Towards an Energy-Efficient Inductively-Powered Ultra-High Frequency Pulsed Neurostimulator

Abstract

Electrical stimulation has emerged as a promising therapeutic modality for the treatment of various neurological disorders such as Parkinson's disease, chronic pain, epilepsy, and depression. In addition, recent studies have provided evidence supporting the effectiveness of electrical stimulation as a treatment option for patients with chronic migraine and cluster headaches who have been unresponsive to other therapies. Specifically, stimulation of the occipital and supraorbital nerves has shown promise in managing these refractory conditions. Current implantable medical devices (IMDs) are battery-powered, which increases the size of the implant and involves lengthy wires to connect with the electrodes. Wireless power transfer (WPT) techniques can be used to reduce the size and omit the battery from the implants. However, conventional power management methods use a rectifier, power converter, and neurostimulator to convert the AC power of the link to DC power and provide electrical stimulus, which lowers implant power efficiency.

This thesis presents a novel neurostimulator circuit that salvages and stores available energy from an inductive link. The circuit design includes an inductor for energy storage and utilizes a buck-boost converter-like topology to deliver the stored energy to the tissue. This topology exploits the capacitive membrane's characteristics and injects the charge into the tissue through ultra-high frequency pulsed stimulation without requiring additional circuits that may contribute to power losses. Operating at the resonance frequency of 6.58MHz, the circuit was designed and simulated to inject approximately 500nC at an 18mm distance from the transmitter. Furthermore, the proposed circuit design incorporates a charge-metering circuit adapted to ensure stimulation efficacy in response to coupling variations. To guarantee safety during stimulation, the residual voltage is carefully monitored and brought close to zero at the end of each stimulation cycle. To achieve this, a charge-balancing scheme has been implemented, which utilizes a comparator to monitor the residual voltage across the electrode.

The proposed circuit was designed on a printed circuit board (PCB) to evaluate its feasibility. To evaluate its performance, a signal generator was used to simulate the input of the inductive link, and an electrode-tissue interface (ETI) model was used at the output. The circuit demonstrated its efficacy in measuring the charge and reducing the residual voltage within established safety limits while maintaining energy efficiency. More specifically, the charge metering measured a charge of 404nC for 450nC of injected charge, which is a 10.22% of error. Moreover, the circuit exhibited a linear response in controlling the injected charge. In the tested range, the charge balancing scheme yielded residual voltages ranging from -20mV to -16mV. The effectiveness of the charge balancing scheme was further confirmed through in-vitro measurements in a phosphate-buffered solution, with residual voltages measuring at -20mV, which falls within the safe range. Finally, the implemented circuit achieved a peak efficiency of 56%.