GoKawiilWhen engineers think about protective materials, like those used in packaging and support, they usually think about strength, stiffness and durability. But what if those same materials could also sense their external environment?
That question emerged unexpectedly for Ling Li, Associate Professor in Materials Science and Engineering, when his lab and colleagues were investigating how sea stars build lightweight yet resilient skeletons.
“We were looking at sea stars to understand how nature creates porous skeletal materials that are both strong and lightweight,” says Li. “Then we discovered lens-like structures embedded in the tips of the sea stars’ arms.”
That surprise became the focus of a study published in Proceedings of the National Academy of Sciences (PNAS), where Li, Ph.D. student and first author Liuni Chen and their collaborators from Penn Engineering, Virginia Tech, MIT, Bowdoin College, the University of South Carolina and the Zuse Institute Berlin, revealed that the skeleton of the sea star Protoreaster nodosus contains specialized mineral structures capable of guiding and concentrating light. The finding suggests that nature may have evolved a way to combine mechanical support and optical sensing within the same material system.
Multifunctional Materials Found in Nature
Li recognizes that the discovery was accidental, but that the multifunctionality of the sea star skeleton is indeed intentional and expected.
“Natural materials often have to do many things at once,” Li says. “They provide structural support, protection, sensing and other functions. We study how these systems are designed and then extract the underlying principles that can inspire future engineering materials, such as lightweight, impact-resistant structures, self-monitoring materials that can sense damage and architected materials for aerospace, transportation and protective applications.”
Like other echinoderms — including sea urchins and brittle stars — sea stars build their skeletons from calcium carbonate, a mineral that engineers know well. Although lightweight and abundant, calcium carbonate is inherently brittle. Making it porous, a common strategy for reducing weight, typically makes it even more fragile.
Yet sea stars somehow achieve the opposite outcome. Their skeletons are highly porous but remain strong, resilient and capable of withstanding the demands of life in the ocean.
Li’s group originally set out to understand how this remarkable skeletal architecture works, which led to the discovery of a unique dual-scale architected microlattice published as a cover story in Science. But when they examined the tips of the sea star’s arms, they noticed something unusual: dozens of smooth, lens-like protrusions embedded within the mineral skeleton.
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