Highly Stretchable Multifunctional Wearable Devices

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20845−20853

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Highly Stretchable Multifunctional Wearable Devices Based on Conductive Cotton and Wool Fabrics Hamid Souri* and Debes Bhattacharyya Centre for Advanced Composite Materials, Department of Mechanical Engineering, The University of Auckland, Auckland 1142, New Zealand

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S Supporting Information *

ABSTRACT: The demand for stretchable, flexible, and wearable multifunctional devices based on conductive nanomaterials is rapidly increasing considering their interesting applications including human motion detection, robotics, and human−machine interface. There still exists a great challenge to manufacture stretchable, flexible, and wearable devices through a scalable and cost-effective fabrication method. Herein, we report a simple method for the mass production of electrically conductive textiles, made of cotton and wool, by hybridization of graphene nanoplatelets and carbon black particles. Conductive textiles incorporated into a highly elastic elastomer are utilized as highly stretchable and wearable strain sensors and heaters. The electromechanical characterizations of our multifunctional devices establish their excellent performance as wearable strain sensors to monitor various human motions, such as finger, wrist, and knee joint movements, and to recognize sound with high durability. Furthermore, the electrothermal behavior of our devices shows their potential application as stretchable and wearable heaters working at a maximum temperature of 103 °C powered with 20 V. KEYWORDS: multifunctional device, wearable strain sensor, wearable heater, graphene nanoplatelets, human motion detection

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INTRODUCTION Flexible and wearable electronics, such as strain sensors,1−7 energy storage materials,8−11 and heaters,12−14 have been rapidly developing in the last decade with considerable demands on maintaining a high level of electrical conductance during deformation. Functional materials, such as graphene, 15−17 carbon nanotubes, 18−24 and carbon black (CB),25,26 have been well researched as alternatives to conventional materials, such as conductive polymers27 and metallic nanomaterials,28−31 used in flexible electronics. The combination of carbon nanomaterials and stretchable elastomers in the form of nanocomposites has been one of the favorable ways to fabricate flexible electronic devices.32 However, considering their poor conductivity at high strain levels and difficulties associated with obtaining a homogenous percolation network within elastomers, a simple and effective fabrication method for flexible and stretchable electronics remains challenging.32 Natural materials, especially textiles, have been extensively explored as active materials for flexible electronics in the recent years.33−37 They offer several advantages over conventional materials, such as natural abundance, environmental friendliness, cost effectiveness, and lightweight. Knitted textiles can be highly stretched, mainly because of the existence of the winding loops in their structure that can expand in various directions. Nevertheless, their electrically insulating nature is a major drawback that has challenged researchers to come up with © 2018 American Chemical Society

methods of making conductive natural materials. Pyrolysis (also known as the carbonization process)35−37 and various coating processes38 of carbon nanomaterials are among the most commonly used methods to turn natural materials into highly electrically conductive materials. Although multifunctional devices based on carbonized natural materials provide a high level of electrical conductivity, the complexity of the setup, high working temperature, and the amount of time consumed by the process are considered as the major drawbacks of the pyrolysis process. These could result in some limitations for mass production of flexible electronics based on natural materials. Coating methods, such as dipcoating,38,39 electrophoretic deposition,40 and spray coating,41 have been utilized to make natural materials conductive; however, the electrical conductivity values were not considerably high for these materials. Hence, a research gap exists in the development of a simple and scalable method for coating the carbon nanomaterials onto natural materials to dramatically enhance their electrical conductivity. We previously introduced a novel and simple coating technique utilizing an ultrasonication bath to make conductive natural fiber yarns by a hybrid system of graphene nanoplatelets (GNPs) and CB in deionized (DI) water with the presence of an anionic surfactant Received: March 23, 2018 Accepted: May 29, 2018 Published: May 29, 2018 20845

DOI: 10.1021/acsami.8b04775 ACS Appl. Mater. Interfaces 2018, 10, 20845−20853

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the preparation of carbon nanomaterial dispersions in DI water; (b) schematic view of the fabrication process of the multifunctional devices; (c−f) photographs of our flexible, stretchable, and wearable multifunctional devices; (g) SEM image related to the surface of the CC and (h) carbon nanoparticles coated on a single cotton fiber; and (i) SEM image related to the surface of the CW fabric and (j) carbon nanoparticles coated on a single wool fiber.

