Controlling personal care device based on skin hydration levels

Abstract

According to one aspect, there is provided a personal care device (500) comprising means (100) for determining a level of hydration in the skin of a subject. The device (100) comprises: a first electrode (102) arranged to contact the skin of a subject; a second electrode (104) arranged to contact the skin of the subject; a radio frequency, RF, generator unit (106) configured to provide an RF voltage between the first electrode and the second electrode at each of a plurality of different frequencies such that current can flow from the first electrode to the second electrode via the skin of the subject; a skin impedance measuring unit (108) configured to measure an impedance of the skin between the first electrode and the second electrode at each of a plurality of different frequencies; and a processing unit (110) in operative communication with the RF generator unit and the skin impedance measurement unit, the processing unit configured to determine a hydration level of the skin based on a gradient of a difference in impedance of the skin at each of the plurality of different frequencies. A control signal for controlling an operating parameter of the personal care device is generated based on the hydration level of the skin.

Description

Technical Field

[0001] The present invention relates to a personal care device and a method for controlling a personal care device based on a determined level of hydration in a subject's skin, and more particularly, to a method for controlling a personal care device based on a level of hydration in a subject's skin determined based on impedance data. Background Art

[0002] The safety and efficacy associated with personal care devices, such as skin treatment devices, can depend on the condition of the skin, such as whether the skin is moist or dry. Thus, the interaction between a personal care device and the skin can depend on the condition of the skin. For example, during use of a personal care device, such as during shaving, radio frequency energy can be used to provide a skin warming experience. The level of radio frequency energy delivered to the skin can depend on a range of factors, such as factors related to the skin condition, which may need to be considered when determining settings for radio frequency energy generation to maintain the safety of a subject to whom the personal care device is applied.

[0003] The present invention aims to address the issues of treatment safety and efficacy associated with moist and dry conditions of the skin. Summary of the Invention

[0004] The level of hydration of a subject's skin can affect parameters related to skin surface friction, optical coupling efficiency, electrical contact, and effective impedance. The object of the present invention described herein is to provide a method in which the level of hydration in a subject's skin can be determined such that the interaction between a personal care device, such as a skin treatment device, and the subject's skin can be adjusted based on skin hydration, thereby taking into account skin-related parameters.

[0005] According to a first specific aspect, there is provided a personal care device comprising means for determining a level of hydration in a subject's skin. The means for determining the level of hydration comprises: a first electrode arranged to contact the subject's skin; a second electrode arranged to contact the subject's skin; and a radio frequency (RF) generator unit configured to provide an RF voltage between the first electrode and the second electrode at each of a plurality of different frequencies such that current can flow from the first electrode, through the subject's skin, to the second electrode. The means for determining the level of hydration further comprises: a skin impedance measurement unit configured to measure the impedance of the skin between the first electrode and the second electrode at each of the plurality of different frequencies; and a processing unit operatively communicating with the RF generator unit and the skin impedance measurement unit. The processing unit is configured to determine the level of hydration of the skin based on a gradient of the difference in impedance of the skin at each of the plurality of different frequencies, and to generate a control signal based on the level of hydration of the skin to control an operating parameter of the personal care device.

[0006] A personal care device may include a motor. The operating parameters generated by the processing unit include parameters of the motor.

[0007] Alternatively or additionally, the personal care device includes an IR light source configured to heat the skin of the subject, and the operating parameters generated by the processing unit include parameters related to the IR light intensity of the IR light source.

[0008] In some embodiments, the personal care device includes a display element, and the operating parameters generated by the processing unit include parameters of the display element.

[0009] In some embodiments, the instruction signal includes a signal for adjusting the RF voltage provided between the first electrode and the second electrode.

[0010] In some embodiments, each of the plurality of different frequencies is in the frequency range of 0.5 MHz to 100 MHz.

[0011] In a preferred embodiment, each of the plurality of different frequencies is in the frequency range of 1 MHz to 10 MHz.

