A soft, adhesive self-healing naked-eye strain/stress visualization patch

To learn about the strain/stress distribution in a material is essential to achieving its mechanical stability and proper functionality. Conventional techniques such as universal testing machines only applies to static samples with standardized geometry in laboratory environment. Soft mechanical sensors based on stretchable conductors, carbon-filled composites or conductive gels possess better adaptability, but still face challenges from complicated fabrication process, dependence on extra readout device and limited strain/stress mapping ability. Inspired by the camouflage mechanism of cuttlefish and chameleons, here we developed an innovative strain/stress responsive hydrogel containing ‘mechano-iridophores’. Force induced reversible phase separation manipulated the dynamic generation of mechano-iridophores, serving as indicators of local deformation. Patch-shaped mechanical sensors made from the responsive hydrogel featured fast response time (<0.4 s), high spatial resolution (~100 μm) and wide dynamic ranges (e.g. 10%-150% strain). The intrinsic adhesiveness and self-healing capability of sensing patches also ensured their excellent applicability and robustness. Importantly, the optical readout allowed strain/stress distributions to be directly identified by naked eyes or smartphone apps, which had not yet been achieved before. The great advantages above are ideal for developing the next-generation mechanical sensors towards material studies, damage diagnosis, risk prediction and smart devices.