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Imaging Between a Rock and a Hard Tooth

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EVANSTON, Ill., Jan. 20, 2011 — Here’s something to chew on: An imaging technique sensitive enough to be used at the atomic scale has been used for the first time to create 3-D maps of the teeth of chitons — a class of marine mollusks that can literally chew rock — looking for clues as to what makes them so strong.

The research at Northwestern University will help scientists understand the structure of hybrid organic-inorganic materials such as teeth and bone, and will allow them to design new materials with similar strength and flexibility. The work is reported in the Jan. 13 issue of the journal Nature.

Chitons continually make new rows of teeth — one a day — to replace mature but worn teeth; in conveyor-belt fashion, the older teeth move down the creature’s tongue-like radula toward the mouth where it feeds.

The black teeth of an Eastern beaded chiton, a sea mollusk, are used for scraping algae from rocks. They are capped with one of the hardest biominerals known, a nanocomposite of magnetite and chitin-based fibers. (Photo: Northwestern)

Chiton teeth resemble human teeth in that they have a hard and tough outer layer – equivalent to our enamel – and a softer core. Instead of enamel, the rock-chewing chitons use magnetite, a very hard iron oxide, which gives their teeth a black luster.

Teeth and bone are important and complex structures in humans and other animals, but little is actually known about their chemical structure at the atomic scale. What exactly gives them their renowned toughness, hardness and strength? How do organisms control the synthesis of these advanced functional composites?

Two Northwestern University researchers used a highly sophisticated atomic-scale imaging tool to peel away some of the mystery of organic/inorganic interfaces that are at the heart of tooth and bone structure. Results are reported in the journal Nature.

“The interface between the organic and inorganic materials plays a large role in controlling properties and structure,” says Derk Joester, an engineering professor and the paper’s senior author. “How do organisms make and control these materials? We need to understand this architecture on the nanoscale level to design new materials intelligently. Otherwise we really have no idea what is going on.”

The researchers set out to find the organic fibers they knew to be an important part of the tooth’s structure, buried in the tough outer layer of the tooth, made of magnetite. Their quantitative mapping of the tooth shows that the carbon-based fibers, each 5 to 10 nm in diameter, also contained either sodium or magnesium ions.

Joester and Lyle Gordon, a doctoral student in Joester’s lab, are the first to have direct proof of the location, dimension and chemical composition of organic fibers inside the mineral.

They were surprised by the chemical heterogeneity of the fibers, which hints at how organisms modulate chemistry at the nanoscale.

“The tooth’s toughness comes from this mix of organic and inorganic materials and the interfaces between them,” Joester says. “While this is in principle well-known, it is intriguing to think we may have overlooked how subtle changes in the chemical makeup of nanoscale interfaces may play a role in, for instance, bone formation or the diffusion of fluoride into tooth enamel.”

The researchers extracted micron-sized samples from the leading edge of the tooth from a chiton. Using a focused ion beam tool, they fashioned the samples into very sharp tips (less than 20 nm across). The process is reminiscent of sharpening a pencil, albeit with a supercharged stream of gallium ions.

The imaging technology the team used is known as atom-probe tomography (APT), which works by applying an extremely high electric field to the sample; atoms on the surface ionize, fly off, and hit an imaging detector (similar to those found in night-vision equipment). The atoms are stripped off atom-by-atom and layer-by-layer, as with peeling an onion.

Computer methods then are used to calculate the original location of the atoms, producing a 3-D map or tomogram of millions of atoms within the sample.

Joester and Gordon now are studying the tooth enamel of a vertebrate and plan to apply APT to bone, which is also made of organic and inorganic parts, to learn more about its nanoscale structure.

The National Science Foundation and the Canadian National Sciences and Engineering Research Council supported the research.

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Jan 2011
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1. In raster scan television, one of the two scans that are interlaced to make up a frame. 2. See field of view.
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