[0012] In some embodiments, the RF voltage includes a value in the range of 5 V to 30 V.

[0013] According to a second aspect, there is provided a computer-implemented method for generating a control signal to control a motor or an IR light source of a personal care device based on determining the hydration level in a subject's skin. The method includes:

[0014] Operating an RF generator to generate an RF voltage to be delivered between a first electrode and a second electrode at each of a plurality of different frequencies such that when the first and second electrodes are in contact with the skin, current can flow from the first electrode through the subject's skin to the second electrode;

[0015] Determining the impedance of the skin between the first electrode and the second electrode at each of the plurality of different frequencies; and

[0016] Generating a hydration level of the skin based on a gradient of differences in impedance of the skin at each of the plurality of different frequencies; and

[0017] Generating a control signal for controlling operating parameters of the personal care device based on the hydration level of the skin

[0018] The operating parameters may include parameters of a motor of the personal care device.

[0019] Alternatively or additionally, the operating parameters may include parameters related to the IR light intensity of an IR light source of the personal care device, the IR light source being configured to heat the skin of the subject, a means for determining the hydration level in the subject's skin.

[0020] These and other aspects will become apparent and be elucidated with reference to the (multiple) embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Exemplary embodiments will now be described by way of example only with reference to the following drawings, in which:

[0022] Figure 1 is a schematic diagram of an example of a device for determining the level of hydration in a subject's skin;

[0023] Figure 2 is a schematic diagram of an example of an electrode arrangement;

[0024] Figure 3 is a graph showing an example of how impedance varies with frequency for dry skin and moist skin;

[0025] Figure 4 is a graph showing another example of how impedance varies with frequency for dry skin and moist skin;

[0026] Figure 5 is a schematic diagram of an example of a personal care device;

[0027] Figure 6 is a schematic diagram of another example of an electrode arrangement; and

[0028] Figure 7 is a flowchart of an example of a method for determining the level of hydration in a subject's skin. DETAILED DESCRIPTION

[0029] The level of hydration of a subject's skin can vary over time, can be different for different regions of a subject's skin, and can be different for the skin hydration level of another subject. Determining the level of hydration of a subject's skin is useful for a variety of reasons, such as setting the operating parameters of a personal care device based on the determined skin hydration level. In particular, prior knowledge of the level of hydration of a subject's skin may be important in controlling the operating parameters of a device, such as a personal care device, to maintain the safety of the subject to whom the device is applied. For example, radio frequency energy may be applied to a subject's skin to provide a heating effect within the skin. However, it may be necessary to set or adjust the operating parameters related to radio frequency generation based on the skin hydration level to avoid the possible formation of hot spots and / or burns.

[0030] The electrical properties associated with a subject's skin can vary significantly with skin conditions, such as whether the skin is wet or dry, and thus measurements of the electrical properties associated with the skin can be used to determine the hydration level of the skin. For example, wet skin can have a relatively low impedance, while dry skin can have a relatively high impedance. The impedance can be different for different regions of the body skin (e.g., face, hands, etc.) and can be different for different people and skin anatomies. However, due to the factors affecting skin hydration mentioned above, the determination of impedance may not be a robust and reliable measure of skin hydration. Accordingly, an object of the present invention is to provide a robust and reliable way in which the hydration level of a subject's skin can be determined based on the electrical properties of the skin. More specifically, another object of the present invention is to provide operating parameters for an RF generator unit in a personal care device such that RF energy is delivered in a safe manner (e.g., such that a comfortable warming experience is provided to the subject to whom the personal care device is applied, thereby preventing the occurrence of hot spots and / or burns, etc.).

