Highly Stretchable, Ultrasensitive, and Wearable Strain Sensors

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Highly Stretchable, Ultrasensitive, and Wearable Strain Sensors Based on Facilely Prepared Reduced Graphene Oxide Woven Fabrics in an Ethanol Flame Biao Yin,† Yanwei Wen,† Tao Hong,‡ Zhongshuai Xie,§ Guoliang Yuan,§ Qingmin Ji,⊥ and Hongbing Jia*,† †

Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, ‡Institute of Bio-Analytical Chemistry, School of Materials Science and Engineering, ⊥Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, P. R. China §

S Supporting Information *

ABSTRACT: The recent booming development of wearable electronics urgently calls for high-performance flexible strain sensors. To date, it is still a challenge to manufacture flexible strain sensors with superb sensitivity and a large workable strain range simultaneously. Herein, a facile, quick, costeffective, and scalable strategy is adopted to fabricate novel strain sensors based on reduced graphene oxide woven fabrics (GWF). By pyrolyzing commercial cotton bandages coated with graphene oxide (GO) sheets in an ethanol flame, the reduction of GO and the pyrolysis of the cotton bandage template can be synchronously completed in tens of seconds. Due to the unique hierarchical structure of the GWF, the strain sensor based on GWF exhibits large stretchability (57% strain) with high sensitivity, inconspicuous drift, and durability. The GWF strain sensor is successfully used to monitor full-range (both subtle and vigorous) human activities or physical vibrational signals of the local environment. The present work offers an effective strategy to rapidly prepare low-cost flexible strain sensors with potential applications in the fields of wearable electronics, artificial intelligence devices, and so forth. KEYWORDS: wearable strain sensor, reduced graphene oxide woven fabric, flame, stretchability, sensitivity

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and nanoscale flexibility from nanomaterials are promising features for the fabrication of highly sensitive and stretchable strain sensors. Flexible strain sensors based on nanomaterials, such as metallic nanowires,13,25,26 metallic thin films,27,28 and carbon nanomaterials (e.g., carbon nanotubes and graphene),22,24,29−31 have been successfully fabricated. In comparison with metallic nanomaterials, carbon nanomaterials have the advantages of easy-processing, cost-effectiveness, superior flexibility, and chemical and thermal stability.11,32 To date, numerous flexible strain sensors based on carbon nanomaterials have been reported.21,22,29−31,33 In spite of these considerable accomplishments, however, the high stretchability and excellent sensitivity of the strain sensors were always difficult to achieve simultaneously, which largely limited their applications. For instance, a graphene thin film-based strain sensor displayed a striking gauge factor (ratio of the relative resistance change to the applied strain) of 1037 at 2% strain34 but stopped working beyond 3.4% strain. In another work, the strain sensor based on carbon nanotubes could accommodate

INTRODUCTION Wearable electronics have drawn widespread consideration and also have gained respectable progress in recent years.1−8 As a pivotal subportion of wearable electronics, flexible strain sensors with features of conformal adhesion to arbitrary and soft surfaces exhibit substantive potential applications in electronic skins,9,10 human-machine interaction,11−13 emotion detection,14,15 sound recognition,16−19 personalized health monitoring,20−23 and others. There are mainly two components of the flexible strain sensor based on the piezoresistive effect, i.e., elastic polymer matrices, which enable flexibility and durability of the whole device, and conductive sensing elements, which predominate the electromechanical properties of strain sensors.12,22 Flexible strain sensors with desirable performance should satisfy the demands of superb sensitivity, extraordinary stretchability, stability, durability, and fast response as well as other demands.24 Among these criteria, studies are focused on the sensitivity and stretchability of strain sensors,9 which determines the threshold sensing level and workable strain range of strain sensors, respectively.24 With the prosperous development of nanotechnology, nanomaterials are employed as functional sensing elements for the design of flexible strain sensors.18 The outstanding electrical properties © XXXX American Chemical Society

Received: July 4, 2017 Accepted: August 30, 2017 Published: August 30, 2017 A

DOI: 10.1021/acsami.7b09652 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the entire procedure for fabrication of GWF. (b) SEM micrographics of GWF, magnification: 40×, 200×, 3000×, and 10 000×.

