Biomedical Engineering Reference
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this might be a reason for the above changing in the growth model
found for HA at pH = 5.0. To conclude, the polynuclear model seems
to be better elaborated among others because it is valid for apatite
dissolution within solution pH of 5.0-7.2, saturation degrees of 0.1
< S < 7, and Ca/P molar ratios of 0.1-20 [58]. However, nothing has
been published on a validity of this model in more acidic solutions.
7.2.3 Self-Inhibition (Calcium-rich Layer Formation)
Model
A model with a self-inhibition layer formation describes a formation
of a calcium-rich layer on the surface of apatite during dissolution
[19, 44-48]. This model was created from the results on dissolution
kinetics of apatite powders in acidic buffers at solution pH within
3.7-6.9 (the constant composition technique was used). Following
concentration of calcium and solution pH, the authors found
that during the initial period (2-5 min) of dissolution release of
calcium into the bulk solution was always less when compared with
consumption of H
+
. When dissolution progressed, rates of calcium
release permanently increased and hydrogen uptake decreased until
the equivalence (consumption of two hydrogen resulted in release
of one calcium) was reached, while the over-all rate of apatite
dissolution permanently decreased [19, 44-48]. Similar decreasing
of apatite dissolution rates were obtained by other research groups
[37-43, 63].
According to this model, apatite is dissolved by ionic detachment
of calcium and orthophosphate ions from the surface to a solution.
When an initial portion of apatite has been dissolved, some amount
of calcium cations (presumably, in connection with anionic counter
ions) is returned from the solution and adsorbed back onto the
surface of apatite. The latter results in formation of a semi-permeable
ionic membrane consisting of a positively charged layer containing
strongly adsorbed calcium ions
i.e. a calcium-rich layer is formed [19,
44-48]. An indirect confirmation of the presence of a calcium-rich
layer was obtained in the most recent paper of this series: for initial
strong calcium undersaturation in solutions, attainment of a pseudo-
steady state was very long [48]. The areas per adsorbed single PO
,
4
3−
2+
and Ca
ions on HA surface at the point of zero charge (pH = 8.5)
were estimated to be 33.1 ± 2.7 and 23.0 ± 2.1 Å
2
, respectively [64].
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