Breathable and Skin-Mountable Strain Sensor with Tunable

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Breathable and Skin-Mountable Strain Sensor with Tunable Stretchability, Sensitivity, and Linearity via Surface Strain Delocalization for Versatile Skin Activities’ Recognition Zhongqian Song,†,§ Weiyan Li,† Yu Bao,*,‡ Fangjie Han,† Lifang Gao,†,§ Jianan Xu,†,§ Yingming Ma,† Dongxue Han,†,‡ and Li Niu*,†,‡

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State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, CAS Center for Excellence in Nanoscience, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Jilin, China ‡ Center for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.R. China § University of Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: Metal film/elastomer-based strain sensors usually exhibit small rupture strain (3 × 108 mL·m2·24 h) even for PU/AU210 with a thick Au layer (Figure 1d). Meanwhile, the strain sensor demonstrates high transepidermal water loss (TEWL) above the human skin level (Figures 1e and S4). The high water and gas permeability endows the strain sensor with high breathability and provides good biocompatibility and comfortable contact with human skin (Figure 1a). The strain sensor was attached onto human skin for 1 day and no obvious discomfort, inflammation, and allergic reactions are observed, indicating good biocompatibility with skin (Figure S5a). In addition, considering the practical applications, the anti-interference ability with the environment such as temperature, humidity, and pressure is also investigated as shown in Figure S5b. The strain sensor exhibits negligible changes in resistance within human body temperature ranges from 30 to 40 °C. Meanwhile, it also shows high anti-interference ability under different humidities and pressures with a resistance change lower than 3% (Figure S5c,d). Moreover, the strain sensor can be designed into diverse shapes for comprehensive monitoring of the human body. Figure 1f shows an illustration of the skin-mountable strain sensors attached onto different places for versatile monitoring of vital signs. These superior properties including high compliance and biocompatibility with human skin as well

onto the PU layer via the electron-beam evaporation method, forming a stretchable conductor. Silver paste is coated on the two ends of the conductive layer for electrical connection with copper wires, obtaining the stretchable strain sensor. The cross-sectional scanning electron microscopy (SEM) image of the strain sensor attached on medical tape indicates that the whole thickness of the strain sensor is 48 μm (Figure 1b). The PU layers-sputtered Au layers for 60, 90, 120, and 210 s are simplified as PU/Au60, PU/Au90, PU/Au120, and PU/ Au210, respectively, and corresponding fabricated strain sensors are denoted as SS60, SS90, SS120, and SS210. The surface sheet resistance of the strain sensors range from 33 to 4.8 Ω □−1 and the thickness of the Au film gradually increases with the increasing deposition time and reaches up to 112 nm for SS210 (Figure S2). The whole fabrication process was carried out at room temperature, which can help to avoid the mechanical properties of the elastomeric substrate from being damaged by high temperature. As shown in Figure 1c(i−iii), a predesigned cruciform biaxial strain sensor attached onto a wrist demonstrates good compliance with human skin even under transverse and longitudinal compression states. The oxygen plasma treatment of a PU stretchable layer endows the Au layer with a high bonding force. As the 3M Scotch tape peeling test illustrates in Figure 1c(iv), no delamination of the Au layer from the PU layer is observed. Meanwhile, because of the strong van der Waals interaction between the PAA adhesive layer and human skin, the strain sensor can adhere tightly to the rugged skin surface. The adhesion force is measured to be 1.9 N with the standard 90° peeling test, which is higher than those of Ecoflex, Solaris, and Spray-on-Bandage.24 The high adhesion force enables the strain sensor to bend and stretch to conform to the C

DOI: 10.1021/acsami.8b14365 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Relative resistance change during the loading−unloading cycle at different strain rates. (b) Relative resistance change during the loading−unloading cycle at a rate of 1 mm s−1, showing slight hysteresis of the strain sensor. (c) Relative resistance change for the strain sensor at 10% strain at different strain frequencies. (d) Relative resistance change at 0.4% strain, showing rapid response and low detection limit. (e) Relative resistance response for the strain sensor during different repeated loading−unloading strains. (f) Relative resistance change at different strains (strain rate of 10% s−1), showing low creep of the strain sensor. (g) Relative resistance change during repeated loading−unloading at a strain of 40% for 1.5 h, showing good durability of the strain sensor.

