In the future, your saliva might power key microelectronics. Researchers at Penn State have devised a micro-sized microbial fuel cells that is power by the organic materials and bacteria present in saliva. The power generated by the tiny fuel cell is very small, though, so don’t expect charging your phone by spitting in it any time soon. It is enough, according to the researchers involved, to power micro chips that diagnose diseases or act as environmental sensors.

MFC. Photo: Bruce Logan/Penn State

How do microbial fuel cells (MFCs) work?  Bacteria need energy to survive, in the same way that humans need food to
live. Bacteria get this energy in a two-step process. The first step requires the removal  of electrons from some source of organic matter (oxidation), and the second step   consists of giving those electrons to something that will accept them (reduction),  such as oxygen or nitrate. If certain bacteria are grown under anaerobic conditions  (without the presence of oxygen), they can transfer electrons to a carbon electrode  (anode). The electrons then move across a wire under a load (resistor) to the cathode  where they combine with protons and oxygen to form water. When these electrons
flow from the anode to the cathode, they generate the current and voltage to make  electricity.

Powered by spit

Microbial fuel cells, known as MFCs for sure, exploit this biological mechanism. Typically, they’re comprised of two chambers to separate the oxygen from the bacteria, which  break down organic material producing a charge that is transferred to the anode. Bruce E. Logan, Evan Pugh Professor and Kappe Professor of Environmental Engineering, Penn State has been studying MFCs for ten years and him and his team recently reported that saliva can be used as the feed for a tiny MFC.

Theirs, however, works differently. For one, it only has one chamber and uses graphene- rather than platinum-coated carbon cloth anode and an air cathode.  The air cathode wasn’t used in the first place, so that the bacteria doesn’t react with oxygen – it doesn’t produce electricity anymore then.

“We have previously avoided using air cathodes in these systems to avoid oxygen contamination with closely spaced electrodes,” said Logan. “However, these micro cells operate at micron distances between the electrodes. We don’t fully understand why, but bottom line, they worked.”

The power produced by the tiny MFC is around 1 microwatt – a joke some might say, but its enough to power ultra-low-power chip-level biomedical electronics, devices able to operate at sub-microwatt power outputs. One such low-power device would be  a tiny ovulation predictor based on the conductivity of a woman’s saliva, which changes five days before ovulation.

To feed of the MFC is saliva, as well as acetate, but any liquid with sufficient organic material can be used. The findings were reported in a recent issue of Nature Publishing Group’s Asia Materials. [via Penn State press release]