A biofuel cell operating at a power density of 50 muW cm(-2) at a 0.5 V cell potential under physiological conditions (air saturated, pH 7.4, 0.14 M NaCl, 37.5degreesC, 15 mM glucose) is described. The cell had a glucose electro-oxidizing anode and an O-2 electroreducing cathode. The anodic electrocatalyst comprised the electrostatic adduct of glucose oxidase (GOx) a polyanion at physiological pH, and the polycationic redox polymer poly (N-vinyl imidazole), partially quarternized with 2-bromoethylamine and partially complexed with [Os(da-bpy)(2)Cl](+/2+), (da-bpy=4,4'-diamino-2,2'-bipyridine) (E-o' = -160 mV vs. Ag/AgCl). The cathode electrocatalyst was the electrostatic adduct of bilirubin oxidase (BOD) also a polyanion at physiological pH, 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' = 350 mV vs. Ag/AgCl). The anode and the cathode were 7 mum diam, 2 cm long carbon fibers, on which the catalytic enzyme-redox polymer adducts were cross-linked. When the miniature cell operated at 0.5 V, its 50 muW cm(-2) initial power density decreased to 30 muW cm(-2) after 2 days of continuous operation at 37.5degreesC. (C) 2003 The Electrochemical Society.