Scientists have developed a new energy storage device that works with fluids in the human body and could lead to more durable and battery-free pacemakers and other implantable medical devices. The bio-friendly energy storage system called biological supercapacitor is powered by charged particles, or ions, from body fluids like blood serum and urine. Pacemakers - which help regulate abnormal heart rhythms - and other implantable devices have saved countless lives.