The biofuel cell developed by chemical engineering professor Adam Heller, graduate student Ting Chen, and coworkers is made from two 7-µm-diameter, electrocatalyst-coated carbon fiber electrodes placed in 1-mm grooves machined into a polycarbonate support. One electrode is a glucose-oxidizing anode, and the other is an oxygen-reducing cathode [J. Am. Chem. Soc., 123, 8630 (2001)].

The electrode areas are 1/60th the size of the smallest reported methanol-oxidizing fuel cell and 1/180th the size of the smallest previously known biofuel cell. The power density of the device is five times greater than that of the previous best biofuel cell. It has a power output of 600 nW at 37 °C, enough to power small silicon-based microelectronics.

The secret to the fuel cell's size and performance is the use of microfibers rather than flat electrodes and the enzyme-based electroactive coatings. This electrode design avoids glucose oxidation at the cathode and O₂ reduction at the anode, Heller points out, eliminating the need for an electrode-separating membrane, which is difficult to produce and enclose when small.

The anode coating is glucose oxidase covalently bound to a reducing-potential copolymer that has osmium complexes tethered to its backbone. The cathode coating is similar but contains the enzyme laccase and an oxidizing-potential copolymer. In the coatings, a network of osmium redox centers electrically "wires" the reaction centers of the enzymes to the carbon fibers.

The enabling breakthrough, Heller says, was the group's earlier development of the "wired" laccase cathode that facilitates the four-electron reduction of O₂ to water near neutral pH (pH 5) at body temperature (37 °C) [J. Am. Chem. Soc., 123, 5802 (2001)]. Reduction of O₂ to water under these conditions has been one of the long-standing problems in electrochemistry, Heller notes. Until now, only noble metal electrodes at pH 0 or carbon electrodes at pH 14 were used for the reduction.

Scientists have been making steady progress at miniaturizing biological sensors, such as a subcutaneous glucose sensor to manage diabetes developed by Heller's group. But the size of the power sources needed to run such implanted devices hasn't been proportionally reduced. The size of the sensors is limited by the size of the battery, the components of which must be enclosed in a relatively large sealed case.

"After many years of research by groups worldwide, Heller's group has achieved record current densities, which allow for extreme miniaturization," notes Robert J. Nowak, an electrochemist who manages advanced energy technology projects at the Defense Advanced Research Projects Agency. "The group has cleverly engineered the electrodes to minimize current-canceling reactions so that a separator, common to most fuel cells, is not necessary. This has significant implications for implanting biofuel cells in tissue."

Nowak envisions that tiny biofuel cells could eventually power "miniature pharmaceutical factories" that utilize natural biochemicals in the bloodstream to deliver drugs as needed. Many technology breakthroughs in sensors, microfluidics, microvalves--and power--are still needed, Nowak adds, but the new biofuel cell "sets a benchmark for future efforts."

REDOX Electron-transferring steps of glucose oxidation and O₂ reduction (diagrammed at right) drive tiny biofuel cell, a section of which is shown.

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