See-Through Mollusk: Scientists Discover What Makes Transparent Capiz Shells So Durable

By Ben Wolford on March 30, 2014 3:59 PM EDT

This Transmission Electron Microscope image shows the way Placuna placenta shells localize damage, like this divot pressed in the surface by MIT researchers. (Photo: MIT)

This transmission electron microscope image shows the way Placuna placenta shells localize damage, like this divot pressed in the surface by MIT researchers. (Photo: MIT)

The windowpane oyster has been used for thousands of years as a natural substitute for glass because of its unique combination of durability and translucence. The Chinese especially were known for outfitting their domiciles with the Asian creature's exoskeleton. But until this week, the exact structure of the mollusk shells of Placuna placenta, often called cadiz in its ornamental uses, was largely unknown.

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A pair of researchers at the Massachusetts Institute of Technology wanted to find out what made the shells so strong, despite being made almost entirely from brittle calcite, MIT News reported Sunday. Their findings, described in a forthcoming issue of Nature Materials, suggest these mollusks evolved a nanostructure that allows light to pass through nearly unobstructed, yet also isolates any penetration damage at the atomic level, preventing cracking. Such a trait would prove useful against predators. For us, this scientific understanding could help improve synthetic materials, such as armor.

To test the mollusk shells, graduate student Ling Li and professor Christine Ortiz used a diamond-tipped utensil to indent the surface. Then by using diffraction and a transmission electron microscope, they observed what happened. As MIT puts it, the particles on the leading edge of the damage broke into two identical blocks, a phenomenon they called "twinning": "This twinning process occurs all around the stressed region, helping to form a kind of boundary that keeps the damage from spreading outward."

"As a first-of-its-kind [demonstration of] the effectiveness of deformation twins in natural materials, this work should have huge practical impact," Huajian Gao, a Brown University engineer not involved with the study, told MIT News. The findings could influence the design of armor in military and manufacturing.

Presently, ceramic armor has limitations: it can stop one bullet, but only at the cost of widespread structural damage. The twinning principle could inspire ways to re-enforce the materials. Plus, given that these mollusk shells can be clear enough to see through, Ortiz says synthetic designs could mimic them to create face masks or armored windows.

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