Apparatus and system

Abstract

A hair drying and / or styling apparatus comprises a heater 6 for heating a tress of hair and drive circuitry 23 for providing power to heat the heater. Sensor circuitry 24, 25 provides sensor data indi

Description

Field of the Invention The present invention relates to a hair drying and / or styling apparatus and system for determining hair health metrics from data gathered during a styling session, and performing control actions in dependence on the hair health metrics. In particular, the instructions may comprise altering a heating profile and / or outputting an alert and / or feedback to a user regarding how to improve their technique. 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. Additionally, it is important to users to ensure that styling causes minimum damage to their hair. The sensitivity of the heaters of the hair styling appliance can facilitate data being gathered during a styling session, which can then be analyzed. The present invention seeks to provide ways in which this analysis can improve hair health and styling technique. Summary of the Invention The present invention is set out in the appended independent claims. Optional features are set out in the appended dependent claims. In the following, any examples and embodiments not falling within the scope of the claims do not form part of the invention and are provided for illustrative purposes only. According to a first aspect, there is provided a hair drying and / or styling apparatus comprising: a heater for heating a tress of hair; drive circuitry for providing power to heat the heater; sensor circuitry for providing sensor data indicative of at least one selected from the group consisting of: power delivered to the heater; the temperature of the tress of hair being dried and / or styled; and the temperature of the heater; and a controller for controlling the drive circuitry to deliver power to the heater in dependence upon sensor data provided by said sensor circuitry; means for determining at least one hair health metric in dependence upon sensor data provided by the sensor circuitry; and means for performing a control action in dependence on the determined at least one hair health metric. This advantageously provides an apparatus adaptable to the hair to be styled or being styled, in particular the ‘health’ of the hair. This can help to minimize any further potential damage to the hair and / or effectiveness of the styling. In some preferable implementations, the control action may comprise controlling power and / or temperature settings of the heater, preferably controlling the setpoint temperature of the heater. This can adapt to the anticipated heating profile of the hair and can help to prevent overheating and damage. In some preferable implementations, the control action may comprise outputting an alert, preferably wherein the alert comprises at least one selected from a group consisting of: a visual output; an audio output; a haptic output; verbal instructions; and a combination thereof. This can alert a user to adapt their technique, typically in order to help to improve styling and / or to reduce damage. In some implementations, the apparatus may further comprise means for: i) determining a heat transfer coefficient between the heater and the tress of hair in dependence upon sensor data provided by the sensor circuitry; ii) determining at least one hair health metric from the determined heat transfer coefficient; and iii) performing a control action in dependence on the determined at least one hair health metric. Heat transfer coefficient can indicate hair health and can indicate how hair will respond to heating; the control of the stylercan therefore be controlled in a manner tailored to the hair. Preferably, the means for determining at least one hair health metric from the determined heat transfer coefficient is configured to retrieve prestored calibration data relating heat transfer coefficient to hair health metric, and to use the retrieved calibration data to determine the at least one hair health metric from the determined heat transfer coefficient. In some implementations, the apparatus may further comprise means for: i) determining a heat capacity of the tress of hair in dependence upon sensor data provided by the sensor circuitry; ii) determining at least one hair health metric from the determined heat capacity; and iii) performing a control action in dependence on the determined at least one hair health metric. Heat capacity can indicate hair health and can indicate how hair will respond to heating; the control of the styler can therefore be controlled in a manner tailored to the hair. Preferably, the means for determining at least one hair health metric from the determined heat capacity is configured to retrieve prestored calibration data relating heat capacity to hair health metric, and to use the retrieved calibration data to determine the at least one hair health metric from the determined heat capacity. In some implementations, the controller may be configured initially to control the drive circuitry to deliver power to the heater in dependence on at least one of: data input by a user; and a predetermined initial value. The apparatus may comprise means for estimating a hair health metric in dependence on the data input by a user. This can be used to determine initial control commands, such as power and / or temperature. The apparatus may comprise means for estimating a heating response of the tress of hair in dependence on the estimated hair health metric, and means for comparing the estimated heating response to a detected heating response detected from sensor data, and preferably wherein the controller is configured to control the drive circuitry to deliver power to the heater in dependence on the comparison of estimated heating response and detected heating response. This can allow the styler to adjust the power according to the detected heating response of the hair. In some implementations, the apparatus may comprise means for comparing the estimated hair health metric and the determined hair health metric, and wherein the performing a control action comprises controlling the drive circuitry to deliver power to the heater in dependence on a difference between the estimated hair health metric and the determined hair health metric. In some preferable implementations, the heater may comprise a plurality of heating zones and the sensor circuitry may be configured to provide sensor data for the plurality of heating zones; and the apparatus may further comprise means for determining which heating zones are in contact with the tress of hair in dependence upon the sensor data. In some implementations, the apparatus may further comprise means for determining at least one score based on the at least one hair health metric, wherein the at least one score relates to use of the appliance and / or hair health, and preferably further comprises means for determining an overall score using a weighted combination of scores. Such a score can provide an overall indication of hair health, based on more than one metric. Preferably, the apparatus further comprises a user interface configured for outputting an alert, and preferably configured for facilitating input of information. In some implementations, the apparatus may comprise means for storing to a user profile the at least one hair health metric and / or the at least one score for each styling session of a user, preferably wherein the means for performing a control action in dependence on the determined at least one hair health metric is further configured to perform a control action in dependence on the at least one hair health metric over time. This can facilitate more information being gathered over time, which can provide a more holistic and tailored review of a user’s hair and / or styling technique. In preferable implementations, the apparatus comprises means for processing the sensor data to identify heating of a tress of hair in dependence on power delivered to the heater. This can assist with determining heating / styling patterns. The means for processing the sensor data may be further configured to determine maximum hair temperature for each tress of hair by determining a difference between a maximum and minimum power delivered to the heater, and preferably further using a mean value of power delivered to the heater. Determination of maximum hair temperature can be used to determine / define the effect of heating on hair (e.g. damage). In some implementations, the apparatus may comprise means for determining a maximum hair temperature for each tress of hair from sensor data indicative of the temperature of the tress of hair being dried and / or styled. The apparatus may further comprise means for determining an average maximum hair temperature in dependence on a mean value of determined maximum hair temperatures for each tress. In some preferable implementations, the apparatus may comprise means for defining new hair damage as a function of maximum hair temperature, preferably average maximum hair temperature. In some implementations, the controller may be configured to determine new damage with reference to the hair metric determined from the heat transfer coefficient. In some implementations, the apparatus may comprise means for processing the sensor data to identify repeated heating of a same tress of hair in dependence on the power delivered to the heater having a damped sinusoidal profile. The apparatus may comprise means for determining number and frequency of repeated heating of a same tress of hair by identifying and processing peaks of the damped sinusoidal power profile. In some implementations, the apparatus may comprise means for determining heat on hair time by summing intervals over which the damped sinusoidal power profile has a negative gradient. In some preferable implementations, the apparatus may comprise sensor circuitry for providing sensor data indicative of motion of the apparatus. This may, for example, comprise: an inertial measurement unit (IMU), a speedometer, a distance meter and / or a proximity sensor. The apparatus may comprise at least two arms moveable relative to one another, wherein at least one of the arms comprises the heater, and may further comprise means for determining configuration of the arms in dependence on the sensor data indicative of motion. The possible configurations of the arms may comprise at least an open configuration and a closed configuration (for example, the arms are in a closed configuration when they are clamped around a tress of hair - i.e. hair is loaded on the heaters). In some implementations, the apparatus may comprise means for determining hair on heat time by summing time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater, preferably in a closed configuration. In some implementations, the apparatus may comprise means for correlating the time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater and the intervals over which the damped sinusoidal power profile has a negative gradient. This can be used to corroborate that the hair was being heated while the styler was closed. In some implementations, the apparatus may comprise means for determining recurrent heating of a tress of hair in dependence on determining, from the sensor data indicative of motion, motion above a threshold speed value coincident with time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater, preferably in a closed configuration. The apparatus may comprise means for comparing heat on hair time with one or more user metrics, preferably wherein the one or more user metrics comprises at least one of: hair length, hair health metric, hair damage metric. 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 such 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. The apparatus may comprise means for scheduling styling sessions to a calendar, preferably configured to output reminder alerts in dependence on the calendar. The apparatus may preferably comprise a database for storing at least one hair health metric to a user profile. The database may further comprise a database of a plurality of user profiles, wherein the apparatus comprises means for performing a clustering algorithm to cluster user profiles in dependence on at least one metric, preferably in dependence on at least one metric from: age, hair length, hair type, average styling time, favourite styles, hair colour, phone model, favourite products, location, styling frequency, and usage behaviour. In some implementations, the apparatus may comprise means for performing control actions in dependence on analysis of user profiles in a same cluster, preferably wherein the performing control actions comprises outputting suggestions. This can provide feedback, advice and / or recommendations to a user, typically based on the analysis and preferably tailored to the user. In some implementations, the apparatus may comprise at least one hair drying and / or styling device and a smart processing device in communication, preferably further comprising a cloudbased processing unit in communication with the at least one hair drying and / or styling device and / or the smart processing device. This can improve the user experience by providing interconnected components. For example, the smart processing device may run an application, preferably wherein the application can provide suggestions and / or alerts. This is preferably based on data from the hair drying and / or styling device. In some implementations, data may be processed by the smart processing device and / or the cloud-based processing unit. The hair drying and / or styling device may be configured to send instructions to the smart processing device, preferably wherein the instructions comprise instructions to output an alert and / or preferably wherein the instructions comprise instructions to open and / or run an application on the smart processing device. In some implementations, the hair drying and / or styling device may be configured to turn off in dependence on the smart processing device moving out of a defined range of the hair drying and / or styling device. Typically, the range may refer to a distance range. According to a further aspect, there is provided a computer program product comprising computer implementable instructions for causing a programmable device to configure one or more processors to implement the features 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. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1a shows an overview of an exemplary hair styling device; Figure 1b shows a hair styling device in use; Figure 2 is a block diagram illustrating the main electronic components of the hair styling device shown in Figure 1; Figure 3a is an exploded view of a heater forming part of the hair styling device shown in Figure i; 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) or via an In Mould Labelling process; 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 is a block diagram illustrating the main electronic components of a further embodiment of the hairstyling device; Figure 14 is an illustrative plot of hair damage against hair heat transfer coefficient; Figure 15 is a further illustrative plot of hair damage against hair heat transfer coefficient; Figure 16 is an illustrative plot of hair temperature over time; Figure 17 is an exemplary plot of a power demand profile over time; Figure 18 is an exemplary plot of a motion metric over time; Figure 19a is a further exemplary plot of a motion metric over time; Figure 19b is a further exemplary plot of a power demand profile over time; Figure 20 is a yet further exemplary plot of a power demand profile over time; Figure 21 shows an exemplary system comprising a smart processing device and cloud-based processing unit in communication with the hair styling device; Figure 22 is a block diagram illustrating the main electronic components of the smart processing device; Figure 23 is a block diagram illustrating connections between components of an exemplary system; Figure 24 shows an exemplary user interface output; Figure 25 shows an exemplary plot illustrating a clustering algorithm; and Figure 26 shows an exemplary calendar output. 