Microcrack/Microscale Decorated Fiber-Based Electronics for Waist Rehabilitation

Engineering ›› 2025, Vol. 55 ›› Issue (12) : 204 -216. DOI: 10.1016/j.eng.2025.07.004

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2025-01-02	2025-07-02	2025-12-27
Issue Date	Revised Date
2025-11-21	2025-05-18

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Abstract

Fiber-based strain sensors have emerged as revolutionary components in flexible electronics owing to their intrinsic compliance and textile compatibility, particularly in human-centric applications ranging from health diagnostics to motion tracking. While substantial progress has been achieved, a critical challenge persists in reconciling the contradictory demands of ultrahigh sensitivity and stable signal transmission through rational structural design. Herein, we develop dual-structure silver (Ag)/polyurethane (PU) fiber-based strain sensors (Ag@PU_x) via an integrated wet spinning and interfacial metal ion deposition (IMID) strategy. Notably, we propose a mechanical pre-stretching strategy that enables precise regulation of strain sensitivity and sensing range through controlled substrate deformation. Systematic characterization reveals that pre-stretched PU fibers form ordered microscale conductive networks, exhibiting exceptional electrical stability (conductivity (σ) = 1.9 × 10⁵ S·m⁻¹; the change in resistance value under external tensile force (ΔR)/the initial resistance of the sensor (R₀) < 0.03 under 360° torsional deformation) with a high quality factor (Q) of 10.1 at 50% strain. In contrast, non-prestretched counterparts develop microcrack-dominated architectures, achieving a high sensitivity (gauge factor (GF) = 7.7) through strain-induced crack propagation and a fracture strain exceeding 660%. A systematic investigation elucidates the underlying mechanisms behind these distinct sensing performances. The Ag@PU_x fiber-based electronics are capable of adapting to various tasks including human motion monitoring, voice recognition, and gesture recognition. Importantly, we developed the Ag@PU_x fiber-based electronics to monitor motion states while stably transmitting electrical signals. Ultimately, the Ag@PU_x show great promise in applications such as motion monitoring, waist rehabilitation, thermal management, electromagnetic shielding, and antibacterial deodorization.

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Fibrous base / Strain sensor / Microstructure / Sensitivity tunable / Mechanical strategy

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Feiyu Tong, Jingmin Shi, Qi Jiang, Ming Li, Ruidong Xu, Ganghua Li, Yuanyuan Liu, Xinyu Zhang, Jinfeng Yang, Mingwei Tian, Yutian Li. Microcrack/Microscale Decorated Fiber-Based Electronics for Waist Rehabilitation. Engineering, 2025, 55(12): 204-216 DOI:10.1016/j.eng.2025.07.004

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1. Introduction

Recent advancements in flexible fabric-based electronics have paved the way for various wearable applications, driving an increasing demand for novel flexible sensors capable of detecting external stimuli across diverse scenarios [[1], [2], [3]]. Specifically, wearable strain sensors, which convert human body movements into machine-readable electrical signals, hold significant potential in fields such as wearable monitoring devices [[4], [5], [6]], human motion capture [[6], [7], [8]], human-machine interaction [[9], [10], [11]], personal health management [[6], [7], [8], [9]], and telemedicine. Fiber-based strain sensors utilizing a variety of structures and sensing mechanisms, including resistive [[6], [7]], capacitive [1,[11], [12]], and piezoelectric types [2,8], have been developed. Among these, resistive strain sensors have garnered substantial attention due to their simple fabrication, convenient signal acquisition, flexibility, stretchability, and weavability. These sensors exhibit high strain sensitivity, broad strain detection range, and rapid response, making them suitable for numerous applications. Consequently, strain sensitivity and operational range are two critical parameters in the design of flexible strain sensors [[13], [14]]. Previous studies predominantly focused on enhancing sensitivity or expanding detection range, with only a few addressing the need for strain sensors to meet specific application requirements for different body parts, namely the tailored design of strain sensitivity and detection range [15]. Such tailored sensors can maximize their effectiveness by leveraging high sensitivity, linearity, and accuracy within their optimal measurement zones. Additionally, fiber-based sensors that are insensitive to strain can serve as conductive wires, ensuring stable signal transmission.

To design and manufacture fiber-based strain sensors with varying strain sensitivities and detection ranges, previous studies [[16], [17]] have explored both material and structural strategies. In the realm of materials, employing different highly conductive materials or adjusting the loading of conductive substances has been demonstrated to effectively create sensors with diverse detection ranges. However, this approach inevitably complicates the fabrication process and introduces challenges related to interfacial coupling. Particularly, mechanical issues such as interface fractures, conductive layer failures, and modulus mismatches exacerbate design complexities and reduce wearable comfort. In terms of structure, research indicates that stretchable configurations like worm-like wrinkled [[18], [19]], serpentine [20], and convolutional structures [[21], [22], [23]] can facilitate spatial movements to achieve strain insensitivity. While these designs boast simple manufacturing processes and advantages, addressing the tunability of strain sensitivity and detection range at the intrinsic level of fiber structures remains unresolved. On one hand, the presence of stretchable structures disperses deformation across the entire stretchable region, leading to reduced strain per unit area. Consequently, the conductive paths may not exhibit significant on-off changes due to strain, thereby diminishing sensor sensitivity. On the other hand, as strain increases, the sensor’s output signal may deviate from a linear relationship with strain, potentially compromising measurement accuracy and necessitating additional corrective measures for nonlinear output compensation. Therefore, achieving tunable working ranges and strain sensitivities through these methods at the fiber’s inherent structural level proves exceedingly challenging.

