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High durability and waterproofing rGO/SWCNTs fabricbased multifunctional sensors for human-motion detection Seong Jun Kim, Wooseok Song, Yoonsik Yi, Bok Ki Min, Shuvra Mondal, Ki-Seok An, and Choon-Gi Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15386 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

High durability and waterproofing rGO/SWCNTs fabric-based multifunctional sensors for human-motion detection

Seong Jun Kim,† Wooseok Song,‡ Yoonsik Yi,† Bok Ki Min,† Shuvra Mondal, †,# Ki-Seok An,‡,* and Choon-Gi Choi†,#,*

†

Graphene Research Lab., Emerging Devices Research Group, Electronics and

Telecommunications Research Institute (ETRI), 218 Gajeongno, Yuseong-gu, Daejeon, 34129, Republic of Korea ‡

Thin Film Materials Research Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea #

School of ICT-Advanced device technology, University of Science and Technology, Daejeon 34113, Republic of Korea *

Corresponding authors E-mail: [email protected], [email protected]

*

corresponding author. Tel.: +82-42-860-6834. Fax: +82-42-860-5211. E-mail address: [email protected] (Choon-Gi Choi), [email protected] (Ki-Seok An). ACS Paragon Plus Environment

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ABSTRACT Wearable strain-pressure sensors for detecting electrical signals generated by human activities are being widely investigated because of their diverse potential applications, from observing human motion to health monitoring. In this study, we fabricated reduced graphene oxide (rGO)/single-wall carbon nanotube (SWCNTs) hybrid fabric-based strain-pressure sensors using a simple solution process. The structural and chemical properties of the rGO/SWCNTs fabrics were characterized using scanning electron microscopy (SEM), Raman and X-ray photoelectron spectroscopy (XPS). Complex networks containing rGO and SWCNTs were homogeneously formed on the cotton fabric. The sensing performance of the devices were evaluated by measuring the effects of bending strain and pressure. When the CNT content was increased, the change in relative resistance decreased, while durability was significantly improved. The rGO/SWCNTs (0.04 wt%) fabric sensor showed particularly high mechanical stability and flexibility during 100,000 bending tests at the extremely small bending radius of 3.5 mm (11.6% bending strain). Moreover, the rGO/SWCNTs fabric device exhibited excellent water resistant properties after ten washing tests due to its hydrophobic nature. Finally, we demonstrated a fabric sensor based motion glove and confirmed its practical applicability.

KEYWORDS: Textile, Fabric device, rGO/CNTs fabric, Strain sensor, Pressure sensor.

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INTRODUCTION Recently, various textile-based hybrid nanomaterials have been extensively investigated for applications as the next-generation wearable and flexible electronics that include solar cells1-4, field effect transistors (FETs)5-7, nano-generators8-11, and various sensors12-19 due to their flexible and human-friendly characteristics. Among these textile-based electronics, strain and pressure sensors in particular have attracted attention because they can be practically used to measure the physical and physiological activities of humans. They can be used, for example, to provide real time monitoring for the diagnosis of stress, and for ongoing health care, by measuring blood pressure and heartbeats, and for correcting posture by detecting human motion. The fabric-based strain-pressure devices that have been studied to date can be classified into piezoelectric20,21, resistive22-28, and capacitive types29,30 based on their sensing mechanism. Among these methods mentioned, resistive type sensors have been widely explored because they offer diverse benefits, including simplicity of fabrication, quick and easy read-out of resistance change, high sensitivity, and stability. Fabric-based resistive strain-pressure sensors have been fabricated by coating metal nanoparticles and carbon-based materials (rGO, CNTs) on fabric via dipping31-33 or by electroless plating process.34-36 While metal nanoparticle coated fabrics provide good electrical conductivity, it is difficult to uniformly coat a surface with them without the use of functional groups. Carbon materials can be used to coat fabrics without functional groups, and can be uniformly coated by van der Waals interaction at the interfaces between the rGO and fabric, but their electric conductivity is lower than that of metal. Given these respective limitations, a novel method is required to simultaneously provide improvements in both the stability and electrical properties of fabric 3



