Stretchable, Transparent, Tough, Ultrathin, and Self-limiting Skin-like

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

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Stretchable, Transparent, Tough, Ultrathin, and Self-limiting Skinlike Substrate for Stretchable Electronics Adeela Hanif,† Tran Quang Trung,† Saqib Siddiqui,† Phan Tan Toi,† and Nae-Eung Lee*,†,‡,§ †

School of Advanced Materials Science and Engineering, ‡SKKU Advanced Institute of Nanotechnology (SAINT), and §Samsung Advanced Institute for Health Sciences and Technology (SAIHST), Sungkyunkwan University, Suwon, Kyunggi-do 16419, Korea

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S Supporting Information *

ABSTRACT: Human skin is highly stretchable at low strain but becomes selflimiting when deformed at large strain due to stiffening caused by alignment of a network of stiff collagen nanofibers inside the tissue beneath the epidermis. To imitate this mechanical behavior and the sensory function of human skin, we fabricated a skin-like substrate with highly stretchable, transparent, tough, ultrathin, mechanosensory, and self-limiting properties by incorporating piezoelectric crystalline poly((vinylidene fluoride)-co-trifluoroethylene) (P(VDF-TrFE)) nanofibers with a high modulus into the low modulus matrix of elastomeric poly(dimethylsiloxane). Randomly distributed P(VDF-TrFE) nanofibers in the elastomer matrix conferred a self-limiting property to the skin-like substrate so that it can easily stretch at low strain but swiftly counteract rupturing in response to stretching. The stretchability, toughness, and Young’s modulus of the ultrathin (∼62 μm) skin-like substrate with high optical transparency could be tuned by controlling the loading of nanofibers. Moreover, the ultrathin skin-like substrate with a stretchable temperature sensor fabricated on it demonstrated the ability to accommodate bodily motion-induced strain in the sensor while maintaining its mechanosensory and thermosensory functionalities. KEYWORDS: skin-like substrate, self-limiting, stretchable electronics, reinforcing filler, ultrathin

1. INTRODUCTION Stretchable electronics that can mimic the fundamental characteristics of human skin are in great demand in robotics,1,2 electronic skin,3,4 artificial intelligence,5 diagnostics,6,7 prosthetics,8,9 human machine interfaces,10 and health monitoring technologies.11−15 In particular, a myriad of stretchable electronic devices have been developed by incorporating novel sensing materials,16−19 structural designs,20−22 and processing methods on stretchable substrates.23−25 Stretchable electronic devices mountable on human skin can meet the requirements for bending, twisting, stretching, and deforming into complex curvilinear shapes.26 Elastomeric materials including poly(dimethylsiloxane) (PDMS),27,28 polyurethane (PU),29,30 Ecoflex,31,32 and poly(styrene-butadiene-styrene)33 have been used as stretchable substrates for the fabrication of stretchable electronic devices. Often, however, these elastomeric substrate materials are not tough enough to endure the device fabrication process, make thin devices for improved conformality, and be handled without damaging them. Furthermore, the large stretchability of the existing conventional elastomeric substrates means that the devices with limited stretchability fabricated on them are not fully protected from excessive stretching when the wearer is engaging in vigorous activity. Interestingly, stretchable human skin has self-limiting mechanical properties due to which skin toughness abruptly increases at a high stretching © XXXX American Chemical Society

strain above a certain value (typically larger than 30%) to prevent rupture.34,35 Therefore, the stretchable substrate must possess durability as well as a self-limiting function for future skin-attachable applications of stretchable electronic devices. In addition, mimicking the properties of human skin is of great interest in the field of stretchable electronics. The primary structural layers in human skin include the epidermis, dermis, and subcutaneous layer (see the schematic drawing in Figure 1a). The dermis is composed of soft elastin fibers and stiff collagen fibers in a proteoglycan extracellular matrix.36,37 The mechanical behavior of skin depends on the properties of the soft and stiff components and is characterized by a low stiffness region at small strain and increased stiffness at large strain in stress−strain curves.34−36,38 The threedimensional network of elastin fibers with high elasticity provides elasticity and softness at low strain. Collagen fibers are arranged in a manner that permits continuous movement of individual fibers, which allows them to absorb minor stress during normal activities,39 whereas the strength of the stiff collagen fibers provides resistance to severe stress.34,35 When collagen fibers are stretched in any direction, the intertwined meshwork of collagen fibers becomes parallel to resist the large Received: May 19, 2018 Accepted: July 24, 2018 Published: July 24, 2018 A

