- Email: [email protected]
Highly sensitive and durable wearable strain sensors from a core-sheath nanocomposite yarn
Composites Part B 183 (2020) 107683
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Highly sensitive and durable wearable strain sensors from a core-sheath nanocomposite yarn Junjie Pan a, Baowei Hao a, Wenfang Song b, Shixian Chen a, Daiqi Li a, Lei Luo a, Zhigang Xia a, Deshan Cheng a, Anchang Xu a, ***, Guangming Cai a, **, Xin Wang c, * a b c
State Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan, 430200, China Garment Digital Engineering Center, School of Art and Design, Guangdong University of Technology, Guangzhou, 510000, China Centre for Materials and Future Fashion, School of Fashion and Textiles, RMIT University, Brunswick, Victoria 3056, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon nanotube Core-sheath Nanocomposite yarn Durability Wearable sensor
With fiber-based wearable electronics being developed and widely used in various fields, the safety and dura bility of nanomaterials in fibrous system become big concerns due to the leakage of nanomaterials and possible failure in functionality. To date, it is still challenging to fabricate wearable strain sensors with sensitivity and durability. Herein, we report a simple strategy for large-scale fabrication of core-sheath nanocomposite yarn (CSCY) based wearable sensor with high sensitivity and superior durability. By dispersing aqueous CNT ink into braided composite yarns as the core (BYs-CNT) and electrospun polyurethane (PU) nanofibers as the sheath, a unique core-sheath micro-nanofibrous structure was fabricated to grant the yarn with extremely high sensing sensitivity (maximum GF up to 980), long-term stability and great durability (against washing and dilute hy drochloric acid). The as-developed CSCY strain sensor was demonstrated to monitor real-time subtle and vigorous human activities. The present work provides insights for developing reliable and stable, highly sensi tive, flexible and durable wearable electronics with potential applications in different areas.
1. Introduction
properties of the fibrous materials, such as reliable stretchability and shape retention performance, elasticity and tenacity together with durability. Therefore, it is highly demanded to develop stretchable, flexible but sensitive wearable sensors with excellent durability for truly wearable applications. For most flexible wearable sensors, the conductive materials are located on the surface of the fiber materials. These conductive materials, usually nanomaterials, are easy to fall off after a long-term usage due to frequent stretching-releasing, bending or twisting and probable washing cycles. Under large mechanical deformations, flexible wearable sensors usually perform poorly owing to the failure in the conductive mecha nism. Hence, durability against washing, deformation and harsh envi ronments has been big concerns for fiber-based wearable sensors [36]. In addition, the detachment of carbon nanomaterials from fibers might cause health problems if the wearable sensors are in contact with the human body [37]. For example, CNTs have serious health hazards when they are inhaled into human body [38–40]. These threats have hindered
Wearable electronics have attracted increasing attention in recent years [1–8]. Flexible strain sensor as an important wearable electronic device has shown great application potentials in various fields including electronic skin [9–11], human-machine interfaces [12,13], and wear able medical devices [14]. Fibers and fibrous materials are often used as the matrix to bring flexibility and stretchability to strain sensors [15–17]. Previous studies have reported the using of conductive mate rials and elastic fiber materials to fabricate strain sensors. For example, graphene [18–21], carbon nanotubes [22–25], silver nanowires [26–29] and PPy were coated on elastic fiber surface to fabricate flexible strain sensors [30]. Graphene and carbon nanotubes functionalized textile yarns were used to fabricate strain sensors [31–33]. Other conductive materials were also deposited on the surface of fabrics for fabricating flexible strain sensors [34,35]. However, the sensitivity and stability of flexible strain sensors are largely determined by the mechanical
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (A. Xu), [email protected] (G. Cai), [email protected] (X. Wang). https://doi.org/10.1016/j.compositesb.2019.107683 Received 5 October 2019; Received in revised form 28 November 2019; Accepted 4 December 2019 Available online 7 December 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
J. Pan et al.
Composites Part B 183 (2020) 107683
2.2. Fabrication
the employment of CNT-coated fibers in e-textiles. Therefore, fabrica tion of flexible strain sensor with high washability and durability has great significance in the application of truly wearable electronics. It has been reported that CNT-based e-textiles by nanosoldering have excellent washability [37], but this method is only applicable to fabrics or non wovens. PDMS has been coated onto TPU/CNTs composite nanofibers for anti-corrosive wearable strain sensors [41]. However, the compact structure of PDMS affects the comfort performance, and this method may not be suitable for large-scale production. Immobilizing CNTs into the fibrous system as a composite yarn by adding CNTs in yarn spinning stage has enhanced the durability and safety of nanocomponents in textiles [31,33]. Furthermore, introducing a polydopamine template between fibers and functional components in the composite yarn resulted in more durable and stable wearable applications [30]. How ever, the structure of the composite yarn must be upgraded with enhanced mechanical properties and a higher level of protection of CNT against leaking before the yarn being applied in real wearable applications. Herein, we report a novel and scalable strategy for developing coresheath structure composite yarns (CSCY). By dip-coating aqueous CNT ink dispersion onto braided composite yarns as the core (BYs-CNT) and electrospinning PU nanofibers as the sheath, the core-sheath composite yarns (BYs-CNT-Nanofiber) have excellent mechanical properties and novel structure for durable and highly sensitive wearable sensing per formance. The nanocomposite yarn can be employed as reliable and stable, highly sensitive and flexible, durable and wearable electronics with potential applications in different areas.
Fig. 1 schematically shows the fabrication process of BYs-CNT. PET filaments were ultrasonically cleaned in deionized water for 30 min followed by winding on bobbins. After that, eight PET filament bobbins and an elastic filament were simultaneously braided into composite yarns (denoted as BYs) were fabricated and winded. The as-fabricated BYs were immersed in acetone under sonication for 30 min and were treated with ethanol under sonication for 30 min. The BYs were then washed with deionized water followed by vacuum drying in an oven at 60 � C. A beaker of 50 mL SWCNT solution was ultrasonically dispersed for 30 min to form CNT ink. The CNT ink was dried in an oven at 80 � C for 6 h to obtain highly viscous CNT solution (20 mL). The concentration of the conductive CNT ink was 0.375 wt% as calculated. The BYs were then dipped into the conductive CNT ink in an ultrasonic bath for 20 min followed by drying at 60 � C for 30 min. The CSCY was fabricated continuously in large-scale by coating PU nanofibers onto the as-prepared BYs-CNT (BYs-CNT-Nanofiber). The electrospinning setup consisted of dual needles pointing at each other, and an earthed collector mounted on a motor was used to wind up the coated yarn [42]. The polymer solution was prepared by dissolving PU pellets in N, N-dimethylformamide (DMF) at a concentration of 25%. The solution was then injected into the two needles which were oppo sitely charged (Fig. 1). While the electrospinning was ongoing, BYs-CNT was passing through the center of the rotating metal collector to collect nanofibers on its surface. As a result of the rotating of the metal plate, nanofibers were twisted onto the yarn. In the end, the CSCY was winded onto the motor collector. Two silver paste were sealed on each end of CSCY to fabricate CSCY strain sensors.
2. Experimental 2.1. Materials Braided composite yarns (BYs) were fabricated in our group with elastic polymers and PET fibers as the main components as per the previously reported method [30]. The core part was elastic polymers (0.5 mm in diameter) and the braiding part was PET fiber. Single-wall CNT (0.15 wt%) with a high purity was sourced from Nanjing XFNANO Materials Tech Co., Ltd., Nanjing, China. Polyurethane pellets (2363-80AE) were provided by Dow, U.S. N,N-dimethylformamide (DMF) was sourced from Sinoparm Chemical Reagent Co., Ltd.
2.3. Measurements and characterizations SEM images were obtained on a scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan). Photos were taken by a camera (OLYMPUS DSX510). Mechanical properties were tested using a universal testing system (Instron 5566) with a gauge length 250 mm and a drawing speed 80 mm/min. A digital multi-meter (Keysight Truevolt 34465A) was used to test the resistance of CSCY. The yarn was mounted between two insulative
Fig. 1. Fabrication process of core-sheath structure composite yarn. 2
J. Pan et al.
Composites Part B 183 (2020) 107683
clamps with a gauge length of 3 cm (see Fig. S-1) while the instant resistance was recorded. The relative change of resistance was then calculated according to the Formula: ΔR/R0¼(R-R0)/R0
after electrospinning of PU nanofibers on its surface, as seen from Fig. 2i–l. Interestingly, the electrospun PU membrane formed a porous micro-nano structure web with some fibers in micro-scale and others in nanoscale. The optical color of BYs-CNT-Nanofiber has changed from black to white as a result of coating with PU nanofibers (Inset of Fig. 2i). The cross-section views in Fig. 2m–p confirm the typical core-sheath structure of BYs-CNT-Nanofiber. There are three parts of the crosssection as seen from Fig. 2m, in which the core is the elastic polymer and the middle layer the twined PET fibers together with the surface layer a nanofibrous network. The difference between micro PET fibers and PU nanofibers can be clearly observed in Fig. 2o and p, and the thickness of the nanofibrous layer is around 50 μm. Detailed view of nanofibrous network in Fig. 2p shows the cutting edge of nanofibers without any interconnecting from cross-sectional view.
