Thiolated Graphene@Polyester Fabric-Based Multilayer

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Surfaces, Interfaces, and Applications

Thiolated Graphene@Polyester Fabric-Based Multilayer Piezoresistive Pressure Sensor for Detecting Human Motions Lin Zhang, Hongqiang Li, Xuejun Lai, Tianyuan Gao, Jian Yang, and Xingrong Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16027 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Thiolated Multilayer

Graphene@Polyester Piezoresistive

Pressure

Fabric-Based Sensor

for

Detecting Human Motions Lin Zhang,a Hongqiang Li,*a,b Xuejun Lai,a,b Tianyuan Gaoa, Jian Yang*c and Xingrong Zeng*a,b aSchool

of Materials Science and Engineering, South China University of Technology, Guangzhou

510640, China bKey

Lab of Guangdong Province for High Property and Functional Polymer Materials,

Guangzhou 510640, China cCollege

of Optoelectrical Engineering, Shenzhen University, Shenzhen 518060, China

KEYWORDS: thiolated graphene, polyester fabric, multilayer-structure, piezoresistive pressure sensor, human motions, electronic skin

ABSTRACT: In the past several years, wearable pressure sensors have arisen the new surge of interest worldwide due to their important applications in the areas of health monitoring, electronic skin and smart robots. However, it is still a great challenge to simultaneously achieve wide pressure sensing range and high sensitivity for the sensors until now. Herein, we proposed an innovative strategy to construct multilayer-structure piezoresistive pressure sensors with in situ generated

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thiolated graphene@polyester (GSH@PET) fabric via one-pot method. Taking advantages of the spacing among the rough fabric layers and the highly conductive GSH, the sensor realized not only wide pressure range (0-200 kPa), but also high sensitivity (8.36 and 0.028 kPa-1 in the ranges of 0-8 and 30-200 kPa, respectively). After 500 loading-unloading cycles, the sensor still kept high sensitivity and stable response, exhibiting the great potential in long-term practical application. Importantly, the piezoresistive pressure sensor was successfully applied to accurately detect different human behaviors including pulses, body motions and voice recognitions. Additionally, the sensing network integrated by the sensors also realized mapping and identifying spatial pressure distribution. Our method to construct the wide-range and high-sensitivity piezoresistive pressure sensor is facile, cost-effective and available for mass production. The findings provide a new direction to fabricate the new-generation high-performance sensors for healthcare, interactive wearable devices, electronic skin and smart robot.

INTRODUCTION In recent years, the increasing standards of the people on health and life quality have accelerated the rapid development of smart wearable devices in both academic and industrial communities.1-3 As the core component, the pressure sensors have been of immense interest due to its excellent tactile sensing capability4,5 and wide applications including healthcare,6 interactive wearable devices,7,8 electronic skin9 and smart robot development.10 As a kind of representative pressure sensor, piezoresistive sensor has the ability to quickly convert the applied pressure into resistance signal.11 Meanwhile, the sensor also possesses many obvious


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advantages such as oversimplified signal acquisition, simple fabrication process and high costeffectiveness.12 According to a large number of reported research works,13-19 the performance of piezoresistive sensor was mainly determined by two crucial factors. One is the active conductive sensing elements which are essential for the construction of conductive paths. The main active sensing elements include conductive polymers,20,21 carbon nanotubes (CNTs),22,23 metal nanoparticles and nanowires,24,25 graphene (containing various reduced graphite oxide),26,27 and so forth. By virtue of the unique advantages of excellent stiffness, extreme electrical conductivity, super flexibility and stretchability, graphene is considered as one of the most suitable materials for fabricating piezoresistive pressure sensors.28 For instance, Boland et al.29 prepared a sensitive sensor with electromechanical property by adding liquid-phase-exfoliated graphene sheets into a highly viscoelastic polymer. Tian et al.30 developed a pressure sensor based on two-layer laser-scribed graphene with rough microstructure. However, the preparation method was relatively tedious and time-consuming, and some special equipment such as lightscribe DVD drive was generally needed, which limited its large-scale production and practical application. The other crucial factor is the microstructure of the piezoresistive pressure sensor which can form the pressure-induced reversible variable conductive pathways. At present, to achieve high sensitivity for greatly changing the contact points or contact area in entire sensing system under the action of pressure, most of the sensors were designed into double-layer rough surface of sensing materials in contact or 3D microporous conductive network.31 For example, by constructing a polypyrrole film having a three-scale nested microstructures, Yang et al.21


