Abstract: The invention provides a voltaic cell for implantation into the body of a subject that oxidizes oxidizable biological material to generate current to do work. The voltaic cell comprises a biologically inert shell having an inner compartment containing a cathodic environment and an anodic environment on the outer surface of the shell that is in contact with bodily fluid. A connector connects the anodic environment to the cathodic environment and has a component that provides resistance between the anodic and cathodic environments. The shell contains at least one salt bridge disposed within the shell that permits passage of small ions between the inner compartment and the bodily fluid, thereby completing the circuit. The invention also provides devices such as a glucometer which continuously detects glucose and transmits a signal to an external device which provides an output, such as blood glucose level. Methods of using the same are also provided.