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Terin Dupre, William J Benjamin; Validity of Oxygen Permeability determined with a Coulometric Method. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1465.
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© 2017 Association for Research in Vision and Ophthalmology.
Samples of silicone-hydrogel (SiHy) contact lens materials are in most cases available only in finished lenses having refractive powers. We evaluated the ability of a coulometric method to achieve a linear relationship between oxygen resistance and sample thickness of powered contact lenses, and to thereby determine the oxygen permeability of SiHy materials using production contact lenses as test samples.
Assessment of oxygen permeability of hydrogel lens materials using a coulometric method requires placement of a reservoir of deionized water against the sample, resulting in a boundary layer. The determination of oxygen permeability (Dk) of a contact lens material therefore depends on a linear relationship between oxygen resistance (t/Dk) and thickness (t), from which the inverted slope of the line is Dk. We found the preliminary oxygen transmissibility (Dk/t) of 3 SiHy lens materials (Balafilcon A, Comfilcon A, and Lotrafilcon A) with a coulometric method using commercially available contact lenses in refractive powers of -3, -2, -1, +1, +2, +3, +4, and +6 D. Three samples of each power in each material were assessed, or 72 test lenses in all. Calibration was performed by evaluation of additional Lotrafilcon A lenses of -1 D power assuming a reference of 95 Fatt Dk units. Thickness (t) was the mean harmonic central thickness of a sample over the area of oxygen exposure derived from the center thickness measured with a low-force gauge, refractive index, base curve, and refractive power.
The coefficients of determination (R2) of linear relationships were derived from the 24 data points for each material. They were 0.835 (Balafilcon A), 0.650 (Lotrafilcon A), and 0.981 (Comfilcon A). The R2 values for curved plots were slightly greater, at 0.853, 0.658, and 0.983.
To correct for the boundary layer, acceptable linear R2 values are at least 0.960 for the coulometric and polarographic methods measuring uniform thicknesses, and for the polarographic method measuring powered samples. Here, a sufficient linear relationship was not achieved for 2 of the 3 SiHy materials when the coulometric method was used with powered samples. Use of the coulometric method was made questionable by the required sample equilibration in deionized water, which preserved the sensitivity of the detector that can be rendered inoperable by contact with saline, but that may also alter the permeability of hydrogel materials.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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