The new fuel cells offer features sought after by manufacturers to provide long-term power for medical devices such as implants that monitor glucose levels in diabetics.
"They present a new paradigm toward the development and manufacture of small fuel cells for medical implants," said lead scientist Tayhas Palmore, associate professor of engineering, biology and medicine. "There is a lot of basic science yet to be worked out. But if successful, this design could help rid a diabetic of the need to monitor blood glucose after each meal, and that would make for a significant advance in the treatment of diabetes."
Fuel cells currently are a hot topic because they are more efficient at converting chemical energy into work than a heat engine, they are simple in design, and they don't pollute the environment. For those reasons, fuel cells are seen as promising alternatives to the combustion engine in automobiles and batteries in portable electronics and medical implants.
A fuel cell consists of two electrodes immersed in fuel-containing fluids separated by an ion-conducting membrane. Power is produced by the fuel cell when electrons are removed from the fuel, transported via an external circuit, and combined with positive ions crossing the ion-conducting membrane and oxygen. Conventional fuel cells run on either hydrogen gas or liquid methanol but more recently, prototype fuel cells have been shown to run on more exotic fuels such as glucose or formate. In theory, fuels cell are amenable to a range of fuels.
The Brown fuel cells do not require an ion-conducting membrane or selective catalysts at the electrodes to separate the fuel-containing fluids – two thorny technological traits of fuel cell design that must be considered in the development of miniature fuel cells. Instead, the new fuel cells exploit the fact that fluids do not mix under certain conditions. "We take advantage of how fuels flow in small channels," said Palmore, "in that they don't mix, which means we can keep fuels separated without a membrane."
The Brown-developed fuel cells work in tandem to provide power under the pulsating conditions that mimic the flow of blood in the body. Until now, fuel cell makers had been stymied in their efforts to produce a membrane-less device that did not short-circuit under pulsed flow.
One of the microfluidic fuel cells fabricated at Brown features a novel branched-channel, which encloses six electrodes. This fuel cell is "most suitable for generating electrical power under conditions of pulsed-flow," said Palmore. "The design of the device makes possible the delivery of power to a chip as a result of changes in the concentration of a fuel, such as glucose," she said. "This power feedback is a necessary component in an imbedded sensor for diabetes."
Palmore discussed the new microfluidic fuel cells March 27, 2003 at the 225th national meeting of the American Chemical Society (ACS) in New Orleans. Coauthors of the work are graduate students Mark Luo, Jiangfeng Fei, and Keng Lim. Brown University, the National Science Foundation and the Office of Naval Research funded the research.