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 or hinge 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. A user interface 11 is provided to allow the user to set user defined parameters and for the device to output information to the user. For example, a desired operating temperature may be set via the user interface 11. The user interface 11 may have a dial, button or touch display for allowing the user to input information to the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. In this embodiment, the user interface 11 also comprises a control button or switch 14 to enable the user to turn the device 1 on or off; and an indicator light 15 to show whether the power is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. 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 source may be an AC mains or a DC power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. In use, the device 1 is turned on, enabling power to flow through the heaters 6 to cause them to heat up. The user then opens the first and second arms 4a, 4b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 40 (which may be clumped) is introduced between the arms 4a, 4b, transversely across the heaters 6a, 6b. The user then closes the arms 4a, 4b so that the length of hair 40 is held between the first and second arms 4a, 4b and then the user pulls the hair through the closed arms (as illustrated in Figure 1b). The outer (hair contacting) surface of the heaters 6 is flat in this embodiment 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 that required to raise the hair temperature above the glass transition temperature of the hair, the hair will not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo significant damage. As such, the device 1 must be able to control the temperature so that the heating surface of the heaters 6 remains within a particular temperature range. Furthermore, it must maintain the temperature range both when hair is frequently and quickly loaded and unloaded onto the heating surface, and when hair is held on the heating surface 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 (not shown). A 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 input. In this example, power is provided to the heaters 6 for heating the user’s 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) which controls the application of an AC mains voltage, or a DC voltage derived from AC mains via a power supply 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 30|jm to 1000|jm (preferably between 75 pm and 300 pm) 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 if the layer 62 does not itself have such non-stick properties. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 into a rigid support 68 which forms a base. These layers may be mounted onto the rigid support after the layers themselves have been bonded together or they may be bonded one at a time (or multiple at a time) onto the rigid support 68. If a flexible heater is desired, then there is no need for the rigid support 68 or if a support is used, this may be a non-rigid support. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 6, and instead, the heaters 6 directly heat the user’s hair. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down much more quickly than prior art stylers. In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. The ends of each of the heater electrodes 64 are electrically connected through the lower dielectric layer 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry 23 that is mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies electrical power to the individual heater electrodes 64 to control the heat generated by each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 3a and 3b as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction). Each of the heater electrodes 64 thus creates an individual heating zone 642 on the hair contacting surface of the heater 6, which spans the width (which we shall refer to as the x-direction) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction) of the heater 6. Figures 4a and 4b show schematic views of different arrangements of such heating zones 642. Figure 4a shows an arrangement corresponding to that of Figures 3a and 3b, in which the heating zones 642-1 to 642-10 are arranged along the y-direction only. Figure 4b shows an alternative arrangement, in which heating zones 642-1 to 642-16 are arranged in both the x- and y-directions. Such an arrangement of heating zones 642 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 3a side by side in the width (x-) direction. The heaters 6 may be separated in this way into any number of heating zones 642 and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 4b shows two zones along the x-direction, a greater number of zones in the x-direction could also be provided. The heating zones 642 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6 such as those shown in Figure 3. The heating zones illustrated in Figure 4 are all the same size. Of course, different sized heating zones 642 may be provided, as illustrated in Figure 5, which shows a heater 6 having seven different sized heating zones (labelled Z1 to Z7). The way in which the heater electrodes 64 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here. The heating zones 642 described above form part of a heater having a flat hair contacting surface. The heater is not limited to flat hair contacting surfaces and can be configured for use in a tubular form (as illustrated in Figure 6a) for example for use in a hair curler device or in a curved form (as illustrated in Figure 6b) for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. The temperature of each heating zone 642 is independently controllable. Each heating zone 642 can be set to a target temperature. The target temperature of each heating zone 642 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 642 which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone 642 can be determined by determining the resistance of the corresponding heater electrode 64. The microprocessor 28 controls the heating in order to reduce the difference between the actual temperature of the heating zone 642 and the target temperature forthat heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 6 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 6 up into plural heating zones. These heating zones can be equally and / or unequally sized and can be arranged regularly and / or irregularly across the width and length of the heater. However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair (which is assumed to be the realistic worst case scenario during operation) and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone in its entirety. That applied power will bring the average temperature of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will be above the average temperature of the heating zone. This temperature increase may be sufficient to cause the unloaded part to overheat. At the same time the loaded part of the heating zone will be below the average temperature causing a reduction in heat transfer and reduced styling performance. This situation is illustrated in Figure 7, which shows a tress of hair 40 overlying heating zones 7Z2, 7Z3 and 7Z4, with heating zone 7Z3 being fully loaded with hair and heating zones 7Z2 and 7Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 64 for each heating zone back to the drive circuitry 23 as well as the number of control switches in the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature within the heater, a maximum size of the heating zones can be defined which depends on the maximum power density to hair that can be extracted from the heating zone and the material characteristics and thicknesses of the layers forming the heating zone. Specifically, if it is assumed that only one half of a heating zone 642 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone 642 can be defined with the equation below: q.W2 T — T -4- -_____ 1 max 1 Tar ' 4 ,-. t 16c. k where, TMax = maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone; TTar = target operational temperature or average temperature (°C) of individual heating zones; q = power density (Wm'2) required to heat hair passing over the surface to the desired temperature for styling; W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and k = the thickness averaged thermal conductivity of the thin layers that constitute the heating zone. If it is assumed that the thickness averaged thermal conductivity of the constituent layers of a heating zone 642 and the total thickness of the layers that form the heating zone 642 are known and fixed (for any given device), then the above equation can be used to determine the required zone width (W) and hence a number of divisions along the length of the heater that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to maintain and / or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that should be positioned along the length of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of the heater materials and without causing the temperature of the unloaded part of a heating zone 642 to exceed the maximum temperature (bTmax) that could cause burning of relatively small bundles / strands of hair that come in contact with such overheated regions of the heating zone. Specifically, the required divisions along the length can be determined from: t. k. (T^gx TTar) Where, L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface); nL = number of zonal divisions along the length of the heater plate; TMax = maximum permitted temperature (°C) on the surface of the heater (which would occur in the unloaded half (worst case) of an individual heating zone) needed to avoid damage to hair or the heater; TTar = target operational temperature or average temperature (°C) of individual heating zones; k = the thickness averaged thermal conductivity (Wm’1oC’1) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and q = maximum power density (Wm2) required to heat hair passing over the surface to the desired temperature for styling. For a hair styling device, the inventors have found the following suitable ranges for these parameters: - Peak power density required for styling dry hair (q) is typically greater than 40,000 W / m2 and less than 100,000 W / m2. - The average thermal conductivity of the layers forming the heating zone (k) (averaged through the depth of the various layers) is between 80 and 200 W / m.K. - The maximum permitted temperature of a heating zone to manage (ideally avoid) hair damage is less than 250°C, more preferably less than 220°C and most preferably less than 200°C. - The total thickness of the layers (t) which make up the heater is less than 300pm but no less than 75pm due to manufacturing limitations. - The target operational temperature of the heater (TTar) is between 150°C and 230°C. Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1.7 cm. Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 4a). If multiple rows of heating zones 642 are provided along the length of the heater (such as is shown in Figure 4b), then each row of heating zones 642 should meet the limits defined above if the above-described overheating problem is to be avoided. Alternative Heater Arrangements A first alternative flexible heater 6’ is illustrated in Figure 8, which shows on the left hand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 8, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive, or diffusion bonding (thermoforming) of the underlying materials to the upper surface of the rigid substrate 68’, or by over-moulding in which the carrier is injection moulded over the back of the flexible heater within the mould. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 8 also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example, thermistors for sensing the temperature of the heating zones 642 of the heater 6’ or fuses that can cut power to the heater electrode of each zone or all the zones in the case of a zone overheating. Figure 8 also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 3 that controls the heating of the different heating zones 642 of the heater 6’. As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm and preferably between 1 pm and 100 pm, more preferably between 10 pm and 70 pm or between 2 pm and 10 pm. The different layers forming part of the heater 6’ are shown in exploded cross-sectional and perspective views in Figure 9. A description of each layer is given below. Low Friction Coating 81 (optional) This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 3 pm) to reduce the thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant. This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. This is needed because the coating is prone to cracking when flexed, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape. Alternatively, this coating may comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm). Heat Spreading Layer 82 (optional) This is also an optional layer and, when provided, helps to spread the heat within each heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics 15 would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 - from the perspective of the control electronics 15, the heating zone 642 would be at the “correct” temperature despite having hot and cold regions. Each heating zone 642 would have its own heat spreader, which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption, reduce warm up time and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 10 illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive material over the corresponding heating zone. The heat spreaders 91 may be separated from each other by a solid material having a thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91, it is difficult for heat from one heating zone 642 to pass into neighbouring heating zones 642. The solid material (dielectric and / or scratch resistant low friction material(s)) that is provided in the gap between adjacent heat spreaders 91 may be provided by a PVD DLC, bond film, coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. Alternatively other suitable methods may be used to provide solid material in the gap between adjacent heat spreaders, such as masking and vapour deposition etc. This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality. Polyimide Separator layer 83 The polyimide separator layer 83 provides electrical insulation 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 impedance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642. Main Heater Electrode &Sensing Layer 84 This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 11 illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source. Each heater electrode 64 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of electrically conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 11 are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. Other processes such as printing, thick film printing, physical vapour deposition and the like could be used to form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at suitable vias 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flowthrough 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 (insulation) 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 or supercapacitors. 