Permeability and diffusivity for water transport through hydrogel membranes

Water transport through soft contact lenses is important to their behavior on the human eye. For the first time, we report gradient-driven diffusion rates of water through HEMA hydrogel membranes prototypical of commercial soft contact lenses. Water is evaporated from one surface of the membrane into known relative humidity air, and the weight loss of a water reservoir at the other membrane surface is followed in time. Typical water permeabilities are around 1–5 cm/s, increasing with the saturated water content of the membrane. Binary Fickian diffusion coefficients are near 10−7 cm2/s. We also garner binary Stefan–Maxwell diffusion coefficients based on Flory–Rehner theory for polymer solutions. Stefan–Maxwell diffusion coefficients are useful because, in general, they depend less strongly on concentration, and they can be readily incorporated into multicomponent transport though membranes. Both the binary Fickian and Stefan–Maxwell diffusion coefficients increase somewhat with the initial water content of the individual lens materials.