When 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.

Collaborating Across Fields to Understand the Sea Star

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 percent 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.”

Combining expertise in materials science, optics, biology, crystallography and computational modeling, the research team aimed to understand not only what the structures looked like, but also how they might function in the animal’s daily life.

Light sensing can help marine organisms detect predators, find shelter, locate food and navigate their environment. Although the exact biological role of the structures remains under investigation, the work demonstrates that the sea star skeleton is doing more than providing support and protection.

For engineers, that multifunctionality may be the most exciting aspect of the discovery.

“In nature, one structure often serves many purposes because materials are expensive for organisms to make,” Li says. “Evolution tends to use the same material system for multiple functions, so adopting that approach in material manufacturing and design would only help our engineered structures perform better and cost less.”

Applying Sea Star Skeleton Lessons to Engineered Materials

Multifunctional design is not a new concept in engineering, but integrating additional capabilities into a material often comes with tradeoffs. Adding sensors, for example, can compromise strength, weight or durability.

What Li’s lab is adding to this engineering approach is the way he looks to nature for the solutions. The sea star’s approach is one that can be a valuable lesson.

“Instead of attaching sensing components to a protective structure, the animal embeds optical functionality directly into its skeletal architecture,” says Li. “The same porous mineral framework that provides support also contains built-in light-guiding elements, and these organisms have had millions of years of evolution to perfect these functions.”

Why Does This Particular Combination of Multifunctionality Matter in Engineering?

This example from nature could inspire future engineered materials like lightweight foams, packaging materials and structural components for spacecraft capable of both protecting and monitoring their surroundings. 

Lightweight cellular materials and foams are widely used in packaging, transportation and structural support applications, but they typically serve only mechanical roles. Inspired by the sea star, engineers could envision future materials that incorporate sensing capabilities without sacrificing structural performance.

“Imagine a protective panel that not only absorbs impacts but also monitors environmental conditions, detects damage or provides feedback about its surroundings,” says Li. “The sea star demonstrates that such combinations may be possible.”

Porosity to Add Strength

The study also highlights another lesson from nature: geometry matters.

Sea star skeletons are composed of intricate networks of pores that minimize material use while maintaining mechanical performance. In conventional engineering materials, adding pores usually reduces stiffness and strength. The sea star overcomes this challenge through carefully controlled architectural design.

“The transitions between pores are extremely smooth on a microscopic level,” Li explains. “That allows stresses to be distributed more uniformly throughout the structure.”

Previous work from Li’s group showed that these skeletal architectures can also localize fractures, preventing small cracks from propagating through the entire system. Together, these strategies allow a brittle mineral to behave in ways that conventional ceramic materials typically cannot.

The discovery of light-guiding structures adds another layer of sophistication to an already remarkable material system. In simulations, the structures not only transmitted light but also enhanced the stiffness of the surrounding skeleton, reinforcing the idea that optical and mechanical functions can coexist within a single design.

Still Uncovering Nature’s Secrets

Discoveries like this underscore the value of studying natural systems with curiosity rather than focusing only on immediate applications.

“Many of the most interesting findings happen by accident,” he says. “There is still so much we don’t know about how these organisms work.”

That sense of exploration continues to guide the lab’s research. The team is now investigating how the light-guiding structures form during growth and regeneration, a process known as biomineralization and morphogenesis. Understanding how sea stars construct these complex mineral architectures could provide additional insights into sustainable manufacturing and materials production.

“Nature builds sophisticated mineral structures at ambient temperatures and pressures, using far less energy than many industrial manufacturing processes,” says Li. “By uncovering the mechanisms behind these biological systems, we can identify new pathways for creating advanced materials more efficiently.”

Learn more about Ling Li’s research here.

This work was supported by the National Science Foundation (NSF) (grants 1942865 and 2432445) and partially supported by a Research Grant from HFSP (Ref.-No: RGP016/2023. 10.52044/HFSP.RGP0162023.pc.gr.168601).