Flexible sensors are prone to the problems of slow recovery rate and large residual strain in practical use. In this paper, a polyurethane functional composite with a gradient change in elastic modulus is proposed as a flexible sensor to meet the recovery rate and residual strain without affecting the motion. Different hard and soft segment ratios are used to synthesize a gradient polyurethane structure. The conductive percolation threshold was obtained between 45 wt% and 50 wt% of flake silver powder. Both gradient polyurethane and gradient polyurethane composites demonstrated that gradient materials can increase the recovery rate and reduce residual strain. The gradient polyurethane composites had a tensile strength of 3.26 MPa, an elastic modulus of 2.58 MPa, an elongation at break of 245%, a sensitivity coefficient of 1.20 at 0–25% deformation, a sensitivity coefficient of 11.38 at 25–75% deformation, a rate of recovery of 1.95 s at a time, and a resistance to fatigue (over 1000 cycles at a fixed strain of 20% showed a stable electrical response). The sensing performance under different cyclic strain frequencies was also investigated. The process has practical applications in the field of wearable skin motion and health monitoring.
In fields ranging from human motion detection to physiological signal monitoring, medical diagnostics, smart clothing, and human–machine interaction, the demand for flexible strain sensors is continuously growing. This demand has spurred active exploration into high-performance flexible strain sensors. Most strain sensors developed through various technologies are based on elastomers with a single elastic modulus serving as the flexible substrate, which determines the recovery time for different strain behaviors. Although such strain sensors are ubiquitous, it has been shown that a single modulus may affect motion or recovery time, thus lacking a balance in the elastic modulus of the flexible substrate. In this study, we prepared a strain sensor with a gradient change in elastic modulus. The sensor features a three-layer flexible substrate with different modulus and a conductive network based on high aspect ratio electroless silver-plated glass fibers. The strain sensor exhibits excellent response performance, including an outstanding detection range (0–200 %), high sensitivity (Gauge Factor, GF max = 7760), rapid response time (140 ms), and exceptional durability (10,000 cycles). The unique gradient change in elastic modulus allows the strain sensor to have low residual strain. During the stretch-release process, the sensor can return to near its initial state, exhibiting low hysteresis. This strain sensor can monitor human movements in real-time, such as finger bending, joint flexing, walking, and running, making it a promising candidate for applications in human motion monitoring and smart wearables.
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