candidate materials is clearly a priority matter. By and large, however, empirical testing has found material implants to be remarkably successful.

Each implant application calls for a specific set of properties. A key property required in implants exposed to blood is nonthrombogenicity; that is, the material must not cause clotting. Polymers being considered for vascular prostheses include poly(ethylene terephthalate) fibers, expanded polytetrafluoroethylene foams, segmented porous polyurethanes, and microporous silicone rubber. Surface treatments include hydrophilic coatings, seeded endothelial cells, immobilized heparin (an anticoagulent), and a garlic extract. The diversity of candidates is impressive. Although none of these materials are completely satisfactory, good blood flow has been maintained for many years in some cases.

Polymers also play a major role in devices used to oxygenate blood. They must operate without blood damage. Silicone rubber and polypropylene have been used successfully in both solid and microporous forms. These materials, in microporous form, are widely used during cardiopulmonary bypass surgery, where blood exposure is relatively short term. For long-term exposure, solid membranes are used. Again, surface treatments, such as immobilized albumin, are providing promising results.

Synthetic, biomimetic phospholipid membranes are under development as coatings that render surfaces compatible with blood. Inclusion of the phosphorylcholine headgroup is thought to be a most promising approach, and it has been employed on poly(vinyl chloride), polyethylene, polypropylene, and other polymers. The phosphorylcholine group can also be added as a plasticizer in polyurethanes and other polymers.

Artificial kidney machines employ polymeric hollow fibers to purify blood by hemodialysis. Cellophane (regenerated cellulose) was introduced early on, and Cuprophan, a form of regenerated cellulose that has been strengthened by cuprammonium solution treatment, remains the material of choice, although many other polymers have been tried. Many factors are involved, including treatment of the dialyzer for reuse and avoidance of removal of desirable factors from the blood. It seems likely that synthetic polymers will eventually come into use, although to date they do not have the proper combination of properties.

Dental materials are dominated by polymers to an increasing extent. Impression materials are made of silicone and polysulfide elastomers that cure rapidly in the mouth and maintain their shape. Denture bases are made from polymers based on poly(methyl methacrylate) (PMMA) that are cross-linked through a free radical process. Fillings that match the teeth in appearance are composed of highly filled difunctional methacrylates that are cured by exposure to blue light. Silane-coated ceramic fillers provide the visual match and the hardness and durability required. The use of photocuring relieves the dentist of the need to work within the limited time allowed by amalgam fillings. The composite has been engineered to minimize contraction during cure, an extremely important aspect of any filling material. Polymers also play a central role in



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