Chiton's armor has transparent ceramic eyes, just like spinel

Two different press releases--from MIT and NRL--describe how biomimetics is being tapped to develop transparent ceramics.

Content Dam Lfw En Articles 2015 12 Chiton S Armor Has Transparent Ceramic Eyes Just Like Spinel Leftcolumn Article Thumbnailimage File

IMAGE: The shell of a chiton (left) when seen close up (right) shows two kinds of sensory organs that cover the shell surface. The eyes are the dark bumps with shiny centers. The exact function of other sensory organs called aesthetes (small bumps with black centers) is not yet known. (Image credit: MIT)

Two different press releases--one from MIT and the other from NRL--describe how biomimetics is being tapped to develop mineral-based transparent ceramics that could improve optics used in rugged, challenging environments.

Sea-dwelling creatures called chitons have tiny eyes embedded within their tough protective shells with transparent lenses made of the same ceramic material as the rest of their shells--and just as tough. These armor-plated eyes could provide a model for protective armor for soldiers or workers in hazardous surroundings, say researchers at MIT, Harvard University, and elsewhere who analyzed the structure and properties of these uniquely hardy optical systems.

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Their work is described in the journal Science by MIT professor Christine Ortiz; recent MIT graduate and Harvard postdoc Ling Li; recent MIT graduate Matthew Connors; MIT assistant professor Mathias Kolle; Joanna Aizenberg from Harvard University; and Daniel Speiser from the University of South Carolina.

These chitons, a species called Acanthopleura granulate, have hundreds of tiny eyes dotting the surface of their tough shells. The researchers demonstrated that these are true eyes, capable of forming focused images. They also showed that unlike the eyes of almost all other living creatures, which are made primarily of protein, these eyes are made of the mineral aragonite, the same ceramic material as the rest of the creatures' shells.

These mineral-based eyes, Li says, "allow the animal to monitor its environment with the protective armor shell. The majority of the shell is opaque, and only the eyes are transparent."

Unlike most mollusks, chitons' shells are made of eight overlapped plates, which allow them some flexibility. The little creatures--about the size of a potato chip and resembling prehistoric trilobites--are found in many parts of the world, but are little noticed, as they resemble the rocks they adhere to. Since these chitons live in the intertidal zone, their eyes need to be able to work in both air and water. Using experimental measurements and theoretical modeling, the team was able to show that the eyes are able to focus light and form images within the photoreceptive chamber underneath the lens in both environments.

It seems that the research could likely lead to more biomimetic materials that provide both physical protection and optical visibility at the same time. Case in point is spinel, a material with similar properties that the Naval Research Laboratory (NRL) has been researching over the last 10 years.

"Spinel is actually a mineral, it's magnesium aluminate," said Jas Sanghera, who leads the research. "The advantage is it's so much tougher, stronger, harder than glass. It provides better protection in more hostile environments--so it can withstand sand and rain erosion."

NRL invented a new way of making transparent spinel using a hot press, called sintering. It's a low-temperature process, and the size of the pieces is limited only by the size of the press. "Ultimately, we're going to hand it over to industry," said Sanghera, "so it has to be a scalable process." In the lab, they made pieces eight inches in diameter. "Then we licensed the technology to a company who was able then to scale that up to much larger plates, about 30 inches wide."

Spinel, like the chiton's eyes, is a mineral that can be mined as a gemstone; a famous example is the Black Prince's Ruby, which is actually spinel with a color dopant. NRL chemists have also synthesized their own ultra-high-purity spinel powder, and other synthetic versions are commercially available. "The precursors are all earth-abundant, so it's available in reasonably low cost," said Sanghera. The spinel NRL makes is a polycrystalline material, or a lot of crystal particles all pressed together. Unlike glass, where "a crack that forms on the surface will go all the way through," spinel might chip but it won’t crack.

The military in particular may want to use spinel as transparent armor for vehicles and face shields. A bullet-proof window today, for example, has layers of plastic and glass perhaps five inches thick. "If you replaced that with spinel, you'd reduce the weight by a factor of two or more," Sanghera said.


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