Highly Flexible and Sensitive Wearable E-Skin Based on Graphite

Oct 17, 2017 - Graphene and nanomaterials based flexible pressure sensors R&D activities are becoming hot topics due to the huge marketing demand on w...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Highly Flexible and Sensitive Wearable E‑Skin Based on Graphite Nanoplatelet and Polyurethane Nanocomposite Films in Mass Industry Production Available Jianfeng Wu,† Huatao Wang,*,† Zhiwei Su,† Minghao Zhang,† Xiaodong Hu,† Yijie Wang,† Ziao Wang,† Bo Zhong,† Weiwei Zhou,† Junpeng Liu,*,‡ and Scott Guozhong Xing*,§ †

School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, 2 West Wenhua Road, Weihai 264209, China ‡ Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, Nottingham NG7 2RD, U.K. § United Microelect Corp. Ltd., 3 Pasir Ris Dr 12, Singapore 519528, Singapore S Supporting Information *

ABSTRACT: Graphene and nanomaterials based flexible pressure sensors R&D activities are becoming hot topics due to the huge marketing demand on wearable devices and electronic skin (E-Skin) to monitor the human body’s actions for dedicated healthcare. Herein, we report a facile and efficient fabrication strategy to construct a new type of highly flexible and sensitive wearable E-Skin based on graphite nanoplates (GNP) and polyurethane (PU) nanocomposite films. The developed GNP/PU E-Skin sensors are highly flexible with good electrical conductivity due to their unique binary microstructures with synergistic interfacial characteristics, which are sensitive to both static and dynamic pressure variation, and can even accurately and quickly detect the pressure as low as 0.005 N/50 Pa and momentum as low as 1.9 mN·s with a gauge factor of 0.9 at the strain variation of up to 30%. Importantly, our GNP/PU E-Skin is also highly sensitive to finger bending and stretching with a linear correlation between the relative resistance change and the corresponding bending angles or elongation percentage. In addition, our E-Skin shows excellent sensitivity to voice vibration when exposed to a volunteer’s voice vibration testing. Notably, the entire E-Skin fabrication process is scalable, low cost, and industrially available. Our complementary experiments with comprehensive results demonstrate that the developed GNP/PU E-Skin is impressively promising for practical healthcare applications in wearable devices, and enables us to monitor the real-world force signals in real-time and in-situ mode from pressing, hitting, bending, stretching, and voice vibration. KEYWORDS: graphite nanoplatelet, polyurethane nanocomposite, wearable devices, electronic skin, static and dynamic pressure sensors, healthcare large-scale production

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INTRODUCTION

There have been some relevant and representative reports addressing flexible and wearable pressure sensors. For instance, Robert et al. developed sensing skins in 2012 by structuring a three-dimensional (3D) carbon nanotube network into three kinds of amorphous thermoplastic matrices, enabling the tailoring of both sensitivity and stability of piezocapacitive responses.31 In 2013, Segev-Bar et al. presented a touch sensor based on monolayer-capped nanoparticles that is low cost, could allow low-voltage operation, and could provide a platform for multifunctional applications.32 In 2014, Cheng et al. introduced a simple approach to fabricate wearable and highly sensitive pressure sensors by sandwiching ultrathin gold nanowire-impregnated tissue paper between two thin poly(dimethylsiloxane) (PDMS) sheets.33 In 2016, Liu et al. reported a simple immersion−swelling approach followed by an

Nowadays, carbon-based sensors have attracted great interest all over the world, due to their applications in physics, chemistry, thermal management, and biology scientific and commercial fields.1−8 Among these applications, wearable pressure sensors are becoming more and more of a focus because of their potential applications in physiological activities, healthcare monitoring, and diagnosis.9−15 Especially when the sensors are flexible and wearable, they can be applied in soft robotics, wearable consumer electronics, and electronic skins (E-Skin).16−29 As the demands for these applications increase, requirements for these sensors will also become more stringent, particularly relating to being lightweight, flexible, low cost, with simple preparation, and having mass-production availability.30 Efforts in the development of pressure and strain sensors have been made worldwide to improve their sensitivity by optimizing the material system, such as graphene-based, carbon nanotube (CNT)-based, or polymer matrix-based, their preparation technology, and sensor structure. © XXXX American Chemical Society