production of conductive natural fiber fabrics. Therefore, sandwich-structured stretchable multifunctional wearable devices based on conductive cotton (CC) and conductive wool (CW) fabrics are developed. We chose cotton and wool fabrics because they are both commonly used natural materials and possess good softness combined with flexibility.43−45 To the best of our knowledge, there have been very few studies on

[sodium dodecylbenzenesulfonate (SDBS)]. This coating technique provides continuous homogenous dispersions with high energy for chemical and mechanical bonds between the well-suspended particles and the surfaces of the natural fibers.42 Herein, we follow our previously reported method of coating carbon nanomaterials onto the surface of natural materials.42 This method is simple, cost-effective, and scalable for mass 20846


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Figure 2. Electromechanical performance of our strain sensors: (a) relative changes of resistance vs different strains under cyclic stretching−releasing at a displacement rate of 3 mm·s−1 for CC strain sensors and (b) for CW strain sensors; (c) relative resistance variations under cyclic stretching− releasing with a strain of 75% at displacement rates of 3, 5, and 10 mm·s−1 for our CC strain sensors and (d) for our CW strain sensors; (e) relative resistance changes for our strain sensors up to 150% of tensile strain; (f) hysteresis curves of five stretching−releasing cycles up to various tensile strains for CC strain sensors and (g) wool strain sensors; and (h) performance of our CW strain sensors under 1000 cyclic stretching−releasing up to 75% of tensile strain at 15 mm·s−1, indicating their stability and durability.

the curing process. Figure 1c−f illustrates our flexible and stretchable multifunctional wearable devices based on CC and CW fabrics. The scanning electron microscopy (SEM) images from the surface of the neat fabrics and their single fibers are shown in Figure S1. The SEM images of the coated fabrics revealed that the hybrid of GNPs and CB particles could cover the surfaces of the CC fabric and its single fiber, as shown in Figure 1g,h, respectively. Covering the surface and its single fiber leads to the formation of compacted networks of conductive particles within the fabric. Compacted conductive networks of particles were mainly formed within the CW fabric (Figure 1i) and on its single fiber (Figure 1j). The hybrid of GNPs and CB particles firmly adhered to the surfaces of the conductive fabrics and their single fibers via mechanical interlocking and hydrogen bonding. The SEM observations clearly confirm the presence of closely compacted carbon nanoparticles on the surfaces and within the fibers, resulting in a low level of surface resistance for the fabrics coated via ultrasonication. The low level of electrical resistance and cost effectiveness of the CC and CW fabrics make them superior materials to be sandwiched within two layers of Ecoflex as a lightweight and highly stretchable elastomer to develop high-performance wearable strain sensors. Before conducting electromechanical tests, the initial resistance of the fabricated CC and CW strain sensors was measured using a two-point probe method. The initial resistances of the samples were found to be 1.95 ± 0.31 and 1.22 ± 0.15 kΩ, respectively. It can be noticed that the overall resistance of the CC and CW fabrics increased when they were encapsulated within Ecoflex layers. This could be due to the penetration of the liquid elastomer within the yarns of the fabrics and coatings, as demonstrated in the cross-sectional SEM images of CW and CC strain sensors (Figure S2). The performance of our strain sensors was investigated under five stretching−releasing cycles at a displacement rate of 3 mm·

exploiting CW fabrics as cost-effective natural-based active materials for stretchable and wearable electronics. Moreover, a research gap exists in developing CW- and CC-based multifunctional devices that can simultaneously function as wearable, stretchable, and sensitive strain sensors as well as high-performance wearable heaters with the aid of hybridized GNPs and CB as active materials. In this paper, the development of multifunctional devices through a simple, cost-effective, and environmentally friendly process using CC and CW fabrics is presented.

