Biomedical Engineering Reference
In-Depth Information
present in skeletal tissues, would contain appreciable amounts of
carbonates, although less than those present in the apatitic phases
of bones [215]. Two other ions that readily incorporate into the ACP
structure are Mg
2+
7
4−
[37, 44, 133, 179, 216-218] and P
O
[215, 219,
2
7
4−
2+
220]. Ions such as P
increase the solution
stability of ACP and, in the case of the latter two ions, could possibly
play an important role in maintaining the presence of ACP in skeletal
tissue. In addition, other ionic substitutions are possible; however,
such inorganic additives alter the ACP composition, which would
enhance the negative effects in the biomedical application of ACP.
Besides, with a few important exceptions, ion-substituted forms of
ACP [34-47] are not discussed here.
O
, carbonate, and Mg
2
2.3.4.1 Other types of ACPs
Little is known on the chemical composition of ACPs, prepared by
other amorphization techniques. For example, various ACP samples
prepared by compressing of several calcium orthophosphates
at very high pressures revealed collapses of their initial crystal
structures but possible changes in their chemical compositions were
not investigated [160, 161]. Interestingly, but the authors found
that in the region below 550 cm
−1
the infrared spectra of DCPD in
amorphous phase resembled that of HA in the crystalline phase and
conversely the spectra of DCPD in the crystalline phase resembles that
of HA in the amorphous phase [161]. In the case of milling, calcium
orthophosphates were found to become amorphous; however, no
additional phases were detected [157, 158]. Presumably, this means
that during amorphization their chemical composition remained
unchanged.
Concerning the ACPs formed in plasma-sprayed HA coatings,
the authors of one study reported that “the amorphous phase
mostly consists of a dehydroxylated calcium phosphate” [174],
which, presumably, meant dehydroxylated HA. If so, the chemical
composition of that particular ACP should be close to amorphous
OA. The authors of another study considered “that the amorphous
phase substance consists of HA molecules” (Ref. [163], p. 227).
However, in the next study, the same authors mentioned that “the
plasma-sprayed amorphous phase is an oxyapatite” [164]. No
further clarification has been provided; however, all these authors
have come to the conclusion on the apatitic chemical composition
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