Bio-Inspired Internal Stress Engineering for Microscale Surface Cracking in Sensitive Strain Sensors

The fabrication of microscale cracks on flexible substrates is a crucial step for potential applications such as human-machine interfaces, biosignal analysis, and structural health monitoring. However, existing crack fabrication methods predominantly rely on externally applied mechanical stimuli such as stretching or bending, which limits the freedom with which cracks can form in terms of their location, density, and geometry. In this study, we introduce a bio-inspired internal stress engineering strategy that enables tunable microscale cracks to be driven by internal stress induced through microsphere expansion. Microscale cracks can be reproducibly formed by exploiting the localized stress concentration and thickness-dependent mechanical property transition of an ultrathin metal film. The crack-based strain sensor exhibits an ultrahigh sensitivity of 24 320 at a strain of 1.6%. The proposed strategy demonstrates excellent sensing performance under both static and dynamic loading conditions, including high repeatability and frequency stability, demonstrating its considerable potential for practical sensing applications. Further, we demonstrate its applicability in wearable sensors for real-time physiological monitoring and structural health monitoring of aircraft structures to verify its versatility across diverse industrial fields.