sensors Article

Monitoring Transepidermal Water Loss and Skin Wettedness Factor with Battery-Free NFC Sensor Syed Muhammad Ali

and Wan-Young Chung *

Department of Electronic Engineering, Pukyong National University, Busan 48513, Korea; [email protected] * Correspondence: [email protected] Received: 7 August 2020; Accepted: 26 September 2020; Published: 28 September 2020







Abstract: The transepidermal water loss (TEWL) and the skin wettedness factor (SWF) are considered parts of a key perspective related to skincare. The former is used to determine the loss of water content from the stratum corneum (SC), while the latter is used to determine the human skin comfort level. Herein, we developed two novel approaches: (1) determination of the TEWL and the SWF based on a battery-free humidity sensor, and (2) the design of a battery-free smart skincare sensor device tag that can harvest energy from a near field communication (NFC)-enabled smartphone, making it a battery-free design approach. The designed skincare device tag has a diameter of 2.6 cm and could harvest energy (~3 V) from the NFC-enabled smartphone. A series of experimental tests involving the participation of eight and six subjects were conducted in vivo for the indoor and outdoor environments, respectively. During the experimental analysis, the skin moisture content level was measured at different times of the day using an android smartphone. The TEWL and SWF values were calculated based on these sensor readings. For the TEWL case: if the skin moisture is high, the TEWL is high, and if the skin moisture is low, the TEWL is low, ensuring that the skin moisture and the TEWL follow the same trend. Our smart skincare device is enclosed in a 3D flexible design print, and it is battery-free with an android application interface that is more convenient to carry outside than other commercially available battery-based devices. Keywords: near field communication (NFC); transepidermal water loss (TEWL); skin wettedness factor (SWF); smart skincare sensor device tag; android application interface

1. Introduction The human body’s skin can be divided into three depending layers: the hypodermis layer, the overlying dermis and the epidermis. The epidermis is the outermost layer of our skin, comprising the inner and outer parts. The outer part is called the stratum corneum (SC) [1] (Figure 1). SC initiates the skin moisture; therefore, it is required to determine the level of skin hydration and other parameters related to its health. Previously, multimodal sensors with soft, ultrathin and skin-like formats were developed for monitoring the hydration level of the skin [2]. Similarly, a wireless epidermal sensor based on passive inductive coupling was reported [3], where the analyzer was connected to a hand-wound copper primary coil, used to measure the hydration of the skin. Recently, an ultra-thin, tape free e-tattoo type sensor was designed, which could measure three features from the human skin, i.e., an electrocardiogram (ECG), the skin temperature and the skin hydration [4]. Similarly, a machine-learning-based noninvasive solution has been purported that uses galvanic skin response (GSR) to detect the hydration level in the human body [5]. The measurements of the transepidermal water loss (TEWL) and the skin wettedness factor (SWF) have been given significant importance in many research areas like dermatology, pharmacology, clinical analysis and cosmetic science. In addressing dry skin problems, one could obtain information about the TEWL rate from the skin, in order to make a choice in selecting relevant cosmetic products. However, individual assessments based on feeling Sensors 2020, 20, 5549; doi:10.3390/s20195549

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the skin, in order to make a choice in selecting relevant cosmetic products. However, individual are too subjective, is are whytoo SWF is used to actually determine human skin comfort level assessments based onwhich feeling subjective, which is why SWF is the used to actually determine the[6]. A portable tester giving information about the TEWL and the SWF could be conveniently used human skin comfort level [6]. A portable tester giving information about the TEWL and the SWF at home outside to provide in the selection of cosmetics. For instance, when showering and could beor conveniently used atguidance home or outside to provide guidance in the selection of cosmetics. For bathing,when the proper kindsand of soap or shower foam kinds must be to themust individual’s skin instance, showering bathing, the proper of chosen soap oraccording shower foam be chosen status, which make the TEWL bewhich low throughout thethe day. according to thecould individual’s skin status, could make TEWL be low throughout the day.

(a)

(b)

Figure (a) Typical skin layer structure; (b) skincare sensor device Figure 1. (a) 1. Typical skin layer structure; (b) skincare sensor device tag. tag.