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 become the primary heaters, and those on the main heater electrode layer 984 would just be used for temperature sensing or vice versa. 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 would reduce the anisotropic thermal conductivity caused by tracks mostly facing one direction, and 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 and electrically insulates 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 present on 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 multilayer heater. As with the other dielectric layers, this backing layer 87 does not need to be a polyimide layer and other dielectric materials could be used. Rear Side Surface Mount Components (Optional) 88 This layer is used to mount components on to the rear of the flexible heater 6. These components may be temperature sensors (e.g. thermistors) or other components involved in providing fusing functionality for the heater (e.g. solder links). This layer may be produced using standard chemical etching methods from the PCB manufacturing process. Additional surface mount components would be added later. High Temperature Adhesive / Bonding Layer 89 (Optional) 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), heat activated adhesive (HAA), thermosetting epoxy films (prepregs and B-stage films). It could also be a thermoplastic bonding film which sets after heat and pressure have been applied in a forming tool. Another method of joining the flexible heater 6’ to the support carrier 68’ is to mount the heater in its final shape and overmould (a form of injection moulding) the carrier directly onto the back. In this case, layer 89 may be a material chosen for moulding compatibility, ensuring the plastic that the support carrier 68’ is made from fuses to the adhesive / bonding layer 89 providing a strong bond between the heater and carrier. Hair damage determination It can be advantageous to alter the styling methodology in such a manner as to preserve hair health. As a basis of this, the extent of damage to hair can be quantified using defined metrics. The very low thermal mass of the styler can aid in facilitating not only agile styling, to ensure hair health is maintained, but also agile sensing of hair condition. Figure 13 illustrates an exemplary block diagram of the hair styling device 1; this corresponds to Figure 2 but with additional optional components of an Inertial Measurement Unit (IMU) 12, an LED display 10, a haptics unit 13, and power measurement circuitry 24. 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 a speedometer and / or distance-meter to measure the speed at which the styler 1 is being moved. The LED display 10 may be the same as or additional to the user interface 11, and comprises a series of LEDs which can be illuminated in different colours. Typically, the LED display 10 illuminates different colours and different patterns in order to convey information, but in some implementations at least some of the LED display 10 may be illuminated for aesthetics. The haptics unit 14 is configured to provide tactile, haptic feedback and typically comprises any kind of device capable of exerting forces on a user to create tactile sensations, thereby providing haptic feedback. The power measurement circuitry 24 is configured to sense the power delivered to the heaters 6. Methods of defining hair damage As illustrated in Figure 14, the inventors have found that hair damage can be modelled as roughly proportional to the heat transfer coefficient and / or the heat capacity or specific heat capacity of the hair. Damage can be modelled as a rate at which the hair heats up, as the more damaged the hair fibres are, the faster the heat transfer. By way of example, and as illustrated graphically in Figure 14, the heat transfer coefficient, h, and the hair damage metric, d, may be related according to: h = m-d + c Where m and c are defined constants. (This is a simple version of the relationship and in some implementations, the relationship is not linear.) The styler 1 has stored within it (typically in the memory 30) calibration data relating hair heat transfer coefficient, h, to hair damage, d. This may be in the form of one or more look up tables and / or stored equations. Figure 14 shows that virgin hair (meaning hair that has never been styled or coloured, especially has never been bleached) has the lowest level of hair damage and so, correspondingly, has the lowest heat transfer coefficient. Hair which is regularly styled using heat has an intermediate level of damage, and so an intermediate heat transfer coefficient. Bleaching hair is widely known to cause damage, and so hair which is regularly bleached has a high level of damage, and therefore a high hair heat transfer coefficient. Knowing the extent of the damage to hair to be styled can be used to adjust the performance of the hair styling device accordingly, in order to mitigate further damage. This can be determined via a combination of user input and measurements made by the hair styling device. This is similarly the case for the heat capacity or specific heat capacity of the hair and so similar equations may be defined with respect to these parameters. The user can typically input information indicating the level of their hair damage; this can either be input directly into the styler 1, via the user interface 11, or via an external program or app in communication with the styler 1 via the communications circuitry 27. In this manner, the controller 28 receives the user’s indication of how they describe their hair damage. This may typically comprise the user inputting information regarding whether their hair is coloured / bleached / dyed, how frequently they heat-style it (Sfreq) and to what temperature they typically style it (Stemp); but it may additionally or alternatively comprise the user directly inputting an assessment of the condition of their own hair. This information may typically be stored in the memory 30 of the styler (or to external storage). Using the user input, the controller 28 can determine a damage metric, d, in dependence on the input data such as the frequency of styling, sfreq, and temperature of styling, stemp, for example according to equations such as: d Cl * Sfreq + b ■ S{emp + c or d e ' Sfreq ■ Sfemp + f Where a, b, c, e and f are constants which may be defined empirically and / or for different users. For example, different sets of constants and / or different equations may be chosen in dependence on whether the hair has been dyed or bleached etc. By way of a particular example, the second equation may typically be used when the user indicates that their hair is coloured / bleached / dyed. For example, the constant e may be 0.5 and the constant f may be 20: ^dyed 0-5 ' Sfreq ’ ^temp + 20 In other instances (i.e. the user indicates that their hair is not dyed), then the first equation may be used. Simply by way of example, the constant a may be 0.25, constant b may be 1.25 and the constant c may be 10. Of course, the constants may take other values, dependent on particular implementations. Using the damage metric, d, as determined from the user input and the stored calibration data, the controller 28 then determines a hair heat transfer coefficient, h, for the user’s hair. The controller 28 may determine the heat transfer coefficient, h, using look-up tables relating the damage metric, d, to heat transfer coefficient, h, and / or may determine the heat transfer coefficient, h, using one or more stored calibration equations. (If the heat capacity or specific heat capacity of the hair are being used, alternatively or additionally, values for these parameters may be determined using stored calibration data relating them to the damage metric.) Optionally, the determined heat transfer coefficient, h, may be used to vary a target operating temperature of the heaters 6. This is typically configured to be an overcorrection, to avoid injecting too much heat into the hair on the first pass. This can avoid accidentally damaging the hair, especially if the user has underestimated their hair damage. In order to try to ensure this, the controller 28 is configured to estimate the anticipated response assuming a higher level of hair damage, d, than determined from the input of the user, and therefore the controller 28 sets a lower target temperature for the heaters, as it would for more highly damaged hair. This means the Styler’s performance is moderated before touching the user’s hair. In some implementations, however, a predefined or set temperature setpoint is used initially. Based on the determined hair heat transfer coefficient, h, and the target operating temperature of the heaters, the controller 28 can predict a heating profile for how a hair tress will heat up over time in response to coming into contact with the heaters 6 of the hair styler 1. This may be based on further stored calibration data. In order to compare the predicted temperature with the actual temperature, the controller 28 is typically configured to compare the predicted temperature (Test) of the hair at a given time point after coming into contact with the heater to the actual temperature of the hair (Tdet) at the given time point after coming into contact with the hair. This given time point, th, may be, for example, a predefined time after the arms of the hair styler close around the hair. Figure 15 shows the estimated hair heat transfer coefficient as determined by the controller 28 based on the user’s input of their assessment of hair damage and Figure 16 shows in the dashed line the anticipated change in hair temperature over time when the user’s hair is heated by the heater 6 during styling based on the estimated hair heat transfer coefficient. The solid line in Figure 16 shows the actual heating profile for the hair as it is heated over time after coming into contact with the heater 6. As discussed above, in order to compare the predicted profile with the actual profile, the controller 28 determines the difference between the predicted profile and the actual profile at a given time point - shown in Figure 16 as time th. The determined temperature difference, ATatr, at time th, between the actual hair temperature, Tdet, and the estimated hair temperature, Test, is indicated as AT® in Figure 16. If the temperature difference, ATdiff, is less than a threshold amount, then the controller 28 determines that the estimated heat transfer coefficient that it calculated from the input provided by the user is correct and the controller 28 may output a message to the user confirming to the user the current level of damage of the hair. If the temperature difference, ATdiff, is greater than the threshold amount, then the controller 28 uses the determined temperature difference or the actual temperature of the hair to work out a more accurate estimate of the hair heat transfer coefficient. This can be achieved by consulting the same calibration data linking the expected temperature profile for the hair and heat transfer coefficient, h, as was used to determine the estimated temperature, Test. This calibration data may be in the form of a look-up table and / or equations. The styler 1 then uses the more accurate value of the heat transfer coefficient, h, and the stored calibration data relating the heat transfer coefficient, h, and the damage metric, d, to determine a more accurate value of the damage metric, d. This can be determined from, for example: h - c d =--- a The value of the determined hair damage metric, d, is typically lower than the ‘estimated’ hair damage metric, d (meaning that the actual hair damage metric indicates a lower level of damage to the hair than the estimated hair damage metric). This can be seen in Figure 15, which shows the exemplary linear relationship between hair damage, d, and hair heat transfer coefficient, h, with the hair damage based on user input (the estimated hair damage metric) plotted at a higher point than the point indicating the actual hair damage (the determined hair damage metric) on the line indicating this linear relationship. This is because the estimated hair damage metric is typically deliberately an overestimation, to ensure a safety margin of the initial heating profile of the tress of hair. Finally, this more accurate determined value of the hair damage metric, d, is output to the user and / or used to control an operating parameter of the styler 1 (such as the operating temperature of the heaters 6). For example, this may comprise altering the setpoint temperature of the heaters 6 to a higher temperature if the hair damage metric, d, is lower than estimated, and vice versa. The controller 28 is typically further configured to send the hair damage information, based on the determined hair heat transfer coefficient, to storage; this may be the internal memory 30 and / or an external storage. In some implementations, the controller 28 is further configured to output to a user via the LED display 10 hair damage information, such as the hair damage metric. In some implementations, this may be accompanied with advice on how to improve hair health (such advice will be described in further detail in a later section). Methods of defining damaging behaviours In addition to determination of a hair damage metric, as described above, in some implementations the processor 29 is configured to determine whether the manner in which the styler 1 is being used is potentially damaging, in other words to determine damaging behaviours. This comprises recording and analyzing parameters of a styling session to identify such behaviours and then typically further comprises defining a corresponding metric. Recurrent heating of a single tress One such potentially damaging behaviour is the recurrent heating of a single, same tress of hair. The high level of heat input into the tress during repeated heat-styling can cause damage. A first method for determining recurrent heating comprises analyzing the ‘power demand’ required to maintain the setpoint temperature of the heaters 6. The controller 28 of the styler 1 controls the power to the heaters 6 in dependence on temperature measurements of the heaters 6, either from temperature sensors or from resistance measurements (as described above). Power is delivered to the heaters 6 by the drive circuitry 23 in dependence on a difference between the temperature of the heaters 6 and the setpoint temperature. The larger the difference in temperature between the heaters 6 and the desired setpoint temperature, the larger the power input by the drive circuitry 23. Accordingly, the load on the heaters 6 can be tracked by reviewing the ‘power demand’. The processor 29 is configured to track this ‘power demand’ by tracking the sensed temperature measurements and the setpoint temperature, and calculating the power delivered accordingly, and / or the power input to the heaters 6 can be sensed directly by the power measurement circuitry 24. In either case, the processor 29 is configured to record the power input overtime, which can be processed to indicate