Biomedical Engineering Reference
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solutions) and high-temperature ACPs. low-temperature ACPs
(described by the chemical formula Ca
H
(PO
)
·
n
H
O,
n
= 3-4.5;
x
y
4
z
2
15-20% H
O) are often encountered as a transient precursor phase
during precipitation of other calcium orthophosphates in aqueous
systems. usually, an ACP is the first phase precipitated from a
supersaturated solution prepared by rapid mixing of solutions
containing ions of calcium and orthophosphate [28, 270-275];
however, other production techniques are known. ACPs are thought
to be formed at the beginning of the precipitation due to a lower
surface energy than that of OCP and apatites [271]. The amorphization
degree of ACPs increases with the concentration increasing of Ca-
and PO
2
-containing solutions, as well as at a high solution pH and
a low crystallization temperature. A continuous gentle agitation of
as precipitated ACPs in the mother solution, especially at elevated
temperatures, results in a slow recrystallization and formation of
better crystalline calcium orthophosphates, such as CDHA [27, 28].
The lifetime of ACPs in aqueous solution was reported to be a function
of the presence of additive molecules and ions, pH, ionic strength
and temperature. Thus, ACPs may persist for appreciable periods
and retain the amporphous state under some specific experimental
conditions [276]. The chemical composition of ACPs strongly depends
on the solution pH and the concentrations of mixing solutions. For
example, ACPs with Ca/P ratios in the range of 1.18 (precipitated at
solution pH = 6.6) to 1.53 (precipitated at solution pH = 11.7) [28,
277] and even to 2.5 [27, 83, 84] have been described. The presence
of poly(ethylene glycol) [278], ions of pyrophosphate, carbonate
and/or magnesium in solution during the crystallization promotes
formation of ACPs and slows down their further transformation
into more crystalline calcium orthophosphates, while the presence
of fluoride has the opposite effect [27-29, 107, 279]. The solution-
mediated transformation of an ACP to CDHA, which can be described
by a ”first-order” rate law, is a function only of the solution pH and
depends upon the experimental conditions which regulate both the
dissolution of ACP and the formation of early HA nuclei [280].
High-temperature ACPs might be prepared using high energy
processing at elevated temperatures. This method is based on a
rapid quenching of melted calcium orthophosphates occurring, e.g.,
during plasma spraying of HA [281-283]. A plasma jet, possessing
very high temperatures (5000-20000°C), partly decomposes HA.
That results in formation of a complicated mixture of products, some
4
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