Heating plate for a contact styler

Abstract

A heating plate for the application of thermal energy to hair in a contact styler comprises a substrate; a plurality of electrical traces mounted on or in the substrate; and a heat application surface configured to contact a user's hair to deliver thermal energy to the user's hair, wherein: each electrical trace of the plurality of electrical traces comprises a material having a positive temperature coefficient of resistance; and the plurality of electrical traces are connected in parzallel. A contact styler such as a hair straightener or curling iron comprising the heating plate is also provided.

Description

[0001] HEATING PLATE FOR A CONTACT STYLER

[0002] BACKGROUND

[0003] When users apply hair straighteners to their hair, they will not always position them in the same area of the styling plates. This creates an uneven demand for heat at the surface of the plates. In most applications, heat is applied uniformly across the plate, so the area not in contact with hair will rise in temperature as no heat is dissipated there. This temperature inhomogeneity can cause issues, from exceeding allowed material temperatures on the hot area, or creating a risk of harm to the user, specifically damage to the hair which contacts the hottest parts of the heated area. In addition the useful power delivered to the hair is limited by the power wasted by heating the unused section of the plates.

[0004] The effects of hotspots can be partially mitigated by the addition of a heat spreader - an additional component adding space, cost and thermal mass. Alternatively a multi-zone heater element can measure and control different segments of the heater independently. This approach adds complexity to the heater design and adds electrical components.

[0005] SUMMARY

[0006] In order to reduce the heating inhomogeneity outlined above without relying on a heat spreader, the present invention proposes a solution which relies on a self-balancing effect arising from the use of materials with a positive temperature coefficient of resistance. Specifically, the present invention provides a heating plate for the application of thermal energy to hair in a contact styler, the heating plate comprising: a substrate; a plurality of electrical traces mounted on or in the substrate; and a heat application surface configured to contact a user’s hair to deliver thermal energy to the user’s hair, wherein: each electrical trace of the plurality of electrical traces comprises a material having a positive temperature coefficient of resistance; and the plurality of electrical traces are connected in parallel.

[0007] When a user’s hair is in contact with a region of the heating plate, the thermal energy from that region of the plate will be transferred to the user’s hair via conduction. As a result, that region of the plate will cool down. Typically, in devices comprising a single heater trace, the control electronics will respond by applying more power to maintain the average temperature of the heated plate. However as the heat is applied uniformly across the surface, causing excessive temperatures in those areas where the fair is not in close thermal contact with the plate.. However, according to the present invention, because the heating plate comprises a plurality of electrical traces connected in parallel, if the temperature in the region of e.g. one of the electrical traces is reduced, the resistance of that electrical trace will decrease (because the traces are made from material having a positive temperature coefficient of resistance). As a result of this, and because the traces are connected in parallel to the power supply, the current in that trace, and the power drawn by that trace will also increase, leading to the temperature of that trace increasing again, essentially replenishing the region of the heating plate in contact with the user’s hair with heat. This means that the heat treatment of the user’s hair continues to be effective, without a runaway increase in temperature in those regions of the heating plate which are not in contact with the user’s hair. This may be referred to herein as a “self-balancing” effect.

[0008] Before discussing various optional features of the invention, we discuss some of the terms used above.

[0009] The first aspect of the invention is directed towards a heating plate for a contact styler. Herein, “contact styler” is used to refer a hair styling appliance which applies some kind of treatment to a user’s hair by bringing the hair into contact with a heated element, in this case the heating plate. Examples of contact stylers include hair straighteners, curling tongs, and curling irons, though it will be appreciated that this is not an exhaustive list. Contact stylers work in much the same way as clothes irons, by drawing the hair into a desired shape, and using heat to maintain the hair in that shape. The application of heat to the hair is also able to impart an attractive sheen to the hair for a “shiny” look. It will therefore be appreciated that it is important that a high temperature is maintained in the regions of the plate which are in contact with the hair, which is achieved by the present invention.

[0010] Herein, “electrical traces” refer to any conductive traces which are able to convey electric current from one place to another. The electrical traces may alternatively be referred to as heating traces, in view of the fact that when an electrical current flows through the electrical traces, they heat up, thereby heating up the surrounding substrate, and the user’s hair. It is stated that the electrical traces are mounted “in or on” the substrate. This refers to arrangements in which the traces are formed on top of the substrate, arrangements in which the traces are fully embedded within the substrate, and intermediate arrangements in which, for example, the traces partially embedded within the substrate, but a top surface of the trace is exposed. The use of the term “mounted” should not be understood to limit the method of manufacture of the heating plate. Heating plates formed by any methods fall within the scope of the invention. For the avoidance of doubt, the term “substrate” is used to refer to any component on which or in which the electrical traces may be mounted, as outlined earlier in this paragraph.

