Core-sheath nanofiber yarn for textile pressure sensor with high pressure sensitivity and spatial tactile acuity

Journal Pre-proofs Core-sheath nanofiber yarn for textile pressure sensor with high pressure sensitivity and spatial tactile acuity Kun Qi, Hongbo Wang, Xiaolu You, Xuejiao Tao, Mengying Li, Yuman Zhou, Yimin Zhang, Jianxin He, Weili Shao, Shizhong Cui PII: DOI: Reference:

S0021-9797(19)31384-0 https://doi.org/10.1016/j.jcis.2019.11.059 YJCIS 25676

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

19 August 2019 6 November 2019 15 November 2019

Please cite this article as: K. Qi, H. Wang, X. You, X. Tao, M. Li, Y. Zhou, Y. Zhang, J. He, W. Shao, S. Cui, Core-sheath nanofiber yarn for textile pressure sensor with high pressure sensitivity and spatial tactile acuity, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.11.059

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Journal of Colloid and Interface Science Core-sheath nanofiber yarn for textile pressure sensor with high pressure sensitivity and spatial tactile acuity

Kun Qia, Hongbo Wanga*, Xiaolu Youa, Xuejiao Taob,c, Mengying Lib,c, Yuman Zhoub,c, Yimin Zhangb,c, Jianxin Heb,c*, Weili Shaob,c, Shizhong Cuib,c a School

of Textile and Clothing, Jiangnan University, 1800 Lihu Road, Wuxi City 214122, Jiangsu Province, China.

b Provincial

Key Laboratory of Functional Textile Materials, Zhongyuan University of Technology, Zhengzhou 450007, China.

c

Collaborative Innovation Center of Textile and Garment Industry, Zhengzhou 450007, Henan, China.

Corresponding author: Hongbo Wang School of Textile and Clothing, Jiangnan University, 1800 Lihu Road, Wuxi City 214122, Jiangsu Province, People’s Republic of China E-mail: [email protected]

Co-corresponding author: Jianxin He P.O. Box 110, College of Textiles, Zhongyuan University of Technology, 41 Zhongyuan Road, Zhengzhou City 450007, Henan Province, People’s Republic of China E-mail: [email protected]

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Graphical abstract

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Core-sheath nanofiber yarn for textile pressure sensor with high pressure sensitivity and spatial tactile acuity Abstract Highly sensitive wearable textile pressure sensors represent the key components of smart textiles and personalized electronics, with potential applications in biomedical monitoring, electronic skin, and human-machine interfacing. Here, we present a simple and low-cost strategy to fabricate highly sensitive wearable textile pressure sensors for non-invasive human motion and physiological signal monitoring and the detection of dynamic tactile stimuli. The wearable textile sensor was woven using a one-dimensional (1D) weavable core-sheath nanofiber yarn, which was obtained by coating a Ni-coated cotton yarn electrode with carbon nanotube (CNT)-embedded polyurethane (PU) nanofibers using a simple electrospinning technique. In our design, the three-dimensional elastic porous nanofiber structure of the forcesensing layer and hierarchical fiber-bundled structure of the conductive Ni-coated electrode provide the sensor with a relatively large surface area, and a sufficient surface roughness and elasticity. This leads to rapid and sharp increases in the contact area under stimuli with low external pressure. As a result, the textile pressure sensor exhibits the advantages of a high sensitivity (16.52 N-1), wide sensing range (0.003-5 N), and short response time (~0.03s). Owing to these merits, our textile-based sensor can be directly attached to the skin as usual and conformally fit the shape deformations of the body’s complex flexible curved surfaces. This contributes to the reliable real-time monitoring of human movements, ranging from subtle physiological signals to vigorous movements. Moreover, a large-area textile sensing matrix is successfully fabricated for tactile mapping of spatial pressure by being worn on the surface of wrist, highlighting the tremendous potential for applications in smart textiles and wearable electronics.

Keywords: flexible pressure sensor, carbon nanotube, electrospun nanofibers, electronic textile, tactile sensing

1. Introduction In recent years, smart electronic textiles (E-textiles) have attracted considerable attention, owing to their potential applications in the next generation of portable and wearable electronic devices [1-7]. In particular, wearable textile pressure sensors are important components of smart E-textiles [8-11]. Compared with traditional bulky or planar structures, wearable textile pressure sensors can be readily incorporated into various kinds of daily textiles, and worn directly against the surface of the skin for close contact with curvilinear surfaces. These have unique advantages in terms of their light weight and portability, softness, breathability, and comfort, and are capable of mimicking the perceptions and responses of human skin to external stimuli [12-14]. Furthermore, these have demonstrated considerable application potential in artificial intelligence, disease diagnosis, exercise tracking, and health monitoring [15-19]. For practical applications, the development of wearable textile pressure sensors with high sensitivity, a fast respond speed, and a wide sensing range based on advanced materials and structural designs is highly desirable for precise capturing and monitoring of various human activities and subtle physiological signals. To date, numerous studies have focused on developing textile-based pressure sensors based on various sensing mechanisms, including piezoresistive [17, 20-21], piezoelectric [22], capacitive [23-25], or triboelectric effects [26]. Among these, textile pressure sensors based on piezoresistive sensing are of considerable interest, owing to their simplicity and low-cost structural 3

