Determination of the oxygen transmissibility and permeability of hydrogel contact lenses

To test the validity of the method of stacked hydrogel contact lenses to obtain the oxygen permeability and transmissibility coefficients of the lenses, the coefficients of one low hydration (38% water) and two high hydration (55 and 58% water) hydrogel contact lenses stacked one to five on an oxygen electrode were determined. From the oxygen diffusion through the lenses, the current intensity in the stationary state was determined, and from this the “instrument” the oxygen transmissibility was obtained. The permeability coefficients of the lenses, corrected for edge effects, were obtained from the slope of the plot of the reciprocal of the transmissibility coefficients versus the lens thicknesses. The oxygen permeability and transmissibility coefficients of the lenses obtained neglected the boundary layers resistance between the stacked lenses and, therefore, these are not the “true” coefficients. This article compares the “apparent” oxygen permeability coefficients of the hydrogel contact lenses, obtained by others, with the “true” oxygen permeability coefficients obtained with a corrected equation that takes into account the boundary layers between the stacked lenses.