A Smart Knee Implant Using Triboelectric Energy Harvesters

Although the number of total knee replacement (TKR) surgeries is growing rapidly, functionality and pain-reduction outcomes remain unsatisfactory for many patients. Continual monitoring of knee loads after surgery offers the potential to improve surgical procedures and implant designs. The goal of this study is to characterize a triboelectric energy harvester under body loads and to design compatible frontend electronics to digitize the load data. The harvester prototype would be placed between the tibial component and polyethylene bearing of a TKR implant. The harvester generates power from the compressive load. To examine the harvester output and the feasibility of powering a digitization circuitry, a triboelectric energy harvester prototype is fabricated and tested. An axial tibiofemoral load profile from normal walking (gait) is approximated as a 1 Hz sine wave signal and is applied to the harvester. Because the root mean square of voltages generated via this phenomenon is proportional to the applied load, the device can be simultaneously employed for energy harvesting and load sensing. With an approximated knee cyclic load of 2.3 kN at 1 Hz, the harvester generated output voltage of 18 V RMS, and an average power of 6 µW at the optimal resistance of 58MΩ. The harvested signal is rectified through a negative voltage converter rectifier and regulated through a linear-dropout regulator with a combined efficiency of 71%. The output of the regulator is used to charge a supercapacitor. The energy stored in the supercapacitor is used for low resolution sensing of the load through a peak detector and analog-to-digital converter. According to our analysis, sensing the load several times a day is feasible by relying only on harvested power. The results found from this work demonstrate that triboelectric energy harvesting is a promising technique for self-powering load sensors inside knee implants

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

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