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Disposable High Power Microfluidic Biofuel Cells On A Chip Formed by Soft Lithography

St. Louis University (St. Louis, MO) earned U.S. Patent 7,709,134  for microfluidic biofuel cells comprising a bioanode and/or a biocathode that are formed using microfluidic principles and soft lithography. The enzymes utilized in the redox reactions at the bioanode and/or the biocathode are stabilized in a micellar or inverted micellar structure. The biofuel cell is used to produce high power densities, say inventors Professor Shelley D. Minteer, Robert S. Martin and Christine M. Moore. 

The microfluidic biofuel cell may be used in any application that requires an electrical supply, such as electronic devices, commercial toys, internal medical devices, and electrically powered vehicles. Further, the microfluidic biofuel cell  may be implanted into a living organism, wherein the fuel fluid is derived from the organism and current is used to power a device implanted in the living organism. 

 A biofuel cell is similar to a traditional polymer electrolyte membrane (“PEM”) fuel cell in that it consists of a cathode and anode generally separated by some sort of barrier or salt bridge, such as a polymer electrolyte membrane. However, biofuel cells differ from the traditional fuel cell by the material used to catalyze the electrochemical reaction. Rather than using precious metals as catalysts, biofuel cells rely on biological molecules such as enzymes to carry out the reaction.
Early biofuel cell technology employed metabolic pathways of whole microorganisms, an approach which provided impractical power density outputs due to low volumetric catalytic activity of the whole organism. Enzyme isolation techniques spurred advancement in biofuel cell applications by increasing volumetric activity and catalytic capacity. Isolated enzyme biofuel cells yield increased power density output by overcoming interferences associated with cellular membrane impedance with electron transfer and lack of fuel consuming microbial growth. 
FIG. 12 is a photograph of a fully integrated biofuel cell on a microchip. 
FIG. 13 is a photograph of an integrated microfluidic bioanode with an external cathode. The cathode consists of a platinum wire in a glass tube with Nafion™ 117 membrane on one end and in phosphate buffer (pH 7.15). 
The microfluidic biofuel cell uses a fuel fluid to produce electricity via enzyme mediated redox reactions taking place at micromolded microelectrodes with immobilized enzymes therein. As in a standard biofuel cell, the bioanode is the site for an oxidation reaction of a fuel fluid with a concurrent release of electrons. The electrons are directed from the bioanode through an electrical connector to some power consuming device. The electrons move through the device to another electrical connector, which transports the electrons to the biofuel cell’s biocathode where the electrons are used to reduce an oxidant to produce water. In this manner, the biofuel cell acts as an energy source (electricity) for an electrical load external thereto. To facilitate the fuel fluid’s redox reactions, the microelectrodes comprise an electron conductor, an electron mediator, an electrocatalyst for the electron mediator, an enzyme, and an enzyme immobilization material.

Unlike a standard biofuel cell, however, the University of Washington biofuel cell utilizes at least one micromolded electrode. In one embodiment, the micromolded electrode has a flow through structure that allows fuel to flow within the microelectrode. When compared to conventional biofuel cell electrodes, this structure yields a higher current density because of the higher amount of microelectrode surface area in contact with the fuel.

 In another embodiment, the micromolded electrode has an irregular topography. Again, the current density of the microelectrode is greater than conventional biofuel cell electrodes because of a higher amount of surface area in contact with the fuel. These features combine with other features to create a biofuel cell with increased current density over conventional biofuel cells from a dimensionally smaller source. Finally, the method of the current invention can advantageously be used to economically produce disposable fuel cells. 
 Suitable electron conductors can be prepared from gold, platinum, iron, nickel, copper, silver, stainless steel, mercury, tungsten, and other metals suitable for colloidal dispersion. In addition, electron conductors which are metallic conductors can be constructed of nanoparticles made of cobalt, carbon, and other suitable metals.

In addition, the electron conductor can be a colloidal semiconductor. Suitable semiconductor materials include silicon and germanium, which can be doped with other elements. The semiconductors can be doped with phosphorus, boron, gallium, arsenic, indium or antimony, or a combination thereof.

Other electron conductors can be metal oxides, metal sulfides, main group compounds (i.e., transition metal compounds), and materials modified with electron conductors. Exemplary electron conductors of this type are nanoporous titanium oxide, cerium oxide particles, molybdenum sulfide, boron nitride nanotubes, aerogels modified with a conductive material such as carbon, solgels modified with conductive material such as carbon, ruthenium carbon aerogels, and mesoporous silicas modified with a conductive material such as carbon.