[0031] According to a first aspect, the present invention provides a personal care device 500 comprising means 100 for determining the hydration level in a subject's skin. Figure 1 An example of such means 100 included in the personal care device 500 is shown. The means 100 includes a first electrode 102 arranged to contact the subject's skin and a second electrode 104 arranged to contact the subject's skin. The electrodes 102, 104 may be referred to as probes or sensing RF skin contact electrodes. In some examples, the first electrode 102 and the second electrode 104 may be connected (e.g., electrically connected). The first electrode 102 and the second electrode 104 may be connected (e.g., via a radio frequency generator unit (described below)). In some examples, the means 100 may include three or more electrodes. For example, the means 100 may include a first electrode 102, a second electrode 104, and a third electrode ( Figure 1 not shown in the figure). The third electrode may be a ground electrode and / or may be located between the first electrode 102 and the second electrode 104. Figure 2An example of a schematic diagram showing an RF electrode configuration including a first electrode 102, a second electrode 104, and a third electrode 202 is shown. The first electrode 102 may be connected to the third electrode 202 and / or the second electrode 104 may be connected to the third electrode 202. In some examples, the first electrode 102 may be connected to the second electrode 104, and the first electrode 102 may be connected to the third electrode 202 and / or the second electrode 104 may be connected to the third electrode 202. The electrodes may have the same or different sizes. For example, the first electrode 104 and the second electrode may each have a size of 25 mm × 4 mm, and the third electrode 202 may have a size of 25 mm × 5 mm. The electrodes may be spaced the same or different amounts. For example, the first electrode 102 may be spaced 4 mm from the third electrode 202, and the second electrode 104 may be spaced 4 mm from the third electrode 202. In some examples, the size (e.g., width) or the shortest side of the electrodes may include values in the range of 0.1 mm to 10 mm. In some examples, the electrodes may include a rectangular shape, a square shape, a circular shape, etc.

[0032] The apparatus 100 further includes a radio frequency (RF) generator unit 106 configured to provide an RF voltage between the first electrode 102 and the second electrode 104 at each of a plurality of different frequencies such that current can flow from the first electrode through the skin of a subject to the second electrode. The RF generator unit 106 may include an RF energy source configured to generate RF energy. The RF generator unit 106 may be referred to as a probe RF generator unit for delivering probe RF energy to the probe RF electrodes. In other words, the RF generator unit 106 may generate RF energy for delivery through the first electrode 102 and the second electrode 104 to the skin of a subject, for example to increase the temperature of the subject's skin by approximately 1 to 4 °C. The RF voltage may include voltages in the range of 5 V to 30 V, 0 V to 30 V, 5 V to 50 V, etc. The voltage signal may include frequencies in the range of 0.5 MHz to 100 MHz, 1 MHz to 10 MHz, etc. In other words, the plurality of different frequencies may include frequencies in the range of 0.5 MHz to 100 MHz, 1 MHz to 10 MHz, etc. In some examples, the RF generator unit 106 may be configured to provide the RF voltage between the first electrode 102 and the second electrode 104 in a pulsed manner, for example each pulse having a pulse duration in the range of 10 ms to 100 ms, etc. The plurality of different frequencies refers to two or more different frequencies (e.g., 10 MHz and 50 MHz). In some examples, when using relatively small electrodes (e.g., <1 mm), a relatively high voltage may be applied to the electrodes.

[0033] In other words, the RF generator unit 106 is configured to supply a voltage between the first electrode 102 and the second electrode 104 such that an electric field can be generated or the electric field can be extended between the first electrode 102 and the second electrode 104. When the first electrode 102 and the second electrode 104 are in contact with the subject's skin (e.g., when the device 100 is in use), due to the polarization of macromolecules (e.g., proteins or cellular components) with respect to the electric field, the skin acts as a capacitor that stores charge. When the voltage of the electrodes is switched (e.g., the first electrode 102 can change from +10V to -10V and the second electrode 104 can change from -10V to +10V), the polarization changes and these charges are released, resulting in a current. This dielectric coupling of the skin reduces the impedance of the alternating current RF current, while the skin, as an insulator for DC current, has a higher impedance value. Additionally, the polarization change of the skin macromolecules results in dielectric loss, leading to dielectric heating. Therefore, when the voltage of the electrodes alternates at a high frequency (e.g., radio frequency), more significant heating can be achieved in the subject's skin.

