Bioengineering Advances Inspired by Coral’s Natural “Jamming System”

Touch the branches of Leptogorgia chilensis, a soft coral found along the Pacific coast from California to Chile, and its flexible arms stiffen. A research team led by University of Pennsylvania scientists has now discovered the mechanism underlying this astonishing ability. Studying the red gorgonian coral as a model system, the team used a variety of techniques, including multiscale three-dimensional (3D) structural characterization, parametric geometrical modeling, 3D printing, mechanical testing, and discrete element simulations, to investigate 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,” said research lead Ling Li, PhD, associate professor in materials science and engineering and in mechanical engineering and applied mechanics. “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 such as sand and coffee grounds, but this marks the first time granular jamming based on hard particles has been observed in a living organism.

“Imagine being able to adjust the stiffness of a surgical instrument or robotic arm,” said first author Chenhao Hu, a doctoral student in Li’s lab. “In this coral’s skeleton, nature has created an incredible material whose principles we can adapt for human use.” The team suggests their discoveries could lead to advances in fields including medicine, robotics, and manufacturing.

They reported on their studies in Proceedings of the National Academy of Sciences (PNAS), in a paper titled “Mineralized sclerites in the gorgonian coral Leptogorgia chilensis as a natural jamming system,” concluding, “The findings in this natural jamming system offer insights for designing synthetic mechanotunable material architectures for a wide range of applications, from soft robotics to mechanical dampeners.”

Granular materials such as sand and grains can undergo a transition from a fluid-like to a solid-like state, the authors wrote. “This phenomenon, known as granular jamming, occurs when the particles in the granular assembly, arranged in either random or periodic patterns, reach a critical density and become locked in a stable configuration.”

For years, Li’s lab has studied the skeletons of undersea creatures, with the motivation that uncovering the basis of their material properties will lead to advances in engineering. “They’re basically made of chalk,” Li explained, referring to calcium carbonate, the same cheap and plentiful white powder that forms eggshells, sticks of chalk, marble, limestone, and pearls. “What gives their skeletons interesting properties is how the calcium carbonate is structured and organized.”

“The soft corals (Cnidaria, Octocorallia), a diverse group of colonial marine invertebrates, can reversibly tune their body stiffness in response to external stimuli,” the team further pointed out in their paper. “This capability is attributed to their dynamic skeletal systems, which consist of thousands of mineralized skeletal elements, called sclerites, embedded within a gel-like matrix that swells/deswells and unjams/jams the sclerites, thus modulating skeletal stiffness.”

So, while marine biologists recognized long ago that soft corals such as L. chilensis have skeletons containing granular particles, the grains’ shapes had primarily been used to classify different species. “Limited work has focused on the functional properties of the shapes themselves, particularly from the granular jamming point of view,” commented Li. As the team further noted, “While sclerite morphology is widely used for species identification, its role in the mechanical performance of a soft coral’s skeletal system is largely unknown.”

Past research has recognized the potential of granular jamming in fields like manufacturing—one group developed a robotic grabber arm whose sand-filled “hand” envelops complex objects, then stiffens to pick them up—but has relied on a few basic grain shapes.

“It’s hard to find the right shape,” said Hu. “They need to jam when they’re close together, which requires friction and interlocking, but still separate easily into a relaxed state.” Because of their varied geometry, sand and coffee grounds make studying the mechanics of the process challenging, while easy-to-manufacture spheres frequently slide past one another due to a lack of friction.

In a sense, nature provides a shortcut: if the Penn researchers could characterize the mineral particles, or sclerites, in L. chilensis, that might point to a novel, and perhaps better, shape for human systems that rely on granular jamming. “In this work, using the red gorgonian coral, Leptogorgia chilensis, as a model system, we elucidate the structure-property relationship of this sclerite-based biological granular jamming system.”

Measuring about a tenth of a millimeter in size, the particles are somewhat cylindrical, like a rod studded with branching outgrowths at regular intervals. The investigators explained, “In this species, the sclerite geometry consists of a cylindrical shaft with two sets of triradiate side branches and two axial branches with their tips covered with small spike-like protrusions.” Hu added, “Once the sclerites get close enough to their neighbors, their branches jam together, holding them in place.”

The researchers explored the material’s properties with advanced imaging techniques, computational modeling, and by poking and prodding preserved samples of the coral. “Our combined analyses revealed that the multiscale geometrical features of sclerites in L. chilensis play a critical role in their jamming behavior,” they found. “When we applied force to the samples,” said Hu, “the material system initially shrank, occupying less volume because the particles were closer together.”

Ultimately, the researchers suggested, their study points toward the benefits of studying nature to find new materials. “We just studied one coral species,” pointed out Li. “But there are many other soft coral species out there, which use different sclerite shapes, with potentially different properties.” In their paper, the team concluded, “The present study investigates sclerite-based gorgonian skeletons as a natural jammable material system, which offers insights for designing particle geometries for granular jamming with improved mechanical strength and structural efficiency.”

In the future, the skeleton of L. chilensis could serve as a point of comparison for other natural systems and inspire human engineers. “There are so many situations where we might want to selectively tune the stiffness of a material,” said Li. “In this coral, nature has given us a blueprint we can follow.” The authors further noted, “Beyond providing insights into the structural complexities of gorgonian skeletal architectures, this work also presents an alternative, bioinspired perspective for the development of innovative synthetic granular jamming-based mechanotunable systems.