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fabricated a piezoresistive pressure sensor, and the sensitivity attained a high value up to 19.32 kPa-1 (0-2 kPa). Cai et al.32 reported a high-sensitivity 3DGF/CNT network sensor through embedding 3D hollow graphene skeleton structure with enabled CNTs in elastomer matrix. Although the fabricated pressure sensors had high sensitivity, the dimensions of the microstructures were usually in a very small range, and the reversible deformation of the structures became saturated and even irreversible under large pressure, resulting in the inaccuracy and even inapplicability of the sensors in many applications accompanying strenuous activities such as human jumping and running. Herein, we propose an innovative strategy to fabricate wide-range and high-sensitivity multilayer piezoresistive pressure sensor with thiolated graphene (GSH) and polyester (PET) fabric. It was based on the utilization of GSH sheets wrapped on PET fabric to construct conductive pathways and the designing of multilayer structure to increase the electric contact points of sensor under pressure. The schematic illustration for fabricating the process of GSH@PET fabric-based piezoresistive pressure sensor is displayed in Figure 1. After dipping plasma-treated polyester fabric in GO dispersion and thiolation reaction, the GSH@PET fabric was obtained for multilayer piezoresistive pressure sensor. The morphology and chemical structure of GSH@PET fabric were characterized, and the relationship between the sensitivity of the sensor and the number of fabric layers was investigated to appear a direct-proportion tendency. The sensor realized a large working range (200 kPa) and excellent sensitivity (8.37 and 0.028 kPa-1 in the ranges of 0-8 and 30-200 kPa, respectively). The sensor also exhibited excellent repeatability even after 500 loading-unloading cycles. Importantly, the sensor was


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successfully applied to detect various human behaviors including pulse, body motions and voice recognitions, and the sensors integrated electronic skin could be used to map and identify spatial pressure distribution. The fabrication approach is facile, cost-effective, and easily produced on a large scale. The wide-range and high-sensitivity piezoresistive pressure sensor shows great potential in the field of health monitoring, wearable devices, artificial intelligence, electronic skin and other special application scenarios. EXPERIMENTAL SECTION Materials. Hydrobromic acid (HBr, 40%), thiourea (CH4N2S) and sodium hydroxide (NaOH) were all supplied by Aladdin Reagent Co., Ltd. (China). Acetone was purchased from Guangzhou Chemical Reagent Factory (China). The commercially available polyester fabric (plain weave fabric, 56 g/m2) was obtained from local supermarket. All chemicals were used as received without further purification. Fabrication of Graphene Oxide@Polyester (GO@PET) Fabrics. GO was prepared by using the modified Hummers’ method (Supporting Information),33 and dispersed in water to obtain the GO dispersion with a concentration of ~2.5 mg/mL by ultrasonic treatment. A piece of polyester fabric (2 cm × 3 cm) was ultrasonically washed in acetone for 0.5 h, and then dried in an air-blast oven at 60 oC for 0.5 h. Subsequently, the cleaned fabric was treated in pure oxygen plasma at 0.2 mbar for 6 min using a plasma machine (Flecto10, Plasma Technology, Germany) under 230 W. Finally, the fabric was immersed in GO dispersion for 1 min, and the