the loading of the heaters 6 over time. Typically, this information is stored in the memory 30 of the styler, and analysis performed by the microprocessor 29; however, in some instances the information may be transmitted to and stored and / or processed by one or more external devices. Figure 17 shows an exemplary profile of ‘power demand’ (the calculated and / or measured power input to the heaters 6) over time, which is indicative of the loading of the heaters 6 over time. When a user repeatedly heats the same tress, the power demand lowers for each successive stroke, as less power is required to heat an already-heated tress (because the difference between the measured temperature and the desired setpoint temperature is smaller). This is illustrated in Figure 17, which shows that the power supplied to the heaters varies over time in a pattern akin to a damped decaying sinusoidal wave. The dashed line of Figure 17 follows the maximum power delivery, and shows that this follows a decay curve. Each wave corresponds to a stroke of the heaters 6 over the hair, whereby the power demand increases when the heater touches the hair, and then drops when the user removes the heater from the hair; the power demand increases again for the next stroke when the user places the heater on the hair once more. For each successive stroke, the power demand is lower, hence the decay profile. This is due to the reduction in power demand to heat the tress as the tress heats up (as the temperature difference between the hair and the setpoint temperature decreases). As such, the processor 29 can identify recurrent heating of the same tress from this characteristic power profile over time, namely a damped decaying sinusoidal wave. The processor 29 is typically further configured to determine the decay rate and count the number of peaks and to use this information to determine the frequency and number of repetitions (np) of the recurrent heating. For example, a decay curve with four peaks indicates the user has passed the heaters 6 over the same tress of hair four times and the time interval between those peaks provides an indication of the time delay between the recurrent passes. A further method for determining recurrent heating of a single tress of hair relates to analysis of user motion, based for example on measurements from an inertial measurement unit (IMU) 12 within the styler 1. The controller 28 receives and processes motion data from the IMU 12. The controller is configured to define a motion metric based on user speed, acceleration, and impulse data received from the IMU 12. The motion metric is typically defined for each particular hair styling device. By way of example, this may be defined in dependence on acceleration, a, speed, s, and ‘jerk’ (which can also be referred to as impulse), j. The absolute net acceleration, aabs_net, can be determined according to: ®abs_net — T ^z — This determines the magnitude of the net acceleration of the styler using measurements of the acceleration in the x-direction, ax, of the styler 1; acceleration in the y-direction, ay, of the styler 1; and acceleration in the z-direction, az, of the styler 1. Unless the axis lies in a horizontal plane, the measured acceleration in each axis (ax, ay and az) will include a component due to gravity, and accordingly, the value for gravity (G) is subtracted from the determined magnitude to remove the influence of gravity from the calculations. The absolute net speed, sabs_net, can be determined from integrating the absolute net acceleration, aabS_net, over the relevant period of time, t, according to: ft Sabsnet = I aabsnet ' dt — D Jo Where D is a drift constant. Additionally or alternatively, speed measurements may be determined by a speedometer or distance-meter. For example, this may be implemented using an optical system capturing images of the hair as the styler passes over it. Using known values of frame rates and dimensions of the styler, the distance travelled over a time interval can be determined and hence the speed determined. Alternative methods could also be used. If speed measurements are determined by a speedometer or distance-meter and received by the controller, then the controller can determine the measured speed vector, smes, directly, the vector in the three orthogonal axes being written as: ^mes ~ [%> Sy ^z] The measured absolute scalar value, smes, can be therefore be found by finding the scalar of this vector, according to: Smes 11 ^mes 11 A value for the average speed can be determined by finding a weighted average of the measured scalar value of the speed, smes, and the absolute net speed, (as determined from integrating the absolute net acceleration, aabs_net). As the measured speed is considered more accurate than the speed calculated from acceleration measurements, this is given a higher weighting. c _ &' ^mes T ' $abs net Simply by way of example, the ratio of the weightings may be 5:2, i.e. the weighting of the measured scalar value of the speed, smes, is 5 / 7 and the weighting of the absolute net speed, sabs_net, is 2 / 7. The average speed therefore calculated according to: Of course, however, different weightings may alternatively be used. Then a value for the upper limit of the speed, sup, can be determined, which defines a maximum value for the speed which a user is moving the styler 1 over the tress of hair. The upper speed, sup, can be determined in dependence on the average speed, measured absolute scalar speed, smes, and the absolute net speed, Sabs_net; in particular, by summing the value for average speed and an error estimation based on a weighted combination of the measured absolute scalar speed, smes, and the absolute net speed, sabs_net according to: sUp = s + (m + n) Of course, again, different weightings may be used. Typically, the weightings will be the same as those used determining the average speed (i.e. k = m and I = n); for the example above, this would mean the equation becomes: sup - 5 + 5(smes - S)2 + 2(sabSnet - S) Alternatively, if no speed measurement data is available or received, then the value of upper limit speed, sup, can be approximated as equal to the absolute net speed, sabs_net, i.e.: Sup Sabsnei A value for jerk or impulse, J, can be determined from the derivative of the vector acceleration, according to: The motion metric can then be defined in dependence on the determined values of the upper limit of the speed, absolute net acceleration and the determined scalar value of impulse: Sup> &abs_net> An exemplary profile of a motion metric over time is illustrated in Figure 18. Such a metric can be used to determine whether a user is moving the styler 1 in an effective manner; for example, if the user is running the plates 6 over a tress of hair too quickly, then the styling is unlikely to be effective, which can lead to a user needing to repeat the motion and thereby heat the hair more than necessary. An upper threshold value of the motion metric can be defined which corresponds to a user moving the stylertoo fast, meaning they will need to repeat the motion. In typical implementations, the threshold is defined as a function of the upper limit of the speed, absolute net acceleration and impulse: T = [Ts,Ta,Tj] Where Ts is the threshold speed value, Ta is the threshold acceleration, and Tj is the threshold impulse. In typical implementations, if any of the thresholds are exceeded, then the user is considered to be moving too fast (i.e. the motion metric is above an acceptable threshold). For example, if the speed, sup >Ts, or if the acceleration, ag3se: >Ta or if the impulse, |(|J |)| >Tj, then the user is considered to be moving too fast. The threshold is indicated by a thick line at a value of 0.2 on Figure 18. In some implementations, there may be an upper threshold indicating an ‘infraction’ zone and a ‘grey zone’ is also defined, which is below the upper threshold and in which the styler 1 is still being moved too fast; this is labelled ‘G’ in Figure 18 and is located between a dotted line at 0.1 and the thick solid line at 0.2. In such an implementation, the upper threshold defines an upper or ‘hard’ threshold, Thard, which is a combination of ‘hard’ thresholds for the speed, acceleration and impulse: >Thj And the grey zone occupies a zone below the upper threshold, Thard, but above a lower threshold, Tgrey, defined according to: T ■ Where each ‘grey threshold’ for speed, acceleration and impulse may be equal to or less than the corresponding ‘hard’ threshold, i.e.: Tgs <Ths\ Tga <Tha', and Tgj <ThJ. The movement of the styler 1 by the user is determined to be in the ‘grey zone’ if any two of the metrics exceed the grey thresholds; while the user is considered to be moving the styler 1 too fast if any of the metrics exceed the relevant ‘hard threshold’ value. Written in another way, the motion metric of the styler 1 is in the grey zone if: f(sup >^s) + f^net >Tga) + f(j >Tgj) 2 Thard — Tha Tgrey Vgs’ Tga Where: f r >T-} = { true L e'when ' X' 1 [o, if false i.e. when (xi <Ti) The motion metric of the styler is above the threshold or in the ‘infraction’ zone if: > Ths) OR (fl-net >Tha) OR (j >T^j) The values illustrated in Figure 18 and described above are of course just exemplary values and the metric bounds can be defined for the particular situation. Similarly, in some implementations, slightly different protocols for the thresholds and grey zone may be implemented. If the processor 29 determines, upon analysing the profile of motion metric over time, that the motion metric is above a threshold value or within the ‘grey zone’, G, then recurrent heating of the same tress is likely. In the case that the styler has two heaters 6 on two arms 4 and hair is styled by placing between the arms 4 and closing the arms 4 around it (as is the case for a hair straightening device, for example), the motion metrics are typically analysed in combination with a determination of whether the styler 1 is open or closed. The processor 29 is typically configured to perform this determination based on feedback from sensors in the arms 4 of the styler 1 (for example, proximity sensors) and / or the data received from IMU 12. Looking again to Figure 18, the solid line illustrates when the styler 1 is closed, and therefore the heaters 6 are likely to be engaged on a tress of hair; and the dashed line illustrates when the styler is open and therefore not engaged with a tress of hair. As such, the motion metrics determined in those intervals in which the styler 1 is closed are of particular interest as they are indicative of how the styler 1 is being moved over the tress. The processor 29 is configured to analyse the IMU data in combination with the data from, for example, proximity sensors regarding whether the arms 4 are open or closed; in this manner, they are configured to determine the time intervals between those periods when the arms 4 are closed (and so when the hair is engaged by the heaters 6). Using these determined intervals, the processor 29 is further configured to determine the frequency of recurrent heating of a tress of hair. In some implementations, the processor 29 is configured to detect recurrent heating of a single tress of hair in dependence on the received data relating to the power delivered over time; or in dependence on the received data relating to the motion of the styler 1 from the IMU 12 (in some cases, in combination with data from proximity sensors); or the processor(s) may be configured to perform both methodologies. The processor 29 is typically further configured to instruct a control action in dependence on this analysis; this may comprise outputting an alarm or alert to a user via the user interface 11 of the styler 1 (or to an external device) and / or outputting control instructions to alter the settings of the styler, for example the setpoint temperature of the heaters 6. Heat on hair time A further behaviour which can be assessed relates to the time over which heat is applied to hair, known as heat on hair time. This provides an estimate of how much time the styler is applying heat to the user’s hair, and thereby provides an estimate for total heat accumulation in the hair. Simply, the longer the user is applying heat to the hair, the more damage occurs to the hair. There are generally two modes of usage that result in heat being applied to hair for too long. The first is when a user moves the heater 6 too slowly over a tress of hair, and so applies heat to hair for a long time on each stroke. The second mode is when a user moves the heater 6 too quickly over a tress of hair and so they need to reapply heat to the same tress of hair to repeat the stroke in order to achieve the desired styling (this is sometimes known as ‘clap-clap’ styling). The heat on hair time can be described by a combination of metrics: single session heat on hair, tH, which is the amount of time heat is applied to hair during one styling session; total heat on hair time, TH, which is the total amount of time heat has been applied to hair; and mean heat on hair time, tHm, which is the mean amount of time heat is applied to hair during each styling session. The single session heat on hair time, U can be determined from the sum of the time during which the styler is closed (which is indicative of the plates 6 of the styler 1 being closed around a tress of hair). The controller 28 receives data from the IMU 12 regarding movement of the styler and from the IMU 12 and / or a separate sensor (such as a proximity sensor) indicating whether the arms 4 of the styler 1 are open or closed. The processor 29 is configured to process this data to determine the heat on hair time. Figure 19a shows the motion metric over time (as described above), with the time intervals over which the styler 1 is closed indicated as tciosedi and tciosed2. The processor 29 is configured to determine the total of these intervals, tm, by summing the time intervals over which the IMU data and / or sensor (e.g. proximity sensor) data indicates the arms 4 are closed: The single session heat on hair time, tH, can also be determined using the profile of power delivered over time. As described previously, the controller 28 controls power input in dependence on the measured temperature of the heaters 6, in particular in dependence on the difference between the measured temperature and the desired setpoint temperature. The processor 29 can therefore track the power delivered to the heater over time, as determined from temperature measurement data received from the temperature measurement circuitry 25 and / or from direct measurements of the power delivered as sensed by the power measurement circuitry 24. The processor 29 is then configured to analyse the profile of power delivered (i.e. power demand, indicating the load on the heaters 6) over time. The single session heat on hair time, can be estimated from the amount of time the power demand has a positive gradient, as power demand drops and the gradient becomes negative when the load is removed (making the assumption that the user does not leave the hair on the heater 6 until it reaches the heater temperature). Figure 19b shows an exemplary power demand profile over time, which has the form of a damped decaying sinusoidal curve. Those time intervals over which the power has a negative gradient are indicated in Figure 19b as tngi, tng2, tng3, tng4 and tng5. The processor 29 is configured to determine the total of these intervals, tp, by summing the intervals over a relevant period (corresponding to the single session): There are therefore two