Received: July 15, 2017 Accepted: October 17, 2017 Published: October 17, 2017 A

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the fabrication process and detecting models of E-Skin based on graphite nanoplates and polyurethane (GNP/ PU) nanocomposite films.

in-situ reduction method to develop a thermoplastic polyurethane (PU) strain sensor composed of a graphite nanosheet/ Ag nanoparticle conductive network and a sandwich structure.34 In 2016, Wei et al. demonstrated a highly stretchable electronic skin based on the facile combination of microstructured graphene nanowalls and a PDMS substrate.35 In 2016, Kwon et al. reported a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a 3D microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a tactile pressure range.36 Most recently, Wang et al. reported a low-cost scalable fabrication for highly reliable, stretchable, and conductive composite yarn as an effective material for human motion monitoring.37 In fact, most sensors can exhibit resistance variation responding to pressure or strain stimulation, and a few of them can also be used as a voice sensor.34,38,39 However, besides their excellent sensitivity and flexibility, some important factors or parameters have to be considered for flexible pressure sensor development that play a paramount role in enabling delivery of large-scale application in future wearable electronics, such as stability for years, production cost, complexity of the fabrication process, and compatibility with industrial massproduction. Therefore, stable and non-expensive conductive fillers with high resistance to oxidation and corrosion can enable sensors to work for years. On the basis of this view, carbon-based nanomaterials, such as carbon nanotubes, carbon black, graphene, and graphite nanoplates (GNPs), are suitable as the conductive filler due to their high conductivity and excellent resistance to oxidation, high temperature, and corrosion. More importantly, sensors with a simple film structure possess good compatibility with industrial level mass-production, leading to manufacture at low cost. In comparison with fundamental sensing devices based on single or multiple layer graphene, graphite and polyurethane composite constructed sensors show compromised sensitivity, but their corresponding unparalleled advantages lie in their facile preparation process with relatively moderate require-

ments of physicochemical conditions in the production, including a lower power, reliable process and great thermal and corrosion stability. Moreover, compared to conventional carbon black rubbery composites with severe nonhomogeneous dispersion issues, the graphite and polyurethane hybrid system possesses much better uniformity with higher flexibility and demonstrates a more eco-friendly nature. Herein, we demonstrate a facile and efficient approach to fabricate highly flexible, sensitive, and wearable E-Skin based on graphite nanoplates (GNPs) and polyurethane (PU) nanocomposites, which has good electrical conductivity and extreme flexibility due to its special microstructure. Our GNP/PU ESkin sensors show excellent sensitivity to both static and dynamic pressure, and can even accurately and quickly detect pressure as low as 0.005 N/50 Pa and momentum as low as 1.9 mN·s. Further investigations show that GNP/PU E-Skin is also highly sensitive to finger bending and stretching, and voice vibration, with a linear relationship between the relative resistance change and the corresponding bending angles or elongation percentage, enabling its application as human action sensors and vibration sensors with good stability and reliability. These performances are comparable with the recent progress on flexible and wearable pressure sensors, possessing advantages of being highly efficient and having a low-cost, simple, and mass-production available fabrication process as well as versatility of detecting pressing, hitting, bending, stretching and voice distinguishing.

