Pyrolytic carbon owes its unusual mechanical properties and its biocompatible nature to a unique microstructure. The material is durable, strong, and resistant to wear. It is also highly thromboresistant, or resistant to blood clotting, and has inherent cellular biocompatibility with blood and soft tissue.
Pyrolytic carbon belongs to the special family of turbostratic carbons. The structure of turbostratic carbons is related to that of graphite, but is subtly different. In graphite, the carbon atoms are covalently bonded in planar hexagonal arrays that are stacked in layers with relatively weak interlayer bonding. For turbostratic carbons, the stacking sequence is disordered and wrinkles or distortions may exist within each layer. This structural distortion is responsible for the superior ductility and durability of PyC.
For heart valves, a silicon-alloyed pyrolytic carbon is used in the form of a thick coating on a polycrystalline graphite substrate. Silicon is added to improve mechanical properties such as stiffness, hardness, and wear resistance, without significant loss in biocompatibility. Components are made by co-depositing carbon and silicon carbide on the graphite substrate by a chemical vapor-deposition, fluidized bed process that uses a gaseous mixture of silicon-containing carrier gas with a hydrocarbon.
Published in the Summer 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.