Sensing temperature is important for a wide variety of applications such as control systems and instrumentation which are integral to various industrial sectors and in research settings. To date, many prior studies have favored the use of the resistive thermistor approach given its simplicity. However, such devices are less sensitive to temperature changes compared to frequency-dependent approaches which are gaining momentum for detection. The importance of high sensitivity and reliable methods using a frequency-based approach for detecting temperature changes should thus be apparent, particularly if such sensors are also fabricated using low-cost approaches which are amenable toward miniaturized wireless platforms at the same time. In this study, Au rectangular single-arm spiral antennas with varying sizes were fabricated and RF S-parameter measurements were conducted over the frequency range of 300 kHz to 20 GHz. Solution-processed, two-dimensional (2D) hexagonal boron nitride (h-BN) was used with cyclohexanone and terpineol as solvents, and the films were characterized using dc current-voltage and frequency-dependent capacitance measurements. We also characterized our solution-processed h-BN films using Raman spectroscopy. The shift in the resonant frequency through the addition of h-BN over the underlying Au antenna was observed as this dielectric was coated on top of the antennas and the temperature response of the resonance frequency was measured. Alongside the experimental measurements, we also present results from our simulation analysis conducted using High Frequency Structure Simulator (HFSS) from ANSYS.
This work presents 3D printed polymer-based flexible electrode substrates exhibiting high surface area and flexibility in reverse electrowetting-on-dielectric energy harvesting for powering patchable human health monitoring sensors. Composite electrode substrates are printed using polydimethylsiloxane (PDMS) polymer and carbon black in 20:1 ratio by weight to provide some mechanical strength to the electrodes. Thin film layers of titanium for current collection and aluminum oxide as dielectric are deposited on the substrates to complete the electrode fabrication process. Without applying any bias voltage, the AC current due to periodic variance in capacitance resulting from mechanical modulation of an electrolyte droplet between two electrodes is measured for a low frequency range that falls within human motion activities. Mechanical integrity of the electrodes are characterized in terms of stress-strain analysis demonstrating robustness of their longevity.
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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