Hydraulic permeability of a hydrogel‐based contact lens membrane for low flow rates

Hydrogels, widely used for contact lenses, are a class of materials that consists of both fluid and solid components. The aqueous fluid phase is supported by a solid polymer matrix. In this study, we characterize the water transport characteristics of an Etafilcon‐A (copolymer of 2‐hydroxyethyl methacrylate and methacrylic acid) membrane. A flow rate‐controlled permeameter consisting of a syringe pump, membrane holder, pressure transducer, and tubing was developed and used to measure the pressure drop across a flat membrane (average thickness ∼ 686 μm ± 40). The relation between velocity and pressure drop was measured. These data were fit to rigid media and biphasic models of 1‐D flow to determine hydraulic permeability, k = 1.80 × 10 −14 m 4 /N s. The results provide insight into the fluid flow properties of this hydrogel under low flow and low pressure conditions (<3 kPa). Physiological implications of these measured parameters on flow and deformation across the lens due to blinking are discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3730–3735, 2007