(1)
where R0 and R represent the initial resistance and current resistance under a given strain, respectively. Then, the gauge factor (GF) of the CSCY strain sensor was calculated by: δ(ΔR/R0)/δε
(2)
where ε is the strain and ΔR/R0 is from Formula (1). The motions of human body were detected by attaching CSCY sen sors (2 cm) onto different positions of body (e.g. finger, wrist, knee, neck and throat). Both ends of the sensors were stuck on skin using adhesive tapes. The electrical resistance was recorded during different move ments, and the ΔR/R0 was calculated subsequently according to For mula (1).
3.2. Mechanical properties Owing to the intrinsic elasticity of the elastic polymer and the special braided structure, BYs show an extension rate of up to 350% and a breaking force of 44 N, as shown in Fig. 3. Dip-coating of CNTs has not affected the mechanical properties of BYs much, as the BYs-CNT exhibits almost the same force-elongation curve as compared with that of BYs (Fig. 3a). It is evident that both BYs and BYs-CNT are highly stretchable and elastic. The breaking force of CSCY is slightly higher than that of BYs and BYs-CNT. BYs-CNT-Nanofiber is stronger due to the interconnected PU nanofibrous network on its surface. However, the breaking elongation of CSCY has sacrificed from 350% to 270% (Fig. 3b). Apparently, the interconnected PU nanofibrous layer has confined the stretchability of BYs-CNT. Nevertheless, >200% elongation would facilitate the wear able sensor applications of CSCY [40]. Due to the excellent mechanical properties, the as-prepared CSCY strain sensor shows high stretchability. As demonstrated in Fig. 3c, The CSCY sensor can be stretched from 0% to 40%, 50%, 100% and 125%, and the sensor becomes longer and thinner. The sensor can return to its
3. Results and discussion 3.1. Morphology Fig. 2a–d shows the typical braided structure of BYs with fiber bundles twining with each other on the surface. A core-sheath structured yarn with the elastic polymer as core and PET fibers as sheath was formed (Fig. 2d). Dip-coating of CNTs has resulted in a nanoscale layers on the surface of BYs, as seen from Fig. 2e. The inserts in Fig. 2a and e indicate that the color of the yarn has changed from white to black after the dip-coating due to the loading of CNT ink. As seen from Fig. 2g, the surface of BYs-CNT is fully covered by the CNT layer with the profile of fibers partly observable. Detailed view of the CNT layer in Fig. 2h sug gests that the CNTs have formed a thick network on the surface. These results confirm that CNT ink has been absorbed onto BYs successfully. An interconnected nanofibrous layer has been jacketed on BYs-CNT
Fig. 2. SEM images of BYs (a–d) and BYs-CNT (e–h); SEM images of CSCY (i–l) and its cross-sectional views (m–p). Insets: optical photos of corresponding samples. 3
J. Pan et al.
Composites Part B 183 (2020) 107683
Fig. 3. Force-elongation curves (a) and their corresponding values of force and elongation at break (b) for BYs, BYs-CNT and CSCY. Photos of the CSCY under different strains (c). (Sample size for tensile test: 5 cm length and ~1.04 mm radius).
original state without obvious permanent deformations (Fig. 3c), and the PU nanofiber network on CSCY is properly maintained without damaging.
3.3. Electromechanical performance The electrical resistance of CSCY is shown in Fig. S-2a. With the in crease of the number of dip-coating cycles the resistance of CSCY de creases sharply owing to the increased loading of CNTs. However, when
Fig. 4. Relative change of resistance for CSCY under increasing strains (a) (Inset: testing method), under cyclic strain of 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 20%, 30%, 40% and 50% (b–c), and under different drawing speeds of 10, 50, 75 and 100 mm/min (d) (30% strain). Time response of CSCY (e) (Insets: detailed views of selected areas). The stability test of the of CSCY under cyclic strain (f) (20% strain). 4
J. Pan et al.
Composites Part B 183 (2020) 107683
the cycle number is higher than three, the resistance of CSCY is stabi lized at 0.12 kΩ/cm. Owing to the electric conductivity of CNT, CSCY is electrically conductive with a linear current-voltage regression (Fig. S2b), exhibiting the ohmic behavior for the yarn. The slopes of both the BYs-CNT and BYs-CNT-Nanofiber are nearly identical, suggesting the same electrically conductive mechanism. The ΔR/R0 of the CSCY strain sensor under different strains is plotted in Fig. 4a. The ΔR/R0 increases linearly in the beginning, and it displays an exponential increase with the increase of strain. Gauge factor (GF) has widely been accepted as the measure of the sensitivity of strain sensors. Fig. 4a shows that CSCY strain sensors possess gauge factors of 46.4, 353 and 980 at the strain range of 0%–15%, 15%–29% and 29%– 44%, respectively. The GFs of reported fiber-based flexible strain sensors are listed in Table S-1 [30,31,33,41,45–55]. A careful comparison of all the GFs suggests that CSCY sensor has the highest GF in 0%–40% strain range. As sensitivity determines the effectiveness of wearable sensors in sensing human motions, it is highly expected that the CSCY possesses a high capacity of sensing human motions. The resistance of CSCY de creases when the strain is larger than 44%. The reason is probably that the neighboring PET fibers are close to each other when the strain is higher than 44%, and the contact areas increase to a level that the in crease in resistance is compromised slightly. Fig. S-3a-c shows the ΔR/R0–ε curve of CSCY under stretching/ releasing of small strains of 1%, 2% and 20%, respectively. The variation of ΔR/R0 is small, suggesting that the elastic polymer allows CSCY to recover its structure even in the 0–20% strain range. CSCY strain sensors are highly responsive to alternating strains. Fig. 4b and c shows the responses of the CSCY strain sensor under strains of 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 20%, 30%, 40% and 50%. As expected, the sensor shows stable responses under each strain. The ΔR/R0 of the sensor increases with the values consistent with the results in Fig. 4a. Fig. 4d shows the ΔR/R0 of CSCY during cyclic stretch with different drawing speeds (30% strain). There is no evident difference in the ΔR/ R0 under drawing speeds, suggesting the excellent adaptivity of the sensor to different stimuli. The excellent adaptivity of the sensor against different drawing speeds is due to its excellent elasticity and special braided structure. Reliable responses under different stimuli are crucial
in determining the reliability of strain sensors. Meanwhile, the response time of the CSCY strain sensor to the loading and unloading is within 200 m s (Fig. 4e) due to the viscoelasticity of the CSCY [43,44]. The electrical resistance of CSCY has no evident effect on the sensitivity and reliability of the strain sensors, as the curves of ΔR/R0 under different dip cycles are almost the same (Fig. S-3d). The stretchability of CSCY is almost the same with different loaded CNTs, thus the ΔR/R0 curves exhibit similar periodic spectra. Fig. 4f shows the stability of CSCY strain sensor under repeated stretching/releasing cycles with a tensile speed of 10 mm/min for 1000 cycles. The resistance of the strain sensor changes in the same way in each cycle with the peaks identical to each other (see Inset), indicating that the strain sensor has long-term stability and superior durability. The SEM images of CSCY before and after 1000 cycles are shown in Fig. S-4, and there is no obvious change in the micro-nanofibrous web. 3.4. The mechanism of strain sensing The sensing mechanism of the CSCY strain sensor is proposed in Fig. 5. Previous reports have shown that structural changes in braided composite yarns during stretching are the main factors for stain sensing yarns [30]. A dense carbon nanotube film has been formed on the sur face of BYs by coating high concentration CNT inks (Fig. 2e–h). When the CSCY is stretched, the PET fiber strands are forced to incline, forming an increased winding angle, as shown in Fig. 5b and c. The overlapping contacts between neighboring PET strands are thus decreased with contacts between neighboring PET fibers within each strand altered (Fig. 5d and e). With the increase of the winding angle, the areas be tween neighboring PET strands are slightly brighter (Fig. 5b and c), suggesting that cracks in the CNT film are formed due to the stretching. As a result, the longitudinal conductive path is changed, forcing elec trons to move within each PET strand and then pass through the over lapped areas (Fig. 5f). With the increase of strain, the cracks between neighboring PET strands are more evident and the overlapped areas are smaller (Fig. 5g). All in all, the resistance of CSCY increases with the increase of strain due to stretching. The excellent tensile properties of PU nanofibrous membrane ensure an intact surface structure of CSCY
Fig. 5. Photos of BYs-CNT under 0%, 20% and 40% strains (a–c), the proposed sensing mechanism (d–g), and the photos of CSCY under 0% and 40% strains (h, i). 5
J. Pan et al.
Composites Part B 183 (2020) 107683
without damage (Fig. 5h and i). Therefore, PU nanofibers play a pro tective role to avoid losing of CNTs during the stretching and releasing. CSCY has excellent elasticity, so it will return back to its original state when the strain is released. As a result, the PET strands interlace together with the winding angle decreased. The cracks within the CNT film rejoin to form an intact film again, the electric conduction is thus recovered with electrical resistance decreased. Therefore, the conduc tive performance of the CSCY is repeatable and reliable to grant the sensing capacity of the CSCY strain sensors.
