Steady-state diffusion of water through soft-contact-lens materials

Abstract

Water transport through soft contact lenses (SCL) is important for acceptable performance on the human eye. Chemical-potential gradient-driven diffusion rates of water through soft-contact-lens materials are measured with an evaporation-cell technique. Water is evaporated from the bottom surface of a lens membrane by impinging air at controlled flow rate and humidity. The resulting weight loss of a water reservoir covering the top surface of the contact-lens material is recorded as a function of time.New results are reported for a conventional hydrogel material (SofLens TM One Day, hilafilcon A, water content at saturation w 10 = 70 weight %) and a silicone hydrogel material (PureVision TM , balafilcon A, w 10 = 36 %), with and without surface oxygen plasma treatment. Also, previously reported data for a conventional HEMA-SCL (w 10 = 38 %) hydrogel are reexamined and compared with those for SofLens TM One Day and PureVision TM hydrogels. Measured steady-state water fluxes are largest for SofLens TM One Day, followed by PureVision TM and HEMA. In some cases, the measured steadystate water fluxes increase with rising relative air humidity. This increase, due to an apparent mass-transfer resistance at the surface (trapping skinning), is associated with formation of a glassy skin at the air/membrane interface when the relative humidity is below 55-75%.Steady-state water-fluxes are interpreted through an extended Maxwell-Stefan diffusion model for a mixture of species starkly different in size. Thermodynamic nonideality is considered through Flory-Rehner polymer-solution theory. Shrinking/swelling is self-consistently modeled by conservation of the total polymer mass. Fitted MaxwellStefan diffusivities increase significantly with water concentration in the contact lens. Water Diffusion through SCLs

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