DOI: 10.1021/acsami.8b08283 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic and characteristics of a skin-like substrate. (a) Schematic illustration of the structure of human skin. Human skin mechanical behavior is mainly controlled by the high-order network of collagen fibers in the dermal layer. (b) Schematic illustration of the ultrathin skin-like substrate. Randomly oriented P(VDF-TrFE) nanofibers as reinforcing fillers are embedded into a PDMS matrix to generate a stretchable, transparent, ultrathin, tough, self-limiting, and skin-like substrate that shows mechanical behavior that is similar to real human skin. (c) The optical transmittance of ultrathin and skin-like substrates. The inset figure (the scale bar is 10 μm) shows a photograph of the substrate, which was sufficiently transparent to allow clear visualization of the view behind the substrate. (d) The image demonstrates that the ultrathin skin-like substrate with a thickness of ∼62 μm (inset) conformally attached on a human hand without detachment induced by repetitive skin deformation. (e) Top-view field emission scanning electron microscopy (FE-SEM) images of electrospun (ES) P(VDF-TrFE) nanofibers. High-magnification FE-SEM image is shown as an inset. (f) Cross-sectional FE-SEM image of the skin-like substrate showing uniform distribution of P(VDF-TrFE) nanofibers inside the PDMS polymer matrix. The nanofibers in the PDMS matrix were delineated by wet etching. The scale bars in (e) and (f) are 10 μm.

applied strain. Therefore, soft human skin becomes mechanically stiff and self-limited in deformation above a certain stretching strain and can be protected from failure by large strain. Human skin also has sensory capabilities due to receptors distributed throughout the skin to detect temperature, strain, pressure, and other complex environmental stimuli.14,40 Similar to human skin, a self-limiting property in a stretchable substrate may help in maintaining the integrity of the layers in the stretchable devices as well as the reliability and level of device performance in response to large mechanical deformations that occur during physical activity of the wearer. Therefore, the development of skin-like stretchable substrates

that have high stretchability at low strain but high toughness at large strain is of great interest.41 Also, it is critical to develop a skin-like substrate that can impart both mechanical and sensory properties.42 As previously mentioned, however, conventional elastomeric substrate materials cannot mimic the mechanical properties of human skin. To increase the stiffness and strength of elastomeric substrates, elastomeric matrices have been mixed with a second phase of nanofillers, such as carbon nanotubes,43 cross-linked membranes,44 graphene oxide (GO),45 graphene,46 or silica nanoparticles.47 Although mixing with a second phase can increase the toughness, it is difficult to tune B


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properties, was the loading amounts of nanofibers in the matrix. For this purpose, the electrospun (ES) volume of P(VDF-TrFE) solution during the electrospinning process of nanofibers was varied from 0.1−0.8 mL in this experiment. A representative substrate sample fabricated with an ES volume of 0.2 mL and a thickness of ∼62 μm showed a high optical transmittance (∼80%) at wavelengths of 400−1000 nm in the visible region (Figure 1c). It is clear from the photograph of the ultrathin skin-like substrate that it was sufficiently transparent to allow clear visualization of a scene behind the substrate (inset in Figure 1c). The optical transmission spectra of the skin-like substrate at higher loadings of P(VDF-TrFE) nanofibers in the PDMS elastomer (ES volume of 0.4 and 0.8 mL), together with that of the pristine PDMS substrate as a reference, are presented in Figure S3. The data in Figures 1c and S3 indicate that the decrease in the optical transmittance with the increase in nanofiber loading was relatively small. This is presumably because of the highly crystalline nature of P(VDF-TrFE) nanofibers and amorphous PDMS that are highly transparent. Thus, the composite maintains its transparency even at higher loadings of the nanofibers. Reinforcement of the P(VDF-TrFE) nanofiber networks allowed the skin-like substrate to be ultrathin, yet still easily handled during the fabrication process without tearing or collapsing. The thickness of the skin-like substrate could be controlled by modulating the spin-coating conditions (see the Supporting Information, Figure S4). The minimum thickness of the substrate for handling without tearing and conformal attachment on human skin was around ∼62 μm (Figure S5), and this thickness was therefore used for all subsequent experiments. The overall thickness of the skin-like substrate consisting of P(VDF-TrFE) nanofiber networks inside a PDMS matrix was measured by field-emission scanning electron microscopy (FE-SEM) as shown in the inset in Figure 1d. Optical microscopy images at a low resolution showed that P(VDF-TrFE) nanofibers made a continuous network of nanofiber. To demonstrate the attachability of the skin-like substrate to human skin, the skin-like substrate with a thickness of ∼62 μm was attached to human skin without using an additional glue layer. The optical image in Figure 1d shows that the skin-like substrate was very conformal and seamlessly contacted the severely curved skin area without detachment from the skin under repetitive deformation of the skin. For comparison, a thin PDMS substrate (∼80 μm) without nanofiber reinforcement was also attached to the human skin region with a crooked geometry. The thinnest PDMS substrate without nanofibers that could be peeled off from the polyimide (PI) carrier film with no damage was ∼80 μm. The PDMS substrate without nanofiber reinforcement was easily detached after a few cycles of skin deformation (see Supporting Information Figure S6 and Video S1). The results indicate that the thinner skin-like substrate showed enhanced conformality even on complex skin geometries. It is important to embed the nanofibers uniformly and fully in the elastomeric matrix to impart mechanical stress during stretching for skin-like mechanical properties. Nanofiber networks of P(VDF-TrFE) were analyzed by cross-sectional field-emission scanning electron microscopy (FE-SEM) to investigate how well the nanofibers are distributed inside the PDMS matrix. We characterized the sample obtained using an ES volume of 0.2 mL, which generated uniformly networked nanofibers having diameters of ∼200 nm (the inset in Figure 1e) without the formation of beads (Figure 1e and the