strain as high as 280% without signal failure,29 while its maximum gauge factor was merely 0.82, which would restrict its applications in identification of subtle strains. Therefore, it is still a challenge to balance the contradictory properties of sensitivity and stretchability. To couple high sensitivity with the large stretchability of strain sensors, one of the basic approaches is to endow brittle carbon nanomaterials with stretchability by structural engineering.24 Theoretically, the brittleness of carbon nanomaterials can give rise to substantial structural changes in response to delicate strain, thus, offering a high sensitivity; the stretchability of carbon nanomaterials ensures the structure to remain intact even under large strain, leading to a large workable strain range of strain sensors. When carbon nanomaterials were assembled into various geometrical configurations like fibers,16,35 foams,10,36 wrinkles,37 thickness-gradient films,24 honeycomb structures,38 and woven fabrics,9,11,21,39,40 their stretchability could be intrinsically improved. Nevertheless, these strategies of structural engineering always involved complicated or timeconsuming techniques, including chemical reduction, chemical vapor deposition (CVD), and thermal annealing. The development of a facile, fast, low-cost, scalable preparation approach is expected for ultrasensitive strain sensors with large stretchability. In this study, we report the performance of a strain sensor based on reduced graphene oxide woven fabrics (GWF).41 The GWF is fabricated using the cotton bandage as the template, which features a macrowoven-fabric geometrical conformation. Graphene oxide (GO, a derivative of graphene) was coated on the cotton bandage through a dip-coating method. Subsequently, the composite is transferred to an ethanol flame to pyrolyze the cotton template and reduce GO synchronously in tens of seconds. To our best knowledge, GWF is first prepared through such an ultrafast, convenient, scalable, and cost-

effective strategy. The strain sensor based on GWF is then tested, which attains a tolerable strain up to 57% and a high sensitivity (gauge factor of 416 within 0%−40% strain and 3667 within 48%−57% strain). The good trade-off between sensitivity and stretchability is attributed to the fantastic hierarchical structure of the GWF. We further demonstrate versatile capabilities of the GWF strain sensor in detection of both vigorous motions (joints bending) and delicate motions (facial expressions, phonation, pulse, etc.) of the human body. Moreover, our sensor can collect signal for sounds of a smart phone speaker and vibrations of an oscillating steel ruler as well. The desirable comprehensive performance of our sensor achieves and even surpasses that of recent sensors with designed device architectures,9,10,22,23,34,42 in addition to having advantages of simplicity, fastness, convenience, low-cost and scalability.

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EXPERIMENTAL SECTION

Materials. Natural graphite powder (average particle size ≤30 μm), H2SO4 (98%), NaNO3, KMnO4, H2O2 (30%), and HCl (37%) were supplied by the Sinopharm Chemical Reagent Co., Ltd. (China) and used as received. The cotton bandage (5 × 10−3 g cm−2) was provided by Winner Medical Co., Ltd. (China), and its number of threads per square inch along the weft and warp directions is 28 × 24. Natural rubber (NR) latex (solid content: 60 wt %) was purchased from Shanghai Nessen international trading Co., Ltd. (China). Fabrication of the GWF. GO dispersions were prepared on the basis of natural graphite powder according to a modified Hummers’ method.43 The cotton bandage was first rinsed using deionized water and ethanol both for several times and dried in an oven at 80 °C. The freshly washed cotton bandage was then dip-coated directly in GO aqueous suspension (2 mg mL−1). Sequentially, the GO-coated cotton bandage was placed in a polytetrafluoroethylene (PTFE) Petri dish and dried at 80 °C to remove the moisture. This dip-coating/drying process was repeated for specific times (ranging from first to fourth), B


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ACS Applied Materials & Interfaces and the samples were further transferred to an ethanol flame (inner flame) for tens of seconds to obtain the GWF. Fabrication of the Strain Sensors. The obtained GWF was tailored into a rectangular shape (3 cm long and 1 cm wide in warp and weft directions, respectively) for the preparation of strain sensors. Both ends of the GWF were connected to copper wires with silver paste. Subsequently, the GWF was encapsulated with NR latex (2 mL) through a droplet-coating method. The whole system was cured at 50 °C to obtain the GWF strain sensors with a thickness of ca. 0.5 mm. Fabrication of the LED Array Device. LEDs were mounted manually between the two pieces of GWF (3 × 1 cm), and the contact resistance between the LED lead frames and the GWF was eliminated with silver paste. After wiring both ends of the two GWF, the GWF integrated with LEDs was encapsulated with NR latex and cured at 50 °C to obtain the LED array device. Detailed processes are illustrated in Figure S1 of the Supporting Information (SI). Characterization. The morphology of GO was observed using atomic force microscopy (AFM, Bruker AXS, U.S.A.) and transmission electron microscopy (TEM, JEOL-2100, Japan). Thermogravimetric analysis (TGA) was conducted in a DTG-60 analyzer (Shimadzu, Japan) under a nitrogen atmosphere at a heating rate of 10 °C min−1. The conformation of GWF was measured by a field-emission scanning electron microscope (SEM, Quanta 250 FEG, FEI, U.S.A.). The structural and chemical attributes of GWF were characterized by a Fourier transform infrared (FTIR) spectrometer (FTIR-8400S, Shimazdu, Japan), an X-ray diffraction (XRD) analyzer (D8-Advanced, Bruker, Germany), and an X-ray photoelectron spectroscope (XPS, VG Scientific ESCALAB 250, Thermo Fisher Scientific, U.S.A.). The tensile tests were performed with a universal testing machine (CMT 4254, Shenzhen SANS, China) at ambient temperature. The electromechanical properties of the GWF strain sensors were determined using a two-point measurement with a digital source meter (Keithley 2400, Tektronix, U.S.A.) at a constant voltage of 3 V. The structure evolution of the GWF strain sensor during stretching was traced by optical microscopy (BM 2100, Jiangnan Yongxin, China) and a digital camera.

