Carbon Nanotubes Core-Spun Yarn as High

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Polyurethane/Cotton/carbon nanotubes core-spun yarn as high reliability stretchable strain sensor for human motion detection Zifeng Wang, Yan Huang, Jinfeng Sun, Yang Huang, Hong Hu, Ruijuan Jiang, Weiming Gai, Guangming Li, and Chunyi Zhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08207 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 26, 2016

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ACS Applied Materials & Interfaces

Polyurethane/Cotton/Carbon nanotubes core-spun yarn as high reliability stretchable strain sensor for human motion detection Zifeng Wang1, Yan Huang1, Jinfeng Sun2, Yang Huang1, Hong Hu2, Ruijuan Jiang3, Weiming Gai3, Guangming Li*4 and Chunyi Zhi*1 1 Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China. 2 Institute of Textiles and Clothing, The Hong Kong Polytechnic University, 11 Hong Chong Road, Hong Kong, China. 3 Shenzhen Municipal Engineering Design & Research Institute Co., Ltd, Shenzhen, China 4 School of Mechanical, Electrical & Information Engineering, Shandong University, Weihai, China

Abstract Smart yarns and textiles are active field of researches nowadays due to their potential applications in flexible and stretchable electronics, wearable devices and electronic sensors. Integration of ordinary yarns with conductive fillers renders the composite yarns with new intriguing functions, such as sensation and monitoring of strain and stress. Here we report a low cost scalable fabrication for highly reliable, stretchable and conductive composite yarn as effective strain sensing material for human motion monitoring. By incorporating highly conductive single-wall carbon nanotubes (SWCNTs) into the elastic Cotton/Polyurethane (PU) core-spun yarn through a self-designed coating approach, we demonstrated that the yarn is able to detect and monitor the movement of human limbs, such as finger, elbow and even wink of eyes. By virtue of the covered structure of the Cotton/PU yarn and the reinforcement effect of SWCNTs, the composite yarn can bear up to 300% strain and could be cycled nearly 300,000 times under 40% strain without noticeable breakage. It is promising that this kind of conductive yarn can be integrated into various fabrics and used in future wearable devices and electronic skins. Keywords: conductive yarn, single-wall carbon nanotube, stretchable strain sensor, wearable devices, core-spun yarn

Introduction Detecting and monitoring the physiological states of human body is one of the most essential goals of various wearable electronics nowadays. A variety of sensors for detecting touch, pressure, temperatures and even perspiration of human beings have been developed recently.1-4 Among those, strain sensors with diverse working mechanisms have been developed for sensing physical movement of human beings. For example, Gong et al. fabricated flexible pressure sensor based on sandwiched ultra-thin Au nanowires filled tissue paper between two thin DMSO sheets.5 Interestingly, the sensor can detect pressure as small as 13 Pa and its sensitivity, defined as: ∆I/Ioff/∆P, was calculated to be more than 1.14 kPa-1. In addition, Pang et al. fabricated highly sensitive strain-gauge sensor materials with the ability to detect minimum pressure of 5 Pa, 1

