Researchers at Penn Engineering have discovered how a coral’s skeleton compacts itself to ward off danger, a novel discovery of ‘granular jamming’ in a living organism.
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When the branches of Leptogorgia chilensis, a soft coral found along the Pacific coast from California to Chile, are touched, its flexible arms stiffen. Engineers at the School of Engineering and Applied Science (SEAS) have discovered the mechanism underlying this ability, one that could advance fields as varied as medicine, robotics, and manufacturing.
Penn Engineering doctoral student Chenhao Hu holding a 3D-printed model of a sclerite, the tiny mineral particles that make up the coral’s skeleton and whose unique shape allows the organism to tune its own stiffness.
(Image: Bella Ciervo)
In a new Proceedings of the National Academy of Sciences paper, a group led by Ling Li, associate professor in materials science and engineering and in mechanical engineering and applied mechanics at SEAS, describes how the coral’s skeleton—made of millions of mineral particles suspended in a gelatinous matrix—compacts itself to ward off danger.
“It’s almost like a traffic jam,” says Li. “When stimulated, the coral’s tissues expel water, shrinking the gel and squeezing the particles closer together until they jam in place.”
Physicists have long studied this phenomenon, known as “granular jamming,” by manipulating grainy substances like sand and coffee grounds, but this marks the first time granular jamming based on hard particles has been observed in a living organism.
Measuring about a tenth of a millimeter in size, the particles are somewhat cylindrical, like a rod studded with branching outgrowths at regular intervals. “Once the sclerites get close enough to their neighbors, their branches jam together, holding them in place,” explains Hu.
The researchers explored the material’s properties with advanced imaging techniques, computational modeling, and by poking and prodding preserved samples of the coral. “When we applied force to the samples,” says Hu, “the material system initially shrank, occupying less volume because the particles were closer together.”
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