Biomedical Engineering Reference
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ratio both in the solution and on the crystal surface or differences
in chemical and structural compositions of apatites studied by
different researchers (both FA and HA are known to be often non-
stoichiometric and calcium deficient, especially on the surface [2-4,
82-84, 90]).
The idea on a great influence of a surface layer is supported by
the data on dissolution of stoichiometric HA (Ca/P = 1.67) in aqueous
solutions within pH 4.90-9.94 [71]. The authors found that after
removal of an irreversible surface, which generated excessive calcium
and orthophosphate solution concentrations, further behavior of HA
became reproducible. A surface of one atom-layer thickness or less
was believed to have formed during the preparation of the solid [71,
p. 697]. On the other hand, dissolution of HA in aqueous medium
appeared to be always non-stoichiometric at the beginning, but
when the solid was successively equilibrated at any given pH, the
solution Ca/P ratio approached a limiting value of 1.67. Once this
value was reached, the solid only maintained this solution ratio by
dissolving stoichiometricaly [71, p. 700]. The authors explained
these phenomena by a previous history of the samples studied. A
similar conclusion was also made in another paper [69]. Thus, the
experimental techniques of apatite preparation might have a great
influence to stoichiometry/non-stoichiometry of dissolution. This is
a serious drawback.
Finally, the results on theoretical analysis of surface layer
formation on HA should be reported [80]. The main point of that
paper is an idea that both the stoichiometric HA (Ca/P = 1.67) and
any non-stoichiometric CDHA (Ca/P ratio within 1.5-1.67) might
be described as the same substance. More precisely, there is a
continuous amount of intermediate compounds within 1.5
≤
Ca/P
≤
1.67 having the same crystal structure which is drawn as a line on
the solubility diagram CaO-P
O [80]. Furthermore, according
to the authors, at the steady state, both HA and CDHA are dissolved
congruently only when both the solution and solid have the same
Ca/P ratio. Therefore, any sample of HA and CDHA is dissolved
incongruently except the only one solution composition having the
similar Ca/P ratio with that particular sample. In all other cases, a
surface layer having a Ca/P ratio less than 1.67, which is often used
as the indication of incongruent dissolution of apatite, will form
as a result (this, however, depends on the accuracy with which the
analyses can be made) [80]. Thus, whether apatites are dissolved
O
-H
2
5
2
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