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Kyle Schulze, Angela A Pitenis, Juan Manuel Uruena, Ryan Nixon, Alison C Dunn, Thomas Angelini, Wallace Gregory Sawyer; Speed-dependent lubricity of high water content hydrogels. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):6094.
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Contact lens material is one of the parameters that is often being modified during the design process in order to increase comfort for the end-user. As such, the inclusion of high water content hydrogels as surface gel layers in contact lenses is currently being incorporated into lens design. In this work, we discover two distinct lubricity regimes in several Gemini interfaces at a wide range of ocular sliding speeds. These regimes are described by their dominating mechanics: thermal fluctuation lubrication and polymer relaxation lubrication. These results further elucidate the mechanisms that affect lubricity in the eye and may aid in the continuing efforts of contact lens design.
For these studies, two hydrogels of different chemistries (n-isopropylacylamide and polyacrylamide) were each molded into a 2 mm radius of curvature probe and a flat 60 mm diameter countersurface for incorporation into a pin-on-disk microtribometer. The apparatus is then operated at discrete translational velocities between 30 µm s-1 and 100 mm s-1 and the normal and friction forces are recorded resulting in coefficient of friction data. Due to the Gemini interface of the hydrogels, which typifies the interfaces in the eye, results were significantly different from historical data found when using glass as a surface or countersurface.
Results showed that between 30 µm s-1 and 5 mm s-1 a speed-independent zone of low friction coefficient (µ=0.01) was found for both hydrogels. At speeds exceeding 5 mm s-1 the coefficient of friction increased at a rate of ½ power. Rheological and drag experiments were conducted to deduce the mechanisms affecting lubricity. We believe that this transition is indicative of when the polymer thermal fluctuations are able to relax local deformations faster than new strains can be exerted on the countersurface, hence denoted as polymer relaxation lubrication. As the material properties of high water content hydrogels are dependent on thermal fluctuations, the speed-independent regime is designated thermal fluctuation lubrication.
These findings allow for a greater understanding of all gel-like interfaces in the eye that are unlike classical lubrication theories currently employed and understood in industry. Further evaluation of these mechanisms can greatly support contact lens design.<br /> <br /> This work was funded by Alcon laboratories.
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