[0034] Skin hydration can depend on the ability of the skin to bind water to the macromolecules of the skin (e.g., the ability to bind water to the keratinized tissue of the skin), which can lead to changes in the dielectric properties of the skin. Additionally, the skin can include water that is not bound to the molecules of the skin (e.g., bulk water), which can contribute to the ionic conductivity of the skin. Therefore, moist skin may be more conductive than dry skin. These changes in the electrical properties of the skin can increase conduction and dielectric heating. Therefore, it may be important to determine the level of skin hydration, for example, in applications where RF energy is applied, so that local heating and / or burning of the skin can be avoided.

[0035] The device 100 further includes a skin impedance measurement unit 108 configured to measure the impedance of the skin between the first electrode 102 and the second electrode 104 at each of a plurality of different frequencies. As previously described, the impedance of the skin can depend on skin hydration and can be determined based on measurements of current and voltage. Figure 3 is a graph showing an example of how impedance varies with frequency for dry skin (represented by line 300) and moist skin (represented by line 302) in the frequency range from 0.001 MHz to 1000 MHz when the first electrode 102 and the second electrode 104 of the device 100 are placed on the subject's skin. Figure 4 is showing the use of the same data as used in Figure 3 in the frequency range from 1 MHz to 10 MHz, for dry skin (line 300) and moist skin (line 302), an example of how impedance varies with frequency. In Figure 3 and Figure 4In the example shown, the resistive behavior of the current in the stratum corneum (i.e., the skin surface) predominates at low frequencies (e.g., <0.1 MHz), resulting in a plateau of high impedance. At Figure 3 and Figure 4 In the example shown, the capacitive coupling across the stratum corneum is maximum at high frequencies (e.g., >100 MHz), resulting in higher current passing towards the dermis, and thus the resistive behavior of the current in the dermis predominates, resulting in a plateau of low impedance. In other words, at low frequencies, the stratum corneum has a high impedance (e.g., the skin surface is highly resistive to current), and as the frequency increases, the impedance decreases (e.g., the skin surface is less resistive to current), and the sub-surface region of the skin predominates in terms of resistivity. At Figure 3 and 4 In the example of, for frequencies in the intermediate range (e.g., between 0.1 MHz and 100 MHz), the capacitive coupling gradually increases, which can be referred to as a transitional state. When the skin is moist, the skin hydrates, which can result in higher capacitive coupling, which in turn can shift the high-impedance to low-impedance transition towards lower frequencies. Thus, in this example, frequencies in the range of 1 MHz to 10 MHz can be closer to the flat plateau of low impedance, such that for this frequency range, the impedance appears relatively constant under moist conditions.

[0036] Apparatus 100 also includes a processing unit 110, which can communicate operatively with the RF generator unit 106 and the skin impedance measurement unit 108. The processing unit 110 is configured to determine the hydration level of the skin based on the gradient of the difference in the impedance of the skin at each of a plurality of different frequencies. For example, the gradient of the difference in the impedance of the skin at each of two different frequencies can be determined using the following equation:

[0037]

[0038] where Z1 is the impedance of the skin at frequency f1 and z2 is the impedance of the skin at frequency f2. In other words, the slope of the impedance as a function of frequency is determined. The gradient or slope of the impedance with frequency can be determined using a slope fitting algorithm, such as a least squares fitting routine, etc. In some examples, the skin can be classified according to Table 1:

[0039]

[0040] Table 1

[0041] In some examples, the apparatus 100 may include a memory for storing impedance data, skin hydration data, etc. The apparatus 100 may include a transmitter configured to transmit data (e.g., impedance data, skin hydration data, etc.) to a memory external to the device (e.g., a server located in the cloud, etc.). In some examples, a processor external to the apparatus 100 (e.g., an external processor, a processor located in the cloud, etc.) may be configured to determine the hydration level of the skin based on the gradient of the difference in the impedance of the skin at each of a plurality of different frequencies. The processor external to the apparatus 100 may be configured to receive data from a transmitter associated with the apparatus 100, from a memory external to the apparatus 100, etc.

