Lubricity of surface hydrogel layers

Many biological interfaces provide low friction aqueous lubrication through the generation and maintenance of a high water content polymeric surface gel. The lubricity of such gels is often attributed to their high water content, high water permeability, low elastic modulus, and their ability to promote a water film at the sliding interface. Such biological systems are frequently characterized as "soft," where the elastic moduli are on the order of megapascals or even kilopascals. In an effort to explore the efficacy of such systems to provide lubricity, a thin and soft hydrogel surface layer (~5 μm in thickness) with a water content of over >80 % was constructed on a silicone hydrogel contact lens, which has a water content of approximately 33 %. Nanoindentation measurements with colloidal probes on atomic force microscopy (AFM) cantilevers revealed an exceedingly soft elastic modulus of ~25 kPa. Microtribological experiments at low contact pressures (6-30 kPa) and at slow sliding speeds (5-200 μm/s) gave average friction coefficients below μ = 0.02. However, at higher contact pressures, the gel collapsed and friction loops showed a pronounced stick-slip behavior with breakloose or static friction coefficient above μ = 0.5. Thus, the ability of the soft surface hydrogel layers to provide lubricity is dependent on their ability to support the applied pressure without dehydrating. These transitions were found to be reversible and experiments with different radii probes revealed that the transition pressures to be on the order of 10-20 kPa.