Biomedical Engineering Reference
In-Depth Information
of other calcium orthophosphates with various chemicals (e.g
.
,
DCPA + CaO). Thus, with the exception of a hydrothermal synthesis
[223-225], in aqueous solutions only CDHA might be prepared [123,
174-176, 226-230]. As apatites (CDHA, HA, and FA) belong to the
sparingly soluble compounds (Table 1.1), simple mixing of calcium-
and orthophosphate-containing aqueous solutions at pH >9 results
in formation of extremely supersaturated solutions and, therefore,
a very fast precipitation of the tremendous amounts of very fine
crystals [231], initially of ACP (see Chapter 2), that afterwards is
re-crystallized into apatites [174-176, 232-235]. The dimensions
of the precipitated nano-sized crystals might be slightly increased
by the Ostwald ripening approach (maturation), that is, by boiling
and/or ambient aging in the mother liquid (Fig. 3.3) [131, 143,
173, 203, 224, 234-239]. Heat treatment of ACP might be applied
as well [240]. Therefore, preparation of nanodimensional and/or
nanocrystalline apatites is not a problem at all and has been known
for many years [131, 132, 241-243]; however, prefix “nano” had
not been used before 1994. On the contrary, with the exception of
a thermally stable FA (thus, big crystals of FA might be produced by
a melt-growth process [244, 245]), manufacturing of big crystals of
both CDHA and HA still is a challenge.
Many different methodologies have been proposed to prepare
nanodimensional and/or nanocrystalline structures [246-253].
Prior to describing them, it is important to stress that in the vast
majority of the available literature on apatites, the authors do not
tell the difference between CDHA and HA. Therefore, getting through
scientific papers, an attentive reader often finds statements, as:
“Because natural bone is composed of both organic components
(mainly type I collagen) and inorganic components (HA), …” [98,
p. 357], “The HA nanorods are synthesized via a wet precipitation
process …” [141, p. 2364], “… (TTCP) has been shown previously
to be an essential component of self-setting calcium phosphate
cements that form hydroxyapatite (HA) as the only end-product. …”
[254, abstract], etc. The matter with distinguishing between CDHA
and HA becomes even much more complicated, when researchers
deal with nanodimensional and/or nanocrystalline apatites because
the assemblies of nano-sized particles give only broad diffraction
patterns, similar to ones from an amorphous material [198, 199].
While composing this review, I always tried to specify whether each
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