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RESULTS AND DISCUSSION Figure 1a schemes the process of preparing the dispersion based on the hybrid of GNPs, CB particles, and SDBS. The trimmed dog-bone-shaped cotton and wool fabrics, 100 mm in length and 6 mm in width, were placed in a bath sonication (60 Hz) in the presence of a well-suspended hybrid of GNPs and CB particles in DI water. Afterward, the coating process started by turning on the bath sonication and setting the deposition time to 20 min. In this process, the carbon nanoparticles could be homogenously dispersed in DI water and deposited on the surfaces of the fibers (refer to the Experimental Section for detailed information). After drying, the surface resistance of each sample was measured five times using a two-point probe method between both ends of the sample. The surface resistances of the CC and CW fabrics were found to be 286.54 ± 24.23 and 232.15 ± 35.21 Ω, respectively. The sandwich-structured multifunctional devices were fabricated, following the procedure exhibited in Figure 1b. First, a thin layer of cured Ecoflex as a substrate was fabricated, and then, the dried conductive fabrics were placed on top of the substrate. Afterward, the fresh mixture of Ecoflex was poured on top of the assembly in the mold to reach the final thickness (4 mm for strain sensors and ∼2.5 mm for heaters). Finally, the multifunctional devices were removed from the mold after 20847


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ACS Applied Materials & Interfaces s−1. Both CC and CW strain sensors showed a noise-free response accompanied by negligible drift for various applied strains (25, 50, 75, and 100%), as shown in Figure 2a,b. One can clearly observe the ascending pattern of resistance changes of the strain sensors as the applied strain increases. The CC strain sensors showed a relatively high resistance change at a specified applied tensile strain, indicating their higher sensitivity. For instance, our CC and CW strain sensors showed about 180 and 60% of resistance change in response to 100% of applied tensile strain, respectively. This behavior can be explained as follows: it can be clearly noticed from the SEM observations that conductive particles could penetrate through the fabrics and cover the fibers (Figure 1h,j). Furthermore, a strong conductive network was formed on the surface of the cotton fabric after the coating process, while particles tended to penetrate within the wool fibers. This could be due to the higher porosity of the wool fabric compared to the cotton fabric (Figure S1). The cracks within the percolative networks of carbon particles on the top surface of CC fabric can be clearly observed in Figure 1g. Therefore, the initiated cracks can gradually propagate by increasing the applied tensile strain on the CC strain sensors, resulting in a higher relative change of resistance for these strain sensors compared to the CW strain sensors. It is important to explain the resistance change pattern of our strain sensors, which can be classified into three various regions. In the first region, the resistance increased with the applied tensile strain, whereas in the second, the resistance slightly decreased, followed by a slight increase in the third region. Finally, the resistance of the strain sensors decreased when they were unloaded. Our strain sensors showed a behavior similar to the recently reported characteristics of sensors made with conductive textiles.46,47 This phenomenon could be interpreted by understanding the deformation of the weft-knitted structure of our CC and CW fabrics under tension. Figure S3 indicates a part of the conductive weft-knitted structure where coated particles on the surfaces and within the fibers contact each other. Initially, the area of the contact points decreased as the applied tensile strain increased, resulting in an increment of the resistance in the first region. In this region, the growth of crack distance within the conducting layer on the surface of CC strain sensors under tensile strain additionally resulted in a larger relative resistance change compared to the CW strain sensors (Figure 1g). In the second region, the contact area and the force between the contact points were enhanced by increasing the tensile load, thereby reducing the contact resistance (Rc) and the overall resistance. This phenomenon may be supported by Holm’s contact theory (refer to the Supporting Information). With further increment in tensile strain, crack growth in the conductive coating networks as well as fracture was induced in the fabric structure. Crack formation resulted in a resistance increase of the strain sensors before removing the load (the third region).46,47 The electromechanical responses of our CC and CW strain sensors at different strain levels with displacement rates of 5 and 10 mm·s−1 are depicted in Figure S4. The response of our CC strain sensors to different displacement rates at a specified tensile strain (75%) is demonstrated in Figure 2c. The relative resistance change of our strain sensors exhibited almost constant values within the range 3−10 mm·s−1. Moreover, our CW strain sensors possess a similar behavior with regard to the same displacement rates (Figure 2d). The responses of our CC and CW strain sensors to stretching−releasing cycles up to

