Biomedical Engineering Reference
In-Depth Information
of a magnetic field during sintering was found to influence the
growth of HA grains [263]. A definite correlation between hardness,
density and a grain size in sintered HA bioceramics was found:
despite exhibiting high bulk density, hardness started to decrease at
a certain critical grain size limit [264-266].
Hot pressing [266-272], hot isostatic pressing [204, 205] or hot
pressing with post-sintering [273, 274] processes make it possible
to decrease a temperature of the densification process, diminish
the grain size, as well as achieve higher densities. This leads to finer
microstructures, higher thermal stability and subsequently better
mechanical properties of calcium orthophosphate bioceramics.
Microwave [108, 275-283] and spark plasma [284-292] sintering
techniques are alternative methods to the conventional sintering,
hot pressing and hot isostatic pressing. Both alternative methods
were found to be time and energy efficient densification techniques.
Further developments are still possible. For example, a hydrothermal
hot pressing method has been developed to fabricate both OCP [293]
and CDHA [294] bulk bioceramics with neither thermal dehydration
nor thermal decomposition. Further details on the sintering and
firing processes of calcium orthophosphate bioceramics are available
in literature [50, 64, 146, 233, 295, 296].
To conclude this part, one should mention on an excellent review
paper on various ceramic manufacturing techniques [297], where
the interested readers are referred to extend their knowledge on the
ceramic processing.
4.4
The Major Properties
4.4.1
Mechanical Properties
Ideally, a bone substitute should be replaced by a mature bone without
transient loss of mechanical support. Unfortunately for material
scientists, a human body provides one of the most inhospitable
environments for the implanted materials. It is warm, wet and both
chemically and biologically active. For example, a diversity of body
fluids in various tissues might have a solution pH varying from 1
to 9. Furthermore, the body is capable of generating quite massive
force concentrations and the variance in such characteristics among
individuals might be enormous. Typically, bones are subjected to
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