Biomedical Engineering Reference
In-Depth Information
can be taken of ACPs' high reactivity to prepare various bioactive
biomaterials. Currently, ACPs are involved as transient or
constitutive phases in several commercial substitute bone materials,
such as plasma sprayed coatings on metal prostheses and injectable
cements for orthopedic applications. ACPs are also used for dental
applications as fillers in ionomer cements to fill cavities or as
colloidal suspensions in toothpastes, chewing gums or mouthwashes
to promote remineralization of carious lesions and/or to prevent
tooth demineralization [31]. Obviously, these examples are just the
initial steps of the biomedical applications of ACPs.
As written above, the atomic structure of amorphous compounds
is difficult to investigate due to the lack of LRO. Thus, in some of
the aforementioned studies on ACPs, an “amorphous” character of
the phases might be due to formation of nanodimensional and/or
nanocrystalline calcium orthophosphates [325, 326] with crystal
dimensions of 2 nm × 2 nm × 2 nm (8 nm
3
) or so (see section 2.2).
Namely, employment of novel high resolution techniques resulted in
doubt of the existence of several calcium orthophosphate samples
in an amorphous state [327, 328]. Therefore, there is still an
unanswered question concerning the structure of the substantial
amounts of nanodimensional and/or nanocrystalline calcium
orthophosphates: whether they appear to be almost amorphous
(according to numerous results of X-ray diffraction studies) due to
their exceedingly small dimensions of well-crystallized structures or
due to a really amorphous (i.e., retaining only a SRO) matter? A good
attempt to discuss this topic is available in literature [329], where
the interested readers are referred to.
To conclude the chapter, the biomedical applications of
ACPs nowadays are limited due to great difficulties in their
consolidations to various 3D structures, which have to possess the
sufficient mechanical properties. As ACPs do not sustain heating at
temperatures exceeding ~ 600°C, they must be consolidated by low
temperature techniques only. Spark plasma sintering approach is
one of them [31, 285]. Another one comprises mixing of thermally
unstable calcium orthophosphates with water soluble porogens,
followed by cold isostatic pressing of the prepared mixture
and dissolving the porogens [330]. Obviously, still other “mild”
consolidation approaches are possible, those need to be developed.
For example, a very interesting approach to fabricate hollow ACP-
based nano-sized tubes has been proposed recently. These nano-
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