Continuously Producible Ultrasensitive Wearable Strain Sensor

Nov 8, 2017 - Wearable devices have been recognized as one of the best ... Polyolefin elastomer (POE) was purchased from Dow Chemical Company...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

Continuously Producible Ultrasensitive Wearable Strain Sensor Assembled with Three-Dimensional Interpenetrating Ag Nanowires/ Polyolefin Elastomer Nanofibrous Composite Yarn Weibing Zhong,† Cui Liu,† Chenxue Xiang,‡ Yuxia Jin,† Mufang Li,‡ Ke Liu,‡ Qiongzhen Liu,‡ Yuedan Wang,‡ Gang Sun,§ and Dong Wang*,†,‡ †

College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620 China Hubei Key Laboratory of Advanced Textile Materials & Application, Wuhan, 430200 China § University of California, Davis, California 95616-8598, United States ‡

S Supporting Information *

ABSTRACT: Fiber-shaped strain sensors with great flexibility and knittability have been tremendously concerned due to the wide applications in health manager devices, especially in human motion detection and physiological signal monitoring. Herein, a novel fiber-shaped strain sensor has been designed and prepared by interpenetrating Ag nanowires (NWs) into polyolefin elastomer nanofibrous yarn. The easy-to-obtain structure and simple roll-to-roll process make the continuous large-scale production of nanofibrous composite yarn possible. The continuous and alternating stretching and releasing reversibly change the contact probability between AgNWs in this interpenetrating network, leading to the variations of electrical resistance of the sensor. The gauge factors of strain sensors are calculated to be as high as 13920 and the minimum detection limit is only 0.065%. In addition, the strain sensor shows excellent durability during 4500 cycles with the strain of 10%. The response times of stretching and releasing strains are 10 and 15 ms, respectively. Furthermore, the strain sensor has been successfully applied in human motion detections both in single yarn and knitted fabrics. The result shows the practicability in applications of monitoring limbs movements, eye motion changes, artificial vocal cords, human pulse, and complex motions, which shows great potential in wearable sensors and electronic skin. KEYWORDS: nanofibrous yarns, large-scale production, strain sensors, silver nanowires, human motion detection

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INTRODUCTION

Actually, as a typical elementary one-dimensional unit for textiles, well-found theory and technology on traditional chemical fiber manufacturing make it possible to fabricate fiber-shaped strain sensor in scale-up production. Moreover, compared with other types of strain sensors, fiber-shaped strain sensors present greater potential in wearable electronics for their virtues of knittability and integrability which enable them to be connected into soft integrated circuit with other essential electronic components.28 Because of the brittle nature of electrical conductive materials, novel elastomer substrates or extraordinarily designed structures, which provide conspicuous restoring force while the strain was removed, to improve cycling performance should be the focus of research. Considerable strain sensors have been operating on the microcrack mechanism which was first proposed by our group29 and then improved by Kang et al.30

Wearable devices have been recognized as one of the best candidates of the next-generation electronics for their unique superior properties such as lightweight, conformability, and portability.1−7 To realize the promising applications on continuous human motion detection, physiological measurements, and disease diagnosis,8−13 plenty of strain sensors with specially designed structures were carried out. In brief, depositing thin conductive layer onto the surface of the stretchable substrates or imbedding conductive polymers into elastomers are common methods to fabric strain sensors.11,14−27 However, there are several obstructive factors which make them extremely difficult for commercial applications. Principally, sophisticated secondary structures such as arrays and interlocking structures are built to improve the sensing performance by rising capacity of strain capturing, whereas such complicated designs lead to exponential growing difficulties in manufacture techniques. Hence, simplification in device structure without performance loss would be the priority to all. © XXXX American Chemical Society

Received: August 3, 2017 Accepted: November 8, 2017 Published: November 8, 2017 A

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of continuous production of ENCY. (b) Schematic illustration of diffusion process of AgNWs into dispersed POE nanofibrous yarn. (c) Photograph of a roll of prepared POE nanofiber yarn which is up to 300 m long. (d) Photograph of school badge which is constructed with POE nanofiber yarn. (e) Photograph of knitted sample with a single POE nanofiber yarn. (f) Photographs of pristine POE that is stretched to 650%−700%. (g) Photographs of emitting of series-wound full-color LED and ENCY strain sensor while the latter was in an unstretched state and stretched with elongation of 3%, 8%, 19%, and 63%.

