Soft contact lenses are made of hydroxyethyl methacrylate (HEMA), which was developed by Wichterle and Lim. HEMA contains a polar OH group that allows water to bind. Contact lenses can have water contents ranging from 38-80%, with lower water content lenses being more stable and easier to handle, and higher water content lenses having greater oxygen permeability but being less stable. Lenses can also be ionic or non-ionic, with ionic lenses being more deposit-prone but better wettable, and non-ionic lenses being less deposit-prone but less wettable. The FDA classifies soft contact lenses into four groups based on water content and ionicity. A lens's Dk, or

1. Soft Contact Lens Material
By Babli Sharma
B.Optom, M.Optom

2. Hydroxyethyl-methacrylate
 PHEMA
 O Wichterle and D Lim
 A polar OH group to which water lipole binds
 38% water content
 Are soft and pliable
 Large lens diameters
 New generation-soft material-improved PHEMA
combination of other polymers 2-3

3. Physical Properties
Water content
Ranges from 38%-80%
>40% are low water content
Dk range 5-8
Low oxygen
permeability
Low water
content
lenses are
stable,
Easier to
handle
More
wettable
40-55% are
mid water
content
Dk range 7-
19
>55% are high water content
Dk range 18-
28
Higher the
water content
greater the
oxygen
permeability
Less stable
Fragile
More deposit
prone
Less
dimension
stability
Thicker due to
low refractive
index

4. Ionicity
Ionicity
Ionic
Net Negative
charge
More prone to
deposits
Better wettability
Nonionic
No net surface
charge
Less deposit
prone
Less wettable

5. FDA Classification Groups
 In 1986 US food and drug administration classified hydrogel
contact lenses into 4 groups
• Non ionic
Low water content 38-50%
Group -1
• Non-ionic
High water content 51-80%
Group-2
• Ionic
Low water content 38-50%
Group-3
• Ionic
High water content 51-80%
Group-4

6. Soft materials and Dk-depends
on
 Water content if higher increases Dk
 Material chemistry
 If pH is acidic it reduces water content
 Hyper tonicity reduces water content

7. Thank you