design and implementation, as well as simple signal measurement [8-9]. In general, it is considered that the typical structure of a piezoresistive pressure sensor mainly consists of two aspects: (1) elastomeric piezoresistive sensing materials and (2) flexible top/bottom conductive electrodes. Recently, piezoresistive sensing layers have mainly been prepared by loading conductive nanomaterials onto flexible substrates, such as carbon nanotubes [27-30], graphene [31-32], nanowires [20, 33], and nanoparticles [14, 34]. The piezoresistive effect of the sensor mainly depends on the force-induced deformation behaviors of the piezoresistive sensing layer and electrode layer, which can change the effective contact area between the sensing materials, resulting in resistance variations in the sensor. Therefore, surface microstructures play an important role in improving the pressure-sensing performance. In particular, to further enhancing the sensitivity and response speed of a device, introducing micro/nano structures on the electrode or piezoresistive layer to achieve pronounced deformation behavior under small pressure is critical in the device design. For example, a variety of surface micro/nanostructures including “micropyramids,”[35] “interlocked microdome arrays,’’ [28, 36-37] “silk-molded microstructures,”[38]“hollow-spheres,”[39-40] “wavy microstructures,” [13, 41-42] and “bionic microstructures” [31, 43] have been widely adopted to enhance the sensitivities of planar pressure sensors. However, most of the aforementioned microstructures often involve complicated and time-consuming fabrication processes. Fortunately, textile structures composed of soft fibers and yarns represent ideal ready-made microstructured substrates for flexible and wearable electronic devices [4, 8]. As a representative example, Wang et al. prepared an all-textile pressure sensor based on CNT-coated cotton fabric and a Ni-coated textile electrode with a high sensitivity of up to 14.4 kPa−1, demonstrating the considerable promise of smart textiles or wearable electronics [8]. In another study, Yuan et al. reported a skin-like sensor based on a graphene/polymer textile with a spring-like mesh network, which not only achieved high sensitivity of 72 kPa−1, but also yielded a low detection of 1.38 Pa [44]. Despite achieving significant device sensitivity, most of these devices adopt conventional textile structures composed of micron fibers on a larger scale. Based on the abovementioned reasons, integrating nanoscale fiber structures into a textile sensor represents a promising strategy for addressing the requirements of enhanced sensing properties. In contrast, electrospinning is a simple, versatile, and scalable approach to fabricating highly interconnected nanofiber structures [45-46]. Note that electrospun nanofiber structures have ultra-high surface areas and easy deformable porous microstructures, and have been widely employed in high-performance wearable electronic devices, achieving significantly enhanced performances [26, 47-49]. However, previously reported nanofiber-based devices have usually adopted randomly oriented nonwoven nanofiber structures [50-52]. These are not capable of weavability, which prohibits their application in smart E-textiles. An effective approach to solving this problem is to develop a one-dimensional (1D) weavable yarn using nanofiber structures, which can be further woven into flexible textiles or integrated into clothes to construct high-performance wearable textile-based pressure sensors with high sensitivity and a stable performance. In this work, we describe a wearable textile pressure sensor woven with 1D weavable core-sheath nanofiber yarns, which exhibits high sensitivity and a wide sensing range for static and dynamic tactile pressure sensing applications. The CNTembedded PU nanofibers, which have urtra-high surface areas and are easy deformable, serve as force-sensing elements, and are uniformly twisted and wound around the surface of a Ni-coated cotton yarn electrode using a simple electrospinning process to achieve the proposed weavable core-sheath nanofiber yarn. Hence, a resistive textile-based pressure sensor is formed at the point-to-point overlapping cross point of the interlaced structure of the core-sheath nanofiber yarn in textile form. In conjunction with the hierarchical fiber-bundled structure of the conductive Ni-coated electrode, the stable coxial structure of the core-sheath nanofiber yarn and three-dimensional (3D) elastic porous nanofiber structure of the force-sensing layer offer a larger contact area and superior deformation behavior under small loads. This enables the textile sensor to simultaneously achieve both a high sensitivity and wide sensing range. The wearable textile sensor sensor is applied for sensitive monitoring of subtle physiological signals and human movements. In addition, large-area textile sensor arrays are successfully fabricated to spatially map tactile stimuli through being worn on the wrist surface, suggesting considerable potential for wearable electronics and smart textiles for biomedical monitoring, electronic skin, and human–machine interfacing.

2. Results and Discussion Fig.1 illustrates the fabrication process of the textile tactile pressure sensor. The fabrication process involves three key steps: (i) Flexible Ni-coated conductive cotton yarns are fabricated as the core electrode. (ii) Core-sheath nanofiber sensing yarns are created by directly helically wrapping the CNT-embedded PU nanofibers on the surface of the core electrode through an improved electrospinning technique. (iii) Large-area wearable textile can be woven plainly using the as-received core-sheath nanofiber sensing yarns, and a piezoresistive-type textile-based pressure sensor is successfully constructed at the

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Fig. 1 Schematics for the fabrication process of the textile pressure sensor. Inset: Optical images of the core-sheath nanofiber sensing yarn (i) and the electronic textile (ii).

