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A highly stretchable nanofiber-based electronic skin with pressure-, strain-, and flexion-sensitive properties for health and motion monitoring Kun Qi, Jianxin He, Hongbo Wang, Yuman Zhou, Xiaolu You, Nan Nan, Weili Shao, Lidan Wang, Bin Ding, and Shizhong Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07935 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
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ACS Applied Materials & Interfaces
A Highly Stretchable Nanofiber-based Electronic Skin with Pressure-, Strain-, and Flexion-sensitive Properties for Health and Motion Monitoring Kun Qia, Jianxin Heb,c*, Hongbo Wanga*, Yuman Zhoua, Xiaolu Youb,c, Nan Nanb,c, Weili Shaob,c, Lidan Wangb,c, Bin Dingb,d, Shizhong Cuib,c a
School of Textile and Clothing, Jiangnan University, Wuxi 214122, China
b
Provincial Key Laboratory of Functional Textile Materials, Zhongyuan University of Technology, Zhengzhou 450007, China c
Collaborative Innovation Center of Textile and Garment Industry, Zhengzhou 450007, China Henan
d
Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China
KEYWORDS: stretchable electronic skin, nanofiber, electrospinning, graphene oxide, PEDOT, health and motion monitoring ABSTRACT: The development of flexible and stretchable electronic skins that can mimic the complex characteristics of natural skin is of great value for applications in human motion detection, healthcare, speech recognition, and robotics. In this work, we propose an efficient and low-cost fabrication strategy to construct a highly sensitive and stretchable electronic skin that enables the detection of dynamic and static pressure, strain, and flexion based on an elastic graphene oxide(GO) doped polyurethane (PU) nanofiber membrane with an ultrathin conductive poly(3,4-ethylenedioxythiophene) (PEDOT) coating layer. The three-dimensional porous elastic GO-doped PU@PEDOT composite nanofibrous substrate and the continuous self-assembled conductive pathway in the nanofiber-based electronic skin offer more contact sites, a larger deformation space, and a reversible capacity for pressure and strain sensing, which provide multi-modal mechanical sensing capabilities with high sensitivity and a wide sensing range. The nanofiber-based electronic skin sensor demonstrates a high pressure sensitivity (up to 20.6 kPa-1), a broad sensing range (1 Pa to 20 kPa), excellent cycling stability and repeatability (over 10,000 cycles), and a high strain sensitivity over a wide range(up to approximately 550%). We confirmed the applicability of the nanofiber-based electronic skin to pulse monitoring, expression and voice recognition, and the full range of human motion, demonstrating its potential use in wearable human-health monitoring systems.
1. INTRODUCTION Artificial electronic skin is a flexible sensor that can mimic the sensory function of human skin by converting various stimuli, such as pressure, strain, shear force, temperature, and humidity, into electrical signals1-6. The device has good application prospects in robot automation, flexible electronics, wearable equipment, the biomedical field, and human-computer interaction7-15. To mimic the properties of human skin, artificial electronic skin must have the ability of conformally covering the arbitrary irregular and moving surfaces to detect multiple mechanical stimuli, such as pressure, strain, and flexion16-19. Additionally, it must satisfy the following requirements: high stretchability, sensitivity, wide sensing range, and fast re-
sponse/recovery speeds. Various previous studies have reported flexible and highly sensitive pressure sensors constructed from conductive polymers, gold nanowires, graphene, and carbon nanotubes as sensing elements and have introduced micro-and nano-structures (pyramids, semispheres, and cylinders) based on different sensing mechanisms(resistive, capacitive, piezoelectric, and triboelectric)20-31. Chung et al. reported a high-performance piezoresistive pressure sensor device based on a bio-inspired hierarchical graphene/polydimethylsiloxane(PDMS) array exhibiting a linear relationship between the applied pressure and output signal, and a high sensitivity of 8.5 kPa−1in the pressure range 0-12 kPa32. Ko et al. developed an electronic skin based on a CNTcomposite-based elastomer film featuring interlocked micro-
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dome arrays, which lead to a giant tunneling piezoresistance and, thus, a high pressure sensitivity (15.1kPa-1, ∼0.2Pa minimum detection)33. However, the practical application of these flexible pressure sensors in continuous and real-time health monitoring is strictly limited, as they cannot stretch like natu-
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ral human skin. To address this issue, various approaches have been proposed to construct highly stretchable and skinattachment sensors using intrinsically stretchable materials and new architectures34-36. Liu et al. reported a high performance
Figure 1. (a)Schematic illustration of the fabrication of the GPPN sensor. (b) Optical images of the original and stretched GPPN membrane. (c)The photograph of the GPPN sensor. (d) Low-and (e) high-magnification SEM images of the GPPN membrane. (f) TEM image of a GPPN.
