The ocean is a perilous environment for a soft-bodied
creature like a sea snail, so nature gives it an
advanced nanostructured armor system that is stiff
and strong yet lightweight. It's called a shell.
the fundamental design principles of natural armor
systems like shells may help engineers design improved
body armor systems for humans in perilous situations,
like soldiers and police officers. At MIT's Institute
for Soldier Nanotechnologies, researchers are studying
the structure and mechanics of the tough inner
layer of mollusc shells, called "nacre" or
mother-of-pearl, at extremely small, nanometer-length
scales (a nanometer is a billionth of a meter).
In an upcoming issue of the Journal of Materials
Research, Professor Christine Ortiz of the Department
of Materials Science and Engineering, Professor Mary
Boyce of the Department of Mechanical Engineering
and doctoral student Benjamin Bruet of materials
science report their results. They show that nature
is indeed an expert nanoengineer.
"The complexity we have observed in nacre at the
nanoscale is quite amazing and seems likely to be
a critical determinant of the toughness of the material," said
is composed of two relatively weak materials: 95
percent calcium carbonate, a brittle ceramic, and
5 percent flexible biopolymer. These materials
are organized into a "brick-and-mortar" structure
with millions of ceramic plates, each a few thousand
nanometers in size, that are stacked on top of each
other like rolls of coins. Each layer of plates is
glued together by thin layers of the biopolymer.
The MIT team has focused its studies on small nanometer-sized
regions of the individual tiny plates.
"Even though the calcium carbonate is very weak
and brittle on its own, one can get enormous increases
in toughness through design at multiple-length scales," said
Ortiz. "Understanding how the material is designed
and functions at the smallest-length scales will
be critical to learning how to create tough biomimetic
Replacing the weak building blocks of nacre with
stronger materials -- in a similar design -- has
the potential to yield much tougher composites for
use in armor systems or structural applications like
automobile panels or plane wings.
The MIT team began its experimental studies by imaging
the tiny plates cut from the nacre of Trochus niloticus,
a sea snail, using a powerful instrument called an
atomic force microscope. They found that each individual
plate also had its own complicated nanostructure
and was divided like a pie into separate sectors,
with cylindrical beams running through the thickness
of the plates, a fine surface of nanosized bumps,
called nanoasperities, which were further organized
into groups, and biopolymer molecules, only about
1 nanometer in height, traversing over and bound
to the mountainous array of nanobumps.
then used a diamond-probe tip only a few hundred
nanometers in size in the Department of Materials
Science and Engineering's Nanomechanical Technology
Laboratory to push into the surface of an individual
plate (a technique called nanoindentation) while "feeling" the
force that resulted. "I was surprised to find that
the tablets were both extremely stiff and strong
at these length scales and that they resisted brittle
crack formation and propagation even at exceedingly
high forces," said Bruet.
Although scientists have studied the properties
of nacre at the macroscale and microscale, Ortiz
says that very little is known about its behavior
at the nanoscale, which is where structure and properties
set the foundation for the material's overall behavior.
The team is currently studying the nanoscale adhesion
forces that exist between the ceramic plates and
flexible biopolymer in the nacre, as well as the
single molecule nanomechanical properties of the
biopolymer. This research may shed light on the longstanding
question of how to create durable interfaces in synthetic
composites that can withstand high forces in water
environments. Ortiz's group is also studying the
nanostructure and nanomechanical properties of other
natural materials, such as bone and cartilage.
"Nature uses nanoscale structural design principles
to produce materials with superior mechanical properties," said
Ortiz. "In many aspects, human engineers have yet
to achieve the same skill. However, as nanotechnology
methods advance, the creation of artificial nacre
-- and other kinds of high-performance armors --
is becoming a more and more realistic goal."
A version of this article appeared in MIT
Tech Talk on September
21, 2005 (download PDF) .
for Soldier Nanotechnologies
morphology and indentation of individual nacre
tablets from the gastropod mollusc Trochus niloticus -
(PDF file, 3 MB)