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taken-out fabric was dried at 60 oC for 0.5 h. After six immersion-drying cycles, the GO@PET fabric was obtained. Fabrication of Thiolated Graphene@Polyester (GSH@PET) Fabrics. The GO@PET fabric was firstly placed in a beaker containing 150 mL of deionized water, and then 12 mL of hydrogen bromide was added and stirred at room temperature for 2 h. After that, 10 g of thiourea was added, and the mixture was heated to 80 oC and stirred for 8 h. Subsequently, 8 g of NaOH was further added and continued to react for 0.5 h. The fabric was taken out and immersed into a large amount of boiling water to remove excess chemical impurities. After washing with acetone for 5 min under ultrasonication and drying at 60 oC for 0.5 h, the GSH@PET fabric was obtained. Preparation of GSH@PET Fabric-Based Piezoresistive Pressure Sensors. Two pieces of square GSH@PET fabric (0.5 cm × 0.5 cm) were firstly stacked, and then the top and bottom layers were respectively connected with a fine copper wire (diameter = 0.1 mm) as electrodes covering with a small amount of silver paste to ensure the flatness of the joint surface. After that, two pieces of polyvinyl chloride (PVC) plate (thickness: 0.2 mm) with the same size was placed on the two layers and fixed by 3M Scotch transparent tape (600-HC33), and the two-layer piezoresistive pressure sensor was obtained. For comparison, one-layer, five-layer and eight-layer piezoresistive pressure sensors were also respectively prepared with different pieces of GSH@PET fabric. Characterizations. Fourier transform infrared spectroscopy (FT-IR) was collected from 600 to 4000 cm-1 on a Tensor 27 spectrometer (Bruker Optics, Germany) using the attenuated total reflectance (ATR) mode, and the resolution was 4 cm-1. Sheet resistances of the GSH@PET fabric


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were measure with a four-point probe conductivity meter (RTS-9, Four Probe Tech., Guangzhou). The surface morphology and the element composition of the fabrics were analyzed by a EVO 18 scanning electron microscope (10.0 kV, Carl Zeiss Jena, Germany) equipped with an energy dispersive X-ray spectrometer (20.0 kV, INCA250, Oxford Instruments, UK). In particular, the conductive GSH@PET was used directly for observation without spaying gold. Raman spectra were obtained by using a Raman confocal microscope (532 nm, Horiba Jobin Yvon, Edison, NJ) in the range of 1000-3200 cm-1. The chemical analysis of the fabrics was examined by a X-ray photoelectron spectroscope (Kratos Axis Ulra DLD, UK) with a monochromated Al Kα source at three electron take-off angles (30o, 60o and 90o). The piezoresistive performance of the pressure sensors was characterized by an all-electric dynamic test instrument (ElectroPuls E1000, Instron, USA) and a semiconductor characterization system (Keithley 4200-SCS, USA) with a constant bias of 5 V, and the tester was a 25-year-old healthy male with a weight of 78 kg and a height of 180 cm. Sensitivity Calculation. The relationship between pressure and resistance was studied by using electronic universal material testing machine and semiconductor characterization system. The sensitivity (S) of the pressure sensors was calculated according to the following equation,34,35 S=

 R /R0  100% P

where ΔR was the absolute value of the resistance change with the applied pressure P on the sensor. R0 represented the initial resistance value of the sensor before applying the pressure, ΔP referred to the change of P. Here, the sensitivity was respectively defined as S1 in the small pressure range (0-8 kPa) and S2 in the large pressure range (30- 200 kPa). RESULTS AND DISCUSSION


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Fabrication and Structural Characterization of GSH@PET Fabric. In this work, GO was firstly promoted to wrap plasma-treated PET fabric through the force of hydrogen bonds between the polar groups of GO and the treated fibers, and then directly reduced into flexible and conductive thiolated graphene in the presence of hydrobromic acid and thiourea at mild reaction conductions to form GSH@PET fabric for piezoresistive pressure sensor. Compared with the white pristine PET fabric, the colors of GO@PET fabric and GSH@PET fabric changed into brown and black, respectively (see Figure S1). The resistance per centimeter on the fabric was about 1.125 kΩ, and the sheet resistance of GSH@PET fabric reached a low value of 479.6 Ω/□. Figure 2 presents the SEM images of plasma-treated PET fabric, GO@PET fabric and GSH@PET fabric. It was obvious to note that the gaps between the fibers were distinct and the fiber surface was very smooth (Figure 2a-a2). After six immersion-drying cycles with GO dispersion, an obvious GO film was observed on the fiber surface, and the gaps between fibers were also covered by GO. Furthermore, it also can be seen that the GO layer on the fiber appeared many tiny curled wrinkles from the amplified image (Figure 2b-b2). After thiolation reaction, the morphology of the fabric surface was almost unchanged and still kept a complete layer on the fiber (Figure 2c). However, the tiny curled wrinkles of the GO film obviously became straight and larger (Figure 2c1-c2), which might be due to the loss of a large amount of bounded water in the transition process of polar GO into nonpolar GSH (see Figure S2).36 The EDS analysis showed a large amount of sulfur was uniformly dispersed on the surface of GSH@PET fabric (Figure 2d-d3). From the FT-IR spectra of different PET fabrics in Figure 3a, after oxygen plasma treatment, the stretching peaks of O-H at 3300 cm-1 and C=O at 1635 cm-1 were greatly enhanced. In the spectrum of GO@PET, the stretching vibrational peaks of C-O epoxy at 1244 cm-1, C-O alkoxy at 1065 cm-1 and C=O of carbonyl/carboxy at 1732 cm-1 all clearly appeared, demonstrating the


