Wet-Pulling Method Induced Self-Folding Layered CNT/WPU Fiber Strain Sensor with Ultrahigh Sensitivity and Wide Working Range for Human Activity Detection

The rapid development of flexible electronics has intensified the demand for strain sensors that simultaneously achieve high sensitivity, wide detection range, and exceptional durability, particularly in human activity monitoring applications. In order to overcome the intrinsic sensitivity─stretchability trade-off in conventional resistive sensors, this work proposes a carbon nanotube (CNT)/waterborne polyurethane (WPU) composite fiber strain sensor featuring a layer-by-layer structure fabricated via a novel wet-pulling method. The synergistic interaction between the self-folding hierarchical layered structure, formed via the wet-pulling method, and surface crack engineering enables an ultrahigh gauge factor (GF = 117222.64) across a wide working range (243.78% strain). This synergy ensures both a rapid dynamic response (60 ms response time, 80 ms relaxation time) and excellent cyclic stability (>6000 cycles). With a CNT content of 28.62% and an electrical conductivity of 369.36 ± 67.99 S·m–1, the fiber accurately monitors subtle physiological signals (e.g., pulse and swallowing) and large-scale joint movements. Moreover, the device exhibits potential for applications in encrypted message transmission. This work establishes a scalable, cost-effective strategy for developing next-generation wearable sensors that integrate high performance with long-term reliability.