by thin films of aligned single-walled carbon nanotubes, but the GF is as low as 0.06. However, the tunable region of strain and GF (green region in Figure 2b) demonstrates both large stretchability and high sensitivity compared with previously reported values (Table S2). To elucidate the underlying mechanism of the tunable sensing properties, the surface morphologies of the strain sensors are investigated as depicted in Figure 2d−g. First, the strain sensors are stretched to 100% at a rate of 10% s−1 to develop the initial interconnected cracks and maintain for 30 s to eliminate the unrecoverable strain. Then, the strain sensors are stretched to 10, 45, and 100% for investigation of surface morphology. As the SS60 and SS90 are stretched from 15 to 100%, small zigzag cracks form perpendicular to the loading direction and then the cracks propagate gradually, forming interconnected island-gap microstructures similar to the previous reports.12,41 We define the crack density as the number of discontinuous cracks per unit area. It can be observed that the stretchability is proportional to the crack density, but the GF decreases with increasing crack density. The SS60 exhibits maximum crack density of 1 μm−2 and minimum width of islands and gaps (Figure 2c). This high crack density can contribute to the enhanced stretchability and linearity of the strain sensors. However, as for SS120 and SS210, the traversed and parallel channels tend to form at a lower strain and prefer to develop into isolated strips (Figure 2f,g), corresponding to low crack density and large islands and gaps (Figure 2c). The illustration model about the crack propagation is proposed as shown in Figure S8. With the

as breathability enable the strain sensor to be an ideal sensing module of stretchable electronic skins. Strain Delocalization-Induced Controllable Stretchability and Sensitivity. The as-fabricated strain sensors exhibit tunable stretchability and sensitivity via regulating the strain distribution in Au films on an elastomeric substrate. The sensitivity of strain sensors can be reflected by the GF, which is calculated via the following equation: GF = (ΔR/R0)/ε, where R0 is the initial resistance of strain sensor and ε is the applied strain. The representative resistance change versus strain for the as-fabricated strain sensors are presented in Figure 2a. Typically, the resistance increases with the applied strain. The SS60 exhibits high sensitivity with a constant GF of ∼7.2 at a strain of lower than 40% and the sensing range can reach up to 140% with a high GF of 358.7. The constant GF of SS60 (0% < ε < 40%) indicates good linearity in the range from 0 to 40% (Figure S6). The stretchability of 140% is higher than some of the previous reported values for metal film-based strain sensors (Figure S7).9−11 Even though the stretchability of SS210 is limited below a strain of 30%, the GF is as high as 474.8. The detailed GF was calculated as shown in Table S1, indicating that the stretchability and sensitivity of the strain sensors can be tuned by regulating the sputtering time of the Au layers. The trade-off relationships between high sensitivity and large stretchability are commonly observed for most of the strain sensors, as the orange- and purple-colored regions shown in Figure 2b. For example, most of the metal film- or carbonbased strain sensors exhibit large GF but very limited sensing range.9−11,14,18,25−40 High stretchability of ∼280% is achieved D


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Figure 4. (a) Illustration of fabrication of the diffraction-induced Au film. (b) Relative resistance change vs strain curves of SS60, SS120, and DFSS. (c) GFs of SS60, SS120, and DFSS at different strains. (d) Photograph of the diffraction-induced Au film and corresponding SEM images of hierarchical microstructures of DFSS under a strain of 15%.