[0042] The RF energy applied to the subject's skin may cause a small amount of heating in the subject's skin. However, the operating parameters of the apparatus 100 (e.g., the voltage applied to the electrodes, the duration of applying the RF energy to the subject's skin, etc.) may be set to avoid hot spots in the skin and / or burns to the skin. For example, applying the RF energy to the subject's skin for 1 second may be sufficient to obtain sufficient data (e.g., impedance data) such that the determination of the hydration level of the subject's skin can be made while avoiding excessive heating in the skin. It may be beneficial to achieve a balance between minimizing the skin heating effect due to the applied RF energy and maximizing the current to minimize the calculation error. For high skin impedance cases, such as when using very small electrodes (e.g., electrodes with a width less than 1 mm), a higher voltage may be preferred. In such examples, to minimize heating, pulses of RF energy with a pulse duration in the range of 10 ms and 100 ms may be used. In some examples, each electrode may have a width between 0.1 mm and 10 mm, an RF voltage between 5 V and 30 V may be used, and an RF frequency between 1 MHz and 10 MHz.

[0043] In some embodiments, the processing unit 110 may also be configured to generate an instruction signal for delivery to a receiving device based on the hydration level of the skin. The receiving device may be a personal care device (e.g., a personal care device including the apparatus 100). In some examples, the receiving device may be external to the apparatus 100 (e.g., a separate) device (e.g., an interactive mirror, a smart phone, a server, a wearable device, etc.). The instruction signal may include a control signal (e.g., a control signal for controlling the operating parameters of the apparatus 100), a signal causing the display of an element in the interactive mirror, etc.

[0044] In some embodiments, the instruction signal may include a signal for adjusting the RF voltage provided between the first electrode and the second electrode.

[0045] For example, a higher frequency (e.g., a higher RF frequency) can result in or be associated with lower skin impedance. Compared to a lower frequency, the lower skin impedance can result in a relatively greater heating effect (e.g., RF heating) (e.g., because for a relatively higher frequency compared to a relatively lower frequency, the current between the electrodes is relatively greater). The current between the electrodes can depend on the voltage applied to the electrodes (e.g., for a larger voltage difference or potential difference between the electrodes, the current between the electrodes may be higher). A relatively low voltage (e.g., an RF voltage) can be applied to the electrodes at a relatively high frequency to minimize heat generation in the skin. In some examples, increasing the current between the electrodes can improve the accuracy of determining the impedance of the skin between the electrodes. For example, the RF voltage can be increased to increase the current, and / or the effective resistance of the system can be reduced, e.g., by using larger electrodes. Increasing the current between the electrodes can result in a relatively greater heating effect (e.g., compared to a relatively lower current between the electrodes). Thus, the voltage applied to the electrodes can depend on the frequency of the voltage applied to the electrodes (e.g., the voltage and / or frequency can be selected based on a minimum desired accuracy level of the impedance). In some examples, a first RF voltage can be provided between the first electrode 102 and the second electrode 104 at a first frequency, and a second RF voltage can be provided between the first electrode 102 and the second electrode 104 at a second frequency. For example, the first RF voltage can be 10V and can have a first frequency of 1 MHz, and the second RF voltage can be 5V and can have a second frequency of 10 MHz.

