|A device based on glucose oxidase, catalase, laccase, and carbon nanotubes delivers the best performance ever seen for an enzymatic glucose biofuel cell.|
Given this need, one can easily envision the utility of a small electrical device implanted in the body that runs on a common molecule present in blood, instead of a conventional battery. Biofuel cells are energing as such a device.
Conventional fuel cells typically depend on rare and expensive metals for energy production. Biofuel cells, based on renewable fuels such as glucose, do not suffer from this limitation, but nevertheless feature low power output and numerous other challenges.
Serge Cosnier (National Center of Scientific Research, France) and coworkers have worked towards improving the performance of glucose biofuel cells. They report a device based on enzymes and carbon nanotubes that is by far the most advanced glucose biofuel cell to date, and with further research (as dicsussed later) it may become a viable technology.
Device construction and function.
The scientists prepared their enzymatic glucose biofuel cells by mixing together multi-walled carbon nanotubes, water, and either (1) glucose oxidase and catalase or (2) laccase. This was followed by pressing either mixture into small disks with an applied force of 10 kiloNewtons, yielding disks that are 3 millimeters thick and 13 millimeters in diameter.
Glue on all but one side repelled water and insulated the disks, and a cellulose film covered the open side. The final biofuel cell was fabricated by joining one disk containing glucose oxidase and catalase and another containing laccase.
Glucose oxidase converts glucose to gluconolactone, laccase converts oxygen to hydrogen peroxide, and catalase converts hydrogen peroxide to oxygen. Electron transfer from these reactions to the carbon nanotubes is the source of electrical energy.
The optimized device features an open-circuit voltage of 0.95±0.05 volts at pH 7, 20°C, in phosphate buffer containing glucose (50 millimolar concentration). The power density is 1.3 milliwatts per square centimeter at 0.6 volts.
After storing it for 30 days, the power density decreases by 4% (as measured at 0.57 volts). Measuring at low (physiological) glucose concentrations of 5 millimolar, the open-cicuit voltage and power density decrease by 2% and 23%, respectively.
Connecting two devices in series yields an open-circuit voltage of 1.8 volts, and a maximum power of 3.25 milliwatts at 1.2 volts. This is well within the power needs of common small implanted medical devices.
Limitations and future work.
It's not clear to me whether the scientists ever tested their device at continuous usage for 30 days, or whether they primarily focused their efforts at 30 days of non-used storage. This is clearly a requirement for a practical, real-world device, and I'm (respectfully) skeptical as to whether enzymes will continue functioning normally over a long period of time.
Having said that, device fabrication is straightforward, and other molecules could realistically replace the enzymes if this is a fundamental limitation to long-term performance. Further research along these general directions looks promising.
NOTE: The scientists' research was funded by the National Center of Scientific Research.
Zebda, A., Gondran, C., Le Goff, A., Holzinger, M., Cinquin, P., & Cosnier, S. (2011). Mediatorless high-power glucose biofuel cells based on compressed carbon nanotube-enzyme electrodes Nature Communications, 2 DOI: 10.1038/ncomms1365