Skin‐Integrated, Stretchable Electronic Skin for Human Motion Capturing and Pressure Mapping

Flexible and wearable electronics have been widely investigated for the extensive applications in real life. Piezoresistive based sensor is one of the common flexible electronics that could be utilized as electronic skin. By simply transducing the external pressure or stretching into resistor signal and integrated with flexible substrate and advanced functional sensing material, piezoresistive based sensors have been applied as the electronic skin. Here, a thin, skin‐integrated, and flexible electronic skin based on piezoresistive working mode is developed. Ultra‐thin polydimethylsiloxane substrate integrated with serpentine‐like Cu electrode could avoid mechanical failing of the device when it is stretched, bended, or twisted. By adopting the advanced function material, MXene, graphene and Ecoflex mixed composite, the electronic skin could sense not only a wide range of pressure from 20.8 to 132 kPa, but also stretching rate from 0% to 20%, allowing the potential application in human motion capturing. Furthermore, a 4 × 4 arrayed electronic skin is fabricated, and demonstrates the prospective application in pressure mapping.


Introduction
Recently, flexible and wearable electronics have drawn many attention for their promising application in the field of health monitoring, [1,2,3] motion detection [4,5,6] and human-machine interface. [7,8,9]Pressure sensors, as one kind of the traditional transduces, have been thoroughly investigated over the past few decades. [10,11]According to the structure and principle, there are three main categories of pressure sensors, such as piezoresistive, [12,13] piezoelectric, [14,15] and capacitive based pressure sensors. [16,17]Among all, piezoresistive pressure sensors, which transduce the pressure signal into resistor signal, have been widely reported for their advantages of low-cost, high DOI: 10.1002/adsr.202300025sensibility, easy signal collection and wide sensing range. [18,19,20]By integrated with flexible substrate and advanced functional materials, piezoresistive pressure sensors have earned an important place in the field of electronic skin (Eskin). [21,22,23]he key to fabricate a wearable piezoresistive pressure sensor is the flexible substrate, conductive electrodes, and active sensing materials. [18,24,25,26]olydimethylsiloxane (PDMS), also known as silicone, is one of the most widely used flexible substrate. [27,28,29]By integrating with stretchable serpentineshaped electrodes, silicon-based flexible electronics equip with high flexibility and stretchability, which are appropriate to be utilized in wearable piezoresistive pressure sensor. [30,31,32]Combined with layered transition metal carbides, nitrides or carbonitrides, MXene, one new class of 2D materials, was first investigated in 2011. [33,34]Because of the excellent electrical conductivity, layered structure, and tunable surface terminations, MXenes have been utilized in various fields, such as energy storage, [35,36] photoelectric devices [37,38] and electrochromic devices. [39,40]However, as 2D materials, pristine MXenes are not suitable for the application in flexible electronics for their drawbacks of easy restacking and small lateral size. [41,42,43,44]To resolve those limitations, MXene-based composites are introduced. [45,46]By assembling the MXenes with other materials, outstanding properties are combined, and drawbacks are remedied. [47,48]Therefore, MXene-based composites are promising candidates for active sensing materials in fabricating wearable electronics. [49,50]Graphene, a two-dimensional material contained with single layer of carbon atoms and arranged in hexagonal lattice structure, has been widely studied since it was first mechanically exfoliated by repeated peeling of graphite. [51,52]With excellent electron mobility at room temperature, graphene exhibits high flexibility and thermal conductivity with good transparency. [53,54]All those extraordinary properties draw the attention of researchers to focus on its applications in conducting electronics, [55,56] flexible field effect transistors, [57,58] photodetectors, [59,60] and biosensors. [61,62]Recently, graphene has been utilized to fabricate flexible sensors. [63,64]As a good candidate to be applied in fabricating E-skins, Ecoflex is one kinds of rubber that equipped with low viscosity, ultraflexibility, and excellent stretchability. [65,66]By mixing MXene, graphene, and Ecoflex, a MXene/Graphene/Ecoflex (MX/Gr/EF) composite could be obtained, which has combined both advantages of excellent electrical property and high flexibility.
Here, we developed a skin-integrated electronic skin (E-skin) based on flexible PDMS substrate, integrated with serpentineshaped Cu electrodes, and utilized with MX/Gr/EF composite as active sensing material, which ensure the sensor to be flexible and stretchable.Our E-skin could be utilized to sense not only the pressure signal, but also the strain change.Even after over 700 times cycling of continuous hitting at the pressure of 132 kPa, our E-skin remain intact and still could work.With high compatibility with human skin and high sensibility, our E-skin could be well attached to human skin.By simply attached on the back of the hand, bending angle of the finger could be detected.Moreover, our E-skin could sense slight activity, like eye blinking.Furthermore, a 4 × 4 array E-skin was fabricated, which indicated that our E-skin has the potential to be applied in pressure mapping.

