Besides simply looking nice when used in jewellery, mother-of-pearl is also one of nature's hardest, stiffest, most stable materials. Scientists have now utilized bacteria to develop a cheap and eco-friendly method of replicating it, for possible use in a variety of areas.
Also known as nacre, mother-of-pearl is the hard iridescent coating found on the outside of pearls, and the inside of certain mollusc's shells. Featuring a brick-wall-like microstructure, it's composed of stacked brick-like calcium carbonate plates, joined together via a biopolymer "mortar."
Synthetic versions of it have already been created, by teams from the University of Cambridge, the CNRS lab, and ETH Zurich. All of the techniques that were used, however, have incorporated either harsh chemicals or large expenditures of energy. Led by Assoc. Prof. Anne S. Meyer, a team at New York's University of Rochester set out to develop a synthetic nacre-production method that was easier on the environment.
The resulting technique involves mixing urea with Sporosarcina pasteurii bacteria and a calcium source, then dipping a glass slide into the solution. A reaction between the urea and bacteria causes a thin layer of calcium carbonate to crystallize onto the slide.
That slide is then placed in a beaker containing a solution of the bacteria Bacillus licheniformis. After that beaker has been left in an incubator for a period of time, the bacteria forms a layer of sticky polymer on top of the existing calcium carbonate layer.
By going back and forth between the two processes, the scientists can build up successive alternating layers of the calcium and the polymer. The final coating is tougher and stiffer than most plastics, yet is also quite lightweight and flexible. It isn't ready quickly, though – one combined calcium/polymer layer currently takes about one day to synthesize, and is just five microns thick.
To that end, Meyer and colleagues are now working on speeding up the production process, and making the layers thicker. It is hoped that the coating could ultimately be applied to a variety of materials, or even produced as a stand-alone material.
Possible applications include its use in lightweight aircraft or other vehicles, as a coating that prevents cracks and corrosion in structures, or as a sustainable food packaging material. Additionally, because the synthetic nacre is biocompatible, in could be used in the construction of implants or artificial bones.
A paper on the research was published this week in the journal Small.
Source: University of Rochester
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