Highly Robust, Transparent, and Breathable Epidermal Electrode

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Highly Robust, Transparent and Breathable Epidermal Electrode You Jun Fan, Xin Li, Shuang Yang Kuang, Lei Zhang, Yang Hui Chen, Lu Liu, Ke Zhang, Si Wei Ma, Fei Liang, Tao Wu, Zhong Lin Wang, and Guang Zhu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04245 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Highly Robust, Transparent and Breathable Epidermal Electrode You Jun Fan1,3,4, Xin Li1,3, Shuang Yang Kuang1,3, Lei Zhang1,5, Yang Hui Chen1,3, Lu Liu1,3, Ke Zhang1,3, Si Wei Ma1,3, Fei Liang1,3, Tao Wu6, Zhong Lin Wang1,3,7, Guang Zhu1,2*

1

CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China. 2

Department of Mechanical, Materials and Manufacturing Engineering, The University of

Nottingham Ningbo China, Ningbo 315100, China. 3

School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing

100049, China. 4

Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China.

5

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of

Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China. 6

New Materials Institute, University of Nottingham Ningbo China, Ningbo 315100, China.

7

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, USA.

*E-mail: [email protected]

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KEYWORDS: nanofibers, silver nanowires, flexible electrode, epidermal electrode, electronic skin

ABSTRACT: Recently emerged electronic skins with applications in on-body sensing and human-machine interface call for the development of high-performance skin-like electrodes. In this work, we report a highly robust, transparent, and breathable epidermal electrode composed of a scaffold-reinforced conductive nano-network (SRCN). Solution-dispersed Ag nanowires, through facile vacuum filtration, are embedded into a scaffold made of polyamide nanofibers. Optical transmittance of 84.9% at 550 nm wavelength is achieved at a significantly low sheet resistance of 8.2 Ω sq-1. The resistance of the SRCN only slightly increases by less than 0.1% after being bent for 3,000 cycles at the maximum curvature of 300 m-1 and by less than 1.5% after being dipped in saline solution for 2,500 cycles. The excellent robustness is attributed to the reinforcement from the nanofibers-based scaffold as a backbone that maintains the connections among the Ag nanowires by undertaking most of the loaded stress. The SRCN not only forms tight and conformal bonding with target surface but also allows the evaporation of perspiration, making it suitable as an epidermal electrode for long-time use. Furthermore, fine and clean-cut circuit patterns with line width on the micrometer scale can be readily prepared, paving the basis for fabricating sophisticated functional electronic skins.

Human skin performs as a sensory receptor and a protective barrier that detect and mitigate insults from the outer environment, respectively, to maintain the internal milieu of human bodies.1 Recently, electronic skins with multiple functions have attracted worldwide attentions due to their promising applications in skin prosthetics, humanoid robotics, and wearable electronics.2-7


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Intimate interactions of the electronic skins with human bodies in vitro or in vivo is particularly useful to achieve high-precision on-body sensing, healthcare monitoring, and human-machine interface.3,8-12 These applications call for the so-called skin-like electrode or epidermal electrode, as a part of the electronic skin, which can have conformal interaction with the genuine skin and endure cyclic deformations, e.g. bending, twisting and stretching. Significant progress in this area has been achieved in recent years.13-17 Rogers et al. has developed a series of ultrathin, stretchable and compliant epidermal electrodes by creating specific structural configurations, such as ‘wavy’ herringbone layout, ‘pop–up’ wave geometry and filamentary ‘serpentine’ mesh.5,18,19 Takao Someya et al. recently reported a substrate-free, inflammation-free, and ultrathin nanomeshes electrode.20 However, there are still major challenges for developing high-performance epidermal electrodes. Normal flexible electrodes with substrates inhibit the permeability of gas and liquid, which is not only unsuitable for long-term wearing but also difficult to achieve tight adhesion to the skin.11,12,21,22,29-32 Although substrate-free electrodes may tackle this problem, their robustness and endurance are usually sacrificed. Furthermore, most previously reported epidermal electrodes are opaque and conspicuous on the skin, which limits their uses for circumstances requiring a high level of optical transmission.23 Therefore, it is highly desirable to develop a type of highly robust, breathable and transparent electrode that has a strong bonding with the skin for conformal interactions. Herein,

we

report

a

scaffold-reinforced

conductive

nano-network

(SRCN).

