Biomedical Engineering Reference
In-Depth Information
commercial bone-substituting product consisting solely of TTCP. For
the comprehensive information on TTCP, the readers are referred to
a recent review [494], while the structure [497], spectra [498] and
solubility [220] of TTCP are well described elsewhere.
There is an opinion [115, 189], that all calcium orthophosphates
listed in Table 1.1 might be classified into three major structural
types: (i) the apatite type, Ca
, which includes HA, FA, OA,
CDHA, OCP and TTCP; (ii) the glaserite type, named after the mineral
glaserite, K
(PO
)
X
10
4
6
2
, which includes all polymorphs of TCP and,
perhaps, ACP; (iii) the Ca-PO
Na(SO
)
3
4
2
sheet-containing compounds, which
include DCPD, DCPA, mCPm and mCPA. According to the authors,
a closer examination of the structures revealed that all available
calcium orthophosphates could be included into distorted glaserite
type structures, but with varying degrees of distortion [115, 189].
4
1.3.14
Multiphasic and Polyphasic Calcium
Orthophosphate Compositions
Calcium orthophosphates might form multiphasic and/or polyphasic
compositions, in which the individual components cannot be
separated from each other. Presumably, the individual phases of such
compositions are homogeneously “mixed” at a far submicron level
(< 0.1 μm) and strongly integrated with each other. Nevertheless, the
presence of all individual phases is easily seen by X-ray diffraction
technique.
The main idea of the multiphasic concept is determined by a
balance of more stable calcium orthophosphate phases (e.g., HA) and
more soluble calcium orthophosphate phases (e.g., TCP). The usual
way to prepare multiphasic calcium orthophosphates consist of
sintering of non-stoichiometric calcium orthophosphates, such as ACP
and CDHA, at temperatures above ~700°C. Furthermore, a thermal
decomposition of the stoichiometric calcium orthophosphates at
temperatures above ~1300°C might be used as well [499, 500];
however, this approach often results in formation of complicated
mixtures of various products including admixtures of CaO, calcium
pyrophosphates, etc. Namely, transformation of HA into polyphasic
calcium orthophosphates by annealing in a vacuum occurs as this:
the outer part of HA is transformed into α-TCP and TTCP, while the
α-TCP phase of the surface further transforms into CaO. Besides, in
the boundary phase, HA is transformed into TTCP [499].
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