strain sensor based on a fish-scale-like graphene-sensing layer, which showed high stretchability and a wide strain-sensing
range (up to 82%), capable of full-range detection of human motions37. In addition, Zhang et al. demonstrated the excellent
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performance of a strain sensor based on carbonized silk fabric that exhibited a large workable strain range (500%)38. Despite these considerable achievements in stress and strain sensors, the construction of a stretchable multi-mode mechanical sensor capable of sensing pressure, strain, and flexion for monitoring human body motions ranging from subtle deformations to substantial movements remains a challenge39-40. Ge et al. reported an electronic fabric with the ability of simultaneously mapping and quantifying mechanical stresses induced by pressure, lateral strain, and flexion. The fabric was based on intertwined sensor electrodes with piezoresistive rubber as the shell sensing element and silver-nanowire-coated elastic thread as the stretchable and highly conductive core electrode41. Here, we propose a cost-effective and large-area-compatible approach for fabricating a highly stretchable and sensitive multi-mode mechanical sensor based on three-dimensional (3D) elastic porous graphene oxide (GO)-doped polyurethane (PU) nanofiber membranes with poly(3,4ethylenedioxythiophene) (PEDOT) coating. This sensor can detect multiple “skin-like” stresses induced by normal pressure, stretching, and bending. The GO-doped PU@PEDOT nanofiber (GPPN) based sensor was constructed using an electrospun GO-doped PU nanofiber membrane as the flexible substrate and in situ polymerization of conductive polymer PEDOT on the as-obtained nanofibers. The highly stretchable structure of the 3D porous GO-doped PU@PEDOT composite nanofiber substrate and the continuous self-assembled conductive pathway in the GPPN sensor provide more contact points for strain sensing and shows a larger deformation space and reversible capacity, thus, providing the nanofiber sensor with high sensitivity and multi-mode mechanical sensing capability over a wide range. The GPPN sensor exhibits a high pressure sensitivity up to 20.6 kPa-1, a broad sensing range (1 Pa to 20 kPa), and excellent cycling stability and repeatability over 10,000 cycles. Furthermore, this GPPN sensor exhibits high stretchability and a broad strain-sensing range (up to 550%). Additionally, we demonstrate the excellent performance of the sensor in monitoring human health (pulse) and full-range human body movement (facial expression, vocalization, and joint movement). This electronic skin sensor device, which combines excellent multi-force sensitivity and stretchability, has a wide range of applications, including medical diagnostics, implantable testing equipment, and even artificial organs and human-computer interaction.
2. RESULTS AND DISCUSSION Figure 1 shows a schematic illustration of the preparation of the GPPN sensor, and the detailed descriptions are presented in the Experimental Section. A highly stretchable and multifunctional sensor was developed by utilizing the 3D elastic porous structure of the electrospun GO-doped PU nanofiber membrane and the excellent thermoelectric properties of PEDOT. The flexible substrate, an electrospun GO-doped PU nanofiber membrane, was coated by in situ polymerization of monomer EDOT on the nanofiber surface to obtain the 3DGO-
doped PU@ PEDOT nanofiber membrane. Firstly, GO-doped PU nanofiber membranes with 1wt% GO mass fraction were prepared by electrospinning. The doping with GO significantly improved the conductivity of the spinning solution, resulting in finer and more uniform nanofibers (Figure S2, Supporting Information). The average diameter of the pure PU nanofibers wasapproximately569 nm, while that of the GO-doped PU nanofibers decreased to 132 nm (Figure S3, Supporting Information). Furthermore, the tensile strength of the GO-doped PU nanofiber mats increased by 162% (from 3.37 MPa to 8.82 MPa) compared with that of the pure PU nanofiber membranes (Figure S6, Supporting Information). The use of GO sheets with a large amount of oxygen-containing functional groups contributed to the production of a large number of carboxyl groups on the surface of the PU nanofiber after lowtemperature oxygen plasma treatment (Figure S7, Supporting Information). This resulted in the transformation of the nanofiber membrane from super-hydrophobic to hydrophilic (Figure S8, Supporting Information), which is beneficial for the chelate adsorption of more oxidant FeCl3 and speeds the bonding between the PU nanofiber substrate and the PEDOT coating layer under the same in situ coating conditions. After the in situ polymerization of PEDOT, the GPPN membrane exhibited a color change from faint yellow to blue-black and also a higher conductivity (213.2 S/cm) than the PU@PEDOT nanofiber (PPN) membrane (22.2 S/cm) for the same coating parameters (Figure S9, Supporting Information). Figure 1d shows the scanning electron microscopy (SEM) images