"Complaints of lens dryness and irritation are among the most common voiced by contact
lens wearers...
The water retention capacity of H2O lenses promises to be benefitial to
both patients and practitioners"
Soft contact lenses are made of water and polymer. Typically, the water makes up
40 to 60% of the weight of a soft lens. An acrylic polymer makes up the rest. Without
water, the acrylic polymer is a hard and brittle material. After absorbing water
the polymer becomes very soft and flexible. The softness and flexibility of a soft
contact lens is the result of the water absorbed and held by the lens polymer. When
soft lenses begin to "dry out" (lose some of their absorbed water) they become less
soft and less flexible.
Soft lens materials are made of polymers that consist of very long chains of repeating
units. When polymers are formed, individual units are connected together end to
end to form polymer chains 2,000 to 10,000 units long. Each long polymer chain is
also connected to two or more other polymer chains. In a soft lens polymer some
or all of the units of the chains have a part that is attracted to water. This is
the hydrophilic part or water loving part of the polymer structure. The rest of
the polymer structure is not attracted to water. When a soft lens absorbs water
and becomes hydrated, the hydrophilic portion of each unit of the polymer chain
becomes attached or bound to water (H2O). When water is absorbed into the polymer
and becomes attached to the hydrophilic parts of the polymer, the polymer becomes
soft and flexible.
There are several different types of hydrophilic parts used in soft lens polymers.
Each type is able to bind a specific number of water molecules (H2O). The capacity
of hydrophilic units to bind a specific number of water molecules is what defines
a polymer material's water content. The graphic below shows a polymer network as
a simplified grid of polymer chains. When the polymer absorbs water, the grid expands,
the polymer becomes soft and flexible. When the polymer loses water, the grid contracts
or shrinks and the polymer becomes less soft and less flexible.