different methods for estimating the total heat on hair time: using data relating to the arms 4 of the styler 1 being closed, and using data relating to the power delivered over time. In some implementations, the processor 29 is configured to determine estimates using both methods and then determine an average estimated value for the single session heat on hair time, tH, by calculating the mean of the two estimates, according to: Using both the power profile and the determination of whether the styler is open or closed in combination enables corroboration that the heat was indeed applied to the hair, rather than the styler simply being closed without the heater 6 being in contact with a tress of hair. The total heat on hair time, TH, is the sum of the heat on hair times for all of the sessions. The processor 29 can therefore be configured to determine an estimation of the total heat on hair time, Th, by summing the determined value for single session heat on hair time, tH, (as described above) over all of the n sessions, according to: n x=l The processor 29 can then be further configured to determine the mean heat on hair, tHm, by dividing the total heat on hair time, Th, by the number of sessions, n, according to: , _ T" — — It can be useful to determine these metrics relating to the time over which hair has been subjected to heat, as the heat on hair time can affect the extent of damage of the hair. It can also provide an insight into the user’s styling technique, typically in combination with other metrics. For example, the heat on hair data can be used in combination with user hair length (typically input by a user) and movement speed (typically determined by the IMU 12) to determine the behaviour the user is exhibiting and how damaging it is. For example, for a user with long hair, the heat on hair time would be expected to be longer, in the region of a few minutes. For a user with shoulder-length or short hair, the heat on hair time would be expected to be shorter. Therefore, if a heat on hair time of a few minutes is determined for a user with shoulder-length hair, then it is likely that too much heat is entering the hair and damage is being done to the hair. By contrast if this same heat on hair time is determined for a user with long hair, the damage is likely to be less. The controller 28 is typically configured to perform control actions in dependence on the determined metrics; this may for example comprise outputting an alert to the user (e.g. via a user interface 11 or LED display 10, or via haptics or audio) and / or altering parameters of the styler 1 to minimise damage and / or optimize styling outcomes, for example by altering the heating profile. An alert may comprise advice to a user how to alter their technique. This data can also be used to determine a ‘persona’ for the user (this will be explained further in later sections). Average maximum hair temperature A further metric which can be defined to describe styling behaviour and potentially damaging techniques is the average maximum hair temperature, Hm. This refers to the average of the maximum temperature to which each tress of hair is raised. The controller 28 is typically configured to determine a value for the average maximum hair temperature, Hm, by analyzing the profile of power delivered (which is used as an indication of ‘power demand’ indicating load on the heaters 6) over time. Figure 20 shows an extended exemplary plot of power demand over time, similar to Figure 17 but extending further across time so as to capture two damped decaying sinusoidal waves. These two damped sinusoidal waves correspond to the styling of two different tresses of hair, which shall be referred to as the first tress and second tress. The amplitude difference between the highest peak and lowest trough of the power demand for each tress corresponds to the largest power input into the tress. These are typically the first peak and first trough as the power demand decreases as the tress heats up. The processor 29 is therefore configured to determine the amplitude difference of power demand for each tress, as indicated in Figure 20 and labelled 5Pdi for the first tress and 6Pd2 for the second tress. In addition, the processor 29 is configured to determine the mean power demand for each tress; this is also indicated in Figure 20 and labelled Pmi for the first tress and Pm2 for the second tress. Using these determined parameters, the processor 29 is further configured to determine maximum temperature of each tress as a function of the power demand amplitude difference, 6Pdx, and the mean power, Pmx: ^mx f (3Pdx> Pmx) Alternatively or additionally, in some implementations the processor 29 may be configured to determine the maximum temperature of each tress by analysis of the heat control algorithm. As the heat control algorithm directly measures the temperature of the hair (and then determines power input in dependence on the difference between this measured temperature and a desired setpoint temperature), the temperature measurements can be analyzed to determine for each particular tress the maximum temperature. As the heating of a single tress is indicated by a characteristic power demand profile and from a characteristic heat profile, the processor 29 is configured to use this data to determine the intervals over which the styler 1 is heating a single, same tress. For example, a first tress heats up when styled and then the next tress will be comparatively cold. Having determined the maximum hair temperature of the tresses by either method described above or both, the processor 29 is configured to determine the average maximum hair temperature, Hm, by calculating the mean value across all the tresses of hair: - 1 V / Hmx n Z—i x=l The average maximum hair temperature, Hm, is a useful metric as it can provide insight into the maximum temperature to which each tress is subjected; as, if this is very high, more damage is likely to have occurred to the hair. Conversely, if this is very low, it may indicate that styling has not been effective. The controller 28 is then configured to perform a control action in dependence on the determined value of average maximum hair temperature, Hm. This may comprise, for example, outputting an alert to the user (e.g. via a user interface 11 and / or LED display 10) and / or altering parameters of the styler 1, such as the heating profile of the heaters 6. Fibres damaged In some implementations, the controller 28 is further configured to determine a percentage value of fibres damaged. For example, this may comprise either the total percentage of fibres damaged or the percentage of new fibres damaged from a particular session. The controller 28 is configured to determine a baseline damage percentage using the hair damage model described above with reference to Figures 14 and 15, in which hair damage is linearly proportional to the determined heat transfer coefficient of the hair. The controller 28 is configured to determine a metric for the baseline damage, Dbi, using the hair damage model: Dht = f(hair damage model) The processor is further configured to determine a metric for new damage, Dn, which can be defined as a function of the average maximum temperature, Hm, applied to hair and of the baseline damage, defined according to the equation: Dn ~ (100 - Djyl) ' ——--- Where: ( 0, ifHM<T\ Y = ) — if Tr >Hm >T2 ifHM>T2 Accordingly, if the average maximum temperature, Hm, applied to the hair is below a first threshold temperature, Ti, then the hair is considered to have undergone no new damage and Dn is defined by the controller 28 as equal to zero. If Hm is between the first threshold temperature, Ti, and a second threshold temperature, T2 (where Ti <T2), then the percentage of new damage, Dn, is defined as proportional to the difference of Hm and the first threshold temperature, Ti. If HM is above the second threshold temperature, T2, then the multiplication factor becomes 1 and the percentage new damage is defined as equal to 100 - the baseline damage, Dbi. The first threshold temperature is typically around normal hair styling temperature. By way of example, the first threshold temperature, T% may have a value of about 180 °C and the second threshold temperature, T2, may have a value of about 230 °C. Of course, other values may also be chosen. The controller 28 is further configured to determine total percentage damage, Dtotai, by summing the baseline damage, Dbi, and the new damage, Dn: ^total — Dn + D^i This total percentage damage, Dtotai, can provide an insight into the condition of the hair. In particular, the determined values can be stored in the memory and so used to track how the damage and condition changes over time. The controller 28 is typically further configured to perform a control action in dependence on the determined damage parameter and, in some instances, on an assessment of how this changes over time. For example, this may comprise outputting an alert (which may comprise outputting one or more determined values and / or advice) and / or updating styler parameters, such as heater setpoint temperature. Typically, the percentage of new fibres damaged from a particular session will be most relevant, as it is dependent on a user’s most recent styling technique. The controller 28 is therefore typically configured to output a value for and / or advice relating to the percentage of new fibres damaged from a particular session. This may, for example, include tips on how to improve styling technique so as to reduce hair damage when styling. The controller 28 is typically further configured to alter parameters of the styler 1 in dependence on the defined baseline damage, Dbi, new damage, Dn, and / or total percentage damage, Dtotai. ‘Scoring’ parameters As described above, each of the parameters calculated can be used by the controller 28 to control the outputting of information to the user and / or control the operation of the heaters during the styling. In addition, the parameters thus calculated can be used to calculate ‘scores’ relating to hair parameters (such as hair damage) and a user’s styling technique, which itself can have a large effect on hair condition and damage. These scores can then be combined to determine an overall ‘hair score’. Once it has calculated the scores, the controller 28 is typically configured to output them to a user, via a user interface 11, typically along with feedback (such as advice) on how to improve their technique so as to improve the scores. In some implementations, the controller 28 is configured to alter styler control parameters (such as heating profiles) in dependence on one or more of the scores. Examples of these scores will now be described. Behaviour score The behaviour score, Sb, relates to how a user has used the styler, and so takes into account the amount of time heat has been applied to the hair, maximum hair temperature etc. In particular, it is a function of the hair on heat time, tH, parameter y (which is itself is a function of maximum hair temperature, Hm) and first and second threshold temperatures, Ti and T2 as outlined above in relation to the new damage, and the number of repetitions, np. c . (t„ Y-np \ SB = mm — + —--— , B A and B are constants, which can be chosen for each particular implementation. Constant A defines the contribution of the hair on heat time, tH, to the behaviour score, Sb. Constant B defines the maximum contribution of the behaviour score, Sb, to the overall hair score. By way of example, A and B may both take the value 30. If, for example, the maximum hair temperature, Hm, is below the first threshold temperature, T-i, then parameter y takes a value of 0, and so the equation simplifies to: / t# \ SB — min I —) \ / 1 / In such a case, the hair on heat time must be equal to or greater than a value of (A x B) in order for the behaviour score, Sb, to take a maximum value (in the case of A and B both having the value 30, this would mean the hair on heat time would need to exceed 900 s). Looking at the alternative extreme, if for example the maximum hair temperature, Hm, is above the second threshold temperature, T2, then parameter y takes a value of (T2 - T-i), and so the equation simplifies to: / t / j \ SR = min--\-n„ ,B ] \A p J In this situation, the number of repetitions also contributes strongly to the value of the behaviour score, SB. Of course, if the maximum hair temperature, Hm, is between the first threshold temperature, Ti, and the second threshold temperature, T2, then the weighting of the number of repetitions, np, will be proportional to the difference between the Hm and Ti. Hair heat score The hair heat score, Sh, is a function of the extent to which hair has been heated, and so is a function of maximum hair temperature, Hm, and the first threshold temperature, Ti (in particular, the extent to which the maximum hair temperature exceeds the first threshold temperature): Constants C and D can be chosen for each particular implementation and / or use case. The value of C defines what range of heat values relative to the first threshold temperature are considered relevant. The value of D determines the extent of the weighting of the hair heat score, Sh, to the overall hair score. Simply by way of example, C may be 5 and D may be 10. In such a case, if the difference between the maximum hair temperature, Hm, and the first threshold temperature, Ti, is 50 °C or larger (either above or below Ti), the hair heat score, Sh, will have its maximum value of 10. However, if the maximum temperature, Hm, is within ± 50 °C of the first threshold temperature, Ti, then hair heat score, H, will have a value of between 0 and 10. The hair heat score, Sh, is therefore a score of the extent to which the maximum hair temperature, Hm, is within a set range of the first threshold temperature, Ti. If, by way of a further example, the value of C were 4 (and D remains 10), then if the difference between the maximum hair temperature, Hm, and the first threshold temperature, Ti, is 40 °C or larger, the hair heat score, Sh, will take its maximum value of 10; and if the maximum hair temperature, Hm, is within ± 40 °C of the first threshold temperature, Ti, then hair heat score, Sh, will have a value of between 0 and 10. As such, the constant C contributes to defining the range around the first threshold temperature, Ti. If the value of the maximum hair temperature, Hm, is outside of this range, then the hair heat score, Sh, will take its maximum value defined by the constant D. The constant D defines the maximum value the hair heat score, Sh, will take; and thus the weighting of the hair heat score, SH, to the overall score. It also contributes to defining the relevant range, in combination with the constant C. For example, if the value of D were instead 20 and the value of C is 5, then the hair heat score, Sh, would take its maximum value of 20 if the difference between the maximum hair temperature, Hm, and the first threshold temperature, Ti, were 25 °C or larger. As such, the constants are chosen in combination. Total damage score The total damage score, Std, is a function of the total damage, Drotai, which is the sum of the baseline damage, Dm, and the new damage, Dn, as calculated in the manner outlined above. The total damage score, Std, is then defined by the equation: _ _ ^total ^TD ~ The constant E defines the contribution of the total damage score, Std, to the overall score. It can be defined for each particular use and / or implementation. By way of example, E may take a value of 20. New damage score The