The teeth and bones of mammals, the protective shells of mollusks, and the
needle-sharp spines of sea urchins and other marine creatures are made-from-scratch
wonders of nature.
Used to crush food, for structural support and for defense, the materials of
which shells, teeth and bones are composed are the strongest and most durable
in the animal world, and scientists and engineers have long sought to mimic
them.
Now, harnessing the process of biomineralization may be closer to reality as
an international team of scientists has detailed a key and previously hidden
mechanism to transform amorphous calcium carbonate into calcite, the stuff of
seashells. The new insight promises to inform the development of new, superhard
materials, microelectronics and micromechanical devices.
In a report today (Oct. 27) in the Proceedings of the National Academy of Sciences
(PNAS), a group led by University
of Wisconsin-Madison physicist Pupa Gilbert describes how the lowly sea
urchin transforms calcium carbonate — the same material that forms "lime"
deposits in pipes and boilers — into the crystals that make up the flint-hard
shells and spines of marine animals. The mechanism, the authors write, could
"well represent a common strategy in biomineralization…."
"If we can harness these mechanisms, it will be fantastically important
for technology," argues Gilbert, a UW-Madison professor of physics. "This
is nature's bottom-up nanofabrication. Maybe one day we will be able to use
it to build microelectronic or micromechanical devices."
Gilbert, who worked with colleagues from Israel's Weizmann Institute of Science,
the University of California at Berkeley and the Lawrence Berkeley National
Laboratory, used a novel microscope that employs the soft-X-rays produced by
synchrotron radiation to observe how the sea urchin builds its spicules, the
sharp crystalline "bones" that constitute the animal's endoskeleton
at the larval stage.
Similar to teeth and bones, the sea urchin spicule is a biomineral, a composite
of organic material and mineral components that the animal synthesizes from
scratch, using the most readily available elements in sea water:
calcium, oxygen
and carbon. The fully formed spicule is composed of a single crystal with an
unusual morphology. It has no facets and within 48 hours of fertilization assumes
a shape that looks very much like the Mercedes-Benz logo.
These crystal shapes, as those of tooth enamel, eggshells or snails, are very
different from the familiar faceted crystals grown through non-biological processes
in nature. "To achieve such unusual — and presumably more functional
— morphologies, the organisms deposit a disordered amorphous mineral phase
first, and then let it slowly transform into a crystal, in which the atoms are
neatly aligned into a lattice with a specific and regular orientation, while
maintaining the unusual morphology," Gilbert notes.
The question the Wisconsin physicist and her colleagues sought to answer was
how this amorphous-to-crystalline transition occurs. The sea urchin larval spicule
is a model system for biominerals, and the first one in which the amorphous
calcium carbonate precursor was discovered in 1997 by the same Israeli group
co-authoring the current PNAS paper. A similar amorphous-to-crystalline transition
has since been observed in adult sea urchin spines, in mollusk shells, in zebra
fish bones and in tooth enamel. The resulting biominerals are extraordinarily
hard and fracture resistant, compared to the minerals of which they are made.
"The amorphous minerals are deposited and they are completely disordered,"
Gilbert explains. "So the question we addressed is 'how does crystallinity
propagate through the amorphous mineral?'"
To answer it, Gilbert and her colleagues observed spicule development in 2-
to 3-day-old sea urchin larvae. The sea urchin spicule is formed inside a clump
of specialized cells and begins as the animal lays down a single crystal of
calcite in the form of a rhombohedral seed, from which the rest of the spicule
is formed. Starting from the crystalline center, three arms extend at 120 degrees
from each other, as in the hood ornament of a Mercedes-Benz. The three radii
are initially amorphous calcium carbonate, but slowly convert to calcite.
"We tried to find evidence of a massive crystal growth, with a well defined
growth front, propagating from the central crystal through the amorphous material,
but we never observed anything like that," Gilbert says. "What we
found, instead, is that 40-100 nanometer amorphous calcium carbonate particles
aggregate into the final morphology. One starts converting to crystalline calcite,
then another immediately adjacent converts as well, and another, and so on in
a three-dimensional domino effect. The pattern of crystallinity, however, is
far from straight. It resembles a random walk, or a fractal, like lightning
in the sky or water percolating through a porous medium," explains Gilbert.
The new work, according to Gilbert, brings science a key step closer to a thorough
understanding of how biominerals form and transform. Knowing the step-by-step
process may permit researchers to develop new crystal structures that can be
used in applications ranging from new microelectronic devices to medical applications.