Oxygen Permeability and Water Content of Silicone Hydrogel Contact Lens Materials

Purpose: To measure the oxygen permeability (Dk) and water content (WC) of silicone hydrogel (Si-Hy) contact lens materials.

Methods: Randomized and masked determinations of the Dk of 5 Si-Hy and two hydrogel materials were made using a modified version of the polarographic measurement method described in ISO 9913-1. Stacks of one to six parallel-sided contact lenses (all -1.00 DS) were evaluated, with each stack measured at least twice. The resulting value for t/Dk was plotted against the thickness (t) of each stack, with Dk calculated as the inverse of the gradient of this relationship. This methodology corrects for boundary effects. A mathematical calculation was used to correct for edge effects. Gravimetric determination of lens WC was conducted at room temperature and 35 degrees C.

Results: Measured values (+/-95% CI) of Dk, and WC at room temperature, with manufacturer-claimed values in parentheses, were Focus Night and Day: Dk 162.0 +/- 9.8 (140), WC 23.0 +/- 3.2 (24); Acuvue Oasys: Dk 107.4 +/- 7.4 (103), WC 36.9 +/- 1.0 (38); O2 Optix: Dk 80.5 +/- 4.9 (110), WC 32.1 +/- 1.2 (33); PureVision: Dk 75.9 +/- 6.6 (91), WC 35.8 +/- 1.3 (36); Acuvue Advance: Dk 75.2 +/- 9.8 (60), WC 46.5 +/- 1.1 (47); 1. Day Acuvue: Dk 21.0 +/- 1.0 (21.4), WC [not measured] (58); and Seequence: Dk 8.2 +/- 0.7 (8.5), WC 36.6 +/- 2.7 (38).

Conclusions: Claimed Dk values for Acuvue Oasys and the two reference hydrogel materials fell within the 95% confidence interval of our measured values. Our measurements of Dk for the other four Si-Hy lenses were not in agreement with claimed values. There is a general inverse relation between Dk and WC (both at 35 degrees C) for Si-Hy lenses. Our modified polarographic methodology can be successfully employed for measuring the Dk of Si-Hy materials.