We present the operational concept, microfabrication, and electrical performance of an enzyme-less direct glucose fuel cell for harvesting the chemical energy of glucose from body fluids. The spatial concentrations of glucose and oxygen at the electrodes of the one-compartment setup are established by self-organization, governed by the balance of electro-chemical depletion and membrane diffusion. Compared to less stable enzymatic and immunogenic microbial fuel cells, this robust approach excels with an extended life time, the amenability to sterilization and biocompatibility, showing up a clear route towards an autonomous power supply for long-term medical implants without the need of surgical replacement and external refueling. Operating in physiological phosphate buffer solution containing 0.1 wt% glucose and having a geometrical cathode area of 10 cm2, our prototype already delivers 20 µ W peak power over a period of 7 days.