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RESULTS AND DISCUSSION E-Skin Fabrication. Figure 1 illustrates the detailed fabrication procedure of GNP/PU E-Skin, including the following steps: (1) expanded graphite (EG) particles were effectively fragmented into GNPs by ultrasonic exfoliation, (2) homogeneous GNP/PU/solvent compounds were prepared by vacuum filtration, re-dispersion, stirring, etc., (3) GNP/PU films were coated by a gap coating method on poly(tetrafluoroethylene) (PTFE) or poly(ethylene terephthalate) (PET) substrates, and (4) GNP/PU E-Skin was assembled and B

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Characterization and schematic microstructures of GNP and GNP/PU films. (a) FTIR spectrum of the GNP/PU nanocomposite film with some characteristic groups from PU, such as N−H, CO, and C−N−H. (b) The schematic drawing of the GNP/PU nanocomposite film microstructure. (c, d) The low and high magnified cross-sectional SEM images of as-prepared GNP/PU nanocomposite films with the thickness of ∼40 μm. The GNPs are uniformly distributed in the PU matrix without the observation of sharp GNP edges. (e) The top-view SEM image of asprepared GNP/PU nanocomposite films. GNPs with the width size of 2−20 μm are completely covered by PU. (f) Low and (g, h) high magnified SEM images of as-prepared GNP films without PU, and GNP shows a very flaky structure with sharp edges.

Moreover, the C−N−H peaks at ∼1527 cm−1 and the −C−O− C− stretching peaks at ∼1134 cm−1 are consistent with previous reports.43 Figure 2b illustrates the main structure of the GNP/PU nanocomposite films. Clearly, the flaky GNPs are nanoscale homogenously distributed in the PU matrix. Figure 2c,d demonstrates the low and high magnified cross-sectional scanning electron microscopy (SEM) images of the as-prepared GNP/PU nanocomposite films with the thickness of ∼40 μm. The GNPs are uniformly distributed in the PU matrix without the observation of sharp GNP edges. The top-view SEM image of the GNP/PU film shows that GNPs with the width size of 2−20 μm are completely covered by PU (Figure 2e), different from the GNP films without PU, in which GNP shows a very flaky structure with sharp edges (Figure 2f−h). It can be noticed that the typical porous structure of GNPs in GNP/PU films has disappeared, and their fracture surface is rough due to the addition of PU. Such a binary synergistic homogenous GNP structure is beneficial to reduce the stress concentration in the PU matrix, and endows the nanocomposite films with extreme

packaged by bonding electrodes and encapsulation.40 The thickness of GNP films can be adjusted from 10 to 100 μm by the gap space of film applicator and the solid content of GNP/ PU/solvent compounds. Lastly, a series of as-obtained GNP/ PU E-Skin sensors were undertaken for complementary pressing, hitting, bending, stretching, and voice vibration tests. Phase Structure and Morphology of As-Prepared GNP/PU Films. The phase structures of as-prepared GNPs were investigated by X-ray diffraction (XRD) and the results are shown in Figure S1 (see Supporting Information for more details). The XRD pattern of GNPs shows sharp peaks of (002) and (004) planes at 26.3 and 54.5° due to the layer-by-layer structure. The (002) diffraction peak exhibits a layer distance of ∼3.34 Å for the GNPs.41 The Fourier transform infrared (FTIR) spectrum of the GNP/PU nanocomposite film is shown in Figure 2a. The characteristic peak of −CO in carbonyl groups of the composite film at ∼1725 cm−1, the C− H stretching peaks at ∼2953 cm−1, the stretching vibrations of N−H groups occurring at ∼3328 cm−1, with the carbonyl bands are indicative of the presence of urethane moieties.42 C

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Photographs of as-prepared GNP/PU nanocomposite films with high flexibility. (a) The initial photograph of the GNP/PU films, (b) which can be easily stretched to ∼2 times the original length, (c) knotted strongly, and can also quickly return to the initial state due to their good flexibility. (d−f) Even after being rubbed again and again, the GNP/PU films can quickly return to the initial state. (g) The photograph of four rolls of 300 m of GNP/PU nanocomposite films fabricated by the film coating machine on PET substrates. (h) The photograph of GNP/PU films with the length of ∼3 m and the width of 0.3 m are tiled on the floor.

