One embodiment of the present subject matter includes an apparatus for storing energy, the apparatus having a first portion comprising a flexible substrate containing a polymer electrolyte and a second portion adapted to provide a conformable housing surrounding the first portion； wherein the apparatus is adapted to provide a source of energy to an implantable device. The apparatus defines a flexible implantable device capable of traversing the circulatory system of a body with minimal obstruction of flow within the circulatory system. Further provided is a method of implanting an power source within the circulatory system of a subject's body.