It is obvious that TEWL cannot measured directly it can measured from It is obvious that thethe TEWL cannot bebe measured directly [7],[7], butbut it can bebe measured from thethe water vapor density skin surface (i.e., at the SC). Typical measurement method devices include water vapor density at at thethe skin surface (i.e., at the SC). Typical measurement method devices include open-chamberand and condenser-chamber condenser-chamber devices, have beenbeen compared with each open-chamber devices,which which have compared withother eachaccording other to their performances [7–10]. No standard referencereference has beenhas given forgiven the TEWL thus a according to their performances [7–10]. No standard been for thevalues, TEWLand values, big difference can be found the calibration of the TEWL instruments, which depends and thus a big difference can beinfound in the calibration of themeasuring TEWL measuring instruments, which solely on the device and the manufacturers [11]. The TEWL level also depends on the skin type. depends solely on the device and the manufacturers [11]. The TEWL level also depends on the skin A lower TEWL is related to healthy skin, while a higher TEWL is related to damaged skin [12]. type. A lower TEWL is related to healthy skin, while higher TEWL is related to damaged skin [12]. study, took eight subjects inside andsubjects six subjects outside to monitor their TEWL. skin’s TEWL. In In thisthis study, wewe took eight subjects inside and six outside to monitor their skin’s The The TEWL is affected by many factors [13–15], including individual and environmental factors like TEWL is affected by many factors [13–15], including individual and environmental factors like age, age, environmental conditions, of a person, sex, skin temperature smoking. direct method environmental conditions, race ofrace a person, sex, skin temperature andand smoking. NoNo direct method can used obtain skin’s TEWL therefore, efficient system which could calibrate can bebe used to to obtain thethe skin’s TEWL [1];[1]; therefore, anan efficient system which could calibrate thethe change of impedance in a moisture-measuring device with the loss of mass during the drying process change of impedance in a moisture-measuring device with the loss of mass during the drying process was previously developed [1,16]. Similarly, another reference, TEWL was calculated with was previously developed [1,16]. Similarly, in in another reference, thethe TEWL was calculated with commercially available evaporimetry devices [17]. In our TEWL measurements, we adopted an almost commercially available evaporimetry [17]. In our TEWL measurements, we adopted an similar approach, but the waswas based on on a battery-less techniques, almost similar approach, but measurement the measurement based a battery-less techniques,i.e., i.e.,involving involvingno nobattery batterysource sourceininthe thedevice. device. The SWF is actually used to determine human comfort [6]. SWF The SWF depends The SWF is actually used to determine the the human skinskin comfort levellevel [6]. The depends on the relative humidity (RH)temperature and temperature thesurface, skin surface, and the ambient humidity (AH) theon relative humidity (RH) and at the at skin and the ambient humidity (AH) and and ambient temperature of The the air. The in increase in is the SWF is responsible for introducing ambient temperature (AT) of (AT) the air. increase the SWF responsible for introducing thermal thermal discomfort, while it is independent of the body temperature [18]. Fukazawa et al. reported discomfort, while it is independent of the body temperature [18]. Fukazawa et al. reported a linear a linear relationship and discomfort thermal discomfort Skinand cooling and sensations thermal sensations relationship betweenbetween the SWFthe andSWF thermal [6]. Skin [6]. cooling thermal could could drastically affect the SWF measurement, where the cooling rate threshold for identifying drastically affect the SWF measurement, where the cooling rate threshold for identifying it as wet it is as wet on is based ontransfer the heatrate transfer from[19]. the The skinskin [19].wettability The skin wettability must be determined based the heat from rate the skin must be determined based on based on physio-chemistry, that the physiochemical a human requires much physio-chemistry, consideringconsidering that the physiochemical behavior of behavior a human of requires much attention attention [20]. Normally, as per the references [6,21,22], the SWF should be less than or equal [20]. Normally, as per the references [6,21,22], the SWF should be less than or equal to 0.3 in ordertoto0.3 in order to ensure a normal level. However, et al. that [23] suggested that the should ensure a normal comfort level. comfort However, Nishi et al. [23]Nishi suggested the SWF should be SWF expressed as the decimal fraction, 0.06the representing minimum with 1 the representing as be theexpressed decimal fraction, 0.06 representing minimum the value, with 1 value, representing maximumthe

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maximum value for a fully wet skin. A number of research studies were previously conducted to analyze insights on the SWF that can be sensed in humans [24–29]. Near field communication (NFC), which is a technology launched in 2004 by three well-known companies (i.e., Nokia, Philips and Sony), constituted the name “NFC–Forum”. However, it was mostly adopted for payment systems. The NFC basically works on electromagnetic induction to establish communication between devices, which operate on an ISM band with a 13.56 MHz frequency [30], within a few centimeters (~4 cm). NFC communication consists of the reader and a tag. With the introduction of the NFC chip in the latest smartphones, NFC has gained serious interest in terms of its usage in Internet of Things (IoT) applications [31–33]. Energy harvesting through RFID or NFC is considered to be the best choice for embedded system applications. Several companies, like TI-, AMS-, NXP- and ST-Microelectronics, commercially introduced very low-power NFC-integrated chips (ICs) to ensure enhanced data writing, reading and storing capabilities. Utmost importance has recently been given to energy harvesting and data communication from NFC, with a focus on the optimal ways to obtain some future directions [34–36]. This study designs a smart skincare device with a novel design approach that could harvest energy from an NFC-enabled smartphone and elaborates on the applications related to skincare. 2. Materials and Methods 2.1. TEWL Calculation The TEWL is generally computed in gm/cm2 ·s, and it refers to the weight (gm) of moisture per unit time (s) per unit area (cm2 ). It is defined as the amount of moisture that evaporates from 1 cm2 of the skin surface per unit second. The TEWL has been given prime importance in dermatology, but it cannot be determined by the naked eye [37]. According to reference [38], the TEWL is described by Fick’s second law, as follows: d T = D C (1) dx where D is the diffusion of water at the SC, expressed in cm2 /s; C is the amount of water concentration in the SC; and x represents the skin membrane thickness (15 × 10−4 cm) from the edge of the SC. The value of D is suggested to be 10−9 cm2 /s [38], and the value of C can be calculated as [39]: C =

4.39 × 10−2 × RH 100

(2)

where RH is the value of the skin moisture taken from the humidity sensor embedded on the skincare device tag. 2.2. SWF Calculation The sensor used in our experiment could also detect the skin temperature; therefore, the skin temperature and moisture are the key parameters for determining the SWF, using the following formula [40]: ρs − ρa SWF = (3) ρs−sat − ρa SWF =

Rhskin. ρskin−sat − Rha. ρa−sat ρskin−sat − Rha. ρa−sat

(4)

where Rhskin represents the RH at the skin surface, which is actually the skin moisture; ρs is the partial vapor pressure at the skin surface; ρa is the partial vapor pressure in the ambient air; ρskin−sat is the saturated vapor pressure at the skin surface; ρa−sat is the saturated vapor pressure of the air at an AT;

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and Rha represents the RH of the air, which is actually the AH. The saturated vapor pressure at the skin surface can be determined as [40]: Sensors 2020, 20, x FOR PEER REVIEW

