Download Contact lenses H tier introduction

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 units
silicone 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 1