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A. C. Dunn, D. J. Dickrell, III, H. A. Ketelson, W. G. Sawyer; Friction Measurements on Hydrogel Contact Lenses: Experiments and Results. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3429.
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Friction of contact lenses is a key issue in modern optical health. The aim of this study is to provide complementary methods of friction and indentation measurements between a hydrogel contact lens and a smooth surface.
A custom micro-tribometer with temperature-controlled wet cell was developed for testing of soft contact lens hydrogels. Normal force loads range 0.01-20mN, and sliding speeds range 10-1000 µm/s within the contact. The contact is provided by a specialized holder pin which holds a punched contact lens portion. Forces are measured by displacement conversion of a mechanical flexure, as measured by a capacitive sensor.Four commercial contact lenses (A,B,C,D) were cycled 6 times from full dehydration to saline solution equilibrium hydration. Control samples were prepared by immersion (>6 hours) in a saline solution with no hydration cycling. The friction tests and indentation tests were performed in saline solution at 34±0.5°C.Friction results are interpreted by measuring the friction forces in both the forward and reverse sliding directions, and then dividing that by the applied normal force (measured simultaneously).Results/
Friction coefficients ranged µ=0.333-0.927. Overall, hydrogel contact lenses which were hydration cycled showed higher values of average friction coefficient as compared to the control. This increase in sliding friction was thought to be caused by a combination of adhesive forces and material responses of the specific hydrogel formulations. This is related to material properties as obtained by micro-indentation experiments on the same lens material.
Friction and indentation experiments were run on 4 hydrogel contact lenses that had been cycled between extreme hydration states, as well as a control. It was found that average friction coefficient is higher on the lenses which had been cycled, as due to their altered adhesive and deformation responses. This scales with properties determined by micro-indentation.
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