Micromachines

This paper studies a novel enhanced energy-harvesting method to harvest water flow-induced vibration with a tandem arrangement of two piezoelectric energy harvesters (PEHs) in the direction of flowing water, through simulation modeling and experimental validation. A mathematical model is established by two individual-equivalent single-degree-of-freedom models, coupled with the hydrodynamic force obtained by computational fluid dynamics. Through the simulation analysis, the variation rules of vibration frequency, vibration amplitude, power generation and the distribution of flow field are obtained. And experimental tests are performed to verify the numerical calculation. The experimental and simulation results show that the upstream piezoelectric energy harvester (UPEH) is excited by the vortex-induced vibration, and the maximum value of performance is achieved when the UPEH and the vibration are resonant. As the vortex falls off from the UPEH, the downstream piezoelectric energy harvester (DPEH) generates a responsive beat frequency vibration. Energy-harvesting performance of the DPEH is better than that of the UPEH, especially at high speed flows. The maximum output power of the DPEH (371.7 μW) is 2.56 times of that of the UPEH (145.4 μW), at a specific spacing between the UPEN and the DPEH. Thereupon, the total output power of the two tandem piezoelectric energy harvester systems is significantly greater than that of the common single PEH, which provides a good foreground for further exploration of multiple piezoelectric energy harvesters system. Full article

Wavefront shaping can refocus light after it reflects from an optically rough surface. One proposed use case of this effect is in indirect imaging; if any rough surface could be turned into an illumination source, objects out of the direct line of sight could be illuminated. In this paper, we demonstrate the superior performance of a genetic algorithm compared to other iterative feedback-based wavefront shaping algorithms in achieving reflective inverse diffusion for a focal plane system. Next, the ability to control the pointing direction of the refocused beam with high precision over a narrow angular range is demonstrated, though the challenge of increasing the overall scanning range of the refocused beam remains. The method of beam steering demonstrated in this paper could act as a vernier adjustment to a coarse adjustment offered by another method. Full article

The design of an upper limb rehabilitation robot for post-stroke patients is considered a benchmark problem regarding improving functionality and ensuring better human–robot interaction (HRI). Existing upper limb robots perform either joint-based exercises (exoskeleton-type functionality) or end-point exercises (end-effector-type functionality). Patients may need both kinds of exercises, depending on the type, level, and degree of impairments. This work focused on designing and developing a seven-degrees-of-freedom (DoFs) upper-limb rehabilitation exoskeleton called ‘u-Rob’ that functions as both exoskeleton and end-effector types device. Furthermore, HRI can be improved by monitoring the interaction forces between the robot and the wearer. Existing upper limb robots lack the ability to monitor interaction forces during passive rehabilitation exercises; measuring upper arm forces is also absent in the existing devices. This research work aimed to develop an innovative sensorized upper arm cuff to measure the wearer’s interaction forces in the upper arm. A PID control technique was implemented for both joint-based and end-point exercises. The experimental results validated both types of functionality of the developed robot. Full article

Fiber-based thermoelectric materials and devices have the characteristics of light-weight, stability, and flexibility, which can be used in wearable electronics, attracting the wide attention of researchers. In this work, we present a review of state-of-the-art fiber-based thermoelectric material fabrication, device assembling, and its potential applications in temperature sensing, thermoelectric generation, and temperature management. In this mini review, we also shine some light on the potential application in the next generation of wearable electronics, and discuss the challenges and opportunities. Full article

