Biomedical Engineering Reference
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of the plasma-sprayed amorphous phases. Besides, these ACPs are
definitively anhydrous contrary to the precipitated ACPs.
To conclude the chemical part, one should mention on solubility of
ACPs. Due to the chemical variations, this value cannot be measured
precisely (Table 1.1). Several different solubility products have been
proposed for various ACPs and the interested readers are referred to
Table 1 of Ref. [31] for the details.
2.3.5
Structure
In general, determination of the atomic structure of amorphous
solids is a non-trivial task. As the structure can be defined
essentially only in terms of unit-cells containing an infinitely
large amount of atoms (as there is no LRO periodic symmetry),
a statistical description appears to be unavoidable. Thus, the
structure of a particular amorphous solid can never be determined
unambiguously and this uncertainty is compounded by the fact that
the structure of a non-crystalline material often depends on the
specific details of preparation techniques [88]. Furthermore, the
chemical composition, namely the Ca/P ratio, of ACPs varies a lot
(1.2 < Ca/P < 2.2 — see Table 1.1), which makes the task even more
complicated. The latter results in the fact that different samples of
ACP possess diverse properties and the data found for one particular
sample appear to be inapplicable to other ACP samples. A possible
solution of this problem seems to be in a wide recognition of the fact
that ACP is not a single chemical compound but represents a group
of calcium orthophosphates having diverse physical and chemical
properties. In other words, depending on the Ca/P ratio and/or
other properties, first of all, any particular sample of ACP must be
ascribed to the existing crystalline phase from Table 1.1 and only
afterward any structural investigations should be performed. This
is the only way to succeed in clarification of the ACP structures from
experimental measurements in future. Current state-of-the-art on
the structure of ACPs is given below.
The first quantitative studies on a synthetic ACP were done in
the mid of 1960s on a material precipitated at pH ~10 [134, 196].
As Watson and Robinson found at neutral pH, the initial phase that
spontaneously formed immediately upon mixing concentrated
alkaline Ca- and PO
-containing solutions was structurally non-
4
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