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.
probably due in part to the influence of silicone in polymers, both
historically in silicone acrylate GP materials and now in silicone
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.
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
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.
problem in sorting this out is complicated as wettability is
measured primarily using the in-vitro captive bubble (for hydrogels)
and sessile drop techniques.
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.
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
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.
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
Continued Study Ahead
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