excellent durability and stability together with reliability. 3.6. Wearable applications The highly flexible and stretchable CSCY sensor has high sensitivity, reliability and durability, thus it can be employed to detect human motions (both subtle and vigorous). The CSCY strain sensor was attached onto finger, wrist, knee and throat to monitor a range of ac tivities. As shown in Fig. 7a, when the finger (Inset) bends to a certain degree rapidly, the ΔR/R0 of the CSCY sensor increases to a certain value, and the higher the bending angle, the higher the value of ΔR/R0. Cyclic bending/extending of the pointing finger leads to a repeated growing/dropping of ΔR/R0, indicating the fast responses of the CSCY sensor. Repeatable responses of the CSCY strain sensor can also be observed when sensing human wrist (as shown in Fig. 7b), as the ΔR/R0 curve shows periodical changes corresponding to repeated bending. Fig. 7c presents the spectra of ΔR/R0 corresponding to handwriting. Biceps brachii movement and clenching fist were also detected by CSCY, proving that light movements of muscle could be detected (see Fig. S-5). The CSCY sensor was fixed on a knee to sense the motions of the joint. The ΔR/R0 value changed immediately corresponding to the extending and bending of the knee, with the signals recorded in Fig. 7d. Fig. S-6 shows that the ΔR/R0 responds swiftly to the shifting between a small and a deep bending of elbow. The CSCY sensor can monitor subtle motions including pronouncing words and drinking water. Fig. 7e shows the responses of the CSCY sensor to the motions of throat when pronouncing “a”, sensor” and “textile”. The ΔR/R0 value of the sensor changes accordingly with a distinctive spectrum for each word. Besides, the motion of drinking the same volume of water (30 mL per mouthful) can be monitored as the spectrum shows similar patterns (Fig. 7f). Meanwhile, the CSCY strain sensor successfully detected the motion of nodding (Fig. S-7).
3.5. Durability Coating of electrospun nanofibrous network on the surface has granted CSCY with excellent durability. Fig. 6a shows the ΔR/R0 of CSCY sensor after repeated folding and relaxing. It can be seen that during the 100 times folding and relaxing, the ΔR/R0 fluctuates around 1 with less than 10% deviation. Besides, the CSCY sensor is durable against washing and treating with hydrochloric acid for a period of time. As shown in Fig. 6b, the resistance of CSCY increases a little bit (<10% deviation) even after washing for 5 cycles (10 min each cycle with ul trasonic frequency 40 kHz). Besides, the resistance of CSCY increases slightly after being placed in 15% dilute hydrochloric acid for 10 h (Fig. 6c). The excellent durability of CSCY has resulted in its stable and reliable sensing performance. Even after bending for 100 times, the sensing spectra of CSCY sensors are almost identical (Fig. 6d). The strain sensing performance of the sensor has been maintained after 5 times of ultra sound washing, as the signals are the same in Fig. 6d. The sensing per formance can resist harsh environment due to the resistance of PU nanofibers. The sensor was placed in 15% dilute hydrochloric acid for 10 h and then its sensing performance was tested at a 10% strain. The ΔR/R0 is nearly the same as that before acid treatment (Fig. 6d). The experimental results show that the as-prepared CSCY sensor has
Fig. 6. The R/R0 of the CSCY sensor after repeated folding and relaxing (bending from 3 cm to 2.5 cm) (a), after repeated washing cycles (ultrasonic washing for 10 min each time under an ultrasonic frequency 40 kHz) (b), and after acid treatment (in 15% dilute hydrochloric acid for 10 h) (c). The strain sensing performance of CSCY sensor under different conditions (folding, washing and treating with 15% dilute hydrochloric acid) (d). 6
J. Pan et al.
Composites Part B 183 (2020) 107683
Fig. 7. Wearable applications of CSCY strain sensors in monitoring human motions including bending finger (a), bending wrist (b), writing (c), bending leg (d), speaking (e) and drinking (f).
Considering the lightweight and flexibility of CSCY, the composite yarn has the potential be woven or knitted into fabrics [31,50], serving as part of a textile or as a whole piece of textile for multiple applications including sensing, heating and thermochromic color changing.
draft. Xin Wang: Conceptualization, Methodology, Investigation, Writing - review & editing.
4. Conclusions
This work was supported by the Foundation of Science Research Program from Hubei Provincial Department of Education (Grant No. Q20181705) and the National Natural Science Foundation of China (Grant No. 51606131).
Acknowledgements
In summary, a highly flexible and sensitive strain sensor with supe rior durability was fabricated from a core-sheath nanocomposite yarn by coating PU nanofibers onto BYs-CNT. The CSCY sensor has flexibility and stretchability based on the excellent mechanical properties of the BYs. Owing to the unique micro-nano structure of CSCY, the obtained strain sensor exhibited high sensitivity and responsivity (maximum GF 980), and long-term stability. The PU nanofibrous network as the pro tective layer on the CSCY granted the strain sensor with superior dura bility and stability against bending, washing and even acidic solution. The CSCY sensor was demonstrated to accurately monitor a range of human motions (both subtle and vigorous). This work provides a novel strategy to fabricate highly sensitive and durable strain sensor devices by designing a novel core-sheath micro-nanofibrous structure.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107683. References [1] Lee YY, Kang HY, Gwon SH, Choi GM, Lim SM, Sun JY, Joo YC. A strain-insensitive stretchable electronic conductor: PEDOT:PSS/acrylamide organogels. Adv Mater 2016;28:1636–43. [2] Guo Q, Huang B, Lu C, Zhou T, Su G, Jia L, Zhang X. A cephalopod-inspired mechanoluminescence material with skin-like self-healing and sensing properties. Mater Horiz 2019;6:996–1004. [3] Song H, Zhang J, Chen D, Wang K, Niu S, Han Z, Ren L. Superfast and highsensitivity printable strain sensors with bioinspired micron-scale cracks. Nanoscale 2017;9:1166–73. [4] Kim S-W, Kwon S-N, Na S-I. Stretchable and electrically conductive polyurethanesilver/graphene composite fibers prepared by wet-spinning process. Compos B Eng 2019;165:571–81. [5] Liao X, Zhang Z, Liao Q, Liang Q, Ou Y, Xu M, Li M, Zhang G, Zhang Y. Flexible and printable paper-based strain sensors for wearable and large-area green electronics. Nanoscale 2016;8:13025–32. [6] Su X, Li H, Lai X, Chen Z, Zeng X. Highly stretchable and conductive superhydrophobic coating for flexible electronics. ACS Appl Mater Interfaces 2019; 10(12):10587–97. [7] Montazerian H, Dalili A, Milani AS, Hoorfar M. Piezoresistive sensing in chopped carbon fiber embedded PDMS yarns. Compos B Eng 2019;164:648–58. [8] Liao X, Zhang Z, Kang Z, Gao F, Liao Q, Zhang Y. Ultrasensitive and stretchable resistive strain sensors designed for wearable electronics. Mater Horiz 2017;4: 502–10. [9] Qiao Y, Wang Y, Tian H, Li M, Jian J, Wei Y, Wang X, et al. Multilayer graphene epidermal electronic skin. ACS Nano 2018;12:8839–46.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Junjie Pan: Methodology, Investigation, Writing - original draft. Baowei Hao: Methodology, Investigation. Wenfang Song: Methodol ogy. Shixian Chen: Investigation. Daiqi Li: Investigation. Lei Luo: Methodology, Investigation. Zhigang Xia: Methodology, Investigation. Deshan Cheng: Methodology, Investigation. Anchang Xu: Conceptu alization, Methodology, Investigation. Guangming Cai: Conceptuali zation, Methodology, Investigation, Supervision, Writing - original 7