Here, we employ an interfacial metal ion deposition (IMID) technique to firmly deposit Ag particles onto the surface of wet-spun PU fibers, thereby fabricating intrinsically highly elastic and conductive fibers. We propose a pre-stretching strategy that constructs microcrack and microscale conductive structures, overcoming the limitations of non-tunable strain sensitivity and detection range. The sensing mechanism induced by the microcrack structure demonstrates high strain sensitivity, quantified by a gauge factor (GF) of 7.7 within 40%-46% strain, accompanied by a remarkable fracture strain exceeding 660%. Its operational principle relies on strain-dependent resistance modulation through microcracks expansion under tensile deformation, enabling applications in human motion monitoring, speech recognition, and gesture detection. Conversely, the sensing mechanism induced by the microscale structure demonstrates exceptional strain insensitivity, quantified by a quality factor (Q) = 10.1 at 50% strain. Its sensing mechanism combines hierarchical structural sliding (Ag islands undergo relative sliding while remaining interconnected under small strain) and crack-bridging mechanism (Ag particles in the grooves form conductive bridges at microcrack edges under large strain), ensuring persistent conductive paths integrity, endowing the material with strain-insensitive properties under 360° torsional deformation (ΔR/R₀ < 0.03, where ΔR represents the change in resistance value under external tensile force, and R₀ denotes the initial resistance of the sensor), making it an ideal conductor for stable signal transmission even amidst complex strain conditions. Such characteristics render it an ideal candidate for stable electrical signal transmission in smart training garment and intelligent thermal therapy belt. Notably, our tunable microstructured Ag@PU_x electronics can be produced continuously and are compatible with textile processing. They possess superior electrical heating, electromagnetic shielding, and antibacterial deodorization properties, showcasing significant potential in the development of multifunctional, interference-resistant wearable smart devices.

2. Experimental section

2.1. Materials

PU was produced by Jining Huakai Resin Co., Ltd, China. N, N-Dimethylformamide (DMF), SnCl₂, HCl, Zn particles, AgNO₃, glucose (C₆H₁₂O₆), and ammonia solution (NH₃·H₂O) were purchased from Qingdao Honghuanxin Environmental Engineering Co., Ltd., China. Deionized water was used throughout the experiment.

2.2. Preparation of fiber-based strain sensor

The fabrication process of the Ag@PU_x primarily consists of three stages: spinning, pre-treatment (sensitization and activation), and IMID. It is crucial to maintain the fibers in a pre-strained condition during both the pre-treatment and the IMID steps. Firstly, PU was mixed with DMF at a ratio of 7:3 and stirred uniformly using a magnetic stirrer. The PU/DMF solution was then injected into a deionized water coagulation bath through a needle with an inner diameter of 4.1 × 10⁻⁴ m at a rate of 3.3 × 10⁻³ mL·s⁻¹. After complete flocculation in water for 30 min, the fibers were dried in an oven. Secondly, the sensitizer SnCl₂ was dissolved in HCl and diluted with distilled water to a concentration of 1.5 × 10⁻² kg·L⁻¹. Zinc granules were added during dilution to prevent the formation of white precipitate. The PU fibers were then immersed in the prepared solution and subjected to ultrasonic agitation at 40 ℃ for 40 min. During this process, a layer of easily oxidizable Sn²⁺ was adsorbed onto the surface of the PU fibers, aiming to reduce Ag⁺ in the activation process. After sensitization, the fibers were rinsed three times with deionized water (the fibers must be washed immediately after sensitization and the washing time should not be too long to prevent oxidation). For activation, an AgNO₃ solution with a concentration of 3.0 × 10⁻⁴ kg·L⁻¹ was used, and the fibers were left to activate at 40 ℃ for 40 min. This step aimed to form catalytically active silver centers on the surface of the PU fibers, thereby reducing the activation energy of the redox reaction. Following activation, the fibers were rinsed three times with deionized water. Notably, the color change of the fibers from white to yellow-brown serves as an indicator of successful activation.

Finally, the metal deposition solution was prepared by separately preparing the reducing agent and oxidizing agent. The reducing agent was selected as C₆H₁₂O₆ with a concentration of 6.0 × 10⁻³ kg·L⁻¹, and the oxidizing agent was selected as AgNO₃ with a concentration of 1.0 × 10⁻³ kg·L⁻¹. NH₃·H₂O (2%) was added dropwise to the AgNO₃ solution, resulting in the gradual formation of reddish-brown precipitates. Continued dropwise addition of NH₃·H₂O caused the precipitates to disappear. The silver ammonia solution is unstable and needs to be freshly prepared before use. When depositing Ag on the interface, both liquids were poured into the plating bath simultaneously, followed by the addition of other reagents while slowly stirring with a glass rod to ensure uniform mixing. Then, the samples to be deposited were placed in the solution. After mixing the reducing solution with the silver ammonia solution and other reagents, the solution turned black-brown within 5 min. The entire deposition process took 30 min. After the deposition process was complete, the samples were rinsed three times with deionized water and placed in an oven at 80 ℃ to dry. This deposition procedure was repeated twice to reinforce the thickness of the Ag layer. After interface deposition, the fiber surface exhibited a bright silver layer with metallic luster. Upon drying and film formation, the pre-strain was released. Fiber-based strain sensors prepared with different pre-drawing ratios were denoted as Ag@PU_x (pre-drawing strain x%, x = 0, 100, 200, and 300).

