SCT: The Rocks in Your Head and in Most Life

The incorporation of protein
with calcium carbonate makes
the weak become strong

This article first appeared in Science & Culture Today.

Calcium Carbonate: How Life Rocks
by David F. Coppedge
Science & Culture Today, January 29, 2016

A revolution in materials science will begin as soon as engineers “make like an oyster.” Calcium carbonate (CaCO₃) is a very abundant mineral on the earth, but its consistency is often weak, like chalk, or brittle, like calcite. Chemical engineers could gain “pearls of wisdom” from a lowly oyster, says the Pacific Northwest National Laboratory. From “How Seashells Get Their Strength”:

Seashells and lobster claws are hard to break, but chalk is soft enough to draw on sidewalks. Though all three are made of calcium carbonate crystals, the hard materials include clumps of soft biological matter that make them much stronger. A study today in Nature Communications reveals how soft clumps get into crystals and endow them with remarkable strength. [Emphasis added.]

The incorporation of protein with calcium carbonate makes the weak become strong. How can two flimsy materials pull this off? Here’s how it works:

The strength of a material depends on how easy it is to disrupt its underlying crystal matrix. If a material is compressed, then it becomes harder to break the matrix apart. Proteins trapped in calcium carbonate crystals create a compressive force — or strain — within the crystal structure.

Unlike the strain that makes muscles sore, this compressive strain is helpful in materials, because it makes it harder to disrupt the underlying crystal structure, thereby adding strength. Scientists understand how forces, stress and strain combine to make strong materials, but they understand less about how to create the materials in the first place.

Life creates such materials with ease, but how they do it leaves engineers baffled. Now, PNNL scientists have a new theory that supplants the old one. “We’ve found a completely different mechanism,” the lead scientist says. It took careful observing with Atomic Force Microscopy (AFM) to hit upon the secret. They seeded a strong solution of calcium carbonate with springy spheres of organic matter. As the solution crystallized into calcite, they watched as the organic balls clung to the edges of “steps” of terraced mineral as the material washed over it. The mineral compressed the organic matter but left a cavity, giving it some compressive strain. The process is explained in detail in their paper in Nature Communications (open access). The paper begins by saying that biology has significantly enhanced their understanding of how to make strong materials.

Soluble additives are widely used to control crystallization processes, providing an experimentally simple and versatile strategy to generate crystals with defined polymorphs, morphologies and sizes. Significant insights into additive-directed crystallization, and the effects of these additives on crystal properties, have come from in vitro studies of biomineralization processes, where the use of advanced characterization methods have demonstrated that the macromolecules active in controlling crystallization become occluded within the crystal structure, modifying crystal texture and lattice strain, causing local disorder and enhancing mechanical properties.

Those macromolecules are typically proteins — long, folded chains of amino acids whose specific structures are encoded by genes. The scientists, however, used simple micelles (spheres of polar molecules) that only “mimic” some of the properties of the biomacromolecules (proteins) used in vivo. To make their doping agents, they dumped copolymer powder into water, stirred it, and adjusted the pH with sodium hydroxide to get the micelles to form. This is a far cry from the elegance of the proteins used in life. They called their resulting materials “artificial biominerals.” The oysters’ biomineral construction is much more complex and elaborate, employing molecular machines to put each molecule where it belongs.

Calcium Carbonate and You

Where in your own body is calcium carbonate used to create a strong material? Try bone. Try teeth. Additionally, your ears have tiny crystals of calcium carbonate called otoliths (“ear stones”). Located in the vestibule of the inner ear near the semicircular canals, these little rocks in your head help you keep your balance….

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Suggested Reading:

• Learn how otoliths send messages to the cerebellum (1 Dec 2018).
• Learn how biominerals in teeth resist cracking (20 April 2002).
• Read about nacre (mother-of-pearl), a calcium carbonate mineral in oysters (5 July 2007).