Ceramics cling to long bacterial strings.

Many scientists have been looking to nature for ideas on how to make useful materials, an approach known as biomimetics. They are not only patterning materials after natural ones but also using organic matter to guide the assembly of inorganic structures--the same strategy that mollusks use to make their shells, for example.

Stephen Mann and his colleagues at the University of Bath in England have now recruited mutant bacteria to help them make ceramic fibers. By dipping long threads of the bacteria into suspensions of inorganic particles that are a few nanometers in size, the researchers have produced fibers of magnetite, cadmium sulfide, and titanium dioxide. The structure of the resulting fibers--inorganic matter on an organic scaffold--varies depending on how the materials interact with the bacteria.

Using bacteria as templates "has great potential," says Richard M. Laine of the University of Michigan in Ann Arbor. "If you can tailor the bacterial microstructure, then you can use biology to direct where the ceramics go. I think that it's a potentially hot area."

The Bath researchers used a mutant strain of Bacillus subtilis provided by Neil H. Mendelson of the University of Arizona in Tucson (SN: 2/12/94, p. 106). Unlike normal B. subtilis, the mutant cells don't separate completely into two cells when they divide. Instead, they remain connected, growing and elongating into a twisty chain.

The chains get easily entangled, Mendelson says, so dragging them out of their growth solution "pulls up thousands of bacterial filaments, which coalesce and dry into a single, hairlike structure up to a meter in length." These strands serve as the templates in the Bath group's study, which appears in the Sept. 21 CHEMISTRY OF MATERIALS.

Each of the three materials the researchers tested interacts differently with the bacteria, depending on whether it carries a positive or negative charge. Negatively charged particles of magnetite, an iron oxide, penetrate the thread, densely packing the spaces between the bacteria. The resulting structure can be suspended from a magnet but has no strength without the bacteria to hold it together. It crumbles if the researchers burn off the organic matter.

Neutral grains of the semiconductor cadmium sulfide don't penetrate the thread as well as magnetite, tending to collect mainly on the outside of the strand. Positively charged particles of titanium dioxide, a white pigment, don't penetrate the thread at all and form a surface coating a few micrometers thick.

Last year, Mann and his group used bacterial templates to make porous silica fibers. The current study takes the technique beyond silica, making it a more general approach, Mendelson says. "The whole idea is that you can use bacterial threads in clever ways to interface with inorganic chemistry."

Bacterial templates allow scientists to design materials with features in the micrometer- and centimeter-size range--larger than is possible with templates made from proteins or other molecules. These nonbacterial templates can generate structures similar to those that Mann's bacteria produced, Laine says, but he admires Mann's technique as "a unique use of biological materials."