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existence of GO layer. Furthermore, the broad absorption peak around 3213 cm-1 attributed to the H-bonded -OH and O-H stretching also proved the coating of GO on fiber surface.37 After thiolation reaction, the characteristic peaks of GO were obviously declined in the spectrum of GSH@PET fabric. Nevertheless, the peak in the range of 2500-2600 cm-1 ascribed to the vibrational band of S-H bonds was not obvious, which was mainly due to the inherent weak vibration mode and the low amount of S-H bond.38 As shown in Figure 3b, the obvious D (breathing mode of sp2-hybridized carbon) and G (graphitic sp2-hybridized carbon) bands could be observed from the Raman spectra of GO@PET fabric and GSH@PET fabric. The D band on GSH@PET fabric exhibited a smaller full-width half-maximum value, demonstrating a less extent of structural disorder. The ID/IG (the peak area ratio of D band to G band) of GSH@PET fabric evidently increased to 1.73 from 1.30 of GO@PET fabric, which was in accordance with Chua’s work.39 Moreover, compared with GO@PET fabric, the peak for the G-band of GSH@PET fabric was shifted from 1603 cm-1 to 1592 cm-1. It indicated that the defective hexagonal network constructed by carbon atoms was recovered at a certain degree.40 To further analyze the chemical composition of GO@PET fabric and GSH@PET fabric, XPS measurement was carried out and the obtained spectra are presented in Figure 3c-f. From the XPS survey scans in Figure 3c, there were 68.92 at.% of carbon and 31.08 at.% of oxygen for GO@PET fabric. Comparatively, except 83.15 at.% of carbon and 14.59 at.% of oxygen, 2.26 at.% of sulfur was also detected for GSH@PET fabric, which were similar with the EDS results (see Figure S3, Table S1 and S2). The C/O atom ratio for GO@PET fabric at 2.22 was obviously lower than that of GSH@PET fabric at 5.70. Furthermore, as can be seen from the high-resolution C 1s core-level spectra (Figure 3d,e), a typical C 1s core-level spectrum was divided into five peaks at 284.6, 285.6, 286.6, 288.2 and 289.2 eV, belonging to the characteristic peaks of C=C, C-C, C-O/C-S, C=O and


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O-C=O, respectively. Compared with GO@PET fabric, the area ratio of C=C peak to C-C peak for GSH@PET fabric sharply increased from 1.67 to 5.26 (see Table S3). Moreover, the intensity of C-O and C=O peaks for GSH@PET fabric appeared an obvious decrease. It was due to the ringopening of epoxide groups and nucleophilic substitution on hydroxyl groups (see Scheme S1). In Figure 3f, the S 2p spectrum was fitted with 2p3/2 components at 163.5 eV (H-S-C), 164.5 eV (RS-C) and 168.0 eV (S-O) and corresponding S 2p1/2 (with a typical splitting magnitude of 1.18 eV).39, 41 Based on the above results, it can be conceivably confirmed that GO layer on PET fiber surface had been reduced into GSH. Fabrication

