The rubber boot chiton is not a glamorous being. The large, lumpy mollusk crawls along the waters of the Pacific coast, dragging its reddish-brown body up and down the coast. It is sometimes referred to as “the wandering meatloaf” for a reason. But the chiton’s modest body hides a series of tiny but impressive teeth. These teeth, with which the creature scrapes algae from rocks, are among the hardest materials that exist in a living organism.
Now a team of scientists has discovered a surprising ingredient in the rock-hard teeth of chitons: a rare iron-based mineral that has previously only been found in real rocks. Tiny particles of the mineral, which is strong but light, help harden the mollusc’s tooth roots, the researchers reported Monday in the journal PNAS.
The discovery could help engineers create new types of materials, according to the scientists who provided evidence of the principle by developing a new chiton-inspired ink for 3D printers.
A chiton feeds by stroking its flexible, ribbon-shaped tongue, known as a radula, over algae-covered rocks. Its ultra-hard teeth are arranged in rows along the soft radula. A long, hollow tube known as a stylus anchors each tooth to the radula.
Scientists had previously discovered that the tips of chiton’s teeth contained an iron ore called magnetite, but they knew less about the composition of the stylus. “We knew that there was iron in the upper part of the tooth,” says Linus Stegbauer, materials scientist at the University of Stuttgart in Germany and first author of the study. “But in the root structure we had no idea what was going on in there.”
In the new study, researchers analyzed chiton’s teeth using a variety of advanced imaging techniques, including various types of spectroscopy, which scientists can use to learn about the chemical and physical properties of a material by observing how it interacts with light and other types of electromagnetic radiation.
They found that the pen contained tiny particles of some type of iron-based mineral suspended in a softer matrix. (The matrix is made up of chitin, the compound that makes up the exoskeletons of insects and crustaceans.)
After further analysis, they were amazed to find that the mineral particles were santabarbaraite, a mineral that had never been observed in living things before. “There were a number of surprises and then they just kept coming,” says Derk Joester, senior writer and materials scientist at Northwestern University.
Santabarbaraite is a hard mineral, but it contains less iron and more water than magnetite, which makes it less dense. The mineral could enable the chiton to build strong, light teeth while reducing its reliance on iron. “Iron is a rare material physiologically,” said Dr. Joester.
The researchers also found that the santabarbaraite particles were not evenly distributed across the entire pen. Instead, they concentrated at the top, closest to the tooth surface, and became more sparse at the bottom, where the stylus joined the soft radula. This distribution pattern created a gradient that made the pen stiffer and harder at the top and more pliable at the bottom.
“The organism has tremendous spatial control over where the mineral goes,” said Dr. Joester. “And that, I think, really got us thinking about how this could be used to make materials. If the organism can model this, can we do the same? “
The researchers decided to create a new 3D printer “ink” inspired by the chiton tooth. They started with a chitin-like compound and then added two liquids: one with iron and one with phosphate. Mixing the ingredients together resulted in a thick paste dotted with tiny particles of a santabarbaraite-like mineral. “And then it’s ready to print – you can just transfer it to your 3D printer,” says Dr. Bridge builder.
The ink cured as it dried, but its final physical properties depended on how much iron and phosphate were added to the mixture. The more that was added, the more nanoparticles were formed and the stiffer and harder the final material became. By adapting the recipe, the researchers were able to create objects that were as flexible and rubbery as an octopus or as stiff and hard as bones.
“It should be possible to mix the color in a ratio that you can change immediately before printing,” said Dr. Joester. “And that would make it possible to change the composition, the amount of nanoparticles and thus the strength of the material in next to no time. This means that you can print materials in which the strength changes very significantly over relatively short distances. “
The technique could be useful in the emerging field of soft robotics, allowing engineers to build machines that are hard and stiff in some places and soft and pliable in others, said Dr. Joester: “I think it would be amazing if you could print all of these gradients into the structure.”