Development of Small and High Efficiency Implant for Deep Local Hyperthermia

Abstract

Designing a small millimeter-sized implant is very difficult because common implant materials such as small ferrite materials, iron materials, ni-chrome wire etc., cannot be heated well by inductive heating when these implant sizes are less than 10 millimeters. This fact is based on the inherent nature of eddy currents which distributes over only the surface of heating material when applying RF magnetic fields to it.
To break through the problems of an inductive deep local heating, a small high efficiency implant has been newly proposed not from conventional material compositions but from electrical circuit theory viewpoint. This new implant is simply composed of a small coil and a microchip condenser. It is heated efficiently based on the resonant circuit theory when RF magnetic field is applied.
In this paper, after the fundamental investigations on an optimum condition for the present implant, rise-time heating characteristics, how to obtain a smaller implant, and how to design an omni-directional implant are described.
A small implant of coil diameter and a coil length being 6.5mm and 1.6 mm, respectively shows a temperature rise more than 20°C within one minute heating in the depth of 10cm at the frequency of 4MHz when using ferrite core applicator with output power of 500 W. Through this development of a small high efficient implant, the essential technology that we are aiming at constituting the wireless thermometer with the function of local heating implant could be paved.