Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 277

with anticoagulants (Okazaki et al. , 1997). There were
no instances of valve thrombosis even though platelets
were present on some of the valve surfaces. But, the
relevance of this observation to clinical valve thromboses
is not clear because human patients with mechanical
heart valves undergo chronic anticoagulant therapy
(Edmunds, 1987) and have a hemostatic system differ-
ent from that of sheep.
A more contemporary version of the mechanism of
pyrolytic carbon blood compatibility might be to reject
the assumption that the surface is inert, as it is now
thought by some that no material is totally inert in the
body (Williaims, 1998), and to accept that the blood-
material interaction is preceded by a complex, in-
terdependent, and time-dependent series of interactions
between the plasma proteins and the surface (Hanson,
1998) that is as yet poorly understood. To add to the
confusion, it must also be recognized that much of the
forementioned conjecture depends on the assumption
that all of the carbon surfaces studied were in fact pure
and comparable to one another.

However, pyrolytic carbon has been the most successful
material in heart valve applications because it offers
excellent blood and tissue compatibility which, com-
bined with the appropriate set of physical and mechan-
ical properties and durability, allows for practical
implant device design and manufacture. Improvements
in biocompatibility are desired, of course, because when
heart valves and other implants are used, a deadly or
disabling disease is often treated by replacing it with
a less pathological, more manageable chronic condition.
Ideally, an implant should not lead to a chronic

condition.
It is important to recognize that the mechanism for
the blood compatibility of pyrolytic carbons is not fully
understood, nor is the interplay between the biomaterial
itself, design-related hemodynamic stresses, and the ul-
timate biological reaction. The elucidation of the mech-
anism for blood and tissue compatibility of pyrolytic
carbon remains a challenge.
It is also worth restating that the suitability of carbon
materials from new sources for long-term implants is not
assured simply because the material is carbon. Elemental
carbon encompasses a broad spectrum of possible
structures and mechanical properties. Each new candi-
date carbon material requires a specific assessment of
biocompatibility based on its own merits and not by
reference to the historically successful General Atomic-
type pyrolytic carbons.

Conclusion

Because the blood compatibility of pyrolytic carbon in
mechanical heart valves is not perfect, anticoagulant
therapy is required for mechanical heart valve patients.

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