Structure
Pyrolytic carbon belongs to the family of turbostratic carbons, which have a similar structure to graphite. Graphite consists of carbon atoms that are covalently bonded in hexagonal arrays. These arrays are stacked and held together by weak interlayer binding. Pyrocarbon and other turbostratic carbons differ in that the layers are disordered, resulting in wrinkles or distortions within layers. This gives pyrocarbon improved durability compared to graphite. (Source)

Pyrocarbon is unlike graphite in that the adjacent layers, made up of hexagonal arrangements of atoms, are small and not ordered with respect to one another. This two—dimensional structure has been termed turbostratic. Because of the strong interatomic links within the layers and the weakness of the bonding between layers, the properties of individual crystallites are very anisotropic. However, grouped together and randomly distributed in an aggregate, the crystallite anisotropy is perfectly averaged and the aggregate appears isotropic. Isotropy is an essential property when coating samples with complex shapes. (Source: White paper: "Medical Applications of Carbonaceous Materials" Authors M. Hassler, S. Ramboud, C. Real, BioProfile.)

History of Pyrocarbon
Pyrocarbon was introduced into the medical world as most biomaterials are: by chance.

Dr. Bokros was using pyrolytic carbon to coat nuclear fuel particles for the General Atomics (GA) gas-cooled nuclear power reactors more than 30 years ago. He discovered its potential for medical uses through what has been called "a lesson in serendipity." In 1966, Bokros read an article by Dr. Vincent Gott, who had been testing carbon-based paint as a blood compatible coating for artificial heart components. Bokros contacted Gott who initiated the collaboration that resulted in making heart valve replacements a practical reality and common-place procedure. In the 1960s, artificial heart valves constructed from plastics and metals suffered from short-term failures due to wearing-out or because of blood clotting. Gott was searching for a material to use in artificial heart valves that did not provoke blood clots and had the mechanical durability to endure for a recipient's lifetime. Pyrolytic carbon, from GA, met both of his needs. However, the initial material used to coat nuclear fuel particles had the needed blood compatibility, but not the durability. GA initiated a development project headed by Dr. Bokros to add the needed durability to the material. This endeavor was successful and the biomedical grade of pyrolytic carbon was rapidly incorporated into the existing heart valve designs. After 30 years, pyrocarbon remains the most widely used material for mechanical heart valves. It has been used in more than 4 million implants in more than 25 different valve designs for a clinical experience on the order of 18 million patient-years. No other material used for long-term blood contacting implants can boast of such a successful clinical experience. ( Source: General Atomic)

In the 1980s and 1990s this isotropic pyrocarbon, known for its excellent biocompatibility, led to the development of orthopedic implants.

Learn more about pyrocarbon manufacturing.