decreasing crack density, the conductive path is prone to disconnect within lower strain ranges, demonstrating low stretchability and high GF (474.8 at ε = 30% for SS210). In contrast, the interconnected island-gap microstructures of SS60 propagate uniformly, leading to high crack density and high stretchability up to 140% (Figure 2a). From the point of conductive pathways, the increasing resistance is ascribed to the progressive separation between the islands. The critical fracture of Au layers results in an ultimate stretchability, where electrons cannot transfer from one island to another. The crack propagation mechanism is proposed on the basis of the information gained from the surface morphological features of the strain sensors. We conclude that the significantly enhanced stretchability is ascribed to the tight bonding and high compliance of the Au film with the elastomeric substrate. 12 The increased sputtering time contributes to increasing thickness and high fracture strength of Au films, leading to a decreased compliance with elastomeric substrates. As for the free-standing Au film, the strain mainly localizes at the defect-induced necks, resulting in a low rupture strain of ∼2%.42 When it is combined with the soft elastomeric substrates, the strain localization can be suppressed by the compliant substrate and evenly distribute onto the whole film, leading to an enhanced rupture strain of the Au film. In addition, because of the high bonding force with the substrate, the Au films exhibit a high compliance with the substrate and can deform uniformly with the substrate. Therefore, the formed necks propagate uniformly and slowly, forming interconnected island-gap microstructures (Figure 2d,e). However, with the increasing thickness of the Au film, the increased fracture strength of the Au film exceeds the bonding force and the compliance with the elastomeric substrates weakens, causing debonding of the Au film from the substrate (Figure S9). The strain distribution locates at few spots and contributes to necking down, local elongation, and eventually breaks similar to the free-standing Au film (Figure 2g). To confirm the proposed explanation, the sensing behavior and

crack propagation process of Au-coated PDMS were further investigated as shown in Figure S10. The Au-coated PDMSbased strain sensor demonstrates a large GF but a very limited stretchability of 5% similar to the previously reported value.10 Because of high Young’s modulus of the PDMS substrate, the strain localization in Au film induces the traversed and parallel channel cracks perpendicular to the stretching direction along the entire cross section of the Au film. The conductive pathways break at a strain of 5%, leading to infinite resistance and failed sensing behavior (Figure S10c). Sensing Performances of the Strain Sensor. Given the relative high GF (12.6 at 30%), high stretchability (∼90%), and wide linear range (from 0 to 40%), SS90 was selected for more detailed investigations of sensing properties. As shown in Figure 3a, SS90 exhibits almost overlapped loading and unloading curves at different strain rates, demonstrating slight hysteresis and good capabilities of elastic recovery. Figure 3b shows the cyclic loading−unloading curves from 20 to 60% at a rate of 1 mm s−1, indicating a 3−10% residual strain that is negligible compared with the total strain during the cycles. To examine the response time of SS90, the frequency test was carried out at the strain frequencies from 0.1 to 1 Hz. It can be observed that the SS90 exhibits a similar resistance change at 10% as the frequency increases from 0.1 to 1 Hz (Figure 3c). As shown in Figure 3d, the SS90 shows a short response time of 33 ms and recovery time of 83 ms during the rapid stretch and release process with a very low detection limit of 0.4%. Meanwhile, the repeatability of the SS90 at different strains is also investigated as shown in Figure 3e. The SS90 is highly reproducible and durable when being stretched at different strains, which is correlated to the low creep of the elastomeric substrate. As shown in Figure 3f, the resistance of SS90 remains stable after being stretched to 10, 20, 30, 50, and 60%. It exhibits only a 0.47% creep strain after keeping a high strain of 60% for 1 min. No obvious creep is observed even at a large strain of 90% for more than 1 min (Figure S11). Repetitive cyclic testing of SS90 from 0 to 40% is performed as shown in E


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Figure 5. (a) Structure illustration of the biaxial strain sensor. (b) Relative change in resistance vs strain in the x-axial and y-axial directions, respectively: x-axial and y-axial resistance change with variable x-axial strains from 0 to 50% (red), x-axial and y-axial resistance change with variable y-axial strains from 0 to 50% (green). (c) Schematic of equivalent circuit for the biaxial strain sensor under the y-axial direction. (d) SEM image of the biaxial strain sensor under y-axial strain, showing compression in the x-axis and stretch in the y-axis because of the Poisson effect. (e) Relative change in resistance along the x-axial and y-axial directions under strain in the x-axis and y-axis, respectively. (f) Biaxial strain sensor attached onto human skin at the wrist. (g) Relative change in resistance vs time for the biaxial strain sensor (f) under different motion states, including clench fist, wrist bending, and clench fist and wrist bending at the same time.

