Pure calcium carbonate—the main component of chalk—crumbles easily. But mix in small amounts of other organic molecules like amino acids, and the resulting material is much harder. In nature, this process is called biomineralization—it is how animals like mollusks create their hard protective shells. Now, scientists have figured out how to manipulate this process in the laboratory to control the hardness of a crystalline material.

By measuring the amount of amino acids incorporated into different calcium carbonate crystals and testing how the impurities influenced the crystals’ hardness, an international team of researchers teased out the relationship between the crystals’ amino acid content and their mechanical and structural properties, as reported in a recent issue of Nature Materials.

The process by which organic molecules like amino acids or proteins become embedded in a larger crystal lattice is called occlusion. These small impurities make the crystal much stronger by disrupting its orderly structure. But in order to harness this phenomenon to design new materials with specific properties, it is important to understand exactly how much of a given substance to add to get the desired structural effect.

“If you study something like calcium carbonate, it’s not so straightforward to determine how much you have occluded,” says Fiona Meldrum, a materials scientist at the University of Leeds, UK, who led the study. “But you have to know this in order to be able to understand the properties.”

Meldrum and her colleagues examined calcite crystals, a form of calcium carbonate that is found in limestone, as a model mineral for their study. They let the crystals form from supersaturated calcium carbonate solutions with different quantities of two amino acids, aspartic acid or glycine, mixed in, each tagged with a fluorescent dye. They then dissolved the crystals and measured how much of the amino acid had been taken up in each trial using reverse-phase high-pressure liquid chromatography.

“It’s possible to determine how much amino acid you’ve added,” says Meldrum, an important benchmark for scientists hoping to engineer materials with certain properties. “We were surprised by the levels of occlusion we could achieve.”

The more supersaturated the starting solution was, the more amino acids worked their way into the crystal structure.

The team also tested the properties of the resulting crystals using nanoindentation tests and x-ray diffraction.

The crystals became harder when more amino acids ended up in the lattice, which occurred when the starting solution had higher amino acid concentrations. But a given amino acid’s effect on calcite crystals depended not just on how much it physically disrupted the crystal lattice, but also on the strength of the covalent bonds it formed with surrounding molecules. Adding amino acids with harder-to-break bonds made the crystals harder, too.

Materials that are harder also tend to be more brittle, as is well known. In nature, few organisms optimize purely for hardness, says Derk Joester, a materials scientist at Northwestern University who was not involved in the study.

But, he adds, “Hardness is a very important technology, and it’s more important if you can differentially harden material—if you could just harden the surface, for instance, without affecting the properties underneath.”

Meldrum says that the process by which the team measured the amount of organic molecules occluded into the calcite crystal matrix to understand how they influenced the crystal’s hardness

is simple enough that it could be used to tune other crystalline materials to a specific hardness as well.

“It’s my belief that it’s really quite widespread—and people just simply haven’t looked,” she says. “This is really something the industry could adopt.”