based devices. Furthermore, to precisely detect human motions or movements such as bending, pressing, stretching, the motions of organs or skin, an integrated sensing system that combines pressure and strain is required, because human movements involve simultaneous changes in pressure and strain. In this study, we demonstrated an rGO fabric-based sensor integrated with strain and pressure capabilities, fabricated by coating graphene oxide (GO) on commercial cotton fabric, followed by subsequent chemical reduction. In addition, single wall carbon nanotubes (SWCNTs) with different concentrations were coated on the rGO fabric to improve the electrical conductivity and durability of the device. The device performance was evaluated by repetitive bending strain and pressure measurements over 100,000 times. Next, water tolerance tests of the fabric-based strain-pressure sensors were conducted by rinsing several times in water. Finally, we fabricated a rGO/SWCNTs fabricbased glove as an application of a wearable device, and evaluated device performance by detecting motion signals when it was pressed, bent, fist-gripped and wrist-turned. This method allows easy manufacturing at low cost and large area using a solution process, and the resulting fabric has the additional merit of being washable, due to the hydrophobic nature of the graphene and SWCNTs. Based on the results of this study, the proposed technology is expected to provide a new path, to invigorate future industry and research fields related to fabric-based wearable electronics.

RESULTS AND DISCUSSION Figure 1a-f shows SEM images of the rGO and rGO/SWCNTs (0.01, 0.02, 0.04 wt%) cotton fabrics (CF), respectively. As shown in the inset of Figure 1a-b, the rGO and rGO/SWCNTs fabrics consisted of bundles of threads several tens of micron-thick. It should 4


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be noted that we confirmed that rGO was uniformly coated on the entire fabric. Long threads of SWCNTs were not observed at such low magnification (Figure 1b), however, a welldispersed SWCNT bundle was observed at higher magnification SEM, as seen in Figure 1d-f. As the concentration of SWCNTs was increased from 0.01wt% to 0.04wt%, the density of the well-dispersed SWCNTs gradually increased, and it was confirmed that the SWCNTs were bridged in the rGO-coated fabrics, forming random networks, with constant small nanogaps. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical structure and arrangement of the GO and rGO coated CFs. Figure 1g shows XPS core level spectra of the C 1s for GO. From the curve fitting of the C 1s core level spectrum for GO, two prominent peaks were observed at binding energies of 284.6eV and 286.7eV, which represent graphitic C-C bonding and C-O bonding states. In addition, peaks including C-OH, C=O, and O-C=O at binding energies of 285.5 eV, 287.5 eV, and 288.7 eV were found.37-39 In the rGO fabric, the C 1s peaks of the same components found in the GO fabric were present. However, the intensity of the peaks related to oxygen-containing functional groups, apart from graphitic C-C bonding, was dramatically reduced by the reduction process (Figure 1h). Figure 1i-k indicates the XPS C 1s core level spectra of the rGO/SWCNTs fabric, adjusted for the concentration of SWCNTs concentration (0.01, 0.02 and 0.04 wt%), indicating that the full-width at half maximum of the C-C bonding state and the intensity of oxygen-containing functional groups related peaks were both reduced, regardless of the SWCNTs concentration. Based on the XPS analysis, the atomic ratio of the C 1s to the O 1s for the rGO fabric increased, compared with that of the GO fabric. In addition, after the hybridization of SWCNTs with the rGO fabric, the atomic ratio slightly increased (Figure 1l). To examine the

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surface states of the fabrics, contact angle measurements were performed to determine the hydrophobicity of the GO, rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabrics, as shown in Figure S1. As a result, the contact angle values of GO, rGO, rGO/SWCNTs (0.01wt%), rGO/SWCNTs (0.02wt%) and rGO/SWCNTs (0.04wt%) fabrics were found to be 56°, 110°, 111°, 115° and 120°, respectively. It is well-known that SWCNTs possess hydrophobic nature. We measured the contact angle of SWCNTs fabric as the change of the concentration of SWCNTs. As a result, as the concentration of SWCNTs increased from 0.01 wt% to 0.04 wt%, the contact angle increased from 111° to 120°, indicating that the hydrophobic property was improved (Figure S1g-i). From these results, it was confirmed that the rGO/SWCNTs fabrics showed excellent hydrophobicity, due to the addition of the SWCNTs. This result suggests that the rGO/SWCNTs fabric can be used in wearable devices which require superior waterproof properties. Raman spectroscopy was used to characterize the structural features of the cotton fabric, and SWCNTs GO-, rGO- and rGO/SWCNTs hybrid fabrics, using a laser excitation wavelength of 532 nm. Figure 1m shows the Raman spectra for pristine cotton fabric. For the GO fabric, the D- and G-bands peaks related to GO appeared at 1347 cm-1 and 1598 cm-1, respectively (Figure 1n). After the reduction of the GO fabric by hydrazine treatment, the intensity ratio of the D- to G-bands increased and the FWHM of the D- and G-bands decreased compared to the GO fabric, as shown in Figure 1o. This can be understood to indicate an improvement in the crystallinity of the GO after the reduction.40,