Solution-dispersed silver nanowires (Ag NWs), through facile vacuum filtration, are embedded into a scaffold made of polyamide nanofibers. Optical transmittance of 84.9% at 550 nm wavelength is achieved at a significantly low sheet resistance of 8.2 Ω sq-1. Excellent robustness in stability is revealed upon being subject to cyclic distortion and soaking in saline solution. The


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resistance of the SRCN only slightly increases by less than 0.1% after being bent for 3,000 cycles at the maximum curvature of 300 m-1 and by less than 1.5% after being dipped in saline solution for 2,500 cycles. The robustness is attributed to the reinforcement from the nanofibers-based scaffold as a backbone that maintains the connections among the Ag NWs by undertaking most of the loaded stress. Due to strong van der Waals force of the nanofibers, the SRCN forms tight and conformal bonding with diverse types of target surface, especially genuine skin. Having a porous structure for air conduction, the SRCN allows the evaporation of perspiration, making it suitable as an epidermal electrode for long-time use. However distorted, e.g. wrinkled, stretched, and twisted, the SRCN can freely follow the deformations of the skin and stably maintain its conducting property. Furthermore, fine and clean-cut circuit patterns with line width on the micrometer scale can be readily prepared, paving the basis for fabricating sophisticated functional electronic skins that are used in a variety of on-body sensing applications. RESULTS AND DISCUSSION The fabrication process of the SRCN is outlined in Figure 1a. Firstly, a scaffold made of polyamide 6 (PA6) nanofibers was electrospun onto a metal collector. The PA6 was used due to its excellent mechanical property, transparency and biocompatibility.24 Secondly, a polyethylene terephthalate (PET) based frame with double-sided adhesive tape was prepared and applied onto the scaffold. Then, the scaffold along with the frame was peeled off from the collector. Subsequently, the scaffold was used as a filter, through which the suspension of Ag NWs was vacuumed. As a result, the Ag NWs were intercepted by the polymer nanofibers. After being dried, the free-standing SRCN was finally obtained. The three-dimensional structure of the SRCN is depicted in Figure 1b. The scaffold comprised of the nanofibers acts as a backbone, while highly conductive Ag NWs supported by the scaffold form a conductive network. As revealed by the


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top-down view of a scanning electron microscopy (SEM) image in Figure 1c, the Ag NWs (marked by arrows) are 30 nm in diameter and 30 µm in length with an aspect ratio of 1,000. The detailed characterization results of the Ag NWs are presented in Figure S1. They are tangled together to form a continuous network on the top of the polymer nanofibers that have an average diameter of 80 nm. The cross-sectional view in Figure 1d explicitly shows that the Ag NWs are embedded in the scaffold. Some Ag NWs have penetrated through the nanofiber scaffold and can be observed from the backside (Figure S2b). As a result, the connections among the Ag NWs are reinforced by the scaffold, which is the key for the excellent mechanical robustness of the SRCN as discussed below. A piece of square-shaped sample with 300 nm thickness is shown in Figure 1e and Figure S2a, which demonstrates excellent optical transmittance. Before the SRCN was applied, a target surface, e.g. human skin, was wetted by water. Then the SRCN was spread onto the wet surface. After being dried, it forms tight and conformal attachment to the genuine skin. Due to a high level of transparency, the SRCN on the skin looks barely visible, as exhibited by the photograph in Figure 1f. The loading quantity of the Ag NWs suspension plays a major role in determining the optical transmittance and the conductivity of the SRCN. As presented in Figure 2a, the sheet resistance of the SRCN significantly decreases with the loading of the Ag NWs. Corresponding surface morphology is presented in Figure S3a-f. If the loaded Ag NWs is 12.5 µg/cm2, the sheet resistance (Rs) reaches as low as 8.25 Ω sq-1, which is superior to most of the recently reported flexible electrodes.13,17,21,25 The overall optical transmittance spectra with different loading quantity are exhibited in Figure 2b. Intuitively, higher transmittance is obtained provided with larger sheet resistance. For the sample having a sheet resistance of 8.25 Ω sq-1, the transmittance at 550 nm wavelength is found to be 84.9%, which has reached the state-of-the-art level as demonstrated in