flame was finally extinguished with a glass Petri dish to obtain the GWF. The template structure of the cotton bandage was found well retained for GWF after the ethanol-flame treatment. During the ethanol-flame process at ca. 550 °C,47 the thermochemical reactions of ethanol oxidation, the decomposition of cellulose molecules and GO, may produce various incomplete combustion intermediates, such as gas molecules and free radicals. Sufficient energy in the ethanol flame would excite and ionize the intermediates to react between themselves, forming a chemical-reducing atmosphere, resulting in the reduction of GO and pyrolysis of the cotton bandage template simultaneously. The ultrafast reducing process (tens of seconds) benefitted from large pore size (ca. 1 and 0.9 mm in warp and weft directions, respectively) and high porosity of the cotton bandage, which assisted in the quick removal of gas molecules and heat transfer during the flame synthesis. When the reaction was completed, the flame-treated GWF held excellent fire resistance and could survive in the flame even after a long time.47,48 Moreover, the reducing atmosphere in the ethanol flame also prevented the GWF from combustion, which ensured the GWF to achieve a stable state.49 On the contrary, the pristine cotton bandage treated with an ethanol flame was partly carbonized and its color turned from white to black or brown (Figure S3). The incomplete pyrolysis of the cotton bandage is due to the absence of GO. TGA confirmed that the presence of GO accelerated the pyrolysis of the cotton bandage (see detailed discussions in Figure S4). The flame process also led to shrinkage and curvature of the plain weave structure of the pristine cotton bandage. Whereas, for the GO-coated cotton bandage, the superb mechanical strength and thermal stability of the flame-reduced graphene sheets facilitated the maintenance of the high integrity of GWF during the pyrolysis of the cotton bandage template. The piece of GWF was easily tailored into desirable shape with a scalpel. As shown in Figure 1a, rectangular GWF (10 mm wide and 30 mm long) suspended over a tweezers was self-supported, demonstrating its mechanically robust structure.41 Additionally, rectangular GWF could rest on a bristlegrass. The gravity loading of GWF did not induce any deformation of the tiny hairs on the bristlegrass, indicating the lightweight structure of GWF.50 SEM images of GWFs are shown in Figure 1b. At the low magnification of 40×, we can observe that rectangular holes (ca. 1 mm long and 0.9 mm wide) were uniformly arranged in GWF. The framework of GWF was constructed by warp and weft yarns (each yarn includes a bunch of crimped and twisted fibers), which were aligned to form crisscross interlaced patterns. Moreover, the yarns were assembled in an overhand-under fashion, as can be seen at larger magnification of 200×. With a more close-up view of the yarns (magnification: 3000× ), gaps were notable among discretely arranged fibers, which would be beneficial to the penetration of rubber latex. At magnification of 10 000×, the fiber presented a crinkled surface due to layer-by-layer assembly of the wrinkled reduced graphene oxide (RGO) sheets. FTIR spectra, XRD measurements, and XPS analysis were further used to characterize the composition of GWF (Figure S5). The results clearly reveal the successful reduction from GO to GWF in company with complete pyrolysis of the cotton bandage template. The merits of the cotton-bandage-templated and flamereduced preparation of GWF can be summarized as follows: (a) an ultrafast, cheap, and scalable process finished in tens of

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RESULTS AND DISCUSSION Figure 1a illustrates the fabrication process of GWF. A piece of cotton bandage was dipped directly in GO suspension. The GO suspension was driven into the cotton-woven fabrics by capillary force. Moreover, the cotton bandage consists of cellulose molecules with abundant hydroxyl groups on their surface, which are supposed to form hydrogen bonds with hydroxyl or carboxylic groups on GO sheets.44−46 Thereby, strong adhesion can be achieved between the cotton bandage and the GO sheets. As shown in Figure S2, the AFM image of GO sheets showed that GO had a single-layer structure with a thickness of 1.5 nm, and the TEM image revealed a wrinkled layer of GO, indicating the excellent flexibility of the GO sheets. After they were dried in an oven at 80 °C, flexible GO sheets wrapped closely around individual fibers of the cotton bandage in a layer-by-layer fashion, thus taking the shape of the cotton bandage. It can be seen that the color of the cotton bandage turned from white to brown, suggesting that it was successfully coated with GO. To be subject to an ethanol-flame treatment, the GO-coated cotton bandage was clipped with two steel meshes (square-pore size 1 cm) beforehand to inhibit burning and/or suppress any curvature of the GO-coated cotton bandage. In the initial seconds, it is noteworthy is that a portion of the GO-coated cotton bandage in the ethanol flame turned its color from brown to black, while the other part that was out of the ethanol flame remained brown. In the following tens of seconds, the color of the GO-coated cotton bandage changed to black over the whole area, implying the full reduction of GO and the completion of the flame process. The C


