Biomedical Engineering Reference
In-Depth Information
of which would be ACPs. Obviously, all types of high-temperature
ACPs are definitively anhydrous contrary to the precipitated ACPs.
Unfortunately, no adequate chemical formula is available to describe
the high-temperature ACPs.
In general, as all amorphous compounds are characterized by a
lack of lrO, it is problematic to discuss the structure of ACPs (they
are X-ray amorphous). Concerning a short-range order (SrO) in ACPs,
it exists, just due to the nature of chemical bonds. Unfortunately, in
many cases, the SrO in ACPs is uncertain either, because it depends on
many variables, such as Ca/P ratio, preparation conditions, storage,
admixtures, etc. It is well known that freshly precipitated ACPs
contain 10-20% by weight of tightly bound water, which is removed
by vacuum drying at elevated temperature [284]. Infrared spectra of
ACPs show broad featureless phosphate absorption bands. Electron
microscopy of freshly precipitated ACPs usually shows featureless
nearly spherical particles with diameters in the range of 20 to 200
nm. However, there is a questionable opinion that ACPs might have an
apatitic structure but with a crystal size so small, that they are X-ray
amorphous. This is supported by X-ray absorption spectroscopic
data (EXAFS) on biogenic and synthetic samples [285-288]. On
the other hand, it was proposed that the basic structural unit of the
precipitated ACPs is a 9.5 Å diameter, roughly spherical cluster of
ions with the composition of Ca
(Fig. 1.7) [28, 277, 289, 290].
These clusters were found experimentally as first nuclei during the
crystallization of CDHA and a model was developed to describe the
crystallization of HA as a step-wise assembly of these units [291]
(see section 1.3.10
(PO
)
9
4
6
) below). Biologically, ion-
substituted ACPs (always containing ions of Na, mg, carbonate and
pyrophosphate) are found in soft-tissue pathological calcifications
(e.g., heart valve calcifications of uremic patients) [27, 83-85].
In medicine, pure ACPs are used in calcium orthophosphate
cements [148-150] and as a filling material in dentistry [269].
Bioactive composites of ACPs with polymers have properties
suitable for use in dentistry [292-295] and surgery [296-299]. Due
to a reasonable solubility and physiological pH of aqueous solutions,
ACP appeared to be consumable by some microorganisms and, due
to this reason, it might be added as a mineral supplement to culture
media. Non-biomedical applications of ACPs comprise their using as
a component for mordants and ackey. In food industry, ACPs are used
for syrup clearing. Occasionally, they might be used as inert filler
. HA
(
or HAp, or OHAp
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