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Figure S4.22,26-28 The optical transmittance of individual components (i.e. nanofibers and Ag NWs) was separately presented in Figure S5a,b. The as-fabricated SRCN also features excellent homogeneity in conductivity. Evidenced in Figure 2c, the sheet resistance of the SRCN is mapped in an area of 3 × 3 cm2, which reveals a uniformity of 4.0% with a standard deviation of 0.33 Ω sq-1 at an average sheet resistance of 8.24 Ω sq-1. In contrast, inferior uniformity of 19% and 11% are yielded from other fabrication methods by simply coating and transferring Ag NWs onto PET substrates, respectively. The detailed data on the uniformity are presented in Figure S6a-c. This merit is mainly attributed to the uniformly dispersed Ag NWs in the suspension. Besides, vacuum filtration also ensures even distribution of the Ag NWs over the nanofiber scaffold. Stability and robustness are the most concerned aspects for flexible electrodes. In this regard, the resistance variation of the SRCN is investigated as it experiences bending, cyclic bending, stretching and exposure to electrolyte. First, the SRCN was spread onto a 100 µm thick PET substrate that was clamped and bent by a step motor. As increasing bending strain was applied onto the SRCN, the resistance between the two clamped edges barely changes. As shown in Figure 2d, the resistance even slightly decreases as a small degree of bending is applied. This is likely attributed to the fact that the nanofiber scaffold becomes more compact as a result of the bending, which then contributes to more contacts among Ag NWs and thus reduced resistance.29 At the maximum curvature of 500 m-1 (i.e. the most right-handed state in Figure 2d), the resistance changes by merely 1.8%, and the morphology of the SRCN at the maximum curvature was shown in Figure S7. Second, the durability of the SRCN against cyclic bending is proved by the long-time testing results in Figure 2e. After being bent for 3,000 cycles at the maximum curvature of 300 m-1, the SRCN almost constantly maintains its resistance with very little fluctuation of less than 0.1%, which is in sharp contrast to conventional sputtered ITO electrode (Figure S8a-c). Third, the


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tolerance of the SRCN against stretching is presented in Figure 2f. As it is elongated under direct uniaxial stretching, the resistance is observed to keep stable until the elongation rate exceeds 10% and then gradually increases, reaching 2.3 times higher than the original value at an elongation rate of 50%. Revealed by the SEM images of insets in Figure 2f, although the nanofibers of the scaffold in the stretched state become oriented along the stretching direction, the network structure is mostly maintained. This observation indicates that displacement among the Ag NWs can occur, which then makes Ag NWs network free of stress. It is believed that this observation is mainly responsible for the obtained robustness, which will be elaborated below. Fourth, even if the SRCN undergoes cyclic wetting by being dipped in saline solution for 2,500 times, the resistance only rises by 1.5% (Figure 2g). Similar observation is also obtained if the saline solution is replaced by deionized water (Figure S8d). Being immune to liquid interaction is a solid evidence that the SRCN is promising to be adopted in electronic skins as its conducting property is barely undermined by exposure to perspiration and water washing. As for long-time exposure to heavy sweat, a very thin layer of parylene-C may be coated onto the PA6 nanofibers by vapor deposition, which further promotes the chemical stability of the scaffold.33 Besides, thermal stability as well as time stability of the SRCN is also examined. Exhibited in Figure 2h, the resistance gradually increases at a rate of 0.15%/℃ below 175 ℃ probably due to the mismatch of thermal expansion coefficient between the polymer nanofibers and the Ag NWs.28 Once the temperature exceeds 175 ℃, the resistance substantially rises. This is likely attributed to the surface oxidation of the Ag NWs and breakage of some Ag NWs at high temperature (right-handed inset in Figure 2h).31 Furthermore, the conductivity of the SRCN is found to be quite stable in ambient atmosphere. The resistance variation of 12.8% is obtained after 100 days (Figure 2i).


