| For soft-contact-lens wearers, corneal health and comfort are highly affected by water, ions, and oxygen transport through the polymeric materials used for contact-lens fabrication. In particular, evaporative water loss at the anterior lens surface is a potential cause of contact-lens dehydration and of post-lens tear-film (PoLTF) depletion, which, in turn, may lead to discomfort, reduced movement of the lens on the eye, lens binding to the cornea, and dry-eye syndrome.; The large variety of recipes for contact-lens materials, the difficult control of in-vivo conditions, the interplay of numerous environmental and geometrical factors, and intersubject variability greatly complicate in-vivo experimental study of SCL dehydration, precluding a full quantitative understanding of the process and the definition of unambiguous SCL design criteria.; In this work, we develop a solution-diffusion model for transport of water through soft-contact-lens (SCL) materials to study evaporative dehydration of a lens on the eye. The transport model elucidates the mechanism of in-vivo contact-lens dehydration; determines the amount of lens dehydration and lens shrinkage during wear; accesses PoLTF depletion and consequent reduced on-eye movement of the lens and SCL adhesion to the cornea surface. Finally, it predicts possible formation of a rigid, possibly uncomfortable, glassy skin on the anterior surface of the lens under highly dehydrating conditions.; For this purpose, we develop an extended version of the Maxwell-Stefan multicomponent diffusion equation for species that differ starkly in size (EMS). Equilibrium and transport parameters underpinning the model have been obtained experimentally. We studied three model soft-contact-lens materials: a low-water-content polymacon SCL, a high-water-content hilafilcon-A SCL, and a silicone hydrogel (balafilcon A) SCL.; We obtain also the SCL glass-transition temperature as a function of water content by modulated differential scanning calorimetry. These data are used in our calculations to predict the formation of a glassy skin on the SCL anterior surface induced by dehydration during wear. (Abstract shortened by UMI.)... |
