Ion transport in silicone hydrogel contact lenses

Transport of ions through contact lenses is essential to maintain lens movement on the eye. We measure the effective diffusivity and partition coefficient of sodium chloride through silicone hydrogels by measuring kinetics of salt release and permeation in a diffusion cell. The results obtained from both approaches are compared and mechanisms related to linearity of transport and dominant transport mechanisms are explored. Also, transport parameters are measured from silicone hydrogels of several compositions to explore the dependency of ion transport on composition. Transport parameters are also correlated with the water fraction in the gel in the context of the free volume theory. Results show that the transport is linear in concentration and satisfies diffusive scaling suggesting that the transport of sodium and chloride ions can be lumped into an effective diffusion model. The permeation approach which is typically used for determining only the permeability can be utilized to yield both effective salt diffusivity and the partition coefficients if the short time transient data is fitted to a diffusion model. The values of diffusivity and partition coefficient obtained from both kinetic and permeation approach are comparable. The partition coefficient changes smoothly with variations in composition but there are discontinuities observed in diffusivity values likely due to changes in microstructure from dispersed to bicontinuous. The diffusivity cannot be related to the water content through the classical free volume theory because of the variations in the microstructure of the gels.