Water Content and Dk in Silicone Hydrogels, Part 2
LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
In part 1 of this article (August 2006), I explored
the relationship between modulus and
water content in silicone hydrogel contact lenses.
The relationship is fairly linear as water content goes up, modulus goes down. Does the Dk/water relationship follow the same path? That
is, does theory dictate that higher-water silicone hydrogels must have lower Dk
I explored this issue with another expert in our field, Philip Morgan,
PhD, MCOptom, who is director of Eurolens Research at the University of Manchester
in the United Kingdom.
For hydrogel contact lens materials, there is an intrinsic relationship
between water content and oxygen permeability. Dr. Morgan together with Professor Nathan
Efron measured the oxygen permeability and water content of 17 hydrogel contact
lens brands and published the relationship as a simple exponential formula. The accurate
prediction given by this exponential formula also confirms that the hydrogels can be
considered to be part of one material family.
Figure 1. Dk vs. water content for silicone
What about Silicone Hydrogels?
Dr. Morgan explains: "Until the launch of the silicone hydrogen contact
lenses, soft lens manufacturers could increase oxygen performance only by adding various
components to soft contact lens materials to elevate equilibrium water content, which
results in a maximum oxygen transmissibility (Dk/t) in the low 30s short of
the values required for extended wear.
"We recently measured oxygen permeability values for five silicone
hydrogel materials (Figure 1). Whereas hydrogel materials exhibit a very tight relation-
between water content and permeability (green dots in Figure 1) this isn't the case for silicone
hydrogels. Although a general relationship exists between these two parameters (with
lower-water-content materials offering higher oxygen performance as these lenses have
higher concentrations of silicone moieties), it's not possible to accurately predict
oxygen permeability from water content values for silicone hydrogels."
Not All the Same
Dr. Morgan says, "the silicone hydrogen contact lens materials currently
on the market are sufficiently different from each other as to be considered from different
material families. Similarities exist between materials from the same manufacturer where
a close relationship exists between water content and permeability, but this doesn't
apply for all the lens brands considered together. For silicone hydrogel contact lenses,
clinicians will need to refer to independently published values or to manufacturer-supplied
data for oxygen permeability."
Dr. Szczotka-Flynn is an
associate professor at Case Western Reserve University Dept. of Ophthalmology
director of the Contact Lens Service at University Hospitals of Cleveland.
Contact Lens Spectrum, Issue: October 2006