100 and 150% at various displacement rates (3−10 mm·s−1) are depicted in Figure S5. These results indicate that the performance of our strain sensors has no dependency on the displacement rates. The sensitivity of a strain sensor can be assessed by gauge factor (GF), which is defined as GF = ΔR/(R0·ε), where ΔR is the electrical resistance change, R0 denotes the initial resistance before any applied strain, and ε is the value of applied strain. Figure 2e depicts a typical plot of resistance change versus applied tensile strain up to 150% when the displacement rate was set to be 3 mm·s−1. Both types of the strain sensors exhibited a monotonic enhancement in resistance with the increment of applied strain. The plot can be divided into two linear parts. The CC strain sensors showed a higher resistance change and therefore higher GFs after about 125% of strain, whereas the resistance of CW strain sensors remained almost constant. This behavior could be due to the propagation of more cracks and fracture points within the conducting networks on top and within the CC fabric compared to the CW fabric. As mentioned earlier, cracks exist within the conducting layer on the top surface of the CC fabric (Figure 1g). The gap among the cracks gradually increases upon stretching. The large gap between the cracks and subsequent fracture of the cotton fabric can cause a dramatic increase in the relative resistance change after applying 125% of tensile strain. In contrast, the percolative networks of CW strain sensors were mainly within the fibers and did not show a dramatic change in their resistance even at a high level of tensile strain. It can be clearly noticed that the CC strain sensors outperform the CW ones in terms of sensitivity. The GFs ranged within 1.67−6.05 for CC strain sensors, whereas those of CW strain sensors were below 0.5. Overall, the sensitivity of our strain sensors was higher than those of some of the recently reported strain sensors (see Table S1).26,48,49 We conducted five stretching−releasing cycles at different strain levels (25, 50, 75, and 100%) at a displacement rate of 3 mm·s−1 to understand the hysteresis characteristics of our strain sensors (Figure 2f,g). Overall, negligible hysteresis in the response of the strain sensors was found up to 50% of strain; however, one can notice that further hysteresis is appeared as the applied percentage strain increases up to 100%. This could be due to the hysteresis characteristics of Ecoflex. At high levels of strains (75−100%), the original resistances of both types of our strain sensors were fully recovered at the end of each cycle. Figure 2h depicts the response of our CW strain sensors under 1000 cycles of stretching−releasing up to 75% of tensile strain at a displacement rate of 15 mm·s−1. Prior to this test, our sample was subjected to rupture training (up to ε = 200% for five cycles). This could result in fractures within the structure of the fabric and boosting the resistance change level of our CW strain sensors. This increased their GFs to a maximum level of 4.26. Our CW strain sensors could display stable properties for 1000 cycles with an insignificant drop in resistance change profile throughout the test. The patterns of the resistance change profile remained similar during the test, as indicated in Figure 2h. A similar test was carried out for our CC strain sensors under the same conditions. There was a significant increase in the resistance change value, leading to the GFs of 20.4 in the last cycles, as shown in Figure S6. Unlike CW strain sensors, CC sensors exhibited a higher resistance change. This could be due to the formation of more cracks within the conducting layer on the top surface of the CC fabric (Figure 1g) and evolution of the existing fractured areas within the CC 20848


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Figure 3. Detection of large-strain human motion (finger movements) by our strain sensors; (a) plots showing the response of a CW strain sensor to the fast movements of each finger’s joints, (b) plots indicating the response of our CW strain sensors to bending-holding of each finger with regard to the joint degree of bending, (c) plots showing the response of a CC strain sensor to the fast movements of each finger’s joints, (d) plots indicating the response of our CC strain sensors to bending-holding of each finger with regard to the joint degree of bending, (e) detection of knee joint movements by our strain sensors, and (f) monitoring the wrist bending movements by our strain sensors.

