Programming Green Light-Responsive Hydrogels: From Photo-Weakening to Photo-Unresponsiveness and Photo-Strengthening

Light-responsive hydrogels are highly valued for their dynamic mechanical properties and biocompatibility. In this study, we present a hydrogel system that can either soften or strengthen on green light exposure, or remain unresponsive to light, depending on the addition of adenosyl cobalamin (AdoCbl) and Co²⁺. These protein-based hydrogels were formed using genetically encoded SpyTag-SpyCatcher chemistry and included green light-sensitive CarH_c protein domains. As previously reported, these hydrogels formed in the dark with the addition of AdoCbl, due to the tetramerization of the CarH_c domains. Upon green light exposure, the CarH_c tetramers disassembled, leading to a rapid transition from gel to sol. Interestingly, we discovered that an excess of AdoCbl leads to photo-strengthening rather than photo-weakening. This occurred because light exposure induces interchain crosslinks between AdoCbl and poly-histidine tags (His₆-tags) of the proteins. Furthermore, incorporating Co²⁺ ions enhanced hydrogel stiffness by coordinating to His₆-tags. This not only suppressed photo-weakening but also promoted photo-strengthening behaviour. These findings highlight the role of His₆-tags in photochemical crosslinking with excess AdoCbl and in coordination to Co²⁺ ions, providing a novel strategy for designing tuneable, light responsiveness in materials.