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minimum shear force abour 10-3 N and 2x10-4 N m for torsion load, respectively, benefiting from the reversible interlocked nanofibers.2 Those works demonstrated that the fabrication of highly sensitive sensor materials is feasible. However, high sensitivity is not the only factor to be considered because of the highly complicated nature of human body and human motions. As we all know, human beings are capable of doing complicated and subtle motions by limbs and other organs, such as finger movements, eye winking and even swallowing. Therefore, in order to integrate materials on human body as wearable sensors, flexibility and stretchability, as well as comfort of wearing and reliability are indispensable requirements for the choice of materials. Indeed, successful integration of strain-sensitive wearable sensors on human body has been reported intensively.1, 3, 6-8 However, the devices suffer from different problems that limit their application as wearable sensors. For example, strain sensors utilize graphene as strain-sensitive material usually provide limited stretchability.6-7 By incorporating strain-sensitive materials, usually electrically conductive fillers into elastic polymer matrix, such as silicone elastomers and natural rubbers, flexibility and stretchability of the whole devices can be obtained.1, 6, 9-14 Kyun Kyu Kim et al even reported multi-dimensional strain sensor based on anisotropic metal nanowires and PDMS.15 However, owing to the nature of those polymeric materials, the composite sensor materials suffer from fatigue and show considerably insufficient cycling performance. In addition, although there were limited numbers of reports aiming at improving the reliability and long-term stability by introducing the self-healing property to the systems, they usually involve complicated fabrication process and it is not easy to realize 100% recovery in the long run.8, 16-17 On the other hand, advances in the textile industry provides a variety of yarns, fabric and cloth, which are ideal candidates to be utilized in wearable devices. However, their applications in wearable devices are impeded by the limited stretchability, sometimes relatively complicated manufacture processes and the cost. Although there are already reports of successful integration of yarn- or textile-based wearable sensors on human body, the device performances are generally limited by the poor stretchability (e.g. less than 30%) of the host material and complication in the fabrication process.18-19 Polyurethane (PU) is known for its elasticity and has been widely applied in the textile industry in the recent years for manufacturing various elastic yarns and fabrics.20-21 Consequently, PU-based composite yarns and fabrics are promising for obtaining wearable sensor with intriguing performances. In the present work, we describe a new type of resistive-type strain gauge sensor based on conductive and elastic core-spun yarn in which the sensor material shows both good stretchability up to 300% and highly reliable long-term cycling performance for nearly 300,000 cycles. More importantly, the simplicity of the self-designed fabrication process enables large-scale scalable production of the yarn, which is different from those that need sophisticated controlling of fabrication parameters or transferring techniques.9, 22-23 In general, the presented approach mimics and utilizes the general manufacturing process in textile industry so that the whole process is universal, which means the choice of materials can be greatly extended. Additionally, it is also found that the yarn sensor materials present tunable gauge factor, for example, the gauge factor can be increased from 0.31±0.06 to 0.65±0.04 by increasing the immersion times of Single-wall carbon nanotubes (SWCNTs). All those evidences demonstrate the flexibility and universality of our method and conductive yarns prepared by this method are highly scalable and cost-efficient. 2


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Materials and Experiments Materials SWCNT was purchased from Beijing BoYu GaoKe New Material and Technology Limited Corporation and used as conductive filler materials. Fabrication process of the conductive yarn As shown in Figure 1, the self-designed fabrication process can be described as follows: the elastic core-spun yarn was fabricated by entwining native cotton yarns around highly elastic urethane continuous filament. The elastic core fibers were stretched first for the consideration of maintaining the elasticity of the polyurethane fibers. Then the cotton sheath fibers were drawn subsequently, covering the twisted polyurethane fibers, forming the elastic core spun yarn, as shown in the schematics. Then the pristine core-spun yarn was ultrasonically washed by acetone and absolute ethanol as well as distilled water successively for 10 min to remove surface contaminants. After drying in air, the yarn was wound on a cylindrical tube prior to use. In a typical fabrication method, 1g SWCNTs and 3g dispersive agent were added into a 200 ml beaker followed by the addition of 100 ml distilled water. The suspension was thoroughly sonicated by tip-sonicator JY92-IIN (Scientz Biotechnology) for 4h. After the sonication, the SWCNTs suspension showed totally black color and 50 ml suspension was poured into stainless steel pan as coating bath. The dried yarn was wound via winding machine and passed through the SWCNTs bath and two strong hot wind dryers were used to quickly evaporate the water of the CNT ink coated on the yarn when the yarn passed through. On the other side, the coated yarn was collected by another cylindrical tube on the winding machine. The whole process was repeated several times to ensure sufficient CNT coated on the yarn.

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Figure 1. Schematics illustration of the self-developed fabrication process of the PU/Cotton/CNT yarn.

Characterization and Sensing Performance Test Scanning electron microscope (SEM) (JEOL JSM 820) and field-emission SEM (JOEL JSM 6335F) were used to investigate the morphology of the yarn. Sensory performance test was conducted on Stanford Research System and LabVIEWTM system. Basically, a strip of yarn was cut and fixed onto the vibration exciter (JZK-2 Sinocera Piezotronics, Inc.). Then a constant voltage provided by DC power supply (UTP3705S) was exerted on the head and tail part of the yarn. During the test, the frequency and amplitude of vibration of the vibration exciter were controlled by YE1311 Signal Generator and YE5871A Power Amplifier, respectively. The current evolution was recorded by MODEL SR570 Low-Noise Current Preamplifier and viewed through the LabVIEW software.

