Polarographic evaluation of highly oxygen permeable silicone rubber: Correction for electrode nonlinearity and implications for contact lenses

Abstract

Accurate values for the apparent oxygen permeability of “superpermeable” silicone rubber were obtained using a polaro-graphic oxygen sensor in conjunction with measuring gases of low oxygen partial pressure. This method reduces errors that result from the nonlinear response of the polarographic cell to high oxygen flux and may call into question all papers measuring highly transmissible samples by the polarographic technique. Data collected with gases of 2–10% oxygen were extrapolated to 0% oxygen in order to correct for electrode nonlinearity. To confirm the accuracy of these apparent permeabilities, we performed thickness analyses of the estimates to eliminate the effect of liquid (boundary) layer resistance. When data for materials of greater than 0. 054 cm thickness were used in the calculation, the final extrapolated value tended to exceed the literature oxygen permeability (Dk) of 600 barrers (one barrer is equivalent to 10 −11 mlO 2(STP) cm/second cm 2 mmHg). However, it was found that such thick sample data were subject to high edge and geometrical effects. When the thick sample data were excluded, Dk values of 561 and 618 barrers were obtained, depending on the thickness analysis used; these values closely match gas/gas results (although with sizable precision errors) and thus independently corroborate our technique. This extension of the usefulness of the polarographic oxygen sensor to superpermeable silicone rubber may also apply to other superpermeable polymers and contact lenses.