Biomedical Engineering Reference
In-Depth Information
longer period of time lose more structural water. Other studies have
shown more amorphous phase located adjacent to the substrate
and a gradient tending toward a lower ACP content at the top of the
coating [31].
To summarize, the amorphous phases in plasma sprayed HA
coatings are in intimated mixtures with both crystalline calcium
orthophosphates and other compounds, such as CaO [9], and up
to now nobody has ever succeed to extract ACPs from the blend.
However, the amorphous regions in plasma sprayed HA coatings
might be mapped using a scanning cathodoluminescence microscopy
technique [170]. Furthermore, due to a number of uncertainties, a
reproducibility of such experiments is poor; thus, the plasma spaying
technique is not considered as a valuable method to produce ACPs. A
flame spray synthesis, when a liquid precursor solution is fed through
a capillary into a burning methane/oxygen supporting flame, seems
to be more preferable to produce ACPs at high temperatures [175,
176]. Further details on and additional examples of ACP preparation
might be found in literature [31].
2.3.2.5 Irradiation
Just a few studies are available on the amorphization of calcium
orthophosphates (up to now, apatites only) by irradiation [177,
178]. However, due to the obvious risks caused by the induced
radioactivity, this amorphization approach is highly unlikely to
be ever used to prepare calcium orthophosphates for biomedical
applications. Unfortunately, it remains unclear in what extent the
structures, compositions, and properties of ACPs prepared by
various production approaches might be mutually compared.
To conclude the preparation part, one should briefly mention on an
interesting attempt to precipitate separately hydroxyapatites of Mg,
Ca, Sr, and Ba from basic supersaturated orthophosphate solutions
containing a 10 : 6 divalent cation / PO
molar ratio [179]. In the cases
of Mg, Ca, and Sr, the first precipitated phase had a 3 : 2 ratio (i.e., that
of a TCP), while only Ba went directly to the stable 10 : 6 HA phase.
Furthermore, the precipitated magnesium orthophosphate was
amorphous and remaining in the mother solution did not convert to
a Mg-deficient HA. The precipitated calcium orthophosphate formed
ACP, which converted to CDHA by a solution-mediated autocatalytic
mechanism [130]. The precipitated strontium orthophosphate was
4
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