Hovden Lab: Real-time, nanoscale view of what makes nacre so tough

Nacre is an extremely resilient biomaterial that has baffled scientists and engineers decades. New visual evidence captured by University of Michigan researchers reveals the mechanisms that give nacre its toughness.

The basics about nacre have been known for decades—it’s made of microscopic “bricks” of a mineral called aragonite, laced together with a “mortar” made of organic material. This bricks-and-mortar arrangement clearly lends strength, but nacre is far more resilient than its materials suggest.

Using tiny piezo-electric micro-indenters, the U-M led team was able to exert force on the shells while under an electron microscope and watch what happened in real time.

They found that the “bricks” are actually multi-sided tablets only a few hundred nanometers in size. Ordinarily, these tablets remain separate, arranged in layers and cushioned by a thin layer of organic “mortar.” But when stress is applied to the shells, the “mortar” squishes aside and the tablets lock together, forming what is essentially a solid surface. When the force is removed, the structure springs back, without losing any strength or resilience.

The research was led by Robert Hovden, assistant professor of materials science and engineering, with Jiseok Gim, materials science and engineering PhD candidate, as the study's lead author.

Their paper, titled, “Nanoscale deformation mechanics reveal resilience in nacre of pinna nobilis shell," was published in Nature Communications: http://dx.doi.org/10.1038/s41467-019-12743-z