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Control of thermal runaway and uniformity of heating in the electromagnetic rewarming of a cryopreserved kidney phantom.

Successful cryopreservation of whole organs may require electromagnetic (EM) heating at much faster warming rates than those at which thermal relaxation can smooth out nonuniform power distribution or suppress hot spots which become unstable due to an increase of power absorbtion with temperature in the material being heated. Calculations of the heating distribution for a uniform spherical workpiece in a resonant EM applicator show that the size of a workpiece for which there will be good uniformity of heating has a sharp upper limit given D < or = 0.3 lambda m, where D is the workpiece diameter and lambda m is the EM wavelength in the material. We also introduce three stability factors which indicate whether thermal runaway can occur; they are dependent solely on the material's dielectric properties and the geometry and orientation of any irregularities. These factors provide clear guidelines to the choice of source frequency and applicator geometry that are favorable for intrinsic control of thermal runaway. Experimental results are presented for spherical phantom workpieces of 36- and 45-mm diameter, rewarmed from -90 to above 0 degrees C in 30 to 150 s, in a 434-MHz resonant cavity applicator. The observed spatial temperature distributions for different E-field configurations clearly indicate the effectiveness of the stability factors in predicting uniformity of temperature, and the significance of the workpiece size limit, in practical EM heating situations.