Biomedical Engineering Reference
In-Depth Information
chloroform solution, which served as a template for the CaCO
3
film formation. The
setup was kept in a “cold” room in which the temperature was maintained at 277 K.
Furthermore, Epple and co-workers reported a facile approach to precipitate large
quantities of ACC which is stable over a long time with and without additives by
using the Kitano method [
65
]. The pure ACC was obtained by bubbling CO
2
through
a saturated calcium hydroxide solution at 273 K. The precipitation was stopped
when the pH reached 8 and the precipitate was filtered and rinsed with cold acetone.
The product was dried under high vacuum at 273 K for 36 h and stored under P
2
O
5
in a desiccator.
6.2.5
Stabilizing Effect of Additives on Amorphous Calcium
Carbonate in Organisms
ACC is the most unstable form of calcium carbonate, it transforms quickly
into more stable crystalline forms under ambient conditions, such as calcite and
vaterite. Analyses of biogenic calcium carbonates have identified the presence of
magnesium, phosphate anions and silicate, which were associated with ACC and
poorly ordered calcite [
66
]. These additives are thought to be able to prevent crystal
nucleation and growth by interfering with the formation of an ordered crystalline
structure of CaCO
3
[
67
]. Moreover, heating ACC is one method to measure its
stability [
68
]. The ACC loses bound water when it is heated, and ACC crystallizes
to calcite (the thermodynamically stable polymorph) when the temperature is high
enough to overcome the kinetic energy barrier.
Addadi et al. found that cystoliths with a high concentration of silicate ions in the
ACC phase are more stable than the cystoliths devoid of silicate [
69
]. A potential
mechanism for stabilization of ACC by the silicate may be the destabilization of
calcite by “geometric frustration” of the crystal lattice (incorporating the planar
carbonate ions) [
70
]. The presence of the large tetrahedral silicate ions with four
negative charges in the ACC may conceivably destabilize calcite both by perturbing
the charge equilibrium and preventing regular packing. On the other hand, many
biogenic ACC examples contain large amounts of phosphate. The phosphate ion
has a tetrahedral structure too and has been suggested to stabilize ACC in the lobster
carapace by a mechanism similar to silicate ion.
6.3
Transformation of Amorphous Calcium Carbonate
Thanks to the ACC is not a stable phase from the viewpoint of thermodynamics, the
temporal stabilization was achieved by the incorporation of organic molecules and
magnesium ions in both biological and synthetic systems. It has been demonstrated
that the ACC could be temporarily stabilized and stored, until its templated
