Seashells, Strong Bones, and Moon Houses

The strongest synthetic materials are often those that directly mimic nature.

Nacre, also known as mother-of pearl, is one such substance; an exceptionally tough, stiff material produced naturally by some mollusks and serving as their inner shell layer. It also comprises the outer layer of pearls, giving them their lustrous shine. Nacre’s unique properties make it an ideal inspiration in the creation of synthetic materials, but most methods used to produce artificial nacre are complex and energy intensive.

Now, a biologist at the University of Rochester has invented a method for making artificial nacre using an innovative component: bacteria. The artificial nacre created by Anne S. Meyer, an associate professor of biology at Rochester, and her colleagues, is made of biologically-produced materials and has the toughness of natural nacre, while also being stiff and, surprisingly, bendable. The method used to create the novel material, in turn, demonstrates an inexpensive and environmentally-friendly fabrication technique that may have applications in medicine, engineering, and even constructing buildings on the moon.

The impressive mechanical properties of natural nacre arise from its hierarchical, layered structure, which allows energy to evenly disperse across the material. In a paper published in the journal Small, Meyer and her colleagues outline their method of using two strains of bacteria to replicate these layers. When they examined the samples under an electron microscope, the structure created by the bacteria was layered similarly to nacre produced naturally by mollusks.

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, Meyer says. “Many people creating artificial nacre use polymer layers that are only soluble in non-aqueous solutions, a nonorganic solvent, and then they have this giant bucket of waste at the end of the procedure that has to be disposed of.”

The nacre produced in Meyer’s lab, however, is produced in a way that is simpler and more environmentally-friendly; all researchers have to do is grow bacteria and let it sit in a warm place.

In order to make the artificial nacre, Meyer and her team 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 (in the human body, urea is the waste product excreted by the kidneys when you urinate). 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 micrometers thick, of calcium carbonate and polymer. Meyer and her team are 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,” Meyer says.

One of the most beneficial characteristics of the nacre produced in Meyer’s lab 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, Meyer says. “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. Because the production of bacterial nacre doesn’t require any complex instruments, and the nacre coating protects against chemical degradation and weathering, it holds promise for civil engineering applications like crack prevention, protective coatings for erosion control, or for conservation of cultural artifacts, and could be useful in the food industry, as a sustainable packaging material.

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, Meyer says. “The moon has a large amount of calcium in the moon 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.”