Biomedical Engineering Reference
In-Depth Information
2.3.4
Chemical Composition
Nowadays, ACPs should be recognized as a special class of biomedically
relevant calcium orthophosphate salts having variable chemical
but rather identical glass-like physical properties. Presumably, all
calcium orthophosphates mentioned in Table 1.1 might somehow be
fabricated in an amorphous state. Therefore, perhaps, sometime in
the future people will deal with an amorphous phase corresponding
to the chemical composition of MCPM (“amorphous MCPM”), an
amorphous phase corresponding to the chemical composition of
DCPA (“amorphous DCPA”), an amorphous phase corresponding
to the chemical composition of TTCP (“amorphous TTCP”), etc.
(in most cases, stabilization procedures will become necessary),
as well as with various mixtures thereof. Currently the majority of
such compounds remain unknown and, in the available literature,
a variety of ACPs is distinguished by the Ca/P ratio only. Since the
greater part of ACPs has the Ca/P ratio close to 1.5 (see below), a term
“amorphous TCP” (ATCP) becomes usual in literature [31, 140, 154,
158, 175, 184-191]. Other terms such as “amorphous carbonated
apatite” [40, 192, 193], “amorphous CaHPO
” [140], which is equal
to “amorphous DCPA” (Table 1.1), “amorphous Ca
4
”
[140], which is equal to “amorphous OCP” (Table 1.1), “amorphous
OCP” [142], “amorphous Ca
(HPO
)
(PO
)
8
4
2
4
4
” [194], which is equal to
“amorphous HA” (Table 1.1) and “amorphous dicalcium phosphate”
[195] are rare but have been already mentioned. One should note,
that ACPs with the Ca/P ratio < ~1.0 currently remain unknown.
(PO
)
(OH)
10
4
6
2
2.3.4.1 Precipitated ACPs
Although first described in 1953 [98], the quantitative chemical
studies on the precipitated ACPs were not reported until 1965,
when methods were devised to isolate large amounts of unstable
solids for analysis. To minimize changing during sample drying,
those methods utilized filtration and/or centrifugation to wash
excess ions from ACP slurries, then freezing wet ACPs under high
vacuum to remove any remaining entrapped solvent by sublimation
[128]. Early chemical studies [134, 196] on ACPs prepared at pH
~10.5, filtered, washed, and lyophilized, showed that the Ca/PO
4
molar ratio was very close to 1.5, suggesting a TCP composition
(as Ca
ions were found in it.
Furthermore, the electron spin resonance spectra of vanadyl (VO
(PO
)
·nH
O [103, 197]). No OH
−
3
4
2
2
2+
)
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