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Figure 1p-r reveals the Raman spectra of the rGO/SWCNTs fabrics with

different SWCNTs concentrations (0.01, 0.02 and 0.04 wt%). The intensity of the D-band decreased significantly after the hybridization with SWCNTs, and the G- and G+-bands for SWCNTs were observed, regardless of the SWCNTs concentration. The G- and G+-bands

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originate from the displacement of carbon atoms along the tube axis and in the circumferential direction, respectively. From these results, we anticipate that the rGO and SWCNTs co-exist without structural deformation, which is related to electrical degradation. The performance of the rGO-, rGO/SWCNTs (0.01, 0.02, 0.04 wt%)- cotton fabric based strain-pressure sensors were evaluated by a bending strain measurement test, as shown in Figure 2. The PET films were placed under the devices as a flexible supporting substrate. When a mechanically homogeneous rGO- and rGO/SWCNTs (0.01, 0.02, 0.04 wt%)- fabric sheets with a thickness d was bent on a flexible PET substrate with a bending radius R, the expansion of outer surface and the compression of inner surface can be expressed by the following equation: ε = d/2R, ε = -d/2R, where ε is the bending strain, d is the thickness of the sample, and R is the bending radius.42 As shown in Figure 2a-d, the applied bending strain was increased by the bending machine from 0 to 11.6% for the rGO and rGO/SWCNTs (0.01, 0.02, 0.04 wt%)- fabric devices. Figure 2e exhibited the relative variation in resistance of the devices according to increase bending strain. Based on these results, the slope of resistance versus strain curve can be defined as a gauge factor, and can be expressed by the following equation. Gauge factor (GF) = (∆R/R0)/ε

(1)

where R, R0, and ε represent the change in resistance, resistance at 0% strain, and applied strain, respectively. Figure 2f shows a histogram of the gauge factor (GF) as a function of the linear slope regions for the GO- and rGO/SWCNTs (0.01, 0.02, 0.04 wt%)- fabric devices. The rGO fabric and rGO/SWCNTs (0.01 wt%) fabric devices had the highest GF values of 6.1 and 5.4, respectively, while both the rGO/SWCNTs (0.02 wt%)- and rGO/SWCNTs (0.04 wt%) fabric devices showed a GF value of about 4 in the bending strain range of 3.3 to 5.5%.

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Furthermore, all the devices exhibited GF value of almost 2.5 and 1 in the bending strain range of 5.5 to 9.3% and 9.3 to 11.6%, respectively. Figure 2g shows the time-dependence of the relative resistance change of the fabric-based strain-pressure sensors under 11.6% bending strain. The rGO fabric device exhibited a higher change in resistance compared with those of the other three rGO/SWCNTs fabric devices. This is because the differences in the average distance between each of the SWCNTs on the rGO/SWCNTs fabric, due to the increasing concentration of SWCNTs, can affect the degree of charge transport.43 In the change in resistance-time curve, all of the devices showed a stable resistance response when a bending strain between 0% and 11.6% was switched repeatedly (Figure 2g). Durability tests of the fabric devices were carried out by repeating bending cycles from 0 to 105, as depicted in Figure 2h. For the rGO fabric device, the relative resistance variation marginally decreased after 104 cycles and then decreased up to 9% after 105 cycles. Interestingly, for the rGO/SWCNTs fabric devices, the degradation in relative resistance change gradually recovered as the SWCNTs content increased. Notably, the rate of change in resistance was almost constant for the rGO/SWCNTs (0.04 wt%) fabric device, indicating extremely high durability. This result suggests that the random network of SWCNTs on the rGO/SWCNTs fabric acts to compensate strain damages, such as cracking and peeling off from the surface, caused by the repeated bending test.

