By looking at individual atoms in tooth enamel, UW and PNNL researchers are learning what happens to our teeth as we age

(Press-News.org) Teeth are essential for helping people break down the food they eat, and are protected by enamel, which helps them withstand the large amount of stress they experience as people chew away. Unlike other materials in the body, enamel has no way to repair damage, which means that as we age, it risks becoming weaker with time.

Researchers are interested in understanding how enamel changes with age so that they can start to develop methods that can keep teeth happier and healthier for longer.

A research team at the University of Washington and the Pacific Northwest National Laboratory examined the atomic composition of enamel samples from two human teeth — one from a 22-year-old and one from a 56-year-old. The sample from the older person contained higher levels of the ion fluoride, which is often found in drinking water and toothpaste, where it’s added as a way to help protect enamel (though its addition to drinking water has recently been a topic in the news).

The team published these findings Dec. 19 in Communications Materials. While this is a proof-of-concept study, these results have implications for how fluoride is taken up and integrated into enamel as people age, the researchers said.

"We know that teeth get more brittle as people age, especially near the very outer surface, which is where cracks start," said lead author Jack Grimm, UW doctoral student in materials science and engineering and a doctoral intern at PNNL. "There are a number of factors behind this — one of which is the composition of the mineral content. We're interested in understanding exactly how the mineral content is changing. And if you want to see that, you have to look at the scale of atoms."

Enamel is composed mostly of minerals that are arranged in repetitive structures that are ten thousand times smaller than the width of a human hair.

"In the past, everything that we've done in my lab is on a much larger scale — maybe a tenth the size of a human hair," said co-senior author Dwayne Arola, UW professor of materials science and engineering. "On that scale, it's impossible to see the distribution of the relative mineral and organic portions of the enamel crystalline structure."

To examine the atomic composition of these structures, Grimm worked with Arun Devaraj, a materials scientist at PNNL, to use a technique called "atom probe tomography," which allows researchers to get a 3D map of each atom in space in a sample.

The team made three samples from each of the two teeth in the study and then compared differences in element composition in three different areas of the tiny, repetitive structures: the core of a structure, a "shell" coating the core, and the space between the shells.

In the samples from the older tooth, fluoride levels were higher across most of the regions. But they were especially high in the shell regions.

"We are getting exposed to fluoride through our toothpaste and drinking water and no one has been able to track that in an actual tooth at this scale. Is that fluoride actually being incorporated over time? Now we're starting to be able to paint that picture," said co-author Cameron Renteria, a postdoctoral researcher in both the oral health sciences and the materials science and engineering departments at the UW. "Of course, the ideal sample would be a tooth from someone who had documented every time they drank fluoridated versus non-fluoridated water, as well as how much acidic food and drink they consumed, but that's not really feasible. So this is a starting point."

The key to this research, the team said, is the interdisciplinary nature of the work.

"I am a metallurgist by training and didn't start to study biomaterials until 2015 when I met Dwayne. We started to talk about the potential synergy between our areas of expertise — how we can look at these small scales to start to understand how biomaterials behave," Devaraj said. "And then in 2019 Jack joined the group as a doctoral student and helped us look at this problem in depth. Interdisciplinary science can facilitate innovation, and hopefully we'll continue to address really interesting questions surrounding what happens to teeth as we age."

One thing the researchers are interested in studying is how protein composition of enamel changes over time.

"We set out trying to identify the distribution of the organic content in enamel, and whether the tiny amount of protein present in enamel actually goes away as we age. But when we looked at these results, one of the things that was most obvious was actually this distribution of fluoride around the crystalline structure," Arola said. "I don't think we have a public service announcement yet about how aging affects teeth in general. The jury is still out on that. The message from dentistry is pretty strong: You should try to utilize fluoride or fluoridated products to be able to fight the potential for tooth decay."

Semanti Mukhopadhyay, a postdoctoral researcher at PNNL, is also a co-author on this paper. This research was funded by the National Institutes of Health, Colgate-Palmolive Company and a distinguished graduate research program between PNNL and UW.

For more information, contact Grimm at jckgrmm@uw.edu, Arola at darola@uw.edu and Renteria at crentb@uw.edu. For questions specifically for Arun Devaraj please contact Karyn Hede at karyn.hede@pnnl.gov.

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