Inquiry 2016, Issue 1

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22 Inqui r y I s sue 1 | 2016 Inqui r y I s sue 1 | 2016 23
ature provides myriad examples of unique
materials and structures developed for specialized
applications or adaptations. An interdisciplinary
group of researchers at Ames Laboratory is trying to
unlock the secrets that organisms use to build such complex
structures so that power can be used to create materials not
found in nature and not capable of being synthesized by
conventional means.
“Naturehas lotsof examplesof thesehierarchical structures
and they’re usually organic-inorganic composite materials,”
said Surya Mallapragada, Ames Laboratory scientist and
Iowa State University Carol Vohs Johnson Chair in Chemical
and Biological Engineering. “A glass sea sponge skeleton is
a perfect example of these structures that are templated by
the organic phase. You have inorganic nanocrystals that form
and it’s a multiscale assembly process, which in most cases
happens at mild temperatures and conditions, such as pH.”
“So we look to nature for inspiration and as a source of
bio-molecules to see how we can recreate some of those
processes that create these wonderful materials with
hierarchical assemblies or uniform structure,” she said.
So far, Mallapragada’s team has been able to replicate the
creation of magnetite by studying magnetotactic bacteria.
These bacteria form magnetic nanocrystals or chains of
nanocrystals that they use to orient themselves with the
Earth’s magnetic field. Using self-assembling polymer
templates and proteins from the bacteria, researchers were
able to grow magnetite crystals.
“We’ve used this approach successfully to grow magnetite
nanocrystals,” Mallapragada said, “but we’ve gone beyond
that, using these techniques to create cobalt ferrite and other
magnetic nanocrystals that are not found in nature. That’s a
great example of templated synthesis.”
The group has also worked with calcium phosphate to try
to mimic the light-weight strength found in bones.
“In some case, we need to come up with synthetic
analogs which can do the same job, but are more robust,”
Mallapragada said. “In many cases, the biomolecules aren’t
as robust. Proteins are fragile molecules so if we can do it
with synthetic polymers, that gives us a lot more flexibility.”
It’s one thing to create nanocrystals. Getting those
nanocrystals to then organize and form microstructures and
then macro-scale structures is something altogether different.
“They’re not at the level of complexity we see in nature
—that’s the Holy Grail,” Mallapragada explained, “but
that’s the inspiration and we’re trying to get there with
synthetic approaches.”
The latest goal for harnessing this natural building process
is the creation of metamaterials, so-called left-handed
materials, that have interesting optical properties that don’t
occur in nature.
“We’re looking at using organic templates to assemble
inorganic particles to get the desired properties,”
Mallapragada. “We have a very strong collaboration with
Ames Laboratory physicists Costas Soukoulis and Thomas
Koschny, and they’ve done some wonderful work with
simulations and predictions of structures and developed
some lithographic structures, but those are only 2D. So it’s
really a perfect case for using these bioinspired approaches
to self-assemble these metamaterials into 3D structures.”
Mallapragada again points to the glass sea sponge for
the type of multiscale assembly that’s required to build 3D
metamaterials.
“The sea sponge has order on multiple scales—nanoscale,
micron-scale, millimeter-scale. It’s a multi-scale assembly—
it looks like the Eiffel tower—and that’s why it has a very
great strength to weight ratio,” she said. “So we need a similar
hierarchy. Define the shapes at nanoscale, but then have an
ordered arrangement of these nanoscale objects in 2D and
then 3D to get the desired properties.”
In addition to using self-assembling polymers, which
provide long-range order, DNA has also been used because
it allows for specificity in the placement of nanoparticles.
To create metamaterials, the team is looking at using both
to control the placement of gold nanoparticles in a specific
pattern, build up layers and then apply a gold film coating to
the entire structure to acquire the desired properties.
“It takes a very interdisciplinary approach,” Mallapragada
said. “We have molecular biologists (Marit Nilsen-Hamilton)
for the DNA side, materials chemists (Mallapragada) for the
polymer synthesis, Soukoulis and Koschny for the theoretical
prediction of the structures and (physicist) Alex Travesset for
modeling the kinds of structures can we get.”
“We need good characterization so David Vaknin is
looking at scattering methods and Tanya Prozorov has
been doing transmission electron microscopy work,” she
continued. “Andy Hillier (chemical/biological engineer) has
been involved in metallization, applying the continuous film
of gold on those nano structured templates. So it’s a multi-
level, multi-step, multi-component synthetic process.”
Mother Nature should be flattered! N
Materials:
Bioinspired
B Y K E R R Y G I B S O N
BORROWING FROM NATURE’S PLAYBOOK
Opposite and top right: A glass sea sponge skeleton showing the complex nature of its structure which provides amazing
strength. (Photos by Michael Monn, Kesari Lab, Brown University.*) Top left: A micrograph of magnetic nanocrystals grown
by Surya Mallapragada’s research group.
*New functional insights into the internal architecture of the laminated anchor spicules of Euplectella aspergillum, PNAS,