Tiny shells can be engineered into a variety of minute devices, claims a team of scientists. The team has demonstrated a way to transform the silicon dioxide shells of diatoms, one-celled algae, into magnesium oxide. The trick could lead to miniature sensors, drug-delivery capsules, and other devices.
For years, Kenneth Sandhage, a materials chemist at Ohio State University in Columbus, has worked to develop ways to modify the chemistry of preshaped ceramic materials. Sandhage's work took him to Germany, where by chance he met marine biologist Monica Schoenwaelder, from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven. After hearing Schoenwaelder describe the elegant microshells created by diatoms, Sandhage realized that they could make "great preforms for three-dimensional microdevices," he says.
The 100,000 or so species of diatoms vary in size from less than a micrometer to a few millimeters and are notable for their intricate features and exotic shapes. "Many of those shapes are not too far off from what you would want for practical applications," says Sandhage. But natural shell material isn't ideal for all nanodevices.
To replace the silica with more useful materials, Sandhage and his colleagues heated Aulacoseira diatom shells to 900 degrees Celsius in an atmosphere of magnesium gas. The magnesium fully displaced the silicon in the shell, leaving a perfectly formed cylinder of magnesium oxide, the team reports in the 18 March issue of Advanced Materials. Although the magnesium oxide "shells" could be useful as heavy metal waste removers or as a friction additive in brake linings, Sandhage says the technique should also work for a wide range of other materials. He has recently teamed up with a pharmacologist to develop minute drug delivery capsules using shells based on calcium oxide compounds, which, unlike silica, can be readily absorbed by the body.
The technique is "very likely" to have real-world applications, says Morley Stone, a biochemist at the U.S. Air Force Research Laboratory at the Wright-Patterson Air Force Base in Dayton, Ohio. "This work fills an important void," says Stone, because it "enables one to make a complicated silica structure and convert it into another oxide with more desirable properties." Eventually, says Sandhage, scientists may even seek to "tinker with the DNA of diatoms to make tailored shapes."