new damage score, Snd, is a function of the new damage, Dn, relative to the baseline damage, Dbi. In other words, it is a function of the effect of the new styling to the hair. It can be defined according to the equation: c _ F ■ Dn Snd “ 100 F is a constant which can be defined for each particular implementation, as it defines the contribution of the new damage score, Snd, to the overall score, Soveraii (i.e. the weighting). As an example, F may take the value 25. Energy score The energy score, SE, is a measure of how efficiently the user has used the styler 1 from an energy use perspective. This is a function of the time over which the styler 1 is turned on, ton, according to the equation: „ _ ton bp — _ E G The constant G again can be defined for each use case. It defines both the contribution to the overall hair score, but also the time range over which the styler 1 is on that is considered normal use, extended use etc. As an example, the value of G may be 60. In such a case, the value of the energy score, SE, increases by 1 for each minute (i.e. 60 seconds) over which the styler 1 is turned on. In some implementations, the energy score is defined such that it also takes into account in the calculation the percentage of the total time the styler 1 is turned on that it is being applied to the user’s hair, it is idle etc. This can be determined based on measurements of the IMU 12 and the heat demand profiles of the heaters 6, as discussed above. Style difficulty In some implementations, it can be useful to include a metric relating to the difficulty of the style or styles a user has attempted. The style is typically defined based on particular known styling techniques and methodologies. The user can input which styling technique they are following, for example, via the user interface 11 or through an app in connection with the styler 1 via the communications circuitry 27 (as will be explained in further detail in later sections). Alternatively or additionally, the controller 28 may be configured to determine the user’s movements to ‘detect’ a particular styling technique from the movement of the styler 1 (using measurements received from the inertial measurement unit, IMU 12). The difficulty of the style is defined by a style difficulty score, Ssd, which is defined by the equation: SSD = (1 to H) Where H is an upper value for the style difficulty score, Ssd. For example, the value of the Ssd may range between 1 and 5. Overall ‘hair score’ The controller 28 can be configured to determine an overall ‘hair score’, Soveraii, by combining the individual scores. The Soveraii can be defined using all of the scores defined above by the equation: ^Overall = 100 — (¾ + + $TD + SND + SE + SSD) Typically, the lowest possible value of the overall score is a number larger than zero. This is to avoid users being disheartened by a very low score. By way of example, if the values of the constants are A = 30, B = 30, C = 5, D = 10, E = 20, F = 25, G = 60, and H = 5; then the lowest possible value of the overall score, Soveraii, is 16. Of course, different combinations of scores can be used and / or weighted as is relevant and suitable for each particular implementation and use. Further and / or alternative scores may also be defined and included. Outputs to a user In dependence on the analysis of styler data and resulting metrics and scores, as described above, the controller 28 is configured to perform control actions. This may typically comprise outputting an alert and / or updating the styler parameters in dependence on the determined metrics and / or scores, for example in dependence on the metrics and / or scores falling outside of predefined acceptable threshold ranges (and thus indicating an issue regarding hair health and / or styling). The alert may be a written warning (output to the user interface 11), a visual alert (such as flashing of the LED display 10), an audio alert, a haptic alert (in particular, via the haptics unit 14), or any combination of these. The alert may comprise instructions how to correct the issue, for example output to the user interface 11. The controller 28 may additionally or alternatively amend the settings of the styler 1 in dependence on the determined metrics and / or scores, for example changing the setpoint temperature of the heaters 6. The alerts may be output to the user interface 11 or the LED display 10 of the styler 1 itself, or the styler 1 may be in communication with further components, to which the alert can be output. In some implementations, the further components are configured to perform at least some of the processing. Figure 21 illustrates an exemplary system 200 within which the styler 1 may be provided. The system 200 comprises at least one styler 1 in communication with at least one smart processing device 210, which is in turn in communication with cloud-based processing unit 220 (such as a server). The at least one smart processing device 210 is typically a mobile phone which runs a specific application (‘app’), but it may also be a computer, smart watch etc. More than one smart processing device 210 may be used in combination; for example, a user may use both a smart watch and a mobile phone. In some optional implementations, the at least one styler 1 is directly in communication with the cloud-based processing unit 220. The one or more stylers 1 communicate with the at least one smart processing device 210 and / or cloud-based processing unit 220 via the communications circuitry 27. Figure 22 illustrates an exemplary architecture of some main components of the smart processing device 210. Communications circuitry 211 of the smart processing device 210 is in communication with the communications circuitry 27 of the one or more stylers 1. The communication of information between the styler 1 and the at least one smart processing device 210 may for example be via Bluetooth™, and the communication circuitry may comprise antennae. Alternatively, in some implementations the communication may be via an internet connection such as WiFi. The communication circuitry 211 is in communication with and sends the received information to a processor 214, which also receives information from a user interface 213. The user interface 213 typically comprises a screen for displaying (outputting) information and buttons, touchscreen capabilities and / or other components which facilitate a user inputting information. The processor 214 is further connected to a memory 215. The memory 215 typically stores an operating system 216 and one or more control modules 217. By way of examples, the control modules 217 may be configured to control communications, and / or to control settings of the styler 1. The memory 215 further comprises data storage 218, which is configured to store presaved data, settings, and data accumulated during use. As shown in Figure 21, the at least one smart processing device 210 is further in communication with a cloud-based processing unit 220. The processor 214 of the smart processing device 210 is configured to receive information and instructions from and send information to the cloudbased processing unit 220 via the communications circuitry 211, typically over an internet connection. The processor 214 of the smart processing device 210 and the processor 224 of the cloud-based processing unit 220 can both be configured to process the data collected by the one or more stylers 1 during use. This may be in addition to or alternatively to processing performed by the processor 29 of the styler 1. That is to say, in the above-described example processes and calculations, the processor 214 of the smart processing device 210 and / or the processor 224 of the cloud-based processing unit 220 may perform the processing of the styler data in addition to or instead of the processor 29 of the styler 1, thereby to determine metrics, values and / or scores. Figure 23 shows a further exemplary architecture of the connections between the smart processing device 210, a first styler 1 and second styler 1’, and the cloud-based processing unit 220. The cloud-based processing unit 220 comprises a database 222 and a processor 224. The database 222 stores user profiles 226 and corresponding information. Typically, the data is stored in a graph-database, linking stylers 1,T, behaviours, and personas to user profiles 226. A user can access their own user profile 226 from the database 222 and load it to one or more smart processing devices 210. Typically, data from the user profile 226 will be downloaded and stored in the data storage 218 of the memory 215 of the smart processing device 210. The user profile 226 can be linked to the user’s particular stylers 1, T. The user profile 226 can be updated via accumulation of data from the usage of those stylers 1, T (the details of the information extracted from the user’s usage of the stylers 1,1’ will be explained in detail below). This data can be stored in the data storage 218 of the memory 215 of the smart processing device 210, which then sends the information to the database 222 of the cloud-based processing unit 220. This allows information to be collected even if the smart processing device 210 is currently offline. The database 222 further typically comprises advice on hair styling techniques; this may comprise, for example, styling routines to achieve particular hairstyles. The processor 224 of the cloud-based processing unit 220 is typically configured to process the information stored in the user profiles 226 in the database 222 relating to how users use their stylers; this can be used to determine trends, etc. The processor 214 of the smart processing device 210 and the processor 224 of the cloud-based processing unit 220 can both be configured to process the data collected for a particular user. The output of the processing of the collected data typically comprises advice, for example on how to improve technique; this will be explained in further detail in the following sections. The processing instructions run by the processor 214 of the smart processing device 210 may be received and / or updated from the cloud-based processing unit 220. Updates may be made in dependence on the results of the processor 224 of the cloud-based processing unit 220 analyzing data from different users sent to and stored in the database 222. It can be beneficial to provide a user with meaningful metrics on hair health and / or styling performance. As illustrated in Figures 21 to 23 and as described above, the styler 1 is configured to communicate with an external smart processing device 210 and cloud-based processing unit 220. The external smart processing device 210 is typically a smart watch, mobile phone, tablet and computer, and its processor 214 is configured to run an application (‘app’) designed to complement use of one or more stylers 1. The app is typically downloadable to the memory 215 of the smart processing device 210 from the database 222 of the cloudbased processing unit 220. The user can create a user profile 226 individual to them, in which their data can be stored. This user profile 226 and its data is typically stored in the cloud-based database 222 so that a user can log into their account (and user profile) from different smart processing devices; for example, on their mobile phone and on their smart watch. The styler 1 is in communication with the smart processing device 210 via their respective communications circuitry 27, 211. Via this connection, data relating to each styling session can be sent to the smart processing device 210 running the app. This data is typically raw styler data, such as the heat demand profile overtime (as illustrated in Figures 27, 19b and 20), etc.; in which case the processor 214 of the device 210 analyses the data according to instructions encoded in the app in order to determine parameters and scores, such as those outlined in previous sections. In some implementations, the controller 28 of the styler 1 may perform some analysis on the raw data before it is transmitted to the smart processing appliance 210. In some implementations, the data may be processed by the cloud-based processor 224. The relevant one or more processors stores the parameters and scores to the user profile 226. The user can link more than one styler to their user profile 226, such that data relating to all of their stylers is collected and saved to their profile 226. For example, a person may own and style their hair with both a hair straightening appliance and a curling tong; data from both of these appliances can be processed and saved to the same user profile 226 (for example automatically saved to the same user profile 226, once these devices have been linked to that profile). The user can also input which interchangeable styling ‘heads’ they have for those stylers. This can aid in building a general picture of the health of the user’s hair. The application (or ‘app’) is configured to provide feedback to a user regarding hair damage, potentially damaging behaviours and / or styling technique. This is typically based on metrics which are defined and determined by the methods described above. On a most basic level, the app can output a simple alert if any of the metrics or scores is out of range of what has been defined to be acceptable values. The alert may be a written warning (such as a push notification), a visual alert (such as flashing of the LED display), an audio alert, or a haptic alert. Alternatively or additionally, the app is configured to output more detailed information on the hair health and styling performance. For example, the individual scores (behaviour score, hair heat score etc.) can be output to a user directly to provide feedback regarding the user’s styling technique and the hair health. Either in addition to or in combination with outputting the individual scores, the overall hair score may be output to a user directly. The scores may be output after each styling session and / or there may be output a weekly, monthly etc. ‘review’ of how the user has used the styler, their hair condition, and / or styling technique (and improvements thereto). Figure 24 shows an exemplary graphical user interface 300 of the app. As shown in Figure 24, the interface 300 provides a weekly summary of the total amount of time a styler has been turned on and breaks this into the time over which heat has been on hair, the styler is moving, and the styler is idle. This is provided both graphically, as a pie chart 302, and verbally in the key 304. The app interface 300 also displays the overall user score 306, with a selectable link 308 to tips regarding how the user can improve their score. The interface 300 may further comprise a ‘persona’ 310 related to their score for that week. In some implementations, the app may output the individual determined scores, such as behaviour score, hair heat score, etc., typically in addition to the overall score. By way of example, the app interface may include a selectable link to a breakdown of the individual scores. In some implementations, the scores will be output with corresponding direct feedback and advice regarding how to improve their technique. By way of example, this may include a message being output regarding how to improve hair health, such as: “Move the styler more slowly for improved hair health”. It may also include a message being output regarding styling technique, for example: “Don’t fully dry your hair for a tighter curl”. In some implementations, the advice is output while the user is styling their hair, in order to give