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Alexander Rudy, Yuchen Huo, Scott S. Perry, Howard A. Ketelson; Surface Mechanical and Tribological Properties of Silicone Hydrogels Measured by Atomic Force Microscopy. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6114.
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© ARVO (1962-2015); The Authors (2016-present)
The surfaces of four types of silicone hydrogel (SH) contact lenses(PureVision®, ACUVUE® Oasys®, ACUVUE® TruEye®, DAILIES® TOTAL1®) and the pHEMA-based ACUVUE® 2 were analyzed using atomic force microscopy (AFM) in aqueous environment. The elastic modulus, frictional, and adhesive properties of each lens were evaluated using calibrated instrumentations, providing a basis for comparing the distinctive surface properties of these lenses at the top 1 μm of the surface.
All samples were prepared by soaking in buffered saline (Unisol® 4) for 24 hours to exchange out the blister pack solutions. Cantilevers modified with 5-μm (diameter) silica colloidal probes were employed in the following experiments. Elastic modulus was measured by indenting the probe into the surface of the hydrogel in a controlled manner (i.e. approach speed and maximum applied force), such that the maximum indentation depth was restricted to sub-micron levels. A modulus value was obtained by fitting the characteristic force versus indentation behavior to a mathematical model. The frictional force was measured for the sliding contact of the probe and the surface at the length scale of 500 nm and with applied loads up 20 nN. The friction coefficient was realized by evaluating the linear dependence of friction force on applied normal load.
The lenses examined exhibited an order of magnitude difference_from the softest to the stiffest sample_in modulus value, generally reflective of the distinct surface treatments they received during manufacturing. For example, the pHEMA-based ACUVUE® 2 was shown to have a modulus between 100 and 130 kPa, whereas PureVision®’s was an order of magnitude higher in value. The frictional properties followed a similar trend with plasma surface treated lenses, such as PureVision®, exhibiting coefficients of friction five times those of a non plasma treated lens, such as DAILIES® TOTAL1®.
The elastic modulus and frictional properties of different lenses have been evaluated on a nanoscopic level by AFM, depicting a strong correlation between the surface treatments and the apparent mechanical behaviors of the lenses.
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