Low-Power Circuits for the Bidirectional Wireless Monitoring System of the Orthopedic Implants

This paper proposes an architecture of the wireless monitoring system for the real-time monitoring of the orthopedic implants, which monitors the implant duty cycle, detects abnormal asymmetry, high amounts of force, and other conditions of the orthopedic implants. Data for diagnosis are communicated wirelessly by the radio-frequency (RF) signal between the embedded chip and the remote circuit. In different working modes, the system can be powered by the RF signal or stiff lead zirconate–titanate (PZT) ceramics which are able to convert mechanical energy inside the orthopedic implant into electrical energy. The power circuits with a variable ratio switched-capacitor (SC) dc–dc converter have been taped out with 0.35-$mu$m complementary metal–oxide semiconductor (CMOS) technology. The test results show that the SC converter can transfer the input voltage that ranges from 5 V to 14 V from the PZT ceramics into the voltage ranging from 2 V to 2.5 V which will be dealt with by a low drop-out circuit in the future work. The total efficiency of the SC converter is from 28% to 42% at full-time working mode. The analog-to-digital converter (ADC) circuits have been fabricated in a 0.18-$mu$m 1P6M CMOS process. The test results show that the ADC chip consumes only 12.5 $mu$W in working mode and 150 nW in the sleep mode. The circuits, including RF circuits, ADC, and the microcontrol unit, have been implemented in a 0.18-$mu$ m CMOS process. Future work includes some clinical experiments test in the application where PZT elements are used for power generation in total knee-replacement implants.