Calibrated oxygen permeability of 35 conventional hydrogel materials and correlation with water content

The relationship between oxygen permeability (Dk) and water content of conventional hydrogel materials was reviewed using upgraded methods to measure both parameters in a masked fashion. This is the first report of conventional hydrogel Dk values that were calibrated using a set of reference materials and determined in the general format of ANSI Z80.20:1998 and ISO 9913- 1:1996.

Calibrated, corrected polarographic Dk values were determined at 35 degrees C for 35 conventional hydrogel materials available as contact lenses, obtained commercially. The refractive index was measured with an Abbe-type refractometer at room temperature and converted to water content by gravimetric adjustment of a volumetric equation derived from the Gladstone-Dale relationship.

Percentages of the water content of the materials determined by gravimetric adjustment (GAWC%) ranged from 33.8% to 70.7% and were strongly correlated with the gravimetric water contents stated by their manufacturers (SWC%). The linear regression was GAWC% = 1.004(SWC%)-2.403, with coefficient of determination (R2) = 0.929. R2 values of the corrected oxygen resistance (t/Dk) versus thickness (t) plots for the 35 materials were 0.96 or greater. Calibrated, corrected Dk values ranged from 8.4 to 44.8 Dk units and were strongly correlated with water content in a semilogarithmic manner. The linear regression was Log(Dk) = 0.01754 (GAWC%) + 0.3897, with R2 = 0.945 over the ranges of water content and Dk noted above.

Water content determined by gravimetric adjustment of the known volumetric relationship was slightly more than 2% less, on average, than that stated by contact lens manufacturers. This was likely the result of the greater tonicity of the storage solution in this study or more pronounced blotting prior to measurement, or both; yet the soft lenses and blotting method were the same as used for Dk assessment. Commercially available soft lenses were consistently manufactured, the blotting method was sufficient, and the polarographic Dk methodology was reliable such that linear resistance (t/Dk) versus thickness (t) plots resulted from lenses of different lots made of material having the same name. The semilogarithmic relationship of Dk and water content survived correction for boundary layers, correction for the edge effect, and calibration. Logically, the next step would be to refine this relationship even further by determining water content at the same temperature as oxygen permeability, 35 degrees C.