Lessons learned from the abalone shell

Sometimes the answers are right in front of your eyes. In a quest to develop lightweight structural materials with excellent strength and toughness (damage resistance), researchers have turned to the abalone shell for inspiration.

We may use strength and toughness interchangeably in everyday conversation, but they actually mean difference things from a materials characterization point of view. The material’s strength measures the force per unit area it can resist when stretched, compressed or bent. On the other hand, the material’s toughness measures the amount of energy or work that is required to break it.

It is important for abalones to have tough shells because predators such as sea otters and octopuses are more likely to break, rather than rip or squish apart their shell. Nacre, the protective material found on the inside of abalone shells and other mollusc shells, is very strong and tough. The toughness of the abalone shell is more than 3000 times greater than that of the individual shell components. This shell toughness far exceeds what could be expected from an average of the components.

Robert Ritchie, a professor at the University of California at Berkeley, wrote in Nature that the robustness of the abalone shell lies in the brick-and-mortar like shell design. The shell is a biological composite consisting of aragonite (a form of calcium carbonate) “bricks” held together by an organic material “mortar”. The mineral “bricks” provide strength, while the organic “mortar” acts like a lubricant between the mineral bricks, allowing the bricks to slide over each other.

Ritchie and his colleagues had previously published a synthetic composite that mimics the design of the abalone shells. They used a freeze-casting method to first make aluminum oxide (Al2O3) scaffolding. The scaffolding was then infiltrated with polymethyl methacrylate (PMMA). Finally the composite was pressed and heated to increase its density, which improves the strength and toughness of the final product.

The team’s synthetic composite show excellent toughness, and is about three times as tough as the abalone shell! Just like the abalone shell, the toughness of the synthetic composite is more the just a sum of its parts. The toughness of the synthetic composite was more than 300 times greater than either Al2O3 or PMMA.

Showing the brick-and-mortar like design in (a) the abalone shell and (b) the synthetic Al2O3/PMMA composite (Ritchie et al. 2011)

The synthetic composite has a similar source of toughness to the abalone shell. Aluminum oxide is hard and brittle, like aragonite. Polymethyl methacrylate on the other hand resembles the organic material in the abalone shell by promoting sliding. Sliding functions as shock absorption and relieves the stress build-up between “bricks” when there’s an impact.

The Al2O3/PMMA composite is an example of how we can use nature as guide to create a material with extraordinary properties. Creating a new material doesn’t always mean you have to reinvent the wheel. Sometimes, you just need to know where to look for ideas.

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Ault, J.S. 1985. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Pacific Southwest)—black, green and red abalones. U.S. Fish Wildl. Serv. Biol. Rep. 82 (11.32) U.S. Army Corps of Engineers, TR EL-82-4. 19 pp.

Munch E, Launey ME, Alsem DH, Saiz E, Tomsia AP, & Ritchie RO (2008). Tough, bio-inspired hybrid materials. Science (New York, N.Y.), 322 (5907), 1516-20 PMID: 19056979

Ritchie RO (2011). The conflicts between strength and toughness. Nature materials, 10 (11), 817-22 PMID: 22020005

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