UCLA

Biomedical implants are designed to last inside the human body for more than a decade. Critical to the longevity of the implants is how power is distributed inside the implant. The first part of the thesis explores how wired power is efficiently delivered inside a distributed implant. AC power must be delivered to avoid tissue polarization. Furthermore, the frequency should be high to reduce the size of rectification capacitors on power receivers. A T-LCC resonant converter topology is presented that can deliver wired AC power efficiently at high frequency while achieving high efficiency.

Distributed implants involve multiple stages of power conversion, all of which introduce losses. Furthermore, the implants must be connected through leads, which involve extra surgeries and are inconvenient. For example traditional lead-based cardiac pacemakers suffer from lead-related complications which include lead fracture, lead dislodgement, and venous obstruction. Modern leadless pacemakers mitigate the complications, but, since they are implanted inside the heart with a small battery, their limited battery lifetime necessitates device replacement. The second part of my thesis presents a leadless and batteryless, wirelessly powered intravenous cardiac pacemaker that mitigates both problems. The pacemaker has a passive wireless power receiver (RX) circuit that receives bursts of power from a transmitter (TX) and stimulates the tissue. The circuit applies monophasic, cathodic, and current/voltage stimulation to the heart with a programmable pulse period. It consumes 1mW power for a 0.5msec stimulation pulse, which qualifies for pacemaker application. A 5V voltage and 5mA current stimulation over 3cm TX and RX distance with controllable pulse width and pulse frequency is demonstrated. In-vivo device characterization demonstrates the potential of the device to advance pacemaker technology.

The cardiac pacemaker has a very small form factor and is implanted in a greater cardiac vein, which is wirelessly powered by a subcutaneous module. The subcutaneous module includes a rechargeable battery and is powered once in a few months from an external module. The third part of this thesis presents a wireless power transfer system (WPT) for power transfer between external and subcutaneous modules. Power is transmitted in bursts between the external transmitter and subcutaneous receiver modules. The duty cycle of the burst is controlled through load modulation to control power flow between the modules. The proposed WPT circuit can regulate output voltage against distance and load variations. No extra communication channel is present to establish feedback between transmitter and receiver, making the circuit suitable for many applications. A 330-milliwatt prototype circuit for use in a pacemaker application is demonstrated.