Chemical hydrogels are commonly prepared in two different
ways: ‘three-dimensional polymerization’ (Fig. 1), in
which a hydrophilic monomer is polymerized in the presence
of a polyfunctional cross-linking agent, or by direct
cross-linking of water-soluble polymers (Fig. 2). Polymerization
is usually initiated by free-radical generating compounds
such as benzoyl peroxide, 2,2-azo-isobutyronitrile
(AIBN), and ammonium peroxodisulphate or by using
UV-, gamma- or electron beam-radiation. However,
three-dimensional polymerization often results in materials
containing significant levels of residual monomers
and therefore purification of these materials has to be performed
thoroughly because the unreacted monomers are
often toxic and could leach out from the hydrogels continuously.
The purification of hydrogels containing residual
monomers is typically performed by extraction into excess
water, and can take up to several weeks to be completed