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1. Passive Frequency Tuning of Kinetic Energy Harvesters Using Distributed Liquid-Filled Mass

   https://doi.org/10.3390/act14020078  

   Adhikari, Rahul; Jackson, Nathan (February 2025, Actuators)

   Micro-scale kinetic energy harvesters are in large demand to function as sustainable power sources for wireless sensor networks and the Internet of Things. However, one of the challenges associated with them is their inability to easily tune the frequency during the manufacturing process, requiring devices to be custom-made for each application. Previous attempts have either used active tuning, which consumes power, or passive devices that increase their energy footprint, thus decreasing power density. This study involved developing a novel passive method that does not alter the device footprint or power density. It involved creating a proof mass with an array of chambers or cavities that can be individually filled with liquid to alter the overall proof mass as well as center of gravity. The resonant frequency of a rectangular cantilever can then be altered by changing the location, density, and volume of the liquid-filled mass. The resolution can be enhanced by increasing the number of chambers, whereas the frequency tuning range can be increased by increasing the amount of liquid or density of the liquids used to fill the cavities. A piezoelectric cantilever with a 340 Hz initial resonant frequency was used as the testing device. Liquids with varying density (silicone oil, liquid sodium polytungstate, and Galinstan) were investigated. The resonant frequencies were measured experimentally by filling various cavities with these liquids to determine the tuning frequency range and resolution. The tuning ranges of the first resonant frequency mode for the device were 142–217 Hz, 108–217 Hz, and 78.4–217 Hz for silicone oil, liquid sodium polytungstate, and Galinstan, respectively, with a sub Hz resolution. 

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2. Passively Tuning the Resonant Frequency of Kinetic Energy Harvesters Using Distributed Loaded Proof Mass

   https://doi.org/10.3390/app14010156  

   Adhikari, Rahul; Jackson, Nathan (December 2023, Applied sciences)

   The inability to tune the frequency of MEMS vibration energy-harvesting devices is considered to be a major challenge which is limiting the use of these devices in real world applications. Previous attempts are either not compatible with microfabrication, have large footprints, or use complex tuning methods which consume power. This paper reports on a novel passive method of tuning the frequency by embedding solid microparticle masses into a stationary proof mass with an array of cavities. Altering the location, density, and volume of embedded solid filler will affect the resonant frequency, resulting in tuning capabilities. The experimental and computational validation of changing and tuning the frequency are demonstrated. The change in frequency is caused by varying the location of the particle filler in the proof mass to alter the center of gravity. The goal of this study was to experimentally and numerically validate the concept using macro-scale piezoelectric energy-harvesting devices, and to determine key parameters that affect the resolution and range of the frequency-tuning capabilities. The experimental results demonstrated that the range of the frequency tuning for the particular piezoelectric cantilever that was used was between 20.3 Hz and 49.1 Hz. Computational simulations gave similar results of 23.7 Hz to 49.4 Hz. However, the tuning range could be increased by altering the proof mass and cantilever design, which resulted in a tuning range from 144.6 Hz to 30.2 Hz. The resolution of tuning the frequency was <0.1 Hz 

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3. Parylene Capping Layer for Embedded Liquid Mass for MEMS Packaging

   https://doi.org/10.1115/IMECE2024-144963  

   Adhikari, Rahul; Jackson, Nathan (November 2024, American Society of Mechanical Engineers)

   Microelectromechanical Systems (MEMS) packaging is over 80% of the cost of a typical MEMS device because there are no standard packaging methods, and each device requires unique packaging. Recently several MEMS devices have illustrated the desire to have a liquid filled cavity within the MEMS device for applications such as biomedical sensors, tunable energy harvesters, or liquid cooling microelectronics. However, embedded liquids in silicon pose a challenge when it comes to packaging. This paper illustrates a novel concept of using a conformal parylene coating to cap or encapsulate the liquid. The concept is validated using various liquids such as various viscosity silicone oils as well as Galinstan a Ga-based liquid metal. The study investigates the packaging reliability through a series of systematic accelerated life-time testing, elevated temperature testing, accelerated soak testing, and mechanical testing (shock and resonant frequency testing). Mass changes were monitored and compared to control (no capping), glass epoxy bonded packaging, and silicone spray coating encapsulation. The results demonstrate the superior mean-time-to-failure of the parylene capping method compared to the other methods. The results confirm that parylene can be used to package embedded liquids in silicon or 3D printed structures. 

