Prorok studies abalone shell architecture for advancements in strength of multilayer composites

When people hear the words snails or slugs, most envision something slow or slimy and not something strong. Bart Prorok, director of Auburn University’s Analytical Microscopy Center and professor of materials engineering, understands something different. He studies abalone, a group of small to very large sea snails or mollusks that are the only animals in the world to make strong single coiled shells. Prorok cultures abalone in a laboratory and uses environmental parameters to control and manipulate how their shell architectures are formed to better understand their toughening mechanisms. The research will have a significant impact in designing a new class of multilayered composite architectures that can improve the performance of mechanically protective applications such as coatings and armor.

An abalone, which spends its life in salt water, possesses a foot muscle that is considered a delicacy and aphrodisiac, particularly in the Far East, where it is considered good luck to eat it. Global demand for this high-protein, low-fat meat has led to its scarcity and an increase in farm-raised abalone. Abalone is also well known for the thick, iridescent inner layer of its shell. This material is used to coat foreign objects that enter the abalone, protecting the animal. This nacre, or mother of pearl, is also harvested to make decorative objects and jewelry.

Prorok and his team of undergraduate and graduate students are interested in the nacre, but not for its cosmetic properties. Rather, they are curious how the architecture of the nacre gives it tremendous strength. The nacre is composed of highly organized layers of aragonite, a form of calcium carbonate mortared together with elastic biopolymers. In wild abalone, these are interspersed by layers of unorganized calcium carbonate. Farm-raised abalones lack the unorganized layers. According to Prorok, the multi-layering of the nacre of wild abalone appears to contribute to the high strength of their shells.

“The cross section of the shell of a wild abalone tells the story of its life. The layers are similar to a tree’s rings in that they represent seasonal patterns of growth,” Prorok said. “Because farm-raised abalone lack seasonal patterns of growth, they also lack the unorganized layers that appear to give the nacre its strength.”

Prorok and his research team will explore this idea by using controlled conditions to manipulate the layering of the nacre into a predetermined architecture. The team will insert a foreign body into the abalone that is later extracted for analysis, and examine whether different water temperatures alter the coating of the body. The strength of the coating will be tested by nanoindentation, which can evaluate the hardness and strength of small surfaces. Such studies will give Prorok insights for improving the design of synthetic structures and advanced compositions. Prorok and his team hope that by manipulating the abalone’s environment and properties, they will discover how to strengthen metals and other materials regularly used by engineers in the automotive, aerospace and defense industries.

This work is being supported by a three-year grant from the United States National Science Foundation.