CONTACT LENSES - The first soft contacts were made in the 1960s of a hydrogel called polymacon or "Softlens." 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. 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. At present, both hydrogel and silicone hydrogel soft contacts are available.

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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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