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Extreme Shrimp Studied as Models for New Materials

By Kevin Wilcox

Understanding how some species of shrimp withstand extreme pressure and heat could lead to future high-performance materials for extreme environments.

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Researchers are working to understand how the Rimicaris exoculata lives in extreme underwater environments in hopes of developing better manmade materials. Wikimedia Commons/Ventus55

August 11, 2015—Deep beneath the surface of the Atlantic Ocean, hydrothermal chimneys on the seabed teem with a species of vent shrimp known as Rimicaris exoculata. The shrimp's exoskeleton is a marvel of natural engineering, able to withstand the extreme pressures at 10,000 ft below the surface, coupled with water temperature near the hydrothermal chimneys of 660°F. Understanding how the shrimp thrives in these harsh conditions holds promise for the development of a new generation of high-performance materials.

Researchers at Purdue University recently compared the exoskeletons of vent shrimp with those of shrimp commonly found in shallow waters and made some interesting discoveries. Their research, "Scale Dependence of the Mechanical Properties and Microstructure of Crustaceans Thin Films as Biomimetic Materials" was recently published in JOM: The Journal of The Minerals, Metals & Materials Society (TMS).

Vikas Tomar, Ph.D., an associate professor at Purdue, authored the article along with Devendra Verma, and Tao Qu, who are graduate research assistants at the School of Aeronautics and Astronautics at Purdue. Tomar explains, "Shrimp is probably one of the few [creatures] found all over the world. [They are] found at multiple ocean depths. So you can actually characterize the survival of the species biologically and physically. We can, in the long term, try to establish a correlation in how the biological survival relates to the physical and material characteristics. Because in different types of environments, you need different biology to survive."

To compare the exoskeletons, the team obtained samples of the vent shrimp from Juliette Ravaux, Ph.D., a biologist and lecturer at the Université Pierre et Marie Curie, who in 2007 conducted a diving exploration dubbed MoMARDREAM, which was dedicated to the biology and geology of a specific site within at the Mid-Atlantic Ridge of the Atlantic Ocean. The team at Purdue examined these exoskeletons through several methods. Strength was tested through nanoindentation, which involves pressing into the surface of a material in small steps until peak force is reached. The researchers also looked at the microstructures of the exoskeletons using scanning electron microscopy and energy dispersive X-Ray spectroscopy.

The researchers discovered that the shrimp have similar microstructures in their exoskeletons, with chemical bonds arranged in helicoidal patterns. The key difference is that in the vent shrimp the layers of these patterns are much thicker and the connections are denser.

The researchers found that in the shrimp living near the ocean surface, although the layers are thinner and the connections are less dense, their exoskeletons are vastly stronger, likely to better withstand predators. The shrimp who live near the deep vents have weaker exoskeletons, but with superior resistance to creep—the small-scale plastic deformation that can occur over long-term exposure to a constant applied load.

"Now, what we are trying to find is how these [qualities] help it to survive at high temperature. Obviously high creep strength is good for high pressure. We are not sure about thermal abilities. So that is a key puzzle that we need to solve. When these species are surviving at very high temperatures, of course, a lot of heat is taken away by the water. How does the combination of the water and the material help it diffuse that large amount of heat in such a very small structural gradient at such a large depth? That is the next puzzle that we are trying to solve."

The solution to that puzzle could point the way to future man-made materials that can directionally propagate heat into extreme environments, protecting structures or devices for very long periods of time and in a very predictable manner.

"Any material is a thing of the future, but what I'm thinking about is whether we can incorporate such materials on robots [or sensors] that can operate in extreme environments for longer times," Tomar says. "How the interfaces in the chemistry play out continues to be a question. Clearly these shrimp have shown that just by the simple [changes in] arrangement—such as compaction or changes in the angle of the connections—one can achieve a lot in terms of wave dispersion and mechanical properties."

Tomar says that engineers designing future materials can learn valuable lessons from such natural materials, in which the sum is always significantly stronger than the parts and often has new multifunctional properties.

"The fundamental constituents of this [exoskeleton] material is much weaker than their combination. So [the strength] is fundamentally the microstructure and how things are arranged together and multiple scale patterns," Tomar explains. "The sum is significantly better than its parts, and that is a key philosophy nature allows that we are unable to [replicate] so far in terms of our industrial materials."



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