Biomedical Engineering Reference
In-Depth Information
intentional variations in composition and/or structure gradually
over volume, resulting in corresponding changes in the properties
of the composite. The main feature of such materials is the almost
continuously graded composition that results in two different
properties at the two ends of the structure. Such composites can be
designed for specific function and applications. Various approaches
based on the bulk (particulate processing), preform processing, layer
processing and melt processing are used to fabricate the functionally
graded materials.
Bone is a biologically formed composite with variable density
ranging from very dense and stiff (cortical bone) to a soft and
foamed structure (trabecular bone). Normally the outer part of long
bones consists of cortical bone with the density decreasing towards
the core, where the trabecular bone is found. The trabecular bone is
porous and the porosity is filled with osseous medulla [25, 26]. This
brief description clearly indicates that bones are natural functionally
graded composites.
The concept of FGM has been increasingly used for biomaterial
design and currently it remains to be an important area of the
research. For example, many studies have been performed to
fabricate porosity-graded calcium orthophosphate bioceramics
in attempts to mimic the porous structure of bones [1099-1102].
This is a structural approach to fabricate FGM. Besides, there
is a compositional approach. For example, powder metallurgy
methods have been used to fabricate HA/Ti functionally graded
biocomposite dental implants offering the biocompatible HA on the
tissue side and titanium on the outer side for mechanical strength
[1103-1105]. The graded structure in the longitudinal direction
contains more Ti in the upper section and more HA in the lower
section. Actually, in the upper section the occlusal force is directly
applied and Ti offers the required mechanical performance; in the
lower part, which is implanted inside the bone, the HA confers the
bioactive and osteoconductive properties to the material [1103].
Since the optimum conditions of sintering for Ti and HA are very
different, HA/Ti functionally graded biocomposites are difficult to
fabricate and the sintering conditions for their mixtures are obliged
to compromise. The expected properties of this implant are shown
in Fig. 6.5 [1104]. Such biocomposites might be both symmetrical
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