Inductive power transfer receiving system for active implantable medical devices

Wireless power transfer (WPT) is turning out as a promising approach to power supply Active Implantable Medical Devices (AIMDs) to recharge their batteries without having to surgically explant them. Exploiting such a technology to recharge the implant’s batteries introduces many advantages in terms of safety, reduced invasiveness and limited power consumption. The design of a personalized recharging method, based on inductive wireless power transfer, was mainly conceived to be integrated into two main devices: a nanofluidic drug delivery system and a bidirectional neural interface for neuroprosthetic applications. This project illustrates all the steps required to realize the receiving side of a WPT system resonating at 13.56 MHz ISM band, from its conceptualization to the final realization on printed circuit board (PCB). Multiple constraints need to be addressed to make this objective feasible, such as the reduced dimensions allowed by the device, thermal heating, radiofrequency (RF) working frequency and proper matching between the source and load impedance. An ideal system-level model was used as the basis for all subsequent steps. The system comprises four main blocks: the receiving coil together with its capacitor needed for tuning the resonance, the matching network, to adapt the load impedance to the voltage source, a signal rectifier made of Schottky diodes to convert the input into a constant voltage and a DC-DC converter, to obtain enough power to recharge Li-Ion battery and supply the overall circuit. The simulations led to the optimization and the efficiency evaluation of each of the single blocks composing the overall circuit. The PCB, designed for a full characterization of the system as a primary goal, is meant to fully integrate all the components of the device to shrink its size and make the implantation process feasible. To this end some strategies were applied, mainly concerning the components’ selection, the separation between different functional areas of the board, the interconnection between top and bottom layer and the introduction of an NTC resistor to assess the amount of power which goes lost in correspondence to the coil. RF resistors were selected in order to ensure high stability measurements even at high frequency ranges. The final PCB board was also equipped with a current sensing measuring system to perform the tests which included current and voltage measurements for a complete characterization of the circuit. The results showed that the measurements performed on PCBs are extremely sensitive to external noise and parasitic capacitances, possibly due to every loop or conductive material posed in close contact to the circuit. Moreover, it was clear that performing these measurements simultaneously could led to a significant decrease in amplitude if comparing the obtained results once acquiring these values separately. For these reasons the necessity to adapt the measuring setup to RF measurements was stressed in the final steps of the project. In the end, an innovative prototype of the receiving circuit of an inductive WPT system for AIMDs applications was developed and characterized, laying the foundations for a top-down approach, starting from the requirements and constraints and propagating down to the circuit and PCB design. From the receiving side perspective, multiple aspects were considered, such as the amount of power delivered from the transmitting side, an integrated test and validation system and finally the influence of the final load.