To evaluate the device performance of the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric pressure sensors, a pressure test was carried out, as shown in Figure 3. A tapping machine (Force Gauge Model M2-10) was employed for the pressure test, as seen in Figure 3a. Figure 3b shows the variations in resistance for the rGO and rGO/SWCNTs fabric

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pressure sensors under applied pressures ranging from 1.27 to 254 kPa. The relative variation in the resistance of the rGO fabric sensor was much higher than that of the rGO fabric sensors containing SWCNTs. In addition, based on the test results, four different pressure sensors can be classified according to three linear resistance variation regions. (1) At low pressures (1.27~12.7 kPa), the slopes for the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric pressure sensors were 0.012, 0.0055, 0.0039 and 0.00054 kPa-1, respectively. (2) At middle pressures (12.7 ~ 63.5 kPa), the slope for the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric pressure sensors corresponded to 2.2, 2.6, 1.5 and 0.71 MPa-1, respectively. (3) At high pressures (63.5~254 kPa), the slopes for the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric pressure sensors were 0.72, 0.87, 0.56 and 0.27 MPa-1, respectively. Accordingly, the slope of pressure sensitivity (S) can be expressed as follows: S = δ(∆R/R0)/δP,

(2)

where ∆R, R0 and P represent the change in resistance, the resistance before applying pressure and the applied pressure, respectively. These results indicate that the pressure sensitivity of the rGO fabric sensor was higher than that of the other rGO-based fabric sensors, including those containing SWCNTs, which his consistent with the above-mentioned results of the bending tests. Figure S2a exhibited the time-dependence of the change in resistance of the rGO and rGO/SWCNTs (0.01, 0.02, 0.04 and 0.06 wt%) fabric devices by the repeated loading-unloading of pressure in 254 kPa. The conductivity of rGO/SWCNTs fabric device improved as the concentration of SWCNTs increased. Figure 3d shows the time-dependence of the relative resistance change of the fabric-based strain-pressure sensors under applied pressure of 254 kPa. When an external pressure of 254 kPa was applied to the rGO- and rGO/SWCNTs-based fabric sensors, as shown in Figure 3d, the physical contact of

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adjacent materials can be improved, resulting in the improvement of electrical conductivity. This pressure-induced improvement in the electrical conductivity allows that the variation in resistance {(R-R0)/R0} becomes a negative value. This phenomenon is also suggested by the previous reports.44, 45 The relative variation in resistance of the rGO fabric device was higher than those of the other three rGO/SWCNTs fabric devices, similar to the results of the previous bending test. Durability testing of the individual fabric sensors using the repeated loading-unloading of pressure in the range of 0-105 cycles under 254 kPa was conducted, as shown in Figure 3e. Interestingly, a trend similar to the previous durability test for bending was observed. All of the fabric sensors exhibited excellent durability characteristics during repeating pressure tests, up to 103 cycles. However, from 104 cycles, a decrease in resistance was observed. Overall, the pressure durability of the rGO/SWCNTs (0.04 wt%) fabric device was robust. Furthermore, we fabricated the rGO/SWCNTs (0.06 wt%) fabric sensor, and a pressure test carried out under applied pressure range of 1.27 to 245 kPa to compare with the rGO/SWCNTs (0.04 wt%) fabric sensor. As a result, resistance and sensitivity were lower than rGO/SWCNTs (0.04 wt%) fabric sensor due to increase of SWCNTs content, but it was found to have slightly improved linearity and conductivity compared to rGO/SWCNTs (0.04 wt%) fabric sensor as shown in Figure S2. Continuously, we fabricated 0.02 wt% SWCNTsbased fabric sensor by dipping the cotton fabrics into SWCNTs solution, and the bending strain and applied pressure tests were performed to compare with those of the rGO/SWCNTs (0.02 wt%)- fabric sensor. As a result, the SWCNTs (0.02 wt%) fabric sensor showed lower electrical conductivity and a smaller variation in resistance compared to those of the rGO/SWCNTs (0.02 wt%) fabric sensor. In addition, the relative variation in the resistance decreases of a bending strain of 7.2% or more and an applied pressure in 127 kPa or more

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(Figure S3 in supporting information). Water stability is a crucial factor for the application of wearable electronics. Accordingly, a washing test of the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric sensors was conducted by rinsing in DI water with a magnetic stirrer, as shown in Figure S4a-d. We confirmed changes in the relative resistance of the fabric devices through measurements under a pressure of 38.1 kPa during the washing experiments, which were conducted ten times. All of the fabric devices showed excellent water resistance properties due to their hydrophobic nature. In particular, the rGO/SWCNTs (0.04 wt%) device exhibited excellent waterproof properties (Figure S4e).

