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Donald Riederer, Charles Scales, Bernardo Santa Maria, Michael Ferran, Zohra Fadli; New Methods for Measuring Water Transport through Hydrogel Contact Lenses. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3071.
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© ARVO (1962-2015); The Authors (2016-present)
The rate of water permeation through a contact lens is important for understanding exchange of the post lens tear film. Evaporation from the surface is one of the routes by which water is removed from the ocular environment. Both of these parameters influence ocular hydration and lens comfort. New approaches are described for measuring permeation and evaporation rates across a series of traditional and silicone hydrogel lens materials. These new methods were also used to measure the influence of adsorbed tear film components on the evaporation rate of water from the lens surface.
The rate of water permeation was measured via two approaches that used saline prepared with isotopically labeled water (n=3 per material). The first used a traditional diffusion cell where water permeation was determined by nuclear magnetic resonance spectroscopy (NMR). The second used a custom cell designed for use with a Fourier transform infrared (FTIR) spectrometer. Evaporation rates (n=3 per material) were determined gravimetrically using a dynamic vapor sorption instrument that provided complete control over the temperature and humidity of the environment. The influence of adsorbed protein and lipid on evaporation was explored by exposing lenses to a tear like fluid (TLF) in the laboratory prior to analysis and by ex-vivo analysis of lenses worn for 30 days.
Both approaches (NMR and FTIR) to measuring permeation rates produced similar values. Permeation rates were found to be material dependent and fell within the range 5 to 50 µL/min/cm2. Evaporation rates were found to be significantly lower (0.5 to 1.5 µL/min/cm2). For some lens types, exposure to protein and lipid caused the evaporation rate to increase by > 30% (p=0.001). For lenses exposed to TLF in the laboratory, the increase in evaporation rate was reversible upon rinsing. For ex-vivo lenses worn for 30 days, the increase in the evaporation rate was irreversible.
Evaporation is the rate limiting step to loss of water from the lens and is material dependent. For some lens materials, adsorption of tear film components can significantly increase the rate of water loss from the lens by evaporation.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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