Modeling and optimization of printed spiral coils in air and muscle tissue environments

Printed spiral coils (PSC) are viable candidates for near field wireless power transmission to the next generation of prosthetic devices with extreme size constraints. Implantable devices need to be hermetically sealed in biocompatible materials and placed in conductive environment with high permittivity, which can affect the PSC characteristics. We have constructed a detailed model that includes the effects of surrounding environment on the PSC parasitic components and eventually on the power transfer efficiency. This model is combined with an iterative design method that starts with a set of realistic design constraints and ends with the optimal PSC geometries. This was applied to optimize the wireless link of a 1 cm² implantable device operating at 13.56 MHz. Measurement results showed that optimized PSC pairs, coated with 0.3 mm of silicone, achieved 72.2% and 30.8% efficiencies at a face to face relative distance of 10 mm in the air and muscle environment respectively. The PSC which was optimized for air could only bear 21.8% efficiency in muscle, showing that considering the PSC surrounding environment in the design process can result in nearly 10% improvement in the power transfer efficiency.