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1730.63

ρskin−sat = 0.13332 × 108.07 − T−39.72 Similarly, the saturated vapor pressure of the air can also be found from [40]: Similarly, the saturated vapor pressure of the air can also be found from [40]: . . = 0.13332 ⨯ 10 . 𝜌 1730.63 ρa−sat = 0.13332 × 108.07 − T−39.72

(5)

(6) (6)

2.3. System Design 2.3. System Design The overall system design structure of the skincare sensor device tag included an NFC-enabled The overall system design structure of the skincare sensor device tag included an NFC-enabled android smartphone, an embedded NFC coil on the skincare device tag to harvest sufficient energy android smartphone, an embedded NFC coil on the skincare device tag to harvest sufficient energy from from the smartphone, an NFC transponder chip for data communication with the smartphone, an the smartphone, an NFC transponder chip for data communication with the smartphone, an ultra-low ultra-low power microcontroller (MCU), schottky diodes for the rectification process and a skin power microcontroller (MCU), schottky diodes for the rectification process and a skin moisture sensor moisture sensor (Figure 2a). In Figure 2b, the main components embedded on the bottom layer of the (Figure 2a). In Figure 2b, the main components embedded on the bottom layer of the FR4 substrate FR4 substrate are depicted, making the device a smart skincare sensing device tag. The MCU are depicted, making the device a smart skincare sensing device tag. The MCU (MSP430G2553, (MSP430G2553, Texas Instruments Inc., Dallas, TX, USA) could consume different amounts of power Texas Instruments Inc., Dallas, TX, USA) could consume different amounts of power (1.8–2.2 V at (1.8–2.2 V at 230 µA) while working on different clock speeds (typically around 1 MHz). The NFC 230 µA) while working on different clock speeds (typically around 1 MHz). The NFC transponder chip transponder chip (RF430CL330H, Texas Instruments Inc., Dallas, TX, USA) is registered with the (RF430CL330H, Texas Instruments Inc., Dallas, TX, USA) is registered with the ISO14443B standard and ISO14443B standard and uses the NFC data exchange format (NDEF) message with a data rate of 848 uses the NFC data exchange format (NDEF) message with a data rate of 848 kbps, while writing data kbps, while writing data to the NFC smartphone. An I2C or SPI message protocol is required by this to the NFC smartphone. An I2 C or SPI message protocol is required by this NFC chip to communicate NFC chip to communicate with the MCU and a sensor while writing data to the NFC smartphone. with the MCU and a sensor while writing data to the NFC smartphone. The chip consumed 250 µA The chip consumed 250 µA while operating on a 400 KHz frequency. Moreover, the skin moisture while operating on a 400 KHz frequency. Moreover, the skin moisture sensor (BME 280, Bosch Sensortec, sensor (BME 280, Bosch Sensortec, Germany), which calculated the TEWL and the SWF, required Germany), which calculated the TEWL and the SWF, required only 1.8 µA at 1 Hz to operate efficiently only 1.8 µA at 1 Hz to operate efficiently on the humidity and temperature modes. on the humidity and temperature modes.

(a)

(b)

Figure Figure2.2.(a) (a)Overall Overallstructure structureofofthe thebattery-free battery-freesmart smartskincare skincaresensor sensordevice devicetag tagsystem; system;(b) (b)main main components componentsofofthe theskincare skincaredevice devicetag. tag.

The Thefabricated fabricatedsmart smartskincare skincaredevice devicetag tagmeasured measured2.6 2.6cm cminindiameter diameter(Figure (Figure3a) 3a)and andwas was enclosed on a flexible 3D-printed object (Figure 3b), enabling it to operate fully and conveniently enclosed on a flexible 3D-printed object (Figure 3b), enabling it to operate fully and conveniently in ina apassive passivemode. mode.This Thislightweight lightweight3D 3Dflexible flexible structure structure made made the the device device more more convenient convenient to totake take outside, outside,especially especiallyfor forthose thosepeople peoplewho whoare areoften oftenquite quiteanxious anxiouswhen whenititcomes comesto tothe theusage usageofofany any skincare device. The operating distance for energy harvesting from the NFC-enabled smartphone skincare device. The operating distance for energy harvesting from the NFC-enabled smartphone was wasapproximately approximately11cm cm(Figure (Figure3c), 3c),making makingthe theskincare skincaredevice devicetag tagwork workefficiently efficientlyon onbattery-less battery-less techniques. The energy harvesting efficiency depends on a number of factors, including the techniques. The energy harvesting efficiency depends on a number of factors, including theantenna antenna dimensions. dimensions. In In our our case, case, the the six-turn six-turn coil coil antenna antenna embedded embedded on on the the skincare skincaredevice devicetag taghad hadthe the following thickness of each turnturn coil: coil: 0.02 cm; the circular antenna: followingdimensions: dimensions: thickness of each 0.02total cm;inner totalradius inner of radius of thecoil circular coil 1antenna: cm; and 1total radius of the circular antenna: cm. The harvested energy cm; outer and total outer radius of thecoil circular coil 1.25 antenna: 1.25 maximum cm. The maximum harvested

energy was measured with an oscilloscope (DS07054A, Tektronix Inc., Beaverton, OR, USA). The peak–peak (Pk–Pk) harvested voltage was 3.06 V when the smartphone was brought very close to the skincare sensor tag device (~1 cm); however, it gradually decreased as we started moving the smartphone away from the skincare device tag (>1 cm) (Figure 4).

