An implantable system is disclosed that includes an encapsulated housing including a radioactive source. The encapsulated housing is internally coated with a light-generating coating that converts the radioactive energy to light, that is then output in vivo to a photodiode positioned proximate to at least part of the encapsulated housing in vivo. The photodiode is configured to generate electrical current from the light, which electrical current is output to a power conditioning circuit that is configured to use the electrical current as an input electrical current and to output power to a load in vivo. The encapsulated may also be internally coated, at least in part, with a light directing coating that causes light to be directed toward an optical lens that focuses the light on the photodiode. The radioactive source may be a gas having a low half-life and which poses no health risk to a patient if released.