In this work, careful experiments reveal how abrasive wear of polyacrylamide hydrogels only temporarily alters the surface properties.

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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.

The structural and mechanical resemblance of hydrogels to cartilage makes them promising candidates for cartilage replacement in load-bearing joints. As with any sliding interface under high loads, the hydrogels in this application would be highly susceptible to wear. Although the fluid component of these biphasic materials mediates low-friction sliding, the hydrophilic polymer network of hydrogels can endure only a limited number of sliding cycles. In this work, a sandpaper-covered probe attached to a microtribometer with a reciprocating stage was used to wear polyacrylamide hydrogels with varying concentrations of polymers and cross-linkers. The wear volumes and surface features of the resulting wear scars were measured using a 3D laser scanning confocal microscope. The roughness of the wear scars and the friction coefficients during wear application increased with increasing cross-linker concentration. We calculated the wear rates for the different compositions and developed a relationship that connects the polymer and cross-linker concentrations to the wear rates. The wear rate scaled with the polymer concentration to the −2 power and with the cross-linker concentration to the 3/2 power. These results suggest that network flexibility through decreasing cross-linkers and network strength through increasing polymers are key elements of a hydrogel to reduce wear. With this relationship between the hydrogel composition and wear resistance, we can better predict the lifespan of hydrogels in different sliding applications and improve their design to be more resilient.