Wireless recharging of battery over large distance for implantable bladder pressure chronic monitoring

This paper presents a wireless power recharging system design for implantable bladder pressure chronic monitoring application. The power recharging system consists of an external 6-turn 15-cm-diameter powering coil and a silicone-encapsulated implantable 18-turn spiral coil with a dimension of 7 mm × 17 mm × 2.5 mm, which encloses a 3-mm-diameter 12-mm-long rechargeable battery, two ferrite rods, an ASIC and a tuning capacitor. For a constant charging current of 100 μA, an RF power of 700 μW needs to be coupled into the implantable microsystem through the tuned coils. Analyses and experiments confirm that with the two coils aligned coaxially or with a tilting angle of 30° and 6 cm offset in coils axes, an external power of 3.5W or 10W is required, respectively, to achieve a large coupling distance of 20 cm at an optimal frequency of 3 MHz.