UC Santa Barbara

Aqueous, low friction interfaces are ubiquitous in biology – from articular cartilage that coats synovial joints to mucin layers that act as a lubricating barrier for epithelial surfaces throughout the body – and have exceptional tribological properties. Hydrogels, three-dimensional networks of crosslinked hydrophilic polymer chains swollen in water, are often utilized as synthetic mimics for these biological systems due to their high water content, lubricity, and tunable mechanical properties. There is a great need to investigate the structure-property relationship of hydrogels to elucidate the lubrication mechanisms driving their behavior in order to design hydrogels with tunable friction coefficients. In this dissertation, I provide a framework to study stimuli-responsive hydrogels and describe multiple methods to engineer hydrogel friction coefficients. Herein, we explore the tribological behavior of four systems: polyacrylamide (PAAm), poly(2-hydroxyethyl methacrylate) (pHEMA), poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)), and poly(N-isopropylacrylamide-co-spiropyran) (pNIPAAm-co-SP). By controlling environmental oxygen during polymerization, we synthesized non-ionic PAAm hydrogels with gradient surface structures. Using a reaction-kinetics model, we predicted the thickness and concentration of this surface gradient layer and investigated the impact of surface structure on the tribological behavior of these hydrogels. The amphiphilic nature of pHEMA hydrogels was explored and their solvophilic transition was exploited to control friction by regulating the ratio of water and ethanol. Charged P(AAm-co-AA) hydrogel friction was controlled by altering solution pH and AA concentration, and we demonstrated two orders of magnitude tunability of the friction coefficient. The surprising pH-dependent friction behavior of PAAm in response to extreme pH was also examined. Light tunable pNIPAAm gels were synthesized through the incorporation of spiropyran-methacrylate, a photoswitchable molecule. By manipulating the swelling and deswelling of the pNIPAAm-co-SP-MA gels with light, friction was tuned an order of magnitude. The investigations herein offer new insight into designing hydrogels with tunable tribological behavior and contribute to a deeper understanding of hydrogel lubricity.