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4. Reverse Electrowetting-on-Dielectric Energy Harvesting Using Inexpensive, Flexible Substrates

   https://doi.org/10.1109/JSEN.2024.3450430  

   Schumacher, Baylee S; Kumar_Sah, Pallav; Kakaraparty, Karthik; Mahbub, Ifana; Reid, Russell C (October 2024, IEEE Sensors Journal)

   Reverse electrowetting-on-dielectric (REWOD) energy harvesting is an effective energy harvesting method at low frequencies such as the frequencies of human motion. Various REWOD energy harvester designs have been presented in prior works, but these generally use rigid and often expensive substrates and time-consuming and/or costly fabrication methods. To address these challenges, in this work REWOD energy harvesters were fabricated consisting of aluminized polyester sheets as the functional layers and with polycarbonate sheets for added mechanical support. The fabrication of these samples eliminates the need for costly materials, clean room technologies, and high-end equipment. Samples were characterized using a flat arrangement and on a test fixture that simulates the repeated bending that occurs on the back of a bending knee. Without applying any external bias voltage, the maximum voltage and current output for the bending samples were determined to be 25.1 mV and 230 nA, respectively, and the corresponding maximum power is 5.77 nW at a bending frequency of 5 Hz. With an estimated cost of U.S. $ 0.28 for each REWOD harvester (U.S. $ 0.03/cm2), the cost per nanowatt of power is U.S. $ 0.05/nW, which is approximately 380 times lower than the approximately U.S. $ 19/nW of our previous REWOD energy harvesters. Our simple devices provide a low-cost, easily fabricated flexible approach to wearable motion sensing and energy harvesting that can be useful for various healthcare applications. 

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5. High surface area reverse electrowetting energy harvesting with power conditioning circuitry for self-powered motion sensors

   https://doi.org/10.1117/12.2587541  

   Adhikari, Pashupati R.; Biswas, Dipon K.; Tasneem, Nishat T.; Reid, Russell C.; Mahbub, Ifana (April 2021, SPIE Defense + Commercial Sensing)

   Dutta, Achyut K.; Balaya, Palani; Xu, Sheng (Ed.)

   Monitoring human health in real-time using wearable and implantable electronics (WIE) has become one of the most promising and rapidly growing technologies in the healthcare industry. In general, these electronics are powered by batteries that require periodic replacement and maintenance over their lifetime. To prolong the operation of these electronics, they should have zero post-installation maintenance. On this front, various energy harvesting technologies to generate electrical energy from ambient energy sources have been researched. Many energy harvesters currently available are limited by their power output and energy densities. With the miniaturization of wearable and implantable electronics, the size of the harvesters must be miniaturized accordingly in order to increase the energy density of the harvesters. Additionally, many of the energy harvesters also suffer from limited operational parameters such as resonance frequency and variable input signals. In this work, low frequency motion energy harvesting based on reverse electrowetting-ondielectric (REWOD) is examined using perforated high surface area electrodes with 38 µm pore diameters. Total available surface area per planar area was 8.36 cm2 showing a significant surface area enhancement from planar to porous electrodes and proportional increase in AC voltage density from our previous work. In REWOD energy harvesting, high surface area electrodes significantly increase the capacitance and hence the power density. An AC peak-to-peak voltage generation from the electrode in the range from 1.57-3.32 V for the given frequency range of 1-5 Hz with 0.5 Hz step is demonstrated. In addition, the unconditioned power generated from the harvester is converted to a DC power using a commercial off-theshelf Schottky diode-based voltage multiplier and low dropout regulator (LDO) such that the sensors that use this technology could be fully self-powered. The produced charge is then converted to a proportional voltage by using a commercial charge amplifier to record the features of the motion activities. A transceiver radio is also used to transmit the digitized data from the amplifier and the built-in analog-to-digital converter (ADC) in the micro-controller. This paper proposes the energy harvester acting as a self-powered motion sensor for different physical activities for wearable and wireless healthcare devices. 

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