A Charge Controlled Switched-Voltage Mode Neurostimulator

Abstract

Development of a visual prosthesis has the potential to help millions of patients with visual impairment around the world. One approach of visual prostheses is intracortical stimulation, where electrodes penetrating the visual cortex are used to create small light dots in the patients visual field. Said therapy requires an implantable neurostimulator that is able to stimulate more than 1000 micro-electrodes for sufficient resolution. Conventional stimulation methods are not suitable for this purpose as they are inefficient in multi-channel stimulation or lack control of injected charge. In this thesis a novel stimulation method is presented based on dynamic duty cycle control of ultrahigh frequency (UHF) pulses. The duty cycle is charge-controlled using a monostable multivibrator control loop. With this novel topology, the injected charge of a stimulation pulse is controlled even when electrode impedance changes. The presented system uses spatio-temporal stimulation to share the stimulator circuit among multiple output channels. Another contribution of this work is the implementation of an active charge balancing method. This method monitors the residual voltage at the electrode-tissue interface in between UHF pulses and stops stimulation when this voltage is brought back close to zero. The charge balancing circuit consists of a single comparator connected to the stimulation path. Overall, the proposed stimulator circuit consists of two comparators, one capacitor, three logic gates, and switches. The circuit is easily scalable depending on the stimulation parameters, requiring only two switches per output electrode. For validation purposes, the circuit has been implemented on a printed circuit board (PCB). The PCB has 8 output pins and operates up to 15V. Using a linear model of the electrode tissue interface, charge injection accuracy of the circuit was measured. The charge injected during a single UHF pulse is scalable with C*V. The implemented PCB uses a capacitor value of C=400pF, while the voltage V is scaled from 62.5mV to 1.25V such that the stimulation intensity ranges from 25pC to 500pC perUHF pulse. Furthermore, the charge balancing method was verified with the linear tissue model. The proposed method successfully reduced the residual voltage to 3.1mV, well within the safety limit of50mV. Measurements on a micro-electrode array confirmed functionality of the circuit for non-linear loads. Finally, the work presents an important insight regarding the development of power efficient neurostimulators. It has been shown that it is important to consider not only power efficiency of the circuit but also the energy efficiency of the stimulation waveform to decrease the power consumption of the system.