Biomedical Engineering Reference
In-Depth Information
properties of pearls have many advantages over corresponding synthetic crystals
[
12
,
13
]. The formation of biominerals is complicated in nature, and organic
templates are believed to be crucial to their crystallization and final morphologies
and functions [
14
]. Inspired by the natural biomineralization procedure, researchers
in the fields of material science and chemistry are attempting to repeat such control
in lab. However, no synthetic materials have until now shown properties that
can compete with those of their natural counterparts. So, many of the secrets of
biomineralization need to be studied and understood.
CaCO
3
is one of the standard model systems for studying biomineralization
because of its abundance in nature and important industrial applications (use as
filler in plastic, paper, and in decorative paints) [
15
]. In parallel, calcium carbonate
has been widely used as a model mineral in biomimetic research, leading to
increased understanding of biogenic control over mineral orientation, morphology,
and polymorph [
16
]. The polymorphs of calcium carbonate include three anhydrous
crystalline phases (calcite, aragonite and vaterite), two hydrated ones (monohydrate
and hexahydrate), and amorphous calcium carbonate (ACC) [
17
-
22
]. Calcite and
aragonite are by far the most common and stable forms, whereas ACC, a least stable
polymorph from the viewpoint of thermodynamics.
ACC has been widely found in biological organisms, where it plays an important
role in the biomineraization of CaCO
3
[
23
-
25
]. It is generally accepted that biogenic
ACC can be broadly as one of two forms: stable or transient. While the stable form
remains non-crystalline, the transient phase can act as a precursor to either calcite
[
26
,
27
]oraragonite[
28
,
29
] (calcite and aragonite are by far the most common in
biological and geological samples). It is found in various organisms like Mollusk
shells, American lobsters, ascidian skeletons, plant cystoliths, and so forth [
30
-
34
].
In addition, evidence has been found to suggest that calcific biominerals, such as the
mollusk nacre and the urchin spine, are formed from an amorphous precursor [
35
-
37
]. The stable ACC is also formed by organisms and used as a skeletal material.
Interestingly, ACC contains various amounts of water, which makes its density
lower than other forms of calcium carbonate [
38
,
39
].
Calcium carbonate crystallization in biological organisms often occurs through
the transformation from the amorphous precursor [
40
]. Due to that 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 [
41
-
43
]. It is believed that the ACC could be tem-
porarily stabilized and stored, until its templated transition to the crystalline form is
induced [
44
]. Biomimetic syntheses of crystalline calcium carbonate thin films and
particles via transient ACC have been reported by many researchers [
45
-
48
].
This chapter will discuss in detail about the syntheses and transformation
of ACC. The aim of the present chapter is to review the synthetic strategies
and crystallization of ACC reported till date. Firstly, a general overview on the
synthesis of ACC will be given. The direct-mixing method, gas diffusion technique,
hydrolysis of carbonate source method and Kitano method will be introduced and
discussed. Subsequently, a general overview on the crystallization of ACC will be
discussed. Finally, the summary and perspectives on this field will be given.