cross points of the textile. First, to obtain a highly conductive and weavable fiber-shaped core electrode, commercial natural cotton yarns were selected as the flexible platform, which were then subject to the electroless deposition of Ni metal coating. Because cotton yarns are hydrophilic and soft, the electroless Ni plating solution can penetrate into the inner space of the cotton yarn, and the Ni was uniformly and densely deposited on the surfaces of both the inside and outside cotton fibers of the yarn, forming a continuous and uniform conductive network. Fig.2a-b and Fig.S1 present typical scanning electron microscopy (SEM) images and EDX mapping of the Ni-coated cotton yarn, showing that each individual cotton fiber is densely and uniformly coated with a layer of Ni nanoparticles. The X-ray diffraction (XRD) pattern of the Ni-coated cotton yarn also confirms that the diffraction peaks of metallic Ni layer appear at 44.9, 52.3, and 76.6°, corresponding to the (111), (200), and (220) peaks of the face-centered cubic Ni metals, respectively(Fig. S2). Remarkably, this hierarchical composite electrode structure not only allows effective charge transportation between the sensing element and Ni surfaces, but also provides large contact areas and sufficient roughness for mechanical deformations of the nanofiber piezoresistive sensing layer under applied pressure. The initial resistance of Ni-coated cotton yarns can be controlled effectively by the electroless nickel plating time. When the electroless nickel plating time is 4 h, the Ni-coated cotton yarns exhibited high metal-like electrical conductivity, with an initial resistance of 3.82 Ωcm-1 (Fig. S3b-c). The cross-sectional SEM image in Figure 2b shows that the thickness of the thin Ni coating was only approximately 817 nm. Thus, the as-prepared Ni-coated cotton yarns are light-weight and can retain the flexibility and weavability of the textile yarns (Fig.S3a). Meanwhile, stress-strain tests proved that the Ni-coated cotton yarns were 25% stronger than pristine cotton yarns (Fig.S3d). To further evaluate the electrical stability and durability of the Ni-coated conductive cotton yarns, bending cycle tests were performed using a linear motor. With larger bending angles of 45°and 75°, the resistance of the Ni-coated cotton yarns only increased by 2% and 5%, respectively, after 2000 bending cycles (Fig.S4a). This can be attributed to the appearances of microcracks in the surface of the Ni coating during the bending

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test (see Fig.S4b-e). The Ni-coated cotton yarns with 4 h electroless nickel plating exhibited unique and promising advantages in terms of their light weight, high conductivity, excellent textile-like flexibility, and good durability. Therefore, the Ni-coated cotton yarns can be utilized as core electrodes of wearable textile sensors.

Fig. 2 (a) SEM image of a Ni-coated cotton yarn with an electroless deposition time of 4 h. (b) high-magnification SEM image of (a). The inset shows the cross-section SEM image of the Ni-coated cotton yarn. The top-view (c) and cross-section (d) SEM images of the core-sheath nanofiber sensing yarn. (e) high-magnification SEM image of (c). (f) TEM image of the nanofiber in (e) with embedded CNT arrays.

CNT-embedded PU nanofibers served as the force sensing elements and were uniformly twisted and wound around the surface of the core electrode by the conjugate principle of electrospinning to produce the core-sheath nanofiber sensing yarns (hereafter noted as NTPNF@NiCY sensing yarn) (see the Fig. S5 in the Supporting Information for fabrication details). As can be observed from the SEM images of the NTPNF@NiCY sensing yarn in Fig.2c-e, the CNT-embedded PU nanofibers were aligned in a regular twist, and tightly wound around the surface of the Ni-coated core yarn, forming a coaxial structure composed of a nanofiber piezoresistive sensing layer and a Ni-coated conductive cotton yarn electrode. The stable coaxial structure contributed to maintaining closer contact between the nanofiber piezoresistive sensing layer and the conductive coating while undergoing mechanical deformations, thereby endowing the final NTPNF@NiCY sensing yarns with an excellent electromechanical performance and stable sensing properties. Transmission electron microscopy (TEM) images of the nanofibers confirmed that the 1D CNT arrays were successfully embedded into the PU nanofibers by utilizing a simple electrospinning process (see Fig.2f). As a force-sensing layer, the CNT-embedded PU nanofibers with an average diameter of 423 nm exhibited a 3D elastic porous nanofiber structure with an ultra-large surface area, which contributed to notable enhancements in the deformation space and reversible mechanical properties under applied pressure, leading to a large variation in the contact resistance. Thanks to the excellent flexibility and mechanical robustness, the core-sheath nanofiber sensing yarn can easily be woven into large-area textile sensors to conformally laminate to human skin for various wearable applications (see the inset i and ii in Fig.1). To study the sensing mechanism of the textile sensor, two nanofiber sensing yarns were perpendicularly stacked together face-to-face to construct a textile-based piezoresistive NTPNF@NiCY sensor unit at the crossing contact site (Fig. 3a). The

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Fig. 3 Pressure sensing capability of textile-based NTPNF@NiCY pressure sensor. (a) The photograph showing the fabricated textile-based pressure sensor unit using two nanofiber sensing yarns and the schematic showing the working principle of the sensor. (b) Resistance changes (R/R0) of NTPNF@NiCY sensors with different CNT concentrations of nanofiber sensing layer as a function of pressure. The inset shows photographs of each NTPNF@NiCY sensor with different CNT concentrations. (c) The relative resistance change (∆R/R0) of the NTPNF@NiCY sensor and NTPNF@Cu sensor with 5% CNT concentration with respect to progressively increasing pressure. (d) Log-log plot of resistance changes (R/R0) of the NTPNF@NiCY sensor with 5% CNT concentration as a function of pressure. (e) Schematic illustration of the contact phenomena and sensing mechanisms of the textile-based NTPNF@NiCY pressure sensor during the pressing process.