J. Pan et al.
Composites Part B 183 (2020) 107683 [33] Cai G, Yang M, Xu Z, Liu J, Tang B, Wang X. Flexible and wearable strain sensing fabrics. Chem Eng J 2017;325:396–403. [34] Lin CW, Zhao Z, Kim J, Huang J. Pencil drawn strain gauges and chemiresistors on paper. Sci Rep 2015;4:3812. [35] Zhang F, Wu S, Peng S, Wang CH. The effect of dual-scale carbon fibre network on sensitivity and stretchability of wearable sensors. Compos Sci Technol 2018;165: 131–9. [36] Chen S, Liu H, Liu S, Wang P, Zeng S, Sun L, Liu L. Transparent and waterproof ionic liquid-based fibers for highly durable multifunctional sensors and straininsensitive stretchable conductors. ACS Appl Mater Interfaces 2018;10:4305–14. [37] Du D, Tang Z, Ouyang J. Highly washable e-textile prepared by ultrasonic nanosoldering of carbon nanotubes onto polymer fibers. J Mater Chem C 2018;6 (4):883–9. [38] Zeng W, Shu L, Li Q, Chen S, Wang F, Tao XM. Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications. Adv Mater 2014;26(31): 5310–36. [39] Lam CW, James JT, McCluskey R, Hunter RL. Pulmonary toxicity of single-wall carbon nanotubes in mice 7 and 90 days after intratracheal instillation. Toxicol Sci 2004;77(1):126–34. [40] Donaldson K, Aitken R, Tran L, Stone V, Duffin R, Forrest G, Alexander A. Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety. Toxicol Sci 2006;92(1):5–22. [41] Wang L, Chen Y, Lin L, Wang H, Huang X, Xue H, Gao J. Highly stretchable, anticorrosive and wearable strain sensors based on the PDMS/CNTs decorated elastomer nanofiber composite. Chem Eng J 2019;362:89–98. [42] Zheng X, Zhang K, Yao L, Qiu Y, Wang S. Hierarchically porous sheath–core graphene-based fiber-shaped supercapacitors with high energy density. J Mater Chem A 2018;6(3):896–907. [43] Pang C, Lee GY, Kim TI, Kim SM, Kim HN, Ahn SH, Suh KY. A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres. Nat Mater 2012;11(9):795. [44] Pu JH, Zhao X, Zha XJ, Bai L, Ke K, Bao RY, Yang W. Multilayer structured AgNW/ WPU-MXene fiber strain sensors with ultrahigh sensitivity and wide operating range for wearable monitoring and healthcare. J Mater Chem A 2019;7:15913–23. [45] Cheng Y, Wang R, Sun J, Gao L. A stretchable and highly sensitive graphene-based fiber for sensing tensile strain, bending, and torsion. Adv Mater 2015;27(45): 7365–71. [46] Li L, Bai Y, Li L, Wang S, Zhang T. A superhydrophobic smart coating for flexible and wearable sensing electronics. Adv Mater 2017;29(43):1702517–24. [47] Zhang MC, Wang CY, Wang HM, Jian MQ, Hao XY, Zhang YY. Carbonized cotton fabric for high-performance wearable strain sensors. Adv Funct Mater 2017;27(2): 1604795–801. [48] Wang CY, Li X, Gao EL, Jian MQ, Xia KL, Wang Q, Xu ZP, Ren TL, Zhang YY. Carbonized silk fabric for ultrastretchable, highly sensitive, and wearable strain sensors. Adv Mater 2016;28:6640–8. [49] Li B, Luo J, Huang X, Lin L, Wang L, Hu M, Mai YW. A highly stretchable, superhydrophobic strain sensor based on polydopamine and graphene reinforced nanofiber composite for human motion monitoring. Compos B Eng 2020;181: 107580. [50] Li Y, Zhou B, Zheng G, Liu X, Li T, Yan C, Guo Z, et al. Continuously prepared highly conductive and stretchable flexible and wearable strain sensing fabrics SWNT/MWNT synergistically composited electrospun thermoplastic polyurethane yarns for wearable sensing. J Mater Chem C 2018;6(9):2258–69. [51] Chen Y, Wang L, Wu Z, Luo J, Li B, Huang X, Gao J. Super-hydrophobic, durable and cost-effective carbon black/rubber composites for high performance strain sensors. Compos B Eng 2019;176:107358. [52] Li L, Xiang H, Xiong Y, Zhao H, Bai Y, Wang S, Sun F, Hao M, Liu L, Li T, Peng Z, Xu J, Zhang T. Ultrastretchable fiber sensor with high sensitivity in whole workable range for wearable electronics and implantable medicine. Sci Adv 2018;5 (9):1800558–66. [53] Liu H, Gao H, Hu G. Highly sensitive natural rubber/pristine graphene strain sensor prepared by a simple method. Compos B Eng 2019;171:138–45. [54] Ren M, Zhou Y, Wang Y, Zheng G, Dai K, Liu C, Shen C. Highly stretchable and durable strain sensor based on carbon nanotubes decorated thermoplastic polyurethane fibrous network with aligned wave-like structure. Chem Eng J 2019; 360:762–77. [55] Lu L, Zhou Y, Pan J, Chen T, Hu Y, Zheng G, Peng H. Design of helically doubleleveled gaps for stretchable fiber strain sensor with ultralow detection limit, broad sensing range, and high repeatability. ACS Appl Mater Interfaces 2019;11(4): 4345–52.
[10] Hu Y, Zhao T, Zhu P, Zhang Y, Liang X, Sun R, Wong CP. A low-cost, printable, and stretchable strain sensor based on highly conductive elastic composites with tunable sensitivity for human motion monitoring. Nano Res 2018;11(4):1938–55. [11] Kim KH, Jang NS, Ha SH, Cho JH, Kim JM. Highly sensitive and stretchable resistive strain sensors based on microstructured metal nanowire/elastomer composite films. Small 2018;14:1704232–41. [12] Zhong J, Ma Y, Song Y, Zhong Q, Chu Y, Karakurt I, Lin L. A flexible piezoelectret actuator/sensor patch for mechanical human-machine interfaces. ACS Nano 2019; 13(6):7107–16. [13] Zhu C, Guan X, Wang X, Li Y, Chalmers E, Liu X. Mussel-Inspired flexible, durable, and conductive fibers manufacturing for finger-monitoring sensors. Adv Mater Interfaces 2019;6(1):1801547. [14] Son D, Lee J, Qiao S, Ghaffari R, Kim J, Lee JE, Song C, Kim SJ, Lee DJ, Jun SW, Yang S, Park M, Shin J, Do K, Lee M, Kang K, Hwang CS, Lu N, Hyeon T, Kim DH. Multifunctional wearable devices for diagnosis and therapy of movement disorders. Nat Nanotechnol 2014;9:397–404. [15] Zhou J, Xu X, Xin Y, Lubineau G. Coaxial thermoplastic elastomer-wrapped carbon nanotube fibers for deformable and wearable strain sensors. Adv Funct Mater 2019;28(16):1705591. [16] Gao J, Wang H, Huang X, Hu M, Xue H, Li RK. A super-hydrophobic and electrically conductive nanofibrous membrane for a chemical vapor sensor. J Mater Chem A 2018;6(21):10036–47. [17] Liu Z, Qi D, Hu G, Wang H, Jiang Y, Chen G, Luo Y, Loh XJ, Liedberg B, Chen X. Surface strain redistribution on structured microfibers to enhance sensitivity of fiber-shaped stretchable strain sensors. Adv Mater 2018;30:1704229. [18] Wang Y, Hao J, Huang Z, Zheng G, Dai K, Liu C, Shen C. Flexible electrically resistive-type strain sensors based on reduced graphene oxide-decorated electrospun polymer fibrous mats for human motion monitoring. Carbon 2018;126: 360–71. [19] Yang H, Yao X, Zheng Z, Gong L, Yuan L, Yuan Y, Liu Y. Highly sensitive and stretchable graphene-silicone rubber composites for strain sensing. Compos Sci Technol 2018;167:371–8. [20] Wang Y, Yang R, Shi Z, Zhang L, Shi D, Wang E, Zhang G. SuperElastic graphene ripples for flexible strain sensors. ACS Nano 2011;5:3645–50. [21] Zhang J, Cao Y, Qiao M, Ai L, Sun K, Mi Q, Zang S, Zuo Y, Yuan X, Wang Q. Human motion monitoring in sports using wearable graphene-coated fiber sensors. Sens Actuators A 2018;274:132–40. [22] Wei X, Cao X, Wang Y, Zheng G, Dai K, Liu C, Shen C. Conductive HerringboneStructure carbon nanotube/thermoplastic polyurethane porous foam tuned by epoxy for High performance flexible piezoresistive sensor. Compos Sci Technol 2017;149:166–77. [23] Jang S, Kim J, Kim DW, Kim J, Chun S, Lee HJ, Pang C. Carbon-based, ultraelastic, hierarchically coated fiber strain sensors with crack-controllable beads. ACS Appl Mater Interfaces 2019;11(1):615079–87. [24] He Z, Zhou G, Byun JH, Lee SK, Um MK, Park B, Chou TW. Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite yarns for ultrasensitive, wearable strain sensors. Nanoscale 2019;11:5884–90. [25] Wang X, Li J, Song H, Huang H, Gou J. Highly stretchable and wearable strain sensor based on printable carbon nanotube layers/polydimethylsiloxane composites with adjustable sensitivity. ACS Appl Mater Interfaces 2018;10(8): 7371–80. [26] Zhu GJ, Ren PG, Guo H, Jin YL, Yan DX, Li ZM. Highly sensitive and stretchable polyurethane fiber strain sensors with embedded silver nanowires. ACS Appl Mater Interfaces 2019;11(26):23649–58. [27] Liao X, Wang W, Wang L, Tang K, Zheng Y. Controllably enhancing stretchability of highly sensitive fiber-based strain sensors for intelligent monitoring. ACS Appl Mater Interfaces 2018;11(2):2431–40. [28] Chen S, Wei Y, Wei S, Lin Y, Liu L. Ultrasensitive cracking-assisted strain sensors based on silver nanowires/graphene hybrid particles. ACS Appl Mater Interfaces 2016;8(38):25563–70. [29] Lee S, Shin S, Lee S, Seo J, Lee J, Son S, Cho HJ, Algadi H, Al-Sayari S, Kim DE, Lee T. Ag nanowire reinforced highly stretchable conductive fibers for wearable electronics. Adv Funct Mater 2015;25:3114–21. [30] Pan J, Yang M, Luo L, Xu A, Tang B, Cheng D, Wang X. Stretchable and highly sensitive braided composite Yarn@Polydopamine@Polypyrrole for wearable applications. ACS Appl Mater Interfaces 2019;11(7):7338–48. [31] Cai G, Yang M, Pan J, Cheng D, Xia Z, Wang X, Tang B. Large-scale production of highly stretchable CNT/Cotton/Spandex composite yarn for wearable applications. ACS Appl Mater Interfaces 2018;10(38):32726–35. [32] Ren J, Wang C, Zhang X, Carey T, Chen K, Yin Y, Torrisi F. Environmentallyfriendly conductive cotton fabric as flexible strain sensor based on hot press reduced graphene oxide. Carbon 2017;111:622–30.
