The fuel cell has been considered a clean alternative to fossil-fuel-based power generation. Conventional fuel cells, however, are large solid-state devices that employ costly mechanical and chemical components and have thus witnessed very limited commercial adoption since their introduction several decades ago. Further, such devices use inorganic fuels, many of which produce substantial carbon footprints when processed and refined. Biofuel cells (BFCs) derive power from organic/biological compounds; e.g., glucose (in blood), lactate (in perspiration), and urea (in urine, wastewater, sewage) - and represent a new, compelling class of energy conversion devices. BFCs have the ability to operate under mild conditions and are envisioned to be applicable as implantable power sources.