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Coatings have simple recipe for success

Cheap approach, natural ingredients may prove useful in foods, medicines

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It’s not often that chemists find a quick, simple and cheap method for making things using widely available ingredients, but researchers have done just that: They’ve created elegant little capsules and coatings in water simply by mixing iron and a compound from plants called tannic acid. The soft coatings form on their own around whatever else is in the water — glass beads, bacteria, gold nanoparticles and more. Just changing the solution’s pH can prompts the coatings to disassemble.

The coatings’ ingredients are considered safe — tannic acid is found in wine, while iron is an important element for living things. That means the capsules might help in delivering drugs in the body or find use in cosmetics or foods, says bioengineer Gregory Payne of the University of Maryland in College Park.

The work fits with an ongoing effort to find biologically friendly, useful materials, Payne says, and it takes advantage of materials that are right under everyone’s noses. “It opens up a lot of opportunities.”

Using ordinary lab equipment, the research team, led by materials scientist Frank Caruso of the University of Melbourne in Australia, create the tiny coatings at room temperature. When the researchers add tannic acid to water, it tends to congregate around surfaces, whether they be a piece of polystyrene or an E. coli bacterium. When the researchers add iron ions to the mix, the iron latches onto the tannic acid molecules, connecting them into a thin film. At a pH of 7.4, the capsules were still intact after 10 days; at a pH of 3, they disassembled within four hours, the team reports in the July 12 Science.

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