Carbon biomedical devices

Abstract Isotropic pyrolytic carbon (LTI carbon) offers a unique combination of properties that has made it the material of choice for use in the construction of artificial heart valves. The biocompatibility of LTI carbon, in particular its thromboresistance, together with its resistance to degradation by wear and fatigue in the biological environment, has greatly reduced the incidence of thromboembolic complications and mechanical failures of prosthetic valves. Since 1969, more than 200,000 valves that use LTI carbon have been implanted. The morphological and/or surface chemical characteristics responsible for the compatibility of LTI carbon with blood remain unidentified. Its surface energy, greater than 50 ergs/cm2, is unusually high for a thromboresistant material, yet LTI carbon surfaces are able to interface with blood through a weakly adsorbed proteinaceous layer without activating clotting via the intrinsic mechanism. The solid carbons have elastic moduli that are in the range often quoted for bone. Thus, for load-bearing applications in dentistry for endosseous dental implants and in orthopedics for joint replacement, the match of moduli provides biomechanical compatibility which can minimize the stress concentrations that often result when stiffer materials such as metals and ceramics are used in bone. The biochemical compatibility and wear resistance of the silicon-alloyed variety of LTI carbon, together with its strength and elastic match with bone, provide an ideal combination of properties for applications in orthopedics as a joint replacement material. Complex shapes not fabricable from isotropic pyrolytic carbon can be fabricated from metals and then coated with a thin, impermeable layer of carbon by vapor deposition. Flexible materials may be obtained by carbon coating thin polymeric sheets or fabrics by vapor deposition or through the use of glassy carbon fibers. The ability of carbon to perform percutaneously is being utilized, for example, in devices used to gain access to the blood stream for dialysis.

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