flexibility, and a uniform distribution also increases conductivity (50 S/m).1,2,4,44 As shown in Figure 3a,b, the as-prepared GNP/PU nanocomposite films are extremely flexible, and can be easily stretched to ∼2 times the original length, knotted strongly, and can also quickly return to the initial state (Figure 3c) once the external force was released with high flexibility. More importantly, such nanocomposite films can quickly recover to their original state even after being rubbed again and again (Figure 3d−f), which fully demonstrates that our developed films are fairly suitable for being equipped upon dedicated healthcare sensors as wearable devices. The corresponding videos can be found in the Supporting Information. Notably, the nanocomposite films can be mass-produced by a film coating machine because of their simple structure, which makes them industry level available and cost effective. Figure 3g shows four rolls of GNP/PU nanocomposite film with hundreds of meters fabricated by the film coating machine on PET substrates. As shown in Figure 3h, the as-fabricated homogenous GNP/PU nanocomposite films with the length of ∼3 m and the width of 0.3 m were tiled on the floor, which clearly illustrates that the fabrication process is scalable, low cost, and industrially amenable for mass production. Response to Both Static and Dynamic Pressure. To characterize the sensing performance of the as-prepared GNP/ PU E-Skin against both static and dynamic pressure, different loading conditions were implemented. The static compressive unloading−loading−unloading cycle tests on as-prepared

GNP/PU E-Skin with loadings of 0.005, 0.01, 0.06, 0.5, and 1 N corresponding to 50, 100, 600, 5000, and 10 000 Pa, were performed, and the plots of relative resistance change rate (ΔR/R) as a function of time (s) are shown in Figure 4a. The left inset shows the corresponding schematic diagram of a heavy object on the E-Skin, whereas the right inset demonstrates the results of 0.01 N (100 Pa) and 0.005 N (50 Pa), as indicated by the shadow area. When the loading was applied on the surface of GNP/PU E-Skin, there is a consequent instant increase in its resistance. If loading is released, the resistance quickly returns to the initial state. It should be noted that GNP/PU E-Skin shows consistent and steady responses to all applied loading levels with a high sensitivity resolution of 0.005 N (50 Pa). Figure 4b shows the corresponding fitting curve of the relative resistance change versus the applied pressure, and the fitting equation of ΔR/R = 2.4P indicates the linear relationship between pressure and (ΔR/R). Figure 4c depicts the plot of relative resistance change rate (10−3) as a function of time under the pressure of 0.5 N (5000 Pa) in several unloading−loading−unloading cycles, the inset photograph is the corresponding physical picture of the loading. The results show that GNP/PU E-Skin exhibits a stable response to cyclical dynamic pressure. Figure 4d,e shows the schematic diagram of the deformability-dependent resistive sensing mechanism. On one hand, GNPs are homogenously covered by PU, meaning that every GNP is separated by the PU layer, as indicated in Figure 2b−e. The thickness of PU layer between one GNP and another GNP along the in-plane D

