Development of Complex Modulatory Microwave Brain Stimulation System

Abstract

In this thesis, a modulated brain stimulation signal generator with microwave carrier was developed and fabricated. Using the fabricated system, the continuous theta burst stimulation protocol signal was applied to mouse hippocampus. Micro-drive wires were used to measure firing rate of brain cell. Stimulation effect was verified by measuring the difference of firing rate before and after stimulation. Brain cell affects each other by neuro-network. To limit the stimulation depth, 6.5 GHz was chosen as a carrier frequency of stimulation signal which has short skin depth in biological tissue. Stimulation protocol frequency is few Hertz(Hz),and 6.5 GHz carrier frequency is in the pulses. Continuous wave in microwave frequency was modulated. To realize the functionality, a modulator coupled voltage controlled oscillator, buffer with variable load, 2-stage differential power amplifier, amplitude detector with DC shifting and transformer were used. Voltage controlled oscillator was used to generate 6.5 GHz microwave carrier. For modulation, the conventional method is to use switches which is connected to signal path. In this thesis, modulator switches are connected to gate of current source n-type metal-oxide-semiconductor (MOS) field-effect transistor (FET). Three switches are connected in parallel to support theta burst stimulation signal using only pulse generator. Modulation is done by turning voltage controlled oscillator on and off. Cross-coupled structure with p-type MOS load is applied to the voltage controlled oscillator. The buffer is used to minimize load variation of oscillator due to power amplifier when voltage swings rail-to-rail. Also, variable load is added using n-type MOSFET on-resistance. Total output power can be controlled by changing the control voltage of the variable load. Since 20 dBm output power is needed for stimulation, the power amplifier is needed to satisfy the power condition. The 2-stage power amplifier is used for both gain and maximum output power satisfaction. 0.5 mm sized FET is used for driver stage and 2 mm sized FET for the main stage. In 6.5 GHz, parasitic components cannot be neglected. The differential structure was applied for minimizing parasitic effect such as source degeneration due to inductance. Cascade structure and Cgs reduction technique are used to overcome breakdown voltage. Further studies can be done by varying waveform of the stimulation signal. For DC shifted waveform, amplitude detector and DC shifting are applied to the system. Amplitude detector uses cascode structure to hold large voltage swing. Diode connected MOS rectifier, and low pass filter are used to make DC which is same as the amplitude of output swing. The transformer was applied to make differential output signal to single-ended for single-ended structure stimulator. The fabricated system was verified by mouse experiment. Continuous theta burst stimulation signal was applied to mouse hippocampus. Micro-drive wires are used to measure the firing rate of hippocampus cells. Stimulation effect was verified by place map difference of before and after stimulation.