Northwestern University researchers have first discovered a rare mineral hidden in the teeth of a chiton, a large mollusk found on rocky shores. Before this strange surprise, the iron mineral santabarbaraite had only been documented in rocks.

The new finding helps understand how the entire chiton tooth – not just the ultra-hard, durable cusp – is designed to withstand chewing on stones in order to feed itself. Based on minerals from chiton teeth, the researchers developed a bio-inspired ink for 3D printing of ultra-hard, rigid and durable materials.

“This mineral has only been observed in very small amounts in geological samples and has never been seen before in a biological context,” said Derk Joester of Northwestern, lead author of the study. “It has a high water content, which makes it strong at a low density. We believe this could make the teeth harder without adding a lot of weight. “

The study will be published in the week of May 31st Procedure of the National Academy of Sciences.

Joester is Associate Professor of Materials Science and Engineering at the McCormick School of Engineering in the Northwest. Linus Stegbauer, a former postdoctoral fellow in Joester’s laboratory, is the first author of the work. While doing research at Northwestern, Stegbauer is now Principal Investigator at the Institute for Interfacial Process Engineering and Plasma Technology at the University of Stuttgart in Germany.

Chiton teeth, one of the hardest known materials in nature, are attached to a soft, flexible, tongue-like radula that scrapes over rocks to collect algae and other food. After studying chiton teeth for a long time, Joester and his team finally turned to Cryptochiton stelleri, a huge red-brown chiton sometimes affectionately referred to as the “wandering meatloaf”.

To examine a Cryptochiton stelleri tooth, Joester’s team worked with Ercan Alp, a senior scientist at the Argonne National Laboratory’s Advanced Photon Source, to use the facility’s synchrotron Mössbauer spectroscopy, and Paul Smeets to do transmission electron microscopy at the Northwestern University Atomic and Center for Nanoscale Characterization and Experimentation (NUANCE). They found santabarbaraite dispersed in the top pin of the chiton, a long, hollow structure that connects the head of the tooth to the flexible radula membrane.

“The stylus is like the root of a human tooth that connects our cusp of our tooth to our jaw,” said Joester. “It is a tough material that consists of extremely small nanoparticles in a fibrous matrix of biomacromolecules, similar to the bones in our body.”

Joester’s group challenged themselves to recreate this material using an ink developed for 3D printing. Stegbauer developed a reactive ink made from iron and phosphate ions mixed into a biopolymer obtained from the chitin. Working with Shay Wallace, a Northwest graduate student in Mark Hersam’s lab, Stegbauer found that the ink printed well when mixed just before printing.

“When the nanoparticles form in the biopolymer, it becomes stronger and more viscous. This mixture can then be used for printing without any problems. The subsequent air drying leads to the hard and stiff end material, ”said Joester.
Joester believes we can continue to learn and develop Chiton Pen-inspired materials that bond ultra-hard teeth to a soft radula.

“We have long been fascinated by the chiton,” he said. “Mechanical structures are only as good as their weakest link, so it is interesting to see how the chiton solves the technical problem of connecting its ultra-hard tooth to a soft underlying structure. This remains a significant challenge in modern manufacturing, so let’s look at organisms like the chiton to understand how this happens in nature, which took hundreds of millions of years to develop. “

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The study “Persistent Polyamorphism in Chiton Tooth: From a New Biomaterial to Inks for Additive Manufacturing” was supported by the National Science Foundation (award numbers DMR-1508399 and DMR-1905982) of the National Institutes of Health (award number NIH-). DE026952), Air Force Research Laboratory (price number FA8650-15-2-5518) and Deutsche Forschungsgemeinschaft (price number STE2689 / 1-1).

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