News, Technique reveals deeper insights into the makeup of nacre, a natural material

Nacre, the iridescent material that lines mollusk shells such as mother-of-pearl and abalone, has long been a prized find of beachcombers and shell collectors, due to the natural beauty and variety of color that can be found therein. But scientists and engineers have also long marveled at and studied nacre; it’s a tough and strong material, composed of alternating layers of aragonite platelets and organic protein-based film. The natural world contains many materials that have evolved over time to optimize strength, durability, and performance. As researchers and engineers look to develop improved and more sustainable building materials, they are increasingly looking to nature for inspiration. The physical makeup of nacre allows it to withstand considerable amounts of pressure and damage along the platelets without causing major damage throughout the whole shell. It has been supposed by some that more is at play of the individual platelets that allows them such extraordinary strength and durability, but researchers have lacked the tools and processes to dig deeper into the relationship between the crystal orientation and the mechanical properties — until now. Over the past two decades, the shells have typically been tested for their strength using techniques such as macroscopic bending test, micro-/nano-indentation, and atomic force microscope. Now, MIT assistant professor of civil and environmental engineering Admir Masic, graduate student Hyun-Chae “Chad” Loh, and five others have combined scanning electron microscopy and micro-indentation with Raman spectroscopy and developed a powerful chemo-mechanical characterization method that allows three-dimensional stress and strain mapping through a technique known as piezo-Raman. “We developed a methodology to extract important chemo-mechanical information from a biological system that is very well known and studied,” explains Masic, whose findings were recently published in Communications Materials. “Correlating micro-indentation and piezo-Raman results allowed us to evaluate and quantify the amount of stress dissipated through the hierarchical structure.” The new approach to quantifying the mechanical performance of the material is enough to be big news on its own, but during the process, Masic and fellow researchers — whom he credits with much of the work in this collaborative effort — were surprised by the results.

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