Extremely Stretchable and Self-Healable Electrical Skin with

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24639−24647

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Extremely Stretchable and Self-Healable Electrical Skin with Mechanical Adaptability, an Ultrawide Linear Response Range, and Excellent Temperature Tolerance Haoxiang Zhang, Wenbin Niu,* and Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, West Campus, 2 Linggong Rd., Dalian 116024, China

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S Supporting Information *

ABSTRACT: Artificial electronic skin (e-skin) that imitates the complex functions of human skin is able to transduce external stimuli into electronic signals. However, it remains challenging to fabricate e-skin sensing materials with extreme stretchability, self-healing, mechanical compliance, extreme temperature tolerance, and an ultrawide linear response range. Here, we demonstrate a new e-skin sensor fabricated by introducing polyvinylpyrrolidone (PVP)-capped Ag nanowires into the chemically and physically cross-linked polyacrylamide−PVP double-network ethylene glycol organogel. The resulting organogel e-skin exhibits extreme stretchability (>22 000%), autonomous selfhealing, as well as mechanical compliance. Particularly, the sensor is capable of antifreezing and antiheating (−20 to 80 °C) and provides an ultrawide linear response range with a gauge factor of 0.15 for 0−430% tensile strain and 0.71 for 430−18 100% tensile strain, respectively. By dynamically accommodating to a curved surface, the e-skin sensor demonstrates comprehensive applications in real-time and in situ tracking of large body deformations, spatial gesture movements, and physiological signals for motion behaviors and health level evaluation, showing great promise in wearable electronics, biomedical devices, and soft robotics. KEYWORDS: electronic skin, extreme stretchability, linear signal response, self-healing, mechanical adaptability

1. INTRODUCTION Human skin is soft, robust, stretchable, self-healable, and sensory, which not only protects internal tissues from injury but also senses external stimuli and conducts tactile signals.1,2 Artificial electronic skin (e-skin) that imitates the complex functions of human skin is able to transduce external stimuli such as temperature, pressure, and strain into electronic signals (e.g., current,3−5 resistance,6,7 and capacitance8,9) that can be easily detected, thus showing great promise in areas ranging from physical motion monitoring, clinical diagnostics, human− machine interaction, to virtual reality technology. To imitate the mechanical and sensory characteristics of the skin, the development of sensing materials and devices with suitable elastic moduli (0.5−1.95 MPa), excellent stretchability, and self-healing capability is significantly required.10−14 For example, Xu and co-workers reported a stretchable (500%) and self-healable polymer composite composed of polyaniline, polyacrylic acid, and phytic acid.15 Using polypyrrole-grafted chitosan as a conductive element, Xing et al. synthesized a mechanically and electrically self-healing hydrogel based on ferric ion and covalently cross-linked poly(acrylic acid) double networks.12 The corresponding sensor exhibited ultrastretchability (1500%), fast resistance response, and autonomous selfhealing property (100% efficiency in 2 min). Analogously, Lee et al. introduced a type of stretchable and self-healable strain sensor by embedding electronic conductors such as carbon © 2019 American Chemical Society

nanotube/graphene in a tetrafunctional borate-crosslinked polyvinyl alcohol hydrogel.16 Such sensing materials and devices have been demonstrated as sensitively wearable devices for mechanical stimuli monitoring, motion detection, and human−machine interaction. Apart from high stretchability and self-healing capability, however, good mechanical compliance, durability, and extreme temperature tolerance are equally important characteristics for practical uses of a soft eskin.9,17−19 For example, an epidermal sensor should possess good mechanical adaptability to fully match curved and dynamic surfaces for detecting biosignals,9 and wearable electronics must resist extreme temperature variation of the natural environments, such as high-temperatures in summer and low temperatures in winter. While artificial e-skins with one or two aforementioned functional properties have been successfully demonstrated, the development of a skin-like sensor, that is simultaneously highly stretchable, self-healing, mechanically adaptive, and extremely temperature resistive, remains a great challenge. In addition, the linear signal response is another important parameter in the practical uses of e-skin sensor because the calibration process of the nonlinear signal is complicated and Received: May 30, 2019 Accepted: June 21, 2019 Published: June 21, 2019 24639

DOI: 10.1021/acsami.9b09430 ACS Appl. Mater. Interfaces 2019, 11, 24639−24647

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of e-skin (a) preparation and (b) properties.