For example, Wang et al.31 proposed a method for scalable preparation of single-wall carbon nanotubes (SWCNTs)/ cotton/polyurethane composite yarn which possesses strain sensitivity by coating core-skin cotton/polyurethane yarn into SWCNTs ink for times; as introduced, the strain sensor displayed high durability as the highest gauge factor (GF) is 2.15. Analogously, Cheng et al.32 reported multifunctional device prepared with conductive yarn which was obtained by dip-coating in the Ag nanowires (NWs)/ethylene glycol dispersion; prestretching is essential to endow the composite yarn with a high density of microcracks which provides strainresponsive ability. However, such designs should be optimized because the microcracks induced by the inhomogeneous prestretching process would raise the indeterminacy of each sample. Meanwhile, the insecure structure may be collapsed under complicated strain field by emerging of microcracks, which would lose the reference value of the original quantitative relation between resistance and strain. Inorganic nanowires,33,34 nanocarbons,35,36 and conjugated polymers37 were applied in constructing scalable, producible, and highly sensitive strain sensors. For example, Lee et al.38 reported a roll-to-roll process to prepare Ag particles/Ag NW/ styrene−butadiene−styrene (SBS) strain sensor while the Ag NW/SBS fibers are obtained through wet spinning and the Ag particles were acquired by reducing immersed Ag precursor. However, the conductivity degration is only 4.4% at the strain of 100%; the dispersion of Ag NW in highly viscous SBS solution as well as chemical reduction of Ag particles makes the techniques complex. Therefore, developing a novel structure of fiber-shaped strain sensor would be vital to achieve high GFs and a facile preparation technique which may contribute to its commercialization. Herein, we proposed a novel continuously producible method for elastic nanofibrous composite yarn (ENCY)

which is composed of polyolefin elastomer (POE) nanofiber yarn and three-dimensional interpenetrating silver nanowires (AgNWs). Typically, POE nanofibrous yarn was dispersed in unique solvents system containing uniformly suspending AgNWs. Then the AgNWs will be immitted homogeneously into the POE nanofibrous yarn because of the concentration diffusion. Shaking or ultrasonication, which would provide extra energy to prompt the uniform diffusion process, makes the whole conductive treatment less than 30 s. The micromorphologies of original POE nanofibrous yarn and obtained ENCY indicates that the AgNWs are uniformly distributed in the POE nanofiber spacing. Continuous network of AgNWs makes the ENCY conductive. After that, ENCY was assembled into wearable strain sensors of which crucial parameters such as GFs, minimum detectable strain limit, response time, and cycling stability are measured. Furthermore, promising applications in human motion capture, expressions diagnosis, artificial vocal cords, and human pulse monitoring are successfully operated, which indicates the non-negligible potential of demonstrated ENCY strain sensors.

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EXPERIMENTAL SECTION

Materials. Polyolefin elastomer (POE) was purchased from Dow Chemical Company. Cellulose acetate butyrate (CAB, butyrate content 35−39%) was obtained from Acros Chemical Co., Ltd. Silver nanowire ethanol solution (with a concentration of 1 wt %/v) was provided by Hefei Vigon Tech, China. Acetone, ethanol, and diethyl ether were supplied by Sinopharm Chemical Reagent Co., Ltd. Preparation of POE Nanofiber Yarn. POE nanofiber yarns were obtained using the method reported previously. It can be illustrated as extraction removal of the CAB from melt extrusion blends which contains 10% POE master batches and 90% CAB powders. Preparation of Stable AgNWs Solution with Dispersive Capacity to POE Nanofiber. Silver nanowire (AgNWs) ethanol suspension was directly mixed with diethyl ether to prepare the stable AgNWs suspension. To investigate the influence of diethyl ether B