comparable stretchability, and a wide linear region. The DFSS exhibits two linear regions and the GF can reach up to 22.62 (0% < ε < 45%) and 40.8 (45% < ε < 115%) (Figure 4c). Even though the GF of SS120 is as high as 314.6 at a strain of 50%, its sensing range is limited within only 56% and the SS120 exhibits worse linearity. The enhanced performance of DFSS is ascribed to the hierarchical microstructures of the Au film. The SEM images of DFSS at 15% were measured to investigate their surface morphology, showing thickness-gradient-induced hierarchical structures. As a result, given the abovementioned strain delocalization, these typical structures in Figure 2d−g are combined together from the regions (i) to (iv) with a gradient width of 200 μm, demonstrating hierarchical structure variations from traversed and parallel channels in region (iv) to small zigzag cracks in region (i). It is expected that the traversed and parallel channels endow the DFSS with a large resistance change at a small strain to the benefit of high sensitivity. However, the zigzag cracks allow the presence of effective conductive paths even at a large strain, leading to large stretchability of strain sensors. When a large strain is applied, the cracks gradually propagate from region (iv) to region (i), contributing to a wide sensing range and linear region. The enhanced linearity of DFSS can simplify the calibration processes and ensure accurate measurements, which is in favor of human motion identification. When the strain sensors are attached onto aa human finger or an air balloon, regular resistance change can be acquired under different bending and deformation states (Figure S16 and Movie S1). Design of a Biaxial Strain Sensor for Detection of Complex Actions. Most strain sensors can only sense uniaxial strain, which limits their capacity to detect complex multiaxial strain, especially for complex action detection of the human body.37,44,45 Therefore, a novel biaxial strain sensor is designed by regulating the surface strain distributions via partial immobilization of the strain sensor via an inelastic positron emission tomography (PET) film (Figure 5a). As shown in

Figure 3g. The SS90 exhibits repeatable sensing performance even after being cycled for 1.5 h from 0 to 40% at a rate of 10% s−1, indicating good durability and reproducibility. The slight decrease in resistance within the initial 600 s could be ascribed to the surface topography formation and stabilization process. After the durability test, the SS90 exhibits slight changes with negligible distance changes between islands and gaps (Figure S12a,b). However, the surface of the strain sensor without pretreatment with oxygen plasma exhibits obvious detachment of the Au film from the substrate (Figure S12c−e). Hence, the pretreatment of oxygen plasma is also crucial to enhance the sensing stability. In addition, the SS90 is able to work normally even at an ultralow working voltage of 1 mV with an energy consumption of 2 nW (Figure S13). Meanwhile, the strain sensors fabricated using PU/Au strips with different widths of 1, 2, and 3 mm exhibit similar sensing performance, because GF refers to the relative change in resistance (Figure S14). Therefore, the strain sensors can be designed into different shapes to adapt with the complex parts of the human body. Owing to the superior sensing performance, the strain sensor exhibits great potential for monitoring physiological vital signs. Diffraction-Induced Au Film-Based Strain Sensor. As discussed above, high sensitivity needs substantial structural variation at a small strain, whereas large stretchability requires maintained conductive structures under large strain.43 Therefore, it is still a challenge to simultaneously integrate large stretchability and high sensitivity together into one stretchable sensing system. Inspired by the diffraction of light, a novel thickness-gradient Au film was fabricated via the electron-beam evaporation method using a grating as a mask (Figures 4a and S15). As shown in Figure 4d, mass production of diffractioninduced Au films on medical tape were obtained. The strain sensor based on diffraction-induced Au film (DFSS) was fabricated by connecting the copper wires with both ends of the film using Ag paste. The relative change in resistance versus strain for DFSS was measured. Surprisingly, as shown in Figure 4b, compared with SS60, the DFSS exhibits enhanced GF, F


