The power for future generations of implanted medical devices is best provided by biofuel cells that harvest energy from the human body, rather than by storage batteries or cumbersome transcutaneous energy transfer. We present the first functional implantable biopile, which utilises an innovative and simple mechanical confinement of enzymes and redox mediators to convert extracellular-fluid into enough energy to power a pacemaker. We are now exploring a wide range of medical applications.