Biomedical Engineering Reference
In-Depth Information
varying the temperature conditions in air and air humidity, without the aid of a
functional template. At high temperatures, ACC is transformed into the crystalline
form via a solid-solid transition, whereas under conditions of high humidity, ACC
tends to dissolve and recrystallize. The crystallization mechanism is expected to be
determined by the increased thermal energy when the ACC film is heated to high
temperature [
91
]. The high temperature will promote the transformation of ACC
by supplying more energy, which will allow the ionic species in ACC to cross the
energy barrier for realignment. On the other hand, the rate of ACC crystallization
increases with increasing relative humidity.
Gower and co-workers reported that a PILP mineralization process may play
an important role in biomineralization [
92
]. The PILP process is induced with
polyanionic polypeptides to produce colloids of an amorphous mineral that are
so highly hydrated that they coalesce into smooth mineral films. Transition bars
have been monitored in situ during the crystallization of ACC films deposited under
Langmuir monolayers and on glass slides via the PILP process. In addition, optical
microscopy was used to observe the amorphous-to-crystalline transformation in
situ. The crystallographic anisotropy of the polymer occlusions and sectorization
of tablets in the PILP system may help to explain some of the crystal textures found
by ex vivo analysis of biominerals (e.g., seminacre and nacre).
In addition, the ACC precipitates are initially formed from highly supersaturated
solutions, which then deposit as films through the cooperation between a soluble
inhibitor and an insoluble matrix [
93
]. The inhibitor and matrix are also found to
affect the morphology, growth, and structure of CaCO
3
crystals by influencing the
transformation of ACC into crystalline forms. The generation of patterned CaCO
3
films with dimensions of several hundreds of micrometers using the photolitho-
graphic properties of an amphiphilic ABA block copolymer and the possibility to
shape CaCO
3
in its amorphous state has also been reported [
94
]. Furthermore, Klok
et al. [
95
] demonstrated the use of brushes to act as ionotropic matrices for the
directed synthesis of microstructured calcite thin films. The acidic macromolecules
are used to form an ionotropic gel, which can stabilize and host a transient
ACC phase and serve to direct crystal growth into geometries defined by the
macromolecular matrix. The effect of chain mobility and functionality of selected
polymer substrates has been investigated by Cho and co-workers [
96
]. It was found
that how the chain mobility and functionality of selected polymer substrates with
ester groups in either the backbone or side chains affect the crystallization of ACC
films fabricated through biomimetic methods. Micropatterned calcium carbonate
films consisted of regularly aligned calcite oriented along the (104) direction can be
selectively obtained on the patterned NDR/PAA multilayer film [
97
].
6.3.4
Crystallization of Precursor on Monolayer
The Langmuir monolayer is widely utilized for investigating biomineralization
on biological membranes. The crystallization process of CaCO
3
on a stearic acid
