Pyrolytic Carbons and the Design of Mechanical Heart Valve Prostheses

Pyrolytic carbons have a long successful history in mechanical heart valve prosthesis applications. Originally pyrolytic carbons had been developed for use in nuclear reactors. But in a chance interaction between a scientist studying nuclear energy and another searching for blood compatible materials, the blood compatibility of pyrolytic carbon was discovered. This discovery of blood compatibility prompted an effort that resulted in the development of a form of pyrolytic carbon specifically tailored for use in mechanical heart valves. This form developed by General Atomic Co. was an alloy of approximately 5 to 12 weight percent silicon codeposited with pyrolytic carbon. Fine silicon carbide particles dispersed in the carbon matrix increased the hardness and wear resistance of the pyrolytic carbon, which compensated for difficulties in manufacturing using the process control capabilities available at the time. Use of pyrolytic carbon instead of polymers in the early valve designs allowed the durability, stability and compatibility needed for true long-term implants. Since the first pyrolytic carbon heart valve component implant in 1968, more than 4 million pyrolytic carbon components in more than 25 different valve designs have been implanted to accumulate a clinical experience on the order of 18 million patient years. The physiochemical and mechanical properties of silicon-alloyed pyrolytic carbon, while enabling the practical utilization of mechanical heart valves, placed some severe restrictions upon design. Silicon-alloyed pyrolytic carbon is an extremely hard and nearly ideal linear elastic material with a strain to failure of approximately 1.2 percent. Traditional machining and joining techniques are not feasible, rather the carbon is prepared as a coating upon a pre-form and the coated components are then finished to size using diamond impregnated tools, grinding forms and abrasive polishing techniques. While the silicon-alloyed material was very successful, design features of known hydrodynamic advantage, such as a flared inlet, were not possible and in some valve designs annular area was sacrificed by the addition of metallic rings used to increase stiffness. As a result, mechanical valve designs in the small aortic sizes tended to be stenotic. In the early 1990’s, pyrolytic carbon coating technology was re-examined and methods of process control were redesigned in order to produce pure carbon. The resulting pure pyrolytic carbon had sufficient hardness and wear resistance, but, in addition, had higher strength and toughness with higher deformability than the silicon-alloyed material. The new material eliminated the need for the silicon and improved the carbon mechanical properties. With the improved mechanical properties, it is now possible to manufacture valve designs with greater hydrodynamic efficiency, and eliminate the need for stiffening rings, thus improving the flow behavior in the small aortic valve sizes. A mechanical valve design utilizing the pure carbon with improved hydrodynamic design features has achieved hemodynamic properties comparable to those of homografts and stentless bioprostheses.