2.3. Characterization

The surface and cross-sectional morphology of the fiber-based strain sensor were characterized using a scanning electron microscope (SEM; JSM6390LV, JEOL, Japan). The distribution of Ag elements was observed by energy dispersive X-ray spectroscopy (EDS; Axis Supra+, Shimadzu, Japan). The breaking strength, tensile elongation, and loading-unloading mechanical properties were tested using a universal testing machine (Instron 5965, USA). All sensing performances of the assembled sensor were evaluated using a system source meter (Models 2601B, Keithley, USA), with the stretching process controlled by a stepping motor controller (DKC-1B) and a sliding module (FSL40-450 mm). The brightness of LEDs under daylight conditions was tested using an imaging luminometer (CX-1000), and the brightness of LEDs connected to optical fibers at different strains was recorded. The applied voltage for LED lamps and optical fibers was 5 V. The equilibrium temperature of fabrics made from silver-plated fibers under different conditions was recorded using an infrared (IR) thermal imager (UTi 120s). Antibacterial evaluation: the antibacterial activity against Gram-negative Escherichia coli (ATCC: 25922) and Gram-positive Staphylococcus aureus (ATCC: 29213) was evaluated by observing the inhibition zone. Additionally, the electromagnetic interference shielding performance of the samples was evaluated using a vector network analyzer (PNA-N5244A; E5071C, Agilent, USA).

3. Results and discussion

3.1. Mechanical strategy for tunable sensitivity of the Ag@PU_x

Fig. 1(a) illustrates a simple mechanical strategy for tuning the sensitivity of fiber-based strain sensors, referred to the roller pre-stretching strategy. Initially, PU fibers produced by wet spinning pass through a coagulation bath filled with deionized water, resulting in a surface texture characterized by grooves and a cross-section with pores (Fig. S1 in Appendix A). This unique structure is a hallmark of the wet spinning process [[24], [25], [26]]. Detailed definitions are available in Note S1 in Appendix A. Subsequently, the pre-stretching ratio of the PU fibers is controlled by adjusting the speed of the drawing roller. The PU fibers then underwent a pre-treatment process involving sensitization and activation, followed by immersion in SnCl₂ and AgNO₃ solutions. Finally, the fibers were introduced into a plating bath where Ag was deposited onto their surfaces using IMID technology. Notably, unlike previously published works, our spun fibers do not require complex procedures such as roughening and degreasing [27] or dopamine modification [[28], [29], [30]] to facilitate the deposition of Ag particles. This is because the rough surface of fibers produced by wet spinning is conducive to metal particle deposition [[31], [32]]. Interestingly, as observed in Fig. 1(b), the fiber surface remains white after sensitization but turns yellowish-brown after activation. This color change results from the oxidation of Sn²⁺ to Sn⁴⁺ during activation and the reduction of Ag⁺ in the activation solution to metallic Ag particles, forming catalytically active silver centers on the fiber surface. Ultimately, fiber-based strain sensors with a silver metallic luster are continuously manufactured using IMID technology (Fig. S2 in Appendix A).

After depositing Ag particles on the surface of PU fibers that have not undergone pre-stretching treatment, the Ag layer exhibits a smooth and uniform surface morphology without microcrack, as shown in Fig. S3 in Appendix A. Due to the higher deformability of the PU substrate compared to the ductility of the Ag layer, stretching Ag@PU₀ results in brittle fracture of the Ag layer, leading to the formation of island-like microcracked Ag structures induced by mechanical stress (Fig. S4 in Appendix A) [33]. Fig. 1(c) left reveals that the application of stress causes the island-like Ag layers on the surface of the Ag@PU₀ to separate. As microcracks progressively expand, the conductive paths are disrupted, leading to an increase in the sensor’s resistance. However, upon the release of the applied stress, the microcracks in the conductive layer gradually narrowed, restoring the previously disconnected conductive paths and consequently reducing the resistance. The resistance of Ag@PU₀ exhibits high sensitivity to the applied strain, with a GF of 7.7 within the strain range of 40%-46% (Fig. S5 in Appendix A), classifying it as a strain-sensitive fiber-based sensor. In contrast, sensors that undergo pre-stretching treatment are more resistant to the effects of strain on resistance in practical applications. This results in a larger range for strain-insensitive detection and enables the intrinsic tensile conductive behavior of the conductor. It is a key step in fabricating elastic electronic components with strain-insensitivity characteristics. Specifically, the pre-stretching process induces a pre-compressive state in the Ag layer during deposition. Upon releasing the pre-stress, both the elastic PU substrate and the Ag layer contract simultaneously. However, due to the larger deformation capability of the PU substrate compared to the limited ductility of the Ag layer, the Ag layer undergoes brittle fracture, forming fragmented Ag islands. The stacking of these Ag islands generates a microscale-structured sensing layer (Fig. 1(c) right). On the other hand, the pre-stretching process reduces the overall diameter of PU fibers and elongates their surface grooves, thereby increasing the specific surface area. During IMID, Ag particles deposit on the expanded groove surfaces (Figs. S3(b)-(d)). After releasing the pre-stress, some Ag particles will be rolled into the groove. When a certain stress is applied to the Ag@PU₃₀₀, the Ag layer maintains the integrity of the conductive paths through hierarchical structure sliding (Ag islands undergo relative sliding while remaining interconnected under small strain) and crack bridging mechanisms (Ag particles in the grooves form conductive bridges at microcrack edges under large strain) to counteract the effects of strain on resistance (Fig. S6 in Appendix A). These mechanisms counteract strain-induced resistance changes, enabling strain-insensitive sensing range up to 80% without complex signal compensation. In contrast, for non-pre-stretched PU fibers, Ag particles deposit only on the PU surface during IMID due to the unexpanded grooves (Fig. S4), resulting in insufficient Ag filling within the grooves. Consequently, the crack-bridging effect is absent during subsequent stretching. Consequently, the resistance of the Ag@PU_x is not sensitive to the applied strain, classifying it as a strain-insensitive fiber-based sensor. This “mechanical pre-programming” strategy provides a novel approach for developing high-performance flexible electronics. Notably, we found that the conductive network constructed with a microscale structure through the pre-stretching strategy can achieve tunable sensing detection range and strain sensitivity. When 0 pre-stretching is applied, the fiber has very high sensitivity and is a strain-sensitive strain sensor; the greater the pre-stretching multiple applied, the more evident the strain-insensitive characteristics of the fiber become.

