Rock-Munching Mollusks A Model For Artificial Bones

To eat, chitons use their teeth, which look like black bulbs with bluish highlights, to grind up rock. i i

To eat, chitons use their teeth, which look like black bulbs with bluish highlights, to grind up rock. Lyle Gordon/Northwestern University hide caption

itoggle caption Lyle Gordon/Northwestern University
To eat, chitons use their teeth, which look like black bulbs with bluish highlights, to grind up rock.

To eat, chitons use their teeth, which look like black bulbs with bluish highlights, to grind up rock.

Lyle Gordon/Northwestern University

A tiny marine creature called a chiton might hold the key to better artificial teeth and bones. This rather unexpected possibility comes from a relatively new branch of science called biomaterials. These are materials that are inspired by what plants and animals make but tweaked to make useful new products.

If you think about, it's rather remarkable that creatures with squishy insides like oysters and clams can make rock hard shells and creatures with squishy insides like us can make bones and teeth.

Derk Joester of Northwestern University is studying how the chiton makes its teeth. Chitons are mollusks. They're small, rather flat and oval in shape. The chiton Joester studies is called Chaetopleura apiculata, and it has a rather odd way of getting a meal.

"This particular organism literally chews rock in order to feed," he says. It grinds down rock to get at algae and other food particles that might be sandwiched in the rock. "And for that, it needs incredibly tough and hard teeth."

Chiton are small, flat mollusks that have eight shell plates.  i i

Chiton are small, flat mollusks that have eight shell plates. Lyle Gordon/Northwestern University hide caption

itoggle caption Lyle Gordon/Northwestern University
Chiton are small, flat mollusks that have eight shell plates.

Chiton are small, flat mollusks that have eight shell plates.

Lyle Gordon/Northwestern University

In fact, Joester says chiton teeth are one of the hardest and toughest materials known in nature. "They also have a very particular structure that allows them to self-sharpen to a certain degree. ... Imagine a knife that keeps its edge forever."

And that's a trick Joester would like to be able to replicate in the lab.

A Closer Look

But before you can contemplate making such a thing in the lab, you need to know how the chiton does it. "For that, we are using one of the most powerful microscopes, the so-called atom probe," Joester says.

He focuses this microscope on the interface between the soft organic molecules of the chiton's innards with the rock-hard inorganic minerals of the chiton's teeth.

"The microscope really works by plucking the sample apart, one ion [a charged version of an atom] at a time," Joester says. As he reports in the journal Nature, Joester says they're now getting a better handle on the organic proteins that are directing how chiton teeth grow.

"This is an unprecedented level of detail," says Lara Estroff, a professor of materials science and engineering at Cornell University.

She says being able to focus on this interface between organic and inorganic molecules is key.

"Anywhere from mollusk shells to teeth to bone, there's always an organic scaffold upon which the inorganic crystals get grown," Estroff says.

She says Joester's atom probe microscope gives a detailed picture of that interface.

Clever Engineering

When materials scientists try to make things in the lab, they frequently have to resort to high temperatures and extreme pressures to force materials into a useful shape. And yet the chiton is able to make its remarkable teeth in regular old seawater and without special equipment. That's why it's worth studying.

"We can start to understand what the important design features are, and then start to develop techniques in the lab that might be able to take some of those features out and replicate them," Estroff says.

Joester says it's not all that surprising that primitive creatures like chitons have developed some clever tricks, such as making self-sharpening teeth.

"Nature has had 500-odd million years of a head start in product development," Joester says. "[Chitons] have been able to find some really creative and efficient solutions to engineering problems."

Engineering problems such as making a better artificial hip joint or a better dental implant.

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