The Effect of Water Hydraulic Permeability on the Settling of a Soft Contact Lens on the Eye

Abstract

Purpose: Silicone-elastomer soft contact lenses (SCLs) adhere to the cornea during wear, whereas silicone-hydrogel soft contact lenses exhibit adequate on-eye movement. One explanation for the observed immunity to binding of silicone-hydrogel lenses is that some interstitial water is expelled during blinking, therefore maintaining a more stable post-lens tear film (PoLTF). We examine quantitatively whether or not water can be squeezed by hydrodynamic flow through a silicone-hydrogel membrane driven by the applied lid force during a blink. Methods: A rigid, porous-disk model of a contact lens was devised to calculate the relative settling rates of a permeable versus a completely impermeable SCL. The settling rate depended strongly on the value of the hydraulic permeability for pressure-driven water flow through the lens. Because the hydraulic permeability of water through silicone-hydrogel materials is not well-known, we measured this value. At steady state, water was forced through flat membranes of representative lens materials under known pressure drops. The resulting volumetric flows were measured by following the transient rise height of water in a vertical, precision-bore glass capillary. Darcy's law permitted calculation of the hydrodynamic permeability. Results: The settling-rate model indicated that tear can be squeezed through a SCL only when the Darcy-law hydrodynamic permeability is greater than about 10 μ m² (i.e., greater than 10 Darcy). Our measurements for silicone and HEMA hydrogel membranes reveal hydrodynamic permeabilities of the order 10⁻⁸ μ m², almost 9 orders of magnitude smaller than that necessary to initiate hydrodynamic flow through a SCL. Conclusions: We conclude that the squeeze-through mechanism cannot quantitatively account for the observed on-eye movement of silicone-hydrogel lenses. Also, we find that the lid-applied pressure cannot squeeze enough water out of a SCL during a blink to stabilize the PoLTF. Neither a squeeze-through nor a squeeze-out mechanism can maintain a stable PoLTF and prevent adherence.

KEYWORDS:

• Darcy hydrodynamic permeability
• lens hydrodynamic squeeze-through and squeeze-out mechanisms
• rigid porous-disk settling model
• soft-contact-lens binding