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was measured with an oscilloscope (DS07054A, Tektronix Inc., Beaverton, OR, USA). The peak–peak (Pk–Pk) harvested voltage was 3.06 V when the smartphone was brought very close to the skincare sensor tag device (~1 cm); however, it gradually decreased as we started moving the smartphone away Sensors 2020, 20, x FOR PEER REVIEW 5 of 14 from the skincare device tag (>1 cm) (Figure 4).

(a)

(b) (a)

(b)

(c) (c)

Figure 3. (a) Dimensions of the smart skincare sensor device tag; (b) skincare sensor device tag Figure (a) Dimensions smart skincare sensor device tag; (b) skincare sensor device tag device tag Figurethe 3. (a) Dimensions ofof thethe smart skincare sensor device tag;distance (b) skincare sensor tag enclosed enclosed within flexible 3D-printed object; (c) operating of the device skincare sensor enclosed within the3D-printed flexible 3D-printed (c) operating of the skincare sensortag device within the flexible object; (c)object; operating distance distance of the skincare sensor device from tag the from the NFC smartphone. from the NFC smartphone. NFC smartphone.

Figure 4. Maximum energy harvested from the NFC-enabled smartphone.

For the skincare device to operate effectively and efficiently, the six-turn coil antenna was first designed and tested on a well-known magnetic simulator (Ansoft Maxwell, ANSYS Inc., Canonsburg, Maximum energy harvested from thethe NFC-enabled smartphone. Figure 4.Figure Maximum energy harvested from NFC-enabled smartphone. PA, USA) to study the 4.electromagnetic simulations. The inductance (3.5887 µH) of the fabricated antenna coil on the device was then measured with a network analyzer (E5062A, Agilent For the Inc., skincare operate effectively efficiently, the six-turn coil(Cantenna Santadevice Clara,to CA, USA) (Figure 5) toand determine the tuning capacitor = 1/4π2f2was L= For theTechnologies skincare device to operate effectively and efficiently, the six-turn coil antenna was first first designed and tested on a well-known magnetic simulator (Ansoft Maxwell, ANSYS Inc., 38.38 × 10−12 ≈ 38 ρF) that should be soldered on the fabricated PCB for proper tuning at the desired Canonsburg, USA) More to study the electromagnetic simulations. The inductance (3.5887 µH) of designed and tested(13.56 onPA, a MHz). well-known magnetic simulator (Ansoft Maxwell, Inc., Canonsburg, frequency importantly, before finding the coil inductance, allANSYS the components the fabricated antenna coil on the device was then measured with a network analyzer (E5062A, were properly on the FR4 simulations. substrate to ensure all the parasitic capacitances PA, USA) to study thesoldered electromagnetic Thethat inductance (3.5887 µH) ofand the fabricated Agilent Technologies Inc.,affect Santa Clara, CA, USA) the (Figure 5) to determine the device tuning coil, capacitor inductances, which might the result of finding inductance of the skincare were antenna coil then measured with ona thenetwork analyzer Agilent −12 ≈ 38 (C =on 1/4π2the f2 L =device 38.38 × 10was ρF) that should be soldered fabricated PCB for proper(E5062A, tuning also added. Technologies Inc., Santa Clara, (13.56 CA, USA) 5) to determine thethe tuning capacitor at the desired frequency MHz). (Figure More importantly, before finding coil inductance, all (C the = 1/4π2f2L = were should properly soldered on the FR4 to ensure PCB that allfor the proper parasitic capacitances 38.38 × 10−12components ≈ 38 ρF) that be soldered on substrate the fabricated tuning at the desired and inductances, which might affect the result of finding the inductance of the skincare device coil, frequency (13.56 MHz). More importantly, before finding the coil inductance, all the components were also added.

were properly soldered on the FR4 substrate to ensure that all the parasitic capacitances and inductances, which might affect the result of finding the inductance of the skincare device coil, were also added.

Technologies Inc., Santa Clara, CA, USA) (Figure 5) to determine the tuning capacitor (C = 1/4π2f2L = 38.38 × 10−12 ≈ 38 ρF) that should be soldered on the fabricated PCB for proper tuning at the desired frequency (13.56 MHz). More importantly, before finding the coil inductance, all the components were properly soldered on the FR4 substrate to ensure that all the parasitic capacitances and inductances, which might affect the result of finding the inductance of the skincare device coil,6were Sensors 2020, 20, 5549 of 14 also added.

Figure 5. 5. Inductance Inductance of of the the embedded embedded coil coil in in the the smart smart skincare skincare device device tag. tag. Figure

