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,” says
Li, whose team published their results in 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 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
byproduct 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 explains, 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,” says 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, 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,” says Li. “Understanding the mechanism is the first step to making, as just one example,
better bullet-proof materials.”
