A wide-band focusing system for bioelectric applications

Since subnanosecond pulses are short compared to the membrane charging time of biological cells, they penetrate the cells, allowing direct electric field interaction with cell substructures. The use of such ultrashort pulses (with risetimes on the order of 100 ps) also provides an opportunity of using antennas, rather than solid electrodes, to deliver the electromagnetic energy to biological targets. For this purpose, we are developing a wideband pulse system that will allow us to focus the electromagnetic energy. The focusing system uses a prolate spheroidal reflector. A spherical TEM wave is launched in a rotationally symmetric structure towards the reflector from the first focal point by using a conical radiator. The TEM wave is then reflected on the prolate spheroidal surface and directed towards the second focal point. The focused electric field at the second focal point has only axial components. The temporal development of the electric fields at the focal point consists of prepulse, impulse and postpulse. Prepulse and impulse have the same polarity and thus contribute to an increased electric field at the focal point. The system is filled with water to achieve a better spatial resolution through compression of the wavelength. Design issues such as optimization of the reflector and the biconical structure are discussed.