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Diffusion limited evaporation rates in hydrogel contact lenses
1Centre for Medical and Health Physics, Queensland University of Technology, Brisbane, Australia.
Purpose:
To systematically measure evaporative water loss for a range of commercial hydrogel contact lens types, correlate the results with measured water self-diffusion coefficients and determine the best way to present and quantify the in vitro evaporation data.
Methods:
Evaporative water loss was measured gravimetrically for the four commercial contact lens types: Acuvue (powers: -0.50 D, -0.75 D), Newvues (powers: -0.50 D, -0.75 D, -1.00 D), CIBAsoft (powers: -0.50 D, -0.75 D, -1.00 D) and SeeQuence 2 (power: -0.75 D), with water contents of 60%, 55%, 38%, and 38%, respectively. Water proton self-diffusion coefficients were measured for these materials using standard nuclear magnetic resonance (NMR) pulsed field gradient techniques.
Results:
The initial rate of evaporation was linear for 5-10 minutes then decreased in all cases. No significant differences were observed in this initial rate between the lenses studied. However, the evaporative rate decreased more slowly with time for the higher water content lenses. The diffusion coefficients were found to be highly dependent on the equilibrium water content, regardless of material composition.
Conclusions:
The most effective way to present/quantify this type of evaporative data is to plot the time-course of the mass evaporation rate. We have found that the decrease in evaporative rate with time is a diffusion-limited effect whereby lenses with higher water content can maintain a higher rate of diffusion of water (to the evaporative surface) due to the presence of higher proportions of mobile water molecules. The NMR diffusion data corroborate these findings.