Abstract
The importance of the microstzructure of silicone hydrogels is widely appreciated but is poorly understood and minimally investigated. To ensure comfort and eye health, these materials must simultaneously exhibit both high oxygen and high water permeability. In contrast with most conventional hydrogels, the water content and water structuring within silicone hydrogels cannot be solely used to predict permeability. The materials achieve these opposing requirements based on a composite of nanoscale domains of oxygen-permeable (silicone) and water-permeable hydrophilic components. This study correlated characteristic ion permeation coefficients of a selection of commercially available silicone hydrogel contact lenses with their morphological structure and chemical composition. Differential scanning calorimetry measured the water structuring properties through subdivision of the freezing water component into polymer-associated water (loosely bound to the polymer matrix) and ice-like water (unimpeded with a melting point close to that of pure water). Small-angle x-ray scattering, and environmental scanning electron microscopy techniques were used to investigate the structural morphology of the materials over a range of length scales. Significant, and previously unrecognized, differences in morphology between individual materials at nanometer length scales were determined; this will aid the design and performance of the next generation of ocular biomaterials, capable of maintaining ocular homeostasis.
Highlights
The design of a contact lens material should not seek merely to create a bio-inert temporary implant, but preferably it should be capable of maintaining the existing homeostasis of the avascular corneal bed in equilibrium with its external environment
According to Domschke et al, ion permeability within contact lenses is a critical parameter for lens movement on the eye[4] and it is thought to be a requirement for post-lens tear turnover and metabolic waste removal.[12]
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
Summary
The design of a contact lens material should not seek merely to create a bio-inert temporary implant, but preferably it should be capable of maintaining the existing homeostasis of the avascular corneal bed in equilibrium with its external environment. The oxygen permeability of commercial SiHys is known to be high.[8] As contact lens technology has evolved, recognition of the desirability of maintaining a pre- and post-lens tear film that mimics the natural tear film (which has a distinct electrolyte composition) has become clear.[9,10,11,12] The permeation of tear electrolytes is, 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 thought to be a requirement for post-lens tear turnover and metabolic waste removal.[12] 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.[7] In addition, studies involving more detailed clinical-based analysis have shown that different materials can affect physiochemical properties which in turn can influence the nature and composition of FIGURE 1 Monomer chemical structures for the four silicone hydrogels. 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
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