Biomedical Engineering Reference
In-Depth Information
of calcium orthophosphates, while the metallic core provides the
excellent mechanical strength. The gradient change from calcium
orthophosphates to the metal is important, for example, from the
point of thermal expansion.
Functionally graded bioceramics consisting of calcium
orthophosphates only [575] have been developed [411, 482,
484, 578-587]. For example, dense sintered bodies with gradual
compositional changes from α-TCP to HA were prepared by sintering
a diamond-coated HA compacts at 1280°C under a reduced pressure,
followed by heating under the atmospheric conditions [578]. The
content of α-TCP gradually decreased, while the content of HA
increased with increasing depth from the surface. This functionally
gradient bioceramics consisting of HA core and α-TCP surface
showed a potential value as bone-substituting biomaterials [578].
Two types of functionally gradient FA/β-TCP biocomposites were
prepared in another study [579]. As shown in Fig. 4.11, one of the
graded biocomposites was in the shape of a disk and contained four
different layers of about 1 mm thick. The other graded biocomposite
was also in the shape of a disk but contained two sets of the four
layers, each layer being 0.5 mm thick controlled by using a certain
amount of the mixed powders. The final FA/β-TCP graded structures
were formed at 100 MPa and sintered at 1300°C for 2 h [579].
Figure 4.11
A schematic diagram showing the arrangement of the FA/β-
TCP biocomposite layers. (a) A non-symmetric functionally
gradient material (FGM); (b) symmetric FGM. Reprinted from
Ref. [579] with permission.
Besides, it is well known that a bone cross-section from cancellous
to cortical bone is non-uniform in porosity and pore dimensions.
Thus, in various attempts to mimic the porous structure of bones,
calcium orthophosphate bioceramics with graded porosity have
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