Biomedical Engineering Reference
In-Depth Information
phase transformation of PSS-ACC in water-ethanol solution at room temperature
[
120
]. The higher PSS content and lower water amounts are two key factors to
improve the formation of a large amount of hollow nanospheres with different sizes.
The higher content of PSS promotes the formation of hollow vaterite nanospheres
with smaller sizes. On the other hand, an increasing of water amount would result
in a formation of less vaterite nanospheres. Interestingly, the formation of hollow
vaterite nanospheres is self-templated by original PSS-ACC precursors.
Furthermore, Yu and co-workers found that 1,3-diamino-2-hydroxypropane-
N
,
N
,
N
0
,
N
0
-tetraacetic acid (H
4
dhpta), a low molecule weight organic molecule
with carboxyl functional groups, could stabilize ACC [
121
]. By interrupting the
reaction at different time intervals, a whole crystallization process was successfully
observed. It is found that the first formed ACC has two forms. One exists in solution
and conglomerates into sphere-like precipitates. The other forms a close packed film
on the substrate. The film is easily ignored as it is so dense and flat that it could be
recognized as a background of the precipitate just like substrates, also for its disap-
pearance after the crystallization finishes. However, this film is the key to inducing
heterogeneous nucleation, as precipitated ACC conglomerates dissolve during the
crystallization process. As various ACC films have been found, we suggest that
the nucleation on ACC films may be the common behavior for heterogeneous
nucleation. Thus, it is possible to combine dissolution-recrystallization [
122
]and
the substrate physical change theory [
123
] into one mineralization system. After the
nucleation, the intermediates at different crystallization stages were also captured.
These intermediates were found to be composed of fibres. A rod-dumbell-sphere
transformation phenomenon was observed (Fig.
6.19
)[
121
]. It is believed that these
are frameworks for the final crystal formation. In situ transformation of each fibre
is brought forward to explain the formation of the final similar calcite hierarchical
structures. As H
4
dhpta here could both serve as ACC stabilizer and crystallization
controller, it is expected to act as the ideal molecule for studying the way to stabilize
ACC, and the crystallization and transformation process of ACC.
6.4
Summary and Outlook
In summary, this chapter provides a general overview on syntheses and transforma-
tion of ACC. It should be noticed that the synthetic strategies and crystallization
of ACC have achieved great progress till now. The main preparation methods for
ACC including the direct-mixing method, gas diffusion technique, hydrolysis of
carbonate source method and Kitano method, have been discussed. Subsequently,
crystallization of ACC in different environments has been discussed, demonstrating
that the transformation of ACC plays an important role in biomineralization. Several
important transformation routes of ACC are selected for more detailed discussion.
Current progress in this field demonstrates that there are a number of potential
opportunities for both syntheses and transformation of ACC, and CaCO
3
crystals
with different morphologies, structural features, and functionalities. In future, it is
