The history of electrochemical power sources shows that batteries or fuel cells were introduced only when the development of new electrical or electronic system demanded these. At this time, the already established feasibility of miniaturization of implantable sensor-transmitter systems to volumes smaller than 1 mm3, and the demand for spatially and temporally resolved information on local temperature, flow, pressure and chemical concentrations, are likely to create a demand for a miniature, low cost glucose-O2 biofuel cell that would power the autonomous sensor–transmitters for a few weeks. Prototypes of these cells are in hand. Their most unique feature which is their structural simplicity, is made possible by the selectivity of their “wired” enzyme catalysts: the cells consist merely of two 7 μm diameter carbon fibers, each coated with a different “wired” enzyme bioelectrocatalyst. On one, catalyzing the two-electron electrooxidation of glucose at a reducing potential, glucose oxidase is co-immobilized in and electrically connected (“wired”) by an electron conducting hydrogel of a reducing redox potential. On the other, catalyzing the four electron electroreduction of O2 to water, bilirubin oxidase is co-immobilized in and electrically “wired” by an electron conducting hydrogel of an oxidizing potential. The cells are the smallest ever built. When the volume of the fibers is 0.0026 mm3, the current of the cell operating at 0.52 V in a physiological buffer solution at 37°C is 8.3 μA. The 4.3 μW power output of the cell is expected to suffice for the operation of implanted sensors and for the intermittent transmission of the data collected to an external receiver.