mechanical moduli at both low strain and high strain for selflimiting or strain-stiffening properties. An ultrathin stretchable substrate mimicking human skin is desirable to overcome these limitations. This can be accomplished by incorporating filler materials having functions similar to collagen nanofibers in skin. Even though mimicry of the sensing capability of human skin via stretchable electronics has been demonstrated by several research groups,6,8,11,13,48 the adoption of stretchable, ultrathin, tough, skin-like substrates to mimic human skin mechanical behavior and sensory function has rarely been demonstrated. Recently, synthetic elastomers mimicking the mechanical behaviors of organ tissues were synthesized by brush and comb-like polymer networks.49 Even though this approach has the advantage of using a single component material, synthesis routes and tuneability of mechanical properties may not be trivial, and sensory function was not employed. Therefore, direct incorporation of skin-like sensory function on/into an ultrathin substrate to mimic the ability of human skin to have self-limiting properties and deformability with the body movement while maintaining its sensing functionalities still remains challenging. Herein, we report the development of a stretchable, ultrathin, mechanically tough, transparent, self-limiting skinlike substrate with a physical sensory function. This skin-like substrate was fabricated by incorporating poly((vinylidene fluoride)-co-trifluoroethylene) (P(VDF-TrFE)) nanofibers as reinforcing fillers and a functional component with piezoelectricity into an elastomeric (PDMS) matrix. P(VDF-TrFE) nanofibers have high strength and toughness, whereas PDMS is an elastomer with a low Young’s modulus (see the Supporting Information, Figure S1). The hybrid structure helps in increasing the strength, toughness, and Young’s modulus of the skin-like substrate. The substrate was still easy to handle due to reinforcement of the substrate with randomly incorporated nanofibers. The ultrathin skin-like substrate was highly stretchable at low strain and became tough at large strain, mimicking human skin. The mechanical behavior of the skin-like substrate could be tuned by varying the loading amount of the nanofibers due to the random orientation of P(VDF-TrFE) nanofibers that initially aligned without resistance at low strain but at a large strain straightened out due to loading of stress, resulting in an increase in modulus. The piezoelectricity of the P(VDF-TrFE) nanofibers provided sensing functionality of the strain induced in the skin-like substrate. Moreover, a stretchable temperature sensor fabricated on the ultrathin skin-like substrate had high stability on human skin, which confirmed the excellent mechanical adaptability of the skin-like substrate under bodily motion. The development of this stretchable, transparent, ultrathin, tough, self-limiting, and skin-like substrate with skin-like sensory functions is a step towards the realization of stretchable electronics.

