Various polymer systems have been investigated for such uses as the preparation of artificial prostheses for biomedical, particularly orthopedic applications. The objective has usually been to duplicate the mechanical properties of the natural counterparts and to meet the requirements of tissue biocompatibility. For example, the preparation of artificial tendons has been pursued basically by the construction of composite structures of fiber-reinforced elastomers, such as poly(ethylene terephthalate) in silicone rubber. Silk and metal fibers have also been used as components in such prostheses. The fabrication of tapes and fiber meshes using conventional polymers, e.g., nylon, dacron, and polyethylene has also been explored. In the latter case, however, nylon has been found to degrade in vivo; dacrons have been found to fail in tissue ingrowth tests; and conventional polyethylene when strained in the working range of human tendons (3% strain), has been unsatisfactory both in its elastic recovery (80-90%) and in the time needed for complete recovery (5 min.)--two important factors in the design of artificial tendons.
Ligament fabrication has been pursued by the design of artificial preparations utilizing ultra-high-molecular-weight polyethylene. Ultra-high molecular weight polyethylene, in contrast to the conventional high-density polyethylenes having average molecular weight up to about 400,000, has an extremely high molecular weight--typically 2 to 6 million--and is intractable. The polymer is supplied as fine powder and is processed into various profiles using compression molding and ram extrusion processes. However, studies made heretofore have suggested that the ultra-high molecular weight polyethylene structures made by such processes do not possess adequate yield, creep, and fatigue properties to meet the requirements of biomedical applications, apparently because of poor interparticle fusion of the powder particles in the raw material during its processing. More recently, new processing systems, including ultra-high speed mixing systems and radio-frequency heating, have been used in injection molding practices to produce ultra-high molecular weight products. However, the products of such processes are obtained also in a temperature range (360.degree.-380.degree. F. 182.degree.-193.degree. C.) in which it has been found that complete fusion of the powder particles does not occur. Furthermore, radio-frequency heating, an effective process for heating polymers such as nylons, PVC, and PVF.sub.2 that have polar molecules to respond to radio-frequency energy is impractical for non-polar polymers such as polyethylene. Radio-frequency heating of such polymers can occur by incorporating in the polymer agents such as Frequon (a trademark of the Phillips Chemical Co.) which are sensitive to radio frequency. However, the use of additives in the polymer, which is known to be biocompatible, may have adverse effects in meeting the requirements of tissue biocompatibility in biomedical applications.
USPTO Backgrounds
USPTO Backgrounds