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Figure 6. (a) Photograph of the strain senor attached onto the front of the ankle. (b) Schematic diagram of the system designed for gait detectios. (c) Monitoring of various human motions during the whole exercise process. (d) Corresponding stride frequency, stride length, and speed during walking, brisk walking, and running. (e−i) Enlarged views of (c). (j−n) Monitoring of some daily activities, including the process from standing up to sitting down, jumping, shaking legs, on tiptoe, and squatting, indicating discriminating signals for different motions.

causing an increased resistance along the x-axis and a decreased resistance along the y-axis. When the wrist is bent, the skin stretching along the y-axis results in the resistance increase in the y-axis and being constant in the x-axis. However, resistance increases simultaneously in the x- and yaxes when the subject clenches his fist and bends his wrist at the same time. Given the independent signals from the two channels, the biaxial strain sensor is able to differentiate the strain in transverse and longitudinal directions simultaneously. It is reasonable to assume that the biaxial strain sensor can be used for tracking and recognition of complex actions of the human body in real time for human−machine interaction and rehabilitation therapy. Gait Detector for Comprehensive Monitoring of Human Motion. Thanks to the characteristics of highadhesion and compliance with deformable skins, it is unnecessary to fix the strain sensor onto joints by medical tape for motion information collecting, which could avoid lots of inconvenience. For example, the strain sensor can be directly attached onto the skin of the ankle as a gait detector for comprehensive recording of human motions (Figure 6a). As shown in Figure 6b, a gait recognition system including the asfabricated strain sensor, analog-to-digital converter (ADC), and microprocessor could be designed. As shown in Figure 6c, various signals for series of actions including sitting, standing up, walking, brisk walking, and running are collected by the strain sensor, showing high capability and repeatability for gait identification. Meanwhile, the stride frequencies are calculated to be 0.544, 0.835, and 1.308 Hz for walking, brisk walking, and running states, respectively (Figure 6d). Combined with the average stride length, the moving speed of the subject can be estimated to be 2.54, 4.5, and 8.48 km h−1. Figure 6e−i

Figure 5b, the resistance along the x-axis increases dramatically on increasing the strain along the x-axis from 0 to 50% (red lines). However, the resistance along the y-axis remains relatively stable with a slight decrease because of the Poisson effect (Figure S17). Similarly, when the biaxial strain sensor is stretched along the y-axis from 0 to 50%, the same behavior of resistance versus strain is observed. Hence, the biaxial strain sensor exhibits independent sensing behavior in the transverse and longitudinal directions. The biaxial sensing mechanism is illustrated in Figure 5c; taking y-axial stretching as an example, the surface strain propagation along the x-axial direction is limited because of the protection of inelastic PET film, contributing to the redistribution of surface strain. Meanwhile, the elastic deformation along the y-axis results in x-axial compression because of the Poisson effect, causing longitudinal recovery of the cracks. Therefore, resistance in the x-axial direction shows slight decrease because of the compressioninduced crack reconnection (Figure S17). The SEM image shown in Figure 5d presents an increased distance between cracks because of the strain along the y-axis and cracks along the x-axis recuperate under the Poisson effect. The changes in resistance along the two axes exhibit different responses during x-axial or y-axial stretching under different strains (Figure 5e). In addition, the biaxial strain sensor exhibits high sensitivity (GF = 32) than that of SS90 (GF = 13.4) when being stretched up to 50% because of the surface strain localization caused by the inelastic PET film. To demonstrate the capability to detect complex actions of the human body, the biaxial strain sensor is attached directly onto the human wrist for complex action detection (Figure 5f). As shown in Figure 5g, the skin stretches along the x-axis and compresses along the y-axis when a human clenches the fist, G


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Figure 7. Applications of the strain sensor for detection of emotional expressions. (a) Photograph of a strain sensor conformably attached onto the lower eyelid. (b) Relative change in resistance for the strain sensor in response to various eye movements in (c). (c) Photograph of various eye movements and corresponding enlarged views of (b). (d) Repeatable characteristic signals of special facial expressions.

