Biomedical Engineering Reference
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The apparent absence of observable crystalline features in ACPs
does not rule out a possibility that ACPs might have a well-defined
local structural unit. The compositional constancy of ACPs over
a relatively wide range of the preparative conditions suggests an
existence of such a core structure [104]. Several lines of evidence,
however, indicate that the local unit of ACPs is not a crypto-structural
variant of one of the crystalline calcium orthophosphates. For
example, although in the majority of cases precipitated ACPs appear
to be closest in composition to TCP, in aqueous solutions ACPs
transform into either OCP or CDHA but not into TCP. If ACP were
a cryptocrystalline TCP, direct growth into observably crystalline
TCP by crystal ripening would be expected to occur. However, a
possibility that ACP might be a cryptocrystalline forms of CDHA
or HA cannot be as easily precluded. Only below pH ~ 9 CDHA can
be ruled out for the same reason as TCP. However, even above this
pH, the finding that CDHA forms primarily as an outgrowth from
the surface of ACP suggests that an
ripening process does
not occur. Furthermore, dissimilarity in composition rules out ACP
as being made up of highly disordered arrays of OCP unit cells. It
is equally unlikely that ACP is an orthophosphate-deficient OCP as
the structural integrity of the latter depends on a full complement
of orthophosphate groups in the unit cell. The finding that the Ca/P
molar ratio remains relatively constant at ~1.5 over a relatively wide
range of pH (7.4-9.25) appears to preclude ACP as a cryptocrystalline
mixture of OCP and CDHA. Instead of remaining constant, a rise in
pH would expect to increase the overall Ca/P molar ratio of such a
mixture as the relative proportion of the OCP and CDHA components
would shift in favor of the latter [20].
Based on the relatively constant Ca/P ratio of ACPs formed under
varying precipitation conditions, such as different concentrations
of calcium and orthophosphate ions, solution pH and different
temperatures, as well as spectroscopic and structural analyzes,
Posner and Betts hypothesized that the initial solid phase of ACPs
(more precisely, of an amorphous TCP) precipitated
in situ
consisted
of spherical particles ranging within 300-1000 Å in diameter with
water molecules in the interstices [104, 224, 225]. Namely, the
defining structural unit of ACP is a spatial subset of the HA unit cell
consisting of a central Ca
in vitro
ion coordinated by the oxygens of six
surrounding orthophosphate groups, which, in turn, are stabilized
by another 8 Ca
2+
2+
ions spherically distributed around the outer
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