Biomedical Engineering Reference
In-Depth Information
because the nucleation barrier is very high at low supersaturations (cf. (
7.16
)and
(
7.21
)). The occurrence of the metastable structure at the nuclei may help to reduce
the nucleation barrier [
61
,
62
]; therefore, it is kinetically more favorable. On the
other hand, the nucleation barrier becomes much lower at higher supersaturations
(cf. (
7.16
)and(
7.21
)), and the structural relaxation of nuclei is not kinetically very
favorable. Therefore, the structure of pre-nucleation clusters can be as ordered as
that of the bulk crystalline phase.
The analyses on the kinetic data [
34
,
36
,
44
] confirm that, at low supersaturations,
the evolution of nuclei from a liquid-like structure to a crystal-like structure will
effectively reduce nucleation barrier and facilitate the nucleation dynamics. In other
words, at low supersaturations, the CNTs overestimate the nucleation barrier. At
high supersaturations, as the nucleation barrier decreases substantially. Adopting
the structure of the bulk crystals during nucleation need not be energetically
unfavorable. Therefore, the dynamic behavior predicted by the CNTs becomes valid.
7.3.4
Multistep Nucleation/Crystallization
in Biomineralization
In the case of biomineralization, it is often found that the most thermodynam-
ically stable calcium phosphate phase, hydroxyapatite (Ca
10
(PO
4
)
6
(OH)
2
or
Ca
5
(PO
4
)
3
(OH), HAP), does not occur first. Instead, the least stable phase, the
amorphous calcium phosphate (ACP) phase, occurs at the beginning and then
transforms into more thermodynamically stable phases in solutions. Pan and Liu
[
63
] applied the combined in situ UV-vis extinction detection and ex situ electronic
microscopy to examine the formation of ACP and the evolution to HAP under
the physiological pH and ionic strength condition in a simulated body fluid. The
UV-vis extinction spectroscopy was applied to monitor the mineralization process
(Fig.
7.7
).
According to the UV-vis extinction curves, the mineralization process can be
divided into four stages (Fig.
7.7
b). In stage I, after the mixing of a calcium solution
and a phosphate solution, ACP occurs, and the solution becomes turbid. In stage II,
the extinction of the solution arises and gradually reaches a plateau. In this stage,
the aggregation of ACP occurred (Fig.
7.8
a). In stage III, an abrupt decrease in the
extinction is observed, corresponding to the transformation of ACP mineral to HAP
crystallites (Fig.
7.8
b, e-g) [
64
,
65
].
Afterward, the crystallites undergo further growth, ripening and aggregation in
stage IV. The final products are some spherulites, composed of platelet crystallites
organized in the radial orientation (Fig.
7.8
c,i). The characteristic four stages in
the UV-vis extinction curves can be found from the solutions of different calcium
and phosphate concentrations [
63
]. The details of the morphological change and
phase evolution of ACP were captured by ex situ EM. The spherical-liked minerals
(215
˙
29 nm) were first formed (within 3 min, Fig.
7.8
a,d). The selected area