Results and Discussions The composite yarn repeatedly coated with SWCNTs shows conductive nature after several times of immersion. Figure 2 (a) schematically shows the morphological change of the yarn after being stretched. The stretchability and the electrically resistive response of CNT-coated core-spun yarn are shown in Figure 2 (b) and (c), in which the yarn dipped 8 times is stretched for 100% elongation and the electric resistance measured by Ohmmeter changes from 61.7 kOhm to 100.6 kOhm. This demonstrates that the electric resistance of the yarn exhibits a positive correlation with strain, i.e., increases with increasing strain level, which allows us to further investigate its strain sensing property. 4


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Figure 2. (a) Schematics illustration showing the stretchability and the morphological comparison before and after stretching; (b) and (c) Pictures show the stretchability of the yarn as well as the corresponding electric resistance before and after stretching (in kOhm). In order to systematically investigate the performance of the yarn sensor, we fabricate series of yarn with incrementing coating times, from 1 to 12 min, and then elaborately characterize them. The morphological structure of pristine polyurethane/cotton core-spun yarn is characterized by SEM. Figure 3 (a) and (b) show that the yarn is woven by three intertwined elastic polyurethane core fibers revolved by bunches of thin cotton fibers. The red arrows in Figure 3 (b) point out the intertwined polyurethane core fibers when the surrounding cotton fibers are carefully stripped. It can be observed from the SEM image of pristine yarn that all the cotton fibers are loosely woven and individual fiber shows nearly no physical bonding with each other. This design gives rise to the stretchability of the core-spun yarn, in which the loose structure facilitates the stretching and recovery of the elastic polyurethane core fibers. On the contrast, after coated with CNT, the yarn shows different morphology compared with pristine yarn. In particular, the originally separated cotton fibers are interconnected by the CNT network and the polymeric dispersive agent. As seen from Figure 3 (c), there are obvious adhesion caused by CNT network and polymer dispersing agent between individual fibers. Magnified SEM observation in Figure 3 (e) and (f) provide better evidence of the existence of physical bonding between fibers and the network of interleaved CNTs (red arrows). It is believed that the polymeric dispersive agent used in this study works as glue that strongly bind CNT and yarn fibers together. By virtue of the excellent adhesion of CNT network on the yarn surface, the sensor materials function well after extremely long-term cycling test, which will be further presented in the following content. When being stretched, however, those disorganized cotton fibers become more or less helically oriented towards a certain direction, which can be seen from low magnification SEM image in Figure 3 (d). It shall be rational to expect an improvement in mechanical properties of the CNT-coated yarns, such as modulus and strength as well as increased strain at break, of which similar works have been reported extensively previously.24-26

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Figure 3. (a) Typical SEM image of the pristine elastic PU/Cotton yarn; (b) Typical SEM image of the PU/Cotton yarn, showing the elastic PU fibers are encapsulated by cotton fiber; (c) Typical SEM image of the 8th immersed PU/Cotton yarn; (d) Typical SEM image of the stretched 8th immersed yarn; (e) Localized SEM image of 8th immersion yarn, showing the physical bonding between individual cotton fibers; (f) Localized SEM image of 8th immersed yarn, red arrows shows the existence of carbon nanotubes. Figure 4 (a) shows the evolution of electrical resistance with respect to immersion times in CNT ink. As the diagram suggests, the average electrical resistivity of the yarn witnesses a dramatic decrease from around 480 kOhm/mm to 1.68 kOhm/mm as the immersion time increases from 1 to 6. However, the decreasing trend of average electrical resistivity seems no longer exist as we keep coating the yarn for 6 more times, reflecting as the fluctuation trend in Figure 4 (a). This could be possibly understood as the gradual decrease of the concentration of CNT in the dispersion as we continuing the coating process, which in return results in less CNT actually coated on the yarn. As a result, the electrical resistivity of yarn immersed for 8 times, denoted as PU/Cotton/CNT_8, and yarn immersed for 12 times, denoted as PU/Cotton/CNT_12, are comparable. These two yarns are chosen for further investigation. In order to understand the electrical resistance evolution characteristics of the conductive yarns upon stretching and recovery, we then test the overall strain-resistance response of the two kinds of yarns. In a typical test, a strip of yarn is pulled till break under an average strain rate of 1.55 mm/s and its electrical resistance is recorded simultaneously. The performance of the yarn being tested is expressed as gauge factor defined as the relative change in resistance with respect to strain: ∆ GF ∆

