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Yuchen Huo, Alexander Rudy, Scott Perry, John Pruitt; Surface Elastic Modulus of a Unique Hydrogel Material Measured with Colloidal Probe AFM. Invest. Ophthalmol. Vis. Sci. 2013;54(15):495.
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This study reports the compressive elastic modulus of a unique water gradient contact lens material, formally known as delefilcon A, which consists of a low water content silicone hydrogel core material that transitions through a gradient to an ultra-soft, high water content surface gel layer via a proprietary chemical anchoring process. Through comparison with two other lens materials, balafilcon A and senofilcon A, the influential role of the surface gel layer is highlighted.
The lenses employed in this study were commercially fabricated and measured approximately 100 μm in thickness at the center. Prior to each experiment, the samples were soaked in saline (Unisol®4, Alcon, Fort Worth, TX) for 24 hours in order to remove the blister pack solutions. Elastic modulus was measured in saline by indenting a colloidal probe into the surface in a controlled manner (i.e. fixed approach speed and maximum applied force), such that the maximum indentation depth was restricted to the nanometer scale. All modulus values were obtained from the anterior surface of the sample. Calibrated cantilevers modified with 5-µm (diameter) silica colloidal probes provided access to quantitative modulus values measured for physiologically relevant contact pressures. A modulus value for each material was determined via a Hertzian analysis of force versus indentation behavior measured in the near surface region.
The lens materials examined exhibited large differences in compressive surface modulus. Balafilcon A, which receives a post production surface plasma oxidation treatment, exhibited the highest elastic modulus, 2000 kPa, followed by senofilcon A, 700 kPa, and the novel delefilcon A, 14 kPa. The large difference in surface mechanical response is correlated to the structure of the near surface region , with delefilcon A achieving the very low modulus through low polymer content, low crosslinking density, and water content greater than 80% by volume in this region.
The surface chemistry of silicone hydrogel contact lens materials is seen to influence the elastic modulus of the lens surface, as measured on the nanometer scale. Specifically, the incorporation of a high water content surface gel (delefilcon A) is seen to produce an exceptionally low modulus.
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