The design of an electronic system for an instrumented hip implant

Abstract

Data that can be collected with the use of instrumented implants can help in diagnosis and treatment of complications during the lifetime of total joint replacements. They can give an accurate reading of the status of the implant from within the body. In this research project an electronic system is designed to
feed sensors with enough energy to be able to collect the data of interest. For the power generation a piezoelectric element is used and placed inside the neck of the implant. The communication from within the implant is achieved with a micro control unit with Bluetooth low energy. As sensors, two thermistors are used and the piezoelectric element can also be used as a force sensor. Multiple experiments were performed to investigate the power generation and power consumption of the circuit. The piezoelectric element was able to harvest 877µW on average during one gait cycle. From the data of the power management integrated circuit it was seen that there was 778mJ available every day. With this energy the circuit is able to measure data from two thermistors every minute and send it three times per day. There is also an option to use the piezoelectric element as a force sensor. The Nominal Root Mean Square Error(NRMSE) when using the element as a force sensor is 0.0314 in the best case. The end result is an instrumented hip implant which completely fits inside a customized hip implant and is able to communicate with a phone via Bluetooth Low Energy.