Biomedical Engineering Reference
In-Depth Information
by the growth of particles. Rather, the inner area minerals were crystallized by
rearrangement of the atoms or clusters inside the particles [
11
]. This means that
the apatite of enamel is formed from ACP by direct transformation.
The artificial protein used in the TR-SLS experiment by Tsuji et al
.
contained
peptide motifs derived from DMP1. These motifs are expressed in bone and dentin
but not in enamel. It would be of interest to examine the mode of crystallization
by using the TR-SLS measurement technique and an enamel-specific protein such
as amelogenin. As described in the previous section, amelogenin, unlike DMP1,
induces HAP deposits without immobilization. This means that it can be simply
applied to SLS measurements. It would also be worthwhile to obtain information on
the crystallization of bone and dentin through in vivo observations.
An important study by Beniash et al. found that, not only the size and shape, but
also a high level of structural organization is formed before the phase transformation
from ACP to HAP [
11
]. This suggests that the body possesses mechanisms for
constructing hierarchical structures by controlling the organization of ACP particles.
Tao et al. observed that organic molecules can control the hierarchical assembly
of HAP by using ACP particles as building blocks [
73
]. HAP is formed by mixing
calcium and phosphate ions in the presence of poly(acrylic acid) (PAA). Under
these conditions, HAP spheres measuring 5 nm in diameter were formed. The
PAA enables apatite spheres to stably exist without growing or assembling. High-
resolution transmission electron microscope (TR-TEM) observation revealed that
these particles consist of a 5 nm HAP core and an ACP shell. When the PAA
was removed, the HAP particles formed colloidal aggregates. Even under colloidal
conditions, the layered structure of the HAP core and ACP shell was maintained, and
the particles were linked via ACP regions. SAED showed diffraction rings, meaning
that the particles were nanocrystalline aggregates packed with a random orientation.
The aggregates were 30 nm in size. When colloidal particles were dispersed in
water in the presence of biological additives, they re-assembled into highly ordered
structures. Needle-shaped single crystals and plate-like single crystals formed in the
presence of Gly (glycine) and Glu (glutamic acid), respectively.
These structures are formed in the following manner. In the presence of Gly,
core-shell aggregates line up. These aggregates are connected by ACP. At this
stage, the direction of the HAP core of each aggregate is random. Since ACP is
unstable and is transformed into HAP, the structures of connected regions of the
aggregates shift from HAP-ACP-HAP to HAP-HAP-HAP. Since ACP has a fluid-
like character, the HAP core within the ACP shell can rotate into the most stable
orientation. Simultaneous rotation of the HAP core and transformation from ACP
to HAP at the junction site results in alignment of adjacent HAP, which stabilizes
the overall structure. Aggregates of nanospheres with the same orientation are
thereby formed and become needle-like single crystals of HAP. This formation of
needle-like single crystals takes approximately 10 days. Because the crystals are
still surrounded by an ACP shell, they aggregate further. Thus, micrometer-sized
HAP crystals resembling enamel HAP crystals are formed. The formation of the
HAP crystals takes approximately 2 months.
