Highly Stretchable Photonic Crystal Hydrogels for a Sensitive Mechanochromic Sensor and Direct Ink Writing

Photonic crystals, which are materials with periodic dielectric constants on the submicroscale, have been the focus of research for an extended period. Photonic soft materials have been extensively developed for use as colorimetric indicators and mechanochromic sensors, but their limited mechanical properties and molding characteristics only suitable for films restrict their practical implementation. Herein we report an approach to synthesize highly stretchable photonic soft materials based on a hydrogel system that is cross-linked by a crystalline colloidal array. The intrinsic inhomogeneous submicroscale structure is exploited for effective reinforcement in the multiphase system of the photonic crystals. The photonic hydrogels exhibit a high deformation capacity, with a stretching deformation above 2800% and compression above 98%. The gel has a full-color tunable range and shows 460 nm photonic shifts that can be reversibly actuated by a small compressive stress (kPa level) and can be ink-written to form patterns and freestanding structures. Mechanochromic sensors are constructed based on the three-dimensional and two-dimensional Bragg diffraction of the gel. Owing to its mechanical strength, formability, and tunable colors, the gel can be used in wearable optical devices, colorimetric tactile sensors, and full-color displays.