Photo-Switchable Hydrogel Friction

Stimuli-responsive hydrogels have a wide range of applications spanning drug delivery to soft robotics. Hydrogel mechanics and swelling are often tuned via external stimuli such as temperature and pH, but there is an increased need for bio-orthogonality in order to improve the biocompatibility of these hydrogels. Recently, light has been used as a stimulus through the incorporation of photo-responsive molecules that isomerize in response to light irradiation. One common photochromic molecule is water-soluble spiropyran, which is copolymerized 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 these photo-switchable hydrogels remain largely unknown. In this work, spiropyran-methacrylate was conjugated with acrylamide to form a copolymerized hydrogel network (p(SP-<i>co</i>-AAm)). In visible light, the photoswitch is hydrophobic in its spiro form (ring-closed). Once irradiated with UV light, the photoswitch isomerizes to its hydrophilic merocyanine form (ring-open), inducing macroscopic hydrogel swelling. 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. In comparison, light-responsive spiropyran-based surfactants (SP-DA-PEG) were introduced in solution to the interface of self-mated poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels, which typically possess high friction. Under UV light irradiation, we demonstrated a decrease in friction coefficient. Based on these findings, we hypothesize that the increase in hydrophilicity with light irradiation is responsible for changes in the tribological properties of photo-switchable hydrogels and surfactants, leading to decreases in friction coefficient. Additionally, incorporation of these photo-switchable molecules at the sliding interface may be all that is necessary for friction reduction rather than incorporation into the bulk material.