Ever since I found out I was a little blind I absolutely hated my glasses. All I asked for for Christmas was contact lenses, and in December of 2014 I got what I had wished for: a pair of silicone hydrogel contact lenses. People with blurred vision are diagnosed with astigmatism, which is the condition of having an irregularly shaped cornea, causing near or far-sighted vision. Having near-sighted vision, I have trouble focusing on objects located anywhere more than 2 feet away from me.
I played a lot of sports growing up, including lacrosse. If you are familiar with the sport you would know that players are required to wear eye goggles, or if you wore glasses you would need special prescription plastic goggles. I refused to be that kid so my parents eventually let me get contacts.
The past five years I've worn contacts everyday, making them a huge part of my life. Therefore, I chose the most widely-used contacts material to study the history and mechanics of.
Origination of Contact Lenses
Over 350 years before contacts were developed, the famous mathematician and inventor Leonardo DaVinci was believed to be the first to produce sketches of the idea to directly put water in contact with the cornea to alter the optics of the human eye. In 1508 DaVinci wrote 'Codex of the Eye' and proposed the possible idea to submerge one's whole head in a glass bowl of water to correct vision defects.
There is some controversy on who created contact lenses first but the first known glass contact lenses were created in 1887. These larger glass contacts were heavy and covered the entire front surface of the eye. They could only be worn for a few hours at a time due to the large reduction in oxygen supply to the cornea.
It wasn't until 1948 that plastic was used to create the lenses and then not until 1959 that soft hydrogel contact lenses were made. In 1999 silicone hydrogel lenses were made and approved by the FDA. Now more than 90% of contact lenses prescribed in the United States are made from silicone hydrogel.
History:
1636: French scientists and philosopher René Descartes invented the first actual contact lens by placing a tube of water in direct contact with the cornea. This is the reason they are called "contact lenses."
1801: British scientist Thomas Young then furthered this idea by using wax to actually glue these water-filled tubes to his eyes.
1887: Glassblower Adolf Gaston Eugen Fick created the first glass contact lens and used a grape-sugar concoction to prevent friction between the eye and glass lens.
1999: Ciba Vision introduces silicone hydrogel contact lenses that feature much higher oxygen permeability.
How They Work
Just like eyeglasses, contact lenses alter the direction of light rays to properly focus light on the eye's retina. Contacts are much smaller and thinner than eyeglasses because the lenses rests directly on the cornea requiring the optic zone to only have to be the same diameter as the pupil.
Think of it as looking out a small window in your house: if you are very close to the window you have a very large unobstructed image of the outside, but if you are far away from the window, your view is very limited, unless your window is very large.
What Are Contacts Made of?
The introduction of silicone hydrogel contact lenses was huge in the optics industry. Of all citizens prescribed contact lenses (CL's) in the United States in 2017, 64% of those were prescribed with silicone hydrogel lenses, making these the most widely used contact lens type.
Silicone hydrogel lenses are polymers and are made from a combination of silicone rubber and hydrogel, deeming CL's a moldable plastic material. The silicone provides high oxygen permeability while the hydrogel facilitates fluid transport.
Material Properties of CL's
Oxygen permeability: This expresses the ability of the lens to let oxygen reach the eye by diffusion. The earliest models of CL's ranged from 6-8 Dk/t while silicone hydrogel CL's today ranged from 110-128.
Friction Coefficient(CoF): A lower CoF gives a smoother more comfortable feeling on the eye. CL's are designed to mimic the interaction between the eye and eyelid.
Elastic Modulus: Also known as "stiffness" is the ratio of stress over strain. This property is relevant because too low of a modulus would cause CL tearing and damage to the eye while too high would cause irritation.
My Understanding of Silicone Hydrogel
My knowledge of this material improved throughout this semester especially after learning more about the mechanical properties of silicone hydrogel. I briefly discussed elasticity modulus being the measure of stiffness of the metal but I not know how to measure and calculate this value through deformations and measuring the stress and strain. I learned about many other properties such as toughness which interestingly can be calculated by finding the area under a stress-strain curve. This property is made up of other properties including strength and ductility that all play a role in characterizing the material of silicone hydrogel. Chapter 6 contained a lot of information about properties and measurement tests such as the tensile test.
We mostly focused on properties and behavior of metallic materials but not as much about plastic materials. Plastics is a huge topic but I would be interested in learning more about how it is manufactured, measured and altered for different purposes. I also wish we had learned more about how materials like silicone hydrogel is applied and used in production.
References
- nih.gov
- science101.com
- jnjvisioncare.co.uk
- allaboutvision.com
- Haunty "Leonardo da Vinci's Camera Obscura"
- Steve Brachmann "The Evolution of Contact Lenses: From Da Vinci to Electronic Lenses"
Credits:
Created with an image by Jakub Dziubak - "untitled image"