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.

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Additive manufacturing is rapidly growing in popularity due to its capability to produce parts with customized mechanical properties. Recent studies show that mechanical properties can be altered in a controlled manner by adjusting the relationship between build orientation and the applied loading from the service. In this work, the effect of build orientation on the tribological properties of 3D printed PLA – Polylactic acid and ABS- Acrylonitrile butadiene styrene is investigated. PLA and ABS samples are printed using material extrusion with three different build orientations. Tribological results show that variation in build direction relative to the sliding direction leads to anisotropic wear properties. The best wear properties are achieved when the layers are oriented orthogonally to the sliding direction. The coefficient of friction remains mostly unaffected by the build orientation. PLA samples demonstrate significantly better tribological properties compared to ABS. Varying the sliding speed between the interacting surfaces also affects the wear properties of both PLA and ABS. The results suggest that optimizing the build orientation with respect to service loading can improve the wear performance of additively manufactured thermoplastics. This presents an additional paradigm when designing additively manufactured parts potentially for functionally graded materials.