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Static and dynamic pressure response. (a) Plots of relative resistance response (ΔR/R, 10−3) as a function of time (s) for five pressures (1, 0.5, 0.06, 0.01, and 0.005 N) applied on the surface of 1 cm2 corresponding to 10 000, 5000, 600, 100, and 50 Pa. The left inset image shows the corresponding schematic diagram of a heavy object on the E-Skin, whereas the right inset image shows the results of 0.01 N (100 Pa) and 0.005 N (100 Pa), as indicated by the shadow area. (b) The fitting curve and equation (ΔR/R = 2.4P, P represents pressure) show the linear relationship between pressure and the relative resistance change rate. (c) Plots of relative resistance response (ΔR/R, 10−3) of the GNP/PU E-Skin as a function of time (s) under the applied pressure of 0.5 N (5000 Pa) in several unloading−loading−unloading cycles. The inserted photograph is the corresponding physical picture of the loading. (d) Schematic diagram of deformability-dependent resistive sensing mechanism. (e) When the distance between one GNP and another GNP along the in-plane direction is increased due to the applied pressure or pulling force, the resistance of the GNP/PU films along the in-plane direction is increased. (f) With pressure, the resistance of the GNP/PU films is increased, leading to smaller current (I2) compared with the initial current (I1). (g) The results of relative resistance change rate vs the ratio of loading area to the surface area of GNP/PU E-Skin. Obviously, the sensitivity indicator (ΔR/R) is linearly increased with the increasing ratio of loading area (Sl) to the surface area (S0) of GNP/PU E-Skin (Sl/S0), due to the greater resistance change produced by the greater loading area. (h) Relative resistance change rate (10−3) vs time (s) profile of the GNP/PU E-Skin with various input impulses. (i) Relative resistance change rate (10−3) vs time (s) profile of the GNP/PU E-Skin with the various finger pressures, indicating excellent sensitivity.

direction of films, which is the passing distance, is increased and subsequently results in higher resistance, if pressure is applied on our films, which is equivalent to when our films are stretched, as indicated in Figure 4e. On the other hand, when a mechanical force is applied on the surface of the E-Skin, the GNP/PU nanocomposite film experiences a compressive deformation in the applied force direction and a tensile deformation along the in-plane direction of films, resulting in a larger resistance, according to the calculation formula of electrical resistance (R = ρ × l/S), in which R, ρ, l, and S represent resistance, resistivity, length, and area, respectively. When a pressure is loading on the sensor’s local surface, the

cross-section area (S) almost remains unchanged (S1 ≈ S2), but the current path along the length direction is increased (L1 < L2) due to the excellent flexibility of the GNP/PU films and the existence of underlying PDMS, leading to greater resistance (R1 < R2) and lower current (I1 > I2). However, from another perspective, the GNPs covered with PU bearing good elastic characteristics, when the pressure is applied on the sensor, certainly become thinner in the vertical direction while still possessing the same amount of conductive path generated upon the GNPs along the in-plane direction due to the PU’s elasticity, consequently leading enhanced overall resistance. E

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Detection of bending and stretching. (a) Relative resistance change rate (10−3) vs time (s) profile of the skin-attachable GNP/PU E-Skin directly above an index finger joint to detect its bending action. The inset images show the various bending actions of the index finger with E-Skin. (b) Relative resistance change rate (%) vs bending angel profile of as-prepared GNP/PU E-Skin. (c) The curvature of GNP/PU E-Skin as a function of time showing the stable recycle bending of 90°. (d) Schematic illustration of the bending process and mechanism of GNP/PU E-Skin. (e) The relative resistance change response to stretching with 10, 20, and 30% elongation, showing that GNP/PU E-Skin has good sensitivity to stretching with different elongation. (f) The fitting curve and equation (ΔR/R = 0.59s) show the linear relationship between strain (elongation) and the relative resistance change rate. (g) The schematic illustration of stretching-dependent resistive sensing mechanism of GNP/PU E-Skin, revealing the whole curve of stretching−recovering process. (h) The enlarged curvature of E-Skin as a function of time showing the stretching−recovering process, indicating that there is a slight rebound on relative resistance change rate during recovering process.

constant (1 N). Obviously, the sensitivity indicator (ΔR/R) is linearly increased with the increasing ratio of loading area (Sl) to the surface area (S0) of GNP/PU E-Skin (Sl/S0), due to the greater resistance change produced by the greater loading area. In fact, most types of applied pressure during the application of E-Skin are not static but dynamic. To further investigate the performance of the as-prepared GNP/PU E-Skin applied in wearable electronics, dynamic loading tests were undertaken. A tiny block with a weight of 2 g was used to simulate various hitting onto the E-Skin. The block fell from a height of 50, 100,

