Durable Mollusk Shells may be Bio-Inspiration for Future Armor

In the future, armor worn by soldiers may take inspiration from the shells of tiny sea creatures.

A team of engineers from Massachusetts Institution of Technology reports that the shells of the mollusk Placuna placenta are not only tough, but also so transparent they are clear enough to read through. The MIT team has learned why these mollusk shells are so durable even though they are made almost entirely of calcite, a very weak mineral.

Writing in the journal Nature Minerals, MIT graduate student Ling Li and materials science professor Christine Ortiz report that the shell's unique properties form a specialized nanostructure that allows for efficient energy dissipation and the ability to localize deformation while still allowing optical clarity.

In the future, bio-inspired armor designed under the principles governing the mollusk shells could be worn by soldiers, the researchers said.

"The properties of this natural armor make it a promising template for the development of bio-inspired synthetic materials for both commercial and military applications -- such as eye and face protection for soldiers, windows and windshields, and blast shields," MIT said in a statement, citing Ortiz.

One marvel about the Placuna placenta shells is that the ceramic-based exoskeleton can withstand multiple penetrating attacks from predators. This fascinates Ortiz and her collaborators because the shell is made of only 1 percent organic material, the rest just calcite, which does not withstand penetration well in other circumstances.

Using a high-resolution analysis of the calcite shells, the researchers were able to determine how the shells respond to penetration from a sharp diamond tip.

"The material initially isolates damage through an atomic-level process called 'twinning' within the individual ceramic building blocks: Part of the crystal shifts its position in a predictable way, leaving two regions with the same orientation as before, but with one portion shifted relative to the other," MIT reported. "This twinning process occurs all around the stressed region, helping to form a kind of boundary that keeps the damage from spreading outward."

Researcher Li said that twinning activates a series of energy dissipation mechanisms that end up preserving and maintaining the mechanical and optical integrity of the surrounding shell material.

This process is 10 times more efficient in dissipating energy than pure calcite can handle.