Dehydration kinetics of soft contact lenses: The hidden impact of early wear.

In contact lens (CL) wear, maintaining adequate CL hydration is essential to prevent ocular dryness and associated discomfort. In this context, dehydration of worn Etafilcon A and Kalifilcon A CLs hydrated by natural tears was monitored using an in-vitro gravimetric method. A kinetic model was applied to derive the water kinetics coefficient, which emerged as a reliable indicator of the on-eye hydration state and water diffusion mechanisms. A significant reduction in water content and a tendency from a polymer-hindered water diffusion toward free diffusion were observed after just five minutes of wear, suggesting a rapid initial adaptation to the ocular environment that alters the material resistance to water evaporation. After two hours of wear, a similar effect was evident in approximately half of the samples investigated, indicating that a new hydration equilibrium may eventually be reached over time.