[0046] Figure 5 An example of a personal care device 500 including the apparatus 100 is shown. The personal care device 500 can include a hair cutting device (e.g., a shaving device, an electric shaver, a beard trimmer, a hair trimmer, etc.), a skin care device (e.g., a skin tightening device, a skin rejuvenation device, a skin cleansing device, etc.), etc. The skin rejuvenation device can include components configured to provide or apply radio frequency energy to the skin. In some examples, the skin rejuvenation device can include a microdermabrasion device for the skin, etc. The skin cleansing device can include a mechanically rotating brush.

[0047] The processing unit 110 is configured to generate a control signal based on the hydration level of the skin to control the operating parameters of the personal care device.

[0048] In some embodiments, the operating parameters may include the operating parameters of the RF generator unit 106 (e.g., the voltage provided between the first electrode 102 and the second electrode 104, the duration for which the alternating voltage is applied to the electrodes (e.g., 1 second), etc.). Adjusting the operating parameters of the RF generator unit based on the determined skin hydration level may result in adjusting the level of heat generated in the skin, the skin temperature, the depth of heating within the skin, the heating rate of the skin, etc. For example, for relatively moist or hydrated skin, a lower voltage or lower potential difference may be provided between the electrodes because moist skin may be associated with a lower impedance, making RF heating more effective. Thus, for safety reasons, the operating parameters of the RF generator unit 106 may be adjusted.

[0049] In some embodiments, the personal care device may include a motor. The operating parameters may include parameters of the motor (e.g., the speed of the motor, the current supplied to the motor, the voltage supplied to the motor, etc.). Skin hydration can affect the skin surface friction, such that it may be desirable to vary the operating parameters of the motor of the personal care device (e.g., the speed of the cutting element of a hair cutting device). In some examples, the cutting element of a hair cutting device may require or desire a lower motor current because wet hair may be softer and easier to cut. Skin hydration can indicate how wet the hair is, and thus the motor current may be adjusted accordingly based on skin hydration.

[0050] In some embodiments, the personal care device may include an infrared, IR light source configured to heat the skin of a subject. The operating parameters may include parameters related to the IR light intensity of the IR light source. Skin hydration can affect the optical coupling efficiency of light (e.g., IR light) entering the skin, the level of light transmission between the light source and the skin, the level of light scattering between the light source and the skin, etc. IR light can be used to provide a skin heating effect. Relatively moist or hydrated skin may be associated with better optical coupling efficiency. Thus, for safety reasons, a lower light intensity may be used for relatively moist skin.

[0051] In some embodiments, the personal care device may include a display element. The operating parameters may include parameters of the display element. For example, the display element may include a light on the personal care device that indicates whether it is safe to use the device. For example, a red light may indicate that the device is not safe to use (e.g., may cause burns), while a green light may indicate that the device is safe to use.

[0052] In some examples, the personal care device 500 can include a hair trimming device (e.g., an electric beard trimmer), which includes a two-way beard trimmer unit mounted on a handheld housing unit and an RF delivery comb attachment including two-way guiding teeth and RF electrodes. The RF generator unit 106 can be mounted in the handheld housing unit. During use, the RF generator unit 106 can provide RF energy to the skin at two different RF frequencies (e.g., 1 MHz and 5 MHz) via the RF electrodes (e.g., the first electrode 102 and the second electrode 104), and the impedance is measured and recorded for these two RF frequencies. After a preset probing time (e.g., 1 second for each frequency), the recorded impedance data is used to determine the gradient or slope of the impedance with respect to frequency. If the gradient has an amplitude of approximately -150 Ω / MHz (e.g., between 140 Ω / MHz and 160 Ω / MHz, etc.), the condition can be considered dry. If the gradient amplitude is approximately -5 Ω / MHz, the condition can be considered moist. Figure 6 is a schematic diagram of another example of an electrode arrangement. The hair cutting device can include Figure 6 the electrode arrangement shown.

