An ultrasensitive and stretchable strain sensor based on a microcrack structure for motion monitoring

Wearable electronics such as electronic skin, human–computer interaction interfaces, health monitoring, and robotics require flexible strain sensors combining high sensitivity with a wide working range. Traditional metal/semiconductor piezoresistive sensors have limited stretchability and conformability. Combining conductive nanomaterials (e.g., CNTs, graphene, AgNWs, MXenes) with elastomeric substrates (e.g., TPU, PDMS, silicone rubber) is a common route, yet direct filler–polymer composites often suffer a tradeoff between sensitivity and strain range due to filler slippage and tunneling-dominated conduction. Inspired by spider slit organs, crack-based architectures can yield ultrahigh sensitivity, but uncontrolled crack propagation often restricts working range. This study aims to resolve the sensitivity–range tradeoff by engineering a microcrack structure on an electrospun TPU mat reinforced with a CNT–AgNW conductive network, targeting ultrahigh sensitivity, wide strain range, fast response, low hysteresis, and durability for wearable motion monitoring.

Prior work illustrates the tradeoff between sensitivity and strain range in flexible strain sensors. Examples include: (1) MXene nanoparticle–nanosheet hybrid networks achieving GF ≈ 178.4 over 53% strain with good durability (Yang et al.). (2) Braided graphene ribbon/dragon skin sensors with adjustable geometry achieving 55.5% working range and high sensitivity (GF > 175.16) (Li et al.). Despite high sensitivity, these have limited working ranges. (3) A polymer-assisted copper deposition plus embroidery approach offered wide range and linearity but low sensitivity (GF ≈ 49.5) (Liu et al.). Crack-based sensors can reach extreme sensitivity (e.g., ultrasound-patterned parallel cracks with GF up to 3.2 × 10^7; Liu et al.) but typically endure only small strains (~20%) suitable for subtle physiological signals. MXene/CNC-coated TPU cloth with adjustable crack density reached 83% range and GF ≈ 3405 (Li et al.), yet controlling crack growth remains challenging. Stress concentration at slits can drive rapid crack propagation and failure of conductive channels (Huang et al.). Hence, there is a need for structural strategies that both control crack morphology and expand the workable strain range without sacrificing sensitivity.

Device fabrication and materials processing: (1) Electrospun substrate: Thermoplastic polyurethane (TPU) solution (30 wt%) prepared in DMF/acetone (1:1 mass ratio), stirred 12 h. Electrospinning at 20–25 °C, DC voltage 10 kV, feed rate 1 ml/h, needle–collector distance 15 cm, using a 1 ml syringe on a micro-injection pump. Fibers collected on a custom plate and air-dried 24 h to form a porous TPU nanofiber mat. (2) Conductive layer deposition: Multiwall carbon nanotubes (MWCNTs) and silver nanowires (AgNWs) dispersed in deionized water; ultrasonication for 2 h. A total of 4 mg MWCNTs and 2 mg AgNWs deposited onto the TPU mat by vacuum-assisted filtration to form a CNT@AgNW layer. Composite mat oven-dried at 120 °C for 30 min. (3) Microcrack formation (prestretching): Composite mat stretched to 100% strain at 4 mm/min using an electronic universal tensile tester (PT-1198GDO), then released at the same speed to induce a uniformly distributed microcrack network. Crack growth is tunable via prestretching speed and strain. (4) Electrode integration: Copper electrodes attached with silver paste; electrode gap 30 mm. The device is referred to as CNT@AgNW/TPU strain sensor (CATSS). Characterization: (a) XRD showed MWCNT peak at 26.1° and AgNW peaks at 38.3° and 44.5°, confirming loading on TPU. (b) FTIR indicated a blueshift of the TPU N–H stretching band (~3330 cm^-1) after CNT@AgNW deposition, attributed to hydrogen bonding between PVP on AgNWs (and oxygen-containing groups in MWCNT dispersion) and TPU, evidencing improved interfacial interactions. (c) SEM revealed porous, bead-free TPU fibers (diameters ~500–750 nm) and, after prestretching, microcracks oriented perpendicular to the stretching direction forming an island-bridge network. Mechanical properties: Tensile tests (rate 5 mm/min) showed the composite mat retained tensile strength similar to neat TPU but with increased strain at break; the composite could be stretched beyond 450% strain. Even at >250% applied strain, the mat surface exhibited no obvious defects. Electrical testing: Resistance (R) and relative change (ΔR) measured under tensile and bending strains; sensitivity quantified via gauge factor GF = [(R − R0)/R0]/ε. Dynamic response assessed via small-strain steps (1–2%), quasi-transient steps (1% with hold), and repeated cycles at large strains (25%, 50%, 75%, 100%).

