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

Abstract

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, CaCl 2 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 (P 60 258 × 10 −8 cm 2 /s) and ice-like water (14%), but narafilcon A (44% EWC) has a low ion permeation (P 60 3.9 × 10 −8 cm 2 /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.

The influence of structure and morphology on ion permeation in commercial silicone hydrogel contact lenses

“…The properties and components of four commercial SiHys namely narafilcon A, comfilcon A, lotrafilcon B, and balafilcon A which were selected based on their water content and ion permeation properties 13 are shown in Table 2 porosity" (black vs. white) of the materials was determined. In principle, the lower the mean atomic number of a sample under a beam, the fewer electrons are generated/detected and the resultant pixel for that region will consequently be dark.…”

“…In this study, four commercial SiHy materials were selected. They enable the study of SiHys with similar water contents but very different ion permeation properties as previously shown by Mann et al 13 Two of the chosen materials have relatively high quoted EWCs: narafilcon A (44%) and comfilcon A (48%), and two have lower water contents: lotrafilcon B and balafilcon A (both 34%). To investigate factors that may influence the disparate ion permeability properties of each material, the structural morphology at different length scales of each of these four materials was investigated by environmental scanning electron microscopy (ESEM) and small angle x-ray scattering (SAXS) together with specific water structuring properties of each lens material.…”

“…Ion permeability in conventional hydrogels is known to be governed simply by water content and water structure, the picture for SiHys is more complex, a point underlined by the huge differences in ion permeabilities found with SiHys of similar water content. 13 In seeking an explanation for the unusual ion transport behavior of SiHys, it is logical to explore the way that differences in the self-assembly characteristics of siloxy moieties leads to complex morphologies in SiHys. These are not found in their conventional hydrogel counterparts and may well reveal unexplored characteristics of this group of materials.…”

“…[9][10][11][12] The permeation of tear electrolytes is, however, much more variable and at the heart of this study, as reported previously ion permeation does not show a uniformly predictable dependence either on equilibrium water content (EWC) alone or the water structuring within the material. 13 According to Domschke et al, ion permeability within contact lenses is a critical parameter for lens movement on the eye 4 and it is also thought to be a requirement for post-lens tear turnover and metabolic waste removal. 12 Similarly, the electrolyte composition of tears, which is quite distinct from that of serum, can be adversely affected by the presence of a barrier, which disturbs ion transport creating a post-lens tear film.…”

“…[14][15][16] Differences in the occurrence of corneal infiltrative events have been identified between commercial SiHys 17 ; materials that have similar high oxygen permeabilities but very different ion permeabilities. 13,18 The influence of polymer phase morphology, which can take many forms, on lens movement on the eye has been highlighted. 19 Structure-property-performance relationships are important.…”