8
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Highly sensitive and durable wearable strain sensors from a core-sheath nanocomposite yarn Junjie Pan a, Baowei Hao a, Wenfang Song b, Shixian Chen a, Daiqi Li a, Lei Luo a, Zhigang Xia a, Deshan Cheng a, Anchang Xu a, ***, Guangming Cai a, **, Xin Wang c, * a b c
State Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan, 430200, China Garment Digital Engineering Center, School of Art and Design, Guangdong University of Technology, Guangzhou, 510000, China Centre for Materials and Future Fashion, School of Fashion and Textiles, RMIT University, Brunswick, Victoria 3056, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon nanotube Core-sheath Nanocomposite yarn Durability Wearable sensor
With fiber-based wearable electronics being developed and widely used in various fields, the safety and dura bility of nanomaterials in fibrous system become big concerns due to the leakage of nanomaterials and possible failure in functionality. To date, it is still challenging to fabricate wearable strain sensors with sensitivity and durability. Herein, we report a simple strategy for large-scale fabrication of core-sheath nanocomposite yarn (CSCY) based wearable sensor with high sensitivity and superior durability. By dispersing aqueous CNT ink into braided composite yarns as the core (BYs-CNT) and electrospun polyurethane (PU) nanofibers as the sheath, a unique core-sheath micro-nanofibrous structure was fabricated to grant the yarn with extremely high sensing sensitivity (maximum GF up to 980), long-term stability and great durability (against washing and dilute hy drochloric acid). The as-developed CSCY strain sensor was demonstrated to monitor real-time subtle and vigorous human activities. The present work provides insights for developing reliable and stable, highly sensi tive, flexible and durable wearable electronics with potential applications in different areas.
1. Introduction
properties of the fibrous materials, such as reliable stretchability and shape retention performance, elasticity and tenacity together with durability. Therefore, it is highly demanded to develop stretchable, flexible but sensitive wearable sensors with excellent durability for truly wearable applications. For most flexible wearable sensors, the conductive materials are located on the surface of the fiber materials. These conductive materials, usually nanomaterials, are easy to fall off after a long-term usage due to frequent stretching-releasing, bending or twisting and probable washing cycles. Under large mechanical deformations, flexible wearable sensors usually perform poorly owing to the failure in the conductive mecha nism. Hence, durability against washing, deformation and harsh envi ronments has been big concerns for fiber-based wearable sensors [36]. In addition, the detachment of carbon nanomaterials from fibers might cause health problems if the wearable sensors are in contact with the human body [37]. For example, CNTs have serious health hazards when they are inhaled into human body [38–40]. These threats have hindered
Wearable electronics have attracted increasing attention in recent years [1–8]. Flexible strain sensor as an important wearable electronic device has shown great application potentials in various fields including electronic skin [9–11], human-machine interfaces [12,13], and wear able medical devices [14]. Fibers and fibrous materials are often used as the matrix to bring flexibility and stretchability to strain sensors [15–17]. Previous studies have reported the using of conductive mate rials and elastic fiber materials to fabricate strain sensors. For example, graphene [18–21], carbon nanotubes [22–25], silver nanowires [26–29] and PPy were coated on elastic fiber surface to fabricate flexible strain sensors [30]. Graphene and carbon nanotubes functionalized textile yarns were used to fabricate strain sensors [31–33]. Other conductive materials were also deposited on the surface of fabrics for fabricating flexible strain sensors [34,35]. However, the sensitivity and stability of flexible strain sensors are largely determined by the mechanical
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (A. Xu), [email protected] (G. Cai), [email protected] (X. Wang). https://doi.org/10.1016/j.compositesb.2019.107683 Received 5 October 2019; Received in revised form 28 November 2019; Accepted 4 December 2019 Available online 7 December 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
J. Pan et al.
Composites Part B 183 (2020) 107683
2.2. Fabrication
the employment of CNT-coated fibers in e-textiles. Therefore, fabrica tion of flexible strain sensor with high washability and durability has great significance in the application of truly wearable electronics. It has been reported that CNT-based e-textiles by nanosoldering have excellent washability [37], but this method is only applicable to fabrics or non wovens. PDMS has been coated onto TPU/CNTs composite nanofibers for anti-corrosive wearable strain sensors [41]. However, the compact structure of PDMS affects the comfort performance, and this method may not be suitable for large-scale production. Immobilizing CNTs into the fibrous system as a composite yarn by adding CNTs in yarn spinning stage has enhanced the durability and safety of nanocomponents in textiles [31,33]. Furthermore, introducing a polydopamine template between fibers and functional components in the composite yarn resulted in more durable and stable wearable applications [30]. How ever, the structure of the composite yarn must be upgraded with enhanced mechanical properties and a higher level of protection of CNT against leaking before the yarn being applied in real wearable applications. Herein, we report a novel and scalable strategy for developing coresheath structure composite yarns (CSCY). By dip-coating aqueous CNT ink dispersion onto braided composite yarns as the core (BYs-CNT) and electrospinning PU nanofibers as the sheath, the core-sheath composite yarns (BYs-CNT-Nanofiber) have excellent mechanical properties and novel structure for durable and highly sensitive wearable sensing per formance. The nanocomposite yarn can be employed as reliable and stable, highly sensitive and flexible, durable and wearable electronics with potential applications in different areas.
Fig. 1 schematically shows the fabrication process of BYs-CNT. PET filaments were ultrasonically cleaned in deionized water for 30 min followed by winding on bobbins. After that, eight PET filament bobbins and an elastic filament were simultaneously braided into composite yarns (denoted as BYs) were fabricated and winded. The as-fabricated BYs were immersed in acetone under sonication for 30 min and were treated with ethanol under sonication for 30 min. The BYs were then washed with deionized water followed by vacuum drying in an oven at 60 � C. A beaker of 50 mL SWCNT solution was ultrasonically dispersed for 30 min to form CNT ink. The CNT ink was dried in an oven at 80 � C for 6 h to obtain highly viscous CNT solution (20 mL). The concentration of the conductive CNT ink was 0.375 wt% as calculated. The BYs were then dipped into the conductive CNT ink in an ultrasonic bath for 20 min followed by drying at 60 � C for 30 min. The CSCY was fabricated continuously in large-scale by coating PU nanofibers onto the as-prepared BYs-CNT (BYs-CNT-Nanofiber). The electrospinning setup consisted of dual needles pointing at each other, and an earthed collector mounted on a motor was used to wind up the coated yarn [42]. The polymer solution was prepared by dissolving PU pellets in N, N-dimethylformamide (DMF) at a concentration of 25%. The solution was then injected into the two needles which were oppo sitely charged (Fig. 1). While the electrospinning was ongoing, BYs-CNT was passing through the center of the rotating metal collector to collect nanofibers on its surface. As a result of the rotating of the metal plate, nanofibers were twisted onto the yarn. In the end, the CSCY was winded onto the motor collector. Two silver paste were sealed on each end of CSCY to fabricate CSCY strain sensors.
2. Experimental 2.1. Materials Braided composite yarns (BYs) were fabricated in our group with elastic polymers and PET fibers as the main components as per the previously reported method [30]. The core part was elastic polymers (0.5 mm in diameter) and the braiding part was PET fiber. Single-wall CNT (0.15 wt%) with a high purity was sourced from Nanjing XFNANO Materials Tech Co., Ltd., Nanjing, China. Polyurethane pellets (2363-80AE) were provided by Dow, U.S. N,N-dimethylformamide (DMF) was sourced from Sinoparm Chemical Reagent Co., Ltd.
2.3. Measurements and characterizations SEM images were obtained on a scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan). Photos were taken by a camera (OLYMPUS DSX510). Mechanical properties were tested using a universal testing system (Instron 5566) with a gauge length 250 mm and a drawing speed 80 mm/min. A digital multi-meter (Keysight Truevolt 34465A) was used to test the resistance of CSCY. The yarn was mounted between two insulative
Fig. 1. Fabrication process of core-sheath structure composite yarn. 2
J. Pan et al.
Composites Part B 183 (2020) 107683
clamps with a gauge length of 3 cm (see Fig. S-1) while the instant resistance was recorded. The relative change of resistance was then calculated according to the Formula: ΔR/R0¼(R-R0)/R0
after electrospinning of PU nanofibers on its surface, as seen from Fig. 2i–l. Interestingly, the electrospun PU membrane formed a porous micro-nano structure web with some fibers in micro-scale and others in nanoscale. The optical color of BYs-CNT-Nanofiber has changed from black to white as a result of coating with PU nanofibers (Inset of Fig. 2i). The cross-section views in Fig. 2m–p confirm the typical core-sheath structure of BYs-CNT-Nanofiber. There are three parts of the crosssection as seen from Fig. 2m, in which the core is the elastic polymer and the middle layer the twined PET fibers together with the surface layer a nanofibrous network. The difference between micro PET fibers and PU nanofibers can be clearly observed in Fig. 2o and p, and the thickness of the nanofibrous layer is around 50 μm. Detailed view of nanofibrous network in Fig. 2p shows the cutting edge of nanofibers without any interconnecting from cross-sectional view.