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the key processes in fabrication and operation of the GWFbased strain sensors. A piece of rectangular GWF was carefully tailored and placed onto a glass substrate. Copper wires were glued onto both ends of GWF with silver paste. Subsequently, the NR latex was poured onto the glass substrate through a droplet-coating method and cured at 50 °C. The cured system was then completely peeled off the glass substrate. Eventually, the GWF-based elastomer strain sensors (ca. 0.5 mm thick) were obtained. An optical image of an as-prepared GWF strain sensor is displayed in Figure 2b. It is visible that the GWF was steadily embedded in the rubber matrix. Figure 2c shows that the encapsulation of NR endowed the GWF strain sensors with conformal adhesion of the curvilinear surface of a human hand because of the flexibility and surface characteristics of NR matrix. During testing of skin motions, the merely ca. 0.5 mm thick film is expected to constrain delamination or partial slipping from skin due to sheer force and serve as a wearable strain sensor for human beings.9 Figure 2d shows the applications of GWF strain sensors to detect macro- and/or microscale motions of the human body at different parts, including facial expressions, phonation, arm bending, pulse, etc. The detailed results will be discussed below. A series of GWFs at varying dip-coating/drying times ranging from first to fourth were prepared to further fabricate the GWF strain sensors. Figure S6a presents the current vs voltage curves of GWF strain sensors, which were linear and conformed strictly to Ohm’s law.60 The corresponding electrical resistance of the GWF strain sensors is shown in Figure S6b; the values were averaged on the basis of four samples prepared on different days. The pristine cotton bandage pyrolyzed with ethanol flame showed a large electrical resistance (over 100 MΩ), which is due to the incomplete pyrolysis of the cotton bandage in the absence of GO. With an increase in the number of times dip-coating was performed, the corresponding GWF strain sensors exhibited a drastic drop in electrical resistance at

seconds, simultaneously reducing GO and pyrolyzing the cotton bandage template; (b) a facile, open, and conveniently available operation environment; (c) controllable conformation of GWF by tailoring the cotton bandage template; and (d) a lightweight and mechanical robust structure of GWF. Until now, various approaches have been established to produce graphene, such as CVD,6,9−11,21,39,41,51,52 chemical reduction,36,53−55 thermal annealing,56,57 hydrothermal methods, etc.42,58−60 Those strategies are generally time-consuming, complicated, or may involve environmentally hazardous reagents. In some cases, the removal of hard porous templates, such as Ni foam and Cu mesh, would require an additional process, which costs a long time and may deteriorate the integrity of graphene monolith. The obtained GWF was composed of brittle carbon materials. Then, GWF was not able to be directly applied as a strain sensor without a polymer substrate. Figure 2a illustrates

Figure 2. (a) Schematic illustrations for preparation of the GWF-based strain sensors. (b) Photograph of the as-obtained strain sensors. (c) Photograph of the strain sensors attached conformably to a curvilinear skin surface of human hand. (d) Illustration of GWF strain sensors to detect various motions of human body at different parts.

Figure 3. (a) The relationships between relative resistance change (ΔR/R0) and applied strain upon quasi-static loading for the GWF strain sensor. (b) Plots of ΔR/R0 under various cyclic stretching−releasing strains. (c) ΔR/R0 under cyclic stretching−releasing with a strain of 7.5% at frequency of 0.02−3 Hz. (d) Retention of ΔR/R0 under cyclic strain upheld at 0 and 7.5% for 10 s each. (e) Performance of the sensor under more than 1000 stretching−releasing cycles of strain variation from 0 to 7.5%, with insets above being signals of the initial and last five cycles. (f) ΔR/R0 plotted against time in the low strain region (less than 0.5% strain). D