2.4. Android Application Design A smart skincare app was designed on the android platform. This app could display the percentage values of the skin moisture taken from the sensor in a circular bar, as well as the skin moisture level (Figure 6a). After taking the skin moisture sensor readings, the values were processed to calculate the corresponding TEWL and SWF values. In android, waterfall graphs with the trendline were later displayed to show the history of the maximum and minimum values of TEWL and SWF in the indoor and outdoor environments, registered at different times of the day. The highest TEWL value noted at 10 AM in indoor environment among the eight subjects was 0.0215 µgm/cm2 ·s, while the lowest one was 0.0162 µgm/cm2 ·s (Figure 6b). Similarly, at 2 PM, 4 PM and 6 PM, the highest (lowest) TEWL readings recorded in the android smartphone were 0.0193 µgm/cm2 ·s (0.0148 µgm/cm2 ·s), 0.0167 µgm/cm2 ·s (0.0139 µgm/cm2 ·s) and 0.0163 µgm/cm2 ·s (0.0109 µgm/cm2 ·s), respectively, in the indoor environment (Figure 6b). The same process was repeated for the outdoor environment, illustrating the highest (lowest) TEWL readings at 10 AM, 1 PM, 3 PM and 5 PM to be 0.0163 µgm/cm2 ·s (0.0117 µgm/cm2 ·s), 0.0170 µgm/cm2 ·s (0.0099 µgm/cm2 ·s), 0.0174 µgm/cm2 ·s (0.0119 µgm/cm2 ·s) and 0.0164 µgm/cm2 ·s (0.0119 µgm/cm2 ·s), respectively (Figure 6c). The TEWL trendline followed a decreasing fashion in the indoor environment. In contrast, an increasing trend, especially during the afternoon times, was observed in the outdoor environment because most of the subjects felt sweaty during that period. The same process was repeated for the SWF calculations. The highest (lowest) SWF values recorded at 10 AM, 2 PM, 4 PM and 6 PM for the indoor environment were 0.41 (0.16), 0.47 (0.14), 0.37 (0.14) and 0.38 (0.01), respectively (Figure 6d). Meanwhile, the highest (lowest) SWF values recorded at 10 AM, 1 PM, 3 PM and 5 PM for the outdoor environment were 0.51 (0.35), 0.53 (0.21), 0.52 (0.29) and 0.48 (0.32), respectively (Figure 6e). Figure 6c,e show that the TEWL and the SWF in the outside environment followed an almost similar trendline; that is, the higher the TEWL, the higher the SWF, and the lower the TEWL, the lower the SWF.

felt sweaty during that period. The same process was repeated for the SWF calculations. The highest (lowest) SWF values recorded at 10 AM, 2 PM, 4 PM and 6 PM for the indoor environment were 0.41 (0.16), 0.47 (0.14), 0.37 (0.14) and 0.38 (0.01), respectively (Figure 6d). Meanwhile, the highest (lowest) SWF values recorded at 10 AM, 1 PM, 3 PM and 5 PM for the outdoor environment were 0.51 (0.35), 0.53 (0.21), 0.52 (0.29) and 0.48 (0.32), respectively (Figure 6e). Figures 6c,e show that the TEWL and Sensorsthe 2020, 20, 5549 SWF in the outside environment followed an almost similar trendline; that is, the higher the7 of 14 TEWL, the higher the SWF, and the lower the TEWL, the lower the SWF.

0.0215

TEWL (µgm/cm2.s)

0.0193

0.0167 0.0163 0.0162 0.0148

0.0139

0.0109

10 AM

2 PM

4 PM

6 PM

Time Sensors 2020, 20, x FOR PEER REVIEW

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(a)

(b) 0.53

0.52

0.51

0.48

0.47

0.0170

0.37

0.38

0.0164

0.0163

0.35

SWF

SWF

TEWL (µgm/cm2.s)

0.41

0.0174

0.32 0.29

0.21

0.0119

0.0117

0.0119

0.16

0.14

0.14

0.01

0.0099

10 AM

1 PM

3 PM

5 PM

10 AM

2 PM

4 PM

6 PM

10 AM

1 PM

Time

Time

Time

(c)

(d)

(e)

3 PM

5 PM

Figure 6. Android appinused in combination theskincare smart skincare sensortag: device tag: (a) Figure 6. Android app used combination with thewith smart sensor device (a) measurement measurement of the skin moisture through a battery-free NFC approach; (b) waterfall graph with a of the skin moisture through a battery-free NFC approach; (b) waterfall graph with a trendline trendline illustrating the TEWL result on the inside environment of eight subjects; (c) waterfall graph illustrating the TEWL result on the inside environment of eight subjects; (c) waterfall graph with a with a trendline illustrating the TEWL result on the outside environment of six subjects; (d) waterfall trendline illustrating the TEWL result on the outside environment of six subjects; (d) waterfall graph graph with a trendline illustrating the SWF result on the inside environment of eight subjects; and (e) with awaterfall trendline illustrating the SWF result on the inside environment of eight subjects; and (e) waterfall graph with a trendline illustrating the SWF result on the outside environment of six subjects. graph with a trendline illustrating the SWF result on the outside environment of six subjects.

3. Results

3. Results

3.1. TEWL in the Indoor and Outdoor Environments

3.1. TEWL in the Indoor and Outdoor Environments

After soldering the components on the PCB and enclosing the device on a flexible 3D object,

After soldering components the PCB and enclosing the device onthe a flexible 3D object, experiments werethe conducted in bothon indoor and outdoor environments by using smart skincare experiments were conducted in both indoor outdoor environments by using the smartNFC skincare sensor device tag. The skincare device tagand harvested sufficient energy from a well-enabled smartphone. skinskincare moisturedevice readings taken through the sensor These readings wereNFC sensor device tag.TheThe tagwere harvested sufficient energytag. from a well-enabled then plugged into moisture Equation readings (2) to calculate water concentration at the Afterreadings finding C, the then smartphone. The skin were the taken through the sensor tag.SC. These were values were then inserted in Equation (1)water to compute the TEWLat levels. For After the indoor experimental plugged into Equation (2) to calculate the concentration the SC. finding C, the values analysis, the TEWL readings(1) on to each subject the wereTEWL taken levels. at 10 AM, PM,indoor 4 PM and 6 PM (Figureanalysis, 7). were then inserted in Equation compute For2 the experimental Similarly, the TEWL readings for the outdoor experiment were taken at 10 AM, 1 PM, 3 PM and 5 the TEWL readings on each subject were taken at 10 AM, 2 PM, 4 PM and 6 PM (Figure 7). Similarly, PM (Figure 8). Some of the subjects felt sweatier outdoors between 2 PM and 4 PM; therefore, the the TEWL readings for the outdoor experiment were taken at 10 AM, 1 PM, 3 PM and 5 PM (Figure 8). increase in skin moisture was observed to be significant, and caused the TEWL to also increase during Somethat of the felt 3sweatier outdoors between 2 PM and 4 PM; therefore, the increase in skin timesubjects period (i.e., PM to 5 PM).