[0011] The more electrical traces in the plurality of electrical traces, the greater granularity with which different power may be drawn by different parts of the heating plate. For example, if there are two electrical traces, the degree of wasted energy used to heat up regions of the heating plate which are not in contact with the user’s hair is likely to be higher than if there are three electrical traces. In other words, an increased number of electrical traces leads to an increased degree of localized control. Accordingly, the plurality of electrical traces may comprise at least three electrical traces, at least four electrical traces, or at least five electrical traces. If there are too many traces, the arrangement of traces within the heating plate may become too complex. Accordingly, the plurality of electrical traces may comprise no more than seven electrical traces, no more than eight electrical traces, no more than nine electrical traces, no more than ten electrical traces, no more than eleven electrical traces, or no more than twelve electrical traces.

[0012] In some cases, at least a subset of the electrical traces may be arranged in pairs on or in the substrate. In other words, at least a subset of the plurality of electrical traces may be connected together in pairs. In some cases, all of the plurality of electrical traces may be connected together in pairs. By connecting the traces in pairs, it is possible to include more traces without needing to increase the number of connection points, thereby simplifying the electronics. Herein, “connected in parallel” takes its usual meaning in the field of electronics. Components which are connected in parallel are connected along separate electrical paths, such that each component (more specifically, the total voltage of the components in each electrical path) has the same voltage across it, which is the voltage across the whole network. The current through the network is equal to the sum of the current in each electric path.

[0013] The term “heat application surface” refers to any surface via which heat may be transferred from the electrical traces to the user’s hair. The heat application surface may comprise at least a surface of the substrate (e.g. in those arrangements in which the electrical traces are fully embedded in the substrate). Alternatively, the heating plate may further comprise a heat transfer body or heat transfer cover which is located on a surface of the substrate, the heat transfer body or heat transfer cover being configured to conduct heat from the electrical traces and the substrate to the user’s hair. The heat application surface may comprise, or may be, an exposed surface of such a heat transfer body or heat transfer cover. A heat transfer body or heat transfer cover may be particularly useful in arrangements in which the electrical traces are exposed at the surface of the substrate, where it may be desirable to avoid direct contact with the hair.

[0014] The “temperature coefficient of resistance” or “thermal coefficient of resistance” of a material is a measure of how much its resistance changes with temperature. A positive temperature coefficient of resistance indicates that the resistance of the material increases with increased temperature. The temperature coefficient of resistance a is generally given by:

[0015] 1 R - Roa = -

[0016] Ro T - TQ

[0017] In which:

[0018] Rois the resistance at To

[0019] R is the resistance at T The temperature coefficient of resistance is measured in units of K'1.

[0020] In order to achieve a sufficiently high temperature dependence on resistance, the plurality of electrical traces may be made from a material having a temperature coefficient of resistance of at least 0.0025 K'1, at least 0.0030 K'1, at least 0.0035 K'1, at least 0.0040 K'1, or at least 0.0045 K'1. The greater the temperature coefficient of resistance, the greater the self-balancing effect. The temperature coefficient of resistance itself may vary with temperature: the given values therefore correspond to the temperature coefficient of resistance as measured at 25 °C, although it will be noted that the value of the temperature coefficient of resistance varies very slowly with temperature.

[0021] The electrical traces may be metallic electrical traces, and may comprise, for example tungsten, iron, platinum, nickel, or suitable alloys thereof..

[0022] The substrate may comprise ceramics such as alumina or aluminium nitride, or high temperature plastics such as polyamide (Kapton) or LCP plastic.

[0023] We now discuss the geometric arrangement of the electrical traces in the heating plate.

[0024] The heat application surface may be elongate, and may have a longitudinal axis and a transverse axis. The longitudinal axis may define a longitudinal direction, and the transverse axis may define a transverse direction. The longitudinal axis may be perpendicular or substantially perpendicular to the transverse axis. And, accordingly, the longitudinal direction may be perpendicular or substantially perpendicular to the transverse direction.

