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Simple Process Devised for Synthetic Mother-of-Pearl

Abalone shell

Abalone shell lined with nacre, or mother-of-pearl (J. Adam Fenster, University of Rochester)

24 Apr. 2019. Researchers in the U.S. and Europe developed a simple, inexpensive, and sustainable process to grow synthetic nacre, also known as mother-of-pearl. A team from University of Rochester in New York, Delft University of Technology in the Netherlands, and other institutions described their process earlier this month in the journal Small.

Nacre is a natural, crystalline substance found on the outer layer of pearls, which gives pearls their high luster. The material also makes up the inner layer of shell fish, such as oysters and abalone, and is strong, lightweight, translucent, and distributes energy evenly across its surface. As a result, nacre has many desirable properties for a host of industrial uses, and because it’s an organic material, nacre can also be deployed where bio-compatibility is important.

Because of these properties, a number of attempts have been made to synthesize nacre. However, as co-project leader and biology professor Anne Meyer explains in in a University of Rochester statement, these previous methods were often expensive and environmentally suspect. “Many people creating artificial nacre,” says Meyer, “use polymer layers that are only soluble in non-aqueous 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.”

Meyer and co-project leader Marie-Eve Aubin-Tam in Delft, created a more sustainable process for synthetic nacre that contains calcium carbonate, the main ingredient in the natural material. However, the team first combines urea, the nitrogen compound filtered out by the kidneys and expelled from the body in urine, with Sporosarcina pasteurii bacteria. The combination of urea and the bacteria form calcium carbonate crystals on a glass slide in the solution. The researchers use another bacteria, Bacillus licheniformis found in the soil, to produce polyglycolide acid, or PGA, a bio-compatible polymer used in medical devices.

The team then arrays alternating layers of calcium carbonate and PGA, each 5 micrometers in thickness, with each layer taking about a day to form in an incubator. The result is a composite material, which in electron microscope inspections resembles natural nacre in structure, and in lab tests shows the toughness and flexibility of the natural material.

And because it’s a bio-compatible material, the synthetic nacre could be used in medical devices and implants, particularly to replace metal. “If you break your arm, for example,” notes Meyer, “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.” Other potential uses are found in transportation, civil engineering, and preserving cultural artifacts.

The authors point out that the process can be applied to produce materials other than nacre. With genetic engineering, say the researchers, bacteria could be designed to produce cost-effective and sustainable materials with varying properties almost on demand. The following video tells more about the process for making synthetic nacre.

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