and

Electromechanical

Performance

of

GSH@PET

Fabric-Based

Piezoresistive Pressure Sensors. Although the GSH@PET fabric processed good electrical conductivity, the change of resistance was very little under exterior pressure, which seriously limits its application in piezoresistive pressure sensors. To enlarge the resistance change, several pieces of GSH@PET were innovatively designed and staked into mini-portable three-dimensional sensor. Thus, the conductive pathways and the electrical conductivity of the sensor will significantly change under exterior pressure. A simple piezoresistive test was conducted with the five-layer piezoresistive pressure sensor. Firstly, the sensor with a resistance of 17.4 kΩ was connected to a 12 V circuit, and 40 light-emitting diode (LED) bulbs emitted a faint light. When put a 200 g weight on the sensor, the resistance quickly decreased to 1.1 kΩ, and the light of the LED lamps was more splendid. After removing the weight, the resistance recovered to 17.3 kΩ (Figure 4a). In addition, when intermittently applying a small finger pressure on the sensor, the brightness of the LED bulbs also alternately increased and decreased (see Video S1). To systematically study the electromechanical performance of the piezoresistive pressure sensor, an electronic universal material testing machine and a semiconductor characterization system were


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utilized to set up the experimental testing platform as shown in Figure 4b. When the cylindrical pressure holddown with a radius of 0.5 cm fixed on a fixture with pressure sensing applies pressure on the sensor, the relationship between the resistance and the pressure can be obtained by matching the data exported from the two above test instruments. In Figure 4c, the sensitivity of the singlelayer sensor was 0 kPa-1 and kept unchanged even under a large pressure. In comparison, the double-layer, five-layer and eight-layer pressure sensors all exhibited a high sensitivity. Meanwhile, as pressure increased, the sensitivity gradually decreased and presented two linear relationship intervals in a small pressure range of 0-8 kPa and a large pressure range of 30-200 kPa. Taking advantage of the relationship between sensitivity and pressure, the sensor can be applied in different applications with a wide pressure range. It was interesting to note that the sensitivity of the sensors also increased with the increase of the number of layers. The reason was mainly ascribed to the spacing among the GSH@PET layers formed by fiber intersection. To conveniently explain the inherent mechanism, the schematic illustration for the structural change process of the two-layer sensor under pressure is displayed in Figure 4d. When a small pressure was applied on the sensor, the number of the conductive contact point increased owing to the reduction even disappearance of the spacing between the two GSH@PET fabrics, showing the high sensitivity of the sensor in the small pressure range. As the pressure increased to a high value, the contact area continued to increase due to the elastic deformation between the fibers, and the electrical resistance further decreased, also exhibiting the high sensitivity in the large pressure range. Therefore, as a result of amplifying the structural change effects, the more the layers of the sensor was, the higher the sensitivity was. Certainly, the roughness and high mechanical resilience of the interwoven PET fibers were also vital for the sensors to quickly recover to the initial state after removing pressure. Considering that the high sensitivities at 7.16 kPa-1 in small pressure


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range and 0.021 kPa-1 in large pressure range, the five-layer sensor was selected for the following electromechanical performance test and applications. Figure 5a shows the current-voltage (I-V) characteristics of the GSH@PET fabric-based piezoresistive pressure sensor under different pressures. It was obvious to note that the I-V characteristic curves all appeared a linear tendency, indicating the “Ohmic” behavior of the sensor. Clearly, as the pressure increased, the resistance correspondingly decreased. It also can be seen that the current values of the sensor at different pressures after 1000 s almost remained the initial ones. Importantly, as shown in Figure 5c, the sensor maintained the stable response after five measurement cycles under the pressures of 4, 8, 20 and 80 kPa, respectively. Meanwhile, the response time of the sensor reached ~159 under the pressure of 20 kPa, and the recovery time also attained ~87 ms (Figure 5d,e). In addition, the effect of dynamic frequency on the electrical cycling response of the sensor was studied. In Figure 5f, the values of current signal were similar with the frequency at 0.1 and 1 Hz, and the signal maintained periodic changes at 2.5 Hz. It was beneficial to greatly expanded the application scenarios of the sensors. Furthermore, the piezoresistive pressure sensor still possessed excellent repeatability even after 500 cycles under a small pressure range of 0-10 kPa and a large pressure range of 120-160 kPa (Figure 5g,h), illustrating the outstanding retentiveness of the spacing between GSH@PET fabrics. Practical Application of Multilayer Piezoresistive Pressure Sensor for Detecting Human Motions. Due to high sensitivity, wide pressure range, fast response and excellent repeatability, the multilayer piezoresistive pressure sensor had important application potential in many practical fields including human healthcare, athletic performance monitoring, soft robotics and artificial intelligence. Herein, the piezoresistive pressure sensor was mainly fixed on various body parts to detect human motions. At first, the sensor was mounted on the tester’s wrist with tape to detect the


