Biomedical Engineering Reference
In-Depth Information
neither normal nor pathological calcifications. It is used in calcium
orthophosphate cements [150, 164-171]. Besides, DCPA might be
implanted [172]. Other applications include using as a polishing
agent, a source of calcium and phosphate in nutritional supplements
(e.g.,
in prepared breakfast cereals, enriched flour and noodle
products), a tabletting aid [173] and a toothpaste component [134].
In addition, it is used as a dough conditioner in food industry.
1.3.5
OCP
Octacalcium phosphate (Ca
O; the chemically
correct name is octacalcium bis(hydrogenphosphate)
tetrakis(phosphate) pentahydrate) is often found as an
unstable transient intermediate during the precipitation of the
thermodynamically more stable calcium orthophosphates (e.g.,
CDHA) in aqueous solutions. Its preparation technique might be
found elsewhere [174-179]. A partially hydrolyzed form of OCP
with Ca/P molar ratio of 1.37 might be prepared as well [179, 180].
The full hydrolysis of OCP into CDHA occurs within ~6 hours [178].
Furthermore, OCP might be non-stoichiometric and be either Ca-
deficient (Ca/P = 1.26) or include excessive calcium (up to Ca/P =
1.48) in the structure [179]. Ion-substituted OCP might be prepared
as well [181]. Crystals of OCP are typically small, extremely platy and
almost invariably twinned.
The triclinic structure of OCP displays apatitic layers (with atomic
arrangements of calcium and orthophosphate ions similar to those
of HA) separated by hydrated layers (with atomic arrangements
of calcium and orthophosphate ions similar to those in DCPD)
[27-29, 182, 183]. A similarity in crystal structure between OCP
and HA [184, 185] is one reason that the epitaxial growth of these
phases is observed. morphologically, OCP crystallizes as {100}
blades of triclinic pinacoidal symmetry, elongated along the
(HPO
)
(PO
)
ยท5H
8
4
2
4
4
2
-axis
and bordered by the forms {010}, {001} and {011}. It is generally
assumed that, in solutions, the hydrated layer of the (100) face is
the layer most likely exposed to solution. The water content of OCP
crystals is about 20% that of DCPD and this is partly responsible for
its lower solubility. New data on OCP solubility have been published
recently [186].
OCP is of a great biological importance because it is one of the
stable components of human dental and urinary calculi [187-190].
a
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