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Alison C Dunn, Thomas Ettore Angelini, John Pruitt, W Gregory Sawyer; Gemini Interfaces in Aqueous Lubrication of Hydrogels. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4656.
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Lubricity of contact lenses is now being used as a comfort metric, as comfort is related to the friction between the eyelid wiper, cornea, and contact lens. However, there is no standard method for measuring this lubricity; proposed methods involve rheometers, microtribometers, and inclined planes. This work measures the friction coefficient in configurations which emulate these methods, and we hypothesize that hydrogels sliding against each other in self-mated “Gemini” contact best replicate the excellent lubricity found in the eye, rather than a hydrogel sliding against an hard impermeable probe. This hypothesis is based on the Gemini interface maintaining water at the surface for aqueous lubrication.
Friction experiments of hydrogel materials were carried out in configurations characterized by contact area motion: the hydrogel surface in intermittent (migrating) or constant (stationary) contact. In a Gemini configuration, both conditions exist simultaneously. Polyacrylamide hydrogels of 92% water were used to emphasize the effects of permeability on friction. The normal force was held constant at 400 µN to apply pressures that typically exist between the eyelid and cornea/sclera, and sliding speed was applied between 100 µm/s-1,500 μm/s. Each experiment was run for 30 reciprocating cycles over a 3mm stroke length.
At 100 μm/s, the steady-state friction coefficients measured were μ=0.366, μ=0.241, and μ=0.015 for migrating, stationary, and Gemini contacts, respectively. The migrating contact friction coefficients decreased as sliding speed increased, characteristic of velocity-dependent lubrication. In contrast, the stationary probe friction coefficients continued to increase over the course of the experiment. In self-mated migrating contact, friction coefficients remained remarkably low μ<0.06 for all sliding speeds over all lengths of experiment.
This work demonstrates lubricity measurements of the same hydrogel in stationary, migrating, and Gemini contact over physiologically-relevant sliding speeds and pressures. The Gemini interface provided the best, most robust lubricity in contrast with higher-friction hydrogel/hard surface pairings; this robust lubricity is seen in the healthy eye. Hydrogels subject to migrating contact show a velocity-dependent lubricity, while hydrogels under constant stationary pressure show a time-dependent response which aligns with water squeeze-out.
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