THE DEVICE: This tiny biological fuel cell, the smallest of its kind with a total volume of just 0.3 microliters, was built using microfluidics and relies on bacteria to produce energy. Bacteria colonize the anode, the negatively charged end of the system, and through their natural metabolism produce electrons that flow to the cathode, creating a circuit. Together, the anode and cathode are only a few human hairs wide, but the tiny circuit generates a consistent flow of electricity.
WHAT'S NEW: In 2008, researchers at the University of Illinois at Urbana-Champaign created a 3 millimeter square hydrogen-powered fuel cell, hailed as the world's smallest fuel cell at the time. The following year, a team at the University of California, Santa Barbara, produced a microbial fuel cell with a volume of 1.5 microliters. This latest fuel cell is five times smaller, making it possible for use in places where larger fuel cells cannot fit or to pack many fuel cells together without space concerns.
The new fuel cell also takes a different approach to the separation of the two fluids in the cell, one in the cathode and the other in the anode. Most microbial fuel cells rely on a semi-permeable membrane to keep the liquids from mixing while still allowing protons to travel from one side to the other. "That's a big issue for microbial fuel cells," said Leonard Tender, a fuel cell researcher at the Naval Research Laboratory in Washington, DC, who was not involved in the research, as engineering tiny membranes and seals into smaller and smaller fuel cells has been a challenge. In this new cell, the researchers solve that problem by simply getting rid of the membrane altogether and relying on microfluidic channels to successfully keep the two liquids apart, like two rivers flowing side by side that don’t mix because one flows at a different rate than the other. "It's a single compartment microbial fuel cell that has no membrane, yet is able to function at presumably good efficiency," said Tender. "It's a very interesting first step."
Furthermore, because microfluidics can be easily mass-produced, this bitty technology is scalable, cheap and easy to construct, said senior author Kelvin Gregory, an environmental engineer at Carnegie Mellon University in Pennsylvania. "Once we had the microfluidics in hand, the assembly became fairly simple to do."
IMPORTANCE: Fuel cells have numerous advantages over chemical batteries as power sources. In addition to being cheaper and lasting longer, they are made of natural components and thus don't run the risk of leaking toxic chemicals into the environment. Microbial fuel cells, in particular, which rely on energy generation from biological systems, also serve as a renewable power source, as they may consume renewable fuels such as organic waste matter. These benefits make such cells attractive alternatives for fueling remotely deployed devices that need to be self-powered over long periods of time, such as underwater sensors or even someday medical implants or sensors, experts say.
"The value is that you can have very small power supplies that run on biological fuels," said Tender.
NEEDS IMPROVEMENT: For now, however, the microbial fuel cells produce only very tiny amounts of electricity -- up to 127 amps per cubic meter, about 7,000 times less than a AA battery, said Gregory. At this level, a single microfluidic microbial cell could potentially power itself as a remote sensor, but for larger applications, many cells would need to be stacked together to increase the power output. Luckily, multiplexing is a common feature of microfluidics, said Philip LeDuc, co-author on the paper and a mechanical engineer at Carnegie Mellon. "It's like computer chips -- you can put a ton of these things in parallel."
Researchers may also be able to improve the cells’ efficiency by tweaking their structure. While the membrane-less microfluidics approach is successful in preventing mixing between the two sides, it may also make it difficult for the protons to move from the anode to the cathode, which may be contributing to the low power output of the device, said Tender. "If that could be addressed, this would be a huge advance."
Z. Li, et al., “Microbial electricity generation via microfluidic flow control,” Biotechnol Bioeng., doi: 10.1002/bit.23156, 2011.