Fabrication the PDMS Based Electrodes
A quartz glass was first cleaned with acetone, ethanol, and deionized water (DI water).Then the cleaned quartz glass was rinsed with sodium stearate aqueous solution and dried on a hot plate at 100 °C for 5 min.The sodium stearate solution was prepared by diluting handwash (Dettol) with DI water at a ratio of 1:4.Thus, a thin layer of dried sodium stearate (10 mL) was spread on the glass sheet, which was served as a sacrificial layer for later device releasing.Afterward, a thin layer of PDMS film (0.4 mm) was spin coated (600 rpm, 10 s) and cured (110 °C, 5 min) to form the stretchable substrate.Another layer of ultrathin PDMS, working as an adhesion agent, was spread over the cured PDMS substrates to firmly adhere Cu electrodes.A thin Cu sheet (6 μm) was gently attached on the PDMS substrate before the ultrathin layer of PDMS fully solidified, then the whole sample was cured (110 °C, 5 min) on a hot plate, following the Cu electrodes with desired geometries was patterned by photolithography.On top of the Cu film, a layer of positive photoresist (PR, AZ 4620, AZ Electronic Materials) was spin-coated (3000 rpm, 30 s), soft baked on a hot plate (110 °C, 5 min), then exposed to ultraviolet light (45 s), and developed (Developer, AZ 400K, AZ Electronic Materials) in the end.After etching the exposed Cu with ferric trichloride, the remaining photoresist was finally removed using acetone, rinsed with DI water, and gently blow-dried with compressed air.

Fabrication of the MX/Gr/EF Composite
As shown in Figure 1B, the sensing materials consist of MXene, graphene and Ecoflex.A commercial MXene, organ-like Ti 3 C 2 was purchased (thickness of 2-10 μm, Beike Nano), graphene powder (thickness of 1-3 nm, diameter of 0.2-10 μm, purity of 98%) was purchased from Suzhou TANFENG graphene Tech Co., Ltd., and Ecoflex (00-30) was from Smooth-on.Organ-like Ti 3 C 2 (0.6 g), graphene powder (0.1 g) and Ecoflex (5 g) were poured into a speed mixer at the speed of 500 rpm for 1 h to form rubbery precursors.Then, the mixture was transferred into an agate mortar and gently grinded for 1h at room temperature.Thus, the aqueous MX/Gr/EF composite was prepared.

Assembly of the E-Skin
The aqueous MX/Gr/EF composite was screen printed onto the flexible PDMS based substrate via screen-printing assisted by a laser cut steel mask (area of 5 mm × 5 mm).After blade-coating the MX/Gr/EF precursor, the device with the steel mask on top of it was heated at 120 °C for 30 min as the MX/Gr/EF composite was competed cured.Finally, a thin layer of PDMS (thickness of 2 μm) was spin-coated on top of the device and cured to fully encapsulate the whole device.