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ACS Applied Materials & Interfaces first, and then the variation became sluggish. The decrease of resistance is because GWFs with a greater number of coating times contained a larger amount of RGO for electrical conductance. The electrical resistance can also be decreased with further reduction processes, such as microwave treatment,61 chemical reduction (hydrazine hydrate or hydrogen iodide),47,54 and thermal annealing in an inert atmosphere.22,23,32,62 To understand the mechanical behavior, the GWF strain sensors were stretched to failure at a speed of 100 mm min−1. The stress−strain curves were recorded and evaluated with the well-known Mooney−Rivlin equation (Figure S7).63 The stress−strain curves show that GWF strain sensors exhibited a two-step stretching process, i.e., an elastic deformation region (0−75% strain) and a plastic deformation region (>75% strain). Detailed discussions are shown in the SI. To investigate the strain-sensing properties, the GWF strain sensors were further stretched under uniaxial tensile loading (10 mm min−1), and the electrical resistance was simultaneously measured upon a constant voltage with a digital source meter. Before their electromechanical properties were tested, the strain sensors were prestretched at a rate of 10 mm min−1 until the signals finally failed. Subsequently, 10 times of prestretching were conducted repeatedly to eliminate the surface effect between GWF and the rubber matrix, and then, stable signals were achieved.64,65 The responsive behaviors of GWF strain sensors are depicted and discussed in Figure S8. To detect multiscale motions, a good balance between sensitivity and stretchability is required for strain sensors. Particularly, for a full-range test of human motions, a stretchability of strain sensors, i.e., ca. 50% strain, should be achieved.26,42 The strain sensor containing GWF at 3 dip-coating times, which presented a tolerant strain of 57% and a relatively high sensitivity, was therefore chosen for the following studies unless otherwise specified. The relative resistance change (ΔR/R0) of the GWF strain sensor plotted against the applied strain is shown in Figure 3a. The ΔR/R0 increased linearly at first and then displayed an exponential dependency upon applied strain. Generally, the gauge factor (ΔR/ΔεR0, where Δε is the change in applied strain) is defined to evaluate the sensitivity of strain sensors. Figure 3a reveals that the GWF strain sensor possessed a gauge factor of 416 (0% < Δε < 40%) and 3667 (48% < Δε < 57%). To test the reproducibility between different GWF strain sensors, four parallel experiments were conducted. As shown in Figure S9, reasonable reproducibility was seen among the four independent samples. Figure 3b shows the curves of ΔR/R0 under various cyclic stretching−releasing strains. The ΔR/R0 showed reproducible and precise signals in response to various strains, manifesting superb electromechanical integrity of the strain sensor in terms of stability and repeatability.28 The strength of signals increased proportionally with the amplitude of cyclic stretching−releasing strains, which was consistent with the results displayed in Figure 3a. Figure 3c illustrates the dependence of signal on the stretching frequency under cyclic 7.5% strain. Notably, the signal of the GWF strain sensor showed almost no frequency dependence (0.02−3 Hz). Figure 3d depicts the retention of signal from the GWF strain sensor maintained at 0% and 7.5% strain for 10 s each. The signal corresponded reasonably well with the strain−time curve. The slight overshoots of signal are due to the tensile stressslackening of the viscoelastic rubber matrix.10,36 Furthermore, the GWF strain sensor was subjected to various strains (0−7.5,

0−15, 0−30, and 0−45% applied strain, respectively) for more than 1000 cycles, and the ΔR/R0 was real-time recorded, as shown in Figures 3e and S10. The signals showed long-time stability under various strains in the approximate 1000 cycles, indicating the excellent durability of GWF strain sensor for applications in our daily life. Besides the detection of large deformation, the GWF strain sensor can also sense delicate strains less than 0.5%. Figure 3f presents the resistance response to a stretching−releasing cycle with a stepwise increase (decrease) in strain. The GWF strain sensor exhibited precise signal, indicative of the reliability of the sensor in testing subtle strains. To make clear the working mechanism for the GWF strain sensor, the structure evolution of the GWF strain sensor was tracked during the aforementioned prestretching and subsequent stretching−releasing. During the prestretching (Figure S11), the warp yarns of GWF elongated, while the weft yarns shrank at strains less than 40%, suggesting the stretchability of GWF. The intrinsically stretchable GWF prevented rupture of the electrical conducting network at low strains. Further stretching led to destruction of GWF in the forms of forming islands and gaps for the warp yards. Whereas, the weft yards retained intact due to the absence of transverse loading. The weft yards and islands kept the integrity of GWF and were beneficial to the reconstruction of GWF during unloading.22,41 During the subsequent measurements (Figure 4a), the gaps enlarged with the augment of strain and narrowed during releasing. The excellent recoverability of the GWF strain sensor can be attributed to the elasticity of the rubber matrix, which

Figure 4. (a) Morphology evolution of a GWF strain sensor during loading and unloading of 55% strain. (b) Plots of experimental and theoretical data of ΔR/R0 upon deformation. E


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Figure 5. Photographs showing the GWF strain sensor fixed onto (a) the elbow joint, (c1) cheek, (c2) corner of an eye, (e) forehead and philtrum of a subject, (g1) wrist, and (h) Adam’s apple. The corresponding time-dependent signals of (b) arm bending at angles of 45, 90, and 135°, (d) cheek bulging and eye blinking, (f1−f4) muscle motions of forehead and philtrum when the subject is crying or laughing, (g2) pulse, (g3) fast Fourier transform of (g2), (g4) typical waveform extracted from pink area of (g2), (i) phonation when the tester pronounced different words, (j1) chewing, (j2) swallowing, (j3) coughing, and (j4) lowering head.

