contact lens materials
How Lens Wettability Affects the Ocular Surface
The influence of material wettability and the measure of wettability has been considered through the years. Today, it seems to have come to the forefront of understanding material properties.
This is probably due in part to the influence of silicone in polymers, both historically in silicone acrylate GP materials and now in silicone hydrogel materials.
Although we intuitively believe that material wettability is associated with on-eye clinical performance, there seems to be little understanding of the impact of lens wettability on the ocular surface.
For our purposes, it's important to consider the formation of dry spots on a contact lens surface as the final result of a series of factors contributing to tear film breakup. In this regard, we should also give consideration to contact lens surfaces relative to impacting patient comfort and success. Factors contributing to tear film breakup might include evaporation, dewetting or adsorption of the film.
Evaporation is the vaporization of a liquid to a gaseous state, whereas dewetting is the process of the rupture of a film resulting in liquid droplets but no net fluid loss. Wettability is just the opposite - it's the process of a liquid film, the pre-lens tear film, spreading over a contact lens surface.
Adsorption could be a contributing factor to tear film breakup, but most would agree that it likely plays little role in the process.
A Complex Process
Understanding the process that leads to tear film breakup over a contact lens is a complex one. Historically, contact lens-related dry eye is classified as evaporative, but again, it's hard to ignore the potential role of wettability in this process.
The problem in sorting this out is complicated as wettability is measured primarily using the in-vitro captive bubble (for hydrogels) and sessile drop techniques.
Ex-vivo studies of wettability suggest that wearing contact lenses alters their wettability (compared to an unworn contact lens), but these studies provide no evidence that wettability contributes to the process of tear film breakup.
To the contrary, imaging of the pre-lens tear film suggests little evidence of actual dewetting, as within about 10 seconds to 15 seconds following the blink, very little film is left. Again, the dewetting process itself, without evaporative forces, is associated with beading of the tear film with no net fluid loss. This isn't what we observe.
If rapid pre-lens tear film breakup occurs, this again may lead to significant and repeated lens surface exposure. This draws attention to other related factors, which include lubricity and friction during wear or tribology, which is the combination of the three.
Lubrication occurs when two adjacent surfaces are separated by a film, which ultimately reduces the friction between the surfaces when they are moving.
The corneal surface isn't smooth as it contains numerous microvilli and microplicae, which might aid in maintaining tear film stability or act as friction reducers themselves. In this regard, it seems as though there would be benefit in a soft lens surface, in addition to lack of surface exposure as described through aforementioned processes.
Continued Study Ahead
As materials and lens care solutions improve, it will be interesting to study their effects on these processes. It seems that our ultimate hope is for the development of lens materials that mimic the corneal surface.