Biomedical Engineering Reference
In-Depth Information
Diamond
Graphite
3.2.11 Pyrolytic carbon for
long-term medical implants
Robert B. More, Axel D. Haubold, and
Jack C. Bokros
Introduction
Carbon materials are ubiquitous and of great interest
because the majority of substances that make up living
organisms are carbon compounds. Although many engi-
neering materials and biomaterials are based on carbon or
contain carbon in some form, elemental carbon itself is
also an important and very successful biomaterial. Fur-
thermore, there exists enough diversity in structure and
properties for elemental carbons to be considered as
a unique class of materials beyond the traditional mo-
lecular carbon focus of organic chemistry, polymer
chemistry, and biochemistry. Through a serendipitous
interaction between researchers during the late 1960s
the outstanding blood compatibility of a special form of
elemental pyrolytic carbon deposited at high tempera-
ture in a fluidized bed was discovered. The material was
found to have not only remarkable blood compatibility
but also the structural properties needed for long-term
use in artificial heart valves (LaGrange et al. , 1969). The
blood compatibility of pyrolytic carbon was recognized
empirically using the Gott vena cava ring test. This test
involved implanting a small tube made of a candidate
material in a canine vena cava and observing the de-
velopment of thrombosis within the tube in time. Prior
to pyrolytic carbon, only surfaces coated with graphite,
benzylalkonium chloride, and heparin would resist
thrombus formation when exposed to blood for long
periods. The incorporation of pyrolytic carbon in me-
chanical heart valve implants was declared ''an excep-
tional event'' (Sadeghi, 1987) because it added the
durability and stability needed for heart valve prostheses
to endure for a patient's lifetime. The objective of this
section is to present the elemental pyrolytic carbon ma-
terials currently is used in the fabrication of medical
devices and to describe their manufacture, characteriza-
tion, and properties.
Fullerene
Bucky Ball
Fig. 3.2.11-1 Allotropic crystalline forms of carbon: diamond,
graphite, and fullerene.
Recently a third crystalline form of elemental carbon, the
fullerene structure, has been discovered. The crystalline
polymorphs of elemental carbon are shown in Fig. 3.2.11-1 .
The properties of the elemental carbon crystalline
forms vary widely according to their structure. Diamond
with its tetrahedral sp 3 covalent bonding is one of the
hardest materials known. In the diamond crystal struc-
ture, covalent bonds of length 1.54 ˚ connect each
carbon atom with its four nearest neighbors. This tetra-
hedral symmetry repeats in three dimensions throughout
the crystal (Pauling, 1964). In effect, the crystal is a giant
isotropic
covalently
bonded
molecule;
therefore,
di-
amond is very hard.
Graphite with its anisotropic layered in-plane hexag-
onal covalent bonding and interplane van der Waals
bonding structure is a soft material. Within each planar
layer, each carbon atom forms two single bonds and one
double bond with its three nearest neighbors. This bond-
ing repeats in-plane to form a giant molecular (graphene)
sheet. The in-plane atomic bond length is 1.42 ˚ , which
is a resonant intermediate (Pauling, 1964) between the
single-bond length of 1.54 ˚ and the double-bond length
of 1.33 ˚ . The planer layers are held together by rela-
tively weak van der Waals bonding at a distance of
3.4 ˚ , which is more than twice the 1.42- ˚ bond length
(Pauling, 1964). Graphite has low hardness and a lubri-
cating property because the giant molecular sheets can
readily slip past one another against the van der Waals
bonding. Nevertheless, although large-crystallite-size
natural graphite is used as a lubricant, some artificially
produced graphites can be very abrasive if the crystallite
sizes are small and randomly oriented.
Fullerenes have yet to be produced in bulk, but their
properties on a microscale are entirely different from those
of their crystalline counterparts. Fullerenes and nanotubes
Elemental carbon
Elemental carbon is found in nature as two crystalline
allotrophic forms: graphite and diamond. Elemental
carbon also occurs as a spectrum of imperfect, turbostratic
crystalline forms that range in degree of crystallinity
from amorphous to the perfectly crystalline allotropes.
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