Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia
ABSTRACT (1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia.
A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t.
(1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye.
(1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.
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ABSTRACT: A series of novel mono-ethylenically unsaturated polycarbosiloxanes macromonomers as compatibilizer materials for soft silicon hydrogels were prepared from the anionic ring-opening polymerization (AROP) of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane followed by methacrylation. The characterization was performed by end-group analysis and included the determination of molecular weight, molecular weight distributions, end-group functionality, and impurity profiles using gas chromatography-mass spectrometry, gel permeation chromatography, nuclear magnetic resonance, and matrix-assisted laser desorption/ionization time of flight mass spectrometry. The synthetic procedure was optimized to minimize the formation of any dimer that would have the potential to act as a low molecular weight cross linker. In addition, the novel di-ethylenically unsaturated polycarbosiloxanes were synthesized by cationic polymerization. Use of these silicone derivatives was explored in the formulation of contact lenses, and the structure–property relationship was examined. When copoymerized with hydrophilic monomers, these were able to give transparent and wettable films with desired properties, particularly a low moduli for contact lenses. Contact lens with high modulus is often shown to impart a higher degree discomfort when worn upon the eye. It was clear from the structure–property relationship that the modulus and the tensile strength of the formulated material depend on the nature and length of the polycarbosiloxane used and may be tuned for the purpose as needed. Copyright © 2013 John Wiley & Sons, Ltd.Polymers for Advanced Technologies 06/2013; 24(6). DOI:10.1002/pat.3115 · 1.96 Impact Factor
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ABSTRACT: Purpose: To define the theoretical oxygen tension behind modern scleral contact lenses (CLs) made of different rigid gas permeable (RGP) materials,assuming different thickness of the tear layer behind the lens. A second goal was to show clinically the effect of the post-lens tear film on corneal swelling. Methods: We simulated the partial pressure of oxygen across the cornea behind scleral contact lenses made of different lens materials (oxygen permeability Dk, 75 to 200 barrer) and different thickness (Tav, 100 to 300 microns). Post-lens tear film thicknesses (Tpost-tear) ranging from 150 to 350 microns were considered. Eight healthy subjects were randomly fitted with a scleral lens with a thin and a thick post-lens tear layer in two different sessions for a period of 3 hours under open-eye conditions. Results: CLs with less than 125 barrer of Dk and a thickness over 200 microns depleted the oxygen availabily at the lens-cornea interface below 55 mmHg for a post-lens tear film of 150 microns. For a post-lens tear film thickness of 350 microns, no combination of material or lens thickness will meet the criteria of 55 mmHg. Our clinical measures of corneal edema showed that this was significantly higher (p<0.001, Wilcoxon signed-ranks test) with the thicker compared to the thinner Tpost-tear (mean±standard deviation, 1.66±1.12 vs 4.27±1.19%). Conclusions: Scleral RGP CLs must be comprised of at at least 125 barrer of oxygen permeability and up to 200 microns thick in order to avoid hypoxic effects even under open eye conditions. Post-lens tear film layer should be below 150 microns in order to avoid clinically significant edema.Investigative Ophthalmology & Visual Science 09/2014; 55(10). DOI:10.1167/iovs.14-14038 · 3.66 Impact Factor
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ABSTRACT: Transport of ions through contact lenses is essential to maintain lens movement on the eye. We measure the effective diffusivity and partition coefficient of sodium chloride through silicone hydrogels by measuring kinetics of salt release and permeation in a diffusion cell. The results obtained from both approaches are compared and mechanisms related to linearity of transport and dominant transport mechanisms are explored. Also, transport parameters are measured from silicone hydrogels of several compositions to explore the dependency of ion transport on composition. Transport parameters are also correlated with the water fraction in the gel in the context of the free volume theory. Results show that the transport is linear in concentration and satisfies diffusive scaling suggesting that the transport of sodium and chloride ions can be lumped into an effective diffusion model. The permeation approach which is typically used for determining only the permeability can be utilized to yield both effective salt diffusivity and the partition coefficients if the short time transient data is fitted to a diffusion model. The values of diffusivity and partition coefficient obtained from both kinetic and permeation approach are comparable. The partition coefficient changes smoothly with variations in composition but there are discontinuities observed in diffusivity values likely due to changes in microstructure from dispersed to bicontinuous. The diffusivity cannot be related to the water content through the classical free volume theory because of the variations in the microstructure of the gels.Journal of Membrane Science 05/2012; 399-400:95-105. DOI:10.1016/j.memsci.2012.01.039 · 4.91 Impact Factor