Result and Discussion
As shown in Figure 1A, a thin and flexible E-skin was fabricated.A thin layer of PDMS was integrated with Cu electrode, with a MX/Gr/EF composite screen-printed working as the sensing material and covered with an encapsulation layer of PDMS eventually.Through photolithography, the Cu electrode was etched with a serpentine-shaped design, which can prevent device failing while mechanical deformation was applied, as shown in Figure 1C.The MX/Gr/EF composite was one-step screen-printed on top of the Cu electrode working as the active sensing material, which is simple and facial, as shown in Figure 1D.Finally, to ensure the stability and integration of the device, a thin layer of PDMS was spin-coated and encapsulated the device.Since the whole device is thin and flexible, it could fit well when attached on human skin as shown in Figure 1E.All procedures were followed in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national).The informed consent of both volunteers was obtained prior to the study.
Our E-skin is equipped with both advantages of good flexibility and stretchability, as shown in Figure 1F, while the device was under stretching, twisting, or bending, it could remain intact.This contributes to the flexible substrate and encapsulation layer of PDMS, the unique serpentine-like Cu electrode, and excellent rubber character of Ecoflex from MX/Gr/EF composite.Besides cheap and stable, PDMS has been widely utilized in the field of flexible electronics because of its excellent flexibility.The PDMS was fabricated with the common ratio of 1:10, which is flexible with some degree of stiff.To eliminate the influence of stiffness as much as possible, we spin-coated a thin layer of PDMS as substrate (0.4 mm) and encapsulation (2 μm) layer.Thus, our E-skin could avoid mechanical failing during stretching, twisting or bending.The serpentine-like Cu electrode also allow moderate outspread, so that the device could remain intact after being stretched, twisted or bended.Moreover, as the binder and substrate of the MX/Gr/EF composite, Ecoflex is equipped with excellent flexibility and stretchability.Therefore, MX/Gr/EF composite would not fall apart after being stretched, twisted or bended.
The electronic performance of our E-skin is shown in Figure 2. As shown in Figure 2A, the rate of resistance change tends to increase with the device being further stretched.This contributes to  both pressure and strain.To investigate the influence of the loading frequency, different frequency of 0.5, 1, and 2 Hz with the same external pressure of 132 kPa are applied on our E-skin.As shown in Figure 2C, there is no obvious fluctuation of rate of resistance change when different frequency of the same external pressure is loaded.This means that the influence of the loading frequency could be neglected.Besides, a cycling test is applied on our E-skin, under the pressure of 132 kPa and frequency of 1 Hz.As shown in Figure 2D, after over 700 times loading of external pressure, our E-skin still could work, which indicates that our E-skin could be used repeatedly.
In addition, our E-skin could be utilized in human motion capturing when attached on human skin, as shown in Figure 3.As shown in Figure 3A, three kinds of external stimuli, touching, tapping, and punching, were applied on our E-skin while it was attached onto a volunteer's arm.The rate of resistance change increased with the applied external pressure increased.When our E-skin was gently touched, the rate of resistance change is 0.27 ± 0.01.This indicates that our E-skin is sensitive enough to respond to gentle touch.Once we punched the E-skin hardly, the rate of resistance change could rise to 0.41 ± 0.01.This demonstrate that our E-skin is equipped with a large scope of pressure sensing.Since the above three kinds of external stimuli belong to external pressure, which could only cause slight local deflection, the responded rate of resistance change is relatively small.Then we placed our E-skin on a volunteer's hand to capture the real-time figure bending angle, as shown in Figure 3B.Initially, the index finger pointed straight ahead, then it gradually pointed down, with the bending angle of 30°, 60°, and 90°, as shown in Figure 3B.When the finger bended for 30°, the rate of resistance change went up to about 0.36, and stable at around 0.18.With the finger continue bended to 60°and 90°, the responded rate of resistance change raised to about 0.61 and 0.77, respectively.Obviously, the responded rate of resistance change of figure bending is much larger than that of external pressure stimuli.As we mentioned before, the rate of resistance change mainly depended on the local deflection, and stretching could cause more deformation than pressure.The action of figure bending could lead to the stretching of our E-skin; thus, the responded rate of resistance change is distinct.Besides being utilized to monitor human motion like touching, tapping, punching and finger bending, our E-skin could subtle slight motion like eye blinking, as shown in Figure 3C.We tightly mounted our E-skin on the canthi of one volunteer, and measured the real-time rate of resistance change, as shown in Figure 3C.Like finger bending, eye blinking could cause mild stretching of the E-skin attached on the canthi.Therefore, our E-skin could sense the motion of eye blinking even though it is fast and insignificant.
Moreover, a 4 × 4 arrayed E-skin was fabricated to realize the function of pressure mapping, as shown in Figure 4.The structure of the 4 × 4 arrayed E-skin imitated the single unit E-skin, as shown in Figure 4A.The bottom PDMS substrate was integrated with serpentine-like arrayed Cu electrode, covered with MX/Gr/EF composite as sensing material, and finally encapsulated with a thin layer of PDMS.The size of the arrayed E-skin is 75 mm × 50 mm × 1 mm, consisting of 16 units, with a 5 mm × 5 mm sensing area each, which has the same sensing area as the single unit of E-skin.As shown in Figure 4B, our arrayed E-skin could be well integrated on human skin, and shows good flexibility and stretchability under stretching, bending, or twisting.Besides, our arrayed E-skin could be utilized in pressure mapping, as shown in Figure 4C.We tapped the pattern of "CITYU" with our figure on the arrayed E-skin, and there showed responded rate of resistance change of around 0.33, which is consistent with the single unit E-skin when it was tapped with human finger, as shown in Figure 4D,F.

