Diffusivity of water through a HEMA-based soft contact lens

Using an evaporation-cell method, molecular diffusion data were obtained for water through a thin membrane of HEMA (2-hydroxyethyl methacrylate) that is a soft contact lens material. Water evaporates from the bottom surface of the membrane that supports a small water reservoir on the top surface. The so-induced chemical potential gradient causes a water flux across the membrane. Measured steady-state water fluxes are interpreted with an extended Maxwell-Stefan model for mixtures whose molecules differ appreciably in size. Flory-Rehner theory accounts for thermodynamic non-ideality of the hydrogel membrane/water mixture. Measured diffusivities are a strong increasing function of water concentration in the membrane.