Figure 2. (a) Schematic diagram of a double-network organogel e-skin. (b) Cross-sectional SEM images of e-skin. (c) Frequency dependence of the storage (G′) and loss (G″) moduli of the as-prepared e-skin. (d) Schematic showing mechanical adaptability and (e) optical photos indicating dynamic adaptability to a prosthetic finger surface.

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Figure 3. Electromechanical performance of organogel e-skin. (a) Stress−strain curves of the e-skin at 16, −20, and 80 °C. (b) Comparison of mechanical properties between e-skin and hydrogel at 16, −20, and 80 °C. The relative resistance change of e-skin as a function of tensile strain at (c) 16, (d) −20, and (e) 80 °C, and the corresponding linear fitting relationship. (f) Schematic illustrating the antifreezing and antiheating properties of e-skin. (g) Linear response mechanism. (h) Comparison of this work to recent works in linear sensing range.21,23,33−41 (i) Compressive stress−strain and relative resistance change−strain curves of e-skin at room temperature, and (j) corresponding pressure sensitivity.

difficult.20 In contrast, the linear response will greatly facilitate the above process and thus improve signal accuracy and reliability.20 To investigate the linear response mechanism, Park et al. fabricated a strain sensor consisting of Ag nanowires on polyimide film.21 It is revealed that the nonlinear response of the electrical signal is usually caused by the non-uniform change in the microstructure of conductive elements under external forces.21,22 Recently, a buckled fiber strain sensor was also designed and fabricated by spraying carbon nanotubes onto prestretched rubber fibers. During stretching, the reduction of contact area between the adjacent buckles increased the resistance to change, showing two linear ranges of 0−200 and 200−600%.23 These existing investigations motivated us to design a skin-like sensor with an ultrawide linear sensing range.

Here, we demonstrate a new electrical organogel e-skin material with extreme stretchability (>22 000%), autonomous self-healing capability, mechanical adaptability, excellent temperature tolerance performance (−20 to 80 °C), as well as an ultrawide linear sensing range with a gauge factor (GF) of 0.15 for 0−430% tensile strain and 0.71 for 430−18 100% tensile strain, respectively. We first introduced polyacrylamide−polyvinylpyrrolidone (PAAM−PVP) double-networks that combine chemical and physical crosslinks to achieve extreme stretchability.24−26 Meanwhile, the presence of reversible hydrogen-bonding interaction between −NH2 in PAAM and OC in PVP endues the gel material self-healing property without functional modification.13,27 Second, ethylene glycol (EG) was used as a liquid constituent instead of water to reduce the physical entanglement density of gel material, thus giving rise to the excellent mechanical adaptability.24 In 24641


