H
Contact lenses
Key Assessment Task information sheet
1 About the eye
To be able to see, light must enter your eye through
your pupil. This means that the eye covering on top of your pupil must be transparent. The covering is called the cornea. It is made of cells which like all living cells need oxygen. Most of your cells get oxygen from your blood. But you can’t see through blood, so your cornea gets oxygen from the oxygen dissolved in your tears.
2 Wearing contact lenses
Soft contact lenses are made of polymers. They cover the cornea so that the only way the cornea can get oxygen is through the contact lens. So a contact lens must be permeable to oxygen (that is, it must allow oxygen to pass through it). This property of the material used to make the contact lens is called its oxygen permeability. It is not the only property to consider when choosing a contact lens material, but it is a very important one.
In a hydrogel polymer (a soft, water-containing plastic), the plastic itself is not oxygen permeable, so the water carries oxygen through the lens to the eye.
Silicone hydrogel polymers contain both water and silicone. Silicone allows oxygen to pass through, so these polymers have a greater oxygen permeability than the hydrogel polymers.
3 Quality testing and results
A laboratory tested lenses to compare the polymers used by different manufacturers. They used lenses that all had the same shape and thickness.
The oxygen permeability was measured. (It was measured in ‘OP units’.) Each material was tested five times. Here are the results.
material / oxygen permeability /OP unitssilicone hydrogel 1 / 98 / 93 / 113 / 106 / 100
silicone hydrogel 2 / 105 / 150 / 106 / 100 / 109
silicone hydrogel 3 / 138 / 142 / 128 / 137 / 130
hydrogel A / 7 / 8 / 10 / 8 / 7
hydrogel B / 21 / 21 / 22 / 21 / 20
© University of York and Nuffield Foundation 2009 C21 IaS1 KAT Contact lenses info sheet page 2