Experimental evaluation of the electrical power output of an uncemented revision hip stem equipped with a piezoelectric element under various everyday activities

Smart implants are intended to achieve a longer lifespan in the future through integrated sensory and therapeutic functions. In this way, insufficient osseointegration and loosening of uncemented total hip replacements should be prevented by electrodes attached to the femoral stem, specifically stimulating bone growth to improve fixation and thus avoiding revision. For this function, an energy source is necessary, whereby an internal, autonomous one is desirable. Accordingly, in a previous study, a piezoelectric element was integrated into an uncemented hip stem to convert mechanical energy into electrical energy. However, the electrical output has only been calculated computationally, while in practical applications, energy losses occur during energy storage. Moreover, there is a lack of research on total hip and knee replacements with integrated piezoelectric elements that evaluate electrical power generation during everyday activities other than gait. Therefore, the present study aimed to experimentally determine the generated average electrical power of an uncemented revision hip stem with equipped piezoelectric element during different everyday activities. Furthermore, the storable energy during the most promising activities was evaluated with a standard energy harvesting (SEH) circuit. A simplified test rig was set up for these measurements. The results indicate that jogging and walking, with an average electrical power of 204 µW and 75 µW, respectively, convert the most energy, followed by stairs up and stairs down. After 20 min of jogging, 20.7 mJ were stored in a 330 µF capacitor, while one hour of walking yielded 12.9 mJ in a 470 µF capacitor. The energy required for two 1 cm long stimulation electrodes is estimated to range from 0.1 mJ (10 µA, 1000 Hz) to 254 mJ (100 µA, 20 Hz) for the recommended daily protocol of three 45 min sessions. Accordingly, the harvested energy of 12.9 mJ may be sufficient to support electrical bone stimulation at up to 20 µA over 20−1000 Hz. To increase the stored energy, significant energy losses in the SEH circuit due to impedance mismatch must be minimized by improving the energy harvesting circuitry.