Biomedical Engineering Reference
In-Depth Information
transition to the crystalline form is induced and biomimetic syntheses of crystalline
calcium carbonate thin films and particles via transient ACC have been widely
investigated by many researchers.
6.3.1
Crystallization of Amorphous Calcium Carbonate
in Biomineralization
Addadi et al
.
[
71
] pointed out that the first suggestion of a transient ACC precursor
in mollusk biominerals by Hamilton and Towe, who back in 1968 observed hollow
“crystals” in the developing nacre of the bivalve, and found that “a possibility exists
that the incipient calcification is not in the form of aragonite but rather in some
other phases”. To our knowledge, the first paper to describe a transitory ACC in
biominerals was that by Beniash et al. in 1997 [
72
], who observed spicule formation
in sea urchin larvae. The spicules are comprised of single-crystalline calcite, but
upon finding the presence of an additional ACC phase which transformed into
calcite with time. This group also shows ex vivo evidence of an amorphous precursor
in the regeneration of the adult urchin spine, as well as the teeth of sea urchins. In
addition, it has been demonstrated that organisms may use ACC as a metastable
precursor to form single crystals with complex shapes. Interestingly, Addadi et al
.
[
73
] found that sea urchin spine regeneration proceeds via the initial deposition
of ACC. Most echinoderms probably all use this same mechanism because they
produce the same type of skeletal material.
Therefore, sea urchin larval spicules have long served for the investigation of
CaCO
3
biomineralization procedure as a model system. The mature larval spicule
is composed of a single crystal of calcite [
74
,
75
]. Furthermore, the study result
showed that even at early stages, when the CaCO
3
is still predominantly amorphous,
it already has a nascent short-range order around the calcium ions similar to that
in calcite. Politi et al. used X-ray photoelectron emission spectromicroscopy with
probing size of 40-200 nm to investigate the transformation mechanism of ACC
into calcite in the sea urchin larval spicule [
76
]. They resolved three distinct mineral
phases: an initial short-lived hydrated ACC phase, followed by an intermediate
transient form of ACC, and the biogenic calcite phase. A secondary nucleation
mechanism was proposed to explain the transformation of ACC into calcite in the
sea urchin larval spicule. Based on the obtained experiment results, they proposed
that a transformation from type 2 amorphous to type 3 crystalline phase propagates
through the spicule via secondary nucleation, in which the crystallization of one
amorphous unit promoted the transformation of the domains in contact with it. In
addition, the propagation pathway through the spicule is inferred to be tortuous and
complex, implying that the transformation rate depends on the size and interface
structure of the amorphous domains.
In order to understand the mechanism of the crystallization, Tremel et al. reported
a time-resolved investigation of the contact-free homogeneous formation and
