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This paper presents the results of tests that demonstrate the feasibility of using piezoelectric (PZT) ceramics to generate in vivo electrical energy for orthopedic implants. Sensors encapsulated within implants could provide in vivo diagnostic capabilities such as the monitoring of implant duty (i.e., walking) cycle, detecting abnormally asymmetric or high forces, sensing misalignment and early loosening, and early detection of wear. Early diagnosis of abnormalities or impending failure is critical to minimize patient harm. However, the routine use of sensors and microprocessors embedded within orthopedic implants for diagnostic and monitoring purposes has been limited by the lack of a long-term self-contained power source capable of lasting the expected 20-year implant lifetimes. By embedding PZT materials within orthopedic implants, a small amount of the mechanical energy generated during normal physical activity can be converted into useful electrical energy. This in vivo energy source can power embedded microprocessors and sensors for a broad range of biomedical uses. The current work investigates the application of this technology to total knee replacement (TKR) implants, but it is applicable to many other implanted biomedical devices.

Piezoelectric materials have long been used as sensors and actuators, however their use as electrical generators is less established. A piezoelectric power generator has great potential for some remote applications such as in vivo sensors, embedded MEMS devices, and distributed networking. Such materials are capable of converting mechanical energy into electrical energy, but developing piezoelectric generators is challenging because of their poor source characteristics (high voltage, low current, high impedance) and relatively low power output. In the past these challenges have limited the development and application of piezoelectric generators, but the recent advent of extremely low power electrical and mechanical devices (e.g., MEMS) make such generators attractive. This paper presents a theoretical analysis of piezoelectric power generation that is verified with simulation and experimental results. Several important considerations in designing such generators are explored, including parameter identification, load matching, form factors, efficiency, longevity, energy conversion and energy storage. Finally, an application of this analysis is presented where electrical energy is generated inside a prototype Total Knee Replacement (TKR) implant.