Biomedical Engineering Reference
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not amorphous but poorly crystallized and readily converted in
solution to a strontium HA. Thus, the smaller alkaline earth cation
systems tended to form the more stable amorphous 3 : 2 compounds
[179].
2.3.3
Morphology of Precipitated ACPs
When viewed by TEM, ACP solids precipitated from aqueous solutions
usually have a curvilinear appearance rather than the faceted and
angular shape of crystalline calcium orthophosphates [180, 181].
However, this curvilinear aspect has only been clearly established
for dried ACP. The morphological form of highly hydrated flocculent
solids that appear initially in freshly precipitated ACP suspensions
is not known. What is observed when drops of these suspensions
are placed on carbon-coated grids, excess solution removed, and
air-dried are irregularly shaped, anastomosing aggregates of low-
contrast, disk-shaped particles varying widely in lateral dimensions
(from ~0.01 µm to 5+ µm) [181, 182]. These highly flattened
particles represent collapsed, de-solvated residues of the initial wet
ACP flocculates.
As ACP suspensions age, high contrast particles with a more
spherical aspect begin to appear, initially evolving as bud-like
extensions from the disks [181, 182]. With time, these spherical
forms become the dominant shape for ACP. Although generally
smaller (20-300 nm in diameter) than the disks they supplant, the
spherical forms, like the disks, frequently aggregate into irregularly
shaped and branching clusters. The progression from disk-shaped
to ball-like particles most probably represents a spontaneous
desolvation
or the initial gel-like flocculates into smaller,
denser, less hydrated structures [183]. That the spherules are
formed in suspension and are not a drying artifact is supported
by the crystallization behavior of ACP preparations. Although the
evolution of a spherical morphology would be favored during
consolidation as this shape minimizes interfacial tension with the
surrounding solution, it also requires that the contracting surface be
isotropic. This is possible for uniformly curved surfaces only when
the enclosed structure remains non-crystalline while desolvating
[20].
in situ
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