Power harvesting integrated in a knee implant for autonomous sensors implanted in human body

This paper describes an energy harvesting system (EHS) composed of an electromechanical generator (EMG) and a dedicated energy management circuit integrable in a human total knee prosthesis, in which the mechanical energy from the knee joint is converted into electrical energy. Since the energy supplied by the proposed EMG is discontinuous in time, a tailored energy management circuit is necessary to adapt the harvested energy to the load energy requirements. The electromechanical generator is composed by two series of NdFeB magnets positioned into each condyle and a coil, placed in the pin of the tibial insert, which collects the magnetic flux variations, generated by the knee movements. A total knee replacement (TKR) prototype has been developed and realized in order to reproduce the knee mechanics. Therefore, electrical performances have been evaluated, at the first, measuring the EMG open circuit voltage by means of a high impedance buffer amplifier, and, subsequently, connecting the EMG to the energy management circuit able to manage the produced energy and to power an implanted circuit for force measurement inside the human knee. The tests showed that the EHS is able to supply the measuring circuit guaranteeing a tension between 2.45 V and 2.15 V for 25 ms almost every 1.5 s with a walking velocity of 1 Hz. The tests successfully demonstrate the possibility to power a measurement circuit transmitting the measurement data outside the prosthesis every about one-step and half.