Effect of proteins on water and transport properties of various hydrogel contact lens materials.

The effect of adsorbed substances on the properties of the water in various hydrogel contact lens materials was examined by exposing contact lenses (Hydron Zero 4, B&L 70, DuraSoft 3, Vistamarc, and Acuvue) to an artificial tear solution for various periods up to 14 days. The only materials affected were the high-water/ionic lenses which adsorbed a large amount of protein, predominantly lysozyme. In the DuraSoft 3 lenses the equilibrium water content (EWC) dropped from 49% to 46% and the freezing water from 28% to 21%. Similar changes were seen with the Vistamarc lenses. After a 10-day exposure of the Acuvue lens to artificial tears, the EWC decreased from 53% to 47% and the amount of freezing water from 33% to 23%. The decrease in the permeability of water seen with these materials was consistent with the decrease of the freezing water, i.e., the water able to participate in diffusion. Since the content of freezing water determines the transport through hydrogels it can be expected that any lens characteristics that depend upon the amount of this portion of water would be affected by the presence of proteins inside the polymer matrix. We extrapolated that an absolute change of 10% in the amount of freezing water could lead to a decrease in oxygen permeability of as much as 7 Dk units. In view of this work more attention should be given to changes in the properties of lenses during wear, in particular, in the high-water/ionic lenses.