Multicolor and Attenuated Light Intensity Responses in Protein Hydrogels Arising from Photoregulated Crosslink Dynamics.

Light-responsive hydrogels enable noninvasive, precise, remote control over material properties with great biocompatibility, yet achieving multicolor addressability and dynamic regulation of mechanical properties remains challenging. Most systems rely on single photoswitches and regulate stiffness primarily through changes in crosslinking density. Here, we present a fully protein-based hydrogel that combines multicolor light responsiveness with optical control of crosslink dynamics. The hydrogel includes two visible light-responsive photoceptors: the cyanobacterial phytochrome Cph1, which enables reversible red/far-red light-controlled crosslinking, and CarH, which introduces irreversible green light-induced gel-sol transitions. Remarkably, Cph1-based hydrogels exhibited an attenuated red light intensity response, forming stiffer networks under low-intensity illumination than under high-intensity light and show a subsequent dark-adaptation with stiffening once red light illumination is stopped. This counterintuitive behavior arises from light-driven bidirectional photoisomerization that modulates crosslink lifetimes without altering the photostationary state composition. Together, these findings establish orthogonally addressable reversible and irreversible crosslinks and photoregulated crosslink dynamics as new design principles for multicolor light-responsive biomaterials.