In brief, the Ag@PU₀, without pre-stretching, exhibits excellent strain sensitivity and can be used in smart training garment to monitor elbow joint movement signals, as shown by the sensor (blue line) on the tester’s elbow joint in Fig. 1(d). When the tester’s arm is extended, the microcracks on the fiber surface of the Ag layer are small, allowing the Ag islands to remain in contact with each other, creating smooth paths for electron transport, causing the blue line to light up. Conversely, when the tester bends their arm, the microcracks in the Ag layer expand, causing the Ag islands to separate, disrupting the electron movement paths, significantly increasing resistance, causing the blue line to turn off. During the cyclic “bending” and “straightening” movements of the arm, the Ag@PU₀ is capable of monitoring and transmitting periodic electrical signals (Video S1 in Appendix A), and the strain-ΔR/R₀ curve (blue line) in Fig. 1(d) demonstrates its strain sensitive sensing mechanism. In contrast, the pre-stretched Ag@PU_x exhibits strain insensitivity and can serve as a conductor for transmitting motion signals (shown as the red wire in the smart training garment in Fig. 1(d)). Even under strain conditions, it ensures stable transmission of electrical signals to the terminal. Whether the arm is “bending” and “straightening”, the Ag layer ensures the integrity of the conductive path through hierarchical structure sliding and crack bridging mechanism, resulting in negligible resistance changes and ensuring stable transmission of electrical signals, as demonstrated by the continuous glowing of the armband (Video S2 in Appendix A). The strain-ΔR/R₀ curve (red line) in Fig. 1(d) demonstrates its strain-insensitive sensing mechanism. Therefore, by employing a pre-stretching strategy to create microcrack and microscale structures, both strain sensitivity and strain insensitivity can be tunably achieved within the intrinsic structure of the fiber, demonstrating promising applications in flexible wearable devices.

3.2. Regulation mechanism of microstructure of the Ag@PU_x

As illustrated in Fig. 2(a), the principle of Ag ions deposition on the interface involves several steps. Initially, during the sensitization process (represented by the white segment fibers in Fig. 2(a)), a layer of Sn²⁺ ions, which are easily oxidized, is adsorbed onto the fiber surface. This layer serves to reduce active metal ions in the subsequent activation step, making the fiber surface more receptive to metal plating. The reaction equation can be seen as Eq. (S1) in Appendix A. Additionally, during the activation process (the yellow-brown segment of the fiber in Fig. 2(a)), Sn²⁺ on the fiber surface is gradually oxidized to Sn⁴⁺, while Ag⁺ in the activation solution gains electrons and is reduced to Ag particles, which deposit on the fiber surface, forming catalytically active Ag centers. The reaction equation can be seen as Eq. (S2) in Appendix A. Finally, during the IMID process (the silver segment fiber in Fig. 2(a)), an oxidation-reduction reaction occurs on the fiber surface, resulting in the deposition of Ag. The reaction equation can be seen as Eq. (S3) in Appendix A.

To gain an in-depth understanding of the microstructure of the Ag@PU_x, we employed SEM to examine their surface morphology. Figs. 2(b)-(c)) illustrates the SEM image of a PU fiber surface prepared by wet spinning, revealing a clear groove structure. By adjusting the take-up speed of the pre-stretching device, we controlled the pre-stretch ratio of the fibers to 0, 100%, 200%, and 300%, thereby fabricating four distinct sensors (mark as Ag@PU_x). Post-IMID, SEM imaging revealed variations in the fiber surfaces under different pre-stretch conditions. For the Ag@PU₀ that underwent direct IMID without pre-stretching treatment, the grooves were not elongated or unfolded, and Ag particles were uniformly deposited on its surface (Fig. S3(a)). During stretching, the Ag layer on the groove surface of this sensor undergoes brittle fracture, forming island-like microcrack, as shown in Fig. 2(d). In contrast, for the Ag@PU₁₀₀, Ag@PU₂₀₀, and Ag@PU₃₀₀ that underwent varying degrees of pre-stretching, the grooves gradually elongated and unfolded, with Ag particles depositing uniformly on the surface (Figs. S3(b)-(d)). After releasing the pre-stress, the elastic substrate returns to its initial size, the elongated and unfolded grooves contract, and some Ag particles are rolled into the grooves. The Ag layer on the groove surface undergoes brittle fracture, forming Ag islands that stack to create a microscale structure sensing layer, as shown in Fig. 2(e). When the Ag@PU₃₀₀ is subjected to large strains, the Ag particles in the grooves play a crack bridging role (forming conductive bridges at the crack edges), ensuring the integrity of the conductive path and stabilizing electrical signal transmission (Fig. S6). The pre-stretching process constructs a microscale structure sensing layer, which can inhibit the crack propagation of the Ag layer during stretching, release part of the interfacial stress, and maintain the stability and continuity of the conductive network, thereby expanding the strain-insensitive detection range.

