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The researchers were particularly interested in the pigmented enamel of the beaver's incisors. The presence of iron gives the teeth a reddish-brown color. (Credit: Brett/Flickr)

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Beaver teeth aren’t pretty, but they’re tough

Even though beavers don’t brush their teeth or drink fluoridated water, they’ve got built-in protection against tooth decay: iron.

A new study finds that this pigmented enamel is both harder and more resistant to acid than regular enamel, including that treated with fluoride.

This finding and others could lead to a better understanding of human tooth decay, earlier detection of the disease, and improving on current fluoride treatments.

Layers of well-ordered hydroxylapatite “nanowires” are the core structure of enamel, but Derk Joester of Northwestern University and his team discovered it is the material surrounding the nanowires, where small amounts of amorphous minerals rich in iron and magnesium are located, that controls enamel’s acid resistance and mechanical properties.

“We have made a really big step forward in understanding the composition and structure of enamel—the tooth’s protective outer layer—at the smallest length scales,” says Joester, lead author of the study and an associate professor of materials science and engineering in the McCormick School of Engineering and Applied Science.

“The unstructured material, which makes up only a small fraction of enamel, likely plays a role in tooth decay,” Joester says. “We found it is the minority ions—the ones that provide diversity—that really make the difference in protection. In regular enamel, it’s magnesium, and in the pigmented enamel of beaver and other rodents, it’s iron.”

‘Rottenness’ is expensive

Dental caries, better known as tooth decay, are the breakdown of teeth due to bacteria. (“Caries” is Latin for “rottenness.”) It is one of the most common chronic diseases and a major public health problem, despite strides made with fluoride treatments.

According to the American Dental Association, $111 billion a year is spent on dental services in the US, a significant part of that on cavities and other tooth decay issues.

A staggering 60 to 90 percent of children and nearly 100 percent of adults worldwide have or have had cavities, according to the World Health Organization.

Improved enamel for humans?

In a series of experiments of rabbit, mouse, rat, and beaver enamel, Joester and his colleagues imaged the never-seen-before amorphous structure that surrounds the nanowires. They used powerful atom-probe tomography and other techniques to map enamel’s structure atom by atom. (Rodent enamel is similar to human enamel.)

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The researchers subjected the teeth to acid and took images before and after acid exposure. They found the periphery of the nanowires dissolved (the amorphous material), not the nanowires themselves.

The researchers next identified amorphous biominerals in the structure, such as iron and magnesium, and learned how they contribute to both the mechanical hardness and resistance of enamel to acid dissolution.

Joester and colleagues were particularly interested in the pigmented enamel of the beaver’s incisors. The research shows it to be an improvement over fluoride-treated enamel in resisting acid. (The presence of iron gives the teeth a reddish-brown color.)

“A beaver’s teeth are chemically different from our teeth, not structurally different,” Joester says. “Biology has shown us a way to improve on our enamel. The strategy of what we call ‘grain boundary engineering’—focusing on the area surrounding the nanowires—lights the way in which we could improve our current treatment with fluoride.”

The National Science Foundation, the Northwestern University Materials Research Center (NSF – Materials Research and Engineering Center), the American Chemical Society Petroleum Research Fund, Northwestern’s International Institute for Nanotechnology and the Institute for Sustainability and Energy at Northwestern (ISEN) supported the research.

The findings appear in Science. A related paper is published in Frontiers in Physiology.

Source: Northwestern University

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