Out-of-phase coupled wireless power transfer system in implantable medical devices for electromagnetic field human exposure reduction

Abstract

Recently, the implantable medical devices (IMDs) market has increased significantly and is being used in various applications. Although these IMDs are excellent for patient symptom monitoring and nerve stimulation, there are still problems such as frequent reoperations due to limitation of battery life, disconnection of wires and infection, and increase in the size of the IMD due to the batteries. The wireless power transfer (WPT) technology can solve problems with conventional systems, such as battery capacity and lifespan and minimization of connection wires. However, due to the nature of the use environment of the IMD, the power transmitter (TX) is located outside the body, and the receiver (RX) is located inside the body, so the human tissue is directly exposed to a strong electromagnetic field (EMF). In this paper, human exposure to EMF reduction design method in the space between the TX and RX systems, where the strongest EMF is exposed to human tissue, is proposed. Based on the equivalent circuit model analysis of the proposed system, the power transfer characteristics and EMF reduction characteristics of the WPT system according to the design variables are comprehensively analyzed. A design procedure is proposed based on the analyses, and the proposed system compared and verified the power transfer efficiency (PTE) and EMF reduction with the conventional system through simulations and measurements. In addition, through simulation, the evaluation of the human exposure to EMF is verified with three indicators: current density, induced electric field, and specific absorption rate (SAR). Since human exposure to EMF can vary depending on the tissue structure and conditions, sensitivity analyses depending on various tissue structures and conditions are performed, and additionally, an anatomical human model-based evaluation is conducted to verify the effectiveness of the proposed system.