[0025] The heat application surface, as defined previously, may comprise a plurality of heating zones. Each heating zone may be defined by a respective trace of the plurality of electrical traces. Each heating zone may be approximately the same area (i.e. the same size). Each heating zone may be approximately the same shape. By having uniform heating zones in this manner, a more uniform self-balancing effect may be achieved across the heat application surface. The number of heating zones corresponds to the number of electrical traces.

[0026] In use the contact styler may be drawn through a user’s hair such that, relative to the heat application surface, the hair moves in the transverse direction. In such cases, the heat application surface may be planar, and may be rectangular or substantially rectangular. The rectangular heat application surface may have a long side corresponding to the longitudinal axis or the longitudinal direction, and a short side corresponding to the transverse axis or the transverse direction. In these implementations, each heating zone may extend across the heat application surface in the transverse direction. For example, the heating zones may extend, in the transverse direction, from a first side of the heat application surface to a second side of the heat application surface, the first side of the heat application surface being transversely opposite from the second side of the heat application surface. The plurality of heating zones may be distributed along the heat application surface in the longitudinal direction. The arrangement of heating zones described in this paragraph is advantageous for heating plates for e.g. hair straighteners, in which the hair is drawn across the heat application surface in a transverse direction. This is because the heat lost from the heating plate to the hair varies longitudinally in these arrangements, because the hair would only contact the heat application surface in a small longitudinal region. In contrast, the hair is likely to be in contact with the heating plate across the full transverse extent of the heat application surface, so the heat lost from the heating plate to the hair is constant, or approximately constant in the transverse direction.

[0027] Each electrical trace may comprise an input terminal at a first end of the electrical trace and an output terminal at a second end of the electrical trace, the first end of the electrical trace being opposite from the second end of the electrical trace. In these cases, the first end of the electrical trace may be located on a first side of the heat application surface, and the second end of the trave may be located on a second side of the heat application surface, wherein the first side of the heat application surface is transversely opposite from the second side of the heat application surface. In other cases, rather than being rectangular and / or planar, the heat application surface may be cylindrical or substantially cylindrical. When the heat application surface is cylindrical, the longitudinal axis of the heat application surface may be the long axis of the cylinder. Similarly, when the heat application surface is cylindrical, the transverse axis of the cylinder may be the circumferential axis of the cylinder. The heat application surface may not be perfectly cylindrical. For example, the heat application surface application surface may be slightly conical or frustoconical. In these cases, the longitudinal axis corresponds to, or is defined by the imaginary line which may be formed by connecting the centres of two circles which intersect the cone or frustum. The angle between the conical surface and the long axis of the conical or frustoconical axis is preferably no more than 20° and more preferably no more than 10°. This geometry is particularly useful for e.g. curling irons or curling tongs, in which the hair is wrapped around a heated central element in order to heat it while in a curled shape.

[0028] The above disclosure relates to a heating plate for a contact styler. A second aspect of the present invention provides a contact styler comprising the heating plate of the first aspect of the invention. The optional features set out above in respect of the first aspect of the invention also apply equally well to the second aspect of the invention, except where clearly technically incompatible, or where context clearly dictates otherwise.

[0029] The heating plate may be a first heating plate, and the contact styler may comprise a second heating plate according to the first aspect of the invention. The first heating plate and the second heating plate may face each other. The contact styler may be movable between a first position in which the first heating plate and the second heating plate are spaced apart from each other, and a second position in which the first heating plate is in contact with the second heating plate. In this way, the user may open and close the contact styler in order to capture the hair between the first heating plate and the second heating plate. The contact styler may be a hair straightener, a hair curler, a curling iron, or curling tongs.

[0030] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] • Fig. 1 shows a schematic of a heating plate according to the first aspect of the invention.

[0032] • Fig. 2 shows a schematic of a heating plate according to the first aspect of the invention.

[0033] • Fig. 3 is a diagram illustrating the energy flows through a heating zone of a heating plate according to the first aspect of the invention.

[0034] • Figs. 4 and 5 show a pair of graphs illustrating the temperature at various points of a heating plate with and without the self-balancing effect provided by the present invention.

[0035] DETAILED DESCRIPTION

[0036] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0037] We begin by describing the structure of some exemplary implementations of heating plates, and then discuss the operation of the heating plates. We finish with some experimental results demonstrating the efficacy of heating plates according to the present invention.