2. RESULTS AND DISCUSSION Stiff (modulus of 1.5 GPa50) and piezoelectric P(VDF-TrFE) nanofibers were uniformly embedded into the elastomeric matrix of PDMS to fabricate a stretchable substrate with skinlike mechanical properties, mechanosensory function, and optical transparency. A conceptual drawing of the skin-like substrate is shown in Figure 1b. The detailed fabrication process is shown in Figure S2 and is described in the Experimental Details section. The most important parameter when tuning its optical, mechanical, and mechanoreceptive C


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Figure 2. Mechanical properties of skin-like substrates with different loading amounts of P(VDF-TrFE) nanofibers. (a) Stress−strain curves of skinlike substrates made with electrospun P(VDF-TrFE) solution volumes of (a) 0.2 mL, (b) 0.4 mL, (c) 0.6 mL, and (d) 0.8 mL. The (e) stretchability, (f) toughness, and (g) strength of the skin-like substrate as a function of ES volume. (h) Young’s modulus at different phases of the skin-like substrates with different nanofiber loadings. As a reference, the moduli of the human abdomen, forehead, and tibia ranging from 0.12−0.6 MPa is provided. D


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P(VDF-TrFE) nanofibers increased, as shown in the FE-SEM images of ES nanofibers (see Supporting Information Figure S10). The stress−strain curves of the skin-like substrates fabricated with ES volumes of 0.4, 0.6, and 0.8 mL are shown in Figure 2b−d, respectively. Three distinct phases were evident in all the curves, and stretchability, toughness, strength, and Young’s modulus were extracted or calculated from stress−strain curve parameters. These results are also summarized, together with information about the PDMS substrate with 0.1 mL ES volume and the pure PDMS substrate for comparison, in Figure S11 and Table S2. The stress−strain curves of the substrate with a 0.1 mL ES volume of P(VDF-TrFE) solution were similar to that of the pure PDMS substrate, which indicates that no self-limiting behavior was imparted due to the small number density of nanofibers inside the elastomer, even though reinforcement of the substrate was observed. An increase in nanofiber loading in the skin-like substrate resulted in a decrease in stretchability (Figure 2e) but an increase in strength (Figure 2f), toughness (Figure 2g), and Young’s modulus (Figure 2h). As the nanofiber content increased, the stiff phase was achieved at a reduced strain level, leading to a decrease in stretchability, and the substrate also resisted fracturing when stressed. We referred to this phase as the strain-limiting phase (phase II). These results indicated that the strength, toughness, and Young’s modulus could increase even when a thin, skin-like substrate was maintained. The Young’s modulus of the ultrathin skin-like substrate could be tuned in the range of 0.04−0.30 MPa by controlling the loading of P(VDF-TrFE) nanofibers. These values are similar to the skin of the abdomen, forehead, and tibia with Young’s moduli ranging from 0.12−0.6 MPa.36 Maximum stress and strain at rupture are increased and decreased, respectively, with an increase in the nanofiber loading due to the increased stress subjected to a higher density of nanofibers near the breaking point. Rupture of the skin-like substrate occurred after reaching the breaking point when embedded P(VDF-TrFE) nanofibers aligned in the direction of the applied force were broken (see Supporting Information Figure S12). The stress−strain hysteresis characteristics were also measured. The results in Figures S13 and S14 show the viscoelastic behavior of the skin-like substrate. When a load is applied to the substrate, embedded nanofibers of P(VDF-TrFE) rearrange their positions to accommodate the stress. Back stress was created in the skin-like substrate that will cause the substrate to return to its original position. The skin-like substrate does not creep because the back stress is of the same magnitude as that of the applied stress. It returns to its initial point even after removing the stress. To compare the durability of the skin-like substrate with that of the PDMS substrate with no nanofibers, we investigated the surface morphology of the skin-like substrate fabricated with an ES volume of 0.2 mL and a pure PDMS substrate after 5000 cycles of cyclic stretching at a strain of 30% using FE-SEM. The results are presented in Figure S15. Wrinkles appeared on both the skin-like substrate and pure PDMS substrate after cyclic stretching. However, cracking of the pure PDMS substrate was severe, in contrast to the lack of cracking in the skin-like substrate. These results demonstrated that the incorporation of P(VDF-TrFE) nanofibers as reinforcing fillers inside the PDMS matrix was effective at protecting the ultrathin substrate from mechanical damage during cyclic stretching.

