The Effect of Misalignment of Induction Coils on Tissue Heating during Wireless Transcutaneous Energy Transfer
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The Effect of Misalignment of Induction Coils on Tissue Heating during Wireless Transcutaneous Energy Transfer E. V. Ryabchenko*, E. A. Mindubaev, and A. A. Danilov
The effect of induction coil misalignment on the heating of surrounding biological tissues was studied for a wire less inductive powering system with 500 mW output power. It was shown that a lateral misalignment of 220 mm can cause an increase in the tissue temperature up to 0.4°C compared with the case of coaxial positioning of the coils. Changes in temperature can be caused by a change in power losses in the elements of the system, as well as by displacement of these elements and the mutual influence of their thermal fields.
Introduction Development of wireless powering systems is a promising modern trend in engineering of implantable medical devices (IMDs) [13]. Energy transfer in such systems is based on inductive coupling [46]. The main advantages of inductive coupling (0.110 MHz frequency range) are weak absorption in tissues and the absence of a pronounced biological effect, which makes it possible to generate ~10 W of power in the receiving part of the sys tem without any harm to the patient. At present, induc tive coupling is the main way of energy supply to cochlear implants and spinal cord stimulators. Inductive coupling systems also hold much promise for powering circulatory support systems, visual prostheses, and brain stimulators. However, a number of scientific and technical problems have still to be solved to make possible a wider use of inductive coupling for powering IMDs. Heating of sur rounding biological tissues is one such problem [711]. Existing standards impose strict requirements on heating: the temperature of the implant surface should not exceed the temperature of the surrounding tissues by more than 2°C [12]. At the same time, for an IMD with inductive energy transfer from an external power source to the implantable part, the problem of heating becomes especially urgent because of an increase in the amount of released heat due to the need to convert energy in the Institute of Biomedical Systems, National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia; Email: [email protected] * To whom correspondence should be addressed.
process of its transfer. In this regard, even devices approved for clinical use may not meet the recommended standards. For example, wireless charging of batteries of modern spinal cord stimulators can increase the temper ature of the surrounding tissues by 4°C [13]. This makes it relevant to study the processes of tissue heating during wireless transcutaneous energy transfer in order to devel op methods for minimizing such heating [7, 8, 10, 13, 14]. In most cases, such studies consider a static situation when the mutual arrangement of the energy receiver and transmitter is fixed. However, operation of systems for transcutaneous inductive energy transfer is often accom panied by relative displacement (misalignment) of the receiver and transmitter coils [4]. In
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