Conclusion
In summary, we fabricated a skin-integrated, stretchable, piezoresistive electronic skin.Ultra-thin PDMS substrate integrated with serpentine-shaped Cu electrode and utilizing MXene and graphene mixed with super soft rubber, Ecoflex, ensure the excellent flexibility and stretchability of our electronic skin.The sensing material, MX/Gr/EF was one-step screen-printed onto the Cu electrode, which is facial and economical.Our E-skin could be utilized in sensing both stretching and pressure, with a wide scope of sensing range.After over 700 cycles of continuous hitting under 132 kPa, our E-skin still could work.Furthermore, by mounted on human skin, our E-skin could sense not only human motions like touching, tapping, punching and finger bending, but also slight human motion as eye blinking.Besides, our E-skin has the potential to be applied in the field of pressure mapping by integrated with arrayed structure.

Figure 1 .
Figure 1.Thin and flexible electronic skin.A) Schematic illustration of the E-skin.B) Schematic of the MX/Gr/EF composite fabrication process.C) Optical images of the electrode, D) encapsulated device, and E) well-attachment on human skin.F) Optical images of the device under stretching, twisting, and bending.

Figure 2 .
Figure 2. Electrical performance of the electronic skin.A) Rate of resistance change with the stretching rate, B) pressure increases, C) and different frequency of external load at the same pressure of 132 kPa.D) The cycling test of the electronic skin at the pressure of 132 kPa and frequency of 1 Hz.

Figure 3 .
Figure 3. Electronic skin for human motion capturing.Optical images of electronic skin placed on A) human skin, B) hand (the scale bars of the inset picture are 2 cm), and C) canthi and the corresponding rate of resistance change under touch, tap, punch, 30°, 60°, and 90°finger bending and eye blinking, respectively.

Figure 4 .
Figure 4.A 4 × 4 arrayed electronic skin.A) Schematic illustration of the 4 × 4 arrayed electronic skin.B) Optical images of the arrayed electronic skin attached on human skin, and under stretching, bending, and twisting.C) The position of the applied external pressure on the arrayed electronic skin, D) their corresponding rate of resistance change, and E) and color mapping.