Hydrogel (or ‘soft’) lenses have had a huge impact on the contact lens market since they were first introduced in the late 1960s. In the UK, a recent survey has indicated that they currently make up 92% of all new contact lens fittings (Morgan & Efron 2002). This trend is also echoed around the world (Morgan et al. 2002). These lenses are termed ‘soft’ since they are made from water-swollen, cross-linked, hydrophilic polymers. 

Polymers are solid materials made from high-molecularweight chains, and consist of repeating units known as monomers. Materials made from one monomer are termed homopolymers and materials made from more than one type of monomer are termed copolymers. Their long chain length, the kinds of atoms they are made up of (eg carbon, oxygen, hydrogen, nitrogen), their geometric arrangement and the stability of the bonds created all combine to give polymers unique and distinctive properties which in turn dictate the material’s particular use and function.