of the GPPN membrane. The figure shows that the GO-doped PU nanofiber membrane exhibits obvious crimping and shrinkage after the in situ polymerization coating of the conductive polymer PEDOT, while the 3D porous network structure of the nanofiber membrane is retained very well. Although the average diameter of the nanofiber increases from 132 nm to 389 nm, it is still obviously smaller than that of PPNs deposited under the same coating conditions (813 nm) (Figure S10, Supporting Information). The magnified SEM image of the single GPPN reveals a rough morphology on the nanofiber surface and that the nanofiber surface was conformally coated by the in situ polymerized PEDOT nanoparticles with size 20-45 nm (Figure 1e). X-ray photoelectron spectroscopy(XPS) spectra of the GPPN membrane confirmed that the characteristic N-signal peaks of PU disappeared while the Ssignal peaks appeared. Moreover, the XPSS2pde-convolution spectra of the GPPN membrane were located at 163.4 eV and 164.8 eV, corresponding to the binding energy of the S2p3/2 and S2p1/2 states of PEDOT, respectively, in agreement with the previously reported results(Figure S11, Supporting Information)42. The mapping images of the S elements of the GPPN displayed in FigureS12 also confirmed that most of the PEDOT nanoparticles were conformally coated on the nanofiber surface (the S element is unique to PEDOT). As seen in Figure1f, the transmission electron microscopy (TEM) image exhibits the obvious core-shell structure of the nanofiber with
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a continuous-coating PEDOT shell layer with an average thickness of ~257 nm. Moreover, some of the GO nanosheets in the core PU nanofiber, which were not completely coated
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by the nanofibers, were inserted into the PEDOT shell layer (Figure S3d, Supporting Information).
Figure 2. (a-c) I-V curves of the GPPN sensor under different pressures, tensile strains, and bending curvatures respectively. (d-f) Comparison of the sensitivity of the sensor with PPN and GPPN membranes under pressures, tensile strains, and bending curvatures respectively.
To assemble a nanofiber-based (GPPN) mechanical sensor, both ends of the GPPN membrane were coated with conductive silver paste and connected with copper wires, then, the upper and lower surfaces of the nanofiber membrane were coated with the PDMS film (Figure 1c, Figure S13). The GPPN sensor demonstrated linear current-voltage (I-V) characteristics with a relatively stable response to pressure, stretching strain, and flexural curvature, indicating multiple forcesensing properties (Figure 2 a-c). The elastic nanofiber membrane with the ultrathin conductive PEDOT coating layer enables a 3D interconnecting conductive network with more contact points, which in turn leads to a tunable effective contact resistance that can capture varied strains applied by pressure, stretching, and bending, thus providing high sensitivity and a wide range of multi-modal mechanical sensing capabilities to the nanofiber-based sensor. To estimate the pressure sensitivity of the GPPN sensor, its relative current change (∆I/I0) was calculated on the basis of the measured value plotted as a function of the applied pressure. The pressure sensitivity of the GPPN sensor is defined as the slope of the curve plotted in Figure 2d,that is, S = ((∆I/I0)/∆P, where ∆I and I0 denote the pressure-induced current change and the initial current of the sensor without pressure loading, respectively, and ∆P is the change in the applied pressure. The pressure-induced current change is determined by the contact resistance of electrode and GPPN membrane, contact resistance of neighbouring conductive nanofibers and resistance changes result from decreased thickness of the compressed sensors. The pressure sensing mechanism in the Figure 3g shows that such an increase in the contact area would decrease the total resistance and more PEDOT conduc-
tive paths can be established when the external force applied, leading to a current improvement. As seen in Figure 2d, the GPPN sensor exhibited a high sensitivity of up to 20.6 kPa-1 in the low-pressure region(1 kPa). Obviously, the current increases instantaneously when subjected to a small external pressure lower than 1 kPa, because the nanofibers are ultrathin and easily deformable, thus, the deformation accumulation of many nanofibers causes a rapid increase in the contact area, which in turn produces a rapid decrease of in the contact resistance. In stage II, where the nanofibers is being deformed (1-8 kPa), the average sensitivity was 0.89 kPa-1, requiring higher pressure to further deform the nanofibers. In stage III (>8 kPa), the sensitivity further reduced to 0.15 kPa-1 as GPPN membrane has a highly compact structure, requiring larger pressure to close the conductive PEDOT particles in the GPPN. In contrast, the PU@PEDOT nanofiber sensor without GO doping has a relatively smaller pressure sensitivity of 2.57 kPa-1 in the lowpressure region (
