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W.J. Benjamin, M.D. Young; The Relationship Between Water Content and Oxygen Permeability for Conventional Hydrogel Materials . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3708.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: 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. Methods: Test contact lenses were equilibrated in saline of pH 7.37 and osmolarity 301.7 mOsm/kg. Calibrated, corrected polarographic Dk values were determined at eye temperature for 35 conventional hydrogel materials available as contact lenses, obtained commercially. 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 Law of Gladstone & Dale. Results: Water contents 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) = 0.945. R2 values of the corrected oxygen resistance (t/Dk) vs. 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 [10-11 (cm2/sec)(ml O2)/(ml x mmHg)] and were strongly correlated with water content in a semilogarithmic manner. The linear regression was Log(Dk) = 0.01752(GAWC%) + 0.3909, with R2 = 0.929 over the ranges of water content and Dk noted above. Conclusions: Water content determined by gravimetric adjustment of the known volumetric relationship was only slightly more than 2 % less, on average, than stated by contact lens manufacturers. This was likely due to the greater tonicity of the storage solution in this study and/or more pronounced blotting prior to measurement, yet the soft lenses and blotting method were the same as used for Dk assessment. Commercially available soft lenses were consistently made, the blotting method sufficient, and the polarographic Dk methodogy reliable, such that linear resistance (t/Dk) vs. 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. A logical next step would be to refine this relationship even further, by determination of water content at eye temperature.
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