Different Scenarios of Autonomous Operation of an Environmental Sensor Node Using a Piezoelectric-Vibration-Based Energy Harvester.

This paper delves into the application of vibration-based energy harvesting to power environmental sensor nodes, a critical component of modern data collection systems. These sensor nodes play a crucial role in structural health monitoring, providing essential data on external conditions that can affect the health and performance of structures. We investigate the feasibility and efficiency of utilizing piezoelectric vibration energy harvesters to sustainably power environmental wireless sensor nodes on the one hand. On the other hand, we exploit different approaches to minimize the sensor node's power consumption and maximize its efficiency. The investigations consider various sensor node platforms and assess their performance under different voltage levels and broadcast frequencies. The findings reveal that optimized harvester designs enable real-time data broadcasting with short intervals, ranging from 1 to 3 s, expanding the horizons of environmental monitoring, and show that in case the system includes a battery as a backup plan, the battery's lifetime can be extended up to 9 times. This work underscores the potential of vibration energy harvesting as a viable solution for powering sensor nodes, enhancing their autonomy, and reducing maintenance costs in remote and challenging environments. It opens doors to broader applications of sustainable energy sources in environmental monitoring and data collection systems.

Fiber-based optrode with microstructured fiber tips for controlled light delivery in optogenetics.

Objective.Optogenetic modulation of neuronal activity requires precise and flexible light delivery to deep brain regions. Flat cleaved optical fibers combined with electrodes are widely used in implantable optogenetic devices for light delivery and electrical monitoring of neural activity. However, the flat fiber tip geometry induces serious tissue damage upon insertion, and makes it difficult to adjust and control the spatial extent of illumination within the brain. With their strongly increased tissue-compatibility and the possibility of spatial illumination control, tapered fibers outperform cleaved fibers in targeted neural photo-stimulation.Approach.In this work, we describe our device concept, and present a novel approach for reproducible fabrication of tapered fiber tips via grinding. Furthermore, we characterize recording electrodes by commenting data obtained from electrochemical impedance spectroscopy (EIS). We also investigate the impact of different cone angles (14°, 30°, 60°, and 90°) on the illumination profile and optical throughput.Main results. We fabricated a fiber-based optrode with cone tip and two deposited electrodes. Custom grinding setup for fabrication of tapered fiber tips with various cone angles is developed as a part of our research. Microscope images showed very good optical quality of cone tips. The results of transmitted optical power measurements performed with integrating sphere suggest that, compared to the flat cleaved optical fiber, transmitted power decreases exponentially with cone angle reduction. Obtained emission profiles (as induced fluorescence in Rhodamine 6G water solution) indicate very strong effect of cone angle on shape and size of illumination volume. Results obtained from EIS show the effect of electrode size on its recording capability.Significance. Compared to optrodes with flat cleaved optical fiber, the demonstrated fiber-based optrode with cone tip allows controlled light delivery with reduced invasiveness. The possibility to fabricate reproducible fiber tips with various cone angles enables control of light delivery in optogenetic experiment. The results presented here give neuroscientists the possibility to choose the appropriate tissue-compatible cone geometry depending on their stimulation requirements.

Multi-Objective Topology Optimization of a Broadband Piezoelectric Energy Harvester.

In recent years, topology optimization has proved itself to be state of the art in the design of mechanical structures. At the same time, energy harvesting has gained a lot of attention in research and industry. In this work, we present a novel topology optimization of a multi-resonant piezoelectric energy-harvester device. The goal is to develop a broadband design that can generate constant power output over a range of frequencies, thus enabling reliable operation under changing environmental conditions. To achieve this goal, topology optimization is implemented with a combined-objective function, which tackles both the frequency requirement and the power-output characteristic. The optimization suggests a promising design, with satisfactory frequency characteristics.

Analysis and Characterization of Optimized Dual-Frequency Vibration Energy Harvesters for Low-Power Industrial Applications.

We present a multiresonant vibration energy harvester designed for ultra-low-power applications in industrial environments together with an optimized harvester design. The proposed device features dual-frequency operation, enabling the harvesting of energy over a wider operational frequency range. It has been designed such that its harvesting bandwidth range is [50, 100] Hz, which is a typical frequency range for vibrations found in industrial applications. At an excitation level of 0.5 g, a maximum mean power output of 6 mW and 9 mW can be expected at the resonance frequencies of 63.3 and 76.4 Hz, respectively. The harvester delivers a power density of 492 µW/cm2. Design optimization led to improved harvester geometries yielding up to 2.6 times closer resonance frequencies, resulting in a wider harvesting bandwidth and a significantly higher power output.