would drive the conductive RGO sheets to their original position under releasing.42 The destruction−reconstruction of GWF formed a tentacle-like pathway and caused reversible resistance changes of the GWF strain sensor.23 Hence, the GWF strain sensors can detect the variations of strain in the form of resistance change.28 In comparison with strain sensors in previous literature, the GWF strain sensor showed a good trade-off between sensitivity and stretchability (Table S1). The contradictory properties of stretchability and sensitivity for the GWF strain sensor were balanced satisfactorily due to the unique hierarchical structure of the GWF. For example, a strain sensor based on graphene films showed tolerable strain of less than 2%,66 which was much smaller than that of the GWF strain sensor with a workable strain of 57%. It is because that the graphene films are too brittle, in which cracks would emerge and propagate to release strain energy under deformation, leading to a limited workable strain range of strain sensors.24 Whereas, the intrinsically stretchable GWF could accumulate the strain, which results in large workable strain range. Another strain sensor based on graphene foams exhibited a large tolerant strain over 70%.10

However, the gauge factor of the strain sensor based on graphene foams was less than 29, which was much less than the GWF strain sensor in our work (gauge factor of 416 within 0− 40% strain and that of 3667 within 48−57% strain). The low sensitivity of the strain sensor based on graphene foams is attributed to its very high stretchability, which would maintain high electrical conductivity of the strain sensor upon applied strain.11 To the contrary, in the present study, the macrowoven-fabric geometrical conformation of RGO could suffer substantial structural changes upon deformation, leading to a high sensitivity of the GWF strain sensor.9 To illustrate the relationship between the hierarchical structure of GWF with its electromechanical property, we drew out some yarns from the cotton bandage template (28 × 24) and then fabricated the GWF strain sensors with different thread counts in the same way. As shown in Figure S12, the sensitivities of the GWF strain sensors with sparser thread counts increased, while their workable strain ranges decreased. The results are similar to the previous reference.22 To elucidate the electromechanical properties of the GWF strain sensor, the underlying mechanisms are herein discussed. F


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ACS Applied Materials & Interfaces The ΔR/R0 plotted against applied strain (Figure 4b) can be divided into two distinct stages, i.e., area 1 (0−40% strain), and area 2 (40−57% strain). In area 1, the adjacent RGO sheets are close, and the resistance is dominated by contact resistance of the RGO layers.28 The contact resistance in area 1 (Rc) can be expressed as67 R c = ρcontact ×

d lw

to measure a full range of human motions, including large and delicate motions. The wide workable strain range ensures the GWF strain sensor to detect large motions, such as bending of human joints. Figure 5a shows that the GWF strain sensor was fixed onto an elbow joint with the aid of adhesive tape to monitor arm bending at varied angles. The amplitude of corresponding signals were discernible for different bending angles (Figure 5b), since larger bending angles led to greater elongations of the sensor.35 Hence, the GWF strain sensor is able to both detect and quantify human joints bending. Additionally, the strain sensor was employed to monitor arm bending at different rates (Movie S1). Since the motions of human joints caused significant resistance changes of the GWF strain sensor, we fabricated a wearable light-emitting diode (LED) array by mounting three LEDs between two pieces of GWF followed by encapsulation of NR (Figure S1). Subsequently, the device was fixed onto a wrist of a human hand. It was found that the repetitive bending of the human hand caused reversible variation of the brightness for the LEDs (Figure S13, Movie S2). The results reveal that the GWF strain sensor promises applications in wearable electronics, organic light-emitting diode display, and human−machine interaction. Due to the highly sensitivity of the GWF strain sensor, it can be used to promptly recognize delicate motions as well. When the GWF strain sensor was attached onto the cheek (Figure 5c1), the corner of an eye (Figure 5c2), and the forearm (Figure S14), tiny muscle motions near the skin, such as cheek bulging and blinking, gave rise to an increase of resistance change. The corresponding characteristic signals were discernible and repetitive, demonstrating the high sensitivity and reliability of the GWF strain sensor (Figure 5d, Figure S14). Similar measurements were conducted on the forehead and philtrum of a subject (Figure 5e). When the subject cried, the frown of the forehead and the wrinkle of the philtrum caused a shrinkage of the GWF strain sensor and resulted in the decrease of the resistance change (Figure 5f1,f2).15 The signal amplitude of the forehead and philtrum were different, which can be attributed to differences in the degree of muscle movements.14 In contrast, when the subject laughed, muscles around the forehead and philtrum stretched, leading to an increase of the resistance change for the GWF strain sensor (Figure 5f3,f4). Therefore, our strain sensor may act as an emotion monitor to recognize intentions and the physiological state of humans. To measure the very subtle motions associated with pulse, the GWF strain sensor was fixed onto the wrist (just over the artery) of a healthy 183 cm tall and 25-year-old male (Figure 5g1). As illustrated in Figure 5g2, the signal for the arterial pulse was stable and periodic, with a Fourier component of frequency ∼1.3 Hz (Figure 5g3), similar to the expected value for a human pulse.68 Figure 5g4 shows a close-up of a typical waveform with three distinctive peaks: systolic wave (PS), point of inflection (Pi), and diastolic wave (PD).10 Generally, two parameters are used to evaluate the physiological conditions of the cardiovascular systems, i.e., the augmentation index AI (AI = ± (Ps −PI)/PP, where PP is the absolute pulse wave magnitude) for characterizing arterial stiffness, and the reflection index RI (RI = h/ΔT, where h is the tester height, ΔT is the time interval between Ps and PD).20,68,69 On the basis of Figure 5g4, the AI and RI value were calculated to be 26.2% and 10.1 m s−1, respectively, which was in good agreement with the literature for a healthy 183 cm tall, 25-year-old male.20,70 The results confirm that sophisticated differences in blood pulses could be identified with our sensor, which possesses