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Sensing mechanism of primary unit when ENCY strain sensor was stretched and released. (b and c) SEM images of original POE nanofibrous yarn with different magnifications. (d) SEM image of ENCY treated with 0.2% AgNWs and 60% diethyl ether. (e) SEM image of ENCY with magnification of 10000 which presents the combined method for POE nanofibers and AgNWs. (f) Schematic illustration of resistance change in elementary unit. (g) Photographs of comparable resistance of same length while the ENCY was straight and tied into a knot. volume fraction on the dispersive capacity and AgNWs loading ability, 40−70% of diethyl ether was added into AgNWs ethanol solution, and then POE nanofiber yarn was drawn in before being extensively shaken. After that, concentrations of AgNWs were adjusted to achieve lower resistance and higher stretchability. AgNWs (0.1−0.4 wt %/v) were prepared to optimize the performance of the conductive yarn and the strain sensor. Characterization and Measurements. The loading mass of AgNWs on each ingredient solution was measured by precision balance. The morphology of pristine POE nanofiber yarn and AgNWs solution treated yarn was observed through a JEOL scanning electron microscope; meanwhile, the EDS was obtained. The mechanical property was tested with an Instron 6957 electronic tensile testing machine. The sensing performances were measured by coordination of Mark-10 using a digital multimeter (UT58) and an Autolab System (PGSTAT302, Switzerland) at room temperature.

Information) that the increase in the volume fraction of diethyl ether from 40% to 70% favors the wetting and separating of the POE nanofiber yarns due to the matched solubility parameter. But the diethyl ether fraction of 70% caused the obvious aggregation of AgNWs. Correspondingly, the unit resistance (resistance per centimeter) of ENCY as a function of the volume fraction of diethyl ether in mixed solvents was recorded in Figure S3 (Supporting Information). When the fraction was 60%, the loading amount of AgNWs in ENCY was about 6% and the electrical conductivity of the obtained ENCY is the highest with a unit resistance of 10 Ω. Figure S4 (Supporting Information) shows the silver content compared to carbon and relative resistance changes (a) as well as distribution of Ag element in different segments (b−d). From the results of three selected representative samples with spacing of 1 m, the loading amount is decreased with the preparation continued. However, the relative resistance changes are no more than 6%, and the silver mapping is uniform, which indicates the feasibility in large-scale production after further optimization. Figure 1c shows the photograph of a roll of prepared POE nanofibrous yarn with the length of about 300 m, which clearly indicated the possibility of large-scale production of continuous and conductive nanofiber composite yarns. The ENCY demonstrated the great potential of being weaved or knitted into flexible and three-dimensional conformable textile based devices by making our university badge and patterns with POE nanofibrous yarn, as shown in Figure 1d,e. After being assembled with Cu wire electrodes, a strain sensor can be fabricated. In Figure 1f the ENCY was carefully stretched to 650%−700% of its original length without break, showing excellent stretchability of POE nanofibrous yarn. Furthermore, the mechanical properties of original POE nanofibrous yarn and ENCY were measured using Instron 5967 electronic tensile testing machine. Results in Figure S5 (Supporting Information) indicate that addition of AgNWs has no obvious effect on the

