Biomedical Engineering Reference
In-Depth Information
2.4 RELATED CALCIUM PHOSPHATES
Apatites and other calcium phosphates that occur in biologic systems are listed in
Table 2.1. The crystallographic properties of synthetic calcium phosphate are
summarized in Table 2.5. In biologic or synthetic systems, non-apatitic calcium
phosphates (e.g., ACP, DCPD, DCPA, OCP,
-TCP) can transform to apatites and
substituted apatites or to other calcium phosphates by dissolution-precipitation
processes as represented in Figure 2.8 [64] .
Non-apatitic calcium phosphates are found in pathologic calcifi cations but
not in normal mineralized tissues (mineral phases of teeth and bones). Non-
apatitic calcium phosphates, except tricalcium phosphate (TCP) and tetracalcium
phosphate (TTCP), can be prepared directly by precipitation or in gel systems
(Figure 2.9) or indirectly by hydrolysis methods [64,69]. From solutions of similar
Ca/P molar ratios, different types of calcium phosphates are obtained depending
on the solution pH, temperature and composition [57,64]. In the presence of F
−
ions, apatite can form even from solutions with low pH. (e.g., pH 4 at 95 °C).
The different types of calcium phosphates are characterized by their Ca/P
molar ratios (Table 2.5), their characteristic morphology and dissolution proper-
ties (Figure 2.10) [22,64,66] However, the morphology and crystal size can be
affected by other ions present in solution (Figure 2.9). For example, the usual
platy morphology of DCPD appears as smaller thick rods in the presence of
P
2
4−
ions [64,69]; OCP can assume a platy or ribbon-like morphology [61,64].
The different types of calcium phosphates also differ in their solubilities
(Figure 2.10) decreasing in the order:
β
ACP
>
DCPD
>
OCP
>
β
-TCP
>
CDA
>
HA
.
2.4.1 Amorphous Calcium Phosphates (ACP)
ACP can be represented by the formula, (Ca, X)
x
(PO
4
, Y)
y
.H
2
O, where X = Mg
2+
,
Zn
2+
, Sn
2+
or Al
3+
ions;
YCO
2
, or
P
2
4−
ions [57,59,64,76,77,90,91]. Depending
on the composition, the Ca/P molar ratio can range from 1.3 to 2.5. At room tem-
perature or 37 °C, ACP can form under any of the following solution (containing
Ca
2+
and
PO
3−
ions) conditions: high pH (pH .10), high CO
3
/P molar ratios, Mg/
Ca molar ratio greater than 0.4, Zn/Ca molar ratio greater than 0.4, small concen-
trations of
P
2
4−
, critical concentrations of Sn
2+
, Al
2+
[64,76,77,91] . ACP is charac-
terized by absence of diffraction peaks in x-ray diffraction profi les except for a
broad peak with a maximum at about 30.40 ° 2
−
=
3
(e.g., shown in Figure 2.6). The
FT-IR spectra of ACP shows lack of resolution of the PO
4
absorption bands. ACP
compounds are represented by hollow spheres in transmission electron
micrographs [64,90]. Ions that promote formation of ACP can act synergistically,
e.g.,
Mg
θ
P
2
4
[64,90]. ACP incorporating other ions besides calcium
and phosphate ions remain stable even after heating at 400 °C [64,90]. Stability of
ACP in solution depends on the ACP composition or solution composition [64,90].
2
+
2
−
−
+
CO
or
+
3
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