The secrets of mother-of-pearl strength
Do you know that the nacre layer, or mother-of-pearl, which is found in pearls and abalone shells is 3,000 times more fracture-resistant than aragonite, the mineral of which it is composed? According to researchers at University of Wisconsin-Madison, this remarkable strength "is due to well-defined nanolayers of organics at the interfaces between micro-tiles of aragonite." "You can go over it with a truck and not break it -- you will crumble the outside [of the shell] but not the [nacre] inside. And we don't understand how it forms -- that's why it's so fun to study," said the lead researcher. But if it becomes possible to harness the mechanism of formation of nacre, it would be possible to produce cars that absorb all the energy at the impact point but do not fracture.
Above is a picture of a "Red Abalone (Haliotis rufescens), a gastropod, showing the polished inner nacre surface. Shell diameter is 20 cm." (Credit: Pupa Gilbert group) Here is a link to a larger version of this photo. You'll find more details about this research by reading this page about nacre set by Pupa Gilbert, a physicist at the University of Wisconsin-Madison.
What's so special about the structure of mother-of-pearl? "Though a mere 5 percent of abalone nacre is organic, this small fraction somehow lays enough foundation for the mineral components to assemble spontaneously," Gilbert says. "Ninety-five percent of the mass of this biomineral is self-assembled, while only 5 percent is actively formed by the organism," she says. "It is one of the most efficient mechanisms you can think of."
In order to learn more about this self-assembly process, Gilbert used synchrotron radiation to look at the structure of abalone nacre.
When used to examine a cross-section of an abalone shell, previously seen to resemble a brick wall with layers of organic "mortar" separating individual crystalline "bricks," the polarized light from the synchrotron revealed that the nacre wall was not uniform. Instead, the wall contained distinct clumps of bricks, each an irregular column of crystals with identical composition but a crystal orientation different than neighboring columns. Since orientation affects how crystals emit electrons, "some of the columns of bricks appear white and others appear black and more appear gray, depending on their crystal orientation," Gilbert explains.
This research work has been published by Physical Review Letters under the name "Architecture of Columnar Nacre, and Implications for Its Formation Mechanism" (Volume 98, No. 26, Article 268102, June 29, 2007). Here is a link to the abstract.
In Inside Mother-of-Pearl, Stephanie Chasteen, who has probably read the full paper, gives additional details (Physical Review Focus, July 2, 2007). "To investigate the nacre structure, Gilbert and her colleagues used a technique that hadn't been applied to biominerals before. They hit a portion of a nacre sample edge-on with a polarized x-ray beam tuned to a resonant frequency of the carbon-oxygen bonds. They then focused the electrons ejected from the sample to make an image--a grayscale map showing a mosaic of differently shaded areas representing regions with different crystal orientations. The team found that the layers were not necessarily aligned with the atomic-scale crystal structure, and neighboring tablets could have different crystalline orientations. But groups of up to 40 tablets with identically oriented structures tended to form ragged columns within the sample, like snaking stacks of quarters."
Chasteen also analyzes how nacre might grow. "It forms layer-by-layer from the bottom up, and some researchers have assumed that each crystalline tablet grows right on top of the one below, with only slight sideways offsets. The tablets may be connected through small holes piercing the scaffold layers. Gilbert and her colleagues propose that the holes--or some kind of nucleation sites that trigger crystal growth--are randomly scattered throughout the scaffold layers and not necessarily vertically aligned. As each tablet grows, it will likely hit the scaffold "ceiling" first but continue to grow horizontally for some time."
Let's return to the UW-Madison news article for the conclusion. "If you understand how [nacre] forms, you could think of reproducing it, producing a synthetic material that's inspired by nature -- a so-called 'biomimetic' material," Gilbert explains. "If we learn how to harness the mechanism of formation, then we could, for example, produce cars that absorb all the energy at the impact point but do not fracture."
Sources: Jill Sakai, University of Wisconsin-Madison news release, July 2, 2007; and various websites
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