[0053] Figure 7 is a flowchart of an example of a computer-implemented method 700 for determining the hydration level in a subject's skin. In some examples, the processor can be configured to perform one or more steps of method 700. In step 702, method 700 includes operating an RF (radio frequency) generator to generate an RF voltage to be delivered between a first electrode and a second electrode at each of a plurality of different frequencies such that when the first electrode and the second electrode are in contact with the skin, current can flow from the first electrode through the subject's skin to the second electrode.

[0054] In step 704, method 700 includes determining the impedance of the skin between the first electrode and the second electrode at each of the plurality of different frequencies.

[0055] In step 706, method 700 includes generating a hydration level of the skin based on the gradient of the differences in the impedance of the skin at each of the plurality of different frequencies.

[0056] In some embodiments, method 700 can include generating an instruction signal for delivery to a receiving device based on the hydration level of the skin.

[0057] In some embodiments, the instruction signal can include a signal for adjusting the RF voltage provided between the first electrode and the second electrode.

[0058] Those skilled in the art, in practicing the principles and techniques described herein, can understand and realize variations of the disclosed embodiments by studying the accompanying drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may implement the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used advantageously. A computer program may be stored or distributed on a suitable medium, such as an optical storage medium or a solid-state medium provided together with other hardware or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A personal care device (500), comprising: means (100) for determining the hydration level in the skin of a subject, said means comprising: A first electrode (102) arranged to contact the skin of the subject; A second electrode (104) arranged to contact the skin of the subject; A radio frequency (RF) generator unit (106) configured to provide an RF voltage between the first electrode and the second electrode at each of a plurality of different frequencies such that current can flow from the first electrode, through the skin of the subject, to the second electrode; A skin impedance measurement unit (108) configured to measure the impedance of the skin between the first electrode and the second electrode at each of the plurality of different frequencies; And A processing unit (110) operably communicating with the RF generator unit and the skin impedance measurement unit, the processing unit being configured to: Determine a hydration level of the skin based on a gradient of differences in the impedance of the skin at each of the plurality of different frequencies, Wherein the processing unit (110) is further configured to generate a control signal based on the hydration level of the skin to control an operating parameter of the personal care device, Wherein the personal care device further includes a motor, and wherein the operating parameter includes a parameter of the motor, or Wherein the personal care device further includes an IR light source configured to heat the skin of the subject, and wherein the operating parameter further includes a parameter related to the IR light intensity of the IR light source.

2. The personal care device (500) according to claim 1, wherein the personal care device further comprises a display element, and wherein the operating parameter comprises a parameter of the display element.

3. The personal care device (500) according to claim 2, wherein the command signal comprises a signal for adjusting the RF voltage provided between the first electrode and the second electrode.

4. The personal care device (500) according to any one of the preceding claims, wherein each of the plurality of different frequencies is in the frequency range of 0.5 MHz to 100 MHz.

5. The personal care device (500) according to any one of claims 1 to 3, wherein each of the plurality of different frequencies is in the frequency range of 1 MHz to 10 MHz.

6. The personal care device (500) according to any one of the preceding claims, wherein the RF voltage comprises a value in the range of 5 V to 30 V.

7. A computer-implemented method (700) for generating a control signal to control a motor or an IR light source of a personal care device based on determining the hydration level in the skin of a subject, the method comprising: Operate (702) the RF generator to generate an RF voltage to be delivered between the first electrode and the second electrode at each of a plurality of different frequencies such that when the first electrode and the second electrode are in contact with the skin, current can flow from the first electrode, through the skin of the subject, to the second electrode; Determine (704) the impedance of the skin between the first electrode and the second electrode at each of the plurality of different frequencies; And Generate (706) a hydration level of the skin based on a gradient of differences in the impedance of the skin at each of the plurality of different frequencies; Generate a control signal for controlling an operating parameter of the personal care device based on the hydration level of the skin, wherein The operating parameter includes a parameter of a motor of the personal care device, or Wherein the operating parameter includes a parameter related to the IR light intensity of an IR light source of the personal care device, the IR light source being configured to heat the skin of the subject.

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