To further confirm the successful deposition of Ag on the surface of PU fibers, EDS mapping revealed a dense layer of Ag with an atomic percentage as high as 66.97% (Fig. 2(f)), substantiating the effective deposition of Ag. The successful deposition of Ag on the fiber surface can also be observed from the cross-sectional SEM image (Fig. S1). Additionally, EDS analysis (Fig. S7 in Appendix A) identified the presence of carbon (C, red) and oxygen (O, blue) elements in the Ag@PU_x, constituting 4.68% and 28.35%, respectively (Fig. S8 in Appendix A), which are integral components of PU. To elucidate the performance disparity between strain-sensitive and strain-insensitive sensors, we compared the relative resistance changes during cyclic loading and unloading at various strains for both Ag@PU₀ and Ag@PU₃₀₀ (Fig. 2(g)). The findings demonstrated that even under 20% strain, the Ag@PU₃₀₀ exhibited negligible change in relative resistance, indicative of its insensitivity to strain; conversely, the Ag@PU₀ displayed significant fluctuations in resistance, highlighting its heightened sensitivity to strain. These outcomes reiterate that microcrack and microscale structures within the sensing layer are pivotal for achieving strain sensitivity and insensitivity, respectively. In conclusion, the sensing layer of microcrack and microscale structures constructed through pre-stretching strategy achieves tunable strain sensitivity and strain insensitivity on fiber intrinsic structures. This approach allows for tunable strain sensitivity and insensitivity within the inherent structure of fibers, offering an effective method for designing sensor sensitivity on demand to suit various application scenarios.

3.3. The electromechanical performance and characterization of the Ag@PU_x

To delve into the electromechanical properties of this tunable strain sensor, we conducted a comprehensive assessment of its mechanical and electrical characteristics on the Ag@PU_x. As illustrated in Fig. 3(a), all specimens exhibited exceptional tensile performance due to the inherently excellent elasticity of the PU substrate (Fig. S2 and Video S3 in Appendix A), with elongation at break consistently exceeding 200%. Notably, the introduction of residual internal stresses through pre-stretching led to a gradual decrease in the elongation at break as the pre-stretch ratio x% increased [34]. For instance, the unstretched Ag@PU₀ had an elongation at break of 668%, whereas the Ag@PU₃₀₀, pre-stretched by 300%, showed a reduction to 275%. This trend demonstrates that higher degrees of pre-stretching result in more significant loss of mechanical properties in the substrate. In the experiments, it was observed that as the number of Ag layers deposited on the interface gradually increased, the conductive pathways also increased, thereby enhancing the material’s electrical conductivity (Figs. S9 and S10 in Appendix A). When three layers of silver coating were reached, the continuity and density of the silver layer were optimized, resulting in the highest electrical conductivity. However, further increasing the thickness of the silver layer led to a decrease in electrical conductivity. This phenomenon may be attributed to excessively thick silver layers generating significant internal stress between the substrate and the coating. Such internal stress can weaken the adhesion between the silver layer and the substrate, thus reducing the overall electrical performance. Consequently, the Ag@PU₃₀₀ ensures high electrical conductivity while avoiding issues related to reduced adhesion strength, demonstrating optimal electrical properties. Fig. 3(b) presents the loading-unloading curves of the Ag@PU₃₀₀ under varying strains (20% to 100%), highlighting its excellent elastic recovery and minor hysteresis, indicative of robust elastic resilience. Remarkably, at a strain rate of 1.0 × 10⁻³ m·s⁻¹, data in Fig. 3(c) shows that the Ag@PU₃₀₀ responds to a 20% strain within just 78 ms and recovers in 190 ms, evidencing rapid response and recovery capabilities. Such prompt responsiveness is crucial for real-time monitoring of physiological deformations, positioning the Ag@PU₃₀₀ favorably for applications in instant health monitoring and motion analysis.

Fig. 3(d) compares the reversible relative resistance changes of the Ag@PU₀ and Ag@PU₃₀₀. Given that the substrate is an elastomer, both exhibit minimal electrical hysteresis [35]. It is evident that the resistance of the Ag@PU₀ increases rapidly with increasing strain (0 to 50%), demonstrating high strain sensitivity. In contrast, the Ag@PU₃₀₀ exhibits a broader strain range (0 to 80%) with relatively stable resistance changes within this range, maintaining a ΔR/R₀ value below 30%. Notably, in the 0 to 30% strain range, the relative resistance change is less than 1%, indicating excellent strain insensitivity. Additionally, Fig. 3(e) presents the initial conductivities of four types of sensors: 460, 670, 1400, and 2000 S·cm⁻¹. This variation in conductivity is attributed primarily to the amount of Ag deposited on the fiber surfaces. With the increase of the pre-stretching ratio, the PU fibers become thinner, resulting in a larger specific surface area that enhances the efficiency of Ag particles loading [36]. The pre-stretching induces micron-scale cracks on the PU fiber surfaces (Fig. S11 in Appendix A). These cracks not only increase the interfacial area for Ag particles loading but also roughen the surface of the PU fibers, providing more “trap” sites to mechanically anchor the Ag particles. Concurrently, the cracks facilitate the penetration of the plating solution and ion diffusion. According to Eq. (S4) in Appendix A for conductivity, a higher amount of Ag on the fiber surface results in lower resistance and correspondingly higher conductivity. Fig. 3(e) also highlights an interesting trend: as the pre-stretch ratio increases, the detection range of Ag@PU_x expands. This occurs because larger pre-stretch ratios result in more microscales Ag layers on the fiber surface after pre-stress release. When strain is applied to the Ag@PU_x, these overlapping microscales Ag layers can unfold and spread over the fiber surface, adapting to larger strains while maintaining the integrity of the conductive pathways. Therefore, under the premise of maintaining intact and restorable conductive pathways, an increased pre-stretch ratio allows the sensor to endure a larger strain range, thereby broadening its detection range. Furthermore, we compared the conductivity of the Ag@PU₃₀₀ with other reported fiber-based strain sensors (Fig. 3(f) [[37], [38], [39], [40], [41], [42], [43], [44]]). The results demonstrated that the Ag@PU₃₀₀ designed in this study maintains a high electrical conductivity of 500 even under 50% tensile strain. This performance significantly exceeds that of most similar sensors. Consequently, our Ag@PU₃₀₀ not only meets the strain requirements for human movement but also exhibits excellent conductivity, making it suitable as a signal transmission wire for motion signals.

