Implantable systems are described that include a stimulation device positionable in vivo and configured to communicatively couple to electrodes configured to stimulate or block body tissue and an auxiliary device positionable in vivo and including one or more coils configured to wirelessly couple, in vivo, to the stimulation device and to wirelessly couple to an ex vivo device. The auxiliary device may include a coil driver and a power source controlled by a processor and memory for storing data instructions for the coil driver and for storing data received from the stimulation device. The auxiliary device may also include a radio transceiver and an antenna. The stimulation device may include a housing, a coil, a power source and an integrated circuit for controlling the electrodes. The stimulation device may be coupled to a cuff via a lead and physically coupled to the auxiliary device.