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RESULTS AND DISCUSSION Figure 1 shows the fabrication process of the ENCY strain sensor. Figure 1a illustrates the roll-to-roll preparation process from well-extracted POE nanofibrous yarn to ENCY. The POE nanofibrous yarns in a roll were drawn and immersed into the AgNWs suspension in the mixed solvents of ethanol and diethyl ether which may saturate and disperse the POE nanofibers well. The volume of AgNWs suspension as high as 10 L which is in excess compared to POE nanofibrous yarn has been applied to reduce the effect of loading amount of AgNWs to the homogeneity of ENCY. A simple shaking device continuously applied the mechanical force to POE nanofibrous yarns in the solvent mixture. The combined effects led to the dispersion of POE nanofibrous yarns from compactly packed bundle into separated fibers. The well-dispersed POE nanofibrous yarn offered the space for the diffusion and interpenetration of AgNWs into the yarn, as illustrated in Figure 1b. The dispersed and swelled POE nanofibrous yarn in the AgNWs/ethanol/ diethyl ether suspension is shown in Figure S1 (Supporting Information). It can be observed in Figure S2 (Supporting C

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Sensitivity of ENCY strain sensors including the detection limit and response time. (a) Plots of relative resistance change and strain. (b) Calculated gauge factors from the definition equation. (c) Minimum detection limit of ENCY strain sensor prepared with AgNWs concentration of 0.2%. (d) Response time of ENCY strain sensor prepared with AgNWs concentration of 0.2% while being stretched and released.

structure of ENCY. In essence, ENCY acquires electrical conductivity by forming perfoliating coherent current pathways with interpenetrated AgNWs. Every contact point of individual AgNWs acted as a strain-controlled switch unit. Figure 2a demonstrates the sensing mechanism which utilizes nanoscale confinement by using a simplified model. The increase in the axial strain resulted in the reorientation of AgNWs interpenetrated with POE nanofibers in ENCY along the axial direction. The POE nanofibers served as the separator between axially aligned AgNWs, lowering the contact possibility of AgNWs. Consequently, the electrical resistance increased. Meanwhile, ENCY is constructed by mass of elementary units illustrated in Figure 2f. The conductive network inside ENCY is composed of a complicated multiple circuit. The resistance of ENCY (R) can be calculated as in eq 1.

elongation at break and Young’s module of ENCY. Figure 1g presents the luminance of full-color LED which was in series circuit with ENCY strain sensor and direct current regulated power supply (0.98 V); this method has been discussed in detail in previous works.39 The elongation of ENCY caused the increase in the electrical resistance, which lowers the voltage attributed to the LED light. As a result, the luminance of LED light becomes darker. The color was changed from blue-green to dark red while the ENCY strain sensor was continuously stretched from 0% to 63%. The corresponding response process has been recorded and presented in a video seen in the Supporting Information. Figure 2 presents the sensing mechanism of ENCY strain sensor, as well as the morphology of original POE nanofiber yarn and ENCY. It can be observed from Figure 2b,c that the overall diameter size of original POE nanofibrous yarn was about 300 μm. The nanofibrous yarn is composed of longitudinally aligned nanofibers with diameters ranging from 250 to 600 nm. The morphology of ENCY is presented in Figure 2d,e. It can be found that the AgNWs were randomly distributed in the POE nanofibrous yarn and interpenetrated with POE nanofibers. The formation of this structure is the result of the concentration diffusion of AgNWs in the mixed solvents, strong interactions between AgNWs and POE nanofibers due to the large interfacial tension of nanoscale materials, and capillary pressure during the drying process of ENCY. Because of the intertwining of POE nanofibers, the AgNW has been trapped in the limited fiber space. The static friction contributes to the construction of nanoscale confinement of AgNW in POE nanofibers. Figure S6 (Supporting Information) shows the cross-sectional SEM image as well as the mapping of silver element. The coherent contents of silver element on the cross section further indicate the uniformity

R=

R1R 2...R n i=n ∑i = 1 R1R 2...R n/R i

(1)

where n refers to the total number of contactors at unstretched state and R1 to Rn refer to the corresponding resistance of n contact points when they are connected. Actually, as the resistance of each contact point is similar, which can be recorded as Rp, the function can be abbreviated to R = Rp/n. As demonstrated, the quantity of contactors reduced drastically, which means the n decreased sharply. It can be concluded that resistance in a stretched state is much larger than the original state. This model could explain the mechanism of similar strain sensor well.40 Figure 2g shows the change in the electrical resistance of straight ENCY and knotted ENCY. As presented, the resistance of straight ENCY was 12.5 Ω, while the resistance of knotted ENCY was reduced to 9.7 Ω due to the larger contact area of D

