Self-Heating Minimization of Implantable Devices with a Wireless Inductive Power Supply System
Keywords:
Wireless Power Transmission, Inductive Charger, Implantable Device, Telemetry, Heating of Implant, Coupling Coefficient, Automatic Power AdjustmentAbstract
Purpose: When using a wireless inductive power system, tissue heating can be caused by the conduction and displacement currents
induced within the body, or by the heat discharged from the LDOs and other resistive components of the receiver module which is designed to recharge the internal battery. The optimum EMF power level depends on the distance between the implanted device and the transmitting coil. Therefore, the main objective of this study was synthesizing an adequate algorithm for the automatic adjustment of the generator power, taking into account the variability of the distance between the inductors defined by particular clinical and experimental tasks. Methods: The study used the calculation methods of complex amplitude and reflected transformer impedance, the calculation of inductor coupling coefficient by the finite difference time-domain method, and an automated simulation experiment. The study was performed on a 9 cm3 model of implant in a polymer casing. At a distance of 25–45 mm from the transmitting coil, this model can provide an output power of 0.5 watts to charge the battery at EMF frequency of 1 MHz. Normal saline was used as the body simulation environment. Results: Analyzing the electrical circuit in the receiver module, an automatic power adjustment algorithm was designed, which is based on several linear dependencies between the current in the transmission coil and the feedback signals, without the need to calculate the current coupling coefficient of the inductors. As a result, three optimal adjustment coefficients were determined, corresponding to the following phases: insufficient power to start charging, increase of the charging current to the nominal value, and excessive power. Comparative analysis of the proportional and relay adjustors showed the advantage of the first under the conditions of frequent changes in the distance between coils, as well as the need for their implementation as a whole, based on the implant heating at a constant EMF power level. The proposed configuration of the experimental stand allowed us to estimate the minimum safe operation time for the system and adjust the value of the charging current, assuming that the implant case temperature increase is limited by 20C. Practical relevance: The described approach increases the safety of using implants with a wireless rechargeable power source in both experimental and clinical practices.