Design and Analysis of a Compliant 3D Printed Energy Harvester Package for Knee Implants

Instrumented implants provide the potential to measure the in vivo tibiofemoral forces that are transmitted through total knee replacements (TKR). The continuous feedback from instrumented implants can be used to objectively justify actions to reduce the risk of implant failure. The main obstacle in developing “smart implants” is reliably powering such devices. Energy harvesting mechanisms, such as the triboelectric effect, can be leveraged to produce usable electricity and measure the transmitted loads in TKRs. A compliant package that interlocks with commercially available TKR components was designed to house triboelectric generators (TEG). Prototypes were more compliant than what was expected from the computational models. During fatigue testing, the prototype failed prematurely due to inherent issues with additive manufacturing. However, these issues can be mitigated with improved post-processing techniques. This package serves as a novel approach to integrating self-powering load sensors in currently available knee implants

Parametric study of a triboelectric transducer in total knee replacement application

Triboelectric energy harvesting is a relatively new technology showing promise for biomedical applications. This study investigates a triboelectric energy transducer for potential applications in total knee replacement (TKR) both as an energy harvester and a sensor. The sensor can be used to monitor loads at the knee joint. The proposed transducer generates an electrical signal that is directly related to the periodic mechanical load from walking. The proportionality between the generated electrical signal and the load transferred to the knee enables triboelectric transducers to be used as self-powered active load sensors. We analyzed the performance of a triboelectric transducer when subjected to simulated gait loading on a joint motion simulator. Two different designs were evaluated, one made of Titanium on Aluminum, (Ti-PDMS-Al), and the other made of Titanium on Titanium, (Ti-PDMS-Ti). The Ti-PDMS-Ti design generates more power than Ti-PDMS-Al and was used to optimize the structural parameters. Our analysis found these optimal parameters for the Ti-PDMS-Ti design: external resistance of 304 M Ω, a gap of 550 µm, and a thickness of the triboelectric layer of 50 µm. Those parameters were optimized by varying resistance, gap, and the thickness while measuring the power outputs. Using the optimized parameters, the transducer was tested under different axial loads to check the viability of the harvester to act as a self-powered load sensor to estimate the knee loads. The forces transmitted across the knee joint during activities of daily living can be directly measured and used for self-powering, which can lead to improving the total knee implant functions

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. Powering such devices throughout the entire life of the knee replacement, however, is a challenge, and selfpowered telemetry via energy harvesting is an attractive solution. Herein, 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 7W at (50-2000)N whereas, under vertical compressive gait loading, the harvesters generated around 10W at average human knee loads of (151-1950)N and 20W 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

Condylar-Stabilized TKR May Not Fully Compensate for PCL-Deficiency: An In Vitro Cadaver Study

© 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Increased-congruency bearing options are widely available in numerous total knee replacement (TKR) systems, with the intended purpose of compensating for posterior-cruciate ligament (PCL) deficiency. However, their ability to provide adequate stability in this setting has been debated. This in vitro joint simulator study measured changes in knee joint kinematics and stability during passive flexion–extension motions and simulated activities of daily living resulting from TKR with condylar-stabilized (CS) TKR without a PCL versus cruciate-retaining (CR) TKR. During passive flexion, the CS TKR resulted in a more posterior tibial positioning than both the intact joint and CR TKR (by 3.4 ± 1.0 mm and 4.8 ± 0.7 mm, respectively). With a posterior tibial force applied, the CS TKR tibia was again significantly more posterior than that of the intact joint and CR TKR (by 4.7 ± 1.3 mm and 5.6 ± 0.8 mm, respectively). Furthermore, there were significant differences in the anterior/posterior kinematics of both TKR with respect to intact knees during gait, and differences between the CS and CR TKR during stair ascent and descent. Overall, there appears to be a reduction in anterior–posterior stability of the PCL-deficient CS TKR knee, suggesting that contemporary increased-congruency bearing surface designs may not adequately compensate for the loss of the PCL. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2172–2181, 2019