pressure response of the textile-based sensor is based on piezoresistive mechanism. The total resistance (R) of the NTPNF@NiCY sensor consists of the resistance of the Ni-coated electrode (RE), the contact resistance (RC) between the electrode/nanofibers and the two NTPNF@NiCY yarns, which defined by the contact area and thickness between two NTPNF@NiCY yarns, as given in Fig.3a of the equivalent circuit schematic. By applying an external pressure, the NTPNF@NiCY yarns are compressed against each other, leading to localized deformations in the 3D elastic porous nanofiber sensing layer and the hierarchical fiber-bundled structure of the electrode. These are respectively composed of numerous highly deformable elastic nanofibers with a huge number of embedded CNTs and many cotton fibers with a Ni nanoparticle coating. Such mechanical deformations result in a dramatic increase in the contact area, which affects the tunneling piezoresistance at the contact site, resulting in a decrease in the device resistance. As the proposed Ni-coated electrodes are highly conductive (Rc >> RE), the force-induced resistance changes in the textile-based sensor mainly depend on the effective contact area variations. By monitoring this electrical resistance change, the applied force can be detected. To demonstrate and quantify the effects of the nanofiber sensing structure on the piezoresistive performance, the pressure sensing capabilities of the textile-based sensors were investigated and characterized in terms of various CNT concentrations, as shown in Fig.3b. The CNT concentrations of the sensors were 3 wt%，4 wt%, and 5 wt%, respectively. All the devices with different CNT concentrations yielded a decreased resistance as the applied pressure increased. Note that a higher CNT concentration results in a larger relative change in resistance (R/R0). This is attributed to the increasing electrical contact area of the nanofiber piezoresistive sensing layer and Ni-coated cotton yarn electrode under applied pressure, because the density

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of the CNT in the nanofibers determines the electrical contact area and contact times. Remarkably, as the applied pressure increased from 0 N to 5 N the sensor with a CNT concentration of 5 wt% exhibited ultra-high resistance changes (R/R0) up to the order of 104. Unless otherwise noted, the discussions below concern samples with a CNT concentration of 5 wt%. The sensitivity is one of the most important performance parameters for pressure sensors. Fig.3c shows the relative resistance variations (∆R/R0) of the textile-based NTPNF@NiCY sensor unit against different applied pressures. To compare the same vertical stacking structures, the sensitivity of the textile-based sensor units constructed by the sensing yarn made of CNTembedded PU nanofibers wrapped onto the copper wire electrode (hereafter denoted as NTPNF@Cu nanofiber sensing yarn) are also illustrated in Fig.3c.The piezoresistive pressure sensitivity (S) is mainly the relative resistance changes induced by applying pressure, which is defined as S = (ΔR/R0)/(ΔP) and can be acquired as the slope of the plot curve in Fig.3c, where R0 indicates the resistance without applied pressure. ΔR and ΔP indicate the pressure-induced change in resistance of the sensors and the applied pressure, respectively. As can be observed from Fig. 3c, similar to most of the reported pressure sensors, these sensitivity plots are composed of multiple regions with different values. The NTPNF@NiCY sensor exhibits a quasi-linear sensitivity up to 16.52 N-1 in a relatively low pressure range under 0.05 N, which is significantly higher than the textile-based NTPNF@Cu sensor unit (4.56 N-1) and a recently reported resistive electronic fabric based on silver nanowires (4.29 N-1) (Table S1)[53]. Fig. 3d presents the log-log plots of the relative resistance (R/R0) as a function of the applied pressure. The relative resistance (R/R0) decreased monotonically as the pressure increases in the larger pressure range (~0.001–5 N), which indicates that the resistance has an exponential relationship with the applied pressure. As expected, the textile-based sensor yields a noteworthy performance, with a highly sensitive resistance response over a wide sensing range. The high sensitivity of our textile-based NTPNF@NiCY sensor unit can be attributed to the rapid and significant variations in the contact points and accumulated contact interface areas, which arise from the deformation of the nanofiber sensing structure and hierarchical fiber bundled structure of the Ni-coated electrode under applied pressure. Fig. 3e schematically illustrates the contact deformation phenomena of textile-based NTPNF@NiCY sensor unit under applied pressure. In the initial state, there are numerous CNT-embedded nanofibers distributed on the core electrode, similar to protruding elastic piezoresistive nanodomes, forming multipoint contacts between the NTPNF@NiCY yarns. In the low pressure region, owing to the highly deformable properties of the 3D porous nanofiber sensing structure and Ni-coated conductive fiber-bundled structure, the flexible sensing yarn can easily deform from a round to a tabular shape under a low applied pressure, resulting in a rapid and significant increase in the contact area between numerous nanofibers and between nanofibers and the Ni-coated cotton fibers of the core electrodes. Meanwhile, the contacting elastic nanofibers also deform separately. Overall, the deformation accumulation leads to a notable increase in the contact area (including the distributed portions of the electrical contact area), which in turn leads to a significant decrease in the resistance, and therefore a high sensitivity. Conversely, for the NTPNF@Cu sensor unit, the copper wire electrode can not be compressed deformation, resulting in smaller contact area changes and a lower sensitivity (Fig. S7). The sensitivity of the sensor unit decreased in the case of a high-pressure region. This is because the nanofiber sensing structures were already highly compressed. For further decreasing the resistance, a higher pressure was required to deform the contact nanofibers to decrease the intertube distance in the CNT percolation. Significantly, the sensor still yielded a highly sensitive response with higher relative resistance ratios (R/R0) in the high-pressure region (see Fig.3d). Compared with the devices composed of various traditional planar or microscale piezoresistive structures and planar electrodes, our proposed sensing yarns were assembled with a combination of a 3D porous elastic nanofiber piezoresistive structure and a hierarchical fiber-bundled electrode structure, featuring abundant surface contact areas and highly deformable properties, endowing the as-prepared textile-based sensor with ultra-high sensitivity under small pressures and a broad sensing range. To estimate the stable and reliable operation of the textile-based sensor, the relative resistance variations were measured under various static and dynamic pressures. Fig.4a-b depicts representative resistive responses of the textile-based sensor. The applied pressure induced immediate deformations of the massive nanofibers and Ni-coated cotton fibers, which in turn apparently resulted in rapid variations in the contact area, and thus the resistance was decreased. Once released, the resistances quickly returned to their original values, indicating the mechanical robustness and excellent reversibility of the NTPNF@NiCY sensing yarn, which benefits from the elastic PU matrix and cotton yarn substrate. Although noise signals were inevitable, our sensor exhibited distinguishable resistive response signals with good cyclic stability and repeatability, even for a small pressure signal of approximately 0.003 N. This indicates that the sensor has a very low pressure detection limit (at least as small as approximately 0.003 N), which makes it suitable for the detection of delicate pressures. Fig. 4c depicts the dynamic instantaneous resistive responses to a series of dynamic pressures in a range from 0.003 N to 4 N, loaded and unloaded on the sensor continuously when applying a constant voltage of 1 V. The results indicate that the sensor can synchronously produce the corresponding resistive response to a sudden change in the loaded pressure, and the corresponding relative resistance (R/R0)

