Highly Sensitive, Stretchable, and Wash-Durable Strain Sensor Based

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Highly Sensitive, Stretchable, and Wash-Durable Strain Sensor Based on Ultrathin Conductive Layer@Polyurethane Yarn for Tiny Motion Monitoring Xiaodong Wu, Yangyang Han, Xinxing Zhang, and Canhui Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01174 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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ACS Applied Materials & Interfaces

Highly Sensitive, Stretchable, and WashDurable Strain Sensor Based on Ultrathin Conductive Layer@Polyurethane 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 Keywords: conductive polymer composite, PU yarn, strain sensor, tiny motion monitoring, functional electronic fabrics

Abstract: Strain sensors play an important role in next generation of artificial 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, cost-efficient and large-area compliant strategy for fabricating highly sensitive strain sensor by coating polyurethane (PU) yarn with ultrathin, elastic, and robust conductive polymer composite (CPC) layer consisting of carbon black and natural rubber. This CPC@PU 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 1

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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 CPC@PU 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 CPC@PU strain-sensitive yarn could be woven into textiles to produce functional electronic fabrics. The high sensitivity and washing durability of this CPC@PU yarn strain sensor, together with its low-cost, simplicity, environmental friendliness in fabrication, open up new opportunities for cost-efficient fabrication of high performance strain sensing devices.

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, large-area compliant and cost-efficient fabrication strategies are desirable. Metallic foil and semiconductor (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 light weight, low-cost and good processability, have been applied as remarkable strain sensing materials 14-21. In CPC-based 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 2


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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 aforementioned 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 crackbased 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 addition, Ko et al. 35 fabricated a novel strain sensor composed of two layers of pre-strained anisotropic silver (Ag) nanowire percolation networks. This sensor was applicable to both 2-dimensional 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-

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area compliant and cost-efficient 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 CPC@PU 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 CPC@PU 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 CPC@PU sensor. Moreover, this CPC@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 nextgeneration cheap and sensitive electronic sensing devices.

2. Result and Discussion Figure 1a illustrates the fabrication process of the CPC@PU yarn. Based on our previous work 40, CNC can stabilize CB to form negatively charged CB@CNC nanohybrid with excellent suspension stability (Figure S1). After mixing with NR 4


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latex, a well suspended CB@CNC/NR suspension could be obtained. The Zetapotential of CB@CNC/NR suspension was measured to be -27.9 mV, confirming that the CB@CNC/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 CB@CNC/NR suspension, allowing electrostatic deposition of CB@CNC/NR onto the surface of PU yarn. After drying, a conductive CPC (i.e. CB@CNC/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 CPC@PU yarn turned to black as shown in Figure 1 d. 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 CPC@PU yarn. As given in Figure 1e, significant drop in electrical resistance can be observed with incipient LBL cycles (< 50 cycles), resulting from the gradual construction of conductive networks on the surface of PU yarn. While, with further increase of LBL cycles, the variation in electrical resistance trends to become sluggish, indicating the formation of conductive networks. When 50 LBL cycles is achieved, the CPC@PU yarn attains an electrical resistance of 4.1 MΩ per centimeter. It should be pointed out that the resistance of this CPC@PU yarn is relatively higher when compared to some other previously reported resistive sensors29, 32, 35. However, the conductive filler content plays an important role for the conductivity of CPC@PU yarn, as shown in Figure S2. Thus, we could improve the conductivity by increasing the content of conductive filler in CPC layer or increasing the LBL numbers of CPC layer. In order to balance the conductivity and

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sensitivity of the prepared sensors, sample of CPC@PU yarn with 50 LBL cycles was employed for the following tests unless otherwise specified.

Figure 1. Schematic illustrations for preparation of the CPC@PU yarn using LBL assembly technique (a). Structural schematic diagram of the CPC@PU yarn strain sensor (b). Photographs of neat PU yarn (c) and CPC@PU yarn (d), a scale is a millimeter. Electrical resistance change of CPC@PU yarn with increasing LBL cycles (e). SEM was used to evaluate the surface structures of PU yarn and CPC@PU yarn. As shown in Figure 2a-b, original PU yarn, which has a diameter of 205±4 µm, consists of bunch of straight PU microfibers. The surface of PU microfibers is very smooth (Figure 2c). After LBL deposition of CPC, as can be seen clearly from Figure 2d-e, a layer of tulle-like CPC is coated on the surface of the PU yarn. In addition, the surface 6


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of CPC@PU yarn becomes rugged (Figure 2f) and CB particles can be observed clearly (Figure S3). These provide the direct demonstration that a CPC layer can be coated on the PU yarn via LBL assembly.

