Wireless battery charge management for implantable pressure sensor

Implantable medical devices intended for chronic application in deep bodily organs must balance small size with battery capacity. Wireless battery recharge of implanted sensors is a viable option to reduce implant size while removing the physical and regulatory hindrance of continuous RF powering. This paper presents wireless battery recharge circuitry developed for an implantable pressure sensor. The circuits include an RF/DC rectifier, voltage limiter, and constant-current battery charger with 150-mV end-of-charge hysteresis. An AM demodulator drawing zero DC current allows for transmission of commands on the recharge carrier. Reception of a time- and value-coded shutdown command places the implantable system into a 15 nanoampere standby mode. The system can be wirelessly activated from standby by reactivating the external wireless recharge carrier. Test results of the wireless system showed a standby current of 15-nA such that the implant standby time is limited by battery self-discharge. Wireless recharge tests confirmed that a constant recharge rate of 200 μA could be sustained at implant depths up to 20 cm, but with low power transfer efficiency <; 0.1% due to small implant coil size. Battery charge measurements confirmed that daily 4-hour recharge periods maintained the implant state of charge and this recharging could occur during periods of natural patient rest.