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is consistent with the sensitivity in Fig. 3c-d, reflecting the high sensitivity and fast response speed of the sensor over a wide pressure detection range. As shown in Fig.4d, our textile-based sensor exhibited a fast response and relaxation time of only 0.03 s to a fast loading/unloading pressure signal of 0.05 N. Compared with devices consisting of planar elastic films with low compressibility and significant viscoelastic behavior, the abundant porous structures of the nanofiber sensing layer and fiber-bundled electrode in our sensor can induce a reduction in the viscoelastic effect, which contributes to the immediate

Fig.4 Static resistive response of the NTPNF@NiCY sensor against the repeated loading-unloading pressures at low (a) and relatively high pressure region (b). (c) Log-lin plot of dynamic instantaneous resistive response of the NTPNF@NiCY sensor to varying pressure ranging from 0.001N to 4.3N. (d) The response and relaxation time of the NTPNF@NiCY sensor to a fast loading/unloading pressure signal of 0.05 N.

deformations of the massive nanofibers under applied pressure, leading to a short response and relaxation time. The rapid response speed is helpful for the real-time monitoring of human physiological signals, such as the pulse and heartbeat. Given the high sensitivity, broad sensing range, excellent flexibility, light weight, and weavability of the 1D structure of the core-sheath nanofiber yarn, our textile pressure sensor are well suited to wearable applications, and can be directly attached to the human skin or integrated into large-area textiles for non-invasive human motion and physiological signal monitoring. As shown in Fig.5a, a wearable textile sensor is attached to the top surface of the wrist to monitor the bending/recovering movement of the wrist joint. The bending motion of the wrist leads to a compression deformation of the sensor, resulting in a rapid decrease in the resistance followed by a short stable period. This is reflected by stable and repeatable rectangular resistance response curves. Once the wrist returns to the initial state, the resistance synchronously returns to the initial value. Furthermore, the intensity and frequency of the wrist movement can be precisely recognized by real-time monitoring of the corresponding changes in the resistance signals of the sensor, indicating that our textile sensor has an excellent sensing capacity for detecting and quantifying the applied pressure stimuli (Movie.S1). On this basis, the textile sensor can also be utilized to detect tactile pressure induced by finger touching (see Fig.5b). When a gentle finger touch is applied to the sensor attached to the wrist, the sensor exhibits a rapidly decreasing resistive response. Notably, when the finger touch is suddenly removed, the resistance of the sensor immediately returns to its original state value. In addition, the sensor synchronously generates corresponding signals of resistance changes with different strengths and frequencies according to strength and frequency of dynamic finger touching

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(Movie.S2). These results demonstrate that our sensor possesses high skin-like sensitivity, a rapid response speed, and a stable and reliable pressure sensing performance, even on arbitrary soft and flexible 3D curved surfaces. Thus, it can potentially be applied in human–machine interfaces and advanced wearable systems to evaluate quantitative data on human movement performance. As shown in Fig. 5c-d, the wearable textile sensor was adhered to a tester's throat to monitor the complex movements of his Adam's apple and neck muscles in a non-invasive manner. When the sensor is pressed by the Adam's apple, the resistance decreases (State 1). When the neck is twisted, the sensor is separated from Adam's apple with the movement of the neck, and

Fig. 5 Application as wearable textile NTPNF@NiCY sensor in monitoring various human motions. (a) Dynamic response signals of wearable NTPNF@NiCY sensor in monitoring wrist bending. Insets: photographs of NTPNF@NiCY sensor attached on the wrist knuckle. (b) Dynamic response signals of the wearable sensor in monitoring finger touching. Inset: photograph of the sensor attached on the wrist with touched by the index finger. The photographs (c,e) and real-time resistance response signals (d,f) of the NTPNF@NiCY sensor mounted on the throat of a test subject for detecting the repeated tiny movements of the Adam's apple when neck twisting and leaning backward.

the resistance returns to the initial state (State 2). Then, the resistance of the sensor synchronously increased and recovered rapidly in accordance with fast and continuous twisting/recovering neck motions (Movie.S3). The reproducible resistancechange signals highlight the excellent stability and reliability of the wearable textile sensor. Notably, when the tester performed a small neck movement involving leaning backward and then recovering, the attached sensor still responded instantaneously to the tiny pressure variations induced by the motions of the Adam's apple, confirming the excellent sensitivity and fast response speed of the sensor (see Fig.5e-f, Movie.S4). Therefore, the high sensitivity of the sensor enables it to sense subtle mechanical changes caused by complex movements of the skin and muscles around the throat during pronunciation. Furthermore, our textile sensor can be applied to detect some subtle movements and physiological signals of the human body. As a demonstration, the sensor was attached to a tester’s forehead to detect related skin and muscular movements during frowning, and subtle movements induced by the changes in human micro-expressions were precisely captured and reflected by

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virtue of the distinctly differentiated resistive response curves (Fig.6a and Movie.S5). As a further example, our sensor was conformally mounted to the 3D curved surface of a human cheek to monitor delicate dynamic facial movements (Fig.6b). Corresponding to the rapid bulging/relaxing movements of the cheek, the sensor synchronously exhibited rapid and reproducible resistive responses, indicating that the sensor yields stable and reliable responses to various dynamic facial movements. The monitoring of pulse signals represents a key basis for the clinical diagnoses of cardiovascular disease, asthma, sudden infant death syndrome, and so on. Benefiting from the high sensitivity to subtle pressures, the sensor can precisely sense the subtle pressure changes induced by heart-beats at the wrist. Thus, we assembled a textile sensor on the wrist artery to monitor the subtle wrist pulse signals of a healthy tester (175 cm high)( Fig.6c). Fig.6d and Movie.S6 records the real-time resistance signals of the sensor attached to the wrist artery over 45 s, demonstrating that the typical pulse waveform was precisely captured, with a corresponding pulse frequency of 68 beats/min. Therefore, our high-performance wearable textile sensor can be utilized for real-time monitoring of the heart rate, pulse, and other vital physiological signals.

Fig. 6 Wearable textile sensor for real-time monitoring of subtle human movements and physiological signals. (a) Photograph image and the real-time resistance response signals of a wearable NTPNF@NiCY sensor attached to the forehead of a male to repeated frowning/relaxation expressions. (b) Photograph image and response curves of a wearable NTPNF@NiCY sensor attached to 3D curved surfaces of a cheek according to the repeated bulging/recovering movement of the cheek. (c) Photograph of a wearable NTPNF@NiCY sensor mounted on the wrist for pulses monitoring. (d) The real-time resistance response signals of the sensor during pulse monitoring.

Thanks to the excellent mechanical robustness, flexibility, weavability, and extensibility of the smart yarns, we can readily integrate the sensing yarn into a large-area electronic textile sensing matrix with multiple pixels using weaving technology. To demonstrate the capability of detecting the spatially resolved pressure distribution, we successfully prepared a textile sensing matrix with 9 × 9 pixels in an interlaced plain structure, with a total area of 25 cm2 (Fig.7a). Fig.7a presents a simplified electrical schematic of the electronic textile, showing the resistance at each intersecting point of the textile’s rows and columns measured before and after the pressure loading (see the Fig. S8 in the Supporting Information for test details). To evaluate the spatial pressure mapping properties of the electronic textile, the two tips of a metal tweezers were used to apply a gentle pressure stimulus to the electronic textile, which introduced localized compression deformations on the contact sites of the electronic textile, resulting in local resistance variation signals. The real-time output resistance mapping of the electronic textile is presented in Fig.7b. The areas with obvious resistance variations correspond well to the specific pressing areas, while the resistances of other pixels show no obvious changes, indicating that the electronic textile can clearly detect and identify the spatial distributions of small input pressure signals. More importantly, to further demonstrate the potential application of the

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electronic textile as wearable electronic skin on an arbitrary irregular curved surface, the electronic textile was worn conformally on a human wrist, and the contact points touched by index fingers were identified (Fig.7c). In Fig.7c, a multidimensional tactile sensing image showing the finger-touch intensity and distribution is depicted through a 3D color mapping of the resistance variations in the textile. Significantly, the color contrast diagram shows visually that the force distribution is very consistent with the actual contact

Fig. 7 Electronic textile sensing matrix for tactile mapping of spatial pressure. (a) Schematic illustration of a textile sensing matrix with 9×9 pixels and the measurement method (Left). Photograph (right) and circuit diagram (bottom) of textile sensing matrix. (b) Photograph of the two tips of a metal tweezer pressed on the textile (top) and the corresponding colored pressure mapping profile. (c) Photograph of the electronic textile worn on a human wrist with the index finger pressed on the textile matrix(top) and the corresponding 3D colored pressure mapping profile of resistance changes for each sensing pixel (bottom).

areas and finger touching shapes. These results indicate that the electronic textile can maintain its pressure mapping performance even when attached to a curvilinear or deformable surface, highlighting the potential applications of our wearable electronic textile in smart textiles, electronic skin, and human–computer interaction interfaces.

