Compared to metal- and polymer-based materials, ceramics can better withstand high temperatures and corrosive environments, but their brittle nature often makes them susceptible to breaking. This brittleness potentially causes problems for innovators trying to create lightweight porous versions of ceramic materials, explaining why ceramic foams are not typically used as structural components.

Facing the challenging task of developing lightweight, high-strength ceramic materials, Ling Li, an assistant professor of mechanical engineering at Virginia Tech, has turned to an unexpected collaborator for design inspiration: the knobby starfish from the tropical Indo-Pacific. By investigating the complex and highly ordered mineralized skeletal system of this unusual marine species, Li and his research team discovered an unexpected combination of characteristics that may lead to developing an entirely new class of high-performance lightweight ceramic composites. They report their findings in a paper in Science.

The automobile and aerospace manufacturing industries have a strong interest in designing materials that are both strong and lightweight, combining the economy of better fuel efficiencies with strength. But they find this balance difficult to strike, since stronger materials commonly possess high densities and thus weigh more.

Nature, through millions of years of evolution, has come up with an ingenious way of solving this problem: using porous materials. The introduction of internal porosity potentially creates both extremely lightweight and mechanically efficient materials.

Several examples of porous materials exist in nature, including the human skeletal system, the stems of plants and the hives of honeybees. Studying these natural materials under a microscope reveals they are filled with tiny voids or chambers. Natural growth forms these porous biological constructions very efficiently, and that formation often results in unexpectedly complex internal geometries.

In the Laboratory of Biological and Bio-Inspired Materials at Virginia Tech, Li and his team are investigating natural lightweight ceramic structures, with the goal of developing new material design principles for addressing the mechanical weakness of ceramic foams and architected materials.

“Our overall goal is to learn and take inspiration from nature to develop novel porous materials,” Li said. “Nature offers many good material lessons for designing porous materials that are both strong and damage-tolerant.”

Previously, the team discovered that the unique chamber-based bioceramic structure of cuttlebone (the internal skeleton of cuttlefish) is simultaneously strong, stiff and fracture-resistant, while still allowing for buoyancy regulation. That project and others like it motivated the team to investigate additional applications for nature’s porous designs at the microscale.

In this work, Li and his team turned their attention to the skeleton of the knobby starfish. Widely distributed throughout the Indo-Pacific region, the species’ dried skeletons are often used for home decoration. These starfish feature cone-shaped projections that rise from their dorsal surface to discourage predators.

While studying samples of these starfish skeletons at the Nanoscale Characterization and Fabrication Laboratory (NCFL), Li and PhD student Ting Yang (co-first author of the paper and now a post-doctoral fellow at MIT) made an observation that piqued their interest. At the microscale, the starfish skeleton exhibited a lattice architecture with very regular arrangements of branches quite different from the porous structures of the cuttlebone and sea urchin spines previously studied.

In fact, they found that the unique skeletal organization of this starfish exhibits the highest structural regularity ever reported for this group of invertebrates. Such regular lattice-like structures display remarkable similarities with the space frame truss structures commonly employed in modern human construction projects.

The team wondered how this natural ceramic lattice material achieved its mechanical properties, since starfish skeletons are made of calcite, a crystalline form of calcium carbonate (chalk). Any child familiar with playing outside knows that sidewalk chalk is very brittle and easily broken. However, the body of the starfish demonstrates high strength and flexibility. Uncovering the underlying principles of this structure may help solve the challenges of making stronger porous ceramics.

What the team found was unexpected. As in other starfish species, the skeleton of the knobby starfish consists of many millimeter-sized skeletal elements called ossicles. These ossicles connect with soft tissue, allowing the animal to be flexible and move. Li and his team discovered that each ossicle is constructed of a microlattice structure so uniform that it can be described mathematically, composed of branches connected through nodes, similar to the structure of the Eiffel Tower. Even more interesting, the team found that the uniform structure of the microlattice, because of the alignment of its atoms, is essentially a single crystal structure at the atomic level.

“This unique material is like a periodic lattice carved from a piece of single-crystal calcite,” Li said. “This nearly perfect microlattice has not been reported in nature or fabricated synthetically before. Most highly regular lattice materials are made by combining materials with small crystals to create composites, but this is new. It’s grown as a single piece.”

This structure allows a starfish to reinforce its skeleton strategically in particular directions, offering enhanced protection. In addition, it appears the animal can thicken branches along selected directions and in particular regions, improving its mechanical performance in a similar manner to how the human body can alter the local geometry of its porous bones to adapt to physical activity. In the starfish, researchers also found regions where the structure appeared to modify the regular lattice pattern of its design, a feature that inhibits crack expansion when the microlattice fractures.

According to Patricia Dove, a professor in the Virginia Tech Department of Geosciences and an expert in biomineralization, this biological discovery could have a major impact on the field of bio-inspired innovation.

“Starfish and other echinoderms living in highly predatory sea floor environments are revealing a world of materials innovations that are critical to survival,” she said. “Using little more than seawater and some organic components, biology directs the formation of remarkable skeletons such as those in starfish. This novel study of the underlying mechanical engineering properties has tremendous potential as a frontier for new materials design.”

Knowing the architecture of natural microstructures represented a huge step forward, but Li and his team had more questions. Was there a key to the way in which the creatures grow their skeletons that might shed some light on a way to reproduce them?

To find out, they used 3D printing to model and generate large-scale versions of these complex lattice structures for both research and educational purposes, a useful approach in understanding the complexity of these unique geometries. While the 3D-printed models created by Li’s team were indeed visually inspiring, the technology needed to bring new, stronger ceramic micro-architectures to market still lay in the future. Currently, 3D printers produce structures at the micrometer level, but printing ceramics still requires firing the final product, which possibly introduces many uncontrolled tiny pores and cracks. These defects make the structures extremely fragile. Li hopes that continued advances in the field of 3D printing and further understanding of the formation mechanisms of biological structures like starfish skeletons could eventually offer a solution.

“Nature is able to assemble mineral precursors to form complex architectures at room temperature and ambient pressure,” Li said. "That is something that modern human technology cannot currently achieve. Virginia Tech has a strong research interest in mineral structures found in nature, and I am hopeful that this exciting research direction may one day lead to the development of a wide range of bio-inspired, stronger and more lightweight materials.”

This story is adapted from material from Virginia Tech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

Ling Li with a starfish skeleton and 3D-printed scale models. Photo: Alex Parrish for Virginia Tech.
Ling Li with a starfish skeleton and 3D-printed scale models. Photo: Alex Parrish for Virginia Tech.