Germs fuel tiny flyers

The US Air Force has long pursued a goal of creating micro-scale flying spy drones, but it has been hampered by the lack of a suitable fuel source. However, new multidisciplinary research it is funding to the tune of $4.4 million could see future tiny flyers powered by bacteria.

A diverse team of microbiologists, engineers and geochemists from the University of Southern California and Rice University are joining forces to produce a flying prototype within 5 years for the US Department of Defense's Multidisciplinary University Research Initiative (MURI).

Rice geochemist Andreas Lüttge has pinpointed bacterium Shewanella oneidensis to attach to and interact with anode surfaces inside the fuel cell as it releases electrons from metals during its metabolic process. To optimise its design, the team must understand how bacteria transfer electrons to anode surfaces under a variety of conditions.

"There are three primary components in the system: the bacteria, the surface and the solution that the bacteria are digesting," said Lüttge. "Any change in one variable will affect the other two, and what we want to do is find out how to tweak each one to optimise the performance of the whole system."

Shewanella oneidensis uses metals to fully metabolise its food and maintain its respiratory metabolism in oxygen-poor environments. In doing so, it is capable of passing electrons directly to solid metal oxides. The researchers expect that it will be able to do the same to the anode of the fuel cell. The collaboration researchers in chemistry, geology, engineering, and evolutionary biology will allow the team to optimise the entire system.

In the fuel cell study, Lüttge will use computer models to estimate how the bacteria will behave under different circumstances. Running tests on the computer will save time and money by allowing laboratory experiments to focus on best candidate approaches.

In addition to the computational modelling, Lüttge will employ an imaging technique called Vertical Scanning Interferometry. The technique, which he helped create in the 1990s, combines information from multiple beams of light to resolve sample features as small as one-billionth of a metre. In previous studies with Nealson, Lüttge used the technique to examine how the cigar-shaped Shewanella attach themselves to crystalline surfaces. The researchers found that Shewanella would lay flat and orient themselves relative to minute defects in the crystal's surface.

"We still have a lot to learn about the chemical cues that the Shewanella use – both individually and in colonies – but they are incredibly efficient at converting organic inputs to electricity, so we are confident that they'll be a great candidate for our fuel cells," Lüttge said.