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W. G. Sawyer, D. J. Dickrell, III, N. V. Dolgova, M. Sarntinoranont, B. G. Keselowsky, S. S. Perry, H. A. Ketelson, D. L. Meadows; Soft, Wet, and Slippery: The Tribology of Hydrogels. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6362.
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To develop an integrated process to non-destructively evaluate the tribology of contact lenses.
Micro-indentation of hydrogel lenses was performed and analyzed using biphasic finite element methods to the contact problem, (e.g. an uncompressed modulus of 50-60kPa and a hydraulic permeability of 2x10-15 m4/(Ns) was found for Etafilcon-A). A continuum mechanics model of viscoelastic contact including adhesion was also derived and fit to the micro- and nano-indentation data. This new capability in contact mechanics was used to design and predict contact areas and pressures for ultra-low pressure micro-tribological friction measurements. These tribology experiments were designed to elucidate the mechanisms of friction in hydrogels. They were conducted at sliding speeds up to 1mm/s and contact pressures below 1kPa; this required robust control of forces below 1mN of applied load. These experiments were conducted on a wide variety of commercially available hydrogels, contact geometries, temperatures, and solutions (all experiments were submerged). The experimental protocol involved soaking the hydrogels for 24hrs in Unisol prior to evaluation. The lenses cleaned and evaluated in a solution of Unisol showed exceptionally high friction coefficient, µ=1.0, and adhesion - demonstrating that the origin of lubricity in hydrogel materials is not simply due to the water content. By varying both load and speed, the contributions to viscous deformation, fluid shear, and surface shear stress were determined. The dominant contributor was found to be the shear stress.
Lenses cleaned in Unisol and evaluated in solutions with 0.1% concentration of hydroxypropyl guar and 1% boric acid (PH 7.8) showed friction coefficients that were 50x lower (µ~0.02). The effects of solution composition, concentration, and temperature will be discussed. Finally, this experimental approach has been extended to enable the evaluation of hydrogels in solution against mature human corneal epithelial cell surfaces.
The initial findings reveal that low friction coefficients (µ=0.03) are common for corneal epithelial cells against hydrogels in a serum environment. Perhaps more intriguing is the finding that the critical contact pressure for these cells is on the order of 1KPa, for experiments run at pressures above 1KPa damage (death) of over 20% of the contacted cells (as measured using a Trypan blue staining) was common.
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