[0038] Fig. 1 shows an example of a heating plate 100 according to the first aspect of the invention. The heating plate 100 is rectangular, having a long side 101a and a short side 101b. A longitudinal axis x is defined in the direction of the long side 101a, and a transverse axis j' is defined in the direction of the short side 101b. The heating plate 100 comprises three heating zones 102a, 102b, 102c, which extend transversely across the heating plate 100, and are distributed longitudinally along the heating plate 100. Each heating zone 102a, 102b, 102c, includes an electrical trace 104a, 104b, 104c. The electrical traces 104a, 104b, 104c are serpentine traces as illustrated in Fig. 1, but it will be acknowledged that the electrical traces 104a, 104b, 104c may take any suitable configuration. Each heating zone 102a, 102b, 102c is defined by its respective electrical trace 104a, 104b, 104c, such that the outer boundaries of the electrical traces 104a, 104b, 104c define an area which is substantially the same as the area of the respective heating zone 102a, 102b, 102c. The electrical traces 104a, 104b, 104c are mounted on or in substrate 103, which may be a ceramic substrate. Herein, the electrical traces 104a, 104b, 104c may be mounted on the surface of substrate 103, may be completely embedded within substrate 103, or may be partially embedded within the substrate 103, for example such that an upper surface of the electrical trace 104a, 104b, 104c is flush with an upper surface of the substrate 103. Each electrical trace 104a, 104b, 104c includes a respective input terminal 106a, 106b, 106c and an output terminal 108a, 108b, 108c. The input terminals 106a, 106b, 106c and the output terminals 108a, 108b, 108c are connectable to suitable electrical contacts (not shown] of a contact styler, most likely but not necessarily during the manufacturing process, in order to enable the provision of electrical current to the heating elements 104a, 104b, 104c.

[0039] Fig. 2 shows an example of another heating plate 200. The heating plate 200 is rectangular, having a long side 201a and a short side 201b. A longitudinal axis x is defined in the direction of the long side 201a, and a transverse axis j' is defined in the direction of the short side 201b. The heating plate 200 comprises ten heating zones 202a-j which extend transversely across the heating plate 200, and are distributed longitudinally along the heating plate 200. Each heating zone 202a-j includes a respective electrical trace 204a- j. The electrical traces 204a-j are serpentine traces as illustrated in Fig. 2, but it will be acknowledged that the electrical traces 204a-j may take any suitable configuration. Each heating zone 202a-j is defined by its respective electrical trace 204a-j such that the outer boundaries of the electrical traces 204a-j define an area which is substantially the same as the area of the respective heating zone 202a-j . The electrical traces 204a-j are mounted on or in substrate 203, which may be a ceramic substrate. Herein, the electrical traces 204a-j may be mounted on the surface of substrate 203, may be completely embedded within substrate 203, or may be partially embedded within the substrate 103, for example such that an upper surface of the electrical trace 204a-j is flush with an upper surface of the substrate 203. Whereas in Fig. 1, each electrical trace 104a, 104b, 104c is connected to its own separate input terminal 106a, 106b, 106c and output terminal 108a, 108b, 108c, in Fig. 2, the electrical traces 204a-j are connected in pairs. Specifically, electrical traces 204a and 204b are connected to input terminal 206a, b and output terminal 208a, b, electrical traces 204c and 204d are connected to input terminal 206c, d and output terminal 208c, d, electrical traces 204e and 204f are connected to input terminal 206e,f and output terminal 208e,f, electrical traces 204g and 204h are connected to input terminal 206g, h and output terminal 208g, h, and electrical traces 204i and 204j are connected to input terminal 206i,j and output terminal 208i,j . The input terminals 206a, b, 206c, d, 206e,f, 206g, h, 206i,j and the output terminals 208a, b, 208c, d, 208e,f, 208g, h, 208i,j are connectable to suitable electrical contacts (not shown) of a contact styler, most likely but not necessarily during the manufacturing process, in order to enable the provision of electrical current to the heating elements 204a-j .

[0040] We now describe the operation of the heating plates 100, 200. As power is supplied to the electrical traces 104a-c, 204a-j via the input terminals 106a-c, 206a,b-i,j and output terminals 108a-c, 208a,b-i,j, a current flows through them. As a result of the current flowing through them, the temperature of the electrical traces 104a-c, 204a-j increases.

