Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels
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REVIEW
Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels Shabnam Z. Bonyadi1 · Md. Mahmudul Hasan1 · Jiho Kim1 · Samsul Mahmood2 · Kyle D. Schulze2 · Alison C. Dunn1 Received: 15 May 2020 / Accepted: 12 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract 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
Keywords Hydrogels · Aqueous lubrication · Lubrication curve Shabnam Z. Bonyadi, Md. Mahmudul Hasan, Jiho Kim, and Samsul Mahmood contributed equally to this work. Extended author information available on the last page of the article
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1 Introduction Tunable soft and aqueous gels, also known as hydrogels, are one of the leading soft matter surrogates for active matter; they are engineered for both temporary and permanent medical prostheses such as catheter coatings, soft contact lenses, and valves [1]. Many of these devices interact directly with dynamic biological interfaces, fully integrated with metabolism, immune response, and other functions, all the while experiencing the mechanical environment of complex sliding speeds, bearing forces, and contact durations. Controlling the motions of slip, that is, the friction and lubrication, is critical to proper function. Crosslinked hydrogels are often thought to be inherently lubricious, but in fact there is a broad spectrum of friction and lubricating abilities. The lubrication of hydrogels was first described by lubricated sliding of engineering materials in that there are regimes of fluid-controlled lubrication and material-controlled lubrication that loosely resemble hydrodynamic and boundary friction, but the similarities past that are scant. Launched in part by seminal work by JP Gong summarized in a 2006 review of hydrogel lubrication [2], the field of hydrogel friction and lubrication has expanded qui
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