In practical applications, long-term durability is a crucial attribute for sensors. Fig. 3(g) illustrates the results of a 10 000 cycle test under 20% strain for the Ag@PU₃₀₀. The output curve maintains high consistency and repeatability even after 10 000 cycle test, demonstrating the excellent cyclic stability of the Ag@PU₃₀₀. Although the pre-stretching strategy may slightly degrade the material’s mechanical properties (Fig. 3(a)), the Ag@PU₃₀₀ with 300% pre-stretching still exhibits remarkable cyclic endurance. Overall, the Ag@PU₃₀₀ ensures stable transmission of electrical signals to the terminal under strain conditions. This feature makes it highly effective in various applications, including human motion monitoring and other scenarios requiring high reliability and stability, providing dependable data. This superior performance is attributed to the unique microscale structure of the sensing layer and the design of the pre-stretching ratio, which enable the sensor to maintain consistent performance over extended use.

3.4. Application of tunable strain sensor

As a proof of concept, Fig. 4(a) demonstrates the variation in brightness of light-emitting diode (LED) bulbs connected to Ag@PU₀ and Ag@PU₃₀₀ under different strains. At 0 strain, both sensors can illuminate the LED bulb with noticeable brightness. However, at 40% strain, the LED bulb connected to the Ag@PU₀ loses its illumination, whereas the Ag@PU₃₀₀ maintains nearly constant brightness even at 80% strain due to minimal changes in the elastic circuit’s length. This illustrates their distinct design objectives: the Ag@PU₀ is highly sensitive to strain, making it suitable for applications requiring high sensitivity, while the Ag@PU₃₀₀ is less sensitive to strain, ideal for environments demanding stability. We employ the Q to evaluate the electrical response of sensors under various strain conditions, defined as the percentage of strain divided by the percentage change in resistance [45].

(1)

Q = (l − l 0) / l 0 (R − R 0) / R 0

Where l is the length of the fiber, l₀ is the initial length of the fiber, R is the resistance of the fiber. A higher Q value indicates a smaller change in resistance for the same strain, signifying greater stability and reliability. Compared to previously reported strain-insensitive sensors with other coating types [[13], [14],[46], [47], [48], [49], [50]], the sensor described in this paper exhibits an acceptable Q value, as shown in Fig. 4(b). Specifically, when a 50% strain is applied, the Q value of the Ag@PU₃₀₀ is 10.1, which is significantly higher than those reported in other literature, demonstrating its superior strain insensitivity. This characteristic allows the Ag@PU₃₀₀ to serve as a stable data transmission wire, showing great potential in wearable and flexible electronic applications. To further verify its practical feasibility, we investigated the relative resistance changes of the Ag@PU₃₀₀ strain-insensitive sensor under different deformation conditions (such as stretching, twisting, and bending) as shown in Fig. 4(c). Fig. 4(d) illustrates that at the initial stage of small deformations (< 20%), the relative resistance change of the Ag@PU₃₀₀ is not significant, and even within a 40% strain range, the relative resistance remains below 2.0. Additionally, during a 360° twist process, the relative resistance only shows minimal changes (Fig. 4(e), ΔR/R₀ < 0.03) and stabilizes after completing a full 360° twist, further confirming the sensor's excellent strain insensitivity. For fibrous materials, bending is a common form of deformation. Our experimental results show that when the Ag@PU₃₀₀ is bent to 60°, the resistance change remains very limited, with an increase not exceeding 1.0 (Fig. 4(f)). These findings indicate that the Ag@PU₃₀₀ maintains relatively stable conductivity under various deformation conditions (stretching, twisting, and bending), meeting the flexibility requirements of textile-based smart electronic devices and ensuring that it does not affect circuit signal transmission quality when used as a wire in smart wearable electronic clothing. This also provides strong support for developing more reliable and functionally rich flexible fabric-based electronic products.

Given the heightened strain sensitivity of the Ag@PU₀, it enables real-time detection and differentiation across a full spectrum of human motion and physiological signals. When affixed closely to a volunteer’s fingertip, the sensor precisely captures subtle variations induced by bending at different angles, manifesting these distinctions through periodic and stable output signals (Fig. 4(g)). Similarly, when positioned on the wrist, the sensor reliably discerns both flexion and torsion movements via its consistent sensing signals (Fig. 4(h)). Installation at the knee joint demonstrated its capability to differentiate lower limb movement patterns such as walking and jogging (Fig. 4(i)), confirming adaptability to complex movement patterns. Of note, beyond limb movements, the Ag@PU₀ also prove adept at detecting finer physiological activities. For instance, in throat movements (Fig. 4(j)), including repetitive swallowing pronouncing the word “morning,” and coughing, the sensor attached to the neck was able to effortlessly capture these minute physiological signals, generating continuous and consistent response waveforms. This demonstrates that our developed strain-sensitive sensor can accurately identify various human movements under different strain conditions while producing stable and repeatable signals (Fig. S12 in Appendix A), making it ideal for real-time strain sensing of a full range of human motions and physiological signals. It holds significant potential for developing next-generation smart clothing or health tracking devices, particularly in applications requiring high-precision dynamic tracking and long-term health monitoring.

