In
nature, the strength of mother-of-pearl is a key to survival for some
shellfish. Now a team led by Xiaodong Li, an engineering professor at
the University of South Carolina,
has posited an explanation for the unusual resilience that this
important defensive shield shows in the face of predatory attacks. Given
the elaborate nanoscale structures that biology naturally incorporates
in mother-of-pearl, the research team believes the findings could serve
as a blueprint for engineering tough new materials in the laboratory.
“For
a long time, we’ve thought that we understood how these nanoscale
biomaterials work—but it turns out we just know a little bit,” said Li,
whose team published their results in a just-released article in Nature
Publishing’s new journal, Scientific Reports.
Mother-of-pearl,
also called nacre, makes up the inner shell lining of pearl mussels and
some other mollusks. Pearls themselves are made of nacre, which is a
composite nanomaterial constructed by the biomachinery of the shellfish.
Tiny crystal grains of calcium carbonate are arranged in a regular,
intricate pattern and bound together by biopolymers in nacre’s
structure, which adds a tremendous amount of stability to the material:
it is some 1000 times more resistant to cracking from impact than the
crystalline form of calcium carbonate (the mineral aragonite) that makes
up the bulk of nacre.
Indeed,
calcium carbonate by itself is perhaps best known as blackboard chalk;
its tendency to crumble undermines any notion that it would serve as an
effective means of stopping a bullet. And yet nature organizes a complex
brick-and-mortar-like structure–with the bricks of calcium carbonate
measuring in the range of nanometers–to create an incredibly tough
material, much stronger than the sum its parts. Mother-of-pearl’s
shimmering quality is a by-product of this structure, because the
visible light that it reflects has wavelengths that are similar in size
to the nanoscale bricks therein.
Nacre’s
strength under pressure, Li explained, is unusual and somewhat counter
to intuition. When squeezed quickly (dynamic loading), it withstands far
more pressure than when squeezed slowly (static loading). “This is a
feature of natural materials with nanoparticle architectures,” said Li,
“Hardly any man-made ceramics have this property, which would be
invaluable in applications like body armor, so understanding how it
works is very important.”
The
increased strength of nacre in the face of rapid pressure has been
known for 10 years, but the reasons underlying it have remained unclear.
So Li’s team set out to understand the mechanism by focusing on the
structure of nacre at the nanoscale. They precisely cut mother-of-pearl
samples from California red abalone and subjected them to both dynamic
and static loading. The nacre that was squeezed rapidly–the ballistic
test, in a sense–put up more than twice as much resistance before
fracturing than that squeezed slowly. Then Li and co-workers, which
included USC researchers as well as contributors from the University of
North Carolina at Charlotte, used transmission electron microscopy to
address the details of the fracturing at the nanoscale level.
Their
results were completely unexpected. Under rapid-compression ballistic
conditions, the nanoscale particles work together to restrain the
buckling of the material. The researchers concluded that deformation
twinning, a process seen in some metals and a particular indicator of
strength in the face of stress, comes into play with the nanoscale
particles of calcium carbonate. But this mechanism was only evident with
ballistic conditions, not under the slower application of pressure.
Li’s team also concluded that partial dislocations within the
nanostructure lend further strength to the material, but again, it only
occurred under the ballistic conditions.
When
confronted by a short, powerful thrust from a predator–an activity
that shellfish have spent many millions of years working out their
defense against–the nanostructural bricks in the overall nacre
structure work together to absorb the impact and maximize resistance.
Stress is first absorbed and dissipated within the nanostructure itself
before the material itself is overpowered and fractures.
Now
that Li’s team has elucidated the means of mother-of-pearl’s heightened
defenses, engineers can try to apply the lessons to synthetic
materials. “The real goal is to be able to design these materials,” said
Li. “Understanding the mechanism is the first step to making, as just
one example, better bullet-proof materials.”
