentially causing damage to the hair. It is therefore typically preferable for a user to move the styler 1 at the correct speed to style the hair effectively on the first time. As such, if the user is moving too fast, the LED strips 8a, 8b and / or LED display 10 may output an indication to slow down by changing illumination colour and / or animation to encourage the user to slow down. The division of the LED strips 8a, 8b can in particular facilitate more complex animations, which can provide intuitive and adaptive feedback to the user. The styler 1 can further be configured to provide feedback on the user’s movements relative to a particular ‘styling protocol’ for a styling technique, where the styling protocol refers to the particular movements (including speed and rotation) and heating required to achieve a particular hair style. For example, there are different styling protocols for straightening hair, styling into curls and for styling into loose waves. The microprocessor 29 can configure the feedback according to the particular styling protocol chosen by a user. In particular, the movements of the styler are compared to the optimum movements for a particular protocol and feedback output via the LED display and / or LED strips 8a, 8b to direct the user to make corrections to their technique. The feedback may comprise particular illumination animations and / or colours of the sections of the LED strips 8a, 8b to direct a user to, for example, change the orientation. The exemplary feedback scenarios described may be provided in any combination - for example, to provide feedback regarding the loading of the heating zones 642 and the speed with which those heating zones 642 are passed over the hair 40. Training / teaching mode The styler 1 can further be operated in a ‘training mode’, in which the heaters 6a, 6b are not at the styling temperature but the user is guided how to move the styler 1 in the correct motions to achieve their desired style, according to the relevant styling protocol. In this mode the user can effectively ‘practice’ the motions without heating the hair, thereby enabling the user to repeat the actions and to avoid accidentally causing damage to the hair (or burning themselves). In this training mode, the LED strips 8a, 8b are illuminated according to a colour scheme different to that used to indicate that the heaters 6a, 6b are heated. This can therefore communicate to a user that the heaters 6a, 6b are not at the styling temperature. In some embodiments, the styler 1 can be connected to an application (‘app’) via the communications circuitry 27 (for example, via Bluetooth). The user can select a particular style via an app connected to the styler 1 or via the LED display 10 and then be taught via visual feedback how to achieve that style. For example, a user may select curling hair using a hair straightener, which requires rotating the styler 1 at a correct angle and speed. The feedback provided via the illumination of different sections of the LED strips 8a, 8b can indicate to the user how to correct their speed and rotation angle. Indicating firmware updates In some implementations, the microprocessor 29 instructs the LED strips 8a, 8b to indicate firmware updates. In this mode, the LED strips 8a, 8b are illuminated in the style of a loading bar, as is illustrated in Figures 20a and 20b. In Figure 20a, one LED strip 8a is illuminated such that a portion corresponding to the download progress is illuminated in a solid colour, while the remainder of the LED strip 8a is illuminated in a colour gradient. The other LED strip 8b is illuminated in a solid colour (alternatively, it may be completely unilluminated). Figure 20b shows the styler 1 once the firmware update has been completed, in which the first LED strip 8a is now illuminated in a gradient. At this stage, the LED strip 8a is also flashing (turning on and off in quick succession). In an alternative implementation as illustrated in Figures 21a and 21b, one LED strip 8a is illuminated such that a portion corresponding to the download progress is illuminated in a solid colour, while the remainder of the LED strip 8a is unilluminated (alternatively, it may be the other way around or the two sections may be illuminated in different colours). This manner of showing progress of the download is illustrated in Figure 21a. Then, when the download is complete, the whole of the LED strip 8a is illuminated the same colour, typically flashing that colour - as is illustrated in Figure 21b. It is typically preferable that the colour scheme in which the LED strips 8a, 8b are illuminated to indicate the progress of a firmware update is not the same as a colour scheme used to portray that the plates are hot. Low light mode In some implementations, a computer vision algorithm (CVA) may be used to determine the movement and orientation of the styler in order to gather information regarding its use (such a use is detailed in the Applicant’s previous application WO2022 / 136866). If the conditions are too dark (i.e. there is a low light level) then computer vision techniques may not work effectively. In such an instance, the light sensor 18 on the styler 1 can detect that the light level is too low, and the microprocessor 29 instructs that the LED strips 8a, 8b be illuminated in a unique pattern, which can allow the styler 1 and its location to be identified more easily using the computer vision algorithm. In particular, the pattern is different on the two sides of the styler 1 so that the computer vision algorithm can determine what side of the styler is being viewed and therefore the orientation of the device relative to the camera frame of reference. This is shown in the example illustrated in Figures 22a and 22b, which show the styler 1 from the left and from the right, respectively. The sections of the LED strips 8a, 8b to the left are illuminated according to a first pattern while the sections of the LED strips 8a, 8b to the right are illuminated according to a second pattern different to the first. Smart power usage The styler 1 can be configured to respond to user behaviour based on accelerometer readings of the IMU 12. The styler 1 can respond to that behaviour by adapting the settings of the styler 1. For example, if the styler is determined to be idle (i.e. not in the hand of a user) as it is detected not to have been moved for more than a specified period of time (for example 30 seconds), the styler 1 is configured to enter a low power mode. This low power mode can be indicated via the LED strips 8a, 8b, which may change to a different colour scheme (for example, a more muted colour scheme) and / or indicated on the LED display 10. User feedback algorithm Figure 23 provides illustrates an exemplary process flow