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 breakage. This behavior potentially causes problems for innovators trying to create lightweight porous versions of these materials, explaining why ceramic foams are not typically used as structural components.

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In this work, Li and his team turned their eyes 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 and discourage predators.

While observing samples of these starfish skeletons at the Nanoscale Characterization and Fabrication Laboratory (NCFL), Li and Ph.D. student Ting Yang (co-first author of the paper and now a post-doctoral fellow at the Massachusetts Institute of Technology), 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, the unique skeletal organization of this starfish exhibits the highest structural regularity ever reported from this group of invertebrates. Such regular lattice-like structures display remarkable similarities with space frame truss structures commonly employed in modern human construction projects.1

What the team found was unexpected. As in other starfish species, the skeleton of the knobby star 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 in similar vein to the structure of the Eiffel Tower. Even more interesting, the team found the uniform structure of the microlattice, because of the alignment of its atoms, is essentially a single crystal structure at atomic level.2

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• 1. The team wondered how this natural ceramic lattice material achieved mechanical protection, 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.
• 2. “This unique material is like a periodic lattice carved from a piece of single crystal of 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.”