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

Wearable devices like electronic skin, human-computer interfaces, health monitors, and robots are rapidly developing. Piezoresistive flexible strain sensors are attractive due to their simple preparation and low cost. However, traditional sensors made of metal or semiconductors have a narrow working range and struggle to conform to human skin. Polymer substrates offer excellent tensile properties making them suitable for flexible electronics. Combining functional conductive materials (CNTs, graphene, AgNWs, MXenes) with elastic polymer substrates (TPU, PDMS, silicone rubber) is a common approach. While some sensors show excellent sensitivity, their small working range limits applications. Others boast wide ranges but lack sensitivity. The challenge lies in balancing sensitivity and working range. Microcrack-based sensors, inspired by spider leg structures, offer high sensitivity but often have limited strain ranges due to uncontrolled crack growth. This research aims to design a sensor that overcomes this limitation.

Existing research on flexible strain sensors highlights a trade-off between sensitivity and working range. Studies using materials like Ti3C2Tx MXene, graphene ribbons, and polymer-assisted copper show either high sensitivity with limited strain range or wide range with low sensitivity. The direct combination of functional materials and elastic substrates doesn't resolve the conflict. Crack-based sensors offer high sensitivity due to controlled microcracks, but their strain range is limited by uncontrolled crack propagation. This paper addresses this challenge.

A flexible strain sensor (CATSS) was fabricated by combining electrospun TPU fibers and a microcrack structure. TPU porous fiber mats were prepared by electrospinning. CNTs and AgNWs were deposited onto the mats via vacuum-assisted filtration. Prestretching to 100% strain created a uniformly distributed microcrack structure. Copper electrodes were attached with silver paste. Material characterization included X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) to confirm the successful loading of CNTs and AgNWs and the formation of hydrogen bonds between the conductive layer and TPU substrate. Scanning Electron Microscopy (SEM) imaged the morphology of the TPU nanofibers and the microcrack structure. Tensile tests determined the mechanical properties of the neat TPU mats and the composite mats. Strain-sensing performance was evaluated by measuring resistance changes under tensile and bending strains. The gauge factor (GF), response time, hysteresis, and durability were determined.

The CATSS sensor achieved a wide working range (0–171% strain) due to the reinforced conductive network limiting slippage and the controlled crack growth. Ultrahigh sensitivity was observed (GF = 691 in 0–102% strain, ~2 × 10⁴ in 102–135% strain, >11 × 10⁴ in 135–171% strain). The sensor exhibited a fast response time (~65 ms), small hysteresis, and excellent durability (>2000 cycles). The increase in GF with strain was attributed to the “island-bridge” structure formed by distributed microcracks, where the conductive paths decrease with increasing strain. The sensor successfully detected small strains (1-2%) and large strains (25%, 50%, 75%, 100%), showing consistent signal waveforms across multiple cycles. The sensor's performance under bending strain was also investigated.

The results demonstrate the successful design and fabrication of a highly sensitive and stretchable strain sensor with a wide working range. The unique microcrack structure significantly improved the sensor's performance compared to existing designs. The “island-bridge” mechanism explains the ultrahigh sensitivity, and the controlled crack growth through prestretching enables the wide working range. The fast response time and low hysteresis make the sensor suitable for real-time motion monitoring applications. The findings address the limitations of previous sensors that struggled to balance sensitivity and working range.

This work presents a novel flexible strain sensor with superior sensitivity and a wide working range, surpassing existing technologies. The microcrack structure and the choice of materials (CNTs and AgNWs on electrospun TPU) were crucial to achieving these results. Future work could explore different conductive materials or polymer substrates to further optimize sensor performance and explore applications in various wearable and robotic systems.

While the sensor demonstrates exceptional performance, further investigation is needed to fully understand the long-term stability and reliability under extreme conditions. The manufacturing process could be further optimized for mass production. The effect of temperature and humidity on sensor performance also requires detailed study.