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Brittlestar uses Mg-rich nanoparticles to forge its calcite armor

By Eva Karatairi March 30, 2018
Brittlestar
(a) Ophiocoma wendtii (credit: Aizenberg et al., J. Mater. Chem., 2004, 14, 2066-2072); (b) scanning electron microscope (SEM) image showing an entire dorsal arm plate and a higher magnification inset (credit: Boaz Pokroy); (c) SEM image of a part of the dorsal arm plate, where the mesh and the array of microlenses are both visible (credit: Aizenberg, et al., Nature 2003, 299, 1205-1208); (d) High resolution transmission electron microscopy image of a thin section from a lens revealing brighter nanodomains, although the fast Fourier transform (FFT) pattern (inset) is that of a single crystal (credit: Boaz Pokroy).

The brittlestar species (Ophiocoma wendtii), a marine organism closely related to starfish, is capable of toughening parts of its brittle calcite skeleton. It does so in a way that resembles the age-hardening technique used for brittle metal alloys, but without the heating and rapid cooling used for the alloys. The international team of researchers who decoded the biostrategy the brittlestar uses believe that the results, published in Science, introduce an elegant way to toughen synthetic ceramic materials at ambient temperature and pressure.

Brittlestar O. wendtii is a light-sensitive member of the phylum of Echinodermata or echinoderms, a group of spiny-skinned marine animals that includes starfish, sea urchin, sea cucumber, and sea dollar. O. wendtii has five elongated, flexible arms, radially attached on its disk-shaped body. It uses calcite, the most stable crystal form of calcium carbonate (CaCO3) in the sea environment, to construct its skeletal elements. More specifically, the small plates that protect the upper part of its arm joints (dorsal arm plates) are made of calcite single-crystals, with calcium and carbonate ions arranged in a periodic pattern. Single crystalline solids are known for their high anisotropy—their physical properties depend strongly on the direction—which leads to a pronounced brittleness. However, significantly, the single crystalline dorsal arm plates of O. wendtii show significant mechanical strength.

A paradigm of fine, multifunctional bio-architecture, the dorsal arm plates consist of a three-dimensional mesh covered by arrays of hemispherical microlenses with diameters of 40–50 μm. It was these microlenses that first attracted the interest of the scientific community back in 2001, when it was shown that they function as optical elements, allowing the organism to sense shadows and light, and protect itself from predators. Researchers led by Boaz Pokroy of the Technion-Israel Institute of Technology decided to investigate the atomic structure of the microlenses.

They found calcite nanoparticles rich in Mg, ~5 nm in diameter, spread regularly inside each microlens. The nanoparticles were coherently aligned with the crystal matrix; in other words, they share the same crystal structure with the matrix, even though the crystal spacings could be slightly different. This interfacial mismatch induces compressive stress on the entire crystal structure, a strategy typically used to strengthen metal alloys.

“This is an extraordinary micro- and nanostructure that has pronounced influence on the mechanical properties. The ability to obtain this in a ceramic, by forming via an amorphous precursor, without heating and quenching, is a new finding which is really exciting!” Pokroy says. His team suggests that the nanoparticles precipitate during or just after the transformation of the supersaturated amorphous calcium carbonate precursor to its crystalline calcite phase, similar to precipitation that takes place during age-hardening of metal alloys (this efficient and well-known toughening procedure involves heating and quenching).

In the case of metals, solute atoms that are deliberately added in the alloy are “pushed” out of the supersaturated solid “solution,” and precipitate coherently with the metallic lattice. The distortion that the clusters create within the alloy matrix and the compressive strains that they induce to it provide some ‘’relief’’ from the supersaturation and help the system achieve a state of equilibrium. As precipitation proceeds, the alloy’s strength increases. The researchers believe that the same mechanism takes place in the O. wendtii.

The findings were unexpected, and the team had to come up with an ensemble of characterization techniques to explain the results. “Dealing with nanoprecipitates of several nanometers, composed mainly of calcium carbonate, within a calcium carbonate matrix is not easy!” Pokroy says, adding that “the samples we had in hand were themselves very small, so mechanical measurements and trying to measure the fracture toughness were not trivial.”

Diffraction measurements showed that there was more than pure calcite in the lenses.  Although transmission electron microscopy (TEM) observations indicated that the entire dorsal arm plate was a calcite single crystal, in the bright-field mode of the TEM, a collection of brighter nanodomains were depicted. “Still, we could not figure out what they were comprised of,” Pokroy says. The next step was to prove the nanodomains were rich in magnesium, with a combination of further x-ray diffraction and TEM measurements, with other spectroscopic techniques and various heating protocols.

“This research uncovered a previously unknown strategy that biology uses for toughening the brittle calcite mineral. It is fascinating that this unique composite nanoarchitecture is structurally akin to that appearing in classical metallurgy, while formed at ambient conditions and via an amorphous precursor phase,” says Johanna Aizenberg, professor of materials science at Harvard University, whose work revealed the optical function of the microlenses.

Pokroy says the team is now looking into whether this phenomenon can also be found in other organisms. Furthermore, they are in a process of translating the observed biostrategy to synthetic materials. According to Giuseppe Falini of the University of Bologna, who has not involved in the study, “This innovative concept can open up new routes to the synthesis of bioinspired functional ceramic materials with increased strength and toughness. In my opinion, the impact of this study is very high and will revolutionize the application of ceramic materials.”

Read the article in Science.