diagram of the algorithm for determining various states of the styler 1, so that corresponding feedback can then be output to a user. This algorithm is typically stored in the memory 30 and implemented by the microprocessor 29. At 100, the process starts - this may be initiated, for example, by a user turning on the styler 1. This triggers at step 102 the booting up of the components of the styler 1, for example the heaters 6 via their drive circuitry 23, along with other components such as the IMU 14, the communications circuitry 27, the temperature measurement circuitry and so on. At step 104 the microprocessor 29 determines if the styler 1 is connected to an external application or ‘app’ (for example of a mobile device) via the communications circuitry 27 (this connection may be made via Bluetooth, for example). If it is determined that there is a connection to an app, then the microprocessor 29 will load the app features to the boot up procedure. This may comprise, for example, particular settings of the styler 1, which may be tailored for a particular styling technique or for the hair characteristics of the user. Once the app features have been loaded in step 106, the process moves to step 108, in which the styler values are determined. Alternatively, if no app connection is determined, the process moves straight to step 108. Reading the styler values in step 108 comprises receiving data from the IMU 14, which provides information regarding the movement and physical arrangement of the styler 1. Receiving the styler values further comprises receiving data relating to the heaters 6, which may be received from the temperature measurement circuitry 25. Typically, this may also comprise information from the drive circuitry 23 and heaters 6 themselves, as the combination of the power data and the temperature data can be used to determine information relating to the thermal load on the heaters 6 (from which can be derived information relating to hair condition and wetness). Relevant IMU data and heater data is collated at 110. At step 112, the received data is used to determine whether the styler is idle. This can be determined from the IMU data indicating that the styler 1 is not being moved by the user, but instead remains stationary. In some cases, the heater plate data can be used instead of or in combination with the IMU data, as the heater plate data will indicate of there being no thermal load on the heaters 6 when idle. If it determined that the styler is idle 132, the process enters the idle mode 130. An idle mode timer keeps track of the length of time in which the styler 1 remains in the idle mode - if it is determined to exceed a pre-set length of time, then the process moves to end at step 120, switching the styler 1 off. In the illustrated process flow of Figure 2, this time period is set as 10 minutes. This causes the styler 1 to turn off automatically when left idle. This provides a safety back stop, for example, if a user forgets to turn off the styler 1. The processing also loops back to step 108 to continue checking the styler values for any indication that the user is now using the device. If the styler values indicate at any point before the end of the pre-set time period that the styler 1 is no longer idle, the process move to step 114 in which the idle mode timer is reset. At step 114, the microprocessor 29 further instructs the drive circuitry 23 to heat the heaters 6 to the target temperature, Ttarget- The microprocessor 29 further implements instructions to illuminate the LED strips 8 and / or the LED display 10 to show the temperature of the heaters, Tpiate. The IMU data is then used to determine whether the styler 1 is closed, and if it is, the process moves to the styling mode 140. During styling mode 140, the microprocessor 29 continues to read the styler values by continuously cycling back through steps 108, 110, 112, and 114. This provides information on the heating and the movement of the styler 1. At step 142, it is determined whether the user is moving the styler 1 too fast. This can be determined from the IMU data alone, or the IMU data in combination with data relating to the thermal load on the heaters, which can indicate whether the hair is being heated sufficiently to cause styling on the first run. If the user is moving the styler 1 too fast, this can mean that styling is not achieved effectively, which can then result in a sub-optimal end result. It can also lead to the user repeatedly using the styler 1 on the same section of hair, which can cause unnecessary damage. As such, if it is determined in step 142 that the user is moving too fast, then at step 144, feedback is output to the user accordingly. Step 144 typically further comprises adjusting (e.g. lowering) the heating of the heaters 6 via the drive circuitry 23. In some instances, positive feedback may be output to the user to indicate that the styler is being performed correctly. Once the user has stopped styling the hair, they will typically initially still be holding the styler 1 but it will no longer be closed. Once this occurs, the microprocessor will at step 116 determine that the styler 1 is not closed and accordingly proceed to step 118 (instead of returning to steps 108 and 142). At step 118, it is determined whether the off button has been pushed. If it has, the styler proceeds to step 120 and the styler 1 is turned off. If the off button has not been pushed, the process then returns to step 108 and the styler values continue to be monitored to determine how the user is using the styler 1. 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. It should be understood that different combinations of feedback components may be provided and that different specific feedback protocols may be implemented. For example, different illumination colour schemes may be implemented. Additionally, the illuminated strips may have a different configuration for different drying and / or styling appliances, typically according to the particular configuration of the heaters and heating zones. Furthermore, further methods of implementing feedback may be implemented in any combination, in particular a combination of visual, haptic and / or audio feedback. 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. 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. 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, wherein the heater comprises independently operable heating zones;a processor; andan illumination display located adjacent to at least one side of the heater and comprising independently operable illumination portions;wherein the processor is configured to receive styler values and control illumination of the independently operable illumination portions in dependence on the styler values.

