Biomedical Engineering Reference
In-Depth Information
seriously affected, i.e., their sizes were reduced
(
Figure 16.11
). The size, number, and distribu-
tion of the
β
sheets seem to determine the
mechanical properties of the silk
[112]
, showing
good agreement between theory and experi-
ment in this particular case.
Similarly, increased mechanical toughness
has been observed when dried collagen is pro-
cessed by ALD diethylzinc or titanium(tetra-
isopropoxide)
[46]
. More in-depth investigation
has shown that the effect results from the incor-
poration of metal into the bulk of the collagen
during the ALD (
Figure 16.12
). Evidently, the
lack of solvents (due to the vacuum-based pro-
cessing) permits a diffusion of the reactive spe-
cies into the collagen structure and induces
small changes with significant effects.
The overall mechanical toughness was increased
threefold, simultaneously increasing the stress
and strain of the measured substrates. The origin
of the property change has not been resolved in
detail yet. However, investigations until now
have shown that, besides the presence of the
metal inside the protein matrix, some chemical
and physical changes occur.
Based on investigations of the infiltrated col-
lagen by XRD, Raman spectroscopy, infrared
spectroscopy, etc., a model has been proposed
that considers chemical bonding of the metal to
the protein helix and the corresponding change
in the crystallinity of the material
[46]
. The struc-
tural and chemical properties of collagen differ
from those of spider silk. Spider silk shows a mix-
ture of amorphous protein and
β
sheets; collagen
FIGURE 16.11
Schematic description of proposed molecular changes in the silk modified by MPI. Together with the
water pulses, Al
3
+
weakens the hydrogen bonds and inserts into the broken bonding sites, thereby resulting in the formation
of metal-coordinated or even covalent bonds with the Al. Reprinted from Ref.
111
. Copyright © 2009, with permission from
the American Association for the Advancement of Science.