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moisture was observed to be significant, and caused the TEWL to also increase during that time period (i.e., 3 PM to20, 5 PM). Sensors 2020, x FOR PEER REVIEW 8 of 14 Sensors 2020, 20, x FOR PEER REVIEW

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Figure 7. TEWL of eight subjectsat different times times of the indoor experiment. Figure TEWL thethe eight subjects atatdifferent the day during the indoor experiment. Figure 7.7.TEWL ofofthe eight subjects times of the theday dayduring during the indoor experiment.

Figure 8. TEWL of the sixsix subjects of the theday dayduring during the outdoor experiment. Figure 8. TEWL of the subjectsatatdifferent differenttimes times of the outdoor experiment. Figure 8. TEWL of the six subjects at different times of the day during the outdoor experiment.

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The sensor sensor readings readings from from the the humidity humidity sensor sensor and and the the calculated calculated TEWL TEWL values values were were then then The compared, and it was observed that the histograms in Figures 9 and 10 depicted how the skin compared, and it was observed that the histograms in Figures 9 and 10 depicted how the skin moisture moisture TEWL valuesthe followed the same trend (i.e., the the skin higher the skinthe moisture, theTEWL higherrate, the and TEWLand values followed same trend (i.e., the higher moisture, higher the TEWL rate, and the lower the skin moisture, the lower the TEWL rate). and the lower the skin moisture, the lower the TEWL rate).

Figure Figure 9. 9. Histogram relationship relationship between between the the skin skin moisture moisture and and the the TEWL TEWL in in the the indoor indoor environment. environment.

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Figure 10. Histogram relationship between the skin moisture and the TEWL in the outdoor environment. Figure 10. Histogram relationship between the skin moisture and the TEWL in the outdoor 3.2. SWF Measurements in the Indoor and Outdoor Environments environment.

The sensor integrated in our skincare device tag could also measure the skin temperature; 3.2. SWF Measurements in the Indoorwere and Outdoor thus, the skin temperature values insertedEnvironments in Equation (5) to determine the saturated vapor pressure the skin surface. in The AT and AHdevice readings using a commercial meter (HTC-1 Theatsensor integrated our skincare tag were couldtaken also measure the skin temperature; thus, Clock/Humidity/Temperature, MCR Inc., Yueqing, Wenzhou, China). Equation (6) was then used to the skin temperature values were inserted in Equation (5) to determine the saturated vapor pressure determine thesurface. saturated vapor the air. Finally, the values of athecommercial AH, RH at the skin(HTC-1 (skin at the skin The AT pressure and AHofreadings were taken using meter moisture), saturated vapor pressure at the skin surface and saturated vapor pressure in the air were Clock/Humidity/Temperature, MCR Inc., Yueqing, Wenzhou, China). Equation (6) was then usedallto inserted intothe Equation (4)vapor to calculate theofSWF. TheFinally, SWF values should be less or the equal to(skin 0.3 determine saturated pressure the air. the values of the AH,than RH at skin tomoisture), ensure a normal comfort The at SWF depending the experimental results in saturated vaporlevel. pressure the was skincalculated surface and saturatedon vapor pressure in the air were the indoor and outdoor environments. The results in Figures 11 and 12 indicate that the subjects had all inserted into Equation (4) to calculate the SWF. The SWF values should be less than or equal to 0.3 different responses, the range of the SWF being betweendepending 0.1 and 0.5. to ensure a normalwith comfort level. The SWF was calculated on the experimental results in

the indoor and outdoor environments. The results in Figures 11 and 12 indicate that the subjects had different responses, with the range of the SWF being between 0.1 and 0.5.

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Figure SWF of of three three subjects subjects at Figure 11. 11. SWF at different different times times of of the the day day during during the the indoor indoor experiment. experiment.

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Figure 12. 12. SWF SWF of of three three subjects subjects at at different different times of the day during the outdoor experiment. Figure

4. Discussion 4. Discussion In this study, we developed a novel smart skincare sensor device tag that could monitor TEWL and In this study, we developed a novel smart skincare sensor device tag that could monitor TEWL SWF values, based on a battery-free humidity sensor capable of harvesting energy from an NFC-based and SWF values, based on a battery-free humidity sensor capable of harvesting energy from an NFCsmartphone. The TEWL and the SWF provide very useful, beneficial information for skincare in the based smartphone. The TEWL and the SWF provide very useful, beneficial information for skincare cosmetics field and can be calculated by taking skin moisture and skin temperature readings from a in the cosmetics field and can be calculated by taking skin moisture and skin temperature readings battery-free skincare sensor device tag. from a battery-free skincare sensor device tag. Our skincare device tag was compact, with small dimensions, (i.e., a diameter of only 2.6 cm), Our skincare device tag was compact, with small dimensions, (i.e., a diameter of only 2.6 cm), and could harvest ~3 V from the NFC-enabled smartphone, where the distance between the smartphone and could harvest ~3 V from the NFC-enabled smartphone, where the distance between the and the skincare device tag was approximately 1 cm. A series of in vivo experiments were conducted smartphone and the skincare device tag was approximately 1 cm. A series of in vivo experiments in both indoor (eight subjects) and outside (six subjects) environments to analyze the data. There was were conducted in both indoor (eight subjects) and outside (six subjects) environments to analyze the a big contrast in the experimental results of the indoor and outdoor environments. This meant that data. There was a big contrast in the experimental results of the indoor and outdoor environments. human skin was more vulnerable to an outside environment, and body sweat also affected skin This meant that human skin was more vulnerable to an outside environment, and body sweat also moisture. Our skincare device tag was enclosed in a flexible 3D-printed object, had a high monitoring affected skin moisture. Our skincare device tag was enclosed in a flexible 3D-printed object, had a resolution and could be easily transported by subjects. This means that our skincare device tag is a high monitoring resolution and could be easily transported by subjects. This means that our skincare very comfortable choice for customers when compared to other commercially available devices. device tag is a very comfortable choice for customers when compared to other commercially available Moreover, an android application was developed to save skin moisture and skin temperature devices. sensing values in an android smartphone. The app could comprehensively and easily provide the Moreover, an android application was developed to save skin moisture and skin temperature history of the calculated TEWL and SWF values for the user. sensing values in an android smartphone. The app could comprehensively and easily provide the history of the calculated TEWL and SWF values for the user.

