Force detection, center of pressure tracking, and energy harvesting from a piezoelectric knee implant

Recent developments in the field of orthopedic materials and procedures have made the total knee replacement an option for people who suffer from knee diseases and injuries. One of the ongoing debates in this area involves the correlation of postoperative joint functionality to intraoperative alignment. Due to a lack of in vivo data from the knee joint after surgery, the establishment of a well-quantified alignment method is hindered. In order to obtain information about knee function after the operation, the design of a self-powered instrumented knee implant is proposed in this study. The design consists of a total knee replacement bearing equipped with four piezoelectric transducers distributed in the medial and lateral compartments. The piezoelectric transducers are utilized to measure the total axial force applied on the tibial bearing through the femoral component of the joint, as well as to track the movement in the center of pressure (CoP). In addition, the generated voltage from the piezoelectrics can be harvested and stored to power embedded electronics for further signal conditioning and data transmission purposes. Initially, finite element analysis is performed on the knee bearing to select the best location of the transducers with regards to sensing the total force and location of the CoP. A series of experimental tests are then performed on a fabricated prototype which aim to investigate the sensing and energy harvesting performance of the device. Piezoelectric force and center of pressure measurements are compared to actual experimental quantities for twelve different relative positions of the femoral component and bearing of the knee implant in order to evaluate the performance of the sensing system. The output voltage of the piezoelectric transducers is measured across a load resistance to determine the optimum extractable power, and then rectified and stored in a capacitor to evaluate the realistic energy harvesting ability of the system. The results show only a small level of error in sensing the force and the location of the CoP. Additionally, a maximum power of 269.1 mu W is achieved with a 175 k Omega optimal resistive load, and a 4.9 V constant voltage is stored in a 3.3 mF capacitor after 3333 loading cycles. The sensing and energy harvesting results present the promising potential of this system to be used as an integrated self-powered instrumented knee implant.