Even without brushing their teeth or drinking fluoridated water, beavers have remarkably strong teeth good for gnawing on wood. A new study shows that their tough teeth are all thanks to a key component built into their chemical structure, and it's iron.

The pigmented enamel of beaver teeth - stained a rusty red from the iron - is harder and much more resistant to acid and decay compared to regular enamel, including enamel treated with fluoride. And for the 60-90 percent of children and 100 percent of adults that have had cavities, this discovery could lead to better treatments for human tooth decay as well as improve current fluoride treatments.

"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," lead study author Derk Joester said in a press release.

Until now, understanding tooth enamel has been challenging due to its complex structure. But this new study from Northwestern University has peeled back the layers, literally. They found that layers of hydroxylapatite "nanowires" comprise the core structure of enamel, but it is the material surrounding the nanowires that gives it its acid resistance and mechanical properties. This surrounding material contains small amounts of amorphous minerals rich in iron and magnesium.

"The unstructured material, which makes up only a small fraction of enamel, likely plays a role in tooth decay," Joester explained. "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."

In a series of experiments of rabbit, mouse, rat and beaver enamel, Joester and his colleagues imaged the never-before-seen amorphous structure that surrounds the nanowires. After subjecting the teeth to harmful acid, they discovered that the outside amorphous material dissolved, but not the nanowires themselves.

"A beaver's teeth are chemically different from our teeth, not structurally different," Joester said. "Biology has shown us a way to improve on our enamel."

Tooth decay is one of the most common chronic diseases in the United States and represents a major public health problem, despite strides made with fluoride treatments. According to the American Dental Association, Americans spend $111 billion annually on dental services.

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