(1)

where ρcontact is the contact resistivity of RGO sheets, d is the RGO interlayer spacing, and l and w are the length and width of the overlap region, respectively. It can be inferred according to eq 1 that Rc inversely increased with w. Given that w presents inversely proportional to the strain, therefore, the ΔR/R0 exhibited approximately a linear dependency upon applied strain in area 1. Upon further straining, the ΔR/R0 experienced a sudden jump in area 2. The higher sensitivity of the GWF strain sensor is due to the separation of overlapped RGO sheets under larger strains, leading to breakage of the electrical current pathway. Hence, the resistance of the GWF strain sensor was conducted by the tunneling effect between neighboring RGO sheets. The tunneling resistance (Rc) can be estimated as12 Rc =

⎞ ⎛ 4πd ⎞ ⎛ L ⎞⎛ 2h2d ⎜ ⎟⎜ ⎟exp⎜ 2mφ ⎟ ⎜ ⎝ N ⎠⎝ 3a 2e 2 2mφ ⎟⎠ ⎝ h ⎠

(2)

where N is the number of conducting paths, L is the number of RGO sheets within a single conductive path, a2 is the effective cross-section area, e is the electron charge, m is the electron mass, φ is the barrier height, h is the Plank constant, and d is the average tunneling distance between adjacent RGO platelets. In area 2, the distance between adjacent RGO platelets is assumed to increase proportionally from d0 to d with strain, in company with the decrease of conducting paths from N0 to N.63 The d, N, and ΔR/R0 can be calculated as

d = d0(1 + bε) N=

(3)

N0 exp[A1ε + B]

(4)

⎤ R − R 0 ⎛ N0d ⎞ ⎡ 4π (d − d0) ΔR = =⎜ 2mφ ⎥ − 1 ⎟exp⎢ ⎦ R0 R0 h ⎝ Nd0 ⎠ ⎣ (5)

where ε is the applied strain, and b, A1, and B are constants. By substituting eqs 3 and 4 into eq 5, ΔR/R0 can be expressed as ΔR = (1 + ε)exp[A 2 ε + C ] R0 with A 2 = A1 +

4πd0 2mφ h

(6)

(7)

where A2 and C are constants that do not have certain physical significances. As shown in Figure 4b, the experimental data of area 2 was fitted with eq 6. It is notable that the theoretical results described the experimental data well with fitted values of A2 = 7.36 and C = 1.74. Due to the superb flexibility, stretchability, sensitivity, reliability, and durability, the GWF strain sensor enables us G


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Figure 6. (a) Photograph of the GWF strain sensor fixed onto a speaker of a smart phone. (b) Recognition signal toward audio of an eagle from a phone speaker, with insets above being the sound-wave profile of the original audio. (c) Photographs of the GWF strain sensor mounted on an oscillating ruler. (d) Recognition signal under damped vibration and enlarged views of the pink areas.

In addition to monitoring human pronunciation, the GWF strain sensor also has excellent performance when sensing the acoustic-driven vibration of a smart phone speaker (Figure 6a). The sound signals propagate as an audible mechanical wave, which can give rise to a resistance change of the attached strain sensor, thereby being collected.12,18 The phone speaker played five kinds of audio, including eagle, elephant, cow, cuckoo, and bear sounds. The results are presented in Figures 6b andS16a− d. Notably, the collected signals exhibited a synchronous response to these original audios. Characteristic peaks of the audios were retained with high fidelity, indicating the superb sensitivity and high strain resolution of the GWF strain sensor. To test the capability of the GWF strain sensor to detect other physical vibrations, the GWF strain sensor was attached onto the oscillating steel ruler (Figure 6c). The recognition signal promptly revealed a damped vibration of the ruler with a response time of less than 20 ms (Figure 6d). Actually, the response time of strain sensors is related to the speed of mechanical stimuli. Generally, mechanical stimuli occurring at a higher speed results in a smaller response time. Mechanical stimuli occurring at a lower speed, such as arm bending (Movie S1), leads to a longer response time. The minimum limit of the response time for strain sensors is often used to evaluate their tracking ability.71 In the present study, the response time (less than 20 ms) is comparable with those of recent sensors (16−70 ms)17,22,68,69,72,73 and faster than those previously reported (larger than 100 ms),30,74,75 indicating the extraordinary tracking ability of the GWF strain sensor. The ultrafast response, superb sensitivity, and high strain resolution enable the GWF strain sensor to detect various physical vibrations in the local environment, which is expected to offer worthy insights into artificial intelligence systems.

