Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels

As soft aqueous hydrogels have moved from new materials to the basis for real engineered devices in the last 20 years, their surface friction and lubrication are emerging as critical aspects of their function. The flexibility to alter and augment their mechanical and surface properties through control of the crosslinked 3D polymer networks has produced materials with diverse surface behaviors, even with the relatively simple composition of a single monomer and crosslink chemistry. Correspondingly with new understandings of the bulk behavior of hydrogels has been the identification of the mechanisms that govern the lubricity and frictional response under dynamic sliding conditions. Here we review these efforts, closely examining and identifying the internal and external influences that drive tribological response in high water content crosslinked hydrogels. The roles of surface structure, elasticity, contact response, charge, water interaction and water flow are addressed here as well as current synthesis and testing methods. We also collect open questions as well as the future needs to fully understand and exploit the surface properties of hydrogels for sliding performance. Graphical Abstract: [Figure not available: see fulltext.].