A basic classification divides materials according to their primary bonding structure into ceramics (ionic bonding), metals (metallic bonding), and polymers (covalent bonding). Modern ophthalmic implants are almost all fabricated from synthetic polymers.
Polymeric materials are composed of long chain molecules (polymers) synthesised from repeat units (monomers) whose chemical character and reactivity determine many bulk properties. Most polymer chains have a covalently bonded backbone of carbon atoms joined to a variety of pendant groups. For siloxanes ("silicone"), an important group of synthetic biomaterials, this backbone consists of alternating atoms of silicone and oxygen. Molecular chains vary in length and are irregularly intertwined, although areas of regular arrangement (crystallinity) may exist. Cross linkage density and the density of secondary bonding further determine bulk properties for a given polymeric material.1
Biological conditioning after implantation
Secondary bonding mechanisms (for example, hydrogen bonds, van der Waals forces) are particularly relevant to biological systems, and are thought to have an important role in modulating protein conditioningthe process by which relatively inert polymeric material surfaces are rendered biologically active by contact with the tissues or body fluids.2
Protein conditioning is partly determined by surface reactivity, which varies between materials. Surface molecules tend to have more unoccupied bonding sites than molecules buried within a material, and are at a relatively higher energy state (Fig 1). Interfacial free energy for a material surface is a measure of the number of free bonding sites per unit area and their reactivity. Soluble proteins can often achieve a lower energy state by occupying these free sites, and synthetic materials are quickly coated after exposure to a biological environment.3
The pattern of protein adsorption varies between materials, and influences subsequent biological interactions.3 Soluble proteins compete for material surface bonding sites after implantation in a "race for the surface".4 Differential adsorption is determined by factors including implant surface chemistry, concentration in the fluid surround, and intrinsic surface reactivity for each protein constituent of the adsorbed film. Adsorption is sufficiently rapid that cells may never encounter an unconditioned material surface.5-7
Changes in tertiary structure (molecular folding) occur after adsorption. Proteins are probably partially denatured but retain modified biological activity.3 7