(1)
where R0 and R represent the initial resistance and current resistance under a given strain, respectively. Then, the gauge factor (GF) of the CSCY strain sensor was calculated by: δ(ΔR/R0)/δε
(2)
where ε is the strain and ΔR/R0 is from Formula (1). The motions of human body were detected by attaching CSCY sen sors (2 cm) onto different positions of body (e.g. finger, wrist, knee, neck and throat). Both ends of the sensors were stuck on skin using adhesive tapes. The electrical resistance was recorded during different move ments, and the ΔR/R0 was calculated subsequently according to For mula (1).
3.2. Mechanical properties Owing to the intrinsic elasticity of the elastic polymer and the special braided structure, BYs show an extension rate of up to 350% and a breaking force of 44 N, as shown in Fig. 3. Dip-coating of CNTs has not affected the mechanical properties of BYs much, as the BYs-CNT exhibits almost the same force-elongation curve as compared with that of BYs (Fig. 3a). It is evident that both BYs and BYs-CNT are highly stretchable and elastic. The breaking force of CSCY is slightly higher than that of BYs and BYs-CNT. BYs-CNT-Nanofiber is stronger due to the interconnected PU nanofibrous network on its surface. However, the breaking elongation of CSCY has sacrificed from 350% to 270% (Fig. 3b). Apparently, the interconnected PU nanofibrous layer has confined the stretchability of BYs-CNT. Nevertheless, >200% elongation would facilitate the wear able sensor applications of CSCY [40]. Due to the excellent mechanical properties, the as-prepared CSCY strain sensor shows high stretchability. As demonstrated in Fig. 3c, The CSCY sensor can be stretched from 0% to 40%, 50%, 100% and 125%, and the sensor becomes longer and thinner. The sensor can return to its
3. Results and discussion 3.1. Morphology Fig. 2a–d shows the typical braided structure of BYs with fiber bundles twining with each other on the surface. A core-sheath structured yarn with the elastic polymer as core and PET fibers as sheath was formed (Fig. 2d). Dip-coating of CNTs has resulted in a nanoscale layers on the surface of BYs, as seen from Fig. 2e. The inserts in Fig. 2a and e indicate that the color of the yarn has changed from white to black after the dip-coating due to the loading of CNT ink. As seen from Fig. 2g, the surface of BYs-CNT is fully covered by the CNT layer with the profile of fibers partly observable. Detailed view of the CNT layer in Fig. 2h sug gests that the CNTs have formed a thick network on the surface. These results confirm that CNT ink has been absorbed onto BYs successfully. An interconnected nanofibrous layer has been jacketed on BYs-CNT
Fig. 2. SEM images of BYs (a–d) and BYs-CNT (e–h); SEM images of CSCY (i–l) and its cross-sectional views (m–p). Insets: optical photos of corresponding samples. 3
J. Pan et al.
Composites Part B 183 (2020) 107683
Fig. 3. Force-elongation curves (a) and their corresponding values of force and elongation at break (b) for BYs, BYs-CNT and CSCY. Photos of the CSCY under different strains (c). (Sample size for tensile test: 5 cm length and ~1.04 mm radius).
original state without obvious permanent deformations (Fig. 3c), and the PU nanofiber network on CSCY is properly maintained without damaging.
3.3. Electromechanical performance The electrical resistance of CSCY is shown in Fig. S-2a. With the in crease of the number of dip-coating cycles the resistance of CSCY de creases sharply owing to the increased loading of CNTs. However, when
Fig. 4. Relative change of resistance for CSCY under increasing strains (a) (Inset: testing method), under cyclic strain of 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 20%, 30%, 40% and 50% (b–c), and under different drawing speeds of 10, 50, 75 and 100 mm/min (d) (30% strain). Time response of CSCY (e) (Insets: detailed views of selected areas). The stability test of the of CSCY under cyclic strain (f) (20% strain). 4
J. Pan et al.
Composites Part B 183 (2020) 107683
the cycle number is higher than three, the resistance of CSCY is stabi lized at 0.12 kΩ/cm. Owing to the electric conductivity of CNT, CSCY is electrically conductive with a linear current-voltage regression (Fig. S2b), exhibiting the ohmic behavior for the yarn. The slopes of both the BYs-CNT and BYs-CNT-Nanofiber are nearly identical, suggesting the same electrically conductive mechanism. The ΔR/R0 of the CSCY strain sensor under different strains is plotted in Fig. 4a. The ΔR/R0 increases linearly in the beginning, and it displays an exponential increase with the increase of strain. Gauge factor (GF) has widely been accepted as the measure of the sensitivity of strain sensors. Fig. 4a shows that CSCY strain sensors possess gauge factors of 46.4, 353 and 980 at the strain range of 0%–15%, 15%–29% and 29%– 44%, respectively. The GFs of reported fiber-based flexible strain sensors are listed in Table S-1 [30,31,33,41,45–55]. A careful comparison of all the GFs suggests that CSCY sensor has the highest GF in 0%–40% strain range. As sensitivity determines the effectiveness of wearable sensors in sensing human motions, it is highly expected that the CSCY possesses a high capacity of sensing human motions. The resistance of CSCY de creases when the strain is larger than 44%. The reason is probably that the neighboring PET fibers are close to each other when the strain is higher than 44%, and the contact areas increase to a level that the in crease in resistance is compromised slightly. Fig. S-3a-c shows the ΔR/R0–ε curve of CSCY under stretching/ releasing of small strains of 1%, 2% and 20%, respectively. The variation of ΔR/R0 is small, suggesting that the elastic polymer allows CSCY to recover its structure even in the 0–20% strain range. CSCY strain sensors are highly responsive to alternating strains. Fig. 4b and c shows the responses of the CSCY strain sensor under strains of 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 20%, 30%, 40% and 50%. As expected, the sensor shows stable responses under each strain. The ΔR/R0 of the sensor increases with the values consistent with the results in Fig. 4a. Fig. 4d shows the ΔR/R0 of CSCY during cyclic stretch with different drawing speeds (30% strain). There is no evident difference in the ΔR/ R0 under drawing speeds, suggesting the excellent adaptivity of the sensor to different stimuli. The excellent adaptivity of the sensor against different drawing speeds is due to its excellent elasticity and special braided structure. Reliable responses under different stimuli are crucial
in determining the reliability of strain sensors. Meanwhile, the response time of the CSCY strain sensor to the loading and unloading is within 200 m s (Fig. 4e) due to the viscoelasticity of the CSCY [43,44]. The electrical resistance of CSCY has no evident effect on the sensitivity and reliability of the strain sensors, as the curves of ΔR/R0 under different dip cycles are almost the same (Fig. S-3d). The stretchability of CSCY is almost the same with different loaded CNTs, thus the ΔR/R0 curves exhibit similar periodic spectra. Fig. 4f shows the stability of CSCY strain sensor under repeated stretching/releasing cycles with a tensile speed of 10 mm/min for 1000 cycles. The resistance of the strain sensor changes in the same way in each cycle with the peaks identical to each other (see Inset), indicating that the strain sensor has long-term stability and superior durability. The SEM images of CSCY before and after 1000 cycles are shown in Fig. S-4, and there is no obvious change in the micro-nanofibrous web. 3.4. The mechanism of strain sensing The sensing mechanism of the CSCY strain sensor is proposed in Fig. 5. Previous reports have shown that structural changes in braided composite yarns during stretching are the main factors for stain sensing yarns [30]. A dense carbon nanotube film has been formed on the sur face of BYs by coating high concentration CNT inks (Fig. 2e–h). When the CSCY is stretched, the PET fiber strands are forced to incline, forming an increased winding angle, as shown in Fig. 5b and c. The overlapping contacts between neighboring PET strands are thus decreased with contacts between neighboring PET fibers within each strand altered (Fig. 5d and e). With the increase of the winding angle, the areas be tween neighboring PET strands are slightly brighter (Fig. 5b and c), suggesting that cracks in the CNT film are formed due to the stretching. As a result, the longitudinal conductive path is changed, forcing elec trons to move within each PET strand and then pass through the over lapped areas (Fig. 5f). With the increase of strain, the cracks between neighboring PET strands are more evident and the overlapped areas are smaller (Fig. 5g). All in all, the resistance of CSCY increases with the increase of strain due to stretching. The excellent tensile properties of PU nanofibrous membrane ensure an intact surface structure of CSCY
Fig. 5. Photos of BYs-CNT under 0%, 20% and 40% strains (a–c), the proposed sensing mechanism (d–g), and the photos of CSCY under 0% and 40% strains (h, i). 5
J. Pan et al.
Composites Part B 183 (2020) 107683
without damage (Fig. 5h and i). Therefore, PU nanofibers play a pro tective role to avoid losing of CNTs during the stretching and releasing. CSCY has excellent elasticity, so it will return back to its original state when the strain is released. As a result, the PET strands interlace together with the winding angle decreased. The cracks within the CNT film rejoin to form an intact film again, the electric conduction is thus recovered with electrical resistance decreased. Therefore, the conduc tive performance of the CSCY is repeatable and reliable to grant the sensing capacity of the CSCY strain sensors.
