Chameleon-Inspired Stress-Responsive Multicolored Ultratough Films

Chameleons have a distinguished talent to rapidly shift their colors by active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. This reversible structural color variation is derived from the birefringence, which is triggered by the special anisotropic structure interacting with natural light. Inspired by the inorganic oligomers and their superiorities for constructing ultratough materials with a special structure, we demonstrated here a chameleon's skin-like film constructed by ultrasmall calcium phosphate oligomers (∼1.75 nm in diameter) as the precursor of crystalline hydroxyapatite nanoline (HNL) arrays and polyvinyl alcohol as the elastic matrix. The resulting films exhibit excellent toughness (48.3 ± 5.2 MJ m^-3). Under the applied cyclic stress, the HNLs embedded in the polymer network can reversibly arrange into a highly ordered crystal arrays owing to the driving action of polymer chains. The intense birefringence of the stretched films is easily observed with the naked eye under crossed polarizers, allowing for rapid and simple measurement of the applied stress. This work provides a pathway for the development of functional composites with super toughness by ultrasmall inorganic oligomers for their potential applications in smart devices for stress detection.