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Cycling stability test of ENCY strain sensor. (a) Current curve of ENCY strain sensor in 4500 cycling at the strain of 10%. (b) Magnified current curve of the signed location from the 4500 times cycling test. (c) 50 times of cycling points with various strains of 2%, 5%, 10%, 20%, and 30%.

the knotted one. It makes the n in eq 1, the number of contact points between AgNWs, significantly increase. As a result, the resistance of knotted ENCY was much lower than that of the straight one. Gauge factor (GF) is employed for evaluating the sensitivity of strain sensors. As demonstrated in other works,28,41,42 GF was defined as the plot slope of the relative resistance change as a function of the strain. It can be defined by the following equations (eq 2 and 3). GF =

∂ΔR /R 0 ∂ε

ΔR = R − R 0

rapidly when stretched. Figure 3b presents the accurate value of gauge factors of the ENCY strain sensors which is the derivative of relative change of resistance and strain. The results show that the strain sensor prepared with higher AgNWs concentration exhibits lower GF. The higher concentration of AgNWs facilitates the formation of more electrically conductive pathways in the strain sensors, obtaining a higher value of n in eq 4. Upon being stretched, the strain sensors with higher concentration of AgNWs displayed reduced probability of disconnections than those with lower concentrations, which decreased the N(ε). It can be found that the GFs sharply increased with the applied strains. SEM images under different strains in Figure S7 (Supporting Information) shows that POE yarn was oriented in a higher degree under larger elongation, which would isolate the AgNWs from each other more unsatisfactorily. This phenomenon results in higher disconnected points under higher strain, N(ε), which is in accordance with eq 4. The chronoamperometry line under the voltage of 0.1 V was presented in Figure 3c. The minimum strain limit of 0.065% could be detected by a repeatable recognizable platform. The strain sensors fabricated demonstrate the excellent detection capability of strains ranging from 0.065% to 64%, which was among the best reported values to our knowledge. Table S1 (Supporting Information) presents the comparison of corresponding parameters such as GFs and detectable limitations between ENCY strain sensor and the latest reported sensors; the result indicates that the ENCY strain sensor exhibits excellent GFs, detection limit, and sensing range. Figure 3d shows an electrical current response cycle when strain was rapidly applied to ENCY strain sensor and then released. The curves of current change as a function of time during stretching and releasing are displayed in Figure S8a,b (Supporting Information). The response time to stretching and

(2) (3)

where R0 refers to the initial resistance of NCY in an unstretched state and R refers to the resistance at the strain of ε. Here, the function can be further generated by substituting simplified eq 1 into eqs 2 and 3. GF =

nN ′(ε) [n − N (ε)]2

(4)

where N(ε) refers to the amounts of disconnected AgNWs contactors at applied strain, N′(ε) refers to the differential derivative of N(ε), and n refers to the total number of contactors in an unstretched state. The N(ε) is less than n. The increase in the applied strain ε leads to more disconnected points of AgNWs, higher value of N(ε). Figure 3a presents the curve of the change in relative resistance as a function of the applied strain ranging from 0% to 64% for strain sensors assembled with ENCY having AgNWs concentrations of 0.2, 0.3, and 0.4 wt %/v. The sensors showed various responses to the applied stains. It can be observed that the relative resistances of the ENCY strain sensors are growing E

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Applications of ENCY strain sensor by attachment to different positions of the body surface. (a) Monitoring process of knee joint movements. (b) Detection process of the second knuckle of index finger motions. (c) Sensing results of variation of visual angle by adhering the sensor on the eye corner. (d) Application of artificial vocal cords to diagnose the phonation. (e) Application on human pulse monitoring and its consequences.