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pulse as shown in Figure 6a. Under the weak pressure by pulse beat on the sensor, the regulated decreases of current by pulse beats were real-time recorded as downward peaks, exhibiting the high sensitivity (see Video S2). Figure 6b shows the real-time curve of electrical current change in 20 s. The pulse rate was calculated to be about 64 times/min. It was very close to the average level of humans. Moreover, three downward peaks marked as P1, P2 and P3 were observed in the representative magnified circulation in Figure 6c, which referred to the percussion wave, tidal wave and diastolic wave, respectively.32 The sensor was also fixed at the eye corner to detect the movements of the eyelid (Figure 6d). With the repeated eye motions accompanying the muscle movements around the eyes such as looking up, closing and looking forward, the generated tiny pressure changes were accurately detected by the sensor to output a series of regular variations of the electrical current (Figure 6e). Additionally, the sensor was also successfully applied in speech recognitions.42,43 As displayed in Figure 6f, the sensor was attached on the tester’s neck for capturing the subtle muscle movements during speaking. When the words “Good”, “Work” and “Hello” were pronounced repeatedly for six times, the sensor exhibited the outstanding sensitivity and distinct signal patterns (Figure 6g-i) generated by the muscle movements around the throat of the tester. Moreover, it was clear that the pronunciations of different words were corresponding to different waveforms of the detection signals, indicating the high accuracy of the piezoresistive pressure sensor in voice recognition. The pressure sensor was further placed on the insole and knee of the tester (Figure 7a,c), and the corresponding real-time response signals of walking and running were recorded (Figure 7b,d). It was clear to note that the response waveforms were regular, stable, repetitive, and easy to be distinguished. Additionally, the sensor was applied to respond to the movement of fingers and the flexion of wrist in a large sensing range (Figure 7e,g). It can be seen that the current appeared the


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quick and stable responses to the finger and wrist at different angles (Figure 7f,h). There is a cubic function relationship between the relative resistance change (ΔR/R0) and bending angles (θ), as indicated by the fitting curves in the insert photograph and equations (see Table S4). To realize the practical applications of the multilayer piezoresistive pressure sensors in humancomputer interfaces applications and e-skin, we designed and prepared the integrated device with 25-pixels sensing networks (Figure 8a) for detecting the distribution of space-resolved piezoresistive effect by measuring the resistance of each pixel.44 The ﬂexible pressure-sensor arrays were exerted with “Point” pressure (Figure 8b) by a metal bent tweezer and “Plane” pressure (Figure 8c) by a finger. Obviously, the “Point” pressure only introduced a local pressure, and the corresponding resistance change of the local pixels could detect the precise position and intensity of the external stimulation on the device (Figure 8d). When the thumb touched the sensor network on the electronic skin, the multidimensional tactile sensing image was successfully obtained corresponding to the pressure distributions (Figure 8e). CONCLUSIONS In summary, we proposed a novel strategy to fabricate GSH@PET fabric-based piezoresistive pressure sensors for detecting human motions. By combining the GSH layer wrapped on PET fibers to construct electrical pathways and the designing of multilayer structure to increase electric contact points under pressure, the sensor exhibited a large detection range with pressure up to 200 kPa and excellent sensitivities of 8.37 kPa-1 in 0-8 kPa and 0.028 kPa-1 in 30-200 kPa, a fast response time of 159 ms and relaxation time of 87 ms. After 500 loading-unloading cycles, the sensor still possessed excellent repeatability. Importantly, the sensor was successfully applied to accurately detect different human motions including pulse, body motions and voice recognitions. Furthermore, the sensors can be integrated into a large-area electronic skin for mapping and


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identifying spatial pressure distribution. The fabrication method is facile, efficient, inexpensive and easily produced on a large scale, and the wide-range and high-sensitivity piezoresistive pressure sensors are highly promising to be applied in the field of healthcare, interactive wearable devices, artificial intelligence, electronic skin and other special application scenarios.


