Analysis of specific absorption rate in biological tissue surrounding transcutaneous transformer for an artificial heart

Abstract

 This paper reports on specific absorption rate (SAR) analysis of biological tissue surrounding a transcutaneous transformer for an artificial heart. An externally coupled type of transformer was used for transcutaneous energy transmission. The secondary (internal) coil of the transformer, which is covered with skin, protrudes from the body surface, forming a skin tunnel. A primary (external) coil closely wound around a toroidal ferrite core is inserted into the skin tunnel. In this transformer, most of the magnetic flux induced by the current of the primary coil is linked with the secondary coil, and the coupling factor between the primary and secondary coils is greater than 0.98, regardless of the skin thickness. The electromagnetic field in the biological tissue surrounding the transformer was analyzed by the transmission line modeling method, and the distributions of the electric field strength and the SAR as a function of output power, the number of coil loops, and the dimensions and output voltage of the transcutaneous transformer were estimated. We found that SARs near the transcutaneous transformer were larger than at other points, that is, the SARs far from the transformer were relatively small. It was also clear that by increasing the dimensions and number of coil loops, and reducing the output voltage, we could reduce the SAR in biological tissue surrounding the transcutaneous transformer. The SAR in the present study was within the limits recommended by the Ministry of Posts and Telecommunications of Japan.