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All of the results above justify the robustness of the SRCN. The fundamental reason lies in the structural reinforcement from the nanofiber scaffold. As revealed in Figure 1d, the Ag NWs are sucked into the scaffold by the vacuum filtration. When the SRCN undergoes stress, the stress is loaded almost solely onto the nanofibers-based scaffold. As for the Ag NWs network, the stress is released within the network because of the displacement among the Ag NWs (Figure 2f). In other words, the Ag NWs network can survive the strain since little stress is loaded. When the strain disappears, the shape restoration of the scaffold can then bring the Ag NWs back to the original positions. Therefore, the contacts among the Ag NWs are supported and stabilized by the scaffold, which is key for the excellent robustness. To apply the SRCN, the target surface was first wetted by water or alcohol. Then the SRCN was spread onto the wetted surface. Because the polyamide-based nanofibers are hydrophilic (Figure S9a, b), the SRCN is quickly soaked with the liquid by capillary effect. As the liquid evaporates, the nanofibers are pulled close towards the surface. Once the liquid is completely dried, the nanofibers become tightly adhered onto the surface by van der Waals force, resulting a strong bonding with the target surface.32 Following the above procedures, here we applied the SRCN onto a piece of paper, a leaf, and a finger joint. As shown in Figure 3a, a piece of paper with the pasted SRCN becomes highly conductive. Connected into an electrical circuit by two clips, the paper can sustain the lighting of a commercial LED. Even if the paper is crumpled into a ball, the brightness of the LED keeps stable as the resistance between the two clippers mildly increases form 26.8 Ω to 32.8 Ω. Similarly, the leaf covered by the SRCN also becomes conductive, and the veins are clearly observable (inset in Figure 3b). After coiled, the conductive leaf experiences a rise of the resistance from 30.2 Ω to 34.3 Ω, which barely affects the brightness of the LED (Figure 3b). Once again, similar observations are obtained if the SRCN is utilized onto a finger knuckle. At


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the state in which the skin is wrinkled (inset in Figure 3c), the resistance of the SRCN is measured to be 29.8 Ω between the two clips. When the knuckle is bent with the skin stretched (Figure 3c), the resistance is found to be 30.4 Ω, which causes negligible effect on the brightness of the LED, as shown in Video S1. In this work, we particularly investigated the SRCN as an epidermal electrode. Presented in Figure 3d, a piece of SRCN with an area of 3 × 2.5 cm2 is attached on the wrist. Owing to the excellent optical transmittance, it is almost invisible. However the skin is distorted, e.g. being wrinkled, stretched, and twisted, the SRCN can freely follow deformations along with the skin in a conformal way without being detached. This is ascribed to not only small mechanical moduli of the SRCN but also its tight bonding with the genuine skin. Attached to the backside of an index finger knuckle that is bent at an angle of 50°, the SRCN is examined in its resistance during the finger bending. The insets in Figure 3e describe different bending states of the finger knuckle in side views (upper row) and top-down views (lower row). From the unbent state (most left-handed state in Figure 3e) to the fully bent state (most right-handed state at a bending angle of 90° in Figure 3e), the variation of the resistance is less than 1.8 %. After 1,600 cycles of quick bending at a frequency of 2 Hz, the increase percentage of the resistance is found to be 19%, as revealed in Figure 3f. It is also observed from the curve in Figure 3f that the increase rate slows down as more bending cycles are conducted, which is beneficial for long-term application of the SRCN. For an epidermal electrode, air permeability is supposed to be an essential character because it has to be breathable so that perspiration can evaporate in time. Unfortunately, this character was normally omitted in previously published works on electronic skins, as described in Table S1. In this work, the porous structure of the SRCN provides channels for air conduction. At a pressure drop of 300 Pa, the air permeability of 6 cm3/s/cm2 is achieved, which is comparable to woven


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fabrics.34 Meanwhile, the resistance remains constant regardless of the air permeability, as illustrated in Figure 3g. This unique feature is another merit of the SRCN for long-time use as an epidermal electrode. As a key building block for electronic skins, the SRCN is also readily compatible to preparing flexible circuit patterns for functional devices. As demonstrated in Figure 4a, a piece of SRCN with a spiral-patterned conducting line is tightly attached to the backside of a hand. The mask-assisted fabrication process is discussed in EXPERIMENTAL SECTION. Shown in Figure 4a, when the two ends of the spiral line are connected into an electric circuit, a LED is powered up. The spiral pattern is explicitly exhibited in Figure 4b. The SEM image of the pattern in Figure 4b reveals the conducting line that has a width of ~ 400 µm. Further magnified image in Figure 4c shows the clear-cut boundary of the line. The region with the Ag NWs (Figure 4c) shows a sharp contrast to the region with the scaffold alone. Therefore, the facile fabrication process of the patterned SRCN highly benefits the assembly of functional electronic skins. Having an interdigitated pattern, the SRCN can be used as a touch sensor (Figure S10, S11). As shown in Figure 4d, the sensor of high transparency is in conformal contact with a fingertip. The separated two parts of the interdigitated structure become electrically connected and allow the flow of electric current once the fingertip touches a conductive object. The measured current experiences dramatic change between the touched and the untouched states, which is independent of the touch frequency (Figure 4e). Furthermore, a type of flexible optical electronic skin (OES) was constructed based on the patterned SRCN as electrodes. As schemed in Figure 4f, a layer of ZnS:Cu/PDMS composite is sandwiched between two layers of the SRCN. The ZnS:Cu phosphor serves as luminescent material while the PDMS performs as an elastic matrix. Figure 4g represents the cross-sectional


