Extremely sensitive mechanochromic photonic hydrogels with ultrahigh mechanical properties for multicolor displays by naked eyes

Abstract

Photonic soft materials, especially responsive photonic hydrogels (RPHs) with periodic permittivity on the submicroscale, have been garnering interest as colorimetric indicators and mechanochromic sensors. However, the fabrication of RPHs with both bright discoloration, satisfactory mechanical properties, and excellent mechanochromic sensitivity to meet the practical applications is still a great challenge. Herein, we develop a novel photonic crystal material using a Fe3O4 array embedded poly(N-isopropylacrylamide-co-acrylamide)/ laponite-XLS (PNA-XLS) hydrogel, which is dual-cross-linked by chemical and physical cross-linkers. Due to the interaction of sparse chemical links and apposite physical links between the polymeric networks, the PNA-XLS exhibits a significant energy dissipation and high deformation capacity, with an enhanced tensile strength above 819 kPa and elongation above 1015 %. It almost has a full-color tunable range and shows bright discoloration that can be reversibly actuated by bending, stretching, and small pressures (Pa level), with extremely high sensitivities of pressure (161 nm kPa−1) and tension (1.52 nm %−1). Moreover, its self-adhesion (133 kPa) would make it be an appealing candidate to develop portable sensors or wearable devices without auxiliary adhesive tools. Besides, elastic hydrogels with programmable and multicolor patterns can be designed by the mask-assisted UV photopolymerization of regional pregel solution. Therefore, such hydrogel has a great application potential in colorimetric indicators, mechanochromic sensors, anti-counterfeiting and wearable devices.