Natural materials such as tooth enamel, nacre (also known as mother-of-pearl), or bone attain their impressive mechanical properties from combining organic and inorganic components into hierarchical composite structures spanning across different length scales. For instance, nacre — the tough, iridescent layer constituting the inner surface of mollusk shells — consists of a tessellated structure of layered calcium carbonate platelets interconnected by an organic matrix. In a paper published in the journal Small, University of Rochester researcher Anne Meyer and colleagues outline their method of using two strains of bacteria to replicate these layers.
In order to make artificial nacre, Spiesz et al use bacteria to create alternating thin layers of crystalized calcium carbonate and sticky polymer. Image credit: J. Adam Fenster / University of Rochester.
“Although nacre-inspired materials have been created synthetically before, the methods used to make them typically involve expensive equipment, extreme temperatures, high-pressure conditions, and toxic chemicals,” Dr. Meyer said.
“Many people creating artificial nacre use polymer layers that are only soluble in nonaqueous solutions, an organic solvent, and then they have this giant bucket of waste at the end of the procedure that has to be disposed of.”
In order to make the artificial nacre, Dr. Meyer and co-authors create alternating thin layers of crystalized calcium carbonate — like cement — and sticky polymer.
They first take a glass or plastic slide and place it in a beaker containing the bacteria Sporosarcina pasteurii, a calcium source, and urea. This combination triggers the crystallization of calcium carbonate.
To make the polymer layer, they place the slide into a solution of the bacteria Bacillus licheniformis, then let the beaker sit in an incubator.
“Right now it takes about a day to build up a layer, approximately 5 µm thick, of calcium carbonate and polymer,” Dr. Meyer said.
“We’re currently looking at coating other materials like metal with the nacre, and we’re trying new techniques to make thicker, nacre-like materials faster and that could be the entire material itself.”
One of the most beneficial characteristics of the nacre produced in the team is that it is biocompatible — made of materials the human body produces or that humans can eat naturally anyway.
“This makes the nacre ideal for medical applications like artificial bones and implants,” Dr. Meyer said.
“If you break your arm, for example, you might put in a metal pin that has to be removed with a second surgery after your bone heals. A pin made out of our material would be stiff and tough, but you wouldn’t have to remove it.”
And, while the material is tougher and stiffer than most plastics, it is very lightweight, a quality that is especially valuable for transportation vehicles like airplanes, boats, or rockets, where every extra pound means extra fuel.
“The nacre might also be an ideal material to build houses on the Moon and other planets: the only necessary ‘ingredients’ would be an astronaut and a small tube of bacteria,” Dr. Meyer said.
“The Moon has a large amount of calcium in the lunar dust, so the calcium’s already there. The astronaut brings the bacteria, and the astronaut makes the urea, which is the only other thing you need to start making calcium carbonate layers.”
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Ewa M. Spiesz et al. Bacterially Produced, Nacre-Inspired Composite Materials. Small, published online April 5, 2019; doi: 10.1002/smll.201805312
