Biomedical Engineering Reference
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through a layer containing calcium with or without counter ions [19,
44-48, 63]. In part, this idea can be found within the ion exchange
model where a monolayer of calcium citrate [114-116] and/
or calcium-acid complexes [117, 118] are assumed to form onto
apatite during dissolution. However, no other model requires such
suggestion. According to the rest of them, formation of any calcium
containing compounds (different from acidic orthophosphates) on
apatite is not discussed at all.
According to the chemical model, the initial stages of apatite
dissolution consist of calcium detachment from the surface and
incorporation of protons instead. Orthophosphate groups are
assumed to keep their positions without any relocation [91-93]. Due
to the fact, that calcium occupies definite lattice positions, whereas
protons are bound to oxygen ions of orthophosphate groups,
removing of each calcium results in decreasing of attraction forces
between the nearest (to this calcium) orthophosphate group, and
rest part of the crystals. When all (or almost all) neighboring cations
of calcium have been removed, orthophosphate groups (as H
PO
4
−
,
2
4
+
CaH
— it is not clear yet) also detach from the surface.
Then they diffuse along the surface away from the dissolution steps
before entering the solution as described above for calcium [126,
140].
In the case of crystal faces with perfectly smooth surface
(dissolution steps are absent), detachment of one or several ions
results in formation of dissolution nuclei, which the polynuclear
model is based on [50-58]. According to this model, all sites in
nuclei edges are the kink positions [57] and, after appearance, the
nuclei grow and spread over the surface with a definite lateral rate
[51], giving rise to formation of dissolution steps. If dissolution
steps already existed on the crystal faces of apatite, detachment of
one calcium or one orthophosphate would result in dissolution step
movement jump-wise over a distance equal to the sizes of these ions
(approximately for 1 and 3 Å, respectively).
The latter is also valid for dissolution of crystal edges and
corners. Since the classical paper by Stranski [149], it is generally
considered, that ions are less strongly bound along edges of a crystal
than in the middle of faces and still less strongly at the corners
[150]. No information about the ionic arrangements on edges and
corners of apatite crystals has been found in literature. Therefore,
two boundary conditions are possible: the edges and corners mainly
PO
or H
PO
2
3
4
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