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Mar 31, 2016 - sensor with good reproducibility over 10 000 cycles and excellent wash- and ... recognition and physiological signal monitoring. Very recently, delicate assembly ..... ACS Publications website at DOI: 10.1021/acsami.6b01174.

Research Article www.acsami.org

Highly Sensitive, Stretchable, and Wash-Durable Strain Sensor Based on Ultrathin Conductive [email protected] Yarn for Tiny Motion Monitoring Xiaodong Wu, Yangyang Han, Xinxing Zhang,* and Canhui Lu* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: Strain sensors play an important role in the next generation of artificially intelligent products. However, it is difficult to achieve a good balance between the desirable performance and the easy-to-produce requirement of strain sensors. In this work, we proposed a simple, costefficient, and large-area compliant strategy for fabricating highly sensitive strain sensor by coating a polyurethane (PU) yarn with an ultrathin, elastic, and robust conductive polymer composite (CPC) layer consisting of carbon black and natural rubber. This [email protected] yarn strain sensor exhibited high sensitivity with a gauge factor of 39 and detection limit of 0.1% strain. The elasticity and robustness of the CPC layer endowed the sensor with good reproducibility over 10 000 cycles and excellent wash- and corrosion-resistance. We confirmed the applicability of our strain sensor in monitoring tiny human motions. The results indicated that tiny normal physiological activities (including pronunciation, pulse, expression, swallowing, coughing, etc.) could be monitored using this [email protected] sensor in real time. In particular, the pronunciation could be well parsed from the recorded delicate speech patterns, and the emotions of laughing and crying could be detected and distinguished using this sensor. Moreover, this [email protected] strain-sensitive yarn could be woven into textiles to produce functional electronic fabrics. The high sensitivity and washing durability of this [email protected] yarn strain sensor, together with its low-cost, simplicity, and environmental friendliness in fabrication, open up new opportunities for cost-efficient fabrication of high performance strain sensing devices. KEYWORDS: conductive polymer composite, PU yarn, strain sensor, tiny motion monitoring, functional electronic fabrics

1. INTRODUCTION Strain sensors have attracted extensive attention for electronic applications in human motion detection,1−4 healthcare,5 speech recognition,6 robotics,7 etc.8−10 At the same time, simple, largearea compliant, and cost-efficient fabrication strategies are desirable. Metallic foils and semiconductors (e.g., ZnO, silicon) are the most common strain sensors with desirable sensitivity. Nevertheless, they exhibit many disadvantages: high-cost, poor flexibility, requirement of temperature compensation, and deterioration of sensitivity with applied strain.11−13 Conductive polymer composites (CPCs), with the merits of lightweight, low-cost, and good processability, have been applied as remarkable strain sensing materials.14−21 In CPCbased strain sensors, variation of the percolated conductive networks under applied strain gives electrical signal output, making them capable of detecting external strain stimuli. To serve this purpose, various strategies have been developed to fabricate CPC strain sensors, including modulating aspect ratios of conductive filler,22 incorporation of hybrid filler,23 surface functionalization of conductive filler,24 orientation and selective distribution of conductive filler,25 etc.15 Despite these pioneering works, however, widespread application of the aforemen© 2016 American Chemical Society

tioned conventional bulk CPCs as strain sensing materials is limited due to their relatively low sensitivity and clumsiness, particularly in tiny human motion detection, such as speech recognition and physiological signal monitoring. Very recently, delicate assembly of conductive fillers and unique microstructure design are proved to be effective strategies in fabricating highly sensitive strain sensors.26−39 For instance, Choi et al.26 developed mechanical crack-based ultrasensitive strain sensors consisting of platinum (Pt) nanoparticles and viscoelastic polymer, which were highly sensitive to strain and vibration. These mechanical crack-based sensors could be applicable to speech recognition and physiological signal detection. Lee et al.31 described a strain sensor which included a sandwich-like stacked nanohybrid film of single-wall carbon nanotubes (SWCNTs) and a conductive elastomeric composite of polyurethane (PU)/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS). These sensors could detect small strains on human skin. In Received: January 28, 2016 Accepted: March 31, 2016 Published: March 31, 2016 9936