repeated cycles, revealing excellent cycling stability at different strains. Since the strain sensor is made of the ENCY, it possesses outstanding flexibility and conformability. The ENCY strain sensor was utilized for continuous monitoring of vigorous joint movements, tiny muscle actions, and subtle physiological character, realizing continuous human motion detection, physiological measurements, and disease diagnosis. Figure 5a−e demonstrates the application examples in the detection of movements with different strain degrees. The real-time resistance variation was obtained by recording the current change with time under the constant secure voltage of 0.1 V. Specifically, Figure 5a shows the test results of the knee joint movement. The ENCY strain sensor was adhered tightly to the stocking; signals that modeled repeated, rhythmed standing up, sitting down, and slow walking were recorded and presented. Figure 5b presents a prepared latex glove with a ENCY strain sensor attached to the second knuckle of the index finger. The electrical current fell down steeply when the bending angle of the index finger regularly increased from 0° to 70°. The current jumped back as the index finger returned to the original state. Figure 5c demonstrates the detection of eye movements by attaching the ENCY strain sensor to the skin at the eye corner. The repeated eye motions of looking forward, closing, blinking, and looking up caused tiny strain changes on the skin around the eyes that were accurately caught by the ENCY sensor and showed the regular variations in the electrical current. In addition to that, the vibrations of the artificial vocal cord were

releasing are 10 and 15 ms, respectively. Such short response time contributes to the dynamic cycling stability of the ENCY sensor and makes the practical applications of sensors possible. Cycling stability of the strain sensor is essential for practical applications. The most sensitive ENCY strain sensor, which was prepared with AgNWs concentration of 0.2%, has been selected to test the cyclability. Figure 4 shows the cycling performance of the strain sensor assembled with ENCY. Figure 4a shows the change in the real time current through the ENCY strain sensor when it experienced repeatedly alternating cycles of stretching and releasing with a strain of 10% at a frequency of 0.9 Hz. During stretching, the electrical resistance of the ENCY sensor increased and the electrical current decreased. After release, the resistance recovered to the unstretched state and the current climbed back. It can be found that within 4500 cycles the current remained almost the same. The detailed electric current change within 4500 cycles is magnified and shown in Figure 4b. The current varies regularly in the range of about 2.4 to 3.4 mA with the alternatingly applied strain of 10%, indicating excellent dynamic cycling stability. The interpenetrating AgNWs network similar to the original state can be easily observed in Figure S9 (Supporting Information) which is the SEM image of ENCY strain sensor after 4500 cycles of stretching and releasing. It indicates the good reversibility of ENCY structure. To investigate the cycling property at different strains, the relative changes in the electrical resistance of ENCY strain sensor at strains of 2%, 5%, 10%, 20%, and 30% were recorded and displayed in Figure 4c. The curves stay almost flat during F

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Human motion detection by using stocking rewoven with ENCY strain sensor. (a) Photograph of rewoven fishnet stocking with a section of ENCY strain sensor. (b) Real-time current curve under 0.1 V while the model rhythmically stands on her tiptoes. (c) Corresponding varying current with body rotation. (d) Current line while the model was gently jumping.

those of gentle jumping. As a result, multiple peaks were observed and the variation range of the electrical current is narrower than that from jumping. The motions of gently jumping and standing on tiptoes are similar, but more vigorous. This difference also can be revealed by sharper and deeper peaks on the jumping curve. The ENCY strain sensor assembled into wearable textiles can correlate different motions with changes in electrical current very well.