shows the enlarged views of signals in Figure 6c, demonstrating different signal shapes for various motions including standing up, walking, brisk walking, running, sitting down, and the process from standing to sitting, which cover most of the common actions in our daily life. In addition, the strain sensor is able to distinguish detailed information, such as landing and pedaling during each action, enabling the ability of action identification. Meanwhile, some complex actions including jumping, shaking legs, on tiptoe, and squatting can be recognized as well (Figure 6j−n). The strain sensor can also be used for monitoring of respiration states via attaching the strain sensor onto the human chest area (Figure S18). The subject under normal state has a respiratory rate of 10 per minute and the inhale and exhale processes exhibit smooth peak during each whole breathing. However, the strain sensor is subject to a large deformation and outputs enhanced signals during deep breathing. It demonstrates excellent capability of the strain sensor for monitoring of vital signs. Comprehensive Detection of Emotional Expressions. With the development of human−machine interaction and virtual reality, the capture and identification of human facial expressions are urgently needed.46 In addition, the recognition of eye movements is greatly meaningful for a machine or robot to read the emotions of human beings, especially for interactions with the patients suffering from amyotrophic

lateral sclerosis. As human emotions could be reflected by eyes, it is supposed that our designed strain sensor could be used for recognition of facial expressions. Hence, a novel arcuate Ushaped strain sensor was designed according to the ocular structure and attached onto the lower eyelid for monitoring facial expressions (Figure 7a). It is expected that the biaxial compression and stretching of the lower eyelid caused by the movements of eyeballs during facial expressions can be detected by the well-designed strain sensor (Figure S19 and Movie S2). As shown in Figure 7b, the strain sensor outputs distinct and repeatable signals in response to series of basic eye movements. The movements of the lower eyelid are associated with the movement of eyeballs because of the contraction and relaxation of the ciliaris. The magnified images in Figure 7c clearly show that various eye movements give rise to the different characteristic peaks, indicating the capability to recognize each subtle eye movement including blinking, moving up, down, left and right, as well as cyclotorsional movement clockwise and anticlockwise (inset in Figure 7c). In addition, as shown in Figure 7d, some emotional expressions related to the subtle movements of eyes can also be distinguished by the strain sensor. These output signals exhibit repeatable characteristic peaks, indicating good capability and high stability of the strain sensor for accurate identification of emotional facial expressions. These results demonstrate that H



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the elaborate strain sensor can be used as a skin-mountable module to read human beings’ emotions for applications in human−machine interactions and virtual reality.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14365.

CONCLUSIONS In conclusion, a skin-mountable and breathable strain sensor with enhanced stretchability and sensitivity was designed and fabricated by regulating the strain localization in Au films on an elastomeric substrate. It is found that strain delocalization of Au films could enhance the sensing range and linearity because of the homogeneous generation and propagation of island-gap microstructures. In addition, the strain sensor exhibits excellent repeatability and durability, as well as low energy consumption (as low as 2 nW). On the basis of a novel diffraction-induced Au film with gradient thickness, the linearity of the strain sensor is significantly enhanced. Meanwhile, the strain sensors are capable to capture the skin activities for monitoring human motions and emotional expressions. The simple yet efficient strategy might be extended to other conductive metal films and stretchable polymer substrates to construct novel wearable strain sensors for human physiological activities’ monitoring, human−machine interfaces, and soft robotics.

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Research Article

Fabrication process of PU/Au stretchable conductor; surface sheet resistance, atomic force microscopy images and thickness of Au films with different deposition times; signal response to the action of clench fist; weight change of water with elapsed time at 35 °C; compatibility and anti-interference ability of the strain sensor; linear relationship of SS60 and SS90; comparison of maximum stretchability of our work with reported works; illustration model of the cracks propagation of the strain sensor; SEM of SS210 stretched at a strain of 15%; relative changes in resistance and SEM images of Au-coated PDMS strain sensor; relative resistance change under large strain of 90%; surface SEM images of strain senor with pretreatment with oxygen plasma; changes in resistance of the strain sensor at different working voltages; sensing properties of the strain sensor fabricated with PU/Au strips with different widths; designed mask for diffraction-induced Au film; bending and deformation detection; enlarged view of Figure 5b; respiration monitoring; special design of strain sensor attached onto the lower eyelid; averaged GF within different strain ranges; comparison of maximum strain and GF with reported strain sensors in Figure 2b (PDF) Resistance response to the folding/unfolding movements of the index finger (AVI) Detection of emotional expressions (AVI)

