Ultrasensitive, highly stretchable and multifunctional strain sensors based on scorpion-leg-inspired gradient crack arrays

•High-performance flexible strain sensors based on scorpion-leg-inspired gradient crack arrays are achieved.•Gradient crack arrays controllably form in periodic thickness-gradient metal films on flexible substrates.•The cracks are progressively open under mechanical strain, behaving a unique sensing mechanism.•Applications for human activity monitoring and information encryption are demonstrated. Wearable flexible strain sensors based on conductive films have shown great application potential in many high-tech fields. They have high requirements for synergistic performances of sensitivity, flexibility, stability and durability. However, overcoming the trade-off between high sensitivity and high stretchability is still a great challenge in this research. Here we develop a facile strategy to prepare high-performance flexible strain sensors based on gradient crack arrays inspired by the gradient slits on scorpion legs. The gradient crack arrays with branched or hierarchical features controllably form in a periodic thickness-gradient metal film prepared by one-step masking technique on a flexible substrate. The cracks are progressively open from the thickest film region to the thinnest one under mechanical strain, behaving a unique sensing mechanism. As a result, the gradient crack-based strain sensors possess excellent comprehensive performances including high sensitivity (up to 21000), wide sensing range (∼70 %), low detection limit (0.02 %), quick response speed (below to 80 ms), excellent stability and durability (up to 15,000 cycles at 5 % strain). As demonstration of applications, the sensors are used to monitor various human activities and to express and encrypt information by bending bidirectionally. This work provides a novel way to fabricate highly sensitive, stretchable, robust, multifunctional, crack-based flexible strain sensors, which would greatly expand the applications in flexible electronics and wearable devices.