2. The appliance of claim 1, wherein individual portions of the illumination display are configured to portray information relating to at least one corresponding heating zone.

3. The appliance of claim 2, wherein the processor is configured to determine thermal load of the heating zones in dependence on the styler values and the individual portions of the illumination display are configured to portray information relating to the thermal load of the at least one corresponding heating zone, preferably wherein thermal load comprises thermal load of hair applied to the heating zone.

4. The appliance of claim 2 or 3, wherein the processor is configured to determine temperature of the heating zones in dependence on the styler values and the individual portions of the illumination display are configured to portray information relating to the temperature of the at least one corresponding heating zone.

5. The appliance of any preceding claim, wherein the illumination display comprises at least one illumination strip comprising a series of the independently operable illumination portions arranged parallel and adjacent to the at least one side of the heater, preferably at least two sides of the heater, and / or preferably adjacent the length of the heater.

6. The appliance of any preceding claim, wherein the portions of the display are located adjacent to heating zones and configured to display information relating to those same heating zones.

7. The appliance of any preceding claim, wherein the styler values relate to progression of a styler process, preferably wherein the styler process comprises: heating up; cooling down; loading firmware; or loading firmware updates.

8. The appliance of any preceding claim, wherein the control of illumination of the illumination display comprises control of colour and / or pattern and / or animation of the illumination.

9. The appliance of any preceding claim, further comprising a light sensor, wherein the processor is configured to control illumination of the illumination display in dependence on detection of a light level below a threshold value.

10. The appliance of any preceding claim, wherein the illumination display comprises a plurality of LEDs, preferably a plurality of LEDs of different colours.

