Amphiphilic gel lubrication and the solvophilic transition

Abstract

Lubrication in biology uses lipids, proteins, and aqueous gels to maintain hydration and provide low shear stress over a range of sliding speeds and contact pressures. The unquestionably amphiphilic nature of proteins and the complexity found in the aqueous solutions suggest that these systems operate near an optimal solvophilic condition. To explore the potential for a solvophilic transition in an amphiphilic gel, we perform tribological and swelling measurements of poly(hydroxyethyl)methacrylate, pHEMA, equilibrated over a range of water-ethanol solutions. Depending on the ethanol concentration, Gemini pHEMA gels achieve either low friction (μ < 0.02) and low adhesion or high friction (μ > 1) and high adhesion. We hypothesize that as the solution becomes increasingly ethanol-rich the alkyl regions of ethanol more fully associate with the aliphatic regions of pHEMA, effectively coating the chains with a hydroxyl presenting surface, promoting hydrogen-bonding and the influx of water and leading to maximum in swelling and mesh size, leading to a dramatic reduction in friction and adhesion. We suggest that the tribological behaviors of amphiphilic Gemini gels reflect the presentation of hydrophobic and hydrophilic domains across the interfaces during sliding. These experiments explore the lubrication and solvophilic transitions in amphiphilic Gemini gels and suggest fundamental mechanisms and solution composition through which biotribological joints leverage lipid and protein-based complex fluids to achieve lubricity.