The fuel cell can harvest electrons from feeding yeast – smaller versions could one day be implanted in the human body to power medical devices. The prototype is 15 millimetres square and 1.4 mm thick (Image: IEEE/Mu Chiao/Chin-Pang-Billy Siu) The yeast grow by budding new cells (indicated) (Image: IEEE/Mu Chiao/Chin-Pang-Billy Siu) Forming ‘micropillars’ on the electrodes increased surface area to improve efficiency (Image: IEEE/Mu Chiao/Chin-Pang-Billy Siu)

Yeast cells feeding on the glucose in human blood might one day power implants such as pacemakers. A living source of power that is able to regenerate itself would eliminate the need for regular operations to replace batteries.

Now that prospect is a step nearer. A team at the University of British Columbia in Vancouver, Canada, has created tiny microbial fuel cells by encapsulating yeast cells in a flexible capsule. They went on to show the fuel cells can generate power from a drop of human blood plasma.

Such fuel cells would be especially useful for devices, such as intraspinal microelectrodes for treating paralysis, which need to be implanted in places where replacing a battery is tricky, says Mu Chiao, who co-authored the paper with Chin-Pang-Billy Siu, also at UBC.

Electron theft

Conventional fuel cells rely on high-temperature catalysts such as platinum to strip electrons from fuels and generate a current. The idea with microbial fuel cells, which are being investigated as large-scale power sources, is to exploit the wide range of low-temperature catalysts – enzymes – found in living cells.

The easiest way to do this is to simply steal the electrons produced when cells start to break down food. This can be done with the help of an “electron mediator” – a chemical small enough to pass into cells, grab some electrons, and diffuse out again.

The new fuel cell consists of a colony of Saccharomyces cerevisiae – the kind of yeast commonly used in brewing and baking – encapsulated in a fuel cell made of a form of silicone called polydimethylsiloxane (PDMS). The prototype is 15 millimetres square and 1.4 mm thick.

Methyl blue – a chemical often used to stain biological samples – is used as the electron mediator. This steals some of the electrons produced when the yeast metabolises glucose and delivers them to the anode side of the cell, creating a small current. On the cathode side, hydrogen ions that diffuse out of the yeast cells combine with oxygen to create water.

Waste problem

To increase the surface area of the electrodes and thus boost the fuel cell’s power output, the team used a silicon etching technique to create “micropillars” roughly 40 micrometers square and 8 micrometers high (see image).

The yeast-based fuel cell produces around 40 nanowatts of power, compared to the microwatt a typical wristwatch battery might produce, Chaio says. That might be enough power for some devices if it were coupled with a capacitor to allow energy to be stored. The yeast could also be genetically engineered to boost its power output.

The work is a step in the right direction, but huge challenges remain, says Lars Angenent, who works on microbial fuel cells at Cornell University.

For instance, to keep the yeast cells healthy, their waste products will need to be removed without allowing any harmful substances to leach out into the blood stream. “I think people will figure this out. This is a first step,” he says.

Journal reference: Journal of Microelectrical Systems (DOI: 10.1109/JMEMS.2008.2006816)