Polymeric implants poly polymers

Implanted polymeric materials can also adsorb and absorb from the body various chemicals that could also effect the properties of the polymer. Lipids (triglycerides, fatty acids, cholesterol, etc.) could act as plasticizers for some polymers and change their physical properties. Lipid absorption has been suggested to increase the degradation of silicone rubbers in heart valves (13). but this does not appear to be a factor in nonvascular Implants. Poly(dimethylsiloxane) shows very little tensile strength loss after 17 months of implantation (16). Adsorbed proteins, or other materials, can modify the interactions of the body with the polymer this effect has been observed with various plasma proteins and with heparin in connection with blood compatibility. 

Poly (e-caprolactone), poly lactides, and polyglycolides have quite unusual properties of biodegradability and biocompatibility. The majority of polymers used in the biomedical field to develop implants, sutures, and controlled drug-delivery systems are the aforesaid resorbable polyesters produced by ring-opening polymerization of cyclic (di)esters. 

Figure 2.3 IgG levels after administration of drug delivery systems in rats. Controlled-delivery systems for antibody class IgG. The insert figures show the release of antibody from the delivery system during incubation in buffered saline. The panel (a) inset shows release from poly(lactic acid) microspheres these spherical particles were 10-100/rm in diameter. The panel (b) inset shows release from a poly[ethylene-co-(vinyl acetate)] matrix these disk-shaped matrices were 1 cm in diameter and 1 mm thick. In both cases, molecules of IgG were dispersed throughout the solid polymer phase. Although the amount of IgG released during the initial 1-2 days is greater for the matrix, the delivery systems have released comparable amounts after day 5. (a) Comparison of plasma IgG levels after direct injection of IgG (open circles) or subcutaneous injection of the IgG-releasing polymeric microspheres characterized in the inset (filled circles). The delivery system produces sustained IgG concentrations in the blood [3]. (b) Comparison of plasma IgG levels after direct intracranial injection of IgG (open squares) or implantation of an IgG-releasing matrix (filled squares) [4]. The influence of the delivery is less dramatic in this situation, probably because the rate of IgG movement from the brain into the plasma controls the kinetics of the overall process.

Because of the instability of the anhydride bond in the presence of water, special properties are required for stable polyanhydride devices. A critical element in the development of polyanhydride biomaterials is controlling hydrolysis within a polymeric device. To obtain implants where hydrolysis is confined to the surface of the polymer, hydrophobic monomers can be polymerized via anhydride linkages to produce a polymer that resists water penetration, yet degrades into low molecular weight oligomers at the poly-mer/water interface. By modulating the relative hydrophobicity of the matrix, which can be achieved by appropriate selection of monomers, the rate of degradation can then be adjusted. For example, copolymers of sebacic acid, a hydrophilic monomer, with carboxyphenoxypropane, a hydrophobic monomer, yield ... 

Most joint replacements utilize polymers to some extent. Finger joints usually are replaced with a poly(dimethylsiloxane) Insert and over h00,000 such replacements are made each year (l). More recently a poly(1, -hexadiene) polymer has been tried in this application (l). Many other parts of the hand, such as the bones, have also been replaced by silicone rubber. Other types of joints, such as the hip or the knee, often involve the contact of a metal ball or rider on a plastic surface which is usually made from high density, high molecular weight polyethylene. These metal and plastic parts are usually anchored in the body using a cement of poly(methyl methacrylate) which is polymerized in situ. Full and partial hip prostheses are implanted about... 

Wo Implantable, artificial lungs exist at this time hut some research has been done on polymeric membranes that could be used In such a device. Extracorporeal blood oxygenators are, however, used In excess of 100,000 times a year (l) and contain a thin, polymeric membrane thru which O2 and CO2 are exchanged. These oxygenators, which exist in several different styles are widely used In by-pass and other operations. The main polymers used are silicone rubber but poly(alkyIsulfones) and some others show promise (l, 50, 5l). 

The growing concern over the toxicity of residual ethylene oxide after sterilization of polymers for use in the field of medicine has led to the rapid growth of the field of radiation sterilization. The medical industry consumes a massive volume of polymeric material in both equipment and implants. As a consequence there is much interest in the effects of radiation on the physical properties and stability of irradiated polymers. The standard dose for radiation sterilization is 25 kGy, which is sufficient to alter the properties of many polymers, being for example close to or above the gel dose of many elastomers. There is also interest in the reaction of oxygen with long-lived radical species formed during irradiation. A common polymer used in medical equipment, poly(propylcne) is susceptible to oxidative degradation, and must be blended with appropriate stabilizers before radiation sterilization. 

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