SOLID MATERIALS WITH HIGH DIELECTRIC-CONSTANTS FOR HYPERTHERMIA APPLICATIONS

The manufacture of solid components with high permittivities epsilon(r ) of 1-100 and differing conductivities sigma of 0-1.0 S/m has practic al significance for fabricating applicators and phantoms in radiofrequ ency hyperthermia. For this purpose, various plastics (resins, polyure thane and silicone) were combined with additives (graphite and metal p owder) and tested to assess their radiofrequency and mechanical charac teristics and to identify manufacturing problems. Most of the plastics could be made highly dielectric and conductive by adding graphite in the range of muscle tissue (i.e. epsilon(r) approximate to 80, sigma a pproximate to 0.8 S/m). However, there are major differences between t he materials with respect to mechanical behaviour, durability, feasibi lity of manufacture, and reproducibility. Manufacturing water-equivaen t plastics (low conductivity sigma < 0.05 S/m and epsilon(r) value of 70-80) is particuarly difficult. A less filled polyester resin in whic h concentration of brass powder can achieve an epsilon(r) value of up to 100 at low conductivity proved to be the only suitable medium. Such a plastic can be used for future applicator designs. Other materials of interest include plastics equivalent to lossy media (e.g. sigma = 0 .45-0.55 S/m, epsilon(r) = 70-80), fat-equivalent plastics (polyuretha ne with graphite) and higher dielectric flexible plastics (silicone wi th brass powder).