As shown in Figure 4 (b), PU/Cotton/CNT_8 and PU/Cotton/CNT_12 yarns exhibit different characteristics in terms of relative change in resistance when being stretched. PU/Cotton/CNT_12 series show much higher tolerable strain, up to 300%, which is nearly doubled compared with those of PU/Cotton/CNT_8, corresponding to up to 150% strain. Besides, it can be obviously seen that PU/Cotton/CNT_12 series exhibit relatively larger response when being stretched, reflecting 6


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as 2-3 fold greater change in resistance, compared with those immersed for 8 times. As a result, the average calculated strain gauge factors, which are the slope of the curves, for PU/Cotton/CNT_12 is 0.65±0.04 while that for PU/Cotton/CNT_8 is 0.31±0.06. The values represent the resistive response of the linear part of the curves, ranging from less than 25% strain to about 240% strain for 12-time-immersed yarns and ranging from less than 25% strain to about 150% strain for 8 time-immersed yarns. This corresponds to the monotonic increase of resistance with respect to increasing strain. The difference in the strain-relative resistance change relation between PU/Cotton/CNT_8 and PU/Cotton/CNT_12 can be ascribed to the coating times. It is expected that the more coating times, the more dispersive agent would finally be coated on the yarn fibers. During stretching and releasing cycles, the ‘glue’ provides extra stretching ability of the yarn so that there is a clear difference in stretchability between the two yarns. The fluctuations and dropping trend of resistance in the test from yarns of 12th series under high strain value, i.e., above 240% strain, are possibly resulted from the realignment of the conductive fillers and contacting of initially loosely packed yarn fibers under highly stretched state.27 Average Resistance/mm (KOhm/mm)

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Figure 4. (a) Evolution of electrical resistance of the PU/Cotton/CNT yarn vs immersion times; (b) Relative change in resistance of 8th and 12th immersed yarn vs strain (%), the relative change in resistance is defined as: ∆R/R ; (c) Aging performance tests of the 8th immersed 0

PU/Cotton/CNT yarn, the curves show the relative change in current at 40th-270,000th cycle respectively, relative change in current in defined as: ∆I/I0. On the other hand, the resistive response of both PU/Cotton/CNT_8 and PU/Cotton/CNT_12 series show similar pattern when being exerted with 0-25% strain, as seen from Figure 4 (b). The average calculated gauge factors for strain value in this range for PU/Cotton/CNT_8 and PU/Cotton/CNT_12 are 1.42±0.56 and 2.15±0.36, respectively. Given the overall performance, our strain sensor is more promising than conventional metallic strain gauge, which exhibits small testable strain range (about 5% maximum strain), and conductive elastomer strain gauge, which exhibits up to 70-80% strain although they have gauge factors ranging from 2-20.9, 28-29 Moreover, we have obtained considerately higher gauge factors compared with the fabricated strain sensor based on CNT and PDMS,1 by which gauge factors of 0.82 (under 0-40% strain) and 0.06 (under 60-200% strain) were obtained. Apart from gauge factors, the durability of the yarn, reflected by the cycling performance, is another key characteristic of sensor materials. To evaluate the cycling performance of the sensor, PU/Cotton/CNT_8 series are chosen for the long-term cyclic tests. In order to mimic the low-frequency and irregular nature of human movement, such as walking, the frequency of the 7



