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San Diego, Calif., Sept. 19, 2019 -- Even millions of chewing cycles and regular contact with rocks and hard shells can’t dull sea urchins’ teeth.
New research from Northwestern and University of California San Diego engineers reveals that the chemical composition and microstructure of sea urchin teeth are optimized for sharpness preservation – findings with the potential to influence the design of tools for mining, boring and machining operations.
“We found that the superior performance of the sea urchin dentition system emerges from tooth self-sharpening during the organism life span,” said Horacio Espinosa, a professor in at the McCormick School of Engineering at Northwestern. “Interestingly, to account for the loss of material, the sea urchin tooth grows continuously.”
The paper, “In situ Wear Study Reveals Role of Microstructure on Self-sharpening Mechanism in Sea Urchin Teeth,” was published Sept. 18 in the journal Matter.
Joanna McKittrick, a professor in the Department of Mechanical and Aerospace Engineering at UC San Diego was the paper's senior author. A team in her research group, led by Michael Frank, analyzed the urchin's mouht to build a robotic claw-like device in 2016.
Biologists previously hypothesized that sea urchins possessed a mechanism for tooth sharpening, but the process was never quantified or directly observed. Through in situ mechanical testing instrumentation and scanning electron microscopy (SEM), the team led by Espinosa were able to image tooth wear in 3D and generate videos revealing the mechanisms involved.
A close look at sea urchins
The feeding part of sea urchins is called Aristotle’s lantern, which is a skeleton muscular system with five separate jaws each holding a tooth firmly in place. The jaws rest in a circular arrangement at the center of the sea urchins’ bodies, working together simultaneously to allow outward protrusion and inward withdrawal of the teeth.
During the research, the team learned that the material on the outer layer of a sea urchin’s tooth, called “the stone,” exhibits controlled chipping of the tooth to maintain its sharpness – a process researchers describe as similar to the sharpening of a knife by selectively removing material from the cutting edge.
“Wear-off does not blunt the tip of the tooth, but it instead sharpens it. Later, the add-up of new materials, via continuous tooth growth, will compensate for the loss during the animal's life span,” said Hoang Nguyen, a Northwestern Engineering PhD student.
“The stone” is made of high aspect ratio, small diameter fibers surrounded by an organic sheath, which allows for high resistance and shearing when damage accumulates. A nearby region called “the plate,” made up of calcite single crystals on the convex surface of the tooth, can be removed when a certain amount of the stone is removed, and this plate chipping preserves the curvature and sharpness of the teeth over time.
“We were awed by the fact that nature has found its way to tailor the design to optimize one animal in their habitat,” Nguyen said.
This discovery could be applied to tooling by imitating the gradient distribution of the materials within the sea urchin structure to design the tip of a carving tool. Once the tip becomes blunt, it could be sharpened by scratching it on a surface of a harder material.
“I am exploring ways to do additive manufacturing of materials that can exhibit the performance of natural materials,” Espinosa said. “Natural materials have an exquisite combination of size scales, morphologies, and special distributions of chemical compositions.”
The work was supported by a Multi-University Research Initiative grant through the Air Force Office of Scientific Research.