Supporting Information, Figure S7). The cross-sectional FESEM image in Figure 1f shows P(VDF-TrFE) nanofibers delineated from the PDMS matrix by selective etching of nanofibers in an N,N-dimethylformamide (DMF) solution. Embedded P(VDF-TrFE) nanofibers were uniformly distributed inside the PDMS matrix, which we attributed to the penetration of PDMS into the nanofiber network by hot pressing during fabrication. As previously mentioned, the loading of nanofibers primarily affects the mechanical properties of elastomeric PDMS embedded with nanofibers. The mechanical behaviors of skin-like substrates with different loadings of nanofibers were analyzed by measuring their stress−strain relationship using an ultimate tensile test machine to elucidate the role of nanofiber loading and tune mechanical behaviors (for details, see the Experimental Details section). The sample size for the ultimate tensile strength test, the measurement procedure, performance, and machine parameters are also described in detail in Figure S8. The stress−strain behavior of skin-like substrates fabricated with an ES volume of 0.2 mL was characterized by three distinct regimes with different Young’s modulus values (Figure 2a). It was apparent from the stress−strain curve of the skinlike substrate shown in Figure 2a that the substrate had high stretchability at low strain and became stiffer at large strain, indicating a self-limiting behavior similar to human skin. When stretching strain was applied to human skin, the elastin fibers inside the skin had a low resistance to the applied strain. At this point, the skin showed isotropic behavior, and collagen fibers remained entangled, resulting in a low Young’s modulus value and linear stress−strain behavior. In the second regime, collagen fibers started to resist deformation, and thus, stretching of collagen fibers began to set in. If further deformed, collagen fibers started to be disentangled, and a linear behavior was observed. At this point, the collagen fibers straightened and became parallel to each other, and thus, more load was required to induce further elongation. This process may continue until complete alignment of collagen fibers in the direction of the applied load occurs. Failure occurred as fibers began to slide past one another. In the case of our skin-like substrate, we attributed the three different phases (regions I, II, and III) in the stress−strain curve (Figure 2a) to the arrangement of P(VDF-TrFE) nanofibers inside the elastomeric PDMS matrix. In region I, under low load, nanofibers remained entangled (see the Supporting Information, Figure S9a), and thus, the strain−stress behavior was primarily governed by the PDMS matrix, resulting in low stress values. The stretchability of the skin-like substrate is defined based on the stress−strain range of region I. In region II, the load increased, and the P(VDF-TrFE) nanofibers straightened out with elongation. This imparted mechanical attributes to the skin-like substrate, and the stress value increased (see Supporting Information Figure S9b). At a high load (region III), the P(VDF-TrFE) nanofibers started to slide past one another, damage started to accumulate, and failure eventually occurred (see Supporting Information Figure S9c). The Young’s modulus values obtained from distinct linear regions increased from region I to region II (Figure 2a). Additional skin-like substrates with ES volumes of 0.4, 0.6, and 0.8 mL and a thickness of ∼62 μm were fabricated to further investigate the effect of different loading volumes of P(VDF-TrFE) nanofibers inside the PDMS matrix on the mechanical properties of the skin-like substrates. As the ES volume of P(VDF-TrFE) solution increased, the density of E


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Figure 3. Sensory response of the skin-like substrate to mechanical force. (a) Time-dependent piezoelectric open circuit output voltage (VOC) of the skin-like substrate under an applied dynamic pressure of 0.15−1.1 kgf in a push tester. Below the dynamic pressure of 0.15 kgf, it was difficult to separate the VOC output from the noise signals. The piezoelectric open circuit output voltage (VOC) of the skin-like substrate under applied (b) dynamic bending by hand and (c) tapping by hand.

In addition to their role as reinforcing fillers to help in tuning the mechanical behavior and durability of the skin-like substrate, P(VDF-TrFE) nanofibers can also potentially function as pressure and strain sensors because of their piezoelectric characteristics. To measure the sensing functionality of the skin-like substrate, we fabricated poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ionic liquid (1-ethyl-3-methylimidazolium tetracyanoborate) electrodes on both sides of a skin-like substrate prepared with an ES volume of 0.2 mL to generate a piezoelectric device. The piezoelectric open circuit output voltage (VOC) of the fabricated device under applied dynamic pressure, bending, and tapping are presented in Figure 3a−c, respectively. VOC increased gradually from 0.05 to 0.19 V with an increase in applied impact pressure from 0.15 to 1.1 kgf at a forcing frequency of 0.2 Hz (Figure 3a and Supporting Information Table S1). The minimum detectable pressure was 0.15 kgf. As shown in the plot of VOC versus applied pressure (see Supporting Information Figure S16), the response is not linear at a low dynamic pressure range. The reason for this is not clearly understood. The device generated a piezoelectric output under manual bending (see Supporting Information Figure S17a) at an average frequency of 2.6 Hz and when tapped by hand (see Supporting Information Figure S17b) with an average frequency of 3.7 Hz (Figure 3b,c, respectively). The photographs of the device under manual bending and tapping by hand are also shown in Figure S17. These results demonstrated that the skin-like substrate mimicked both the mechanical behaviors of human skin and the ability of human skin to sense dynamic mechanical stimuli. To mimic the ability of human skin to detect temperature, a stretchable temperature sensor with a resistor structure was