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view of the OES. Driven by an ac power, electroluminescence from the ZnS:Cu phosphor is obtained. Owing to the stable conducting property of the SRCN, the electroluminescence is maintained however the electronic skin is distorted, including flexing, stretching and twisting (Figure S12). The OES for display can be adhered onto the genuine skin, as shown in Figure 4h.

CONCLUSIONS In summary, we demonstrate a highly robust, transparent and breathable epidermal electrode that is composed of a scaffold-reinforced conductive nano-network. It possess both high transparency and low sheet resistance. Owing to the structural reinforcement from the nanofibers-based scaffold backbone, the conducting Ag NWs network exhibits excellent robustness as it experiences cyclic deformations and exposure to saline solution. Due to strong van der Waals force and small mechanical moduli, the SRCN forms intimate, conformal and endured bonding with target surface, especially genuine skin. The good mechanical compliance allows it to freely follow repeated deformations of the skin, and stable conductivity is observed. Besides, the fabrication process is compatible to the formation of flexible circuit patterns with line width on the micrometer scale. This feature provides a facile approach for constructing patterned electrodes in electronic skins, which is demonstrated here in a touch sensor and an optical device. More importantly, the internal channels and pores make the SRCN breathable as they allows the evaporation of sweat, which is essential for its long-time use as an epidermal electrode. Therefore, the SRCN reported here shows great promise for applications in electronic skins and potentially other flexible devices. EXPERIMENTAL SECTION Synthesis of the Ag NWs. The Ag NWs were synthesized by using a polyol process.28,35,36 In the experiment, ethylene glycol (EG, Aladdin) was used as a solvent, and also rendered the


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reduction of Ag+ ions. 3.35 g of poly vinylpyrrolidone (PVP, Mw: 58000, Aladdin) and 0.60 g of KBr (potassium bromide, Aladdin) were dissolved in 100 ml of EG in a flask, and the mixture was heated and thermally stabilized at 170 ℃ under gentle stirring. The PVP was genetically dissolved with the increasing temperature, and the solution became canary yellow. Then, 0.117 g of NaCl (sodium chloride, Aladdin) and 0.5 ml of deionized water were added consecutively into the flask and stirred rapidly. Simultaneously, 0.34 g of AgNO3 (silver nitrate, Aladdin) was dissolved in solution for the initial nucleation of silver seeds. After 5 min, 1.35 g of AgNO3 was slowly added as the source for the Ag NWs growth. The temperature was kept at 170 ℃ for about 30 min. Upon the completion of the reaction, the solution was cooled in ambient air, and then washed by acetone and deionized water to remove the ethylene glycol was well as the PVP. The washed solution was then centrifuged at 2000 rpm for 20 min to collect the Ag NWs of high aspect ratio and to discard other residues. After that, the supernatant containing the Ag NWs of high aspect ratio was centrifuged at 5000 rpm for 10 min to remove Ag nanoparticles. The collected Ag NWs in the sediment were then dispersed in alcohol. Fabrication of the nanofiber scaffold. For PA6 nanofibers,37-39 the precursor was made by dissolving PA6 particles (PA6, Aladdin) into formic acid (Aladdin) at a weight ratio of 15%, and then stirred for about 12 h at room temperature. The electrospinning device (Ucalery, ET-2535H) was used to produce nanofibers with a positive voltage of 16 KV and a negative voltage of 2 KV, and the PA6 nanofibers of 3 µm thick were collected on aluminum foil wrapping around a revolving roller. Then a PET frame with double sided tape was used to peel off the PA6 nanofibers from the collector. Fabrication of the SRCN. The SRCN was fabricated by a vacuum filtration method. A piece of nanofiber scaffold with the PET frame was used as a filtration membrane, and diluted


