Abstract

The world has stepped into the fourth industrial revolution in many ways such as using the internet of things (IoT) in various applications, and removing the rechargeable power sources seeking for batteryless systems. Since energy is widely abundant around us and it is going to waste, numerous of recent studies have been conducted to propose sustainable solutions to harvest the free ambient power from the surrounding and convert it into electricity. Heat energy, kinetic energy, and radio waves are examples of these potentially harvestable energy sources. Mechanical vibrations represent one of the most plentiful forms of kinetic energy that can be scavenged by different techniques, such as electromagnetic and piezoelectric energy harvesters. As an application, utilizing the human body to harvest energy for wireless autonomous medical applications is under investigation. Therefore, developing safe, efficient, and biologically-compatible energy harvesters to be implanted inside the human body motion is critical to the success of such applications. Triboelectric generators to be installed inside the knee implant in the Total Knee Replacement (TKR) and the hip implant in Total Hip Replacement (THR) for self-powered load monitoring have been proposed.

Duo to the inability of traditional scavenging techniques to generate enough energy from low-frequency ambient vibrations, a frequency up-converter vibration energy harvester is proposed. The harvester converts low-frequency vibrations to high-frequency self-oscillation through a mechanical frequency up-converter using a magnetic coupling, thus providing more efficient energy conversion at low frequencies. The harvester consists of two cantilever beams with tip magnets facing each other at the same polarity. The low-frequency beam is made of polymer, while the high-frequency beam is made of Aluminum. The high-frequency beam is a bimorph fully covered with piezoelectric layers. A lumped parameter of the two degrees of freedom model (2DOF) is utilized to simulate the dynamic behavior and the generated voltage signal. The static response of the resonators shows a threshold distance of 15mm between the two magnets where the system has monostable oscillations above the threshold and bistable oscillations below the threshold. Furthermore, the dynamic behavior of the resonators is investigated at monostable, threshold, and bistable regions for different excitation levels. The harvester’s output voltage at different resistance values is extracted from the model. The frequency up-converter was found to effectively scavenge energy from low-frequency external vibrations by mechanically up-converting the ambient vibrations to high-frequency self-oscillations.

Validating the proposed frequency up-converter experimentally has been done with modifying some of the simulation parameters such as the physical dimensions and converting the high frequency beam (HFB) into a unimorph fully covered with a single piezoelectric layer. The static analysis of the system reveals a threshold distance of 15mm that divides the system into a monostable regime for weak magnetic coupling and a bistable regime for strong magnetic coupling. Hardening and softening behaviors were observed at the low-frequency range for the mono and bistable regimes, respectively. In addition, a combined nonlinear behavior of softening and hardening behaviors was captured for low frequencies at the threshold distance. Furthermore, the proposed system generates voltage showing 100% increment at the threshold compared to the monostable regime. Lowering the separation distance to reach the bistable range, d ≤ 8mm, will increase the generated voltage compared to the voltage generated in the monostable and threshold. The simulated and experimental results were in good agreement. Moreover, the effect of changing the external resistance was investigated, and setting the external resistance to 25MΩ was found to maximize the generated voltage.

To satisfy biomedical implants continuous need for improvement; triboelectric energy harvesters continue to show promising and efficient performance in transferring mechanical energy into electrical energy, making them a prime candidate for biomedical implants. TKR is a widely used surgery worldwide and, more so, in the United States. Therefore, Triboelectric harvester performance in biomedical applications was investigated in TKR. In this study, performance of two new configurations a triboelectric energy harvester in TKR were compared as self-powered implanted sensors for load measurement. The first configuration is a full knee harvester, covering the whole area of the tibial tray. The second configuration consists of two harvesters at the lateral and medial locations. Both configurations are to be fit in the knee implant. Performance of both configurations experimentally was evaluated while subjected to an axial cyclic load applied by a dynamic tester at different frequencies. Also, the lateral and medial generators were tested for load imbalance detection producing promising results. Findings from this study would contribute to the improvement of TKRs by transforming them from passive to smart TKRs using the proposed energy harvesters, which will lead to better health monitoring.

Similarly, Total Hip Replacement (THR) involves a conventional medical implant where many interacting factors could cause patient dissatisfaction, sometimes leading to lengthy and risky procedures based on guesses. Energy harvesting from natural human motion is being investigated to create a reliable source that will power smart implants and monitor performance simultaneously without any replacement or exchanges. A novel Triboelectric Energy Harvester (TEH) design was proposed to retrofit a TEH to the THR implant, making it a smart implant. A custom femoral head was designed to incorporate grooves onto the THR femoral head, maximizing energy production without increasing the overall size of them.

The TEH consists of two Titanium layers separated by a polydimethylsiloxane (PDMS) insulator. The Finite Element Analysis shows that the mechanical spring maintains the contact separation motion which is the working cycle of the of the TEH for voltage generation. A theoretical model of a single-degree-of-freedom system with piece-wise functions was proposed based on the FEA results to model the contact and release modes and voltage estimations. This study can open the door and lead to new research in load monitoring for total hip replacement.

Date of publication

Spring 5-3-2022

Document Type

Thesis

Language

english

Persistent identifier

http://hdl.handle.net/10950/3987

Committee members

Alwathiqbellah Ibrahim, Ph.D., Neal Barakat, Ph.D., Tahsin Khajah, Ph.D.

Degree

MS Mechanical Engineering

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