Role of the water matric potential (ΨM) and of equilibrium water content (EWC) on the water self-diffusion coefficient and on the oxygen permeability in hydrogel contact lenses

This paper illustrates a new thermodynamic and kinetic model that describes the relationship between the water self-diffusion coefficient, Dw/g*, in hydrogel contact lenses, in terms of water matric potential (ΨM) and equilibrium water content (EWC). Experimental measurements on commercial contact lenses yield water thermodynamic activity values ranging between 0.996 and 0.999. The corresponding ΨM matric potential is, respectively, between −8 and −2 J/mol at temperature 35°C. Comparison between water self-diffusion coefficients derived in this paper and those suggested by other authors shows that our values are greater (25%–50%) than the previous ones. The impact of this model on the nature of the oxygen permeability, Π, in the lenses has been evaluated and the changes of Π with ΨM and EWC are predicted and compared with direct experimental measurements. For the contact lenses investigated, the oxygen permeability turns out to be only a quadratic function of equilibrium water content, despite the fact that the fraction of the “free” water molecules can be as high as 50%.