Biomedical Engineering Reference
In-Depth Information
consist of a graphene layer that is rolled up or folded
(Sattler, 1995) to form a tube or ball. These large mole-
cules, C
60
and C
70
fullerenes and (C
60
รพ
18
j
) nanotubes,
are often mentioned in the literature (Sattler, 1995)
along
The term ''pyrolytic'' is derived from ''pyrolysis,''
which is thermal decomposition. Pyrolytic carbon is
formed from the thermal decomposition of hydrocar-
bons such as propane, propylene, acetylene, and meth-
ane, in the absence of oxygen. Without oxygen the typical
decomposition of the hydrocarbon to carbon dioxide and
water cannot take place; instead a more complex cascade
of decomposition products occurs that ultimately results
in a ''polymerization'' of the individual carbon atoms into
large macroatomic arrays.
Pyrolysis of the hydrocarbon is normally carried out
in a fluidized-bed reactor such as the one shown in
Fig. 3.2.11-2
. A fluidized-bed reactor typically consists of
a vertical tube furnace that may be induction or re-
sistance heated to temperatures of 1000-2000
C
(Bokros, 1969). Reactor diameters ranging from 2 cm
to 25 cm have been used; however, the most common
size used for medical devices has a diameter of about
10 cm. These high-temperature reactors are expensive to
operate, and the reactor size limits the size of device
components to be produced.
Small refractory ceramic particles are placed into the
vertical tube furnace. When a gas is introduced into the
bottom of the tube furnace, the gas causes the particle
bed to expand: Interparticle spacing increases to allow
for the flow of the gas. Particle mixing occurs and the bed
of particles begins to ''flow'' like a fluid. Hence the term
''fluidized bed.'' Depending upon the gas flow rate and
volume, this expansion and mixing can be varied from
a gentle bubbling bed to a violent spouting bed. An
oxygen-free, inert gas such as nitrogen or helium is used
with
more
complex
multilayer
''onion
skin''
structures.
There exist many possible forms of elemental carbon
that are intermediate in structure and properties be-
tween those of the allotropes diamond and graphite.
Such ''turbostratic'' carbons occur as a spectrum of
amorphous through mixed amorphous, graphite-like and
diamond-like to the perfectly crystalline allotropes
(Bokros, 1969). Because of the dependence of properties
upon structure, there can be considerable variability in
properties for the turbostratic carbons. Glassy carbons
and pyrolytic carbons, for example, are two turbostratic
carbons with considerable differences in structure and
properties. Consequently, it is not surprising that carbon
materials are often misunderstood through over-
simplification. Properties found in one type of carbon
structure can be totally different in another type of
structure. Therefore it is very important to specify the
exact nature and structure when discussing carbon.
Pyrolytic carbon (Pyc)
The biomaterial known as pyrolytic carbon is not found in
nature: it is manmade. The successful pyrolytic carbon
biomaterial was developed at General Atomic during the
late 1960s using a fluidized-bed reactor (Bokros, 1969).
In the original terminology, this material was considered
a low-temperature isotropic carbon (LTI carbon). Since
the initial clinical implant of a pyrolytic carbon compo-
nent in the DeBakey-Surgitool mechanical valve in 1968,
95% of the mechanical heart valves implanted worldwide
have at least one structural component made of pyrolytic
carbon. On an annual basis this translates into approxi-
mately 500,000 components (Haubold, 1994). Pyrolytic
carbon components have been used in more than 25
different prosthetic heart valve designs since the late
1960s and have accumulated a clinical experience of the
order of 16 million patient-years. Clearly, pyrolytic
carbon is one of the most successful, critical biomaterials
both in function and application. Among the materials
available for mechanical heart valve prostheses, pyrolytic
carbon has the best combination of blood compatibility,
physical and mechanical properties, and durability.
However, the blood compatibility of pyrolytic carbon in
heart-valve applications is not perfect; chronic anticoag-
ulant therapy is needed for patients with mechanical
heart valves. Whether the need for anticoagulant therapy
arises from the biocompatible properties of the material
itself or from the particular hydrodynamic interaction of
a given device and the blood remains to be resolved.
Feed
rate
Add
particles
Pressure
sensor
Controller
Bed
reaction
Remove
particles
Hydrocarbon
gas
Withdraw
rate
Fig. 3.2.11-2 Fluidized-bed reactor schematic.
