Polymers In Advanced Technology Applications In Health applications: Part 1

Implants and Medical Devices
Development of medical implants has been limited by many factors. The synthetic nondegradable materials needed in such products as orthopedic joints, heart valves, vascular prostheses, heart pacemakers, neurostimulators, and ophthalmic and cochlear implants must meet many technical requirements, including being stable and biocompatible in the host environment for moderate to long lifetimes.

However, the fact that most of the polymers currently used in implants were not initially designed for medical use means that those polymers may not meet such requirements. This carries with it inherent risks, such as those dramatically brought to light in the course of recent litigation concerning silicone breast implants. Also, toxic breakdown products have been reported for certain polyurethanes under consideration for an artificial heart pump design. Development of new techniques for screening and testing the biological response of 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.

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.

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.

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 dental adhesives. Further advances in dental materials can be expected as polymer systems are designed and engineered to satisfy the complex needs of the area.

source and credit: https://www.nap.edu/read/2307/chapter/4#40