resulting in a slight increase in resistance of the strain sensors; afterward, it was fully bent such that a further increment in the resistance profile appeared. Finally, the resistances of the strain sensors recovered their original values when the finger was straightened. The sensors were able to fully detect the largestrain movements caused by bending the knee and wrist of the volunteer (Figures S7c,d). Figure 3e,f demonstrates the resistance change profile of the strain sensors similar to those obtained for finger joint movements. We evaluated the sound signal recognition from a speaker by our strain sensors. For this test, both types of strain sensors were attached by a piece of strong double-sided tape on a speaker and a DMM was connected to the strain sensor to record the data. Figure S8a illustrates the corresponding resistance change signals generated by CW and CC strain sensors when a unique track was played on the computer. Both strain sensors responded synchronously to the sound wave of the played track, and most of the characteristic peaks were recognized. More distinct output signals for CW strain sensors can be observed compared to those of CC strain sensors. This behavior could be due to the higher sensitivity of CW strain sensors at very low strain levels compared to that of the CC strain sensors. The response of both strain sensors under a low tensile strain level but at a high displacement rate (10 mm·s−1) was investigated. High displacement rates could induce sudden acceleration on the strain sensors and simulate their performance when detecting the music beats. The plots of relative resistance change versus applied tensile strain (up to 0.5%) for both CC and CW strain sensors are shown in Figure S8b. It can be noticed that the plots for CW and CC strain sensors can be divided into two linear regions with GFs of 7.23 and 3.17 between 0 and 0.057%, and 1.89 and 1.84 between 0.057 and 0.5%, respectively. Higher GFs for CW strain sensors under very low strain level (0.05%) show their ability to detect minor beats in a played music track with higher sensitivity, leading to higher peaks in the output signals. As shown in our experiments, these devices detected various signals caused by

fabric after the rupture training. This resulted in a gradual increase of the relative resistance change for the CC strain sensors. Nevertheless, the CW fabrics that possess higher stiffness showed a consistent relative resistance profile. This indicates that there were fewer cracks in their structures during rupture training and durability testing. From our results, it can be summarized that our CC-based strain sensors offer higher sensitivity and lower hysteresis compared to the CW-based strain sensors. Both strain sensors show excellent durability and large stretchability; however, the CW strain sensors show higher stability in the results. Both strain sensors showed no dependency on the displacement rate of applied tensile strain; therefore, their application in monitoring various human movements was investigated to analyze their performance as wearable strain sensors. The movements of different finger joints were monitored by both types of strain sensors attached on the surface of a laboratory-grade nitrile glove on a volunteer’s finger. A piece of strong double-sided tape (3M) was used to make sure that they would not slide during finger movements (Figures S7a,b). The resistance data points were stored every 25 ms through the connection of a digital multimeter (DMM) to both ends of our strain sensors. The responses of CW and CC strain sensors to the fast bending movements of the various fingers are depicted in Figure 3a,c, respectively. Similar to the results of cyclic stretching−releasing tests, the resistance change profiles show an increase in resistance with bending fingers. This was followed by a slight decrease due to the drop in contact resistance (Rc) when the fingers were bent further. A further increment in resistance appeared when the fingers were fully bent before the final drop in resistance due to straightening of the fingers. A similar trend was observed for all related resistance change profiles in response to the finger bending at a faster rate. The relative resistance changes of our CW and CC strain sensors under different bending angles for each finger are shown in Figure 3b,d, respectively. Initially, the volunteer’s finger was slightly bent (θ < 45°) and held for a few seconds, 20849


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Figure 4. Time-dependent temperature changes of (a) CW and (b) CC wearable heaters under various input powers (0.32, 0.72, 1.28, and 2.05 W); (c) cyclic electrothermal performance of (c) CW and (d) CC heaters under zero tensile strain under an applied current of 50 mA, showing the heating stability of our wearable heaters; (e) elevation of the temperature of a CW strain sensor (resistance ≈ 1.2 kΩ) under stepwise strains of 0− 40% at a constant dc voltage of 50 V; and (f) elevation of the temperature of a CC strain sensor (resistance ≈ 1.9 kΩ) under stepwise strains of 0− 50% at a constant dc voltage of 50 V.

show that these devices are suitable for practical applications as wearable heaters. The cyclic electrothermal test for these heaters under a similar constant applied current was performed for more than 10 cycles (Figure 4c,d). The initial temperature of the devices was set to 30 °C by running a trial cycle. The heaters were then Joule-heated for 30 s under an applied current of 50 mA, followed by a natural cooling process to the initial temperature (30 °C). The CC heaters showed a stable dynamic electrothermal behavior with the maximum temperature of approximately 70.5 °C at each cycle by an input power of 3 W, whereas the CW heaters showed a lower surface temperature (∼40 °C) during Joule heating at each cycle because of a lower level of input power (1.25 W). The difference in the input power is due to the variation in the resistance level of the heaters. The natural cooling process to the initial temperature took approximately 150 and 90 s in each cycle for CC and CW heaters, respectively. The natural cooling process was mainly from the surface of Ecoflex, and hence an almost identical cooling time for each cycle of each heater was found. Further investigations on these multifunctional devices were carried out to understand the effect of applied tensile strain on their electrothermal behavior. For this purpose, we utilized our highly stretchable CC and CW strain sensors with the initial resistances of 1.95 ± 0.31 and 1.22 ± 0.15 kΩ, respectively, to stretch them on a motorized moving stage while they were subjected to an input voltage of 50 V. Simultaneously, the surface temperature was monitored using a thermometer. Figure 4e presents the temperature profile of a CW strain sensor under 0, 20, and 40% of strains. It can be noticed that the maximum surface temperature reached 91.6 °C at 0% strain; the surface temperature of the CW strain sensor increased to 100.7 °C by 20% of tensile strain. Further tensile strains on the sample up to 40% resulted in the maximum surface temperature of 114.6 °C. On the other hand, the temperature profile of a CC strain sensor showed the maximum

