Post-lens tear-film depletion due to evaporative dehydration of a soft contact lens

For a soft-contact-lens (SCL) wearer, corneal health and comfort are strongly influenced by water transport through the polymeric materials used in 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 depletion, which in turn, may lead to discomfort, dryness syndrome, and/or lens adhesion. We present a solution–diffusion model for transport of water through soft-contact-lens materials to mimic evaporative dehydration from a contact lens during blinking and to access possible SCL adhesion to the corneal surface under a variety of environmental conditions (e.g., wind speed and relative humidity). To describe the water-transport process, we use an extended version of the Maxwell–Stefan multicomponent diffusion equation for species that differ starkly in size (i.e., water and the polymer matrix). To describe thermodynamic properties of the soft-contact-lens/water mixture, we use a modified Flory–Rehner theory for polymer solutions. The proposed transport model is applied to two typical SCL materials: a low-water-content (38wt.%) polymacon SCL (SofLens® 38), and a high-water-content (70wt.%) hilafilcon A SCL (SofLens™ One Day). We calculate that a SCL on the eye loses water within a few minutes from lens insertion until it reaches a periodic steady state, with an average water content a few percent lower than the initial saturated water content. When the external relative humidity is low and the wind speed is high, the periodic-steady net flux of water from the post-lens tear film (PoLTF) through the contact lens toward the environment is comparable to the supply of water to the PoLTF from the eye anterior chamber. Thus, PoLTF depletion may occur at these conditions, leading to undesired, reduced SCL on-eye movement or, perhaps, to SCL adhesion on the ocular surface. Also, our calculations show that, at the most dehydrating conditions, the high-water-content hilafilcon A lens is more prone to dehydration and PoLTF depletion than is the low-water-content polymacon lens with the same thickness. However, at the least dehydrating conditions this trend is reversed. Relative humidity, wind speed, and lens thickness significantly influence SCL dehydration. [Copyright &y& Elsevier]