Biomedical Engineering Reference
In-Depth Information
have potential in immunoassays
[101]
. The same
activity has been observed with CeO
2
and mag-
netic Fe
3
O
4
nanoparticles
[102, 103]
. Deposited
by ALD, all of these materials are potential
enzyme-analog catalysts
[104, 105]
.
However, investigations into the catalytic
activities of metal nanoparticles synthesized by
ALD are mainly focused on technological appli-
cations, such as for fuel cells
[94, 95, 106]
. The
reason lies in the fact that most research groups
making use of ALD have a background in vari-
ous nonbiomedical areas, such as electronics. In
addition, the majority of the recent ALD pro-
cesses were developed and optimized to satisfy
the needs of the electronic industry, which until
now is the only large-scale industrial application
field of ALD.
Further development of the precursors and
processes is required for biomedical applica-
tions. The best candidates are Ag and Au, which
can in principle be deposited by ALD, but either
the processes are still very demanding or the
required precursors are not commercially avail-
able. ALD as enabler technology for biomedical
applications (e.g., enzyme mimetics) is an undis-
covered field with great potential. Compared
with conventional nanoparticle synthetic meth-
ods (e.g., wet chemistry), ALD provides comple-
mentary approaches, including precise control
of both size and processing of complicated struc-
tures. Future growth is envisaged.
matrix of the jaws of the marine polychaete
worm
Nereis
contributes to its hardness, showed
a good example of metal incorporation to change
mechanical properties
[109, 110]
.
Interactions of the ALD precursors with organic
functional groups present on or in the maternal
substrates may occur in many different ways.
Such interactions are often the origin of certain
improved or unexpected properties. This is to a
certain extent in good agreement with many natu-
rally occurring processes of biomineralization.
The pioneering work of Lee
et al
. showed an
infiltration of metals into a protein by means of
diffusion of ALD precursors into a spider silk
[111]
. The spider silk was chosen because the
mechanical properties of natural spider silk are
remarkable. In toughness it outperforms most
humanmade materials, such as carbon fibers,
poly-aramides, nylon, etc.
The materials of choice for the ALD process
are Al2O3, TiO2, and ZnO because of their pro-
cessability at low temperatures. Exposure of the
spider silk to the first precursor is seriously
extended from subseconds to minutes to enable
the precursor to diffuse into the protein matrix
and attach to functional groups such as alcohols,
amines, etc. Metals became homogeneously dis-
tributed within the silk. Instead of expected
hardening and stiffening, the silks turned more
ductile and stronger, resulting in a tenfold
increase in toughness (
Figure 16.10
).
A series of characterization experiments,
including nuclear magnetic resonance (NMR),
X-ray diffraction (XRD), transmission electron
microsopy-energy-dispersive X-ray spectros-
copy (TEM-EDX), Raman spectroscopy, etc.,
showed that the metal originating from the ALD
precursor is indeed also found in the bulk of the
spider silk. This proves infiltration of the protein
from the gas phase. The proposed mechanism is
a diffusion of the precursor into the bulk pro-
tein, interruption of hydrogen bonds between
some protein chains, and insertion of metals into
those bonds. After the metal infiltration was per-
formed by ALD, the
β
sheets appeared to be
16.3.2 Biomineralization
Bio-organic molecules or materials show
defined but complicated chemical compositions
or arrangements. The adaptation of biomaterials
to specific needs leads to insertion of metals into
protein structures
[107, 108]
. Those insertions
are in most cases related to the enhancement
of mechanical properties. Proteins containing
certain metal ions or compounds can become
hard, stiff, tough, and so on. Bryan and Gibbs,
who found that Zn incorporated into the protein