also tested and the results are shown in Figure 5d. When the words “nanofiber” and “sensor” were pronounced, the vibrations were collected though the ENCY strain sensor overlaid outside the throat. Each word was repeatedly read three times; the word “nanofiber” was easily read twice and “linking” read once. The similar unique waveform can be easily identified, which clearly indicated the accuracy of voice recognition. At last, the human pulse was detected by pasting the ENCY strain sensor on the wrist as exhibited in Figure 5e. Real-time electrical current change curve lasting about 20 s was shown and a representative circulation was magnified. As the ENCY strain sensor is tightly attached to the wrist, the pulse beat would lead to various degrees of strain to the sensor. These strains contribute to the decreases of current which has been real-time recorded and the regulated pulse beats would be recorded as downward peaks. From Figure 5e, three downward peaks that were labeled as P1, P2, and P3 in each circulation refer to the percussion wave, tidal wave, and diastolic wave.17 Here, the percussion wave would lead to variation of about 4% on ΔR/R0. With these potential applications testing on human activities with diverse movement ranges, it is determined that the facilely prepared ENCY strain sensor demonstrated great potential for monitoring human activities and healthcare management. To investigate the integrating performance of ENCY strain sensors, an ENCY strain sensor was assembled into fashionable fishnet stocking. Figure 6a shows the photograph of knitted stocking which contains the ENCY strain sensor at the acupuncture point of chieh hsi. Different movements caused corresponding deformations of the knitted stocking, which was then transferred into strain changes and applied on the ENCY sensor. Figure 6b shows the electrical current changes when a model girl rhythmically stands on her tiptoes. This action elongated the ENCY; as a result, the current exhibited periodic downward peaks. Figure 6c,d shows the responses of ENCY strain sensor when she rotated and gently jumped, respectively. It can be found that different motions could generate corresponding curves of electrical current from ENCY sensors. The complexity and motion amplitude of rotation is less than

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SUMMARY

In conclusion, wearable ENCY strain sensor was fabricated by combining AgNWs and separated POE nanofibrous yarn through facile shaking in specific mixed solvents. Owing to the unique structure of three-dimensional interpenetration, the ENCY strain sensor can run with a novel sensing mechanism, demonstrating superior performance. The GFs of ENCY strain sensor grows exponentially as the strain rises. The ENCY strain sensor prepared with 0.2 wt %/v AgNWs presents highest GF, reaching as high as 13920 at the strain of 64%. Additionally, the minimum detectable limit strain was 0.065%; the response times upon applying and releasing strains were as fast as 10 and 15 ms, respectively. The cycling stability of ENCY strain sensor was shown in 4500 cycles. The ENCY strain sensor was successfully applied in monitoring human activities by being attached to the different areas on the human body, including knee joint, the second knuckle of the index finger, skin from the corner of the eye, throat, and wrist. The movements such as standing up, sitting down, and slowly walking, bending and stretching of the index finger, variation of visual angles, pronunciation of words, and human pulse were obtained. This ENCY sensor fabricated by a roll-to-roll method exhibits excellent sensing performance, and provides the possibility of large-scale production and great potential for practical applications. G

DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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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.7b11431. Photographs of dispersed POE nanofibrous yarn, AgNW loading ability and unit resistance of ENCY, electrical parameter and Ag mapping of samples from different sections, mechnical property, cross-sectional morphology, SEM images under different strains, response times of ENCY strain sensor, and SEM image of ENCY after 4500 cycles of stretching and releasing (PDF) Video of the response process of ENCY strain sensor in the circuit with full color LED (AVI

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Tel.:+86-27-59367691. ORCID

Dong Wang: 0000-0002-8139-8502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Natural Science Foundation of China (Grant Nos. 51473129, 51503157, and 51503160), National Science and Technology support program (2015BAE01B01), and Nature Science Foundation of Hubei Province (Nos. 2016CFA076 and 2016CFB386). The authors also express gratitude for financial support from the plan for Scientific and Technological Innovation Team of Excellent Young Investigator from Education Department of Hubei Province of China under Grant No. T201408 and Innovation Team from Science and Technology Department of Hubei Province of China under Grant No. 2015CFA028.

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DOI: 10.1021/acsami.7b11431 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX