Keshi pearls,
sourced from
Akoya pearl
oysters, can
exhibit up to 2,615
layers of nacre,
built up over a
span of 548 days.March/April 2022 25PH
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Deep Math
5
// BY LEIL A SLOM A N //This Natural
Treasure
Contains
the Secret
to Stronger
Armor
W
HEN IT COMES TO BUILDING
tough, resilient materials without
wasting energy, few human manu-
facturers can compete with Mother
Nature. One of her most intriguing
achievements is nacre, commonly
known as mother-of-pearl, a mate-
rial that is much stronger than the sum of its parts.
“[Nacre] is effectively chalk, but it doesn’t break
like chalk. When I think of chalk, I think of a very
brittle, delicate material,” says Robert Hovden, a
professor of materials science at the University
of Michigan and one of the researchers on a study
involving nacre. Last year, his team uncovered
new information about the nano-level structure of
nacre that could open new frontiers in the world of
human-made supermaterials.
The study, published in Proceedings of the
National Academy of Sciences, focuses on nacre’s
structure at the nanoscale level—lengths of a
billionth of a meter, even smaller than a wave-
length of light. Using an electron microscope, the
researchers noticed that inconsistencies in the
nacre’s brick-and-mortar structure led to “cor-
rections” in future layers. When an asymmetrical
nacreous layer corrupted the pearl’s symmetry,
future layers “adjusted” their thicknesses to make
the pearl more spherical.
Clams and other mollusks construct pearls
by blanketing a “bead nucleus,” or a small object
inside the pearl sac, with tiny alternating layers
of calcium carbonate and organic protein. These
layers make up nacre. “It’s sometimes referred to
as brick and mortar,” explains Neil H. Landman,
curator emeritus at New York City’s American
Museum of Natural History. “The bricks are the
nacreous [calcium carbonate] tablets; the mortar
are the organic sheets between the tablets.”
Hovden and seven other researchers analyzed
the “periodicity” of this brick-and-mortar struc-
ture, or how uniform the layers of nacre were.