A piezoelectric energy harvesting concept for an energy-autonomous instrumented total hip replacement

Abstract

To improve the clinical outcome of total hip replacements (THRs), instrumented implants with sensory functions for implant monitoring and diagnostics or actuators for therapeutic measures are a promising approach. Therefore, an adequate energy source is needed. Batteries and external power supplies bring shortcomings e.g. limited lifetime or dependency on external equipment. Energy harvesting has the clear benefit of providing continuous and independent power for fully autonomous implants. Our present study evaluates by means of finite element analysis (FEA) the capabilities of a concept of a piezoelectric energy harvesting system (ring shaped multilayer piezoelectric element of 5 mm diameter and 2.5 mm height) integrated in a femoral hip stem. The deformations from a modified load-bearing implant are used to generate electric power for various instrumentation purposes. Besides the expected amount of converted energy, the influence on the stress distribution of the instrumented implant is analysed. The results show that the local stress increase for the modified implant geometry does not exceed the stress of the original reference model. The maximum generated open circuit voltage of 11.9 V can be processed in standard energy harvesting circuitry whereas an average power output amounts up to 8.1 µW. In order to increase the electric power in an upcoming design optimization, a sensitivity analysis is performed to identify the most important influencing parameters with regard to power output and implant safety.

Performance of a Piezoelectric Energy Harvesting System for an Energy-Autonomous Instrumented Total Hip Replacement: Experimental and Numerical Evaluation

“…All power curve progressions corresponded to the typical shape of graphs of generated power against load resistance, also found in other works on piezoelectric energy harvesting in load-bearing implants [27,28,41,56,64]. The location of the power maximum depended on the capacity of the piezoelectric element and was shifted towards smaller resistance values for a higher capacity, clearly visible for the different simulations and also for the data of a single element [41] or a customised geometry with comparable piezoelectric element structure [64]. The maximum calculated power for the stacked configuration differed from our previous work (31.1 µW) [41] despite a comparable maximum force level of F 33 acting on the piezoelectric element's end faces (see Table 5).…”

“…The distal tip of the total hip stem was placed on the femoral axis and the neck axes of both components were defined to lie in the same plane. The femoral head resection was comparable to the previous simulation of a human femur, maintaining the greater trochanter [41]. The bone cement layer was initially defined as a 3 mm-thick layer around the total hip stem.…”

“…To compare the new setup to our previous physiologically based reference simulations [41], we considered the following output parameters:…”

“…To the best of our knowledge, no further work on energy harvesting for THRs was published until we recently proposed our new concept [ 41 ]: Here, a piezoelectric multilayer element was integrated into a total hip stem and loaded by the deformation of the implant. For this reason, a cavity was milled from the metal component.…”

“…Results of the iterative study to reproduce the implant and piezoelectric element loading situation from previous work[41] for single and stacked configurations. Final output values of the new loading situation with absolute and relative differences from the previous model.…”