3.5. Thermal management performance of the Ag@PU_x

Prolonged sitting or desk work can lead to chronic tension in the lumbar spine and surrounding muscles, increasing the risk of diseases such as lumbar muscle strain and disc herniation. Heat therapy, a non-invasive treatment method, has been shown to effectively alleviate pain symptoms in patients with lumbar diseases [51]. Based on Joule’s Law [52]:

(2)

E = U 2 R t

where E, U, R, and t represent the generated heat, input voltage, resistance, and heating time, respectively, the passage of electrical current through a resistive conductive fiber generates heat, raising the temperature of the conductive yarn and thus achieving a heating effect. With constant voltage, lower resistance results in higher current, producing more heat and a more pronounced heating effect. Utilizing the Ag@PU₃₀₀ sensor’s characteristic of minor resistance changed under strain, which meaning the resistance remains relatively constant even when stretched, we developed a smart thermal therapy belt that can connect via Bluetooth to a smartphone for wireless data transmission, as shown in Fig. 5(a). This device features four adjustable temperature settings, allowing users to find their optimal comfort zone based on personal preferences and varying environmental conditions. The heating process and four-level temperature control are demonstrated in Video S4 in Appendix A. Compared to traditional heating clothing using metal wires or films, our smart thermal therapy belt offers several advantages: firstly, it is designed from breathable knitted fabric, providing uniform heating along the spine (Fig. 5(b)), addressing issues of localized overheating and poor air circulation found in conventional methods; secondly, it supports both wireless and wired operation for four-level temperature control, enabling connection to smartphones via Bluetooth for wireless data transmission; finally, it includes elastic recovery properties that monitor user movement, such as bending posture (Fig. 5(c)). This feature is achieved by integrating the strain-sensitive sensor (Ag@PU₀), which exhibits significant resistance changes when stretched, into the belt. The peak values in Fig. 5(c) correspond to the wearer's bending angle, alerting them to maintain proper posture and prevent excessive lumbar strain, thereby reducing the risk of spinal diseases. These innovative functionalities not only enhance product comfort and safety but also open new avenues for personalized health management.

Uniform thermal distribution is considered a crucial standard for electric heaters [[53], [54]]. Using an infrared camera, we investigated the surface thermal distribution of fabric under various strains. The results show that at a constant input voltage of 3 V, the fabric exhibits uniform thermal distribution within the strain range of 0-50% (Fig. 5(d)). Additionally, when the fabric is stretched to 10%, 20%, and 50%, the temperatures reach 37.4, 38.0, and 38.3 ℃, respectively, with the unstrained fabric temperature set at 36.8 ℃. It can be observed that as the strain increases, the belt’s surface temperature remains stable and uniform, maintaining local temperature stability near the skin. This confirms its stability under conditions of physical activity. Moreover, it validates the strain-insensitive characteristic of our prepared Ag@PU₃₀₀. Notably, the smart thermal therapy belt can achieve a high temperature under a driving voltage of 3 V, which is significantly lower than the 36 V safety threshold, thus allowing for portable battery operation. This indicates that the conductive fiber fabric developed in this study can be used stably over long periods for thermal therapy, offering significant potential in personal health management and physiological monitoring during physical activities.

3.6. The antimicrobial efficacy and electromagnetic shielding capabilities of Ag@PU_x sensor fabrics

In recent years, the frequent occurrence of infectious epidemics has heightened public awareness of the health benefits of antimicrobial clothing. For daily health management, such clothing can prevent the combination of fabric with human metabolic substances and inhibit microbial growth, thereby reducing the risk of disease transmission and odor production. It is well-known that Ag exhibits broad-spectrum antibacterial properties. The antimicrobial mechanism of Ag@PU_x is illustrated in Fig. 6(a). Detailed definitions are available in Note S2 in Appendix A. Notably, wet-spun PU fibers have a rough surface [[24], [25], [26]] with grooved structures (Fig. 2(b)) [[31], [32]], which allows for a tight binding of Ag particles to the fiber surface. This results in a slow release of Ag⁺, maintaining an effective concentration over an extended period. Given that Escherichia coli (E. coil) is one of the primary bacteria causing gastrointestinal infections in humans and animals, while Staphylococcus aureus (S. aureus) commonly colonizes human and animal skin, nose, and throat, Gram-negative E. coli and Gram-positive S. aureus were selected to evaluate the biocidal activity of fabrics woven from Ag@PU_x conductive fibers. As shown in Fig. 6(b), the antibacterial behavior of fabrics with a diameter of 6 mm was analyzed using an inhibition zone test. The fabrics included three types, Fabric I: PU fiber fabrics without Ag deposition on the fibrous interface; Fabric II: Ag@PU_x sensor fabrics; Fabric III fabrics from Fabric II washed 30 times. In Fig. 6(b), no inhibition zone is observed around the Fabric I fabric, indicating that the fabric itself does not possess antibacterial properties. Conversely, significant inhibition zones of approximately 4 mm are observed around both Fabrics II and III fabrics, suggesting that fabrics woven from Ag@PU_x conductive fibers exhibit excellent antibacterial properties and washing durability.