[0041] Fig. 3, which illustrates the flow of energy into and out of a heating zone, e.g. any of heating zones 102a-c and 202a-j. Within each heating zone 102a-c, 202a-j, electrical power is supplied via the input terminals 106a-c, 206a,b-i,j and output terminals 108a-c, 208a,b-i,j to the electrical traces 104a-c, 204a-j . The heat dissipated by the electrical traces 104a-c, 204a-j as a result of current flowing through them is then transmitted to the substrate 103, 203 by conduction. The heat is then lost via a number of mechanisms: heat flow to adjacent heating zones 104a-c, 204a-j, heat loss to the hair tress, and residual heat leak through the contact styler itself. Each of these heat losses may be modelled using Fourier’s Law:

[0042] . kAAT

[0043] Q— In which k is the thermal conductivity of the heater material, A is the cross-sectional area along the heating plate, I is the segment length, and A 7' is the temperature difference with the neighbouring segment.

[0044] The heat lost to adjacent heating zones 104a-c, 204a-j, and the residual heat lost to the product may be approximated as being constant along the length of the heating plate 100, 200. However, the heat lost from the heating zones 102a-c, 202a-j varies along the length of the heating plate 100, 200. This is because the heating plate 100, 200 is much longer (in the longitudinal direction x) than the typical width of a tress which passes over the heating plate 100, 200 in a hair styling operation.

[0045] As the heat from the heating plate 100, 200 is lost, its temperature decreases. According to the invention, the electrical traces 104a-c, 204a-j are made from a material having a positive temperature coefficient of resistance. This means that, as the temperature of the electrical traces 104a-c, 204a-j decreases, so does the resistance, according to the relationship:

[0046] The terms have been defined earlier in this application.

[0047] As a result of the decrease in resistance, the current flowing through the electrical traces 104a-c, 204a-j increases, and correspondingly, so does the power drawn by the electrical traces 104a-c, 204a-j. This can be seen by considering the relationship:

[0048] Where P is the power drawn by the electrical trace in question, and V is the (constant) voltage supplied to the electrical trace in question. This means that as heat is lost to the tress, and the temperature of the heating zone(s) 102a- c, 202a-j in contact with the tress decreases (thereby reducing the effectiveness of the contact styler), the resulting decrease in resistance causes the heating zone(s) 102a-c, 202a- j to draw more power, thereby providing a self-balancing effect. Consider, in contrast, a heating zone 102a-c, 202a-j over which a tress of hair does not pass: in this case, there will be no reduction in temperature, and the resistance remains high. No additional power is therefore drawn, and the temperature of such a heating zone 102a-c, 202a-j does not increase unnecessarily, thereby giving rise to wasted energy and risks of injury to the user.

[0049] By separating the heating plate 100, 200 into a plurality of heating zones 102a-c, 202a-j, each heating zone 102a-c, 202a-j comprising a respective electrical trace 104a-c, 204a-j, and the electrical traces 104a-c, 204a-j being connected in parallel, each electrical trace 104a-c, 204a-j is able to exercise the self-balancing regime described in detail above independently of the others, ensuring that only those heating zones 102a-c, 202a-j of the heating plate 100, 200 over which the tress of hair is passing draw more power from the power source in order to replenish the heat lost to the hair.

[0050] This technique was shown to be effective in simulations. Simulations were run using a heating plate 200 as illustrated in Fig. 2. In the simulations, the residual heat lost through the product was estimated to have a thermal resistance of 1000 KW'1per heating zone, i.e. 100 KW'1in total, taking approximately 2 W to maintain a 200 K elevation. The thermal resistance used to represent hair in contact with the heating plate is 15 KW'1per segment, which is around 1.5 KW'1total, taking approximately 133 W to maintain a 200 K elevation. Power is input into each heating zone 204a-j according to the output from a PID control, which uses as its input the average temperature of all 10 heating ones 204a-j, but is modulated by a term in 1ZR(7), where R(T) is the temperature dependence of the resistance. In reality, the power may be supplied at constant voltage and controlled using some form of pulse width modulation.

[0051] The electrical traces 204a-j were assigned a temperature coefficient of resistance of 0.0033 K'1, which is representative of tungsten traces in a co-fired ceramic heater. Bulk tungsten has a temperature coefficient of resistance of 0.0045 K'1, but the lower value is used to take account of the fact that it is in a co-fired ceramic heater. Each electrical trace 204a-j was assigned a room temperature resistance of 12 , for a total resistance across ten electrical traces 204a-j in parallel of 1.2 .