- Working range: Reliable sensing over 0–171% tensile strain with monotonically increasing resistance across three regimes (0–102%, 102–135%, 135–171%). - Sensitivity (gauge factor): GF ≈ 691 (0–102%), ~2 × 10^4 (102–135%), and >11 × 10^4 (135–171%). Rapid GF escalation attributed to microcrack propagation and reduction of conductive pathways. - Response time: ~65 ms for a 1% quasi-transient step strain (fast response and recovery). - Small-strain detection: Clear, stable resistance changes at 1% and 2% strain with consistent waveforms; flat plateaus when holding strain for ~1 s indicate stable signal recognition. - Hysteresis: Minimal; loading/unloading curves nearly coincide in single tensile–recovery cycles. - Durability: Superior cycling stability (>2000 cycles as stated in the abstract). Multi-cycle responses at large strains (25%, 50%, 75%, 100%) showed high consistency. - Mechanical robustness: Composite mat stretchable beyond 450% strain; at >250% strain no obvious surface defects, supporting wide-range sensing. - Structural/chemical validation: XRD confirmed presence of MWCNTs and AgNWs; FTIR blueshift indicates hydrogen-bond-mediated interfacial bonding; SEM verified microcrack network perpendicular to stretch forming an island-bridge conduction structure.

The engineered CNT@AgNW conductive network on a porous electrospun TPU mat, combined with a controlled prestretch-induced microcrack architecture, addresses the traditional sensitivity–range tradeoff. Interfacial hydrogen bonding (PVP–TPU and oxygenated MWCNT–TPU) and physical entanglement of CNTs with AgNWs reinforce the conductive network and limit filler slippage, ensuring that applied tensile load is preferentially released via distributed microcrack nucleation and propagation rather than catastrophic network failure. The microcracks form an island-bridge conductive topology: under strain, narrowing bridges and expanding islands drastically reduce the number and cross-section of conductive pathways, amplifying resistance changes and thereby sensitivity. Because out-of-plane bending/distortion is restricted by good interfacial bonding, crack propagation is controlled spatially, allowing cumulative crack opening widths to exceed the elastic limit of the substrate, thus extending the effective working range to 171% without loss of signal monotonicity. Compared with conventional filler-in-matrix sensors limited by tunneling and slippage, this architecture yields ultrahigh gauge factors, fast response, small hysteresis, and robust cycling. Demonstrations of small-signal and large-joint motion detection, as well as vibration monitoring, highlight applicability to electronic skin and wearable health-monitoring systems.

This work presents a flexible CNT@AgNW/TPU strain sensor (CATSS) leveraging a prestretch-induced microcrack network on an electrospun TPU scaffold. Key contributions include: (1) simultaneous achievement of ultrahigh sensitivity (GF up to >11 × 10^4) and a wide working range (0–171% strain), (2) fast response (~65 ms), low hysteresis, and durability (>2000 cycles), and (3) a mechanistic understanding of island-bridge microcrack conduction enabling controlled crack propagation. The device effectively monitors subtle and large human motions and vibration signals, indicating strong potential for electronic skin and wearable health monitoring. Potential future research directions include: refining prestretch/crack engineering to tailor linearity and dynamic range; investigating long-term environmental stability (humidity, temperature, sweat) and washability; integrating soft packaging and wireless readout for on-body systems; and exploring scalable manufacturing and patterning for arrayed sensing and multimodal integration.

Explicit limitations are not detailed in the provided text. Potential constraints include limited reporting of ultra-long-cycle endurance beyond >2000 cycles, incomplete characterization under varying environmental conditions (humidity, temperature), and limited quantitative data on bending sensitivity and performance under complex multiaxial deformations, which may affect generalizability to diverse real-world wearable scenarios.