Stimuli-responsive hydrogels have a wide range of applications spanning drug delivery to soft robotics. Hydrogel mechanics and swelling can be tuned via external stimuli such as temperature and pH. However, there is an increased need for bio-orthogonality, including the use of light to control hydrogel properties. Recently, water-soluble spiropyran photoswitches have been incorporated into hydrogel networks to control the swelling and deswelling kinetics, resulting in light-induced actuation and bending. However, the tribological properties (e.g., friction and lubrication) of light-responsive hydrogels remain largely unknown. In this work, spiropyran-methacrylate was conjugated with acrylamide to form a copolymerized hydrogel network (p(SP-AAm)). In visible light, the photoswitch is hydrophobic in its spiro form (ring-closed). Once irradiated with UV light, the photoswitch isomerizes to its merocyanine form (ring-open) and becomes hydrophilic, inducing swelling within the hydrogel. Using a custom-built microtribometer, we characterized the friction coefficient before and after irradiation and demonstrated spatiotemporal control over the tribological properties of these hydrogels. These results are in good agreement with our earlier findings of solvophilic transitions in friction behavior for self-mated amphiphilic hydrogels (polyhydroxyethylmethacrylate, pHEMA) swollen in ethanol-water solutions. Based on our prior results and the findings herein, we hypothesize that the deswelling-swelling transition is responsible for changes in the tribological properties of the light-responsive hydrogel, leading to a decrease in friction coefficient.

Light-tunable hydrogel friction

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