the movements of various spots on the human skin, suggesting their human−machine interface capability. Another interesting application of our multifunctional devices is their potential to be used as wearable heating elements. For the purpose of manufacturing a heater, a fabrication process similar to that of our wearable strain sensors was used, although the thickness of the heater devices was reduced to ∼2.5 mm. The SEM images from the cross section of our wearable heaters clearly show the relatively lower thickness of the heater devices compared to that of our strain sensors (Figure S2c,d). The initial resistances of the CW and CC wearable heaters between the both ends of the samples were found to be 0.52 ± 0.03 and 1.19 ± 0.04 kΩ, respectively. Prior to electrothermal testing, thermogravimetric analysis (TGA) for CC and CW fabrics was conducted to understand the decomposition temperature of the fabrics (refer to the Supporting Information, Figure S9). Figure 4a,b indicates the typical time-dependent surface temperature changes of the CW and CC heaters under constant input powers of 0.32, 0.72, 1.28, and 2.05 W for 10 min. The plots can be divided into three phases, which include elevating temperature (heating process), steady-state temperature (equilibrium), and decaying temperature (cooling process). It can be noted that the CW and CC heaters could reach their steady-state temperature in less than 150 s with the highest input power of 2.05 W. It is worth noting that the presence of the elastomer could delay the heating stage reaching the steadystate temperature because of its low thermal conductivity.47 After the steady-state stage, the power supply was switched off, allowing the heaters to naturally cool down to ambient temperature for 5 min. Almost similar maximum temperatures of 93.4 and 97.2 °C attributed to the CW and CC heaters with an applied input power of 2.05 W were obtained. The slight differences in the maximum temperatures of CC and CW heaters for each input power could be due to the slight differences in the thickness of the active materials. The results 20850


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ACS Applied Materials & Interfaces temperatures of 78.8, 80.9, 83.7, and 90.1 °C under applied tensile strains of 0, 10, 30, and 50%, respectively. Our results indicate the stable electrothermal characteristics of the multifunctional devices under various applied tensile strains with a relatively small variation, as shown in Figure 4f. In contrast to the recently reported stretchable heaters,50,51 the surface temperature of our multifunctional devices was elevated under applied tensile strains. The increase in the temperature of our stretchable multifunctional devices could be due to the resistance variation patterns under external tensile load, as outlined previously. Most of the previous studies in the literature reported decaying temperatures under applied tensile strains, whereas our multifunctional devices show almost stable electrothermal performance under large tensile strains. Overall, the electrothermal performance of our wearable heaters is comparable to those of recently reported ones (see Table S2). Our CW and CC heaters were also fixed onto a volunteer’s wrist to demonstrate the distribution of temperature of our wearable heaters with the aid of an infrared camera (Figure S10). Both heaters showed a homogenous temperature profile on their surfaces after reaching a steady-state temperature. However, higher temperatures can be observed around the location of electrodes because of the generated heat by the contact resistance between the electrodes and heaters. Figure S11 indicates the tendency of Tmax to the increment of input power. A linear trend between the Tmax and the input power values (Pin = V2/R) can be seen. Here, Pin is the value for the input electric power for each input voltage (V) and R represents the electrical resistance between the electrodes.52 By fitting a tread line, Tmax can be well defined with regard to the input power using eqs 1 and 2, for CC and CW heaters, respectively: Pin = 35.15(Tmax ) + 25.91

(1)

Pin = 34.15(Tmax ) + 24.14

(2)

liness of our multifunctional wearable devices, we believe that these wearable devices can contribute to the development of industrial fields related to flexible electronics.

