Understanding the surface chemical and mechanical properties of hydrogel materials for contact lens applications

The surface chemical properties of commercially available silicone lenses were analyzed with respect to monitoring changes following adsorbed block copolymer molecules of ethylene oxide-co-butylene oxide (EO-BO). X-ray photoelectron spectroscopic (XPS) data confirmed the presence of physisorbed EO-BO on all lens surfaces following solution treatment, and the extent of adsorption greatly differed between the lenses. Friction measurements with atomic force microscope (AFM) employing a colloidal probe in aqueous environment corroborated the adsorption of EO-BO by demonstrating the reduction of the friction coefficient following EO-BO treatment. In a separate study, based on the bulk elution of EO-BO from the lenses and the complementary XPS adsorption data, an EO-BO molecular concentration gradient was evidenced for each lens type. These results suggest that the overall interaction between EO-BO and a lens material depends upon both the surface and bulk composition, and the tribological behavior of the polymer surface can be altered by chemical modifications.;The surface elastic moduli of three different silicone hydrogel lenses were examined with colloidal probe AFM. Fitting of the force-indentation plots of the lenses to a Hertzian contact model revealed the disparity in both the magnitude of modulus and the mechanism of deformation as a result of the different surface chemical treatments. Particularly, the "graded" properties arising from the structure of delefilcon A's surface gel exemplifies the potential of molecular-level design to achieve depth-dependent, tunable surface mechanical properties.;Furthermore, to gain a more fundamental perspective, hydrogels of various chemistry and water content were fabricated into films and analyzed using AFM. For a given gel composition, the magnitude of surface modulus varied as much as one order of magnitude for a water content difference of 15%. The dependence of modulus on water content is slightly lower at the surface than in the bulk as predicted by scaling laws. For poly(N-isopropylacrylamide) (PNIPAM), one order of magnitude of increase in surface elastic modulus was observed when solvated chains collapsed following a characteristic phase transition. These findings underscored the importance of the molecular structure and dynamic interaction with surrounding medium in predicting elastic modulus of hydrogel materials at surface.