Biomedical Engineering Reference
In-Depth Information
FIGURE 16.12
Speculative conformational change of collagen chains after metal infiltration. The repeating sequence is
Gly-Pro-Hyp. The
z
coordinates are listed (left side). The right side shows a schematic description conjectured from WAXS
and Raman spectra. Reprinted from Ref.
46
. Copyright © 2010, with permission from the American Chemical Society.
mainly consists of protein helices. The hydrogen
bonding between the functional groups of the col-
lagen seems to be affected by intercalation of the
metals, which eventually leads to new reflections
observed by XRD as well as a new bonding situ-
ation that was observed by Raman spectroscopy.
Again, indications could be found that chemi-
cal interactions of the metal-organic precursor
with the protein occur. One may argue that this
process will inevitably lead to a coating of the
fiber in parallel and that the ceramic (titania or
alumina) coating may have strong impact on the
mechanical properties of the resulting product.
For this reason, the same process was also per-
formed in a regular coating mode (exposure
time equal to pulsing time) with very minor
influence on the mechanical properties of the
fiber
[113]
. The process with extended exposure
times for infiltration of the proteins with metals
was named
multiple-pulsed vapor phase infiltration
(MPI) to distinguish between the original coat-
ing mode and the infiltration mode.
Processes of improving or modifying mechan-
ical properties by controlled and organized incor-
poration of inorganic minerals into an organic
matrix exist in biology. Such processes are col-
lectively known as
biomineralization
. A very com-
mon example is the formation of bone. The
different mechanical properties of various types
of bone (woven bone, cortical bone, etc.) are
derived from the varying amount of mineral con-
tent (e.g., HA) within a matrix of collagen and
other types of proteins
[114]
. Because the ALD
processing of spider silk and collagen leads to
similar outcomes, this process mimics biominer-
alization. Another example of biomineralization
mimicry was discussed in Section
16.2.1
: the ALD
of TiO
2
within the cavity of apoferritin. The pro-
cess is in a sense comparable to the mineraliza-
tion of apoferritin to ferritin
[53]
.
Those initial approaches toward the mimicry
of biomineralization by ALD were rather curiosity-
driven. The infiltration is a very interesting side
effect of the ALD process to be considered if soft