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vibration is set at 4 Hz while a moderate working strain of 40% is chosen as testing strain.30 Due to the incompressible nature of the yarn, a pre-strain for each of the test is applied. We have tested the yarn sensor continuously for more than 100,000s, which corresponds to more than 400,000 cycles and the yarn is still functioned well without any observable damage. However, as limited by the data storage capacity of the system, only as much as 67,864 seconds of testing, corresponding to 271,456 cycles of testing can be recorded at one time. As shown in Figure 4 (c), the relative change in current ∆I/I0 was recorded with respect to testing time and the yarn was continuously tested for nearly 300,000 cycles. One second of cycling that contains 4 repeated stretching-releasing cycles at around 40th cycle, 400th cycle, 4,000th cycle, 40,000th cycle, 100,000th, 200,000th cycle and 270,000th cycle are representatively shown in the diagram. However, it should be noted that ∆I/I0 as a function of time experiences a slight increase from 40th cycle to 40,000th cycle, as the amplitude of the curves are enlarged. After 40,000 cycles of testing, however, the performance gets stabilized, as the amplitude of the subsequent testing curves are no more fluctuated. Similar phenomenon has been reported by Darren J. Lipomi and colleagues, where they fabricated an elastic transparent thin film filled with CNT as strain sensation material.31 After being stretched and released for 12,500 cycles at 25% strain, there is fluctuation in resistance observed, which could be attributed as the ‘programming effect’ that the nanotubes adopt their preferable morphology or orientation to facilitate to strain. Also there would be disconnection between conductive CNT networks as a result of long-term cycling. In order to provide further evidence to support the theories, SEM images of the yarn before and after the cycling tests are took and compared, as seen from Figure S2 (a) and (b). It can be clearly observed from Figure S2 (b) that detachment of CNT and wrapping polymer on adjacent cotton fibers exist, which leads to a certain extent of disconnection and separation of CNT conductive networks. Inset figure in Figure S2 (b) shows the morphology of the yarn at the middle point of the yarn being tested. Apparently, the yarn is been tightly stretched at the center after long-time stretching-releasing cycles, which is different from the loose morphology of the yarn before the tests in Figure S2 (a) inset. Although there is disconnection and loss of conductive network after long-term stretching-releasing tests, it is insignificant as the material is still well functioned and the responsive sensory signals are even more stable, as seen from curves after 40,000th cycling test in Figure 4 (c). The process is an aging process necessary for obtaining the optimized working condition for the sensor material. Additionally, the polymeric dispersive agent, acting as glue, provides CNT with strong interfacial bonding on the yarn surface so that the electrical conductivity of the yarn can be well-maintained even after extremely long-term cycling test. Later tests and investigations are based on aged yarn materials showing stabilized cycling performance.

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Figure 5. (a)-(b) Typical exciter sensory test results of Cotton/PU/CNT_8th at 5%-100% strains, respectively; (c) Shows the one period of typical sensory test results of Cotton/PU/CNT_8th at 4-15Hz frequencies, respectively. Figure 5 shows the sensory response of the yarn at different strains. Basically, PU/Cotton/CNT_8 series of yarns are tested by exciter at 4 Hz. The strains are exerted by changing the length of the yarn being tested. Due to the limitation of experimental setup, the maximum strain being tested is 100%. As seen from Figure 5 (a) and Figure 5 (b), three seconds results representing 12 cycles of stretching-releasing tested at increasing strain values, from 5% to 100%, are presented with respect to relative change in current. According to the amplitude of relative change in current in each of the test, it can be estimated that the relative change in current shows roughly monotonic increase as the strain value increases, which is in consistency with the trend showing in Figure 4 (b). Additionally, the sensory signal as a function of time shows good repeatability and asymmetric characteristics. At each of the test, the current signals show periodic maxima and minima values in accord with the physical changes of the yarn during the test. Before being stretched, the electrical resistance of the yarn is of minimum value, corresponding to the maximum or the peak value of current, while the electrical resistance reaches maximum upon being stretched to a certain strain value, which in return corresponds to the minimum value in current signals. The signals show good repeatability by virtue of the excellent recoverable stretchability of the yarn so that both the physical and electrical states of the yarn can be nearly totally recovered. As a result, even at 100% testing strain value the sensory signals of the yarn show highly consistent characteristics. The asymmetric characteristics of the current signals can be ascribed to the incompressible nature of the yarn so that the current signals of the yarn cannot be further increased by compression. All those characteristics mentioned above show that the electrical signal evolutions can directly reflect the physical changes on the yarn, which is essentially the fundamental working principle of the yarn sensor. Then we test the sensory performance of the PU/Cotton/CNT_8 yarn materials at increasing frequencies to substantiate their stability at high frequency and quick signal response. As shown in Figure 5 (c), one second of normalized sensory signals of the yarns tested at 4-15Hz frequencies at 10% strain value is presented. It is obvious that the number of signals match perfectly with the number of cycles tested, showing quick response nature of the yarn sensor to mechanical stimuli. Higher frequencies do not degrade the performance of the yarn sensor, as reflected by the good repeatability and recoverability in the signals. What’s more, according to the frequency in each of the test, the period can be calculated to vary from 250 ms to 67 ms, representing the time interval during which the sensor works. This indicates that our sensor material is able to exhibit fast 9



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enough response to the high frequency mechanical stretching and releasing, although those frequencies are already too high for human motions. More importantly, it is of great potential that the yarn sensor could enable the sensation of even higher frequency motions, although we did not conduct further experiments.