real-time feedback so that the user can adjust their styling technique. The app is typically configured to store information regarding a user’s previous hair condition and styling techniques, for example by storing a log of the metrics and scores for each use. This typically comprises the processor 214 of the smart processing device 210 sending the information to the cloud-based database 222 to store the information to the user’s profile. In this manner, the app can ‘track’ the user’s hair health and / or styling technique over all the styling sessions, and update after each use. This can be used to output personalized recommendations to the user. It can also be used to determine and output to a user any improvement (or otherwise) in their hair health and / or styling technique. The data detected and stored to the user profile 226 during use of the styler(s) can further be processed to determine and track the user’s favourite styles. This may be in dependence on inputs by the user via a styler user interface 11 and / or the user interface 213 of the smart processing device 210; alternatively, one or more processors may detect the users most frequently used styles based on data from the IMU 12 and / or temperature profiles of the heaters 6. Tailored recommendations can be output in dependence on this information, for example other hairstyles which may be of interest to the user. Furthermore, in some implementations the app is configured to output suggestions for further styles based on information about the user. This information may typically comprise a combination of data input by the user (for example age, described hair type, estimated hair damage, etc.) and metrics determined during use (for example, styling frequency, determined hair type, determined hair damage, etc.). If multiple stylers are linked to the same profile or account, then one or more processors, implementing instructions encoded in the app, can analyze the cumulative effect of their use and create and output recommendations accordingly. This is typically based on a knowledge bank stored in the cloud-based database 222, which comprises knowledge regarding particular styles and styling techniques in relation to different hair types and in relation to different tastes of users. The database 222 stores the user profiles as a graph-database, linking products, behaviours, and ‘personas’ to user profiles 226. Using a clustering algorithm, users are grouped into ‘personas’. Figure 25 illustrates a relatively simple two-dimensional algorithm, which clusters users into three different personas in dependence on hair length and age. A clustering algorithm can also be implemented in a higher dimensional space using additional features such as hair type, average styling time, favourite styles, hair colour, phone model, favourite products, location, styling frequency, usage behaviour, etc. The clustering algorithms are used to offer tips, styles, and products that benefit similar users. In this manner, the app outputs recommendations based on what similar users are doing. In some implementations, the app may have access to the user’s calendar or comprise a calendar itself to schedule practice sessions for a styling technique and / or to advise on frequency of heat styling. Figure 26 shows an example of such a calendar; in the illustrated scenario, a user wants to practice a particular style for a wedding in a couple of months. The calendar schedules fifteen practice runs (indicated by patterned dates) and a final dress rehearsal (indicated by the hatched date) before the wedding (indicated by the cross-hatched date). The app comprises instructions to output a push notification to the user interface 213 of the smart processing device 210 to remind a user to practice the style. The data (including the metrics) from the styling sessions is saved to the user profile 226 to aid in tracking their progress. In some implementations, the user can upload photos of the finished style after each session to aid in visually tracking their progress over time. The photos can typically be uploaded from the photo storage of the smart processing device 210 to the user profile 226, which is saved on the cloud-based database 222. Additionally, the app may be configured to allow a user to control styler parameters directly; for example, setpoint temperature, idle switch off time, LED display colours, etc. The relevant instructions may be stored in the control modules 217 of the smart processing device 210. The processor 214 of the smart processing device 210 can then send these instructions relating to the settings to the controller 28 of the styler 1. In some implementations, the settings are determined in dependence on the analysis of data from the styler 1, for example in dependence on the determined metrics and scores. By way of example, the processor 214 of the smart processing device 210, implementing the instructions encoded in the app, may instruct the controller 28 of the styler 1 to amend the temperature setpoint of the heaters 6 in dependence on the determined scores and metrics. For example, if the analysis determines a high level of damage to hair, the temperature setpoint may be lowered correspondingly. Furthermore, in some implementations, the app is configured to be connectable to social media and / or a community platform and / or webpage, to enable users to interact with one another remotely via their own individual smart processing devices, for example sharing tips and photos. This connection is typically implemented via an internet connection. The community platform and / or webpage may form a networking platform within the app itself. Social media and / or the community page typical comprise capability to enable a user to upload ‘style recipes’, which can then be saved by other users to their own profiles, so that they can then also follow them. In some implementations, the styler 1 communicates with the smart processing device 210 to open the app or output a prompt (such as a push notification) upon connection to the styler 1. Some smart devices may have security measures that prevent external devices from opening apps automatically; in which case, a prompt can be output to open the app. Other smart devices may be configured such that the styler 1 can directly open the app (this may be in the ‘background’). The styler 1 is typically configured such that it can communicate the relevant instructions to a connected smart processing device 210. The communication between the styler 1 and the app on the smart processing device 210 is configured to enhance the ease of use. For example, the app can indicate whether the styler 1 is turned on or off. Additionally, when the smart processing device 210 goes out of a predefined range of the styler 1, the styler 1 can be configured to turn off automatically. The app can indicate when the styler 1 was last turned off, for example via a relevant tab within the app (for example, an “is my styler off?” tab). Typically, the smart processing device 210 may be a user’s mobile phone and / or smart watch, and it can provide the user with peace of mind that the styler 1 automatically turns off when the smart processing device 210 goes out of range (for example, the user has left their house with the smart processing device 210) and that they can check when the styler 1 was last turned off. Typically, if the smart processing device 210 is out of the predefined range of the styler 1, the styler 1 can still be operated as normal; for example, it can be switched on, used, and switched off as normal (so that the phone does not have to be within the defined range of the styler 1 in order for the styler 1 to be used). 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 hair styling device 10 may comprise a single heater 6, or may alternatively comprise two or more heaters 6. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. The expressions “to dry hair”, “drying hair” or “decrease a moisture level of hair” and the like, as used in the present disclosure, can refer both to the removal of “unbound” water that exists on the outside of hair when wet, or the removal of “bound” water, which exists inside individual hairs, and which can be interacted with when heat styling hair. The “bound” water need not necessarily be removed when drying hair, although removal of some bound water may occur during a drying or styling process. In addition to or alternatively to the above-described methodologies for determining a value for the hair heat transfer coefficient, h, and so the damage metric, d, based on user inputs, the styler 1 may be configured to use measurements during styling to calculate values for these parameters directly rather than relying on such user inputs. As a general methodology, this may comprise the steps of: 1) At some time ti, the styler 1 determines the temperature of the hair tress; 2) The styler determines which heating zones 642 are providing heat to the hair at time ti by detecting which heating zones 642 dropped in temperature when hair was loaded in the styler 1; 3) The styler monitors the electrical power delivered to the heaters at time ti; 4) Assuming that the electrical power delivered to the heaters 6 approximately equals the thermal power delivered to the hair, the styler 1 determines a value for the hair heat transfer coefficient, h, by dividing the thermal power delivered at time ti by the area of the heating zones that are delivering heat to the hair and by the difference between the measured hair temperature and the temperature of the heater(s) 6 at time ti; 5) The styler then uses this determined value of the hair heat transfer coefficient, h, to determine a value for the hair damage, d, using the stored calibration data (for example, as shown graphically in Figure 14); 6) The hair damage information is then output to the user and / or used to control an operating parameter of the styler (such as the operating temperature of the heaters 6). According to this example, the heat transfer coefficient, h, can be calculated by the processor 29 by dividing the heat flux, q, by the temperature difference between the heater 6 and the tress of hair, ATm, according to: Based on the determined hair heat transfer coefficient, the processor 29 is then configured to determine a more accurate heat transfer coefficient, h, of the hair, which can be used to determine a more accurate value of the damage metric, d, using the stored calibration data (such as that displayed graphically in Figure 14). In some implementations, the styler 1 may be configured to define the damage parameter with respect to heat capacity and / or specific heat capacity of the hair. In such an implementation, the same methodology as previously described, using user input data, can be performed, but using calibration data relating the damage metric, d, to the heat capacity, C, and / or specific heat capacity, c. Again, this may be a linear or non-linear relationship, and the calibration data may comprise look-up reference tables and / or equations. If the heaters 6 are in contact with the hair, the expected change in hair temperature, AThair, from the power input can be determined from: P ■ At Where P is the power delivered over a time interval, At, and AThair is the temperature difference over this time interval of the tress of hair, which has a mass mhair. This equation can then be used by the controller 28 to determine an estimated change in temperature, AThair, of the hair tress within interval At. This is based on an estimated value of the specific heat capacity, c, determined from calibration data relating specific heat capacity to the value of damage metric, d, based on user input. Once the heaters 6 are applied to the hair, temperature measurements can be used to determine the actual temperature difference of the hair, ATdet, from two temperature measurements made by the temperature measurement circuitry 25 at the interval At. When making these calculations, the controller 28 may use a predefined average value for the mass of the tress of hair, mhair. The chosen predefined value may be dependent on user input, for example, dependent on the type of hair a user has indicated that they have or it may be a factory set value for a typical tress size. The controller 28 then compares the estimated value of AThair to the actual determined temperature change, ATdet and if the difference is greater than a threshold, then the controller 28 can determine a more accurate value for the specific heat capacity, c, using the equation as above. This can then be used to determine a more accurate value of the damage metric, d, using calibration data relating the specific heat capacity, c, and the damage metric, d. Alternatively, in some implementations, the styler 1 may skip the estimation steps and simply determine a value for the specific heat capacity using the equation above and measured values for the temperature difference of the hair, AThair, and power measurements, P, and estimated tress mass, mhair. In some implementations, the controller 28 may be configured to determine the heat capacity, C, of the tress of hair (in addition to or alternatively to the specific heat capacity, c), according to: P-kt C = ^T~ hair This avoids the need to use an estimate of the tress mass, mhair, but effectively makes the assumption that all tresses will have a similar mass. This may be used as a reasonable approximation for other tresses of the same user’s hair, based on an assumption that the tresses are likely to be of a similar size (and so similar mass). As described for the specific heat capacity, c, the heat capacity, C, may be used in a methodology based on estimation and then altering of damage metric, d, in dependence on measured values and / or it may simply be used to determine a damage metric value, d, directly. In general, the styler 1 is configured to determine (and / or determine a metric indicative of) the manner in which hair absorbs heat from a heater. It is then typically configured to define and / or determine an associated damage parameter or metric. This is based on the principal that more damaged hair absorbs more heat (reaches a higher temperature for a particular heating profile). Different methodologies can be used to provide an indication of how hair responds to heat applied to it, for example including determination and / or estimation of: heat transfer coefficient, heat capacity, and / or specific heat capacity. In some implementations, the controller and / or processors may define the values of the constants used in determination of the ‘scores’ (i.e. constants A to H) in dependence on information input by the user and / or determined by components of the styler. For example, this may relate to hair type and / or original hair condition. 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 apparatus comprising:a heater for heating a tress of hair;drive circuitry for providing power to heat the heater;sensor circuitry for providing sensor data indicative of at least one selected from the group consisting of: power delivered to the heater; the temperature of the tress of hair being dried and / or styled; and the temperature of the heater; anda controller for controlling the drive circuitry to deliver power to the heater in dependence upon sensor data provided by said sensor circuitry;means for determining at least one hair health metric in dependence upon sensor data provided by the sensor circuitry; andmeans for performing a control action in dependence on the determined at least one hair health metric.