Efficient design optimization of a miniaturized thermoelectric generator for electrically active implants based on parametric model order reduction.

This research focuses on the design of a miniaturized thermoelectric generator (TEG) for electrically active implants. Its design optimization is performed using the finite element method. A simplified TEG model is obtained by replacing the thermocouple array with a single representative thermopile, which considers the number and fill factor of the thermocouples as parameters. Instead of rebuilding the geometry of a detailed model with multiple thermocouples, the simplified model adapts the material properties of its representative thermopile, facilitating design optimization. We extend the model by integrating the simplified TEG together with a housing inside a human tissue model for thermoelectric analysis. For computation efficiency and applicability of model order reduction (MOR), a thermal model is derived from the thermoelectric one, with the Peltier effect being considered through an effective thermal conductivity. Through parametric MOR, two parametric reduced-order models are generated from the full-scale thermoelectric and thermal model, respectively. Furthermore, we demonstrate the design optimization of TEG both in full-scale and reduced-order model for maximal power output and sufficient voltage output.

Towards efficient design optimization of a miniaturized thermoelectric generator for electrically active implants via model order reduction and submodeling technique.

Thermoelectric generators (TEG) convert the thermal energy into electrical energy and are under investigation as a power supply for medical implants. To improve the performance of TEG, the design optimization process through finite element model simulation is preferred by biomedical engineers. This paper aims to provide an efficient method of speeding up the design optimization process of TEG. A three-dimensional realistic human torso model incorporating the TEG is investigated, where the internal heat transfer in human tissue is characterized by Pennes bioheat equation. In addition, convection, radiation, and evaporation effects at the skin surface are applied to identify the heat transfer effects between the human body and the environment. To speed up finite element simulation of the large-scale human torso model, projection-based model order reduction (MOR) is applied for generation of a compact but highly accurate model. Parametric MOR (pMOR) further enables generating a parameter-independent compact model. For an efficient design optimization of TEG, this compact human torso model is applied within a thermal submodeling approach. Its temperature distribution results are back-projected and used as boundary conditions for the TEG submodel. The achieved speed-up in simulation time, demonstrated in this work, clearly indicates that the design optimization process of TEG is more efficient with the combination of MOR and submodeling techniques.

System-Level Model and Simulation of a Frequency-Tunable Vibration Energy Harvester.

In this paper, we present a macroscale multiresonant vibration-based energy harvester. The device features frequency tunability through magnetostatic actuation on the resonator. The magnetic tuning scheme uses external magnets on linear stages. The system-level model demonstrates autonomous adaptation of resonance frequency to the dominant ambient frequencies. The harvester is designed such that its two fundamental modes appear in the range of (50,100) Hz which is a typical frequency range for vibrations found in industrial applications. The dual- frequency characteristics of the proposed design together with the frequency agility result in an increased operative harvesting frequency range. In order to allow a time-efficient simulation of the model, a reduced order model has been derived from a finite element model. A tuning control algorithm based on maximum-voltage tracking has been implemented in the model. The device was characterized experimentally to deliver a power output of 500 µW at an excitation level of 0.5 g at the respected frequencies of 63.3 and 76.4 Hz. In a design optimization effort, an improved geometry has been derived. It yields more close resonance frequencies and optimized performance.

Magnetic Frequency Tuning of a Multimodal Vibration Energy Harvester.

In this paper, we present a novel vibration-based piezoelectric energy harvester, capable of collecting power at multiple operating frequencies and autonomously adapting itself to the dominant ambient frequencies. It consists of a compact dual-frequency resonator designed such that the first two fundamental natural frequencies are in the range of [50, 100] Hz, which is a typical frequency range for ambient vibrations in industrial environments. A magnetic frequency-tuning scheme is incorporated into the structure, which enables the frequency agility of the system. In contrast to single frequency harvesters, the presented approach combines multi-resonance and frequency tunability of both modes enabling a larger operative bandwidth. We experimentally demonstrate independent bi-directional tunability of our dual-frequency design. Furthermore, a control algorithm based on maximum amplitude tracking has been implemented for self-adaption of the system. The latter has been demonstrated in a system-level simulation model, which integrates the dual-frequency resonator, the magnetic tuning, and the control algorithm.

Fabrication of gap-optimized CMUT.

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 microm between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature.