1. Introduction
Chemical protection in high-risk environments is crucial for ensuring human health. Insufficient body protection from leakage or splashing of chemical solutions in laboratory accidents, chemical plant accidents, and workshop incidents can result in direct injuries or even disabilities for numerous personnel [
1], [
2], [
3]. Thus, chemical resistant textiles play a vital role in personal protection, essentially ensuring the safety and health of individuals [
4], [
5]. However, chemical resistant textiles for real-time motion detection and ideal personal thermal management in high-risk environments are currently lacking.
Recently, various techniques, such as material compounding [
6], [
7], [
8], [
9], structure optimization [
10], [
11], [
12], [
13], [
14], and novel manufacturing [
15], [
16], [
17], [
18], have been applied to enable the sensing of yarns/textiles with excellent durability, high sensitivity and stretchability, and fast response time. Although yarn-based strain sensors have been studied extensively in flexible sensing [
19], [
20], [
21], little attention has been paid to their compatibility with chemical resistance for personal protection. Protective films have been applied to sensing clothing using various methods to incorporate efficient sensing and chemical resistance functions into smart textiles. These methods include emulsion dipping, laminating composites, and sputter coating [
22], [
23], [
24], [
25], [
26]. However, the limited air permeability and thermoregulatory properties of these chemical resistant textiles hinder the release of heat when the human body sweats. Consequently, wearers may experience discomfort, reduced working efficiency, and heat-induced fatigue [
27], [
28], [
29], [
30]. Moreover, the demand for advanced textiles that can offer multifunctional integration and superior performance to meet complex application requirements in extreme environments is high [
31], [
32]. In this regard, advanced smart clothing with chemical resistance, human motion detection, and dynamic thermal management capabilities can protect the human body from chemical hazards, enable operators to make timely judgments regarding potential hazards, and provide wearers with a tunable thermal management option. However, such clothing has not yet been realized.
Herein, a chemical resistant yarn with hierarchical core-shell structures was fabricated to integrate chemical-harm protection, perform real-time motion detection, and offer tunable thermal management (
Fig. 1 ) using a continuous and scalable textile engineering process. The stretchable hierarchical core-shell yarns (HCSYs) consist of elastic yarns, wrapped conductive yarns (stretchable sensing layer), and network-covered polytetrafluoroethylene (PTFE) yarns (protective layer). The protective surface of the PTFE layer protects the human body from chemical hazards, given that the composite yarn is stretchable. Changes in electrical signals and thermal dispersion can result from stretching-induced hierarchical structure variations when HCSYs are woven as hierarchical core-shell structured yarn-based textiles (HCSTs), thus achieving functional compatibility of operator vital physiological sign monitoring, human-machine interaction, and user-tunable thermoregulability. Thus, multifunctional HCSTs with flexibility, scalability, and durability offer a promising approach for next-generation smart antichemical protective textiles to protect those working in laboratories, chemical plants, special workshops, and emergency sites
2. Experimental details
Materials: The silver-plated nylon conductive filaments (280 Denier (D), 18% silver; resistance < 40 Ω·m) were purchased from Kazhtex Silver Tech Co., Ltd. (China). The elastic yarn (Spandex, 200 D) was purchased from Hubei Chemical Fiber Co., Ltd. (China). The PTFE (1000 D/3) were purchased from Changzhou Huafu Fluoroplastic Materials Co., Ltd. (China). Sodium hydroxide solution (NaOH: 40 wt%) and sulfuric acid solution (H 2SO4: 99 wt%) were obtained from China National Medicines Co., Ltd. (China). A commercial chemical protective fabric (CCPF) was obtained from a commercial chemical protective suit (CCPS) of Dupont™ Tyvek™ 1422A (USA).
Fabrication of HCSY: The silver-plated nylon conductive yarns of 280 D were produced using a twisting machine with conductive multifilaments. A hollow-spindle fancy twister was used for fabricating wrapped silver-plated nylon/elastic spandex-sensing yarns with a core-sheath structure. Furthermore, the core-sheath-structured sensing yarns were cross-braided by multiple groups of PTFE yarns to form a hierarchical core-shell structure with a modified pre-loading tension.
Fabrication of HCST: The HCST samples were woven using HCSYs as weft yarns and PTFE yarns as warp yarns to create a stretching-induced switching structure wrapped with silver-plated nylon/elastic spandex sensing and surface-structured PTFE protective layer domains between the adjoined and separated states, finally forming a hierarchically structured HCST. Subsequently, the HCST piece was integrated with a commercial protective unit under the armpit for a multifunctional demonstration.
Characterizations: The morphologies of the HCSTs were observed using optical microscopy (RH2000; HIROX, Japan). An SL200KS goniometer (KINO Industry Co., Ltd., USA) was used to record the changes in the contact angle of the stretching-induced HCSTs. The tensile properties of the as-prepared samples were analyzed using a universal material testing system (Instron 5943; Instron, USA). Relative resistance changes were measured using a Digital Multimeter (Keysight 34470A; Keysight, USA). A G571 air permeability tester (Standard International Group (HK) Limited, China) was used to test the air permeability of the various textiles with a testing area and pressure of 20 cm
2 and 200 Pa, respectively, according to GB/T 5453—1997 [
33].
The water vapor transmission rate of the various textiles was tested using a moisture permeability tester (TS3301; Standard International Group (HK) Limited) with the upright cup method, in which the sample size was 25 cm
2, temperature was (38 ± 0.6) °C, and relative humidity was (90% ± 2%) based on American Society of Testing Materials (ASTM) E398 [
34]. A thermal constant analyzer (Hot Disk TPS 2500S; Hot Disk AB, Sweden) was used to measure the thermal conductivity using the transient planar heat source method adapted from International Organization for Standardization (ISO) 22007-2 [
35]. A thermocouple (HH506RA; Omega, USA) and hygrometer (RHXL5SD; Omega) were used to record the temperature and relative humidity of the artificial skin and human body.
3. Results and discussion
3.1. Fabrication and characterization of HCSYs
Based on the above multifunctional structure design principle, proof-of-concept materials were selected; these include silver-plated nylon, elastic spandex, and PTFE yarns. Silver-plated nylon filament yarn was adopted because of its good electrical conductivity (approximately 40 Ω·m
−1) [
36], and the PTFE yarn has excellent acid and alkali resistance. Herein, scalable textile engineering techniques (
Fig. 2 (a)), namely hollow-spindle spinning and braiding, enabled functional selectivity for the continuous manufacturing of our HCSYs by realizing a hierarchical core-shell design. To produce the sensing layer ( Fig. S1 in Appendix A ), the elastic spandex was consistently guided and fed into the hollow spindle using a tension controller. Subsequently, it was helically wrapped with silver-plated nylon yarns, which had a fineness of 280 D. The wrapping density of the silver-plated nylon yarn was effectively controlled by adjusting the twist level and winding speed. To create HCSYs, PTFE yarns were cross-braided on a tension-controlled helically wrapped sensing layer while the disc rotated in the high-speed braiding machine. The fabrication process of the HCSYs during the operation of the braiding machine can be seen in Movie S1 in Appendix A. Large-scale preparation of HCSYs was achieved, thus allowing continuous winding on cones without any length limitations ( Fig. S2 in Appendix A ). Owing to this continuous production process, composite yarns of any length can be produced or connected in parallel for wearable electronics, thus enhancing their compatibility with textile processing methods, such as weaving, knitting, and braiding [
37].
The stretching-induced morphology change of the as-fabricated HCSYs is shown in
Fig. 2 (b), and a cross-sectional photograph of the desired hierarchical core-shell structures is shown Fig. S3 in Appendix A. Tensile testing for the characterization of the representative engineering force versus strain response and typical optical images at different strains obtained for the composite HCSYs are shown in Fig. S4 in Appendix A. Engineering force versus strain curves reveal that HCSYs can readily withstand substantial strain deformation of 50% with a strain force of approximately 0.8 N (± 10%). The optical images in Fig. S5 in Appendix A demonstrate the flexibility and stretchability of HCSTs, thus reflecting its excellent wearable performance as a textile. As illustrated in
Fig. 2 (c) and Fig. S6 in Appendix A, the optical images show that the network-structured PTFE layer on HCSTs surfaces transitioned from a nearly continuous adjoined state to a separated state with gapped domains when stretched along the axis of HCSYs. Further, stable hydrophobicity was endowed by the surface-braided PTFE layer from the slightly changed contact angles (from 108° to 86°) of the chemical droplets on various stretched HCSTs. These aforementioned observations suggest that our HCSTs demonstrates adaptive chemical protection and morphology that can be reconfigured under strain.
3.2. Chemical resistance property of HCSTs
Chemical resistance is critical to ensure safety in high-risk environments. Herein, 10 mL 40 wt% NaOH and 99 wt% H
2SO
4 were dropped on HCSTs and common cotton fabric. Evidently, the cotton fabric gradually disintegrated, whereas the chemical droplets rolled off the surface of the HCSTs, which retained its original appearance and mechanical properties (
Fig. 2 (d); Figs. S7 and S8 and Movie S2 in Appendix A ). When the entire pieces of HCSTs and the cotton fabric were immersed in 40 wt% NaOH and 99 wt% H
2SO
4 solution for 30 min, the cotton was damaged by the NaOH and H
2SO
4 solution, whereas the HCSTs remained in its original state ( Fig. S9 in Appendix A ). In addition, the chemical droplets still rolled off the surface of the HCSTs at different strain levels up to 50% ( Fig. S10 in Appendix A ), thus indicating that the hierarchical core-shell structure endows both chemical resistance and stretchability to the HCSTs. The results in Fig. S11 in Appendix A demonstrate the long-term chemical resistance of the HCSTs, which remains stable even after friction and washing, thus indicating its suitability for prolonged use.
To further verify the chemical performance of the HCSTs, we designed a purpose-made chemical resistance test device consisting of a glass container, test sample, and corrodible material (
Fig. 3 (a)).
Fig. 3 (b) shows the test samples covering the corrodible materials for the chemical resistance test, and
Fig. 3 (c) shows the optical images of the fabric surface when in contact with a 99 wt% H
2SO
4 for 10 min. The cotton fabric immediately exhibited breakage at the contact location with the 99 wt% H
2SO
4; gradually, a large hole formed after 10 min. Further, H
2SO
4 solution spread on the surface of a CCPF. By contrast, the H
2SO
4 droplets concentrated on the surfaces of HCST-0 (0 stain) and HCST-50% (50% stain), thus demonstrating that their surfaces are chemically repellent.
Fig. 3 (d) shows the chemical-protective performance of the fabrics, in which the corrodible materials covered by the cotton fabrics are completely corroded. Slight corrosion was observed on the corroded material when it was covered with the CCPF. By contrast, the corrodible materials remained in their initial state under the protection of the HCST-0 and HCST-50% samples, thus showing the potential for advanced protective clothing with chemical resistance. This long-term chemical resistance is optimized because the chemical solution can be efficiently blocked from the sensing layer by the network-structured PTFE protective layer, and the functional design of the PTFE protective layer can decrease the gap between the braided PTFE yarns during the stretching process for ideal chemical-harm protection.
3.3. Sensing performance of HCSY
The strain-sensing performance of the HCSY was investigated, and the measurements showed the relative resistance variation (Δ
R/
R 0) of the yarn samples under various strain-loading conditions. Fig. S12 in Appendix A illustrates the experimental setup diagram for evaluating the electromechanical performance of the HCSY sensors. Evidently, Δ
R/
R 0 increased with an increase in the wrapping density from 0-4 T·cm
−1 (T: twist), thus suggesting that the HCSY with a wrapping density of 4 T·cm
−1 has a relatively higher sensitivity (
Fig. 4 (a)). By contrast, a high wrapping density of 5 T·cm
−1 made the helical structure more compact. This explains the smaller resistance change and separation hysteresis resulting from the densely arranged silver-plated nylon yarns. In particular, the Δ
R/
R 0-strain curve of the aforementioned HCSY exhibited three distinct intervals (strain ranges of: ① 0-5% with a gauge factor (GF) of 5.9, ② 5%-23% with a GF of 3.4, and ③ 23%-50% with a GF of 2.2), as shown in
Fig. 4 (b). Note that the sensitivity and strain range of the HCSYs are the determining factors for wearable sensors to detect human motion ( Table S1 in Appendix A ) [
38], [
39]. As shown in Appendix A Fig. S13 (a), the hysteresis experiment results revealed that the HCSY sensors exhibited a slight hysteresis of 2.12% at 50% strain, thus indicating excellent stability and reliability of the HCSY sensor for full-range strain-sensing detection. The HCSY sensors exhibited a rapid resistance response within 40 ms ( Fig. S13 (b) in Appendix A ). The inset images in
Fig. 4 (b) show that the brightness of the light-emitting diode (LED) is consistent with the change in resistance of the HCSYs; the smaller the deformation of the HCSYs, the brighter the illuminated LED. As shown in Appendix A Figs. S14 and S15, the HCSYs with varied braiding numbers and pitches of PTFE yarns exhibited similar strain-sensing performances before contact with the chemical solution. However, only the HCSY with 16 braided PTFE yarns and a pitch of 24 mm was stably maintained with a slight resistance loss after exposure to H
2SO
4 for 10 min, thus indicating that the HCSY sensors are highly chemically resistant for long-term use.
The change in resistance was attributed to the deformation of the helical conformation of the silver-plated nylon yarn when subjected to tensile strain [
40], [
41]. The resistance variation in the HCSYs can be attributed to the mechanism in which neighboring coils of the silver-plated nylon yarn separate owing to stretching. Consequently, the stretching force disrupts the conductive pathway between the twisted staple fibers, thereby leading to a decrease in the helical angle (
Fig. 4 (c)) [
42], [
43]. A simple circuit model was also developed to understand the resistance-changing process, in which
R 1 is the resistance of the neighboring coils of the silver-plated nylon yarn, and
R c refers to the resistance of the stapled fibers from the twisted structure. The increased distance between the gapped silver-plated fibers is expected to decrease the conductive capability, thereby causing an increase in the resistance of each contact area [
44]. In our cyclical stretching/releasing experiment, HCSY was exposed to a strain of 50% for over 15 000 tensile cycles (
Fig. 4 (d)), during which the resistance change was nearly constant and the hysteresis of the basic resistance was only 5.36%. The irreversible change in resistance can be attributed primarily to the viscoelastic properties of the elastic yarns and the incoherent deformations occurring between the wrapped silver-plated nylon, which result in interconnect damage and internal wire slippage. Fig. S16 in Appendix A illustrates the resistance evolution during cyclic stretching following exposure to the chemical solution. The results indicate the excellent sensing stability of the HCSY after undergoing the chemical resistance test. This can be attributed to the surface-structured PTFE protective layers, a barrier in constructing anti-chemical protective suits. Fig. S17 in Appendix A illustrates that the resistance signal of the HCSY gradually increases with increasing spiral angles and bending distances when the sensor is subjected to torsion and bending. Notably, the 10 cm long HCSY device demonstrated remarkable and consistent sensing properties across various strains, frequencies, and relative humidities, as depicted in Appendix A Fig. S18. The adaptivity of the sensor is crucial in wearable applications as it ensures a reliable response to different external stimuli, thus guaranteeing the sensor’s overall reliability.
3.4. Human-machine interaction of HCSY
Real-time monitoring of human physiological signals in extreme environments is crucial for chemical protective suits [
12], [
15], [
37], [
43]. These include motions associated with walking, shaking the beaker, or formulating chemical reagents ( Figs. 5 (a)-(c); Fig. S19 in Appendix A ). These results confirm the exceptional strain-sensing performance of the HCSY, thus ensuring their suitability as wearable devices for detecting various human physiological activities. More importantly, this electrically responsive HCSY can remotely operate a manipulator after it is connected to a series circuit to prevent safety hazards in extreme environments [
45], [
46], [
47]. Herein, we demonstrated the collective application of five HCSY sensors as a practical wearable sensor platform in a human-machine interface. The strain sensors were sewn onto the surface of five knuckles of a glove. The real-time relative resistance change signal was generated as the finger was bent, and the recognition result was obtained with an accuracy of 94.16% ( Figs. 5 (d) and (e)) after 1000 iterations of training and testing using the
K-nearest neighbor (KNN) model. Based on the high-precision gesture recognition function, the corresponding fingers of the manipulator received the changing signal and quickly repeated the same action when the human forefinger was bent, whereas the other parts remained still, as shown in
Fig. 5 (f). The five fingers of the manipulator and the corresponding five human fingers bent simultaneously, thereby allowing for a high-sensitivity grip on either a beaker or a pipette gun ( Movie S3 in Appendix A ). Therefore, the HCSY has broad application potential in controlling instruments by monitoring the movement of human limbs when facing potential chemical dangers.
3.5. Tunable thermoregulation function and thermal management of HCSTs
The thermal management of functional textiles has gained considerable importance in ensuring wearable comfort [
29], [
47], [
48]. The excellent tensile properties of HCSTs provide not only stretchable sensing performance but also enable adjustable thermoregulation functions. First, we conducted measurements on the air and water vapor permeabilities of HCST pieces to confirm their tunable thermal and moisture management capabilities. At a strain of 50%, the presence of separating domains within the gapped PTFE layer facilitated the easy flow of gas and water molecules, thus leading to higher air permeability and water vapor transmission in the HCST (
Fig. 6 (a)). In addition, we examined the thermal resistance of HCSTs at different strain levels. The results revealed that the HCSTs exhibited adjustable thermal conductivity (
Fig. 6 (b)). As shown in the inserted photograph of
Fig. 6 (b), Fig. S20 and Movie S4 in Appendix A, the working mechanism of the HCSTs relies on the dynamic formation of thermally conductive interfaces and inter yarn spaces upon mechanical actuation. Consequently, implementing a hierarchical structural design facilitates the efficient transfer of heat from the human body to the surrounding environment [
49]. Herein, the cooling performance of HCSTs at different strains was compared based on a purpose-designed experimental setup (
Fig. 6 (c)), in which artificial sweat was added continuously at 37.6 °C at a rate of 100 μL·min
−1 onto the artificial skin and power density of the artificial skin was maintained constant at the temperature of 37.6 °C (power density, 80 W·m
−2). The prewet artificial skin was covered with various fabric samples to monitor and record the near-skin temperature. As sweat evaporated and heat dissipated, the skin temperature steadily decreased. The near-skin temperature with HCSTs changed from 37.6 to 31.2 °C after the application of 50% strain (
Fig. 6 (d)), thus demonstrating that the human body can effectively evaporate sweat and dissipate body heat to achieve lower skin temperatures owing to its functional, structural design [
40], [
41], [
42], [
43], [
44], [
45], [
46], [
47], [
48], [
49], [
50], [
51], [
52]. After contact with H
2SO
4 for 10 min, hardly any decrease in thermoregulatory function was observed ( Fig. S21 in Appendix A ), possibly because the braided PTFE protective layer effectively blocked chemical penetration and maintained gapped domain control. Thus, even after undergoing chemical treatment, the thermal management of the HCSYs remains uncompromised.
To test the actual wearing performance of the tunable heat management, a piece of HCST was sewn into the underarm of a CCPS and examined at 28 °C and 62% humidity (
Fig. 6 (e)). Additionally, a heat-humidity sensor was placed near the underarm area to monitor near-skin temperature and humidity changes. During strenuous exercise, when the HCST piece was slightly stretched, the rate of increase in near-skin temperature and humidity, as measured by the individual wearing the protective clothing integrated with HCSTs, was slightly lower than that of the CCPS. After 600 s, the HCST was fully stretched when the participant stood with raised arms to dissipate heat. Compared with the value of CCPS after 900 s, the temperature at the HCST was approximately 1.1 °C lower, and the humidity was reduced by 7.6% (
Fig. 6 (f)), thus clearly demonstrating the effectiveness of the thermoregulation function of HCSTs.
The synergistic effects observed were chemical resistance of the braided-structured PTFE protective layers from the outer layer, stretch-introduced resistance sensing of helically wrapped silver-plated nylon yarns from the inner layer, and dynamic thermoregulation of the strain-reconfigurable structure from the hierarchical functional design in the HCSYs. Consequently, the desired HCSYs/HCSTs can effectively overcome the trade-off between chemical harm protection, real-time motion detection, and adjustable heat diffusion. This integration allows superior chemical protection, strain sensing, and thermal management capabilities.
4. Conclusions
Herein, we proposed a multifunctional intelligent yarn and fabric featuring unique hierarchical core-shell structures. These structures can be created through continuous and large-scale fabrication processes. The outcomes of our research contribute to an innovative chemical hazard protection textile. This textile combines real-time motion detection and adjustable heat management properties. The hierarchical structural design of the helically wrapped silver-plated nylon sensing layer resulted in electrical signal variations, which were utilized for monitoring human motion, essentially offering outstanding sensitivity, stability, and durability. Furthermore, the HCST includes a surface-structured PTFE protective layer, which addresses acid and alkali resistance, and provides user-tunable thermoregulability properties from the stretching-induced variations in yarn spacing. Therefore, durable, flexible, and scalable smart protective yarns with hierarchical core-shell structures can be efficiently manufactured and developed as next-generation chemical protective textiles for use in chemical laboratories, plants, special workshops, and emergency sites.
Acknowledgments
Funding was received from the National Key Research and Development Program of China (2022YFB3805800), the National Natural Science Foundation of China (52173059 and U21A2095), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX223203), the Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (21KJA540002), and the Key Research and Development Program of Hubei Province (2021BAA068).
Compliance with ethics guidelines
Duo Xu, Yingcun Liu, Can Ge, Chong Gao, Ze Chen, Ziyi Su, Haoran Gong, Weilin Xu, and Jian Fang declare that they have no conflict of interest or financial conflicts to disclose.
PDF
(3736KB)
