Aqueous salt transport through soft contact lenses: An osmotic-withdrawal mechanism for prevention of adherence

In addition to improving oxygen permeability, modern silicone-hydrogel (SiHy) soft contact lenses (SCLs) exceed a limiting diffusive ion permeability to aqueous sodium chloride. Below the ion-permeability threshold, siloxane-based SCLs are prone to bind against the corneal epithelium. Salt permeability is argued to reflect indirectly water hydraulic permeability. However, no quantitative explanation is available to date for a threshold salt permeability. We hypothesize that molecular salt diffusion through a SCL supports the postlens tear film (PoLTF) by enhancing water flow into the PoLTF from the cornea. Higher salt concentrations in the PoLTF raise the osmotic pressure there relative to that in the cornea increasing osmotic water withdrawal from the cornea. The proposed osmotic-withdrawal mechanism successfully predicts a self-consistent threshold lens salt permeability when thin-film attractive binding forces are introduced. For the first time, we present a quantitative picture for the possible origin of a threshold salt permeability in SCL manufacture.