biofuel cell operating under physiological conditions (20 mM phosphate, pH 7.4, 0.15 M chloride) at a power density of 244 muW cm(-2) at 0.36 V (37 degreesC). Unlike earlier cells operating under physiological conditions, the cell operates without a membrane separating its anode and cathode compartments. The cell is 180 times smaller than earlier reported biofuel cells operating under physiological conditions while its power density exceeds eightfold that of the highest reported. The anodic electrocatalyst comprised the electrostatic adduct of glucose oxidase (GOx), a polyanion at physiological pH, and the polycationic redox polymer poly(vinylpyridine) complexed with [Os(4,4'-dimethoxy-2,2'-bipyridine)(2)Cl](+/ 2+) (E-o' = -69 mV vs. Ag/AgCl). The electrocatalyst of this cathode is the cross-linked electrostatic adduct of bilirubin oxidase from Myrothecium verrucaria, a polyanion at >pH 4.1, and the polycationic redox copolymer of polyacrylamide and poly(N-vinylimidazole), complexed with [Os(dcl-bpy)(2)Cl](+/2+), where dcl-bpy=4,4'-dichloro-2,2'-bipyridine (E-o' = 340 mV vs. Ag/AgCl). (C) 2003 The Electrochemical Society.