Researchers from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, and Penn State University published a paper in Nature Magazine showcasing their work on a saliva powered microbial fuel cell. According to the researchers, the fingernail-sized fuel cell is capable of powering lab-on-a-chip diagnostic devices in rural settings or battlefields with the 1 microwatt of power it generates.
Microbial fuel cells use bacteria to generate electricity from waste. Electrons are released during a process which breaks down the bacteria inside a fuel cell. The electrons are then collected at an anode and then travel to an external circuit to the cathode to produce the electrical current.
Microbial fuel cells can be found at factories and wastewater plants where they are used to produce clean water and electricity while cutting down on the sludge produced. A cathode and anode chamber separated by a proton-exchange membrane is usually contained within a conventional microbial fuel cell. Anaerobic bacteria break down organic matter from liquids at the anode, a process which releases carbon dioxide, electrons and protons. The electrons then flow to the cathode through an external circuit while the protons flow through the membrane.
The 25 microliter device created by the researchers differs vastly from the conventional one widely used as it is made of carefully chosen electrodes and fuel sources, says Muhammad Hussain, professor of electrical engineering at KAUST. Unlike conventional microbial fuel cells that contain carbon-based anodes and cathodes made of carbon brushes or cloth, the cells created by the researchers contain graphene anodes and an air cathode. The researchers also managed to get rid of the membrane used in conventional cells, “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,” said Hussain. “At the same time, current generation depends on the internal resistance of the whole fuel cell. Without the membrane you reduce resistance.”
A rubber spacer measuring 1×1 cm is placed on top of a sheet of graphene film of the same dimensions. A hole of 5×5 mm, which acts as an anode chamber, is cut through the center of the spacer. The device is then loaded with wastewater bacteria and saliva is inserted by syringe tips into both sides of the rubber.
Acetate is widely used as a fuel source in microbial fuel cells. The researchers tested out saliva as an alternative fuel source as it easily accessible. “Soldiers in a battlefield don’t have time to put chemicals in fuel cells to make it operational,” says Hussain. “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 researchers found that their device generates greater current densities than other micron-sized microbial fuel cells that have been made. The graphene anode they used also generates forty times as much power as the carbon cloth usually used.
“Our study is the first to show that saliva (and most probably other highly concentrated organic fuels) can be used to power bioelectronics devices,” says Hussain. “By producing nearly 1 microwatt of power, our micron-sized microbial fuel cell is already good enough to run ultralow-power lab-on-a-chip devices – such as an EEG seizure-detection system, to name but one example.”
The researchers discovered an unexpected application of their device, an ovulation predictor. “It is well known that around five days before ovulation, saliva’s conductivity decreases sharply – most likely because of a peak in estrogen levels. The new cell could measure this change in conductivity and so identify when a woman is most fertile. The power generated by the device could also perhaps be harnessed to send the data to a smartphone. Such an application could therefore help in better family planning in a non-invasive, easy-to-use way” said Hussain.
The researchers are investigating how to make the device power within the megawatt range. Hussain believes a more efficient air cathode may be the solution.