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Ag NWs dispersion was poured onto the scaffold membrane and vacuum filtrated through the membrane by a sucking pump. For the creation of circuit patterns, the SRCN was covered by a layer of mask (made of PET films) during the vacuum filtration. Conductivity testing of the SRCN. To test the bending stability, a piece of SRCN with the size of 2 × 2 cm2 was pasted on a 100 µm PET film. Copper wires were attached at the opposite sides of the SRCN by silver paste. The electrical resistance was monitored by a sourcemeter (Keithley 2400). Cyclic bending was conducted at a frequency of 0.5 Hz, and the largest bending curvature was about 300 m-1. To test the stretching stability, a piece of the SRCN with the size of 2 × 2 cm2 was pasted on a 100 µm cured PDMS film. Testing of the touch sensor. The overall size of the pattern is about 2 ×1.5 cm2. The width of teeth is 400 µm, and the interval between adjacent teeth is about 800 µm. A metal bar was put into contact with the finger, and the current response to the touch was measured by an electrometer Keithley 6514 with an applied voltage of 0.1 V. Fabrication of the flexible OES. The ZnS:Cu phosphor was mixed with the PDMS (10% curing agent) at a weight ratio of 1:1.40,41 The mixture was coated on a flat substrate and cured at 80 ℃ for 60 min, obtaining a 120 µm thick luminescence layer. Subsequently, two layers of the SRCN with identical patterns were wetted and spread onto the opposite sides of the luminescence layer with the patterns aligned. Then, the optical device was dried in ambient air.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1-S12, Table S1 and Video S1. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Y.J.F. and X.L. contributed equally to this work. ACKNOWLEDGMENT This research was supported by the National Key R & D Project from Ministry of Science and Technology, China (Grant Nos. 2016YFA0202701 and 2016YFA0202703), National Science Foundation of China (Grant No. 51572030), Natural Science Foundation of Beijing Municipality (Grant No. 2162047), and China Thousand Talents Program. REFERENCES 1.

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Hammock, M. L.; Benjamin, A. C.; Tee, C. K.; Tok, J. B. H.; Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress. Adv. Mater. 2013, 25, 5997-6038.

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Bauer, S.; Gogonea, S. B.; Graz, I.; Kaltenbrunner, M.; Keplinger, C.; Schwödiauer, R. 25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters. Adv. Mater. 2014, 26, 149-161.

10. Liu, Y.; Pharr, M.; Salvatore, G. A. Lab-on-Skin: A Review of Flexible and Stretchable


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Electronics for Wearable Health Monitoring. ACS Nano 2017, 11, 9614-9635. 11. Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D. -H.; Brooks, G. A.; Davis, R. W.; Javey, A. Fully Integrated Wearable Sensor Arrays for Multiplexed In Situ Perspiration Analysis. Nature 2016, 529, 509-514. 12. Zhao, S.; Zhu, R. Electronic Skin with Multifunction Sensors Based on Thermosensation. Adv. Mater. 2017, 29, 1606151. 13. Liu, Y.; He, K.; Chen, G.; Leow, W. R.; Chen, X. Nature-Inspired Structural Materials for Flexible Electronic Devices. Chem. Rev. 2017, 117, 12893-12941. 14. Jin, J.; Lee, D.; Im, H. -G.; Han, Y. C.; Jeong, E. G.; Rolandi, M.; Choi, K. C.; Bae, B. S. Chitin Nanofiber Transparent Paper for Flexible Green Electronics. Adv. Mater. 2016, 28, 5169-5175. 15. Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A. Nanowire Active-Matrix Circuitry for Low-Voltage Macroscale Artificial Skin. Nat. Mater. 2010, 9, 821-826. 16. Xu, J.; Wang, S.; Wang, G. -J. N.; Zhu, C.; Luo, S.; Jin, L.; Gu, X.; Chen, S.; Feig, V. R.; To, J. W. F.; Gagné, S. R.; Park, J.; Schroeder, B. C.; Lu, C.; Oh, J. Y.; Wang, Y.; Kim, Y. -H.; Yan, H.; Sinclair, R.; Zhou, D., et al. Highly Stretchable Polymer Semiconductor Films Through the Nanoconfinement Effect. Science 2017, 355, 59-64. 17. Han, B.; Huang, Y.; Li, R.; Peng, Q.; Luo, J.; Pei, K.; Herczynski, A.; Kempa, K.; Ren, Z.; Gao, J. Bio-Inspired Networks for Optoelectronic Applications. Nat. Commun. 2014, 5, 5674. 18. Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. -S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.;


