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Biomineralization may show nanotechnology how to align atoms in regular arrays with unusual morphologies

A better understanding of how biomineralization converts ordinary minerals to biological mineral structures with extraordinary hardness and fracture resistance may lead to superhard materials for nanotech applications. In the case of sea urchin spicules, the process involves a fractal-like or random walk process by which 40-100 nm particles of amorphous calcium carbonate are converted into a single crystal of calcite with a very unusual geometry. From the University of Wisconsin-Madison, via AAAS EurekAlert “Sea urchin yields a key secret of biomineralization“:

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.

—Jim

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