Nanoscale View Finally Reveals What Makes 'Nacre' Shell Material So Resilient

Nacre is an extremely resilient biomaterial found in the shells of mollusks that has perplexed scientists and engineers for decades. It's been known that the material is made of microscopic “bricks” of the mineral aragonite and laced together with a “mortar” made of organic material; now, new visual evidence captured by University of Michigan researchers reveals the exact mechanisms that give nacre its resiliency. Using tiny piezo-electric micro-indenters, the team was able to exert force on the shells while under an electron microscope and watch what happened in real time. In research published in Nature Communications  , the researchers explain 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.

Transcript

00:00:00 (background music) HOVDEN >> Why is it that a piece of chalk that has the same atoms as mollusk shells is so brittle, it breaks so easily, yet a shell is really, nearly impossible for you to crack with your hands? NARRATOR >> The hidden toughness of mollusk shells comes from a bio super-material called nacre, which has captured the imagination of scientists and engineers for decades. But previous limitations have kept researchers from understanding exactly how this material works at the nanoscale. HOVDEN >> Our human bodies, as well as mollusk shells -- a wide variety of animals -- actually make nanotechnology. They're building mineral systems at the

00:00:36 nanometer length scales. It's not something you would find in a rock. It's unique. It's made intentionally to give superior performance. NARRATOR >> Using electron microscopy and mechanical deformation techniques Hovden and his team at the University of Michigan have, for the first time, captured visual evidence that helps explain how nacre responds to strain. HOVDEN >> So, nacre is built with tablets. It's like a brick and mortar structure, and they're glued together by very thin organics. And it can go from a regime where each tablet deforms individually, to the tablets fully locked and deforming as one whole unit. So this transition between local deformation and large-scale deformation

00:01:18 is what makes nacre so tough. NARRATOR >> This new understanding gives scientists and engineers a framework to begin developing new classes of tough super-materials that are based on the structure of nacre. HOVDEN >> Anytime we create a material that's tougher, stronger, more heat resistant, it takes us to new places. Studying naturally-occurring bio-minerals is so fascinating because we can find inspiration. (background music intensifies)