According to the above deformability-dependent resistive sensing mechanism, if the ratio of the deformation volume to the whole volume of GNP/PU films is higher, the relative resistance change (ΔR/R) will be higher. This confirms that the sensitivity is improved with the ratio of loading area to surface area GNP/PU films. Figure 4g shows the plot of relative resistance change rate as a function of the ratio of loading area to the surface area of GNP/PU E-Skin. The inset images indicate the E-Skin area (Sl) and loading area (S0) with various Sl/S0 ratios of 30, 60, and 90%, respectively. The loading is F

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Real-time and in-situ detection of voice vibrations. (a) Schematic diagram of voice vibration and the corresponding detecting mechanism. (b) Measurement of voice vibration three times spelling “H, I, T”, by the skin-attachable GNP/PU sensors directly attached on the skin of the volunteer’s throat.

Figure 5d shows the schematic illustration of the bending process and mechanism of as-prepared E-Skin. When the ESkin is bent to a certain degree, its cross-section area (S) almost remains unchanged, but the current path along the length direction is increased due to the excellent flexibility of the GNP/PU films, leading to greater resistance. Interestingly, asprepared GNP/PU E-Skin was directly stretched by hands with elongation percentages of 10, 20, and 30%, respectively. Figure 5e shows the curvature of the relative resistance change rate (%) versus time (s), showing that the E-Skin has good sensitivity to stretching with different elongation. Further, the resistance is quickly increased when stretching is applied. Obviously, GNP/PU E-Skin is quite sensitive to elongation. When elongation is larger, the relative resistance change rate is higher. The gauge factor is about 0.9 when the strain is 30%. As indicated in Figure 5f, the fitting curve and equation (ΔR/R = 0.59s, s represents strain) show the linear relationship between strain (elongation) and the relative resistance change rate. Curiously, the enlarged curvature of GNP/PU E-Skin as a function of time shows the stretching−recovering process, indicating that there is a slight rebound on relative resistance change rate during the recovering process, as shown in Figure 5h. Such a phenomenon is attributed to the corresponding stretching-dependent resistive sensing mechanism as illustrated in Figure 5g, revealing the whole curve of the stretching− recovering process. We believe that there could be some interfacial separation phenomena occurring when the sensor is undergoing the bending progress, further investigations upon micro-delamination during bending were undertaken, and more results and discussions will be presented and carried out in our future following works.45−47 As shown in Figure 5g, as-prepared GNP/PU E-Skin is mainly composed of PDMS layers and GNP/PU films, which form a sandwich structure with GNP/ PU films covered by PDMS (Figure 5g(i)). In fact, the elasticity of GNP/PU films is lower than the elasticity of PDMS layers, and PDMS layers play a critical role in protecting GNP/PU films. During the stretching process, GNP/PU films can be simultaneously stretched with PDMS layers, in which the length of GNP/PU films is increased largely leading to greater resistance (Figures 5g(ii)), corresponding to Steps 1 to 4 in Figure 5h. During the recovering process, PDMS layers shrink more quickly than GNP/PU films due to the better elasticity of

