R&D DIGEST

Yeast cells that feed on the glucose in human blood might be able to create biological microelectromechanical systems that can power implanted devices. These living power sources, which are able to regenerate themselves, could eliminate the need for batteries during regular functions of implants such as pacemakers.

A team at the University of British Columbia in Vancouver, has created tiny microbial fuel cells by encapsulating yeast cells in a flexible capsule. They have also shown that the fuel cells can generate power from a drop of human blood plasma.
The yeast, Saccharomyces cerevisiae, converts the chemical energy stored in glucose in the bloodstream to electrical energy. A microfabricated polydimethylsiloxane (PDMS) microbial fuel cell houses electrodes and the yeast. An anode and a cathode are separated by a Nafion 117 proton exchange membrane. These electrode surfaces feature more than 70,000 8-μm-high micropillar structures in a 1.2 × 1.0-cm geometric area. The prototype measures 15 mm² and is 1.4 mm thick.
The fuel cell is flexible and has a biocompatible structure. Using just 15 µl of blood, it demonstrates a maximum open circuit potential (OCP) of 488.1 mV, a maximum current density of 30.2 µA/cm², and a maximum power density of 401.2 nW/cm².
When the fuel cell operates continuously for 60 minutes, it shows an average OCP of 297.4 mV, average current density of 4.3 µA/cm², and average power density of 42.4 nW/cm² at 1-kΩ load. The 40 nW is about the level of power needed to run a wristwatch, says study author Mu Chiao.
The power might be enough for some devices if coupled with a capacitor to allow energy to be stored. Alternatively, the yeast could also be genetically engineered to boost its power output.
Challenges remain, says Chiao. For instance, the team is concerned with keeping yeast cells healthy and also with removing waste products from the body. Otherwise, harmful substances could be released into the bloodstream.
The study was published in the December 2008 issue of the Journal of Microelectrical Systems.
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