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Islam, A.; Yu, K. J.; Kim, T.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H. -J.; Keum, H.; Cormick, M. M.; Liu, P., et al. Epidermal Electronics. Science 2011, 333, 838-843. 19. Yeo, W. H.; Kim, Y. -S.; Lee, J.; Ameen, A.; Shi, L.; Li, M.; Wang, S.; Ma, R.; Jin, S. H.; Kang, Z. Multifunctional Epidermal Electronics Printed Directly onto the Skin. Adv. Mater. 2013, 25, 2773-2778. 20. Miyamoto, A.; Lee, S.; Cooray, N. F.; Lee, S.; Mori, M.; Matsuhisa, N.; Jin, H.; Yoda, L.; Yokota, T.; Itoh, A. Inflammation-Free, Gas-Permeable, Lightweight, Stretchable on-Skin Electronics with Nanomeshes. Nat. Nanotechnol. 2017, 12, 907-913. 21. Im, H. -G.; Jin, J.; Ko, J. -H.; Lee, J.; Lee, J. -Y.; Bae, B. -S. Flexible Transparent Conducting Composite Films Using a Monolithically Embedded AgNW Electrode with Robust Performance Stability. Nanoscale 2014, 6, 711-715. 22. Deng, B.; Hsu, P. C.; Chen, G.; Chandrashekar, B. N.; Liao, L.; Ayitimuda, Z.; Wu, J.; Guo, Y.; Lin, L.; Zhou, Y.; Aisijiang, M.; Xie, Q.; Cui, Y.; Liu, Z.; Peng, H. Roll-to-Roll Encapsulation of Metal Nanowires between Graphene and Plastic Substrate for High-Performance Flexible Transparent Electrodes. Nano Lett. 2015, 15, 4206-4213. 23. Hattori, Y.; Falgout, L.; Lee, W.; Jung, S. -Y.; Poon, E.; Lee, J. W.; Na, I.; Geisler, A.; Sadhwani, D.; Zhang, Y.; Su, Y.; Wang, X.; Liu, Z.; Xia, J.; Cheng, H.; Webb, R. C.; Bonifas, A. P.; Won, P.; Jeong, J. -W.; Jang, K. I., et al. Multifunctional Skin-Like Electronics for Quantitative, Clinical Monitoring of Cutaneous Wound Healing. Adv. Healthcare Mater. 2014, 3, 1597-1607. 24. Abdal-hay, A.; Abdelrazek Khalil, K.; Al-Jassir, F. F.; Gamal-Eldeen, A. M. Biocompatibility Properties of Polyamide 6/PCL Blends Composite Textile Scaffold Using


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EA.hy926 Human Endothelial Cells. Biomed. Mater. 2017, 12, 035002. 25. Sannicolo, T.; Lagrange, M.; Cabos, A.; Celle, C.; Simonato, J. P.; Bellet, D. Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: A Review. Small 2016, 12, 6052-6075. 26. Li, J.; Hu, L.; Wang, L.; Zhou, Y.; Grüner, G.; Marks, T. J. Organic Light-Emitting Diodes Having Carbon Nanotube Anodes. Nano Lett. 2006, 6, 2472-2477. 27. Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes. Nano Lett. 2009, 9, 4359-4364. 28. Hu, L.; Kim, H. -S.; Lee, J. Y.; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4, 2955-2963. 29. Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H. Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network. Adv. Mater. 2012, 24, 3326-3332. 30. Kim, G. M.; Michler, G. H.; Ania, F.; Calleja, F. J. B. Temperature Dependence of Polymorphism in Electrospun Nanofibres of PA6 and PA6/Clay Nanocomposite. Polymer 2007, 48, 4814-4823. 31. Peng, P.; Hu, A.; Huang, H.; Gerlich, A. P.; Zhao, B.; Zhou, Y. N. Room-Temperature Pressureless Bonding with Silver Nanowire Paste: Towards Organic Electronic and Heat-Sensitive Functional Devices Packaging. J. Mater. Chem. 2012, 22, 12997-13001. 32. Pugno, N. M. Towards a Spiderman Suit: Large Invisible Cables and Self-Cleaning Releasable Superadhesive Materials. J. Phys.: Condens. Matter 2007, 19, 395001. 33. Lu, B.; Zhu, D.; Hinton, D.; Humayun, M. S.; Tai, Y. C. Mesh-Supported Submicron