In addition, the Ag@PU_x sensor fabric not only retains the softness and breathability of textiles but also exhibits the electromagnetic shielding properties characteristic of metallic fabrics, offering potential applications in electronic equipment coverings, military gear, and electromagnetic radiation protection clothing [[55], [56], [57]]. The effective electromagnetic shielding provided by the Ag@PU_x sensor fabric can be attributed to two primary reasons. From a macro perspective (Fig. 6(c)), the fabric’s excellent conductivity results in significant reflection and attenuation losses of electromagnetic waves. Electromagnetic induction generates faradic currents on the fabric surface, creating a new electromagnetic field that interferes with and diminishes the incident electromagnetic wave when their directions are opposite. From a microscopic perspective, the PU fibers produced via wet spinning feature grooves on the surface and internal pores. This groove-pore structure enhances the reflection and scattering of electromagnetic waves. As illustrated in Fig. 6(d) part 1, when electromagnetic waves encounter the groove-pore material surface, multiple reflections off the fiber surface reduce the waves’ energy. Some of the remaining waves return to free space, while others penetrate the fiber and encounter further reflections and scattering at internal walls. The waves reflected within the fiber eventually escape into space, while those that pass through the pores undergo multiple reflections and vibrations, leading to substantial energy loss. This process repeats until the waves reach the other side of the fiber, where they refract back into free space (Fig. 6(d) part 2). Therefore, effective electromagnetic interference shielding is achieved through the combination of macroscopic fiber conductivity and microscopic multiple reflections and refractions within the fiber’s groove-pore structure. It is noteworthy that the shielding effectiveness (SE) value is commonly used as the standard for evaluating anti-electromagnetic shielding performance (Table S1 in Appendix A). Fig. 6(e) displays the SE values of different samples measured using the flange coaxial method. From the figure, it can be observed that: ① The SE value of the fabric is close to zero, indicating that pristine PU fibers have almost no electromagnetic shielding capability; ② the SE value of around 40 suggests that the fabric can be used as an anti-electromagnetic shielding material; ③ although the SE value decreases slightly, it remains around 40, demonstrating that the fabric woven from Ag⁺ deposited PU fibers maintains good shielding efficacy and acceptable water-wash stability.

4. Conclusions

In summary, we demonstrate two innovative Ag@PU_x fiber sensors with microstructured Ag/PU interfaces fabricated via IMID. This methodology enables precise modulation of strain sensing ranges and sensitivities, addressing a critical limitation of conventional fiber-based sensors that lack adaptability to application-dependent strain response requirements. Specifically, the microcrack structured Ag layer achieves strain sensitivity with a high GF of 7.7 within 40%-46% strain through crack propagation. In contrast, the microscale structured Ag layer maintains conductive path integrity via two mechanisms: hierarchical structural sliding (where Ag islands slide relatively while remaining connected under small strains) and crack bridging (where Ag particles in grooves form conductive bridges at crack edges under large strains). This design ensures high conductivity while demonstrating insensitivity to strain (Q = 10.1 at 50% strain) and torsion (ΔR/R₀ < 0.03 under 360° torsional deformation), rendering it an ideal stable conductor for electrical signal transmission. Finally, the high-performance Ag@PU_x electronic devices we fabricated allow continuous production and textile integration. These devices exhibit exceptional multifunctionality, including electrothermal heating, antibacterial/deodorization, and electromagnetic shielding properties. These features highlight the immense potential of the Ag@PU_x electronics in developing multifunctional, interference-resistant smart wearable systems.

CRediT authorship contribution statement

Feiyu Tong: Visualization, Software, Conceptualization, Writing - original draft, Validation, Data curation. Jingmin Shi: Investigation, Validation. Qi Jiang: Validation, Data curation. Ming Li: Conceptualization, Software. Ruidong Xu: Formal analysis, Investigation. Ganghua Li: Software, Visualization. Yuanyuan Liu: Software, Validation. Xinyu Zhang: Visualization. Jinfeng Yang: Investigation. Mingwei Tian: Writing - review & editing, Methodology, Formal analysis, Supervision, Funding acquisition. Yutian Li: Writing - review & editing, Funding acquisition, Supervision, Conceptualization.

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.

The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for this journal and was not involved in the editorial review or the decision to publish this article.

Acknowledgments

This work was supported by the National Key Research and Development Program (2022YFB3805802), the Key Research and Development Program of Shandong Province (2024CXGC010411), the National Natural Science Foundation of China (52473307, 22208178, and 62301290), the Taishan Scholar Program of Shandong Province in China (tsqn202211116), the Shandong Provincial Universities Youth Innovation Technology Plan Team (2023KJ223), the Natural Science Foundation of Shandong Province of China (ZR2023YQ037, ZR2023QE043, ZR2022QE174, and ZR2024ME012), the Shandong Province Science and Technology Small and Medium sized Enterprise Innovation Ability Enhancement Project (2023TSGC0344 and 2023TSGC1006), the Natural Science Foundation of Qingdao (23-2-1-249-zyyd-jch and 24-4-4-zrjj-56-jch), the China Postdoctoral Science Foundation (2024M761560), the Anhui Province Postdoctoral Researcher Research Activity Funding Project (2023B706), the Qingdao Key Technology Research and Industrialization Demonstration Projects (23-1-7-zdfn-2-hz), the Qingdao Postdoctoral Funding Project (QDBSH20240201011), the Suqian Key Research and Development Plan (H202310), and the Systems Science Plus Joint Research Program of Qingdao University (XT2024202).

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2025.07.004.

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