3. Conclusion We have developed a simple, low-cost, and scalable strategy for fabricating a highly sensitive and wearable textile pressure sensor based on 1D weavable core-sheath nanofiber yarns that are capable of sensing static and dynamic tactile pressure and performing spatial mapping in a wide pressure range. The 1D weavable core-sheath nanofiber yarns were prepared by coating CNT-embedded PU nanofibers onto a Ni-coated cotton yarn electrode as force sensing layers, using a simple electrospinning technique. Here, the 3D porous elastic CNT-embedded PU nanofibers serve as a force-sensing layer, and the Ni-coated cotton yarn acts as the conductive core electrode. Thanks to the reasonable structural design and material composition, the resulting textile sensor not only exhibits excellent flexibility and weavability, but also possesses a high pressure sensitivity (16.52 N-1), a low detection limit (~0.003 N), a fast response speed (~0.03 s), and excellent stability. More remarkably, our textile sensor based on the 1D yarn structure demonstrated the advantage that it can be not only directly attached to the skin, but also worn on the body following integration into large-area textiles, for non-invasive full-range human motion and physiological signal monitoring and the detection of dynamic tactile stimuli. We believe that these weavable core-sheath nanofiber yarns will

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facilitate the design of novel smart textiles and personal wearable electronics for personalized healthcare and artificial intelligence applications in the future.

4. Experimental Section 4.1. Materials. Polyurethane (MW = 160000 with density 1.12 g/cm3 from BASF), dimethylformamide (DMF, 99.9%, Aladdin), tetrahydrofuran (THF, 99.9%, Aladdin), nickel sulphate (99.9% metals basis ， Aladdin), sodium hydroxide (AR, ≥ 96.0%, Aladdin), sodium borohydride (98%, Aladdin), hydrochloric acid (AR, 36.0-38.0%, Aladdin), sodium hypophosphite (AR, 99.0%, Aladdin), ammonium chloride (AR, 99.5%, Aladdin), trisodium citrate (AR, 99.0%, Aladdin) and ammonia solution (AR, 25-28%, Aladdin) anhydrous ethyl alcohol (AR, 99.5%), CNT powder (Multiwalled carbon nanotubes, 10-30 nm in diameter, Tanfeng) and PDMS elastomer kit (Sylgard 184, Dow Corning) were used as purchased without further purification. 4.2 Fabrication of Ni-coated cotton yarns Firstly, the commercial cotton yarns were thoroughly cleaned with 100 mL of a NaOH (10 g L-1) aqueous solution for 1 h at 80 ℃ and then dried at 60 ℃ in a vacuum oven. Subsequently, the pre-treated cotton yarns were dipped into in 100 mL of a nickel sulfate (0.05 g mL-1)/hydrochloric acid (0.02 g mL-1) aqueous solution for 10 min, removed, and immediately dipped into 100 mL of a sodium borohydride (0.01 g mL-1)/sodium hydroxide (0.01 g mL-1) aqueous solution for 10 min. Then, the resulting cotton yarns were cleaned with deionized water for several times. Secondly, the electroless nickel plating solution was prepared by the mixed aqueous solution of nickel sulfate (30 g L-1), sodium hypophosphite (12 g L-1), ammonium chloride (45 g L-1), trisodium citrate (25 g L-1) and aqueous ammonia (2.5 mL). The samples were then immersed into the electroless nickel plating solution (100 mL) for 2-6 h, washed with deionized water, and then dried at 60 ℃ in a vacuum oven. Finally, the flexible Ni-coated cotton yarn electrode was readily fabricated. 4.3 Fabrication of core-sheath nanofiber sensing yarn First, the CNT dispersion was first prepared by dispersing CNTs (0.16g) and dispersing agent (0.48g) in 20 g of a DMF/THF mixture (mass ratio of 1:1) by probe sonication for 5 h and then PU (3.2g) was dissolved in the stable CNT dispersion under magnetic mixing for 8 h at room temperature until a homogeneous transparent spinning solution with 5% CNT concentration was obtained, other spinning solutions with different CNT concentrations were also prepared through the same method. As shown in the schematics (Fig.S5), the Ni-coated cotton yarn was used as core yarn, then the CNT-embedded PU core-sheath nanofiber yarn (NTPNF@NiCY nanofiber yarn) was prepared by the conjugate principle of electrospinning at a voltage of 15 kV, a flow rate of 0.3 mL/ h, and a receiving distance of 15 cm with a metal rotary funnel of a rotation speed of 400 rpm as the receiving device. For comparison, NTPNF@Cu nanofiber yarns with 5% CNT concentration were fabricated by wrapped the CNT-embedded PU nanofibers onto the copper wire core yarn through the same electrospinning method. 4.4 Integration of the core-sheath nanofiber sensing yarn into wearable electronic textile Firstly, the wearable textile-based pressure sensor was fabricated by sandwiching the core-sheath nanofiber yarns between two PDMS films. Copper lead was connected on the ends of electrode of the core-sheath nanofiber yarns by silver paste and copper tape for further measurement of electric signals. PDMS (Dow Corning Sylgard184; ratio of base to cross-linker: 10:1 by mass) mixture was mixed, degassed, poured onto the silicon substrate and solidified at 80 °C for 2 h, and then peeled off from the silicon substrate to obtain the flexible PDMS thin film. For textile sensing matrix, the obtained core-sheath nanofiber yarns were used as warp and weft yarns for the construction of a wearable electronic textile sensing matrix with 9 × 9 pixels in an interlaced plain structure with a total area of 25 cm2. 4.5 Characterization and measurements The surface morphology of the as-prepared nanofiber yarns, nanofibers and cotton yarns was collected using a field-emission scanning electron microscopy (Hitachi S-4800, Japan). The TEM image of CNT-embeded PU nanofiber was observed using transmission electron microscopy (Hitachi H-800, Japan).The XRD pattern was collected using Rigaku MAX-2200 (filtered Cu Kα radiation). The mechanical properties of cotton yarns were tested with a versatile testing machine (Instron5565A). To

