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Scott S. Perry, Clay Argenbright, Yuchen Huo, Howard A. Ketelson; Surface Segregation of Chemical Moieties in Silicone Hydrogels. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6115.
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This objective of this study was to determine the extent to which compositional variation occurs within the near-surface region of silicone hydrogels as a function of chemical environment. As few analytical techniques are available for in-vivo lens characterization, it is necessary to understand the influence of measurement environment on surface properties, as similar effects may impact the in-eye performance of these hydrogel lenses.
X-ray photoelectron spectroscopy (XPS), a vacuum-based surface analytical technique was used to measure the chemical composition of the near-surface region (~10 nm) of three silicone hydrogel (SH) contact lenses ( DAILIES® TOTAL1®, ACUVUE® TruEye®, and PureVision®). Lenses were introduced into a vacuum load lock in a fully hydrated state and then cryogenically cooled to 100K under a 1 atm nitrogen environment. The frozen lens was then transferred to an UHV environment while maintaining the lens temperature below 140K so as to avoid the complete desorption of the covering ice layer. The lens was then annealed to specific temperatures to sequentially remove water from the interface. The core level spectra of the lens elemental constituents were recorded as a function of temperature and used to calculate relative compositions as a function of lens dehydration.
The temperature-dependent surface compositions, correlated with lens dehydration, depict the mobility of chemical moieties within the hydrogel structure and the migration of species to the interface as dehydration occurs. For example, a 5 atomic % increase in N was observed upon dehydration of the DALIES® TOTAL1® lens. For the entire series, it is noted that only single digit changes are observed over the entire range of temperatures. Variations in the extent of compositional changes are consistent with known surface treatments of the different lens types.
XPS represents a powerful probe of silicone hydrogel surface composition. While performed in a vacuum environment, temperature-programmed dehydration of the lens surface demonstrates that only minor changes in composition are observed in moving from a fully hydrated to fully dehydrated state.
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