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Figure 6. (a)-(d) show the typical sensory signals for detecting finger movements, elbow motions, walking and winking, respectively. Insets are pictures of the wearable sensory yarns on human fingers, elbow, knee and forehead, respectively. In order to demonstrate the potential applications of our yarn sensor in wearable devices to monitor human motions, the conductive yarns are fixed onto copper electrodes and then adhered to commercially available bandages to be assembled as device. By virtue of the flexibility of the yarn, it can be readily cut into appropriate length and assembled. The sensors devices are then being attached on human fingers, elbow, legs and even forehead and connected with our measurement instruments. Figure 6 (a)-(d) demonstrate the ability of the yarn sensor as wearable devices to monitor human motions. When being attached to fingers, the yarn sensors are able to detect the movement as well as the speed of movement by the evolution in electrical signals. The changes in current can be directly read out from the LabVIEW software along with the test, which allows the real-time monitoring of the motions. It can be observed from Figure 6 (a) that the sensor is able to distinguish the amplitude and frequency of the movement, reflecting as the amplitude and frequency of the current signals. Fast finger movements leads to fast changes in current signals and movement of larger amplitude can be observed as larger changes in current signals. This is also valid when using the yarn as sensor for detecting the motion of human arms and legs, as shown in Figure 6 (b) and (c). Because of the high sensitivity of our yarn sensor, we also try to 10


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apply it to detect small movement of human beings, such as winking of eyes, which induces much smaller strain in the yarn. Interestingly, the sensor performs surprisingly well as the signals match the normal frequency of human eye winking, around 4-5s per winking. Additionally, similar with what we have demonstrated in finger and legs, the short response time of the sensor enable the detecting of high frequency winking of eye, as shown in Figure 6 (d). What’s more, although being directly attached to human body, we do not observe any signal variation because of the heating effect caused by body heat during the whole testing process. Figure S5 shows the electrical resistance of the conductive yarn measured at different temperatures, ranging from root temperature to 70oC, indicating the stability of the signal against temperature change and showing the minor impact of the negative temperature coefficient of CNTs. As a result, there is no need to worry about long-term inaccuracy or calibration of the signals, which could be a potential problem in thermoelectric material or graphene related sensor devices.3, 32

Conclusions In this research, we developed a highly reliable and stretchable strain sensor based on SWCNTs-coated PU/Cotton core-spun yarn via a scalable fabrication process. The special core-spun structure of the yarn as well as the reinforcement of CNTs renders the material with high durability during nearly 400,000 continuous stretching-releasing cycles without noticeable damage. The sensitivity of the sensor, expressed as gauge factor can be tuned from 0.31±0.06 to 0.65±0.04 by adjusting the coating times of CNT. The highly stretchable and durable nature of the yarn renders the yarn sensor as promising wearable strain sensor for real-time monitoring of human movements and the potential integration with human beings in the future.

Acknowledgement This work was supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China (CityU 109213), the Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140419115507579) and the Hong Kong Polytechnic University (1-BBA3). Supporting Information. Typical scanning electron micro-scope (SEM) images of Spandex core-spun yarn coated with 2 times, 4 times and 8 times SWCNT suspensions, respectively, showing the existence of carbon nanotube and the encapsulation effect caused by polymeric dispersive agent. SEM morphologies of the yarns before and after 270,000 cycles test. Other supportive information can also be found accordingly. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D. N.; Hata, K., A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6 (5), 296-301. (2) Pang, C.; Lee, G.-Y.; Kim, T.-i.; Kim, S. M.; Kim, H. N.; Ahn, S.-H.; Suh, K.-Y., A Flexible and Highly Sensitive Strain-Gauge Sensor Using Reversible Interlocking of Nanofibres. Nat. Mater. 2012, 11 (9), 795-801. 11



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Table of Content

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Carbon Nanotubes Core-Spun Yarn as High

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