6.5-GHz Brain Stimulation System Using Enhanced Probe Focusing and Switch-Driven Modulation

This article, for the first time, presents the design, fabrication, and measurement results of a novel microwave brain stimulation system enabling efficient probe focusing of rectangular-pulse enveloped 6.5-GHz waves. While the conventional ON/OFF stimulation systems in literature employ low frequencies below 0.5 GHz, the proposed system employs 6.5 GHz that can achieve more spatial energy focusing and a moderate level of energy penetration depth. In the proposed system, the ON/OFF modulated microwave signal is generated by a single chip consisting of a voltage-controlled oscillator (VCO) and power amplifier (PA). The VCO is driven by a switch at the current source to generate modulated signals with over 20-dB isolation between the high and low states. The probe with a center-opened aperture surrounded by a symmetric loading enables low power reflection toward the brain and focuses the field in the square-shaped aperture of 1-mm
²
area. Finally, it is demonstrated that the 20-min stimulation of an
in vivo
mouse brain using microwave signals with 1-Hz repetitive pulse envelopes and 1% duty cycle enables the normalized firing rate to reach up to 0.2 while the normalized firing rate stays just within ±0.05 under no stimulation. This suggests that the proposed brain stimulation system can achieve a dramatic change in the activity of individual hippocampal neurons.