fabricated on a skin-like substrate prepared using an ES volume of 0.2 mL. A stretchable conductive PEDOT:PSS/PU dispersion (PUD) composite was directly spin-coated and patterned on the skin-like substrate to form source−drain electrodes. A reduced graphene oxide (R-GO)/polyurethane (PU) nanocomposite having excellent temperature responsivity and durability was spin-coated as a temperature sensing layer51 on the source−drain electrodes. Details of the fabrication process and a schematic illustration of the device structure are presented in Figures S18 and 4a, respectively. To study the responsivity of the device to temperature, the current of the device was measured at various temperatures ranging from 30 to 100 °C at intervals of 5 °C. The results in Figure 4b clearly show that the current increased as the temperature increased. This phenomenon can be explained by a charge transport mechanism, including carrier hopping and tunneling conduction at the boundaries between the adjacent R-GO nanosheets that formed a conductive network inside the PU matrix.51−54 Charge transport by means of hopping and tunneling across barriers at the nanosheet junctions increased at elevated temperatures, thereby increasing the conductance of the R-GO/PU nanocomposite channel. In turn, the current increased due to the increase in conductance. The responsivity of the device was defined as (ΔR/R0 = (R − R0)/R0) × 100%, where R0 and R are the resistances at 30 °C and the set temperature up to 100 °C, respectively. The responsivity of the temperature sensing device is shown in Figure 4c. The temperature sensitivity of the stretchable temperature sensor extracted from the linear fit in Figure 4c was about 0.5%/°C. To evaluate the effect of mechanical deformation on the temperature sensing capability of the device on the skin-like substrate, the ΔR/R0 of the device was measured from 30 to 40 F


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Figure 4. Transparent stretchable temperature sensor fabricated on a skin-like substrate. (a) Schematic illustration of a stretchable temperature sensor on the skin-like substrate. (b) The current−voltage (I−V) curves of the temperature sensor at applied temperatures ranging from 20 to 100 °C. (c) The resistance change, ΔR/R0 (%), of the temperature sensor as a function of temperature. (d) The ΔR/R0 (%) responsivity of the temperature sensor to various temperatures at applied strain ranging from 0 to 40%. (e) Optical images of the stretchable temperature sensor on skin-like substrates under applied strain ranging from 0 to 40%.

°C under static stretching from 0 to 40% where the skin-like substrate has a low Young’s modulus (region I). The ΔR/R0 value of the temperature sensor up to 40% static stretching is shown in Figure 4d. The optical images in Figure 4e show the device loaded in a custom-built stretching tool under applied strain from 0 to 40% elongation. There were no significant changes in temperature responsivity values with increased stretching strain (Figure 4d), and there was no mechanical damage to the temperature sensor. The slight difference in the responses in Figure 4c,d is presumably due to variations in the responsivity from different samples. A comparison of the change in the current of the stretched devices on the skin-like substrate with an ES volume of 0.2 mL and a pure PDMS substrate indicates less change in the current for the device on the skin-like substrate stretched up to phase II (∼60%)

compared to that of the device on the pure PDMS substrate (see Figure S19). To confirm the conformality of the skin-like substrate on human skin, the device was attached to human skin and was tested to investigate motion-induced effects caused by the movement of the human body during sensing. A photograph of the skin-like substrate with a temperature sensor attached to a hairy human wrist is shown in Figure 5a. The current response to skin temperature is shown in Figure 5b. When the temperature sensor was initially attached to human skin, current I increased immediately and became stable at skin temperature. While moving the wrist up and down, there was no significant current change in the stretchable temperature sensor. This is because long and continuous nanofibers in the PDMS matrix had minimum fiber edges, and thus presented G