200, 300, and 400 mm, respectively, which supplied various momenta or impulses of 1.9, 2.7, 3.8, 4.7, and 5.4 mN·s, as indicated by the inset image of Figure 4h. As shown in Figure 4h, the GNP/PU E-Skin exhibited quick and obvious responses to the different impulses. Compared with static pressure, the sharp peaks of relative resistance change revealed the quick responses to the momenta. The height of the peaks (i.e., ΔR/ R) can accurately identify the amounts of applied momenta. Significantly, the response of GNP/PU E-Skin to momenta is stable and quick. The time for one cycle of resistance to increase and decrease is usually around 1 s, revealing that the response time of the as-prepared GNP/PU E-Skin is as short as less than 0.5 s. If the response time is longer than 0.5 s, it is not possible to identify the amounts of momenta. Therefore, the sensing results of both the static and the dynamic tests showed that the as-prepared GNP/PU E-Skin had a quick and stable response to pressure, which makes it suitable to be used as a wearable pressure sensor. Thus, random hitting or pressing from human fingers were detected by the GNP/PU E-Skin, and the results are shown in Figure 4i. This demonstrates that the E-Skin is sensitive to quick hitting and various pressure, and suitable to be used as wearable electronics. Detection of Bending and Stretching. To further investigate the potential application of the as-prepared GNP/ PU E-Skin, it was bonded to fingers and the fingers’ bending was detected. The relative resistance change was recorded while bending the finger to a certain degree and then returning it to the initial state. Figure 5a shows the corresponding relative resistance change rate (10−3) versus time (s) profile. The insert photograph indicates the various bending positions of the index finger with E-Skin. Obviously, the E-Skin behaves with a quick and obvious response to the bending of fingers. The resistance is quickly increased when the finger is bent, and subsequently, the resistance is rapidly decreased and returns to the initial state when the finger returns to the initial state (straight). Moreover, the E-Skin is also sensitive to the bending angles, which are reflective to the relative resistance change rate (ΔR/R), as shown in Figure 5b. Obviously, there is a linear relationship between the relative resistance change and bending angles (θ), as indicated by the fitting equation of ΔR/R = 0.035θ. Figure 5c illustrates the curvature of the E-Skin as a function of time, revealing the stable response under the recycle bending of 90°. G

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ACS Applied Materials & Interfaces the PDMS layers, resulting in the decreasing of resistance of the GNP/PU films, corresponding to Steps 5 and 6 in Figure 5h; at the same time, some interfacial separation phenomena occurred. However, the combining force between PDMS layers and GNP/PU films in some areas could change violently, due to the inconsistent contraction of PDMS layers and GNP/PU films, which may produce instantaneous tension to prevent the contraction of GNP/PU films, resulting in the slight rebound on resistance during the recovering process, corresponding to Steps 6 and 7 in Figure 5h. After the slight rebound, GNP/PU E-Skin would continuously recover to the initial state, corresponding to Steps 7 and 8 as indicated in Figure 5h. Similar phenomena can also be found in other sandwich structure’s bending cycles.26,38 We first introduce such a phenomenon and explain its mechanism with the sandwich model. Detection of Voice Vibrations. Furthermore, the skinattachable GNP/PU sensors show excellent sensitivity to voice vibration. As shown in Figure 6a, the relative resistance change was recorded when the monolayer GNP/PU E-Skin film without PDMS was directly exposed to a volunteer’s voice vibrations. The stress caused by vibration causes the sensor to receive stress in different directions. Herein, the vibration caused the resistance change in the monolayer GNP/PU E-Skin film.48 Obviously, there were three characteristic peaks in the curve of ΔR/R versus time, when the volunteer pronounced the three letters of “H, I, T” as shown in Figure 6b. Noticeably, there are two peaks during pronunciation of “H”, and there was one peak during pronunciation of “I” and “T”, indicating that the E-Skin had a superb distinguishing response to voice vibration. Moreover, the relative resistance change was quite stable even after re-pronouncing of “H, I, T” three times, revealing the good reliability in monitoring voice. All results showed that as-prepared GNP/PU E-Skin had a good repeatability, stability, and sensitivity to pressure, hitting, bending, stretching, and even voice vibration, indicating that it is very promising for practical applications in wearable devices.

application in future wearable electronics such as electronic skin, human action sensors, and sound detecting skin.