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In the future, this smart skincare sensor device tag could be fabricated on a flexible design PCB, which could be more convenient for use, and, similarly, a considerable amount of effort may also be required in order to make the dimensions of the proposed flexible PCB be smaller and more compact. Author Contributions: Conceptualization, S.M.A. and W.-Y.C.; methodology, S.M.A.; writing—original draft preparation, S.M.A.; writing—review and editing, S.M.A. and W.-Y.C.; supervision and funding acquisition, W.-Y.C. All authors have read and agreed to the published version of the manuscript. Funding: This work (2019R1A2C108P139) was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT). Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2.

3.

4. 5. 6. 7. 8.

9.

10.

11. 12.

13. 14.

15. 16.

Sahle, F.F.; Gebre-Mariam, T.; Dobner, B.; Wohlrab, J.; Neubert, R.H.H. Skin diseases associated with the depletion of stratum corneum lipids and stratum corneum lipid substitution therapy. Skin Pharmacol. Physiol. 2015, 28, 42–55. [CrossRef] Krishnan, S.; Shi, Y.; Webb, R.C.; Ma, Y.; Bastien, P.; Crawford, K.E.; Wang, A.; Feng, X.; Manco, M.; Kurniawan, J.; et al. Multimodal epidermal devices for hydration monitoring. Microsyst. Nanoeng. 2017, 3, 17014. [CrossRef] Huang, X.; Liu, Y.; Cheng, H.; Shin, W.-J.; Fan, J.A.; Liu, Z.; Lu, C.-J.; Kong, G.-W.; Chen, K.; Patnaik, D.; et al. Materials and designs for wireless epidermal sensors of hydration and strain. Adv. Funct. Mater. 2014, 24, 3846–3854. [CrossRef] Wang, Y.; Qiu, Y.; Ameri, S.K.; Jang, H.; Dai, Z.; Huang, Y.; Lu, N. Low-cost, µm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts. Npj Flex. Electron. 2018, 2, 1–7. [CrossRef] Rizwan, A.; AbuAli, N.A.; Zoha, A.; Ozturk, M.; Alomainy, A.; Imran, M.; Abbasi, Q. Non-Invasive Hydration Level Estimation in Human Body Using Galvanic Skin Response. IEEE Sens. J. 2020, 20, 4891–4900. [CrossRef] Fukazawa, T.; Havenith, G. Differences in comfort perception in relation to local and whole body skin wettedness. Eur. J. Appl. Physiol. 2009, 106, 15–24. [CrossRef] Nilsson, G.E. Measurement of water exchange through skin. Med. Biol. Eng. Comput. 1977, 15, 209–218. [CrossRef] Imhof, R.E.; De Jesus, M.E.P.; Xiao, P.; Ciortea, L.I.; Berg, E.P. Closed-chamber transepidermal water loss measurement: Microclimate, calibration and performance. Int. J. Cosmet. Sci. 2009, 31, 97–118. [CrossRef] [PubMed] Farahmand, S.; Tien, L.; Hui, X.; Maibach, H.I. Measuring transepidermal water loss: A comparative in vivo study of condenser–chamber, unventilated–chamber and open–chamber systems. Skin Res. Technol. 2009, 15, 392–398. [CrossRef] [PubMed] Fluhr, J.W.; Feingold, K.R.; Elias, P.M. Transepidermal water loss reflects permeability barrier status: Validation in human and rodent in vivo and ex vivo models. Exp. Dermatol. 2006, 15, 483–492. [CrossRef] [PubMed] Alexander, H.; Brown, S.; Danby, S.; Flohr, C. Research Techniques Made Simple: Transepidermal Water Loss Measurement as a Research Tool. J. Investig. Dermatol. 2018, 138, 2295–2300. [CrossRef] [PubMed] Akdeniz, M.; Gabriel, S.; Lichterfeld-Kottner, A.; Blume-Peytavi, U.; Kottner, J. Transepidermal water loss in healthy adults: A systematic review and meta-analysis update. Br. J. Dermatol. 2018, 179, 1049–1055. [CrossRef] [PubMed] Rogiers, V. EEMCO Guidance for the Assessment of Transepidermal Water Loss in Cosmetic Sciences. Skin Pharmacol. Phy. 2001, 14, 117–128. [CrossRef] [PubMed] Du Plessis, J.; Stefaniak, A.; Eloff, F.; John, S.; Agner, T.; Chou, T.-C.; Nixon, R.; Steiner, M.; Franken, A.; Kudla, I.; et al. International guidelines for the in vivo assessment of skin properties in non-clinical settings: Part 2. transepidermal water loss and skin hydration. Skin Res. Technol. 2013, 19, 265–278. [CrossRef] [PubMed] Darlenski, R.; Sassning, S.; Tsankov, N.; Fluhr, J. Non-invasive in vivo methods for investigation of the skin barrier physical properties. Eur. J. Pharm. Biopharm. 2009, 72, 295–303. [CrossRef] [PubMed] Valentin, B.; Mundlein, M.; Chabicovsky, R. Evaluation of a novel transepidermal water loss sensor. In Proceedings of the IEEE Sensors Conference, Vienna, Austria, 24–27 October 2004; pp. 115–118.