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EXPERIMENTAL SECTION

Preparation of Materials. In this research, commercially available weft-knitted cotton (measured fabric thickness ≈ 0.55 ± 0.05 mm, measured yarn diameter ≈ 223.9 ± 27.4 μm, measured fiber diameter ≈ 15.1 ± 0.8 μm, and area density ≈ 0.22 kg/m2) and wool (measured fabric thickness ≈ 0.7 ± 0.06 mm, measured yarn diameter ≈ 509.7 ± 34.1 μm, measured fiber diameter ≈ 49.8 ± 4.6 μm, and area density ≈ 0.38 kg/m2) fabrics were locally purchased as products of ALSCO and Inter-weave Ltd, New Zealand, respectively. GNPs were provided by EMFUTUR, Spain, with the nominal sizes of 5 nm and 5 μm in thickness and width, respectively. In addition, CB, with an average particle size of 30 nm, was purchased from Degussa, Germany. We used DI water as an eco-friendly and chemical-free solvent. To disperse carbon nanomaterials in DI water, SDBS, as a proprietary product of Sigma-Aldrich, was employed. The two components of Ecofelx (0030) were obtained from Fiberglass shop, New Zealand, as products of Smooth-On Inc., USA. We chose Ecoflex (0030) as a skinsafe, super soft, highly stretchable (εbreak = 900%), and very strong (tensile strength ≈ 1.38 MPa) elastomer with a specific gravity of 1.07 g/cm3, as it possesses suitable properties for fabrication of multifunctional wearable devices.49 Fabrication Process of Conductive Natural Fabrics Using Ultrasonication. The hybrid of GNPs/CB dispersion was prepared in accordance to our previous study.42 The schematic view of the preparation of the dispersion is shown in Figure 1a. In brief, a 1 wt % hybrid system of GNPs and CB (1:1 by weight) well suspended in DI water was prepared with the SDBS to the hybrid of GNPs and CB ratio of 1:3. We also tried a ratio of 2:3; however, the values for sheet resistance of the conductive fabrics were higher. First, the combination of 5 g of GNPs and 5 g of CB was bath-sonicated for 30 min in DI water (∼1 L). Next, SDBS (3.33 g) was added to the dispersion and stirred for 4 h at 60 °C. Finally, the dispersion was bath-sonicated for another 2 h to obtain a more homogenous dispersion. To fabricate conductive fabrics, pieces of cotton and wool fabrics were trimmed in course direction into a small-scale dog-bone shape (Figures 1c and S3). Here, a novel coating technique using an ultrasonication bath (Techspan S30H, 100 W, 60 Hz) was utilized for the coating process of pristine trimmed fabrics according to our previous study.42 The pristine cotton and wool fabrics with a shape similar to dog bone (Figure 1b) with a planar area of 100 mm × 6 mm were put into the ultrasonication bath loaded with our well-suspended hybrid of GNPs/ CB particles in DI water. We coated the fabrics for 20 min to obtain highly conductive fabrics.42 Finally, the coated fabrics were removed from the bath and placed on a piece of tissue to remove the residual water and then fully dried in an oven at 110 °C for 1 h. Fabrication of Highly Stretchable and Flexible Multifunctional Devices. Figure 1b illustrates the fabrication process of the proposed multifunctional wearable devices based on the CC and CW fabrics. First, the two components of Ecoflex were thoroughly mixed with a ratio of 1:1 (by volume). Next, the mixture was poured into a mold up to 1.5 mm in thickness. For curing, the mold was moved into an oven and maintained at 90 °C for 45 min. Following cure, the conductive dog-bone-shaped fabrics were attached on the upper surface of the Ecoflex substrate, and then, a fresh mixture of Ecoflex was added from top of the fabrics to fill the mold with a thickness of 4 mm. Finally, the prepared devices were demolded after curing under similar conditions (45 min at 90 °C). The stretchable and flexible multifunctional devices are shown in Figure 1c−f. In the case of wearable heaters, the final thickness of the devices was lowered to be ∼2.5 mm to reduce the level of electrical resistance of the devices. Characterization of the CC and CW Fabrics. The study on the surface morphology of the CC and CW fabrics was carried out using SEM (Hitachi SU-70) images. Platinum sputtering was conducted on the samples prior to SEM analysis. The surface resistance of the samples after drying was measured using a two-point probe method

