Charging Current Characteristics and Effect of Casing Material in Wireless Recharging of Active Implantable Medical Devices Using Transcutaneous Energy Transfer System

Batteries inside an active implantable medical device (AIMD) need to be replaced every few years. However, rechargeable batteries can enhance the life of such devices to a large extent. Transcutaneous Energy Transfer System (TETS) is a promising method for recharging these batteries inside medical devices. These devices are generally made of metal casings to avoid fluid ingress and provide better mechanical strength. However, the metal cases when being present in the path of electromagnetic energy induces eddy current thus producing excessive temperature rise due to thermal loss. Thus, the selection of an interface casing material plays a significant role in the performance of the wireless recharging. In this paper, the performance of a transcutaneous energy transfer system for recharging an AIMD with different axial gaps and casing materials is reported. The effect of these variations on the output voltage, recharge current, and efficiency of operation was quantified. It has been found that, with TETS the charging current of 0.3 A to 0.5 A can be obtained to charge the implanted battery within 180 minutes. It was found that the induced voltage in the secondary coil is substantially reduced with the presence of titanium casing compared to epoxy encapsulation. Thermal studies were performed with titanium casing material of various thicknesses. The casing temperature rose to above 70 degrees C within the first 10 minutes for 0.5 mm thickness and within 50 minutes in the case of 0.25 mm. With epoxy encapsulation, the casing temperature rose to only 30 degrees C. The charging voltage of 5 V and charging current of more than 0.3 A were obtained with epoxy encapsulation. A polymeric material casing or epoxy encapsulation is the best choice in the interface region to get a high recharging current in the case of wireless recharging of implantable medical devices. With the proposed design modification, wireless energy transfer and recharging implanted batteries shall be done in a more energy-efficient manner with less thermal damage to nearby tissues.