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FIGURES

Figure 1. Schematic illustration for fabricating GSH@PET fabric-based piezoresistive pressure sensor.


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Figure 2. SEM images of (a) plasma-treated PET fabric, (b) GO@PET fabric and (c) GSH@PET fabric at 100 ×, and the corresponding SEM images with higher magnifications at (a1, b1, c1) 800 × and (a2, b2, c2) 10000 ×. (d) SEM image of GSH@PET fabric for EDS analysis and the corresponding mappings of (d1) sulfur, (d2) carbon, and (d3) oxygen elements.


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Figure 3. (a) FT-IR spectra of pristine PET fabric, plasma-treated PET fabric, GO@PET fabric and GSH@PET fabric. (b) Raman spectra of GO@PET fabric and GSH@PET fabric. (c) XPS survey scans of GO@PET fabric and GSH@PET fabric. C 1s XPS spectra of (d) GO@PET fabric and (e) GSH@PET fabric. (f) S 2p XPS spectrum of GSH@PET fabric.


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Figure 4. (a) Resistance change of five-layer piezoresistive pressure sensor under pressure. (b) Digital photos of the instrument for resistance measurement. (c) Resistances of the different sensors (one-layer, two-layer, five-layer and eight-layer) with pressure increasing. (d) Schematic illustration for the structural change of the two layers of GSH@PET fabric under small and large pressures.


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Figure 5. (a, b) I-V characteristics and stability determination of the five-layer sensor. (c) Response test of the five-layer sensor at different pressures. (d) Response time and (e) recovery time of the five-layer sensor. (f) Relative current response of the five-layer sensor under different frequencies. (g) Repeatability performance of the five-layer sensor during 500 loading-unloading cycles in the pressure ranges of 0-10 and 120-160 kPa. (h) Enlarged image from (g).


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Figure 6. (a) Real-time measurement of the pulse and (b) the corresponding signal curves with the sensor mounted on the wrist. (c) Enlarged view of the selected area from (b). (d,e) Sensing results of variation of visual angle by attaching the sensor at the eye corner. (f) The sensor pasted on throat for voice recognitions and the response curves when the tester spoke (g) “Good”, (h) “Work” and (i) “Hello”, respectively.


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Figure 7. The photographs and the corresponding response curves for the tester when (a, b) walking and (c, d) running by respectively placed sensor on the knee and insole. Detection of (e, f) the second knuckle of index finger motions and (g, h) the wrist movements at different bending angles.


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Figure 8. (a) A wearable interactive tactile-sensing integrated device (5 × 5 array) with multilayer piezoresistive pressure sensors as e-skin. (b, c) Photographs of “Point” and “Plane” pressures on the device by a bent tweezer and a finger, respectively. (d, e) The distribution of “Point” and “Plane” pressures on the device.

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Additional preparation of graphene oxide. Photograph of pristine PET fabric, GO@PET fabric and GSH@PET fabric (Figure S1). Optical images for the static water contact angles of GO@PET fabric and GSH@PET fabric (Figure S2). EDS spectra of plasma-treated PET fabric, GO@PET fabric and GSH@PET fabric (Figure S3). Relative surface chemical compositions of different fabrics from EDS analysis (Table S1) and XPS analysis (Table S2), the relative peak areas of C 1s spectra of different fabrics from XPS analysis (Table S3), and the fitting cubic function equation between the relative resistance change rate (ΔR/R0) and bending angles (θ) of finger and wrist (Table S4). Thiolation reaction of graphene oxide (Scheme S1). (PDF) Change process of the light brightness of the LED bulbs in the circuit when intermittently applying a small finger pressure on the sensor (Video S1) (AVI)


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Real-time monitoring of pulse signal with semiconductor instruments by attaching the sensor to the tester’s wrist (Video S2) (AVI) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The work was financially supported by the Science and Technology Planning Project of Guangdong Province, China (2017B090915002), the Science and Technology Planning Project of Guangzhou City, China (201804010381), the Guangdong College Students’ Science and Technology Innovation Foster Special Funds (2018pdjhb0032) and the Science and Technology Project of Shenzhen City, China (JCYJ20170817094728456). REFERENCES


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TOC


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Thiolated Graphene@Polyester Fabric-Based Multilayer

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