Photoswitchable Cross-Linking in Polymer Gels: Effects on Surface Creasing and Network Relaxation during Swelling

Polymer gels with photoresponsive cross-links enable tunable mechanics and surface morphologies, making them promising for adaptive materials. While prior work on coumarin cross-linked gels has focused on photomediated events in dilute solution, their network-level mechanical responses remain unclear. Herein, we design PEG hydrogels with both permanent covalent and dynamic coumarin cross-links, allowing in situ modulation of cross-linking under wavelength-specific UV light. Real-time FTIR and dynamic mechanical analysis (DMA) show that postcure 365 nm irradiation drives rapid dimerization, increasing storage modulus by up to 69%, whereas cleavage of coumarin cross-links via 254 nm postcure irradiation has a more limited effect due to attenuation in bulk samples. Surface imaging reveals that dynamic cross-linking governs swelling-induced crease formation and evolution. Together, these results establish design principles for hydrogels with programmable mechanics and adaptive surface topographies, enabling light-addressable coatings, mechanically lockable soft actuators, and dynamic biomaterial interfaces.