Poor electron transfer and slow mass transport of substrate are significant rate-limiting steps in electrochemical systems. It is especially true in biological media where the concentrations and diffusion coefficients of substrates are low, hindering development of power systems for miniaturized biomedical devices. Here we show that the newly engineered porous microwires comprised of assembled and oriented carbon nanotubes (CNT) overcome both the limitations of small dimensions and large specific surface area. Their improved performances are demonstrated by comparing the electroreduction of oxygen to water in saline buffer on carbon and CNT fibers. Under air, and after several hours of operation, we show that CNT microwire exhibit more than ten-fold higher performances than conventional carbon fibers. Consequently, under physiological conditions, the maximum power density of a miniature membrane less glucose/O2 CNT biofuel cell exceeds by far the power density obtained for current state of the art carbon fiber biofuel cells.