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 Co. These protein-based hydrogels were formed using genetically encoded SpyTag-SpyCatcher chemistry and included green light-sensitive CarH protein domains. As previously reported, these hydrogels formed in the dark with the addition of AdoCbl, due to the tetramerization of the CarH domains. Upon green light exposure, the CarH 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 (His-tags) of the proteins. Furthermore, incorporating Co ions enhanced hydrogel stiffness by coordinating to His-tags. This not only suppressed photo-weakening but also promoted photo-strengthening behaviour. These findings highlight the role of His-tags in photochemical crosslinking with excess AdoCbl and in coordination to Co ions, providing a novel strategy for designing tuneable, light responsiveness in materials.