excellent durability and stability together with reliability. 3.6. Wearable applications The highly flexible and stretchable CSCY sensor has high sensitivity, reliability and durability, thus it can be employed to detect human motions (both subtle and vigorous). The CSCY strain sensor was attached onto finger, wrist, knee and throat to monitor a range of ac tivities. As shown in Fig. 7a, when the finger (Inset) bends to a certain degree rapidly, the ΔR/R0 of the CSCY sensor increases to a certain value, and the higher the bending angle, the higher the value of ΔR/R0. Cyclic bending/extending of the pointing finger leads to a repeated growing/dropping of ΔR/R0, indicating the fast responses of the CSCY sensor. Repeatable responses of the CSCY strain sensor can also be observed when sensing human wrist (as shown in Fig. 7b), as the ΔR/R0 curve shows periodical changes corresponding to repeated bending. Fig. 7c presents the spectra of ΔR/R0 corresponding to handwriting. Biceps brachii movement and clenching fist were also detected by CSCY, proving that light movements of muscle could be detected (see Fig. S-5). The CSCY sensor was fixed on a knee to sense the motions of the joint. The ΔR/R0 value changed immediately corresponding to the extending and bending of the knee, with the signals recorded in Fig. 7d. Fig. S-6 shows that the ΔR/R0 responds swiftly to the shifting between a small and a deep bending of elbow. The CSCY sensor can monitor subtle motions including pronouncing words and drinking water. Fig. 7e shows the responses of the CSCY sensor to the motions of throat when pronouncing “a”, sensor” and “textile”. The ΔR/R0 value of the sensor changes accordingly with a distinctive spectrum for each word. Besides, the motion of drinking the same volume of water (30 mL per mouthful) can be monitored as the spectrum shows similar patterns (Fig. 7f). Meanwhile, the CSCY strain sensor successfully detected the motion of nodding (Fig. S-7).
3.5. Durability Coating of electrospun nanofibrous network on the surface has granted CSCY with excellent durability. Fig. 6a shows the ΔR/R0 of CSCY sensor after repeated folding and relaxing. It can be seen that during the 100 times folding and relaxing, the ΔR/R0 fluctuates around 1 with less than 10% deviation. Besides, the CSCY sensor is durable against washing and treating with hydrochloric acid for a period of time. As shown in Fig. 6b, the resistance of CSCY increases a little bit (<10% deviation) even after washing for 5 cycles (10 min each cycle with ul trasonic frequency 40 kHz). Besides, the resistance of CSCY increases slightly after being placed in 15% dilute hydrochloric acid for 10 h (Fig. 6c). The excellent durability of CSCY has resulted in its stable and reliable sensing performance. Even after bending for 100 times, the sensing spectra of CSCY sensors are almost identical (Fig. 6d). The strain sensing performance of the sensor has been maintained after 5 times of ultra sound washing, as the signals are the same in Fig. 6d. The sensing per formance can resist harsh environment due to the resistance of PU nanofibers. The sensor was placed in 15% dilute hydrochloric acid for 10 h and then its sensing performance was tested at a 10% strain. The ΔR/R0 is nearly the same as that before acid treatment (Fig. 6d). The experimental results show that the as-prepared CSCY sensor has
Fig. 6. The R/R0 of the CSCY sensor after repeated folding and relaxing (bending from 3 cm to 2.5 cm) (a), after repeated washing cycles (ultrasonic washing for 10 min each time under an ultrasonic frequency 40 kHz) (b), and after acid treatment (in 15% dilute hydrochloric acid for 10 h) (c). The strain sensing performance of CSCY sensor under different conditions (folding, washing and treating with 15% dilute hydrochloric acid) (d). 6
J. Pan et al.
Composites Part B 183 (2020) 107683
Fig. 7. Wearable applications of CSCY strain sensors in monitoring human motions including bending finger (a), bending wrist (b), writing (c), bending leg (d), speaking (e) and drinking (f).
Considering the lightweight and flexibility of CSCY, the composite yarn has the potential be woven or knitted into fabrics [31,50], serving as part of a textile or as a whole piece of textile for multiple applications including sensing, heating and thermochromic color changing.
draft. Xin Wang: Conceptualization, Methodology, Investigation, Writing - review & editing.
4. Conclusions
This work was supported by the Foundation of Science Research Program from Hubei Provincial Department of Education (Grant No. Q20181705) and the National Natural Science Foundation of China (Grant No. 51606131).
Acknowledgements
In summary, a highly flexible and sensitive strain sensor with supe rior durability was fabricated from a core-sheath nanocomposite yarn by coating PU nanofibers onto BYs-CNT. The CSCY sensor has flexibility and stretchability based on the excellent mechanical properties of the BYs. Owing to the unique micro-nano structure of CSCY, the obtained strain sensor exhibited high sensitivity and responsivity (maximum GF 980), and long-term stability. The PU nanofibrous network as the pro tective layer on the CSCY granted the strain sensor with superior dura bility and stability against bending, washing and even acidic solution. The CSCY sensor was demonstrated to accurately monitor a range of human motions (both subtle and vigorous). This work provides a novel strategy to fabricate highly sensitive and durable strain sensor devices by designing a novel core-sheath micro-nanofibrous structure.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107683. References [1] Lee YY, Kang HY, Gwon SH, Choi GM, Lim SM, Sun JY, Joo YC. A strain-insensitive stretchable electronic conductor: PEDOT:PSS/acrylamide organogels. Adv Mater 2016;28:1636–43. [2] Guo Q, Huang B, Lu C, Zhou T, Su G, Jia L, Zhang X. A cephalopod-inspired mechanoluminescence material with skin-like self-healing and sensing properties. Mater Horiz 2019;6:996–1004. [3] Song H, Zhang J, Chen D, Wang K, Niu S, Han Z, Ren L. Superfast and highsensitivity printable strain sensors with bioinspired micron-scale cracks. Nanoscale 2017;9:1166–73. [4] Kim S-W, Kwon S-N, Na S-I. Stretchable and electrically conductive polyurethanesilver/graphene composite fibers prepared by wet-spinning process. Compos B Eng 2019;165:571–81. [5] Liao X, Zhang Z, Liao Q, Liang Q, Ou Y, Xu M, Li M, Zhang G, Zhang Y. Flexible and printable paper-based strain sensors for wearable and large-area green electronics. Nanoscale 2016;8:13025–32. [6] Su X, Li H, Lai X, Chen Z, Zeng X. Highly stretchable and conductive superhydrophobic coating for flexible electronics. ACS Appl Mater Interfaces 2019; 10(12):10587–97. [7] Montazerian H, Dalili A, Milani AS, Hoorfar M. Piezoresistive sensing in chopped carbon fiber embedded PDMS yarns. Compos B Eng 2019;164:648–58. [8] Liao X, Zhang Z, Kang Z, Gao F, Liao Q, Zhang Y. Ultrasensitive and stretchable resistive strain sensors designed for wearable electronics. Mater Horiz 2017;4: 502–10. [9] Qiao Y, Wang Y, Tian H, Li M, Jian J, Wei Y, Wang X, et al. Multilayer graphene epidermal electronic skin. ACS Nano 2018;12:8839–46.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Junjie Pan: Methodology, Investigation, Writing - original draft. Baowei Hao: Methodology, Investigation. Wenfang Song: Methodol ogy. Shixian Chen: Investigation. Daiqi Li: Investigation. Lei Luo: Methodology, Investigation. Zhigang Xia: Methodology, Investigation. Deshan Cheng: Methodology, Investigation. Anchang Xu: Conceptu alization, Methodology, Investigation. Guangming Cai: Conceptuali zation, Methodology, Investigation, Supervision, Writing - original 7
