Bioinspired, Ultrasensitive and Wide-Range Flexible Strain Sensors Based on Dual-Gradient Crack Structures

High-performance flexible strain sensors are crucial in various emerging fields including stretchable electronics, soft robots and wearable devices. However, traditional strain sensors often face challenges in achieving both high sensitivity and wide detection range simultaneously, typically sacrificing one characteristic to enhance the other. Here, we present a novel design of strain sensors featuring a dual-gradient crack structure by film thickness modulations. The first gradient (local scale) facilitates the gradual crack opening in the film during the stretching process, enabling precise mechanical response. The second gradient (global scale) modulates the degree of the local gradients across the entire surface, where small local gradients ensure sensitivity and large local gradients extend the working range. Phase field simulations confirm the stable and controlled crack propagation modulated by film thickness gradients, aligning with the experimental results. The dual-gradient crack strain sensor exhibits ultrahigh sensitivity (∼9 × 10⁶) and wide detectable strain range (∼80%). Moreover, the sensor features a low detection limit (0.02%), short response time (∼60 ms), and excellent durability over 22,000 test cycles. The dual-gradient crack sensor excels in detecting human motions, from subtle facial expressions to large joint movements, highlighting its promising potential for applications in flexible electronics and health monitoring systems.