tremendous potential in diagnostic applications to monitor human’s health. Furthermore, the GWF strain sensor was attached onto the Adam’s apple to monitor the muscle movements of the vocal cords of a healthy male (Figure 5h, Movie S3). As shown in Figure 5i, when a volunteer was asked to speak a word (e.g., one, three, five, seven), the GWF strain sensor exhibited repeatable signal patterns. Signal patterns were distinguishable among different words. Actually, every individual word has its own unique amplitude, duration, and thus signal pattern originating from a particular motion form of the vocal muscle.39 When monosyllabic words (e.g., one, three, five) were pronounced, a corresponding single pattern was observed, while polysyllabic words (e.g., seven) generated multipeak patterns.15 The signal was also associated with the volume of pronunciation. When a volunteer pronounced “one” with different voice volumes, a louder voice would strengthen the vibration of the signal due to the use of more muscle movements (Figure S15a). Some other tests on monitoring words and phrases were shown in Figure S15b,c. In addition, the GWF strain sensor was attached onto the front neck of a male and a female, respectively (Figure S15d). When pronouncing the same word, signals from the two volunteers displayed similar features in spite of different speech organs between the male and female (Figure S15e,f). The sensitive and reliable detection of speech endows the GWF strain sensor with potential applications in phonetic recognition. Besides phonation, the GWF strain sensor attached to the Adam’s apple was also capable of monitoring other tiny physiological motions, such as chewing, swallowing, coughing, and head lowering, respectively (Figure 5j1−j4). H


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Author Contributions

CONCLUSIONS In summary, we reported a flexible strain sensor based on reduced graphene oxide woven fabrics, which were fabricated through a cotton-bandage-templated and ethanol-flame-reduced process, during which the template was pyrolyzed simultaneously. This novel strategy for preparation of reduced graphene oxide woven fabrics was facile, ultrafast, low-cost, and scalable. Due to the unique macro-woven-fabric structure of the reduced graphene oxide woven fabrics, the obtained strain sensor exhibited a good trade-off between the contrary properties of sensitivity and stretchability, ensuring it to measure multiscale motions. Moreover, the strain sensor displayed reliable signal when upheld at a certain strain as well as good durability in long-time testing. On the basis of its excellent electromechanical properties, the strain sensor could monitor a full range of human activities, such as large motions (joints bending) and delicate motions (facial expression, pulse, pronouncing, coughing, etc.). Furthermore, sound signals from a smart phone and damping vibrations of a steel ruler were also detected simultaneously and precisely. The response time was less than 20 ms, which is comparable with recent strain sensors. We foresee that the strain sensor based on reduced graphene oxide woven fabrics, which were prepared via a facile, cheap, and ultrafast method, will open up widely practical applications in wearable electronics, human−machine interaction, sound recognition, personalized health monitoring, and artificial intelligence as well as others.

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H.J., B.Y., and Y.W. conceived the experiments. H.J. supervised the project. B.Y., Y.W., T.H., and Z.X. conducted the experiments. B.Y., T.H., G.Y., and Q.J. analyzed the data and cowrote the paper. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Jiangsu Province Key Project (BE2015158), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Graduate Student Research and Innovation Project of Jiangsu Province (KYLX16_0434), and the Aerospace Science Foundation of China (2016ZF59009).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09652. Preparation of an LED array device; AFM and TEM images of GO; photographs of the cotton bandage before and after the ethanol-flame treatment; TGA, FTIR, XRD, and XPS of cotton bandage, GO, and GWF; electrical, mechanical, and electromechanical properties of GWF strain sensors containing GWFs at varying dip-coating times; reproducibility and durability of the GWF strain sensor; optical images of a GWF strain sensor during initial prestretching; table of the strain sensors reported in the literature; strain-sensing properties for the GWF strain sensors with different thread counts; brightness vibration of a wearable LED array device; real-time monitoring of muscle movements; resistive signals of phonation with different volumes, with varied words, and by different people; recognition signals toward audio from a phone speaker (PDF) Movie S1: capability of the GWF strain sensor in monitoring arm bending (AVI) Movie S2: reversible variation of the brightness for the LED array device (AVI) Movie S3: Capability of the GWF strain sensor in monitoring phonation (AVI)

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ABBREVIATIONS GWF, reduced graphene oxide woven fabrics GO, graphene oxide CVD, chemical vapor deposition PTFE, polytetrafluoroethylene AFM, atomic force microscopy TEM, transmission electron microscopy TGA, thermogravimetric analysis DTG, derivative thermogravimetric analysis SEM, scanning electron microscope FTIR, Fourier transform infrared XRD, X-ray diffraction XPS, X-ray photoelectron spectroscopy RGO, reduced graphene oxide LED, light-emitting diode AI, augmentation index RI, reflection index REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*H.J.: E-mail: [email protected]. ORCID

Qingmin Ji: 0000-0001-7810-3438 Hongbing Jia: 0000-0003-2127-3983 I


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Highly Stretchable, Ultrasensitive, and Wearable Strain Sensors

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