Both heaters showed excellent electrothermal behavior working at a low range of input power. An almost similar electrothermal behavior was observed by both heaters in terms of maximum steady-state surface temperature by identical input power. The absence of any major variations in maximum and minimum temperatures in dynamic heating performances demonstrates the high stability of both heaters. On the basis of our results, we believe that our multifunctional devices have great potential as stretchable and flexible heaters for wearable thermal-therapy and heating element devices.

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CONCLUSIONS In summary, a simple, cost-effective, and scalable technique to develop highly stretchable multifunctional wearable devices using electrically conductive natural materials by an ultrasonication bath has been presented. Strain sensors with GFs up to 20.4 and high stretchability (applied strain more than 100%) can be achieved by encapsulating CC and CW fabrics between an elastomer. Our strain sensors possess stability, low hysteresis characteristics, good sensitivity, and high durability at high strain levels. They can also detect various human movements and sound waves, making them suitable for human−machine interaction electronics. Our stretchable heaters can reach up to 103 °C with an applied voltage of 20 V. Moreover, our wearable heaters show excellent electrothermal performance under an applied tensile strain up to 50%. Considering the simple fabrication process, cost effectiveness, and environment friend20851


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ACS Applied Materials & Interfaces from both ends of the samples five times using a DMM (Keithley DMM 2100). All measurements were taken at room temperature. TGA of the pristine and conductive fabrics was performed by a TA Instrument (Q5000 model) in a N2 atmosphere with a temperature ramp of 10 °C·min−1 up to 800 °C. Characterization of Our Stretchable and Wearable Multifunctional Devices. The electromechanical performance of the devices was evaluated by gripping the samples on a motorized moving stage to understand their sensing behavior in stretching−releasing cycles (Figure S12). The displacement rates and the applied strain profiles were controllable while the resistance data were stored using a DMM (Keithley DMM 7510). The applied cyclic tensile strains ranged from 25 to 100% while the displacement rates were set to be 3, 5, 10, and 15 mm·s−1 depending on the tests. The durability tests were carried out for more than 1000 stretching−releasing cycles under 75% of applied tensile strain on the same motorized moving stage. Prior to the durability tests, the devices were stretched up to 200% for five cycles, so the coated fabrics were partially fractured within the elastomer, resulting in a higher resistance change. In addition, the electrothermal behavior of the devices was characterized by applying various input voltages using a power supply (Agilent E3649A) and simultaneous monitoring of the resultant temperature on the surface of the wearable heaters using a thermometer (CENTER 309 data logger thermometer). An infrared camera (FLUKE Ti20) was used to observe the temperature over the surface of the devices. Moreover, the changes in temperature at a constant applied voltage (50 V using Agilent E3649A) between the both ends of the strain sensors (thickness of the samples = 4 mm) were monitored by using a thermometer while various strains were applied on the strain sensors by the motorized moving stage. All measurements were taken at room temperature.

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and all other staff and technicians at the Centre for Advanced Composites Materials (CACM) for their assistance. In addition, the authors would like to acknowledge the Ministry of Business, Innovation & Employment, New Zealand, for their financial support (grant number: UOAX1415).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04775. TGA results, electromechanical responses of our multifunctional devices, SEM images from the cross section of our multifunctional devices, photographs of our multifunctional devices detecting various human motions and sound, photographs captured by an IR camera from our wearable heaters, and additional data related to the electrothermal performance of our multifunctional devices (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +64-27-445 9880. ORCID

Hamid Souri: 0000-0002-7971-1045 Author Contributions

H.S. and D.B. proposed, planned, and supervised the research. H.S. performed the experiments and worked on the data analysis. H.S. and D.B. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Associate Professor K. C. AW, Dr. Tim Giffney, and Dr. Mahtab Assadian for providing the motorized moving stage, Morteza Amjadi and Ryan Calder for their helpful comments, Dr. Velram Balaji Mohan for providing carbon nanomaterials, Dr. Nam Kim for providing wool fabrics, 20852


Research Article

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