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dioxythiophene polystyrene sulfonate)) (1−1.3 wt %, Clevios PH 1000 from Heraeus) were used as stretchable transparent materials for electrodes as purchased at a ratio of 28:72 (wt %). To measure the piezoelectric response, a 0.3 wt % solution of ionic liquid (1-ethyl-3methylimidazolium tetracyanoborate by Merck) in PDOT:PSS solution was prepared to form stretchable electrodes. Polyimide (PI) (Sigma-Aldrich) was used as a carrier substrate in the complete process. 3.2. Fabrication of Skin-like Substrate, Temperature Sensor and Piezoelectric Device. A DMF and acetone mixture at a volume ratio of 1:1 was used as a solvent to prepare nanofiber mats from P(VDF-TrFE) by electrospinning. The P(VDF-TrFE) solution was used at a concentration of 15 wt %. Aluminum foil (10 × 10 cm) was used as a collector. Electrospinning conditions included a voltage of 18 kV, a flow rate of 1.5 mL/h with a 21 G needle, and a distance of 15 cm between the tip and the collector. After collecting nanofibers on aluminum foil, the resulting nanofiber mat was left at ambient temperature to allow evaporation of the residual solvent. The nanofiber mat was then transferred to a PI substrate, and the samples were covered with 1 mL PDMS and kept in a vacuum for bubble removal. The PDMS-coated samples were then spin-coated at 3500 rpm for 40 s and cured in an oven for 45 min, after which they were placed in a hot press for 1 h at 80 °C to ensure embedding of nanofibers inside the PDMS elastomer. A clear substrate with a total thickness of ∼62 μm was fabricated by controlling the spinning conditions. PEDOT:PSS/ionic liquid solution was spin-coated on both sides of the skin-like substrate to fabricate a metal−insulator− metal structure to assess the piezoelectric response of the fabricated skin-like substrate to tapping. To fabricate a temperature sensor, PEDOT:PSS (28 wt %) and a PU dispersion (PUD) (72 wt %) composite coating was used to form source−drain and gate electrodes. A 5 nm thick Ni thin layer was deposited by means of e-beam evaporation through a shadow mask onto the PEDOT:PSS/ PUD composite film as an etch mask. To pattern source−drain electrodes, the area of the PEDOT:PSS/PUD composite not masked by Ni was exposed to oxygen microwave plasma at 640 mTorr in a chemical dry etching system using 500 W of microwave power. RGO/PU was patterned on top of the source−drain electrodes as a sensing channel layer under a N2 atmosphere. The sensor response was measured by varying temperature values and attaching the sensor to human skin. 3.3. Characterization. The morphologies of the nanofibers and the thicknesses of the skin-like substrates were evaluated using fieldemission scanning electron microscopy (FE-SEM, JEOL JSM-6500F) and a surface profiler (Alpha-Step XP-100). Optical transparency was measured using a UV−visible spectrometer (S-3100, Scinco). Fluorescence imaging was performed by using a confocal microscope (FLUOVIEW FV3000, Olympus). An ultimate tensile tester (LR10KPlus) was used to measure the stress−strain behavior of the skin-like substrate. A push tester (ZPS-100) was used to evaluate the output response of the skin-like substrate under periodic tapping at various vertical pressures and constant frequency. An oscilloscope (Tektronix DPO 3052) was used to record the generated open circuit voltage (VOC). VOC output was measured under a 10 mm bending radius using an oscilloscope (Tektronix TDS-3014B). Temperature sensor response was measured using a semiconductor parameter analyzer (HP4145B, Agilent Technologies).

Figure 5. Suppression of motion-induced signal changes in a stretchable temperature sensor attached to the human body. (a) Optical images of the stretchable temperature sensor attached to a hairy human wrist moving up and down. (b) Current response of the temperature sensor to the skin temperature of the wrist during continuous movement of the wrist up and down for 35 s. Skin temperature was obtained from the pre-calibration curve (see Supporting Information Figure S20).

few stress concentration points inside the skin-like substrate. Transfer of stress from the PDMS polymer matrix to the P(VDF-TrFE) nanofibers may occur during body movement, which thereby reduces stress concentration points in the substrate, increases the stability of the device on the skin-like substrate, and presumably results in minimization of motioninduced effects in sensing signals during movement. In the time interval between when the movement stopped and when the device was detached from the skin, there was a small decrease in the current, most likely due to the adjustment of heat to control the body temperature after movement. A standard curve for temperature values is provided in Figure S20.

