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W. G. Sawyer, Alison C. Dunn, Juan M. Uruena, Howard A. Ketelson; Robust Contact Lens Lubricity using Surface Gels. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6095.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the effects of a soft surface gel on the lubricity of contact lenses. This study examines the friction response of the gel layer under varied pressures and correlates the frictional behavior and frictional responses to the mechanical properties of the gel layer.
Gradient gel surfaces (2-3 um thick) were formed on the surfaces of a new type of silicone hydrogel daily disposable contact lens (DAILIES TOTAL1®). These layers were minimally crosslinked and enveloped the entire lens. Friction measurements were carried out on a precise micro-tribometer constructed specifically to probe low contact pressure sliding in aqueous environments. This apparatus can reliably perform experiments at contact pressures approaching 1 kPa, and can go up to pressures in excess of those seen on the human eye during ocular movements and blinking. Experimental soft contact lenses (20) were mounted into a custom conformal holder, placed in a bath of borate-buffered saline (Unisol®), and heated to physiological temperatures. Lubricity experiments were performed at a range of contact pressures (1-10kPa) and sliding speeds (10um/s - 600um/s).
This gel layer was designed to enhance lubricity and comfort of the contact lenses by acting as a compliant foundation to spread contact, reduce pressure, and promote boundary lubrication. Contact mechanics experiments and indentation reveals that the gels have a low elastic modulus (<200KPa), and at contact pressures below 18 kPa the friction coefficients are mu=0.03. The friction coefficient is robust to pressure changes under the 18 KPa pressure threshold and does not show any effect of sliding speed over the range examined.
Tribological probing of the surface gels indicates that they are compliant but mechanically stable to pressures below 18 KPa. At pressures such as those seen in the eye (3-5 kPa) the gel layers can support smooth sliding and provide a lubricous surface with friction coefficients of mu=0.03 in the boundary lubrication regime. These findings indicate that surface gels can be a route to promote lubricity in contact lenses.
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