Permeability and partition coefficient of aqueous sodium chloride in soft contact lenses

Abstract

Transport of physiologic saline through soft contact lenses is important to on-eye behavior. Using a specially designed Stokes-diaphragm cell, we measure aqueous NaCl permeabilities through commercial soft contact lenses at 35°C. The permeabilities increase exponentially with the water content of the lenses spanning a range from 10-7 to 10-5 cm2/s. Equilibrium partition coefficients are obtained by the back-extraction of lenses initially immersed in 1M aqueous NaCl. Partition coefficients also increase with lens water content but over a smaller range, from 0.1 to 0.7. Because the partition coefficient values are smaller than the water content of the lenses, ideal theory is not followed. Donnan exclusion, bound water, and excluded volume are proposed explanations. The diffusion coefficients of aqueous NaCl through soft contact lenses increase with increasing lens water content following free-volume theory. Aqueous NaCl diffusivities in the lower water-content lenses are smaller than the diffusion coefficient of NaCl in water by factors up to 100 indicating very tortuous diffusion paths. © 2011 Wiley Periodicals, Inc.