The most commonly used synthetic polymers in corneal implants are reviewed from the viewpoint of their properties and tissue tolerance. Special attention is given to the properties of these materials in regard to their application as artificial corneal epithelium and endothelium. It is postulated that an ideal artificial corneal membrane must exhibit, as closely as possible, the physical properties and the functional characteristics of the natural membrane. Of all the known artificial materials, homogeneous hydrogels seem to offer the best possibilities as membranes for corneal surgery. Three types of hydrogels were investigated. Poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(glycery1 methacrylate) (PGMA) are covalently linked tridimensional nonionic networks.
The third type of hydrogel has multiple ionic crosslinks and belongs to the family of the polyelectrolyte complex hydrogels, made by reacting poly(vinylbenzy1trimethylammonium chloride) with sodium poly(st,yrenesulfonate). The acrylic hydrogels are very well tolerated by living tissues, but the polyelectrolyte complexes gave inconsistent results. Membranes of either type were made having permeability to water similar to that of the corneal layers. Most hydrogels are friable and do not hold sutures; this difficulty was solved by attaching the hydrogel membrane to the cornea by means of alkyl cyanoacrylate adhesives. n-Heptyl and n-butyl 2-cyanoacrylate gave very good results in this application.