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Contact
Lenses
What Are Contact
Lenses Made Of?
Contact Lens Chemical Composition
By Anne Marie Helmenstine, Ph.D.
Millions
of people wear contact lenses to correct their vision, enhance their
appearance, and protect injured eyes.
The
success of contacts is related to their relatively low cost, comfort,
effectiveness, and safety.
While
old contact lenses were made of glass, modern lenses are made of
high-tech polymers.
Take a
look at the chemical composition of contacts and how it's changed over time.
Key Takeaways: Contact Lens Chemistry
· The
first contact lenses were hard contact made of glass.
· Modern
soft contact lenses are made of hydrogel and silicon hydrogel polymers.
· Hard
contacts are made of polymethyl methacrylate (PMMA) or Plexiglas.
· Soft
contacts are mass-produced, but hard contact lenses are made to fit the wearer.
Composition of Soft Contact Lenses
The
first soft contacts were made in the 1960s of a hydrogel called polymacon or
"Softlens."
This is
a polymer made of 2-hydroxyethylmethacrylate (HEMA) cross-linked to ethylene
glycol dimethacrylate.
The
early soft lenses were about 38% water, but modern hydrogel lenses may be up to
70% water.
Since
water is used to allow oxygen permeation,
these lenses increase gas exchange by getting larger.
Hydrogel
lenses are highly flexible and easily wetted.
Silicone hydrogels
came on the market in 1998. These polymer gels allow for higher oxygen
permeability than can be obtained from water, so the water content of the
contact isn't particularly important.
This
means smaller, less-bulky lenses can be made. The development of these lenses
led to the first good extended wear lenses, which could be worn overnight
safely.
However,
there are two disadvantages of silicone hydrogels.
Silicone
gels are stiffer than the Softlens contacts and are hydrophobic, a characteristic that makes
it difficult to wet them and reduces their comfort.
Three
processes are used to make silicone hydrogel contacts more comfortable. A
plasma coating can be applied to make the surface more hydrophilic or
"water-loving".
A
second technique incorporates rewetting agents in the polymer. Another method
lengthens the polymer chains so they are not as tightly cross-linked and can
absorb water better or else uses special side chains (e.g., fluorine-doped side
chains, which also increase gas permeability).
At
present, both hydrogel and silicone hydrogel soft contacts are available.
As the
composition of lenses has been refined, so has the nature of contact lens
solutions.
Multipurpose
solutions help wet lenses, disinfect them, and prevent protein deposit
build-up.
Hard Contact Lenses
Hard
contacts have been around for about 120 years.
Originally,
hard contacts were made of glass. They were thick and uncomfortable and
never gained widespread appeal.
The
first popular hard lenses were made of the polymer polymethyl methacrylate,
which is also known as PMMA, Plexiglas, or Perspex.
PMMA is
hydrophobic, which helps these lenses repel proteins. These rigid lenses don't
use water or silicone to allow for breathability.
Instead, fluorine is added to the polymer, which
forms microscopic pores in the material to make a rigid gas permeable lens.
Another option is to add methyl methacrylate (MMA) with TRIS to increase the
permeability to the lens.
Although
rigid lenses tend to be less comfortable than soft lenses, they can correct a
wider range of vision problems and they are not as chemically reactive, so they
can be worn in some environments where a soft lens would present a health risk.
Hybrid Contact Lenses
Hybrid
contact lenses combine the specialized vision correction of a rigid lens with
the comfort of a soft lens. A hybrid lens has a hard center surrounded by a
ring of soft lens material.
These
newer lenses can be used to correct astigmatism and corneal irregularities,
offering an option besides hard lenses.
How Contact Lenses Are Made
Hard
contacts tend to be made to fit an individual, while soft lenses are
mass-produced. There are three methods used to make contacts:
1.Spin Casting - Liquid silicone is spun on a revolving mold, where
it polymerizes.
2.Molding - Liquid polymer is injected onto a rotating mold.
Centripetal force shapes the lens as the plastic polymerizes. Molded contacts
are moist from start to finish. Most soft contacts are made using this method.
3.Diamond Turning (Lathe Cutting) - An industrial diamond cuts a disk of polymer to shape
the lens, which is polished using an abrasive. Both soft and hard lenses can be
shaped using this method. Soft lenses are hydrated after the cutting and
polishing process.
A Look to the Future
Contact
lens research focuses on ways to improve the lenses and solutions used with
them to reduce the incidence of microbial contamination.
While
increased oxygenation offered by silicone hydrogels deters infection, the
structure of the lenses actually makes it easier for bacteria to colonize the
lenses.
Whether
a contact lens is being worn or being stored also affects how likely it is to
be contaminated.
Adding silver to lens case material is one way
to reduce contamination. Research also looks at incorporating antimicrobial
agents into the lenses.
Bionic
lenses, telescopic lenses, and contacts intended to administer drugs are all
being researched.
Initially,
these contact lenses may be based on the same materials as current lenses, but
it is likely new polymers are on the horizon.
Contact Lens Fun Facts
· Contact
lens prescriptions are for particular brands of contacts because the lenses
aren't quite the same. Contacts from different brands aren't the same thickness
or water content.
Some
people do better wearing thicker, high water content lenses, while others
prefer thinner, less hydrated contacts.
The
specific manufacturing process and materials also affect how quickly protein
deposits form, which is more of a consideration for some patients than others.
· Leonardo da Vinci proposed the idea of
contact lenses in 1508.
· Blown
glass contacts made in the 1800s were shaped using cadaver eyes and rabbit eyes
as molds.
· Although
they had been designed some years earlier, the first plastic hard contacts were
commercially available in 1979. Modern hard contacts are based on the same
designs.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
Education
Ph.D., Biomedical Sciences, University of
Tennessee at Knoxville
B.A., Physics and Mathematics, Hastings
College
Introduction
Ph.D. in biomedical sciences from the University
of Tennessee at Knoxville - Oak Ridge National Laboratory.
Science educator with experience
teaching chemistry, biology, astronomy, and physics at the high school,
college, and graduate levels.
ThoughtCo and About Education chemistry
expert since 2001.
Widely-published graphic artist,
responsible for printable periodic tables and other illustrations used in
science.
Experience
Anne Helmenstine, Ph.D. has covered
chemistry for ThoughtCo and About Education since 2001, and other sciences
since 2013. She taught chemistry, biology, astronomy, and physics at the high
school, college, and graduate levels. She has worked as a research
scientist and also abstracting and indexing diverse scientific literature for
the Department of Energy.
In addition to her work as a science
writer, Dr. Helmenstine currently serves as a scientific consultant,
specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr. Helmenstine holds a Ph.D. in
biomedical sciences from the University of Tennessee at Knoxville and a
B.A. in physics and mathematics with a minor in chemistry from Hastings
College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive
chemical detection and medical diagnostic tests.
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