[0052] Simulations with uneven heat loads were applied. Specifically, simulations were then run in a scenario in which a heat load was applied to heating zones 202f-j, with no heat load applied to zones 202a-e. In a first case, the simulation was run assuming that the electrical traces 204a-j had zero temperature coefficient of resistance, i.e. in which there is no selfbalancing effect. In these cases, the maximum temperature of the heating zones 202a-e (i.e. the heating zones which do not experience the thermal load of the hair tress) was 313 °C, and the power drawn by the heating zones 202f-j which are in contact with the hair tress was 33W. These results are illustrated in Fig. 4. In a second case, the temperature coefficient of resistance was set at 0.0033 K'1, as discussed above. In this case, the maximum temperature of the heating zones 202a-e was reduced to 292°C, corresponding to an 8% decrease. The power delivered to the parts of the heater in contact with the hair tress increased to 40W, corresponding to a 25% increase. These results are illustrated in Fig. 5. This stimulation demonstrates the effectiveness of the self-balancing effect in (a) reducing the maximum temperature seen on the parts of the heating plate 200 where no hair tress absorbs the heat (to limit risk of injury of discomfort, or damage to any small amounts of hair in that area), and (b) increasing the power delivered to the parts of the heater in contact with the hair tress, which increases the power actually being used to heat and style the hair.

[0053] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0054] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0055] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0056] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0057] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0058] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

CLAIMS1. A heating plate for the application of thermal energy to hair in a contact styler, the heating plate comprising: a substrate; a plurality of electrical traces mounted on or in the substrate; and a heat application surface configured to contact a user’s hair to deliver thermal energy to the user’s hair, wherein: each electrical trace of the plurality of electrical traces comprises a material having a positive temperature coefficient of resistance; and the plurality of electrical traces are connected in parallel.

2. The heating plate of claim 1, wherein: the plurality of electrical traces comprises at least two electrical traces.

3. The heating plate of claim 1 or claim 2, wherein: the plurality of electrical traces comprises no more than ten electrical traces.

4. The heating plate of any one of claims 1 to 3, wherein: at least a subset of the plurality of electrical traces are connected together in pairs.

5. The heating plate of any one of claims 1 to 4, wherein: the plurality of electrical traces are made from a material having a temperature coefficient of resistance of at least 0.0025 K'1.

6. The heating plate of claim 5, wherein: the plurality of electrical traces are made from tungsten, iron, platinum, nickel, or suitable alloys thereof.

7. The heating plate of any one of claims 1 to 6, wherein: the substrate is made from alumina, aluminium nitride, polyamide or LCP plastic.

8. The heating plate of any one of claims 1 to 7, wherein: the heat application surface is elongate and has a longitudinal axis and a transverse axis defining, respectively, a longitudinal direction and a transverse direction.

9. The heating plate of any one of claims 1 to 8, wherein: the heat application surface comprises a plurality of heating zones, each heating zone defined by a zone-defining portion of a respective electrical trace of the plurality of electrical traces.

10. The heating plate of claim 9, wherein: each heating zone extends transversely across the heat application surface, and the plurality of heating zones are distributed longitudinally along the heat application surface.

11. The heating plate of claim 9 or claim 10, wherein: the heat application surface is planar and rectangular, with a long side corresponding to the longitudinal axis and a short side corresponding to the transverse axis.

12. The heating plate of any one of claims 1 to 11, wherein: each electrical trace comprises an input terminal at a first end of the electrical trace and an output terminal at a second end of the electrical trace; the first end of the electrical trace is located on a first side of the heat application surface, and the second end of the electrical trace is located on a second side of the heat application surface; and the first side of the heat application surface is transversely opposite the second side of the heat application surface.

13. The heating plate of any one of claims 8 to 11, wherein: the heat application surface is cylindrical; the longitudinal axis corresponds to a long axis of the cylinder; and the transverse axis corresponds to a circumferential axis of the cylinder.

14. A contact styler comprising the heating plate of any one of claims 1 to 13.

15. A contact styler comprising a first heating plate according to any one of claims 1 to 13 and a second heating plate according to any one of claims 1 to 13.

16. The contact styler of claim 14 or claim 15, wherein: the contact styler is a hair straightener or a curling iron.

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