Solid materials with high dielectric constants for hyperthermia applications.

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Abstract
The manufacture of solid components with high permittivities εr of 1–100 and differing conductivities s` of 0–1.0 S/m has practical significance for fabricating applicators and phantoms in radiofrequency hyperthermia. For this purpose, various plastics (resins, polyurethane and silicone) were combined with additives (graphite and metal powder) and tested to assess their radiofrequency and mechanical characteristics 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. εr ≈ 80, s` ≈ 0.8 S/m). However, there are major differences between the materials with respect to mechanical behaviour, durability, feasibility of manufacture, and reproducibility. Manufacturing water-equivaent plastics (low conductivity s` < 0.05 S/m and εr value of 70–80) is particuarly difficult. A less filled polyester resin in which concentration of brass powder can achieve an εr 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. s` = 0.45–0.55 S/m, εr = 70–80), fat-equivalent plastics (polyurethane with graphite) and higher dielectric flexible plastics (silicone with brass powder).