Investigating the permeation properties of contact lenses and its influence on tear electrolyte composition

The health of the cornea is paramount; the aim of this study was to assess the permeation of essential tear electrolytes through a range of commercial contact lenses. Donor/receiver conductivity measurements were recorded using a dual‐chamber diffusion system which allowed material permeability profiles and coefficients to be calculated. Water structuring properties of the contact lenses were measured by differential scanning calorimetry. Freezing water was subdivided into “ice‐like” water (free, non‐bound and has a melting point close to that of pure water) and polymer‐associated water (free but loosely bound to the polymer matrix). Each material interacts differently with each of the three salts, for example; lotrafilcon B (34% equilibrium water content [EWC]) shows a higher and larger range of receiver concentrations post KCl, NaCl, CaCl2 permeation (76, 59 and 42 mM, respectively) compared with the lower and tighter range exhibited by lotrafilcon A (22% EWC) (36, 22, and 18 mM, respectively). Additionally, in terms of the relationship between permeation and water structure, balafilcon A (34% EWC) has a high KCl permeation (P60 258 × 10−8 cm2/s) and ice‐like water (14%), but narafilcon A (44% EWC) has a low ion permeation (P60 3.9 × 10−8 cm2/s) and significantly less ice‐like water (4%). The permeation trends for the silicone hydrogel materials could not be fully explained by water content and structuring. Composition and, in particular, the microstructure and morphology of these materials must impart a greater influence on permeation capability.