Biomedical Engineering Reference
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and orthophosphate concentrations (from ~0.002 to ~1 mol/l), as
well as at temperatures within 0°C to 50°C. However, the Ca/P ratio
of the mixing reagents (classically, Ca(NO
)
is typically kept within 1.50-1.67, while a basicity of the mixing
solutions is frequently created by NH
)
·4H
O and (NH
)
HPO
3
2
2
4
2
4
OH addition [48, 116, 128-
132]. At acidic pH, crystalline calcium orthophosphates normally
are precipitated. However, in presence of stabilizers (magnesium
and/or citrates), ACP could be precipitated at solution pH within
6.0-6.5 [133]. No information on ACP precipitation from even more
acidic aqueous solutions has been found in literature. The obtained
precipitates should be collected shortly after the preparation (the
sooner, the better), because in aqueous media ACP is spontaneously
converted to the crystalline calcium orthophosphates, mainly to
CDHA [116, 134]. Furthermore, it was shown that the final calcium
orthophosphate (a dry powder) would be amorphous if, beside the
appropriate key factors of the synthesis (a high concentration of
reagents, a basic solution pH, a rapid mixing, and a low temperature),
both a high addition rate and a mandatory freeze-drying of the
precipitates were employed [40, 116, 132, 135].
In all wet-precipitation techniques, the amorphous precursors,
although related to the final CDHA phase, are differed from the final
phase in atomic structure, particle morphology, and stoichiometry.
For example, the X-ray diffraction pattern of ACP (Fig. 2.2, bottom),
if compared to those of CDHA (Fig. 2.2, middle) and HA (Fig. 2.2,
top), shows a single and a very broad diffraction peak, typical for
amorphous materials, which lack the atomic LRO characteristics of
all crystalline materials, including HA [104]. The precipitated ACP
phases appear to be spherical (Fig. 2.3a,b) in an electron microscope
(diameter ca. 30-100 nm), unlike the needle-like crystals of CDHA
(Fig. 2.3c,d). The solution pH, concentration of the mixing reagents,
and a preparation temperature all affect the ACP particle sizes;
namely, a higher supersaturation produces smaller ACP particles
[129]. Although ACP can be prepared with a Ca/P molar ratio as
low as ~1.2 (at low pH — see Fig. 2.4) or as high as 1.7 (at high
supersaturation), the departure from a Ca/P of ~1.5 has been shown
to be due to surface-adsorbed soluble phases those can be washed
away or to occluded Ca, respectively [104].
4
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