Biomedical Engineering Reference
In-Depth Information
the aragonite crystals. In a more homogeneous
material the cracks would propagate and cause
a failure, but the proteinaceous layers absorb
energy by deforming elastically and distribute
part of the energy as crack formation in many
other aragonite crystals. The result is that not
only is a failure prevented, but many small
microcracks are created. However, to avoid that
repeated attacks from predators break the shell,
the microcracks need to heal. This is assumed to
happen as self-healing caused by a combination
of the very small dimensions of the cracks and
the energy being released from the proteina-
ceous layers.
Addadi
et al
.
[53]
describe a model for how
the layers are produced and aragonite crystals
are formed. In order to make the layered struc-
ture, a biological cascade with several steps is
followed. The mollusc mantle secretes the highly
cross-linked protein layer that is called the
peri-
ostracum
from its ectodermic (epithelial) cells. In
the space between the mantle and the periostra-
cum, the epithelial cells in the mantle form a
gel-like framework matrix of various macromol-
ecules. It is in this matrix that the aragonite min-
eral forms. The matrix consists of hydrophobic
proteins (polysaccharide
β
-chitin) and hydro-
philic proteins that are rich in aspartic acid.
Within the matrix, colloidal particles of chemi-
cally unstable amorphous calcium carbonate
(ACC) may also be present. Possibly, they are
isolated from the aqueous environment by vesi-
cle lipid membranes. Definitely at the first peri-
ostracum layer, nucleation sites are laid out.
Each nucleation site equals an aragonite crystal
that has a typical size of 10
×
10
×
0.5
μ
m
3
. The
crystals grow from the nucleation site upward,
as shown in
Figure 13.19
. As the growing crystal
reaches the upper periostracum layer, the
growth continues laterally until the crystal
meets other crystals. Between the crystals,
β
-chitin is trapped. Maybe new nucleation sites
are being laid out on each periostracum layer or
the crystallization continues through small holes
in the periostracum.
FIGURE 13.19
The formation of the aragonite-protein
structures. 500-nm-thick layers of gel are laid out between
30-nm-thin protein layers. Aragonite crystals grow from
nucleation sites, first upward (a) and then sideward
(b). Protein particles in the gel are “pushed” in front of the
crystal and form the vertical protein borders between cells
(c). Drawing based on the description in Ref.
53
.
The micro- and nanostructure in nacre is inter-
esting for material development in many areas.
Much effort has gone into making materials with
better strength and toughness properties as well
as materials for biomedical applications such as
bone and dental analogs. A less explored area is
of photonic multilayer structures, where selective
reflection of narrow or broadband light through
interference could make possible coatings that
reflect light like metal without having the thermal
or electric conducting properties typical of metals
[57, 58]
. A process of layer formation and bio-
mineralization similar to nacre production would
be attractive if the translucent calcium carbonate
could be replaced with a transparent substitute.
See Chapter 11 on structural colors.
Munch and colleagues describe a technique
called
ice templating
of ceramic alumina and
polymethyl methacrylate (PMMA) into a very
