Materials for low Curie temperature induction heating of tumors (hyperthermia).

Abstract

Use of electroless nickel plating for self-regulating low temperature induction heating of tumors (hyperthermia) was investigated. The desired magnetic properties for the material were: (1) a Curie temperature, T(C), in the range of about 52-62°C, (2) high induced power above T(C), and (3) an abrupt drop in induced power at the Curie temperature. An amorphous ferromagnetic material would provide the highest corrosion resistance and superior magnetic properties, while cylindrical geometry is necessary for clinical considerations and for maximum heating. Electroless Ni-P containing near 11-12 atomic percent phosphorus (Curie temperature 45-60°C) was plated to thicknesses exceeding three skin depths (calculated for nickel) onto 1 mm diameter wires. Power produced by the plated wires was low and no sharp drop in power was seen in the range of 20-80°C. High internal stress, which decreases magnetic permeability, and thus reduces power, can be reduced by annealing at 150°C. The lack of a sharp temperature drop was attributed to inherent inhomogeneity in the plating, determined by x-ray microanalysis. Stainless steel tubes filled with amorphous high permeability material heated well in a magnetic field, while no heating was obtained using the same amount of amorphous material packed into plastic tubing or using empty stainless steel tubing. The heat produced per unit length by the composite implants was greater than that produced by solid 1 mm diameter NiSi, but less than that estimated for stranded NiSi implants, which are comprised of optimum diameter strands to maximize eddy current heating. Electroless Ni-P alone cannot be used to provide high power implants, but it or other biocompatible conductive coatings could possibly be used on the outside of a flexible implant filled with high permeability material. This would allow the possibility of producing a flexible, biocompatible device which is thermally self-regulating and produces high induced power. It also opens up the possibility of using induction heating and radiotherapy sequentially or simultaneously if the radiation sources could be loaded with the high permeability material.