Development of pulsed antennas for noninvasive exposures

Abstract

Electric pulses in the subnanosecond range can be radiated with pulsed antennas. Large antennas, such as prolate spheroidal antennas, allow the pulses to be focused in biological tissues, but the field intensity is generally low as the pulses suffer the losses including the one proportional to the distance of the targeted tissue, the reflection loss at the air-tissue interface, and the attenuation loss in the tissue. Thus, the field intensity may not be sufficient to cause significant biological effects such as stimulation or electropermeabilization. Small antennas, such as a dielectric rod antenna, can be used to improve the field intensity. The dielectric antenna has a conical wave launcher, a rod wave guide, and a conical emitter. It can make direct contact with tissues, eliminating the propagation in free space and reducing the reflection loss occurring at the air-tissue interface. Since the antenna is physically small and the distance to the targeted tissue is short, the propagation loss (which increases with distance) can be lowered. From the antenna perspective, small antennas are not as efficient as large antennas as their sizes are typically smaller than the spatial distribution of the pulse, and therefore, a high-voltage source needs to be employed to drive the antenna. But small antennas have the advantages of higher coupling efficiency to tissue and smaller footprint on the tissue surface. In a case that a subnanosecond pulse (100 ps) was delivered for transcranial stimulation, the simulation results showed that the penetration depth is approximately 2 cm and the field is concentrated in an area of 1×1 cm. A pulsed power system capable of generating a few hundreds of kilovolts is expected to be sufficient to drive the antenna in order to generate electric fields sufficient for biological responses.