J. Pan et al.
Composites Part B 183 (2020) 107683 [33] Cai G, Yang M, Xu Z, Liu J, Tang B, Wang X. Flexible and wearable strain sensing fabrics. Chem Eng J 2017;325:396–403. [34] Lin CW, Zhao Z, Kim J, Huang J. Pencil drawn strain gauges and chemiresistors on paper. Sci Rep 2015;4:3812. [35] Zhang F, Wu S, Peng S, Wang CH. The effect of dual-scale carbon fibre network on sensitivity and stretchability of wearable sensors. Compos Sci Technol 2018;165: 131–9. [36] Chen S, Liu H, Liu S, Wang P, Zeng S, Sun L, Liu L. Transparent and waterproof ionic liquid-based fibers for highly durable multifunctional sensors and straininsensitive stretchable conductors. ACS Appl Mater Interfaces 2018;10:4305–14. [37] Du D, Tang Z, Ouyang J. Highly washable e-textile prepared by ultrasonic nanosoldering of carbon nanotubes onto polymer fibers. J Mater Chem C 2018;6 (4):883–9. [38] Zeng W, Shu L, Li Q, Chen S, Wang F, Tao XM. Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications. Adv Mater 2014;26(31): 5310–36. [39] Lam CW, James JT, McCluskey R, Hunter RL. Pulmonary toxicity of single-wall carbon nanotubes in mice 7 and 90 days after intratracheal instillation. Toxicol Sci 2004;77(1):126–34. [40] Donaldson K, Aitken R, Tran L, Stone V, Duffin R, Forrest G, Alexander A. Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety. Toxicol Sci 2006;92(1):5–22. [41] Wang L, Chen Y, Lin L, Wang H, Huang X, Xue H, Gao J. Highly stretchable, anticorrosive and wearable strain sensors based on the PDMS/CNTs decorated elastomer nanofiber composite. Chem Eng J 2019;362:89–98. [42] Zheng X, Zhang K, Yao L, Qiu Y, Wang S. Hierarchically porous sheath–core graphene-based fiber-shaped supercapacitors with high energy density. J Mater Chem A 2018;6(3):896–907. [43] Pang C, Lee GY, Kim TI, Kim SM, Kim HN, Ahn SH, Suh KY. A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres. Nat Mater 2012;11(9):795. [44] Pu JH, Zhao X, Zha XJ, Bai L, Ke K, Bao RY, Yang W. Multilayer structured AgNW/ WPU-MXene fiber strain sensors with ultrahigh sensitivity and wide operating range for wearable monitoring and healthcare. J Mater Chem A 2019;7:15913–23. [45] Cheng Y, Wang R, Sun J, Gao L. A stretchable and highly sensitive graphene-based fiber for sensing tensile strain, bending, and torsion. Adv Mater 2015;27(45): 7365–71. [46] Li L, Bai Y, Li L, Wang S, Zhang T. A superhydrophobic smart coating for flexible and wearable sensing electronics. Adv Mater 2017;29(43):1702517–24. [47] Zhang MC, Wang CY, Wang HM, Jian MQ, Hao XY, Zhang YY. Carbonized cotton fabric for high-performance wearable strain sensors. Adv Funct Mater 2017;27(2): 1604795–801. [48] Wang CY, Li X, Gao EL, Jian MQ, Xia KL, Wang Q, Xu ZP, Ren TL, Zhang YY. Carbonized silk fabric for ultrastretchable, highly sensitive, and wearable strain sensors. Adv Mater 2016;28:6640–8. [49] Li B, Luo J, Huang X, Lin L, Wang L, Hu M, Mai YW. A highly stretchable, superhydrophobic strain sensor based on polydopamine and graphene reinforced nanofiber composite for human motion monitoring. Compos B Eng 2020;181: 107580. [50] Li Y, Zhou B, Zheng G, Liu X, Li T, Yan C, Guo Z, et al. Continuously prepared highly conductive and stretchable flexible and wearable strain sensing fabrics SWNT/MWNT synergistically composited electrospun thermoplastic polyurethane yarns for wearable sensing. J Mater Chem C 2018;6(9):2258–69. [51] Chen Y, Wang L, Wu Z, Luo J, Li B, Huang X, Gao J. Super-hydrophobic, durable and cost-effective carbon black/rubber composites for high performance strain sensors. Compos B Eng 2019;176:107358. [52] Li L, Xiang H, Xiong Y, Zhao H, Bai Y, Wang S, Sun F, Hao M, Liu L, Li T, Peng Z, Xu J, Zhang T. Ultrastretchable fiber sensor with high sensitivity in whole workable range for wearable electronics and implantable medicine. Sci Adv 2018;5 (9):1800558–66. [53] Liu H, Gao H, Hu G. Highly sensitive natural rubber/pristine graphene strain sensor prepared by a simple method. Compos B Eng 2019;171:138–45. [54] Ren M, Zhou Y, Wang Y, Zheng G, Dai K, Liu C, Shen C. Highly stretchable and durable strain sensor based on carbon nanotubes decorated thermoplastic polyurethane fibrous network with aligned wave-like structure. Chem Eng J 2019; 360:762–77. [55] Lu L, Zhou Y, Pan J, Chen T, Hu Y, Zheng G, Peng H. Design of helically doubleleveled gaps for stretchable fiber strain sensor with ultralow detection limit, broad sensing range, and high repeatability. ACS Appl Mater Interfaces 2019;11(4): 4345–52.
[10] Hu Y, Zhao T, Zhu P, Zhang Y, Liang X, Sun R, Wong CP. A low-cost, printable, and stretchable strain sensor based on highly conductive elastic composites with tunable sensitivity for human motion monitoring. Nano Res 2018;11(4):1938–55. [11] Kim KH, Jang NS, Ha SH, Cho JH, Kim JM. Highly sensitive and stretchable resistive strain sensors based on microstructured metal nanowire/elastomer composite films. Small 2018;14:1704232–41. [12] Zhong J, Ma Y, Song Y, Zhong Q, Chu Y, Karakurt I, Lin L. A flexible piezoelectret actuator/sensor patch for mechanical human-machine interfaces. ACS Nano 2019; 13(6):7107–16. [13] Zhu C, Guan X, Wang X, Li Y, Chalmers E, Liu X. Mussel-Inspired flexible, durable, and conductive fibers manufacturing for finger-monitoring sensors. Adv Mater Interfaces 2019;6(1):1801547. [14] Son D, Lee J, Qiao S, Ghaffari R, Kim J, Lee JE, Song C, Kim SJ, Lee DJ, Jun SW, Yang S, Park M, Shin J, Do K, Lee M, Kang K, Hwang CS, Lu N, Hyeon T, Kim DH. Multifunctional wearable devices for diagnosis and therapy of movement disorders. Nat Nanotechnol 2014;9:397–404. [15] Zhou J, Xu X, Xin Y, Lubineau G. Coaxial thermoplastic elastomer-wrapped carbon nanotube fibers for deformable and wearable strain sensors. Adv Funct Mater 2019;28(16):1705591. [16] Gao J, Wang H, Huang X, Hu M, Xue H, Li RK. A super-hydrophobic and electrically conductive nanofibrous membrane for a chemical vapor sensor. J Mater Chem A 2018;6(21):10036–47. [17] Liu Z, Qi D, Hu G, Wang H, Jiang Y, Chen G, Luo Y, Loh XJ, Liedberg B, Chen X. Surface strain redistribution on structured microfibers to enhance sensitivity of fiber-shaped stretchable strain sensors. Adv Mater 2018;30:1704229. [18] Wang Y, Hao J, Huang Z, Zheng G, Dai K, Liu C, Shen C. Flexible electrically resistive-type strain sensors based on reduced graphene oxide-decorated electrospun polymer fibrous mats for human motion monitoring. Carbon 2018;126: 360–71. [19] Yang H, Yao X, Zheng Z, Gong L, Yuan L, Yuan Y, Liu Y. Highly sensitive and stretchable graphene-silicone rubber composites for strain sensing. Compos Sci Technol 2018;167:371–8. [20] Wang Y, Yang R, Shi Z, Zhang L, Shi D, Wang E, Zhang G. SuperElastic graphene ripples for flexible strain sensors. ACS Nano 2011;5:3645–50. [21] Zhang J, Cao Y, Qiao M, Ai L, Sun K, Mi Q, Zang S, Zuo Y, Yuan X, Wang Q. Human motion monitoring in sports using wearable graphene-coated fiber sensors. Sens Actuators A 2018;274:132–40. [22] Wei X, Cao X, Wang Y, Zheng G, Dai K, Liu C, Shen C. Conductive HerringboneStructure carbon nanotube/thermoplastic polyurethane porous foam tuned by epoxy for High performance flexible piezoresistive sensor. Compos Sci Technol 2017;149:166–77. [23] Jang S, Kim J, Kim DW, Kim J, Chun S, Lee HJ, Pang C. Carbon-based, ultraelastic, hierarchically coated fiber strain sensors with crack-controllable beads. ACS Appl Mater Interfaces 2019;11(1):615079–87. [24] He Z, Zhou G, Byun JH, Lee SK, Um MK, Park B, Chou TW. Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite yarns for ultrasensitive, wearable strain sensors. Nanoscale 2019;11:5884–90. [25] Wang X, Li J, Song H, Huang H, Gou J. Highly stretchable and wearable strain sensor based on printable carbon nanotube layers/polydimethylsiloxane composites with adjustable sensitivity. ACS Appl Mater Interfaces 2018;10(8): 7371–80. [26] Zhu GJ, Ren PG, Guo H, Jin YL, Yan DX, Li ZM. Highly sensitive and stretchable polyurethane fiber strain sensors with embedded silver nanowires. ACS Appl Mater Interfaces 2019;11(26):23649–58. [27] Liao X, Wang W, Wang L, Tang K, Zheng Y. Controllably enhancing stretchability of highly sensitive fiber-based strain sensors for intelligent monitoring. ACS Appl Mater Interfaces 2018;11(2):2431–40. [28] Chen S, Wei Y, Wei S, Lin Y, Liu L. Ultrasensitive cracking-assisted strain sensors based on silver nanowires/graphene hybrid particles. ACS Appl Mater Interfaces 2016;8(38):25563–70. [29] Lee S, Shin S, Lee S, Seo J, Lee J, Son S, Cho HJ, Algadi H, Al-Sayari S, Kim DE, Lee T. Ag nanowire reinforced highly stretchable conductive fibers for wearable electronics. Adv Funct Mater 2015;25:3114–21. [30] Pan J, Yang M, Luo L, Xu A, Tang B, Cheng D, Wang X. Stretchable and highly sensitive braided composite Yarn@Polydopamine@Polypyrrole for wearable applications. ACS Appl Mater Interfaces 2019;11(7):7338–48. [31] Cai G, Yang M, Pan J, Cheng D, Xia Z, Wang X, Tang B. Large-scale production of highly stretchable CNT/Cotton/Spandex composite yarn for wearable applications. ACS Appl Mater Interfaces 2018;10(38):32726–35. [32] Ren J, Wang C, Zhang X, Carey T, Chen K, Yin Y, Torrisi F. Environmentallyfriendly conductive cotton fabric as flexible strain sensor based on hot press reduced graphene oxide. Carbon 2017;111:622–30.
8