3. EXPERIMENTAL DETAILS 3.1. Materials. P(VDF-TrFE) (65% vinylidene fluoride, Piezotech S.A.) powder was dissolved in N,N-dimethylformamide (DMF) and acetone (Sigma-Aldrich) solvents mixed at a ratio of 1:1 to prepare a P(VDF-TrFE) solution for electrospinning. The P(VDF-TrFE) solution was directly spun into nanofibers using a custom-built electrospinning machine. Poly(dimethylsiloxane) (PDMS) with a ratio of silicone elastomer base/silicone elastomer curing agent (10:1) (Sylgard 184 silicone elastomer kit, Dow Corning Corporation) was spin-coated on the P(VDF-TrFE) nanofiber mat to prepare a skin-like substrate. The Hummers’ method was used to prepare GO nanosheets. GO nanosheets in dimethylacetamide (DMAC, SigmaAldrich) were dispersed and mixed with polyurethane (PU, SG 85 A from Tecoflex). PU was used to fabricate the temperature responsive GO/PU nanocomposite channel layer. Granules of PU were first dissolved in DMAC, and then the PU solution was kept at 80 °C under stirring for 1 h. On cooling, GO was added drop by drop to PU under continuous stirring. PUD (4 wt % Alberdingk U3251 from Alberdingk Boley) and a PEDOT:PSS solution (poly(3,4-ethylene

4. CONCLUSIONS We synthesized an ultrathin and durable skin-like substrate that was highly stretchable, optically transparent, tough, ultrathin, and had self-limiting characteristics by incorporating P(VDF-TrFE) nanofibers with a high modulus and piezoelectricity into the low modulus matrix of PDMS. P(VDFTrFE) nanofibers functioned as reinforcing fillers, which helped the skin-like substrate to mimic the mechanical behavior of human skin. The piezoelectricity of the P(VDFTrFE) nanofibers enabled the skin-like substrate to detect H



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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program (2016R1D1A1B039-34709) of the National Research Foundation of Korea funded by the Ministry of Science, ICT, and Future Planning and also by the GRRC program of Gyeonggi province [GRRC Sungkyunkwan 2017-B04, Development of wearable chemical sensor].

dynamic pressure and strain. The skin-like substrate had a high optical transparency of 80%, and the stretchability, toughness, and Young’s modulus of the substrate could be tuned by controlling the loading amount of P(VDF-TrFE) nanofibers in the PDMS matrix. We directly fabricated a stretchable temperature sensor on the skin-like substrate to generate a stretchable electronic device capable of mimicking the mechanosensory ability, thermosensory ability, and mechanical behavior of human skin while being resistant to the effects of mechanical deformation based on the electrical signal change of the sensor device on the substrate. The stretchable, transparent, ultrathin, tough, and self-limiting skin-like substrate with skin-like sensory functions described here will facilitate the realization of stretchable electronics.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08283. Stress−strain curves at various ES volumes, a schematic illustration of the process flow for fabrication of a skinlike substrate, optical transmittance of skin-like substrates at different loadings compared with PDMS, crosssectional FE-SEM images under various spin-coating conditions, a comparison between PDMS and skin-like substrates, photographs of a substrate loaded for the measurement of tensile stress−strain in a universal testing machine, confocal images showing behavior of nanofibers inside PDMS, self-limiting characteristics, FESEM images showing the morphology of P(VDF-TrFE) nanofibers at different loadings corresponding to ES volumes, stress−strain curves of skin-like substrates with different ES volumes of P(VDF-TrFE) solution, a FESEM image showing P(VDF-TrFE) nanofibers inside PDMS elastomer up to 0.8 mL ES volume, a FE-SEM image showing P(VDF-TrFE) nanofibers inside a PDMS elastomer at 0.8 mL ES volume after reaching a breaking point during ultimate tensile measurements, stress− strain hysteresis curves of skin-like substrates with different ES volumes of P(VDF-TrFE) solution, comparison of stress−strain hysteresis of skin-like substrates, VOC comparison with respect to applied pressure, FE-SEM images of skin-like substrate before cyclic stretching and after cyclic stretching of 5000 cycles at a strain of 30%, standard curves for temperature values, a schematic illustration of process flow for fabricating a stretchable temperature sensor on a skinlike substrate and a comparison of the mechanical properties of skin-like substrates with a pure PDMS substrate (PDF) PDMS substrate without nanofiber reinforcement (MPG) Thinner skin-like substrate showed enhanced conformality even on complex skin geometries (MPG)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nae-Eung Lee: 0000-0002-6539-5010 Notes

The authors declare no competing financial interest. I


Research Article

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Stretchable, Transparent, Tough, Ultrathin, and Self-limiting Skin-like

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