Sensors 2020, 20, 5549

17. 18. 19. 20.

21. 22.

23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

14 of 14

Gioia, F.; Celleno, L. The dynamics of transepidermal water loss (TEWL) from hydrated skin. Skin Res. Technol. 2002, 8, 178–186. [CrossRef] Gagge, A.P.; Stolwijk, J.A.J.; Hardy, J.D. Comfort and thermal sensations and associated physiological responses at various ambient temperatures. Environ. Res. 1967, 1, 1–20. [CrossRef] Filingeri, D.; Redortier, B.; Hodder, S.; Havenith, G. The role of decreasing contact temperatures and skin cooling in the perception of skin wettedness. Neurosci. Lett. 2013, 55, 65–69. [CrossRef] Eudier, F.; Savary, G.; Grisel, M.; Picard, C. Skin surface physio-chemistry: Characteristics, methods of measurement, influencing factors and future developments. Adv. Colloid Interface Sci. 2019, 264, 11–27. [CrossRef] Gagge, A.P.; Stolwijk, J.A.J.; Nishi, Y. The prediction of thermal comfort when thermal equilibrium is maintained by sweating. ASHRAE Trans. 1969, 75, 108–122. Gonzalez, R.; Gagge, A.P. Magnitude estimates of thermal discomfort during transients of humidity and operative temperature and their relation to the new ASHRAE effective temperature (ET). ASHRAE Trans. 1973, 79, 88–96. Nishi, Y.; Gagge, A.P. Effective temperature scale useful for hypo- and hyperbaric environments. Aviat. Space Environ. Med. 1977, 48, 97–107. [PubMed] Sweeney, M.M.; Branson, D.H. Sensorial comfort: Part I: A psychophysical method for assessing moisture sensation in clothing. Text Res. J. 1990, 60, 371–377. [CrossRef] Sweeney, M.M.; Branson, D.H. Sensorial comfort: Part II: A magnitude estimation approach for assessing moisture sensation 1. Text Res. J. 1990, 60, 447–452. [CrossRef] Li, Y. Perceptions of temperature, moisture and comfort in clothing during environmental transients. Ergonomics 2007, 48, 234–248. [CrossRef] Daanen, H.A.M. Method and System for Alerting the Occurrence of Wettedness. U.S. Patent No. 8,697,935 B2, 15 April 2014. Niedermann, R.; Rossi, R.M. Objective and subjective evaluation of the human thermal sensation of wet fabrics. Text Res. J. 2012, 82, 374–384. [CrossRef] Lee, J.Y.; Nakao, K.; Tochihara, Y. Validity of perceived skin wettedness mapping to evaluate heat strain. Eur. J. Appl. Physiol. 2011, 111, 2581–2591. [CrossRef] Youbok, L. Antenna Circuit Design for RFID Applications; Microchip Tech. Inc.: Chandler, AZ, USA, 2003. Marrocco, G. Pervasive electromagnetics: Sensing paradigms by passive RFID technology. IEEE Wirel. Commun. 2010, 17, 10–17. [CrossRef] Babar, A.; Manzari, S.; Sydanheimo, L.; Elsherbeni, A.; Ukkonen, L. Passive UHF RFID tag for heat sensing applications. IEEE Trans. Antennas Propag. 2012, 60, 4056–4064. [CrossRef] Fernández-Salmerón, J.; Rivadeneyra, A.; Martínez-Martí, F.; Capitán-Vallvey, L.F.; Palma, A.J.; Carvajal, M.A. Passive UHF RFID tag with multiple sensing capabilities. Sensors 2015, 15, 26769–26782. [CrossRef] Boada, M.; Lázaro, A.; Villarino, R.; Girbau, D. Battery-less soil moisture measurement system based on a NFC device with energy harvesting capability. IEEE Sens. J. 2018, 18, 5541–5549. [CrossRef] Lázaro, A.; Boada, M.; Villarino, R.; Girbau, D. Color measurement and analysis of fruit with a battery-less NFC sensor. Sensors 2019, 19, 1741. [CrossRef] [PubMed] Boada, M.; Lazaro, A.; Villarino, R.; Girbau, D. Battery-less NFC sensor for pH monitoring. IEEE Access. 2019, 7, 33226–33239. [CrossRef] Lieb, L.M.; Nash, R.A.; Matias, J.R.; Orentreich, N. A new in vitro method for transepidermal water loss: A possible method for moisturizer evaluation. J. Soc. Cosmet. Chem. 1988, 39, 107–119. Nagayama, K.; Kurihara, T. Numerical Simulation of Skin Formation: The Relationship between Transepidermal Water Loss and Corneum Thickness. J. Appl. Math. Phys. 2018, 6, 1757–1762. [CrossRef] Stockdale, M. Water diffusion coefficients versus water activity in stratum corneum: A correlation and its implications. J. Soc. Cosmet. Chem. 1978, 29, 625–639. Sun, Y.; Jasper, W.J. Numerical modeling of heat and moisture transfer in a wearable convective cooling system for human comfort. Build. Environ. 2015, 93, 50–62. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).