Chitons are ocean mollusks that feed by scraping algae off rock, and the cutting tips of their teeth are among the hardest materials made by any animal. A 2021 study from Northwestern University looked at the stalk that attaches each tooth to its flexible base and found a mineral called santabarbaraite, which had been seen before only in rocks. Its structure appears to help the stalk stay strong while staying light.
This is a Lined Chiton, Tonicella lineata. This picture was taken at about 50 feet depth on the west side of Whidbey Island, Washington.Attribution:
Kirt L. Onthank
What is a chiton?
Chitons are flat, oval mollusks with eight overlapping shell plates. They cling to rocks along coastlines. Instead of jaws, they have a radula, a flexible, tongue-like ribbon lined with rows of small teeth. As the teeth wear down from scraping rock, new ones move forward to replace them.
What question did the researchers ask?
The hard caps of chiton teeth had been studied for years. Less was known about the stylus, the long stalk that connects each hard tooth head to the soft radula. Joining a very hard material to a very soft one without the joint failing is a difficult engineering problem, and the team wanted to know how the chiton does it.
They studied the giant Pacific chiton, Cryptochiton stelleri, using high-energy X-ray methods at Argonne National Laboratory along with electron microscopy.
What did they find?
The upper part of the stylus is a mineralized tissue, not just soft protein.
The mineral is santabarbaraite, an iron phosphate with a disordered, non-crystal structure. It had never been found in a living thing.
A second iron phosphate in a similar disordered form sits nearby. Keeping two such forms side by side requires tight biological control.
Over a distance of a few hundred micrometers, about the width of a few hairs, the mineral content and stiffness of the stylus change three- to eightfold. This gradual change lets the stalk blend from soft at one end to hard at the other.
Santabarbaraite is low in iron and high in water, which may make the stylus strong for its weight.
The team then used these ideas to make inks for 3D printing composite materials whose stiffness can be tuned.
What are the limits?
This is basic materials science on one species. It explains how a structure is built. It does not test anything in people.
Why would a dentist care about mollusk teeth?
Human teeth face a similar problem. Hard enamel sits on softer dentin, and teeth are held in bone by a flexible ligament. Where hard and soft materials meet is often where fillings, crowns and bonded restorations fail. Studying how animals grade one material into another gives dental materials researchers ideas for stronger joins between a restoration and the tooth.
If you are curious how printing is already used in dental care, see our post on 3D printing and dentistry. For another look at teeth in the animal world, read about shark teeth and human dental biology.
Sources
Stegbauer L, Smeets PJM, Free R, et al. Persistent polyamorphism in the chiton tooth: from a new biomineral to inks for additive manufacturing. Proc Natl Acad Sci U S A. 2021;118(23):e2020160118. doi:10.1073/pnas.2020160118. Retrieved via PubMed.
Northwestern University. Press release on santabarbaraite in chiton teeth, May 2021.
About the author. CJ Henley, DMD, is a general dentist in Jacksonville whose practice focuses on cosmetic and restorative dentistry, dental implants and dental care around head and neck cancer. Meet Dr. Henley.
CJ Henley, DMD, PA · 3675 Hendricks Avenue, Jacksonville, FL 32207 · 904-398-1549
Updated October 2026.