Irvine, Calif. — Researchers at the University of California, Irvine and Japan's Okayama and Toho universities conducted a first-of-its-kind study to understand how chitons, mollusks that feed on algae growing on intertidal rocks, develop such hard, wear-resistant and magnetic teeth, and what they learned is inspiring new ways to produce advanced materials for a variety of applications. The results were published today in Science.
In its study, the team unveiled the process by which chiton-specific, iron-binding proteins called RTMP1 are transported into newly forming teeth through nanoscopic tubules called microvilli. Where and when the proteins are deposited is precisely controlled, ensuring that the creatures develop a hard, strong and tough dental architecture that enables them to perform the repetitive abrasive motions on which their lives depend.
"Chiton teeth, which consist of both magnetite nanorods and organic material, are not only harder and stiffer than human tooth enamel, but also harder than high-carbon steels, stainless steel, and even zirconium oxide and aluminum oxide – advanced engineered ceramics made at high temperatures," said co-author David Kisailus, UC Irvine professor of materials science and engineering. "Chiton grow new teeth every few days that are superior to materials used in industrial cutting tools, grinding media, dental implants, surgical implants and protective coatings, yet they are made at room temperature and with nanoscale precision. We can learn a lot from these biological designs and processes!"
There are more than 900 different chiton species worldwide, mostly dwelling within intertidal coastal regions. They can be found in places like Crystal Cove and Laguna Beach near the UC Irvine campus, but Kisailus said the ones investigated in this study are much larger and live in Northwest coastal areas of the United States and off the coast of Hokkaido, Japan. The research team learned that the RTMP1 proteins exist in chitons at disparate locations around the world, which suggests "some convergent biological design in controlling iron oxide deposition," according to Kisailus.
He said that when he and his collaborators began, they were not aware of how and when these iron-binding proteins were conveyed into the chiton teeth. But by using a combination of advanced materials and molecular biological analyses, they discovered that these specialized proteins that were initially found within tissues surrounding immature, nonmineralized teeth were directed through nanostructured tubules into each tooth.
Once inside, the proteins bind to preassembled scaffolds of chitin nanofibers, the structural biopolymer that controls the architecture of the magnetite nanorods in the teeth. Concurrently, iron stored in ferritin, another protein found in the tissue outside the teeth, is released into each tooth, where it binds to the RTMP1, leading to the precise deposition of nanoscale iron oxide, which continues to grow during the tooth maturation into highly aligned magnetite nanorods that ultimately yield the ultrahard teeth.
Kisailus said this project has improved humanity's understanding of cellular iron metabolism while providing insight into the synthesis of next-generation advanced materials.
"The fact that these organisms form new sets of teeth every few days not only enables us to study the mechanisms of precise, nanoscale mineral formation within the teeth, but also presents us with new opportunities toward the spatially and temporally controlled synthesis of other materials for a broad range of applications, such as batteries, fuel cell catalysts and semiconductors," he said. "This includes new approaches toward additive manufacturing – 3D printing – and synthesis methods that are far more environmentally friendly and sustainable."
Setting this study apart, according to Kisailus, was the blending of state-of-the-art materials science techniques, including ultra-high-resolution electron microscopy, X-ray analysis and spectroscopy, with biological methods such as immunofluorescence, gene expression tracking and RNA interference to reveal the full molecular choreography of chiton tooth formation.
"By combining biological and materials science approaches through wonderful, global efforts, we've uncovered how one of the hardest and strongest biological materials on Earth is built from the ground up," Kisailus said.
His collaborators on this project were Michiko Nemoto, Koki Okada, Haruka Akamine, Yuki Odagaki, Yuka Narahara, Kiori Obuse, Hisao Moriya and Akira Satoh of Okayama University and Kenji Okoshi of Toho University.
Kisailus' contributions to this work were funded by the U.S. Air Force Office of Scientific Research.
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