Aqueous gels are often selected as model materials for fundamental biotribological studies owing to their high water content, ease of synthesis, and optical transparency. Synthetic hydrogels are highly tunable and can be designed to emulate the mechanical and transport properties of common biological components (biopolymer gels, cells, and tissues) by controlling the water content. Increasing the water content increases the characteristic mesh size, the average spacing between neighboring polymer chains in a three-dimensional hydrogel network. In 1989, Canal and Peppas showed an empirical relationship between water content and mesh size (measured by wide-angle X-ray scattering) for charge-neutral, covalently cross-linked hydrogels. In 2015, Urueña et al. systematically varied the water content of polyacrylamide hydrogels by increasing the polymer concentration and used small-angle X-ray scattering to measure the mesh size, which ranged from about 1 to 10 nm.  Urueña et al. then performed a suite of self-mated or “Gemini” experiments using a custom-built microtribometer and found an inverse relationship between friction coefficient and mesh size, and thus water content. Here, we explore the scaling models that reveal high water content gels are optimized for lubricity as well as contact pressure, shear stress, and resistance to draining.

Water content and lubricity in aqueous gels

2019 Fall National Meeting

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