Three important factors determining the wear comfort of contact lenses are:
Since the recent advent of silicone hydrogels, contact lenses can now be fabricated from a soft material with extremely high oxygen permeability. Unfortunately, hydrogels are also inherently “hydrophobic”, meaning they have poor wettability characteristics that can cause discomfort during lens wear. To overcome this problem, the lenses are surface modified by plasma treatment, rendering them wettable to tear fluid and maximizing lens wear comfort.
The surface characteristics of silicone hydrogel are altered by plasma treatment to render it wettable, or hydrophilic. This can be achieved by plasma oxidation of the silicone hydrogel to form thin silicate islands across the surface that remain intact when the hydrogel is hydrated (causing it to swell by 10 – 20% by volume) and autoclaved. Another method involves first plasma activating the lens material, followed by a plasma induced polymerization process of an organic species, which is then oxidized on the surface of the contact lens by an additional plasma step. Since polymerization requires covalent bonding to the polymer backbone of the substrate, the plasma induced coating adheres so well it is not removed from the surface by hydration and autoclaving, or through normal wear and handling, such as digital rubbing.
Tear fluid carries lipids and proteins which can deposit on the surface of the lens. The accumulated deposition of these materials can result in biocompatibility issues. In addition to improving patient wear comfort, the hydrophilic plasma treatment of hydrogel contact lenses also reduces their affinity for lipids.
Fluorinated polymers, be they hydrogels or non-hydrogels, have been recognized as having very high oxygen permeability characteristics (denoted in the industry by their Dk value). As such they are particularly useful for extended continuous wear contact lenses as well as relatively thick lenses. Similar to traditional contact lens materials, fluoropolymers are inherently very hydrophobic. Plasma oxidation of these surfaces can result in weak boundary conditions that can result in delamination of the hydrophilized layer. Plasma solves this problem by inducing surface defluorination as a preliminary step to hydrophilization.
In multilayered hybrid lenses, plasma can be used to ablate the surface layer to expose an underlying layer. Here, the application of plasma has replaced difficult, time consuming and costly wet chemical methods.
