"Design and Tribological Characterization of Stimuli-Responsive Hydrogels"

A. Chau^a,e, C. Edwards^b,e, P. Getty^a,e, A. Rhode^a, K. Karnaukh^c, L. Mansson^a, C. Pugsley^c, M. Helgeson^b,e, M. Valentine^d,e, C. Eisenbach^e, C. Hawker^a,c,e, C. Bates^a,e, J. Read de Alaniz^c,e, A. Pitenis^a,e   

^aMaterials Department / ^bDepartment of Chemical Engineering / ^cDepartment of Chemistry & Biochemistry / ^dDepartment of Mechanical Engineering / ^eMaterials Research Laboratory / University of California, Santa Barbara, 93106

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. 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 tribological properties (e.g., friction) of hydrogels to elucidate the lubrication mechanisms driving their behavior in order to design hydrogels with tunable friction coefficients.

In this work, I explored the structure-property relationships of stimuli-responsive hydrogels, controlling their mechanical and tribological properties by tuning mesh size via swelling and synthesis. Herein, we explore the tribological behavior of three systems: polyacrylamide (PAAm), poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)), and poly(N-isopropylacrylamide-co­-2-acrylamido-2-methylpropane sulfonic acid-co-spiropyran methacrylate) (pNIPAAm-co-AMPS-co-SP). By controlling environmental oxygen during polymerization, we synthesized 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 pH-responsive behavior of P(AAm-co-AA) was investigated, and we demonstrated two orders of magnitude tunability of the friction coefficient by regulating acrylic acid concentration and pH. Light tunable p(NIPAAm-co-AMPS) gels were synthesized through the incorporation of spiropyran-methacrylate, a photoswitchable molecule. By manipulating the swelling and deswelling of the pNIPAAm-co-AMPS-co-SP gels with light, friction was tuned an order of magnitude. The investigations herein can be applied to situations that require tunable tribological properties, such as in soft robotics or haptics, and contribute to a deeper understanding of hydrogel lubricity.