11. The appliance of any preceding claim, wherein the appliance further comprises a control display, preferably wherein the control display is configured to allow a user to control settings of the styler.

12. The appliance of any preceding claim, further comprising an inertial measurement unit (IMU), wherein the processor is further configured to receive styler values from the IMU and to control illumination of the illumination display in dependence on the orientation and / or speed of the styler.

13. The appliance of claim 12, wherein the processor is configured to determine from the styler values from the IMU that the styler is idle and to control the illumination of the illumination display to portray that the styler is idle, preferably wherein the illumination comprises an alternative colour scheme.

14. The appliance of any preceding claim, wherein the appliance is configured to operate in a styling mode and a training mode, wherein the heater is set to a higher temperature in the styling mode than in the training mode, and wherein illumination of the illumination display is controlled according to different colour schemes in the styling mode and the training mode.

15. The appliance of any preceding claim, further comprising a haptics unit for outputting haptic feedback and / or a speaker for outputting audio feedback.

16. The appliance of any preceding claim, wherein the heater has a heat up rate greater than 30 °C per second.

17. The appliance of any preceding claim, 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 haircontacting 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 30pm and 2mm; andwherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 15 W / m.K and greater than 0.1 W / m.K.

18. A method of operating a hair drying and / or styling appliance comprising a heater comprising independently operable heating zones and an illumination display located adjacent the heater and comprising independently operable illumination portions; wherein the method comprises: receiving styler values;processing the styler values to determine at least one styler status; and controlling illumination of the independently operable illumination portions to portray information relating to the styler status.

19. The method of claim 18, wherein the at least one styler status comprises a series of statuses of the independently operable heating zones and the controlling the illumination comprises controlling illumination of individual portions of the illuminated display to portray information relating to the status of at least one corresponding heating zone.

20. The method of claim 19, wherein the series of statuses comprises thermal loads of the heating zones and controlling the illumination comprises controlling illumination of individual portions of the illuminated display to portray information relating to the thermal load of at least one corresponding heating zone, preferably wherein thermal load comprises thermal load of hair applied to the heating zone.

21. The method of claim 19 or 20, wherein the series of statuses comprises temperatures of the heating zones and controlling the illumination comprises controlling illumination of individual portions of the illuminated display to portray information relating to the temperature of at least one corresponding heating zone.

22. The method of any of claims 19 to 21, wherein the status comprises progress of a styler process, preferably wherein the styler process comprises: heating up; cooling down; loading firmware; or loading firmware updates.

23. The method of any of claims 19 to 22, wherein the controlling of the illumination is performed according to a different colour scheme in dependence on a mode of the styler, preferably wherein the mode may comprise at least one of: styling mode, training mode, low light mode and idle mode.

24. The method of any of claims 18 to 23, wherein the hair drying and / or styling appliance is the hair drying and / or styling appliance of any of claims 1 to 17.

25. A hair drying and / or styling appliance comprising:a heater for providing heat for drying and / or styling hair, wherein the heater comprises a plurality of independently controllable heating zones arranged along a length of the appliance;a processor; andan illumination strip located adjacent to at least one side of the heater and comprising a plurality of independently controllable illumination portions corresponding to the plurality of heating zones;wherein the processor is configured to control illumination of the independently controllable illumination portions and to control heating of the independently controllable heating zones.

26. The hair drying and / or styling appliance of claim 25, wherein each independently controllable illumination portion is positioned adjacent a corresponding one or more of the independently controllable heating zones.

27. The hair drying and / or styling appliance of claim 25 or 26, wherein each independently controllable illumination portion is aligned with a corresponding one or more of the independently controllable heating zones.

28. The hair drying and / or styling appliance of any of claims 25 to 27, wherein the processor is configured, in a first mode, to control the plurality of independently controllable illumination portions in dependence upon a temperature or a loading of the corresponding independently controllable heating zones.

29. A computer program product comprising computer implementable instructions for causing a programmable device to carry out the method of any of claims 18 to 24.

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