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observe the responses of the textile sensor against pressure stimuli, the resistances of our devices were measured using a Keithley 2400 source meter at a constant bias voltage of 1 V. The pressure sensing capability of the electronic textile sensor were characterized by using an Instron-5565 versatile testing machine combined with a custom-built universal mechanical testing platform at room temperature.

Acknowledgements This work was supported by a grant from the National Natural Science Foundation of China (No. 21671204, 51203196, and U1204510), the Natural Science Foundation of Henan (No. 162300410339), and the Program for Science & Technology Innovation Talents in Universities of Henan Province of China (No. 15HASTIT024). The Program for Science & Technology Innovation Teams in Universities of Henan Province of China (No. 16IRTSTHN006) and the Plan for Scientific Innovation Talent of Henan Province are also gratefully acknowledged. We would like to thank Editage (www.editage.cn) for English language editing.

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Figure Caption Fig. 1 Schematics for the fabrication process of the textile pressure sensor. Inset: Optical images of the core-sheath nanofiber sensing yarn (i) and the electronic textile (ii). Fig. 2 (a) SEM image of a Ni-coated cotton yarn with an electroless deposition time of 4 h. (b) high-magnification SEM image of (a). The inset shows the cross-section SEM image of the Ni-coated cotton yarn. The top-view (c) and cross-section (d) SEM images of the core-sheath nanofiber sensing yarn. (e) high-magnification SEM image of (c). (f) TEM image of the nanofiber in (e) with embedded CNT arrays.

Fig. 3 Pressure sensing capability of textile-based NTPNF@NiCY pressure sensor. (a) The photograph showing the fabricated textile-based pressure sensor unit using two nanofiber sensing yarns and the schematic showing the working principle of the sensor. (b) Resistance changes (R/R0) of NTPNF@NiCY sensors with different CNT concentrations of nanofiber sensing layer as a function of pressure. The inset shows photographs of each NTPNF@NiCY sensor with different CNT concentrations. (c) The relative resistance change (∆R/R0) of the NTPNF@NiCY sensor and NTPNF@Cu sensor with 5% CNT concentration with respect to progressively increasing pressure. (d) Log-log plot of resistance changes (R/R0) of the NTPNF@NiCY sensor with 5% CNT concentration as a function of pressure. (e) Schematic illustration of the contact phenomena and sensing mechanisms of the textile-based NTPNF@NiCY pressure sensor during the pressing process. Fig.4 Static resistive response of the NTPNF@NiCY sensor against the repeated loading-unloading pressures at low (a) and relatively high pressure region (b). (c) Log-lin plot of dynamic instantaneous resistive response of the NTPNF@NiCY sensor to varying pressure ranging from 0.001N to 4.3N. (d) The response and relaxation time of the NTPNF@NiCY sensor to a fast loading/unloading pressure signal of 0.05 N. Fig. 5 Application as wearable textile NTPNF@NiCY sensor in monitoring various human motions. (a) Dynamic response signals of wearable NTPNF@NiCY sensor in monitoring wrist bending. Insets: photographs of NTPNF@NiCY sensor attached on the wrist knuckle. (b) Dynamic response signals of the wearable sensor in monitoring finger touching. Inset: photograph of the sensor attached on the wrist with touched by the index finger. The photographs (c,e) and real-time resistance response signals (d,f) of the NTPNF@NiCY sensor mounted on the throat of a test subject for detecting the repeated tiny movements of the Adam's apple when neck twisting and leaning backward. Fig. 6 Wearable textile sensor for real-time monitoring of subtle human movements and physiological signals. (a) Photograph image and the real-time resistance response signals of a wearable NTPNF@NiCY sensor attached to the forehead of a male to repeated frowning/relaxation expressions. (b) Photograph image and response curves of a wearable NTPNF@NiCY sensor attached to 3D curved surfaces of a cheek according to the repeated bulging/recovering movement of the cheek. (c) Photograph of a wearable NTPNF@NiCY sensor mounted on the wrist for pulses monitoring. (d) The real-time resistance response signals of the sensor during pulse monitoring. Fig. 7 Electronic textile sensing matrix for tactile mapping of spatial pressure. (a) Schematic illustration of a textile sensing matrix with 9×9 pixels and the measurement method (Left). Photograph (right) and circuit diagram (bottom) of textile sensing matrix. (b) Photograph of the two tips of a metal tweezer pressed on the textile (top) and the corresponding colored pressure mapping profile. (c) Photograph of the electronic textile worn on a human wrist with the index finger pressed on the textile matrix(top) and the corresponding 3D colored pressure mapping profile of resistance changes for each sensing pixel (bottom).

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