A reconfigurable implantable system for ultrasonic power control and telemetry, the system comprising: a) a charging device including: an ultrasonic transducer configured to transmit ultrasonic signals and to receive ultrasonic signals transmitted through a biological body; and a signal generator to drive the ultrasonic transducer to transmit an ultrasonic charging signal through the biological body; and b) an implantable device implantable in the biological body and configured to communicate wirelessly with the charging device through the biological body via an ultrasonic communication link between the implantable device and the charging device, the implantable device including: an implantable ultrasonic transducer to receive the ultrasonic charging signal from the charging device and to transmit ultrasonic signals through the biological body; a power unit coupled to the ultrasonic transducer, the power unit configured to harvest energy from the received ultrasonic charging signal when the implantable device is in an energy harvesting mode; a sensing or actuation unit powered by the harvested energy; and a communication unit coupled to the power unit and the sensing or actuation unit, the communication unit configured to switch the implantable device between the energy harvesting mode and an ultrasonic communication mode, the communication unit configured to read data from the sensing or actuation unit and transmit the data through the implantable ultrasonic transducer when the implantable device is in the ultrasonic communication mode, wherein switching between the energy harvesting mode and the ultrasonic communication mode includes selectively connecting, under control of the communication unit, the ultrasonic transducer to the power unit or the communication unit. a) a charging device including: an ultrasonic transducer configured to transmit ultrasonic signals and to receive ultrasonic signals transmitted through a biological body; and a signal generator to drive the ultrasonic transducer to transmit an ultrasonic charging signal through the biological body; and an ultrasonic transducer configured to transmit ultrasonic signals and to receive ultrasonic signals transmitted through a biological body; and a signal generator to drive the ultrasonic transducer to transmit an ultrasonic charging signal through the biological body; and b) an implantable device implantable in the biological body and configured to communicate wirelessly with the charging device through the biological body via an ultrasonic communication link between the implantable device and the charging device, the implantable device including: an implantable ultrasonic transducer to receive the ultrasonic charging signal from the charging device and to transmit ultrasonic signals through the biological body; a power unit coupled to the ultrasonic transducer, the power unit configured to harvest energy from the received ultrasonic charging signal when the implantable device is in an energy harvesting mode; a sensing or actuation unit powered by the harvested energy; and a communication unit coupled to the power unit and the sensing or actuation unit, the communication unit configured to switch the implantable device between the energy harvesting mode and an ultrasonic communication mode, the communication unit configured to read data from the sensing or actuation unit and transmit the data through the implantable ultrasonic transducer when the implantable device is in the ultrasonic communication mode, wherein switching between the energy harvesting mode and the ultrasonic communication mode includes selectively connecting, under control of the communication unit, the ultrasonic transducer to the power unit or the communication unit. an implantable ultrasonic transducer to receive the ultrasonic charging signal from the charging device and to transmit ultrasonic signals through the biological body; a power unit coupled to the ultrasonic transducer, the power unit configured to harvest energy from the received ultrasonic charging signal when the implantable device is in an energy harvesting mode; a sensing or actuation unit powered by the harvested energy; and a communication unit coupled to the power unit and the sensing or actuation unit, the communication unit configured to switch the implantable device between the energy harvesting mode and an ultrasonic communication mode, the communication unit configured to read data from the sensing or actuation unit and transmit the data through the implantable ultrasonic transducer when the implantable device is in the ultrasonic communication mode, wherein switching between the energy harvesting mode and the ultrasonic communication mode includes selectively connecting, under control of the communication unit, the ultrasonic transducer to the power unit or the communication unit.