Characterization of a packaged triboelectric harvester under simulated gait loading for total knee replacement.

Load sensing total knee replacement (TKR) implants are useful tools for monitoring prosthesis health and providing quantitative data to support patient claims of pain or instability. However, powering such devices throughout the entire life of the knee replacement is a challenge, and self-powered telemetry via energy harvesting is an attractive solution. In this study, we implemented vertical contact mode triboelectric energy harvesters inside a knee implant package to generate the power required for embedded digitization and communications circuitry. The harvesters produce small-scale electric power from physiologically relevant loads transmitted through the knee. Experiments were performed on a joint motion simulator with an instrumented package prototype between the polyethylene bearing and tibial tray. The amplitude and the pattern of the power output varied with the input loadings. Under sinusoidal loading the maximum apparent power harvested was around 7µW at (50-2000)N whereas, under vertical compressive gait loading the harvesters generated around 10µW at average human knee loads of (151-1950)N and 20µW when the maximum applied load was increased by 25%. Full six degrees of freedom (6-DoF) gait load/motions at 0.67Hz produced 50% less power due to the slower loading rate. The results show the potential of developing a triboelectric energy harvesting-based self-powered instrumented knee implant for long-term in vivo knee joint force measurement.

Effect of Dielectric Material and Package Stiffness on the Power Generation in a Packaged Triboelectric Energy Harvesting System for Total Knee Replacement.

The objectives of this study are to experimentally investigate the effects of the dielectric material and the package stiffness on the durability and the efficiency of a previously developed triboelectric-based instrumented knee implant prototype. The proposed smart knee implant may provide useful information about prosthesis health and its functionality after a total knee replacement (TKR) by routine monitoring of tibiofemoral load transfer without the need for any external power source. The triboelectric powered load sensing by the proposed TKR system needs to be functional throughout the entire life of a knee replacement. The power output of the triboelectric system depends on the surface charge generations and accumulations on its dielectric material, and the force that transmits through its housing into the tribo-materials. The properties of the dielectric material and the package stiffness can significantly influence the reliability of the proposed device. For such a TKR system, a compliant mechanism with the ideal material selection can improve its state of the art. We investigated the performance of three vertical contact mode triboelectric generators made with three different dielectric materials: polydimethylsiloxane (PDMS), fluorinated ethylene propylene (FEP), and polytetrafluoroethylene (PTFE). To investigate the effect of package stiffness, we tested two Ti-PDMS-Ti harvesters inside a polyethylene and a Ti6Al4V package. At 1500 N of sinusoidal loads, the harvesters could generate 67.73 µW and 19.81 µW of mean apparent power in parallel and single connections in the polyethylene package, which was 32 and 17 times greater than the power recorded in the Ti assembly, respectively.

Design and analysis of a compliant 3D printed energy harvester housing for knee implants.

Instrumented implants have the potential to detect abnormal loading patterns which could be deleterious to implant longevity, indicating a need for intervention which could reduce the need for more complicated revision surgeries. Reliably powering such devices has been one obstacle preventing widespread usage of instrumented implants in clinical populations. This study presents a 3D-printed titanium interpositional device designed to integrate triboelectric generators (TEGs) into a commercially available total knee replacement (TKR). The device's stiffness, durability, and efficacy as a TEG housing were determined. Surprisingly, the stiffness of the 3D printed prototype was 73% less than what was calculated in a corresponding computational model, and under long-term durability testing failed after approximately 30,000 cycles of simulated gait loading. Under cyclical compressive loading, TEGs embedded in the device were able to generate 10.05 µW of power which is sufficient to run the frontend electronics for a load measurement system. The stiffness discrepancy between the computational and experimental models and the premature fatigue failure are suspected to be a result of internal porosity, unfused material and surface roughness of the 3D printed metal. Further refinements in design and manufacturing of the compliant device are required to improve its durability and TEG power output.

Self-Powered Load Sensing Circuitry for Total Knee Replacement.

There has been a significant increase in the number of total knee replacement (TKR) surgeries over the past few years, particularly among active young and elderly people suffering from knee pain. Continuous and optimal monitoring of the load on the knee is highly desirable for designing more reliable knee implants. This paper focuses on designing a smart knee implant consisting of a triboelectric energy harvester and a frontend electronic system to process the harvested signal for monitoring the knee load. The harvester produces an AC signal with peak voltages ranging from 10 V to 150 V at different values of knee cyclic loads. This paper demonstrates the measurement results of a PCB prototype of the frontend electronic system fabricated to verify the functionality and feasibility of the proposed approach for a small range of cycling load. The frontend electronic system consists of a voltage processing unit to attenuate high peak voltages, a rectifier and a regulator to convert the input AC signal into a stabilized DC signal. The DC voltage signal provides biasing for the delta-sigma analog-to-digital converter (ADC). Thus, the output of the triboelectric harvester acts as both the power signal that is rectified/regulated and data signal that is digitized. The power consumption of the proposed PCB design is approximately 5.35 µW. Next, the frontend sensor circuitry is improved to accommodate a wider range of cyclic load. These results demonstrate that triboelectric energy harvesting is a promising technique for self-monitoring the load inside knee implants.

Analysis of mechanical deformation effect on the voltage generation of a vertical contact mode triboelectric generator.

One of the associated factors that controls the performance of a triboelectric generator (TEG) is the mechanical deformation of the dielectric layer. Therefore, a good contact model can be a prominent tool to find a more realistic and efficient way of determining the relationships between the contact and electrical output of the generator. In this study, experiments are conducted on a vertical contact mode triboelectric generator under an MTS machine. The open-circuit voltages are measured at different loads imposed by the MTS by controlling the cyclic displacement of the top tribo layer of the generator. A finite-element-based theoretical model is developed to explain the behavior of the generator during the experiments. The 2D-contact problem of the micro-structured tribo layers is simulated and then the contact results are integrated into 3D to find the actual contact area between the two surfaces. These numerical contact results improve the existing theoretical model by evaluating the correct surface charge density and contact area as a function of the input parameters. The excellent agreement between our experimental and theoretical results illustrates that theoretical modeling could be used as a robust approach to predict the mechanical and electrical performance of TEGs. In addition, some parametric studies of the harvester are presented here for different geometrical parameters of the microstructures.