EXPERIMENTAL SECTION

Fabrication of the Strain Sensor. The PET protective layer of medical tape (3M Tegaderm) was peeled off and then the PU layer (36 μm) was pretreated with oxygen plasma (Harrick Plasma, PDC002) at a high power level for 5 min for tight bonding with the Au layer. The flow rate of the air was 60 mL min−1. Immediately, the Au layer was sputtered onto the PU layer via the electron-beam evaporation method (Cressington Sputter Coater 108) at a current of 70 mA and a vacuum of 36 mbar. The obtained Au film-coated medical tape can be tailored into desired shapes, including strips with different widths, U-shape, and cruciform via a scalpel. Two copper wires were connected with the two ends of the Au layer using silver paste. For fabrication of a biaxial strain sensor, the cruciform sensing film (2 mm × 2 cm) was transferred onto another medical tape and the four endpoints and central part of the cruciform film were coated with a square 4 mm × 4 mm PET film with a thickness of 25 μm. To enhance the binding force between the PET film and medical tape, the PET film was pretreated with the oxygen plasma. In addition, the obtained biaxial strain sensor was compressed with a pressure of 1 MPa to ensure the strong junction of the two films. The same procedure was adopted for fabrication of the PDMS/Au strain sensor. The PDMS substrate (SYLGARD 184, Dow Corning) was prepared in a 10:1 weight ratio of the polymer base and curing agent. The PDMS film was fabricated via the spin-coating method on a glass plate and cured at 80 °C for 1 h. The diffraction-induced Au film was fabricated with the same procedure, instead of putting a stainless-steel grating mask onto the PU layer with a distance of 0.6 mm from the PU layer. The DFSS was obtained by connecting two copper wires with the two ends of the diffraction-induced Au film using silver paste. Characterizations. SEM was performed using a Phenom ProX at an acceleration voltage of 10 kV. The loading and unloading tests were carried out via a home-made computer-controlled movable stage (Lianyi XYZM100H-50, Shanghai) via attaching the two ends of the strain sensor on the stage. The change in resistance of the strain sensor was recorded by a digital source meter (Keithley 2560) at a sample rate of 50 Hz with a working voltage of 1 V. The GF of the strain sensor was defined as (ΔR/R0)/ε, where ΔR is the change in resistance, R0 is the initial resistance, and ε is the applied strain. The gas permeability was measured with a PERME G3/132 instrument at 25 °C and the TEWL was estimated according to a previous method.47 The adhesive force was measured by attaching a strain sensor (9 cm2) on human skin with a pressure of 10 kPa and tested with a standard 90° peeling test under a constant speed of 50 mm min−1. All stretching tests were conducted under ambient conditions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.B.). *E-mail: [email protected] (L.N.). ORCID

Li Niu: 0000-0003-3652-2903 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC, China (21527806, 20727815, 21622509, and 21475122), the Department of Science and Techniques of Jilin Province (21602010087GX, 20170203004SF, 20170101183JC), Jilin Province Development and Reform Commission (2016C014 and 2017C053-1), and Science and Technology Bureau of Changchun (15SS05).

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REFERENCES

(1) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C.-K.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z. Skin-like Pressure and Strain Sensors Based on Transparent Elastic Films of Carbon Nanotubes. Nat. Nanotechnol. 2011, 6, 788−792. (2) Chortos, A.; Liu, J.; Bao, Z. Pursuing Prosthetic Electronic Skin. Nat. Mater. 2016, 15, 937−950. (3) Ho, M. D.; Ling, Y.; Yap, L. W.; Wang, Y.; Dong, D.; Zhao, Y.; Cheng, W. Percolating Network of Ultrathin Gold Nanowires and Silver Nanowires toward “Invisible” Wearable Sensors for Detecting Emotional Expression and Apexcardiogram. Adv. Funct. Mater. 2017, 27, 1700845.

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