Research

Researchers leverage rare iron mineral found in “wandering meatloaf” mollusk teeth for 3D printing bioinks

Researchers from Northwestern University have discovered a rare type of iron mineral in mollusk teeth that has previously only been observed in rocks.

The mineral, Santabarbaraite, was found in the stylus – a long, hollow structure that connects a tooth to a soft membrane – in a Cryptochiton stelleri mollusk, also known as the “wandering meatloaf” mollusk due to its reddish-brown color, shape and size. 

The researchers leveraged the discovery to develop bioinks suitable for 3D printing that mimic the mollusk stylus’ strength, stiffness, and connective properties. 

“This mineral has only been observed in geological specimens in very tiny amounts and has never before been seen in a biological context,” said Derk Joester, Senior Author of the study and Associate Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering

“It has high water content, which makes it strong with low density. We think this might toughen the teeth without adding a lot of weight.”

Cryptochiton stelleri or "wandering meatloaf" mollusk in the wild. Photo via Jerry Kirkhart.
Cryptochiton stelleri or “wandering meatloaf” mollusk in the wild. Photo via Jerry Kirkhart.

The “Wandering Meatloaf” mollusk

Nicknamed the “Wandering Meatloaf” due to its reddish-brown color and body that can grow up to 14 inches in length, the Cryptochiton stelleri mollusk is also referred to as the giant Pacific chiton and giant gumboot chiton.

Chiton teeth are reportedly made up of one of the hardest known materials in nature, designed to endure chewing on rocks to feed. Chiton teeth are attached to a soft, flexible, tongue-like radula which scrapes over the rocks to collect algae and other food.

The researchers were particularly interested in the stylus, which connects the ultrahard tooth to the soft radula, and believe that there is much to learn from this natural material.

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

To examine a tooth from a Cryptochiton stelleri, the researchers used the synchrotron Mössbauer spectroscopy capabilities at the Argonne National Laboratory’s Advanced Photon Source, and leveraged transmission electron microscopy at Northwestern’s Atomic and Nanoscale Characterization and Experiment (NUANCE) Center.

Through this, they found santabarbaraite dispersed throughout the chiton’s upper stylus, a rare iron mineral that until now has only been observed in rocks. 

Rendering of a virtual section of a single tooth head and upper stylus, generated from a 3D reconstruction as determined by synchrotron microcomputer tomography. Image via Northwestern University
Rendering of a virtual section of a single tooth head and upper stylus, generated from a 3D reconstruction as determined by synchrotron microcomputer tomography. Image via Northwestern University

Leveraging santabarbaraite for 3D printing

After discovering the rare mineral in the chiton’s stylus, the researchers embarked upon recreating the material in a bioink designed for 3D printing. They combined iron and phosphate ions with a biopolymer derived from the chiton to create a reactive ink that “printed well” when mixed immediately before printing.

“As the nanoparticles form in the biopolymer, it gets stronger and more viscous,” said Joester. “This mixture can then be easily used for printing. Subsequent drying in air leads to the hard and stiff final material.”

The ability to replicate the strength, stiffness and connectivity properties of the chiton’s stylus via bioink development and 3D printing is promising for future learning and material development inspired by nature, Joester believes. 

“We’ve been fascinated by the chiton for a long time,” he said. “Mechanical structures are only as good as their weakest link, so it’s interesting to learn how the chiton solves the engineering problem of how to connect its ultrahard tooth to a soft underlying structure.

“This remains a significant challenge in modern manufacturing, so we look to organisms like the chiton to understand how this is done in nature, which has had a couple of hundred million years of lead time to develop.”

More information on the study can be found in the paper titled “Persistent polyamorphism in the chiton tooth: From a new biomaterial to inks for additive manufacturing”which will be published in the Proceedings of the National Academy of Sciences journal later this week. 

3D printing of bioinspired composites. Photo via Northwestern University.
3D printing of bioinspired composites. Photo via Northwestern University.

Bioinks inspired by nature

Researchers and scientists are increasingly looking to replicate materials and processes found in the natural world that exhibit desirable properties unobtainable by purely man-made means. Bioink development for 3D printing is playing a key role in harnessing these properties, and there has been a plethora of recent research breakthroughs in this area. 

Soft robotics applications have received particular attention, with researchers from Rutgers University having 3D printed camouflage-ready robots inspired by the adaptable cells found in squid, cuttlefish, and octopuses, and Yamagata University scientists having developed a 3D printed actuator that could form the basis of a jellyfish-like soft robot. Elsewhere, Dutch scientists have 3D printed a novel adhesive material with a microscopic mushroom-like design that could be deployed to help soft robots walk vertically.

The medical sector has also seen new nature-inspired developments, including novel dragonfly-inspired 3D printed “spiky joints” for treating wrist injuries, and 3D printed microneedle patches for painless drug delivery that replicate the hierarchical structure of a limpet

Other 3D printing research developments and applications inspired by natural materials include biomimetic cell-like structures with unique energy absorption capabilities based on the skeleton of a cuttlefish, 3D printed concrete structures inspired by lobster shell patterns, and shatterproof materials based on the natural properties of spiderwebs.

Mouth and protruding anterior end of the radula. Photo via David Young, Victoria High School.
Mouth and protruding anterior end of the radula. Photo via David Young, Victoria High School.

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Featured image shows Cryptochiton stelleri or “wandering meatloaf” mollusk in the wild. Photo via Jerry Kirkhart.

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