2. The apparatus of claim 1, wherein the control action comprises controlling power and / or temperature settings of the heater, preferably controlling the setpoint temperature of the heater.

3. The apparatus of claim 1 or 2, wherein the control action comprises outputting an alert, preferably wherein the alert comprises at least one selected from a group consisting of: a visual output; an audio output; a haptic output; verbal instructions; and a combination thereof.

4. The apparatus of any preceding claim, comprising means for: i) determining a heat transfer coefficient between the heater and the tress of hair in dependence upon sensor data provided by the sensor circuitry; ii) determining at least one hair health metric from the determined heat transfer coefficient; and iii) performing a control action in dependence on the determined at least one hair health metric.

5. The apparatus of claim 4, wherein the means for determining at least one hair health metric from the determined heat transfer coefficient is configured to retrieve prestored calibration data relating heat transfer coefficient to hair health metric, and to use the retrieved calibration data to determine the at least one hair health metric from the determined heat transfer coefficient.

6. The apparatus of any preceding claim, comprising means for: i) determining a heat capacity of the tress of hair in dependence upon sensor data provided by the sensor circuitry; ii) determining at least one hair health metric from the determined heat capacity; and iii) performing a control action in dependence on the determined at least one hair health metric.

7. The apparatus of claim 6, wherein the means for determining at least one hair health metric from the determined heat capacity is configured to retrieve prestored calibration data relating heat capacity to hair health metric, and to use the retrieved calibration data to determine the at least one hair health metric from the determined heat capacity.

8. The apparatus of any preceding claim, wherein the controller is configured initially to control the drive circuitry to deliver power to the heater in dependence on at least one of: data input by a user; and a predetermined initial value.

9. The apparatus of claim 8, comprising means for estimating a hair health metric in dependence on the data input by a user.

10. The apparatus of claim 9, comprising means for estimating a heating response of the tress of hair in dependence on the estimated hair health metric, and means for comparing the estimated heating response to a detected heating response detected from sensor data, and preferably wherein the controller is configured to control the drive circuitry to deliver power to the heater in dependence on the comparison of estimated heating response and detected heating response.

11. The apparatus of claim 9 or 10 when dependent on any of claims 4 to 7, comprising means for comparing the estimated hair health metric and the determined hair health metric, and wherein the performing a control action comprises controlling the drive circuitry to deliver power to the heater in dependence on a difference between the estimated hair health metric and the determined hair health metric.

12. The apparatus of any preceding claim, wherein the heater comprises a plurality of heating zones and the sensor circuitry is configured to provide sensor data for the plurality of heating zones, and further comprising means for determining which heating zones are in contact with the tress of hair in dependence upon the sensor data.

13. The apparatus of any preceding claim, further comprising means for determining at least one score based on the at least one hair health metric, wherein the at least one score relates to use of the appliance and / or hair health, and preferably further comprising means for determining an overall score using a weighted combination of scores.

14. The apparatus of any preceding claim, further comprising a user interface configured for outputting an alert, and preferably configured for facilitating input of information.

15. The apparatus of any preceding claim, comprising means for storing to a user profile the at least one hair health metric and / or the at least one score for each styling session of a user, preferably wherein the means for performing a control action in dependence on the determined at least one hair health metric is further configured to perform a control action in dependence on the at least one hair health metric over time.

16. The apparatus of any preceding claim, comprising means for processing the sensor data to identify heating of a tress of hair in dependence on power delivered to the heater.

17. The apparatus of claim 16, wherein the means for processing the sensor data is further configured to determine maximum hair temperature for each tress of hair by determining a difference between a maximum and minimum power delivered to the heater, and preferably further using a mean value of power delivered to the heater.

18. The apparatus of any preceding claim, comprising means for determining a maximum hair temperature for each tress of hair from sensor data indicative of the temperature of the tress of hair being dried and / or styled.

19. The apparatus of claim 17 or 18, comprising means for determining an average maximum hair temperature in dependence on a mean value of determined maximum hair temperatures for each tress.

20. The apparatus of any of claims 17 to 19, comprising means for defining new hair damage as a function of maximum hair temperature, preferably average maximum hair temperature.

21. The apparatus or system of claim 20 when dependent on claim 4, wherein the controller is configured to determine new damage with reference to the hair metric determined from the heat transfer coefficient.

22. The apparatus of any preceding claim, comprising means for processing the sensor data to identify repeated heating of a same tress of hair in dependence on the power delivered to the heater having a damped sinusoidal profile.

23. The apparatus of claim 22, comprising means for determining number and frequency of repeated heating of a same tress of hair by identifying and processing peaks of the damped sinusoidal power profile.

24. The apparatus of claim 22 or 23, comprising means for determining heat on hair time by summing intervals over which the damped sinusoidal power profile has a negative gradient.

25. The apparatus of any preceding claim, further comprising sensor circuitry for providing sensor data indicative of motion of the apparatus.

26. The apparatus of claim 25, comprising at least two arms moveable relative to one another, wherein at least one of the arms comprises the heater, and further comprising means for determining configuration of the arms in dependence on the sensor data indicative of motion.

27. The apparatus of claim 25 or 26, comprising means for determining hair on heat time by summing time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater, preferably in a closed configuration.

28. The apparatus of claim 27 when dependent on claim 24, comprising means for correlating the time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater and the intervals over which the damped sinusoidal power profile has a negative gradient.

29. The apparatus of any of claims 25 to 28, comprising means for determining recurrent heating of a tress of hair in dependence on determining, from the sensor data indicative of motion, motion above a threshold speed value coincident with time intervals over which the arms are determined to be in a configuration indicative of the tress of hair being loaded on the heater, preferably in a closed configuration.

30. The apparatus of any of claims 24 to 29, comprising means for comparing heat on hair time with one or more user metrics, preferably wherein the one or more user metrics comprises at least one of: hair length, hair health metric, hair damage metric.

31. The apparatus 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 such that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes:a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; andat least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer;wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; andwherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 15W / m.Kand greater than 0.1 W / m.K.

32. The apparatus of any preceding claim, comprising means for scheduling styling sessions to a calendar, preferably configured to output reminder alerts in dependence on the calendar.

33. The apparatus of any preceding claim, further comprising a database for storing at least one hair health metric to a user profile.

34. The apparatus of claim 33, wherein the database further comprises a database of a plurality of user profiles, and wherein the apparatus comprises means for performing a clustering algorithm to cluster user profiles in dependence on at least one metric, preferably in dependence on at least one metric from: age, hair length, hair type, average styling time, favourite styles, hair colour, phone model, favourite products, location, styling frequency, and usage behaviour.

35. The apparatus of claim 34, comprising means for performing control actions in dependence on analysis of user profiles in a same cluster, preferably wherein the performing control actions comprises outputting suggestions.

36. The apparatus of any preceding claim, comprising at least one hair drying and / or styling device and a smart processing device in communication, preferably further comprising a cloudbased processing unit in communication with the at least one hair drying and / or styling device and / or the smart processing device.

37. The apparatus of claim 36, wherein the hair drying and / or styling device is configured to send instructions to the smart processing device, preferably wherein the instructions comprise instructions to output an alert and / or preferably wherein the instructions comprise instructions to open and / or run an application on the smart processing device.

38. The apparatus of claim 36 or 37, wherein the hair drying and / or styling device is configured to turn off in dependence on the smart processing device moving out of a defined range of the hair drying and / or styling device.

39. A computer program product comprising computer implementable instructions for causing a programmable device to configure one or more processors to implement any of claims 1 to 38.

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