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Parylene-C Membranes for Culturing Retinal Pigment Epithelial Cells. Biomed. Microdevices 2012, 14, 659-667. 34. Ogulata, R. T. Air Permeability of Woven Fabrics. J. Text Apparel Technol. Manage. 2006, 5, 1-10. 35. Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Crystalline Silver Nanowires by Soft Solution Processing. Nano Lett. 2002, 2, 165-168. 36. Yang, C.; Gu, H.; Lin, W.; Yuen, M. M.; Wong, C. P.; Xiong, M.; Gao, B. Silver Nanowires: From Scalable Synthesis to Recyclable Foldable Electronics. Adv. Mater. 2011, 23, 3052-3056. 37. Fong, H.; Liu, W.; Wang, C. S.; Vaia, R. A. Generation of Electrospun Fibers of Nylon 6 and Nylon 6-Montmorillonite Nanocomposite. Polymer 2001, 43, 775-780. 38. Ryu, Y. J.; Kim, H. Y.; Lee, K. H.; Park, H. C.; Lee, D. R. Transport Properties of Electrospun Nylon 6 Nonwoven Mats. Eur. Polym. J. 2003, 39, 1883-1889. 39. Aussawasathiena, D.; Teerawattananon, C.; Vongachariyac, A. Separation of Micron to Sub-micron Particles from Water: Electrospun Nylon-6 Nanofibrous Membranes as Pre-filters. J. Membr. Sci. 2008, 315, 11-19. 40. Wei, X. Y.; Wang, X.; Kuang, S. Y.; Su, L.; Li, H. Y.; Wang, Y.; Pan, C.; Wang, Z. L.; Zhu, G. Dynamic Triboelectrification-Induced Electroluminescence and Its Use in Visualized Sensing. Adv. Mater. 2016, 28, 6656-6664. 41. Wang, J.; Yan, C.; Chee, K. J.; Lee, P. S. Highly Stretchable and Self-Deformable Alternating Current Electroluminescent Devices. Adv. Mater. 2015, 27, 2876-2882.


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Figure 1. Fabrication and structure of the SRCN. (a) Schematics of the fabrication process. (b) Three dimensional scheme of the SRCN. (c) Top-down and (d) cross-sectional views of the SRCN in SEM images, respectively. (e) Photograph of the SRCN showing great optical transmission. (f) Photograph of the SRCN applied on the backside of a hand.


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Figure 2. Optical and electrical characterizations of the SRCN. (a) Sheet resistance of the SRCN with different Ag NWs loading quantity. (b) Transmittance spectra of the SRCN with different sheet resistance. (c) Area mapping of the sheet resistance on the SRCN. (d-h) Normalized resistance variation when the SRCN is subject to bending, cyclic bending, uniaxial strain, repeated wetting in saline solution, and heating, respectively. (i) Normalized sheet resistance of the SRCN that is set in ambient air for 100 days, and corresponding absolute values of the sheet resistance in the inset.


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Figure 3. Conducting performance of the SRCN on target surface. (a-c) Photographs of electrical circuits for powering LEDs, in which the SRCN is applied onto a piece of crumpled paper, a coiled leaf, and a


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bent finger knuckle, respectively; insets: corresponding unstrained states. (d) Photographs of the SRCN on the wrist at unstrained state (left) and at the states of wrinkling (upper-right), stretching (middle-right) and twisting (lower-right). (e,f) Normalized resistance variation of the SRCN on a finger knuckle bending from 0° to 90° in a single cycle and for a number of cycles, respectively. (g) Pressure drop and normalized resistance at different air permeability.


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Figure 4. Patterned SRCN for fabricating a touch sensor and an OES. (a) Photograph of an electrical circuit for powering a LED, in which a spiral-patterned SRCN on the hand serves as a conducting line. (b) Magnified view of the pattern and one end of the pattern in a SEM image (right). (c) A line boundary in the pattern and magnified SEM images of the regions with Ag NWs (upper-right) and without Ag NWs


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(lower-right). (d) Photograph of the interdigitated SRCN on a fingertip as a touch sensor (left), and corresponding magnified photograph of the pattern (upper-right) and the overall pattern (lower-right). (e) Measured electric current from touch sensor as it contacts with a conductive object in different frequencies. (f) Scheme of the OES. (g) Cross-sectional views of the OES (left) and magnified views on the phosphor (upper right) and the SRCN (lower right). (h) Photograph of the OES on the hand with the luminescent pattern ‘SKIN’.

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Highly Robust, Transparent, and Breathable Epidermal Electrode

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