Biomedical Engineering Reference
In-Depth Information
A similar high level of organization was seen in the presence of amelogenin
as well. Surprisingly, while 2 months were required to form this high level of
organization in the presence of Gly, the process completed in only 3 days in
the presence of amelogenin. This accelerated formation of needle-like aggregates
of HAP crystals parallel to the
c
-axis mediated by amelogenin was reported by
several groups [
49
,
50
,
53
]. This characteristic of amelogenin was confirmed in an
experiment using HAP core-ACP shell particles.
On the other hand, in the presence of a Glu, HAP core—ACP shell particles were
seen to reorganize into a different type of hierarchical structure and form a plate-
like single crystal with a large surface (0001), which resembled the HAP crystals
found in bone. In a control experiment, no highly organized structure was formed
even with the addition of Glu, Gly, or amelogenin when pure HAP crystal particles
without an ACP shell were used.
Although several experiments showed crystal formation due to the aggregation of
nanocrystals [
73
,
74
], the one by Tao et al. showed something particularly important:
ACP connects HAP at the different crystal surfaces and forms various hierarchical
structures depending on the condition of various biomolecules. Biomolecules added
to the reaction determine the assembly orientation of the nanocrystals and accelerate
the aggregation and transformation.
The role of Gly and Glu in the control of the morphology of nanocrystal
aggregates was clarified using molecular dynamics simulation. The interfacial
energies of the (0001) face and the (1
1
00) face of HAP in water are similar. The
effects of Gly and Glu adsorption on these faces on their interfacial energies were
examined. Adding Gly markedly raised the energy of the (0001) face and slightly
reduced that of the (1
1
00) one. Thus, HAP (0001) faces become unstable in Gly
solution, resulting in packing on the (0001) face and formation of a hierarchical
structure along the
c
-axis. In contrast, simulation showed that the (0001) face is
significantly stabilized by Glu while the (1
1
00) face is not affected. This indicates
that packing takes place at the (1
1
00) face, resulting in aggregation along the axis
vertical to the
c
-axis, with preferential formation of a plate-like superstructure.
The simulation results explain the experimental observations, i.e., formation of
needle-like HAP crystals and plate-like HAP crystals with Gly and Glu, respectively
(Fig.
4.4
).
4.7
Controlling Mineralization with Artificial Proteins
and Peptides
An important molecular biological method is display technology. Typical display
technologies include phage display [
75
], cell surface display [
76
], ribosome display
[
77
], and mRNA display [
78
,
79
]. Among these, phage display and cell surface
display have been widely used and have been applied to inorganic materials [
80
].
A phage is a virus that infects
E. coli
and has a structure in which its genome is
