Influence of water states in hydrogels on the transmissibility and permeability of oxygen in contact lens materials

The properties of water in soft contact lenses such as the water content, free-to-bound water ratio were key determinants of their oxygen transmissibility characteristics. Differential scanning calorimetry (DSC) was used for measuring the different states of water in hydrogel materials. The convenient apparatus was devised for testing the oxygen permeability coefficient of hydrogel membranes. DSC data showed that the content of freezing water in hydrogel increased with growth of N-vinyl pyrrolidone (NVP) percentage, and the non-freezing water in hydrogel increased with growth of 2-hydroxyethylmethacrylate (HEMA) percentages. It was found that the transmissibility and permeability of oxygen in hydrogel could be described according to the solution–diffusion model. The systematic variations in chemical structure and volume fraction of water in polymer bring about significant changes in oxygen permeability coefficients. The permeability of the material was affected by the freezing water in hydrogel rather than by the non-freezing water.