Tiny Microbial Fuel Cell Runs On Spit

Photo: KAUST

A tiny saliva-powered microbial fuel cell

Researchers have made a fingernail-sized microbial fuel cell that runs on saliva. The cell generates 1 microwatt of power, enough to power lab-on-a-chip diagnostic devices in rural settings or battlefields, the researchers say.

Conventional microbial fuel cells contain a cathode and anode chamber separated by a proton-exchange membrane. At the anode, anaerobic bacteria break down organic matter from liquids, releasing carbon dioxide, electrons and protons. The electrons flow to the cathode through an external circuit while the protons go through the membrane. Employed at factories or wastewater treatment plants, microbial fuel cells could produce clean water and electricity while significantly cutting down sludge produced.

The new cells are nothing like their liter-size cousins, though. These 25-microliter devices have a radically different design featuring unique, carefully chosen electrodes and fuel sources, says Muhammad Hussain, a professor of electrical engineering at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. Hussain and his colleagues at KAUST and Penn State University published their results recently in the journal Nature Asia Materials.

Conventional microbial fuel cells contain carbon-based anodes and cathodes made of carbon brushes or carbon cloth. The essential requirement for the electrodes is that they be conductive and have a high surface-to-volume ratio so that most of the bacteria have access to the waste material.

Graphene meets both of those conditions, so the KAUST researchers used that for their anode. They went with an air cathode, which is commonly used in large-scale microbial fuel cells. Most importantly, the researchers got rid of the expensive membrane. “We figured you don’t need the membrane, you just need to bring anode and cathode as close as possible, which becomes much easier on the micro scale,” says Hussain. “At the same time, current generation depends on the internal resistance of the whole fuel cell. Without the membrane you reduce resistance.”

The team starts with a 1-by-1-centimeter sheet of graphene film. They place a rubber spacer with the same dimensions on top and cut a 5-by-5-millimeter hole in the center of the spacer. The hole acts as the anode chamber. The researchers loaded the device with bacteria from wastewater and then added saliva via syringe tips inserted into both sides of the rubber. The device can easily be built on plastic, Hussain points out.

Acetate is a common fuel source for microbial fuel cells. But Hussain and his colleagues wanted an easily accessible fuel, so they tested saliva. “Soldiers in a battlefield don’t have time to put chemicals in fuel cells to make it operational,” he says. “People in rural areas might not have access to specialty chemicals. So the easiest thing is saliva. Saliva’s organic content is much higher than known chemicals like acetate, making it a good fuel source.”

The device generates higher current densities than other micron-sized microbial fuel cells made so far. The graphene anode also generates 40 times as much power as its carbon cloth counterpart.

The researchers are now exploring ways to increase their device’s power output by making more efficient electrodes and stringing multiple cells in series.

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