Gastric apparatus (18) is provided, including one or more sensors (70, 72, 74), adapted to generate respective sensor signals responsively to a gastrointestinal (GI) tract physiological parameter of a GI tract of a subject, and an implantable control unit (90), comprising a rechargeable battery and a first set of one or more transducers. The implantable control unit (90) is adapted to receive the sensor signals, using one or more of the transducers of the first set, and transmit data responsively to the sensor signals, using one or more of the transducers of the first set. An external control unit (200), including a power source and a second set of one or more transducers, is adapted to drive the power source to inductively transfer energy via one or more transducers of the second set, so as to recharge the battery, and receive the transmitted data, using one or more transducers of the second set.
