Biomedical Engineering Reference
In-Depth Information
TABLE 2.7
Mechanical Properties of Carbon Fiber-Reinforced Carbon
Fiber Lay-Up
Property
Unidirectional
0-90° Crossply
Flexural modulus (GPa)
Longitudinal
140
60
Transverse
7
60
Flexural strength (MPa)
Longitudinal
1200
500
Transverse
15
500
Interlaminar shear strength (MPa)
18
18
Source:
Adapted from Adams D. and Williams D.F. 1978.
J. Biomed. Mater. Res
. 12:38.
Carbons exhibit excellent compatibility with tissue. Compatibility of pyrolitic carbon-coated devices
with blood has resulted in extensive use of these devices for repairing diseased heart valves and blood
vessels (Park and Lakes, 1992).
Pyrolitic carbons can be deposited onto finished implants from hydrocarbon gas in a
fluidized bed
at a controlled temperature and pressure. The anisotropy, density, crystallite size, and structure of the
deposited carbon can be controlled by temperature, composition of the fluidized gas, the bed geometry,
and the residence time (velocity) of the gas molecules in the bed. The microstructure of deposited car-
bon should be highly controlled, since the formation of growth features associated with uneven crys-
tallization can result in a weaker material (Figure 2.5). It is also possible to introduce various elements
into the fluidized gas and co-deposit them with carbon. Usually silicon (10−20 w/o) is co-deposited (or
alloyed) to increase hardness for applications requiring resistance to abrasion, such as heart valve disks.
Recently, success was achieved in depositing pyrolitic carbon onto the surfaces of blood vessel
implants made of polymers. This type of carbon is called ultra-low-temperature isotropic (ULTI) carbon
instead of low-temperature isotropic (LTI) carbon. The deposited carbon has excellent compatibility
with blood and is thin enough not to interfere with the flexibility of the grafts (Park and Lakes, 1992).
The vitreous or glassy carbon is made by controlled pyrolysis of polymers such as phenolformalde-
hyde, Rayon (cellulose), and polyacrylonitrite at high temperature in a controlled environment. This
process is particularly useful for making carbon fibers and textiles which can be used alone or as com-
ponents of composites.
2.3 Biodegradable or Resorbable Ceramics
Although Plaster of Paris was used in 1892 as a bone substitute (Peltier, 1961), the concept of using syn-
thetic resorbable ceramics as bone substitutes was introduced in 1969 (Hentrich et al., 1969; Graves et al.,
1972).
Resorbable ceramics
, as the name implies, degrade upon implantation in the host. The resorbed
material is replaced by endogenous tissues. The rate of degradation varies from material to material.
Almost all bioresorbable ceramics except Biocoral and Plaster of Paris (calcium sulfate dihydrate) are
variations of calcium phosphate (Table 2.8). Examples of resorbable ceramics are aluminum calcium
phosphate, coralline, Plaster of Paris, hydroxyapatite, and tricalcium phosphate (Table 2.8).
2.3.1 Calcium Phosphate
Calcium phosphate has been used in the form of artificial bone. This material has been synthesized
and used for manufacturing various forms of implants, as well as for solid or porous coatings on other
implants (Table 2.9).
Calcium phosphate can be crystallized into salts such as hydroxyapatite and β-whitlockite depend-
ing on the Ca:P ratio, presence of water, impurities, and temperature. In a wet environment and at
