contact
lens materials
Modulus, Water Content and Dk in Silicone Hydrogels: Part
1
BY
LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
Silicone hydrogel lenses are entering a new
era with daily and flexible wear outcompeting 30-day continuous wear. The newest
wave of products promotes lower modulus and higher-water-content materials
as a way to address comfort, which may or may not affect the material's oxygen permeability.
In this first of a series of columns, I explore the relationship
between water content, modulus and Dk together with experts in this field.
In silicone hydrogel technology, traditional theory governs that
the higher-water-content materials will have a lower modulus, potentially enhancing
initial comfort, and higher-water materials will in turn translate to lower Dk values.
But is this theory set in stone? Can modulus and water content be dissociated in
silicone hydrogels?
In Part 1, I pose this question to Noel Brennan, PhD, of Brennan
Consultants in Melbourne, Australia, who has been researching contact lenses for
more than 20 years.
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Figure 1. Relationship between modulus and
water content of various materials.
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Exploring the Relationship
Dr. Brennan responds to my question:
"Polymers are, by name and nature, composed primarily of chained
molecules comprising long series of repeating units. There is an inherent flexibility
to long chains depending upon the nature of the primary units. The units may consist
of a simple monomer or of an almost endless combination of different monomers and
even combined polymers. The polymer chemist introduces crosslinkers, fillers and
other materials to pack the whole thing together in a desired fashion; these plus
the nature of the primary units impact both the water holding properties as well
as the stiffness of the material.
"Within
the context of the subunits and crosslinking agents, one can fiddle around with
these parameters in a quasi-independent manner," Dr. Brennan continues. "So, even
with relatively unsophisticated polymers such as familiar hydrogel materials used
for soft contact lenses, we see considerable variability in the modulus/water content
relationship. For example, etafilcon A has a low modulus compared to other materials
of similar water content. The degree of sophistication in silicone hydrogels is
much greater and potentially allows even greater flexibility in this regard.
"To summarize, modulus is loosely but not absolutely related to
water content," Dr. Brennan concludes.
How Much Oxygen?
In silicone hydrogels, the relationship between water content
and modulus is fairly linear as traditional theory dictates. Figure 1 shows this
relationship.
The real question is: How much oxygen reaches the cornea amidst
these various material property combinations? We'll explore that question in future
columns.
Dr. Szczotka-Flynn is an
associate professor at Case Western Reserve University Dept. of Ophthalmology and
is director of the Contact Lens Service at University Hospitals of Cleveland.
Contact Lens Spectrum, Issue: August 2006