The fabricated rGO/SWCNTs fabric based wearable strain and pressure sensor is very light, with superior mechanical flexibility and can be easily attached to any clothing material, making it practical for applications to detect human motion signals. Although the rGO/SWCNTs fabric device has a lower sensitivity than the rGO fabric device, the linearity of a variation in the resistance is significantly improved with increasing the SWCNTs content. These results suggest that the addition of SWCNTs improves the stability of the device, which is a decisive factor for actual application. Furthermore, rGO/SWCNTs (0.04 wt%)fabric sensor exhibited the excellent durability characteristics than other sensors. In this study, the rGO/SWCNTs (0.04 wt%) fabric sensors were attached to a generic white cotton glove and metallic wires were connected to both ends of the fabric sensors to construct an rGO/SWCNTs fabric-based motion glove. As shown in Figure 4, the sensing performance of the fabric-based motion glove was demonstrated by various hand motions related to pressing, bending, grabbing and wrist actions (see Supplementary Information, Movie S1). Figure 4a shows the change in relative resistance when several repetitions of pressing and relaxation

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were performed on an object with the sensor attached to the fingertips of the fabric gloves. As the pressure and relaxation were repeated, a resistance variation of about 40% was observed. Additionally, when bending and relaxation of the index finger was repeated, a resistance variation of about 20% was confirmed (Figure 4b). In the case of grabbing, shown in Figure 4c, the change in resistance corresponded to 15%, which is significantly lower when compared to the previous pressing and bending motions. Finally, we investigated the resistance signals generated by wrist motions (Figure 4d). Interestingly, when a positive or negative bending or rotation of the wrist was conducted, noise signals such as a small sawsharped peak appeared for each. First, when the wrist was bent in a downward direction, the rGO/SWCNTs fabric was subjected to tensile strain with a resistance change rate of about 5%. Next, when the wrist bent in an upward direction, a resistance variation of about 10% exhibited after a small saw-sharped peak related to noise signal appeared. Finally, when the wrist was rotated 360 degrees, a relative resistance signal of about 10% was observed with a narrow and sharp noise peak resulting from the rotation of the fast wrist. These studies helped us to estimate the signals related to various hand motions based on the respective different resistance changes, which occurred when moving and holding an object with the fabric-based motion glove. In the future, we expect this feature will allow the technology to be used to calibrate any posture during exercise, or to move robot arms via remote control in a work space dangerous to humans.

CONCLUSION In summary, we demonstrated a simple solution process to fabricate a rGO/SWCNTs fabric based strain-pressure sensor. The structural features of the rGO/SWCNTs fabric were 12


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investigated by SEM, Raman and XPS analysis. From bending strain and pressure measurements, the rGO fabric sensor exhibited higher device performance compared to that of the rGO/SWCNTs (0.01, 0.02, 0.04 wt%) fabric devices. On the other hand, the durability of the rGO/SWCNTs fabric device was much better than that of the individual rGO fabric device because of the CNT network, which compensated the generation of defects. In addition, the rGO/SWCNT fabric device exhibited negligible damage despite rinsing several times in water. Finally, we demonstrated a motion glove based on the rGO/SWCNTs fabric for detecting human movements, including pressing, bending, grabbing and wrist actions. The movements from a variety of hand gestures produced distinguishable characteristics in terms of changes in resistance in the fabric sensors. We believe that our study will pave the way for applications of future fabric-based wearable sensors.

METHODS Preparation of the rGO/SWCNTs fabrics. The three main steps of the fabrication process of the rGO/SWCNTs fabric used for the proposed strain-pressure sensors are illustrated in Scheme 1. First, white cotton fabric was rinsed several times with distilled water to remove the surface treatment chemicals and impurities commonly found on commercial fabric (Scheme 1a). Then the fabric was immersed in a 3 mg/ml GO in deionized water solution for 10 min and subsequently dried in an oven at 80° C for 30 min. After repeating this process five times, a GO-coated fabric was consequently formed, as shown in Scheme 1b. Next, a 20-ml vial bottle filled with 1/3 hydrazine solution was placed in a glass petri dish and the GO fabric was placed next to the opening of the vial bottle, to reduce GO to rGO. 13



Another glass petri dish was placed over the sample to completely seal in the hydrazine fumes. After heating overnight in an oven at 80° C the color of the GO fabric had turned from light brown to black, which indicated the formation of rGO fabric, as shown in Scheme 1c. Finally, a 200 ml solution with concentrations of 0.01, 0.02, and 0.04 wt% SWCNTs was prepared for hybridizing the rGO fabrics. Then samples of the rGO fabric were dipped into the different concentrations of SWCNTs solutions for 10 minutes and dried at 80° C for 30 min using a hot plate (Scheme 1d). The rGO/SWCNTs (0.01, 0.02, 0.04 wt%) fabrics were eventually completed by repeating this process 5 times (Scheme 1e).