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EXPERIMENTAL DETAILS

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ASSOCIATED CONTENT

Preparation of GNP/PU E-Skin. Natural flake graphite with an average diameter of 180 μm was used for preparing the expanded graphite (EG). Acetic acid, concentrated sulfuric acid, and potassium permanganate were used as chemical intercalator and oxidizer to prepare graphite intercalation compounds. Graphite nanoplates (GNP) were fabricated by a surfactant-assisted ultrasonic exfoliation method. After ultrasonication assisted by dispersion agent, EG particles in a mixture of ethanol and deionized water were effectively fragmented and exfoliated into graphite nanoplates. After vacuum filtration, homogeneous GNP dispersions were obtained for the preparation of E-Skin. After drying, GNPs were uniformly mixed with polyurethane (PU) in organic solvent with the weight ratio of GNPs to PU of 10:2. Then, the GNP/PU nanocomposite film was coated by a gap coating method on poly(tetrafluoroethylene) (PTFE) or release PET substrates. After drying and the evaporation of organic solvent, free-standing GNP/PU nanocomposite films with a thickness of 10− 100 μm were peeled off. Subsequently, the electrodes were prepared by using silver paste as the adhesive to bond the copper wire at the temperature of 80 °C for 3 h. Typically, as-obtained GNP/PU E-Skin with the length (l), width (w), and thickness (h) of 20 mm, 15 mm, and 40 μm, respectively, has excellent flexibility and good electrical conductance (∼50 S/m). However, as-obtained GNP/PU E-Skin was extremely flexible and thin and difficult to be operated by hand; it had to be encapsulated by poly(dimethylsiloxane) (PDMS) to take the pressing, hitting, bending, and stretching tests more easily. It should be noted that the GNP/PU E-Skin used for the voice test was not encapsulated by PDMS to improve its sensitivity. Characterization and Measurement Details. Scanning electron microscopy (SEM, Zeiss, Merlin Compact) was used to reveal the sample morphology and microstructure. The X-ray diffraction (XRD) patterns and IR spectra of samples were acquired on an X-ray diffractometer (DX-2700, Dandong Haoyuan Instrument Co., Ltd.) with Cu Kα radiation (λ = 1.5418 Å), and FTIR Spectrometer (Thermo Electron Nicolet 380), respectively. Electrical resistivity (ρ) of the samples was measured by a four point probe meter (RTS-8, Guangzhou 4 Probes Tech.). The resistance change was detected by the Digital multimeter (ROGOL, DM3068).

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CONCLUSIONS In summary, we have developed a simple, low-cost, and industrially available approach to produce a new type of highly flexible, sensitive, and wearable E-Skin based on GNP/PU nanocomposites. As-prepared GNP/PU E-Skin with good electrical conductivity and extremely high flexibility possesses a steady and consistent response to both static and dynamic pressure, and it can even accurately and quickly detect pressure as low as 0.005 N/50 Pa and momentum as low as 1.9 mN·s. Further investigations show that GNP/PU E-Skin is also sensitive to finger bending and stretching with a linear relationship between ΔR/R and the corresponding bending angles or elongation percentage. Moreover, the skin-attachable GNP/PU sensors exhibit a steady and high-resolution response to voice vibration. It was demonstrated that as-prepared GNP/ PU E-Skin is very promising for practical applications in wearable devices that enable us to monitor in real-time and insitu the real-world force signals from pressing, hitting, bending, stretching, and voice vibration. More importantly, the entire fabrication process of the E-Skin is simple, low cost, and industrially available without complex processing and expensive equipment. We believe that our methodologies open a new route to industrially available pressure sensors with potential

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10316. XRD pattern of GNPs (PDF) Stretching of GNP-PU nanocomposite films (Movie S1), knotting of GNP-PU nanocomposite films (Movie S2), rubbing of GNP-PU nanocomposite films (Movie S1) (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (J.L.). *E-mail: [email protected]; guozhongupenn@gmail. com(S.G.X.). ORCID

Huatao Wang: 0000-0002-2807-7881 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. H

DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grants 51302051 and 51502060), Natural Science Foundation of Shandong (ZR2012EMQ 007) for their financial support.

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DOI: 10.1021/acsami.7b10316 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX