Biomedical Engineering Reference
In-Depth Information
to fluidize the bed, and the source hydrocarbon is added
to the gas stream when needed.
At a sufficiently high temperature, pyrolysis or ther-
mal decomposition of the hydrocarbon can take place.
Pyrolysis products range from free carbon and gaseous
hydrogen to a mixture of C
x
H
y
decomposition species.
The pyrolysis reaction is complex and is affected by the
gas flow rate, composition, temperature and bed surface
area. Decomposition products, under the appropriate
conditions, can form gas-phase nucleated droplets of
carbon/hydrogen, which condense and deposit on the
surfaces of the wall and bed particles within the reactor
(Bokros, 1969). Indeed, the fluidized-bed process was
originally developed to coat small (200-500 micrometer)
diameter spherical particles of uranium/thorium carbide
or oxide with pyrolytic carbon. These coated particles
were used as the fuel in the high temperature gas-cooled
nuclear reactor (Bokros, 1969).
Pyrolytic carbon coatings produced in vertical-tube
reactors can have a variety of structures such as laminar
or isotropic, granular, or columnar (Bokros, 1969). The
structure of the coating is controlled by the gas flow rate
(residence time in the bed), hydrocarbon species, tem-
perature and bed surface area. For example, an in-
adequately fluidized or static bed will produce a highly
anisotropic, laminar pyrolytic carbon (Bokros, 1969).
Control of the first three parameters (gas flow rate,
hydrocarbon species, and temperature) is relatively easy.
However, until recently, it was not possible to measure
the bed surface area while the reactions were taking
place. As carbon deposits on the particles in the fluidized
bed, the diameter of the particles increases. Hence the
surface area of the bed changes, which in turn influences
the subsequent rate of carbon deposition. As surface area
increases, the coating rate decreases since a larger surface
area now has to be coated with the same amount of
carbon available. Thus the process is not in equilibrium.
The static-bed process was adequate to coat nuclear fuel
particles without attempting to control the bed surface
area, because such thin coatings (25-50
m
m thick) did
not appreciably affect the bed surface area.
It was later found that larger objects could be
suspended within the fluidized bed of small ceramic
particles and also become uniformly coated with carbon.
This finding led to the demand for thicker, structural
coatings, an order of magnitude thicker (250-500
m
m).
Bed surface area control and stabilization became an
important factor (Akins and Bokros, 1974) in achieving
the goal of thicker, structural coatings. In particular, with
the discovery of the blood-compatible properties of py-
rolytic carbon (LaGrange
et al.,
1969), thicker structural
coatings with consistent and uniform mechanical prop-
erties were needed to realize the application to me-
chanical heart-valve components. Quasi-steady-state
conditions as needed to prolong the coating reaction were
achieved empirically by removing coated particles and
adding uncoated particles to the bed while the pyrolysis
reaction was taking place (Akins and Bokros, 1974).
However, the rates of particle addition and removal were
based upon little more than good guesses.
Three of the four parameters that control carbon de-
position could be accurately measured and controlled,
but a method to measure and control bed surface area
was lacking. Thus, the quasi-steady-state process was
more of an art than a science. If too many coated particles
were removed, the bed became too small to support the
larger components within it and the bed collapsed. If too
few particles were removed, the rate of deposition de-
creased, and the desired amount of coating was not
achieved in the anticipated time. Furthermore, there
were considerable variations in the mechanical properties
of the coating from batch to batch. It was found that in
order to consistently achieve the hardness needed for
wear resistance in prosthetic heart valve applications, it
was necessary to add a small amount of b-silicon carbide
to the carbon coating. The dispersed silicon carbide
particles within the pyrolytic carbon matrix added suf-
ficient hardness to compensate for potential variations
in the properties of the pyrolytic carbon matrix. The
b-silicon carbide was obtained from the pyrolysis of
methyltrichlorosilane, CH
3
SiCl
3
. For each mole of sili-
con carbide produced, the pyrolysis of methyltri-
chlorosilane also produces 3 moles of hydrogen chloride
gas. Handling and neutralization of this corrosive gas
added substantial complexity and cost to an already
complex process. Nevertheless, this process allowed con-
sistency for the successful production of several million
components for use in mechanical heart valves.
A process has been developed and patented that
allows precise measurement and control of the bed sur-
face area. A description of this process is given in the
patent literature and elsewhere (Emken
et al.
, 1993,
1994; Ely
et al.
, 1998). With precise control of the bed
surface area it is no longer necessary to include the silicon
carbide. Elimination of the silicon carbide has produced
a stronger, tougher, and more deformable pure pyrolytic
carbon. Historically, pure carbon was the original objec-
tive of the development program because of the potential
for superior biocompatibility (LaGrange
et al.
, 1969).
Furthermore, the enhanced mechanical and physical
properties of the pure pyrolytic carbon now possible with
the improved process control allows prosthesis design
improvements
in
the
hemodynamic
contribution
to
thromboresistance (Ely
et al.
, 1998).
Structure of pyrolytic carbons
X-ray diffraction patterns of the biomedical-grade
fluidized-bed pyrolytic carbons are broad and diffuse
because of the small crystallite size and imperfections. In