Conflict of Interest: The authors declare no competing financial interest.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional data including the measurement of water contact angle for GO-, rGO-, rGO/SWCNTs (0.01, 0.02 and 0.04 wt%)- and SWCNTs (0.01, 0.02 and 0.04 wt%)- fabrics, Time-dependence of the change in resistance of the rGO and rGO/SWCNTs (0.01, 0.02, 0.04 and 0.06 wt%) fabric devices, the relative resistance change {(R-R0)/R0} with increasing an applied pressure in the rGO/SWCNTs (0.02 wt%)- and rGO/SWCNTs (0.06 wt%)-fabric based devices, The relative resistance change {(R-R0)/R0} with increasing bending strain and applied pressure of the rGO/SWCNTs (0.02 wt%)- and SWCNTs (0.02 wt%)-CFs based devices, respectively, the washing tests of the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 14


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wt%) fabric devices (Figure S1-S4). Movie files for the sensing performance of the fabricbased motion glove (Movie S1).

Acknowledgement. This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (No: 17ZB1300) and STEAM Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017M3C1A9069590). W. Song and K. -S. An were supported by Nano/Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Korea (NRF-2016M3A7B4900119).

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FIGURE CAPTIONS

Scheme 1. Schematic illustration of the rGO/SWCNTs fabric fabrication process for preparing strain-pressure sensors.

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Figure 1. Representative SEM images of (a) rGO-CF and (b) rGO/SWCNTs-CF (scale bar: 50 µm). SEM images of (c) rGO-CF, (d) rGO/SWCNTs (0.01 wt%)-CF, (e) rGO/SWCNTs (0.02 wt%)-CF and (f) rGO/SWCNTs (0.04 wt%)-CF (scale bar: 2 µm). XPS C1s core level spectra of (g) GO-CF, (h) rGO-CF, (i) rGO/SWCNTs (0.01 wt%)-CF, (j) rGO/SWCNTs (0.02 wt%)-CF and (k) rGO/SWCNTs (0.04 wt%)-CF. (l) The extracted atomic ratios of C 1s/O1s for rGO-CF and rGO/SWCNTs (0.01, 0.02, 0.04 wt%)-CF. Raman spectroscopy recorded at an excitation wavelength of 532 nm (m) bare CF, (n) GO-CF, (o) rGO-CF, (p) rGO/SWCNTs (0.01 wt%)-CF, (q) rGO/SWCNTs (0.02 wt%)-CF and (r) rGO/SWCNTs (0.04 wt%)-CF.

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Figure 2. Photographs of bending tests with various applied bending strains (ε = (a) 0, (b) 4.4, (c) 7.2 and (d) 11.6 %). (e) The relative resistance change ((R-R0)/R0) with increasing bending strain for the rGO- , rGO/SWCNTs (0.01 wt%)-, rGO/SWCNTs (0.02 wt%)- and rGO/SWCNTs (0.04 wt%)-CF based devices. (f) Gauge factor (GF) in the linear slope region of the rGO and rGO/SWCNTs fabric based devices. (g) The time-dependence of relative resistance change in the the rGO and rGO/SWCNTs fabric based devices. (h) Relative resistance change as a function of the number of bending cycles under an applied 11.6 % bending stain. 24


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Figure 3. (a) A photograph of the tapping machine used for pressure measurement. (b) The plot of relative resistance response vs. pressure curves for the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric devices, respectively. (c) The sensitivity under 3 linear slope regions for the rGO fabric device, and the rGO/SWCNTs fabric devices with different SWCNTs content, respectively. (d) Time-dependence of the relative change in resistance for the rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric devices at a pressure of 254 kPa. (e) The durability tests of rGO and rGO/SWCNTs (0.01, 0.02 and 0.04 wt%) fabric devices under repeated cycles of pressure from 0 to 105 at an applied pressure of 254 kPa.

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Figure 4. Relative resistance changes with various movements of the motion glove containing the fabric-based sensors, such as (a) pressing, (b) bending, (c) grabbing and (d) up, down and rotation of the wrist.

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TABLE OF CONTENTS

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SWCNT ... - ACS Publications

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