Figure 2. SEM images of neat PU yarn (a-c) and CPC@PU yarn (d-f) under different magnifications, 50 × (a, d), 200 × (b, e), 2000 × (c, f). For intuitive observation, a portion of CPC layer was peeled off from the PU yarn. The boundary of CPC layer and PU yarn was observed using SEM. As given in Figure 3a-b, in the boundary region, smooth PU yarn and rugged CPC layer can be distinctly distinguished. Next, CPC@PU yarn was fractured in liquid nitrogen to observe its fracture surface. As shown in Figure 3c, the PU substrate consists of bunch of peanut-shaped microfibers. A layer of CPC with the thickness of only several micrometers is coated on the surface of PU substrate (Figure 3d). It is noticed that the CPC layer are well combined with the PU microfibers, benefiting from the electrostatic interaction between negatively charged CB@CNC/NR and positively charged CS. This desirable combination between CPC layer and PU substrate avoids 7



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the exfoliation of CPC layer, and guarantees the synchronized deformation of the substrate and functional layer. This is of great importance to realize an excellent reproducibility of the CPC@PU yarn sensor. To intuitively observe the CPC layer, we removed the PU yarn substrate, and directly observed the CPC layer. As given in Figure 3e-f, a CPC sheath can be clearly seen. The thickness of the CPC layer is measured to be 3±1 µm, which is in accordance with that in Figure 3d. The mass ratio of CPC layer to PU yarn is calculated to be 1:16, while the mass ratio of conductive filler (i.e. CB) in CPC layer to CPC@PU yarn is only 1:67 (i.e. CB content in CPC@PU is 1.5 wt%). This means that only very little CB was used in fabricating this CPC@PU yarn strain sensor. The low consumption along with the low price and easy availability of CB would dramatically reduce the fabrication cost of this CPC@PU yarn strain sensor.

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Figure 3. SEM images: CPC@PU yarn after peeling off a portion of CPC layer (a-b), the cross section of the CPC@PU yarn (c-d), the CPC sheath after removing the PU yarn (e-f). Magnification: 2000× (a), 4000× (b),300× (c), 1000× (d), 250× (e), 1000× (f). Moreover, we evaluated the robustness of the functional CPC layer in various harsh conditions, including boiling water, detergent solution, acid condition and alkaline condition. Figure 4a shows that after boiled in water for 5 h, the CPC@PU yarn exhibits only a slight resistivity increase (showing a relative increase by 37%, which 9



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does not exceed one order of magnitude). After washed in a commercially available detergent solution with magnetic stirring for 24 h, the CPC@PU yarn also shows a small increase in resistivity (relatively increased by 23%). Even exposed to acid condition and alkaline condition for 24 h, the conductivity of CPC@PU yarn changed very little. These data indicate the good corrosion resistance of the CPC@PU yarn against water, detergent, acid and alkali, as well as mechanical resistance against stirring. This could be attributed to following reason: in CPC layer, the CB-based conductive network is embedded in NR matrix (Figure 4b) rather than directly attached to the PU substrate, which could be revealed from Figure S4 and Figure S5. The NR matrix of CPC layer, which has good corrosion resistance and cushioning effect to mechanical impact, protects the CB conductive network against exfoliation. Thus, a robust functional CPC layer is firmly coated on PU yarn substrate. However, if the CPC layer consists of only CB and CNC without NR, the conductivity of CPC layer would be decreased by 900% after boiled in water for 5 h, revealing the protective effect of NR component. The desirable robustness of the CPC layer might endow the CPC@PU yarn strain sensor with good reproducibility, and promote its utilization in various functional electronic textiles.

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Figure 4. Relative resistance change of CPC@PU yarn during treatment in boiling water, detergent, acid solution and alkaline solution, respectively (a). Microstructure diagram of the cross section of the CPC@PU yarn (b). In order to assemble a strain sensor, both ends of CPC@PU yarn were coated with conductive silver paste and connected with copper wires. Then the CPC@PU yarn was embedded into a polydimethylsiloxane (PDMS) matrix to form the final strain sensor, as shown in Figure S6a. For piezoresistive behavior test, a constant voltage was applied on the CPC@PU yarn based strain sensor, and the electrical current of the prepared sensor was recorded with variation of strain (Figure S6b). The normalized current responses of the strain sensor to repeated tiny strain (

Highly Sensitive, Stretchable, and Wash-Durable Strain Sensor Based

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