Researchers have discovered that the skeleton of the sea star Protoreaster nodosus contains specialized mineral structures capable of guiding and concentrating light, suggesting natural mechanical support and optical sensing in a singular system.
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A discovery emerged unexpectedly for Ling Li, associate professor of materials science and engineering at Penn’s School of Engineering and Applied Science, 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 lenslike 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, where Li, Ph.D. student and first author Liuni Chen, and 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.
Ling Li holds the skeleton of a sea star (left), which is composed of numerous millimeter-sized skeletal elements called ossicles. These biomineralized ossicles are primarily made of calcium carbonate, the same chemical composition as the geological calcite crystal shown for comparison on the right.
(Image: Courtesy of Penn Engineering)
“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.”
Using high-resolution imaging and optical experiments, the researchers found that these structures—called light-guiding structures—extend deep into the skeleton like tiny mineral cones. Rather than serving only a mechanical purpose, they can transmit and focus incoming light into an internal cavity within the skeletal element. Optical simulations showed that individual structures can guide roughly 70% of incident light while concentrating it at their base. Working together as an array, the structures collect light across a wide field of view and produce a signal several times stronger than any single structure alone.
The finding helped solve a longstanding mystery. Similar lens-like features had been observed decades ago in sea stars and brittle stars, but their function remained unclear. “A different group had already identified related lens structures in brittle stars,” says Li. “When we found similar features in sea stars, we were able to connect the dots and investigate them systematically.”
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