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mechanism was further confirmed by the fact that there is a substantial hysteresis loop with a residual strain cycle and no pronounced hysteresis in the first and the subsequent loading− unloading cycles (Figure S9). The e-skin exhibited good antifreezing and antiheating properties in a wide temperature range because of the high boiling point and low saturated vapor pressure of EG (Figure 3f). To quantify the temperature tolerance, the mechanical properties of gel e-skin were tested immediately after storing at −20 and 80 °C for 30 min. The stress−strain curves revealed that the elongations at break were 22 000% at −20 °C and reduced to 17 000% at 80 °C due to slight evaporation of EG, indicating that the mechanical properties of organogel e-skin in this temperature range are well maintained. In contrast, the hydrogel e-skin was frozen and fragile at −20 °C and was hardened and lost elasticity at 80 °C, as intuitively displayed in Figure 3b. This is because the low freezing point of EG prevents the formation of crystal regions at low temperature, while high boiling point avoids its rapid evaporation at a high temperature.17,18 As a result, organogel e-skin exhibits good stability in a wide temperature range, which could be further improved by elastomer encapsulating (such as polydimethylsiloxane) to retard volatilization of EG,19 thus widening the scope of seasonal and geographical applications considerably. Notably, good linear response throughout sensing range is another prominent feature of our e-skin. Figure 3c shows the relative resistance changes (ΔR/R0= (R − R0)/R0, where R0 is initial resistance and R is stretched resistance) as a function of tensile strain, in which e-skin exhibits ultralarge sensing range. Interestingly, there is good linearity with two sensing ranges (0−1000 and 1000−18 100%). The tensile sensitivity (GF) was calculated to be 0.21 (0−1000% tensile strain) and 0.71 (1000−18 1000% tensile strain), respectively, according to GF = (ΔR/R0)/ε from the slope of relative resistance change (ΔR/R0) versus strain (ε).23,33−42 Particularly, the e-skin maintained high linearity at both low and high temperatures (Figure 3d,e), demonstrating extreme temperature tolerance. Such linearity with two sensing ranges may be due to the tunneling effect and the response of organogel itself after Ag NWs network breaking completely (Figure 3g). In the initial state, the tunneling effect between Ag NWs conductive channel dominates the relative resistance change, and the resistance enhanced with increase in the distance between Ag NWs within a tensile strain 22 000% because of the presence of reversible hydrogen-bonding interaction and hybrid chemically and physically cross-linked PAAM−PVP double-networks. Notably, the ultrawide linear signal response was realized in an extreme temperature range from −20 to 80 °C. On the basis of these merits, we demonstrated its application as a sensor to monitor human behaviors and health level by mechanically adapting to the curved or dynamic surface, which can not only identify gesture movements and spatial directions (e.g., fingers configuration and wrist moving directions) but also clearly distinguish physiological signals (e.g., respiration and wrist tremor trembling with Parkinson’s disease). This work may open a door to construct diverse novel gel materials with unique functions by introducing other 0-, 1-, and 2-dimensional nanomaterials, and we anticipate such a multifunctional e-skin can be highly attractive to emerging wearable, healthcare, medical, and human−machine interfacing applications.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b09430. SEM images of Ag NWs and organogel; FT-IR spectra; mechanical properties and cyclic tensile test of e-skin; photographs of e-skin before and after stretching; electromechanical performance of organogel; relative resistance change of pure organogel without Ag NWs doping; electromechanical properties of organogel e-skin with a 1.25 wt % of PEGDA400 cross-linker; cyclic tensile test of e-skin at −20 and 80 °C; mechanical and electrical self-healing properties of e-skin; conductive loop using e-skin as a flexible conductive circuit; and cyclic tensile of e-skin before and after self-healing (PDF) As-prepared e-skin exhibiting extreme stretchability (MP4)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wenbin Niu: 0000-0001-5507-1592 Shufen Zhang: 0000-0003-3390-4199 Author Contributions

4. EXPERIMENTAL SECTION

All authors are contributed to this work, and the manuscript was written through the contributions of all authors.

4.1. Materials. PVP (M̅ = 58 000 g/mol, K30) and polyethylene glycol diacrylate (PEGDA400) were purchased from Sigma-Aldrich Chemical Reagent. AAM and EG (chromatographic grade) were purchased from Kermel. 4.2. Preparation of the Skin-like Gel Material. Ag NWs were prepared according to reported methods.28,31 Typically, 50 mL EG was heated to 151 °C for about 1 h under magnetic stirring. Then, a CuCl2 EG solution of 400 μL (4 × 10−3 M) was introduced followed

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by Key Program of National Natural Science Foundation of China (21536002), 24645


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National Natural Science Foundation of China (21506023), the fund for innovative research groups of the National Natural Science Fund Committee of Science (21421005), Innovation Research Team in University (IRT_13R06), and the Fundamental Research Funds for the Central Universities (DUT19JC14).

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Extremely Stretchable and Self-Healable Electrical Skin with

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