DOI: 10.1021/acsami.6b01174 ACS Appl. Mater. Interfaces 2016, 8, 9936−9945

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustrations for preparation of the [email protected] yarn using LBL assembly technique (a). Structural schematic diagram of the [email protected] yarn strain sensor (b). Photographs of neat PU yarn (c) and [email protected] yarn (d), a scale is a millimeter. Electrical resistance change of [email protected] yarn with increasing LBL cycles (e).

addition, Ko et al.35 fabricated a novel strain sensor composed of two layers of prestrained anisotropic silver (Ag) nanowire percolation networks. This sensor was applicable to both 2dimensional controlling devices and areal strain detection. However, scalable application of these highly sensitive strain sensors was limited due to their sophisticated fabrication processes and use of very expensive materials (such as graphene, SWCNTs, noble metal, etc.). As massive strain sensing materials would be required in the near future of artificial intelligence, simple, large-area compliant and costefficient fabrication of highly sensitive strain sensing materials still remains a great challenge. In this work, we proposed a simple, cost-efficient, and environment friendly approach to fabricate highly sensitive strain sensors based on ultrathin CPC layer-decorated PU yarn. Specifically, a negatively charged carbon black (CB)@cellulose nanocrystals (CNC)/natural rubber (NR) nanohybrid was coated on the surface of PU yarn to form an ultrathin conductive CPC layer via layer-by-layer (LBL) assembly using positively charged chitosan (CS) as a mediator. The resultant [email protected] yarn strain sensor exhibited excellent sensitivity with a gauge factor (GF) of 39 and detection limit of 0.1% strain as well as good reproducibility over 10 000 cycles. Due to the desirable sensitivity, our [email protected] yarn strain sensor was qualified for tiny motion monitoring, including speech recognition, pulse monitoring, and expression detection. Other normal physiological activities (e.g., swallowing, coughing, chewing, raising head, lowering head) could also be monitored using this [email protected] sensor. Moreover, this [email protected] PU strain-sensitive yarn could be woven into textiles to product functional electronic fabrics. The high sensitivity of our sensor, which is comparable to those of recently reported strain sensors which require complicated fabrication process,26,28,29,35 together with its significant advantages of low-cost, easy fabrication, washing durability, and environmental friendliness, makes it

promising in fabricating next-generation cheap and sensitive electronic sensing devices.

2. RESULT AND DISCUSSION Figure 1a illustrates the fabrication process of the [email protected] yarn. On the basis of our previous work,40 CNC can stabilize CB to form negatively charged [email protected] nanohybrid with excellent suspension stability (Figure S1, discussed in Supporting Information, SI). After mixing with NR latex, a well suspended [email protected]/NR suspension could be obtained. The Zeta-potential of [email protected]/NR suspension was measured to be −27.9 mV, confirming that the [email protected]/NR suspension is negatively charged. Then, cleaned PU yarn was alternately dipped into positively charged CS (Zeta-potential: + 48.3 mV) solution and negatively charged [email protected]/NR suspension, allowing electrostatic deposition of [email protected]/NR onto the surface of PU yarn. After drying, a conductive CPC (i.e., [email protected]/NR) layer was formed on the surface of PU yarn as illustrated in Figure 1b. Compared with original white PU yarn (Figure 1c), which was used as the substrate, the [email protected] yarn turned to black as shown in Figure 1d. The dark visual appearance is an indication of the successful deposition of CPC on the surface of PU yarn. Besides, the deposition of conductive CPC layer on PU yarn can be revealed from the change in electrical resistance of the resultant [email protected] yarn. As given in Figure 1e, significant drop in electrical resistance can be observed with incipient LBL cycles (

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