The study of the influence of the nanoparticle volume fraction and aspect ratio of microchannels on the fluid flow and heat transfer characteristics of nanofluids in microchannels is important in the optimal design of heat dissipation systems with high heat flux. In this work, the computational fluid dynamics method was adopted to simulate the flow and heat transfer characteristics of two types of water-Al₂O₃ nanofluids with two different volume fractions and five types of microchannel heat sinks with different aspect ratios. Results showed that increasing the nanoparticle volume fraction reduced the average temperature of the heat transfer interface and thereby improved the heat transfer capacity of the nanofluids. Meanwhile, the increase of the nanoparticle volume fraction led to a considerable increase in the pumping power of the system. Increasing the aspect ratio of the microchannel effectively improved the heat transfer capacity of the heat sink. Moreover, increasing the aspect ratio effectively reduced the average temperature of the heating surface of the heat sink without significantly increasing the flow resistance loss. When the aspect ratio exceeded 30, the heat transfer coefficient did not increase with the increase of the aspect ratio. The results of this work may offer guiding significance for the optimal design of high heat flux microchannel heat sinks. Full article

The unique properties of two-dimensional (2D) materials make them increasingly attractive in various fields, especially for energy harvesting, conversion, or storage. Simultaneously, numerous synthetic methods have been rapidly developed. Recently, topochemical strategies were demonstrated, and they show tremendous promising potential for synthesizing 2D materials due to their simplicity, scalability, and high efficiency. Considering the suitability of material structures and their synthesis methods, as well as the relationship between material properties and applications, it is necessary for researchers to comprehensively review and determine the prospects of 2D materials based on topological chemical synthesis methods and their related applications. Therefore, in this review, we systematically summarize and analyze the representative topochemical strategies for synthesizing 2D materials, including salt-templating methods for non-layered 2D materials, molten Lewis acid etching strategy for novel MXenes, and the chalcogen vapors etching and substituting strategy for phase-controlled 2D materials and so on, with the application of these 2D materials in energy-related fields including batteries, supercapacitors, and electrocatalysis. At the end of the paper, the corresponding perspective was also illustrated, and we expect that this could provide a reference for the future research in the field. Full article

This paper shows how fused decomposition modeling (FDM), as a three-dimensional (3D) printing technology, can engineer lightweight porous foams with controllable density. The tactic is based on the 3D printing of Poly Lactic Acid filaments with a chemical blowing agent, as well as experiments to explore how FDM parameters can control material density. Foam porosity is investigated in terms of fabrication parameters such as printing temperature and flow rate, which affect the size of bubbles produced during the layer-by-layer fabrication process. It is experimentally shown that printing temperature and flow rate have significant effects on the bubbles’ size, micro-scale material connections, stiffness and strength. An analytical equation is introduced to accurately simulate the experimental results on flow rate, density, and mechanical properties in terms of printing temperature. Due to the absence of a similar concept, mathematical model and results in the specialized literature, this paper is likely to advance the state-of-the-art lightweight foams with controllable porosity and density fabricated by FDM 3D printing technology. Full article

Cell bioprinting technologies aim to fabricate tissuelike constructs by delivering biomaterials layer-by-layer. Bioprinted constructs can reduce the use of animals in drug development and hold promise for addressing the shortage of organs for transplants. Here, we sought to validate the feasibility of bioprinting primary adult sensory neurons using a newly developed laser-assisted cell bioprinting technology, known as Laser-Induced Side Transfer (LIST). We used dorsal root ganglion neurons (DRG; cell bodies of somatosensory neurons) to prepare our bioink. DRG-laden- droplets were printed on fibrin-coated coverslips and their viability, calcium kinetics, neuropeptides release, and neurite outgrowth were measured. The transcriptome of the neurons was sequenced. We found that LIST-printed neurons maintain high viability (Printed: 86%, Control: 87% on average) and their capacity to release neuropeptides (Printed CGRP: 130 pg/mL, Control CGRP: 146 pg/mL). In addition, LIST-printed neurons do not show differences in the expressed genes compared to control neurons. However, in printed neurons, we found compromised neurite outgrowth and lower sensitivity to the ligand of the TRPV1 channel, capsaicin. In conclusion, LIST-printed neurons maintain high viability and marginal functionality losses. Overall, this work paves the way for bioprinting functional 2D neuron assays. Full article

This study investigated the combined effects of proton irradiation and surface pre-treatment on the current characteristics of Gallium Nitride (GaN)-based metal-insulator-semiconductor high-electron-mobility-transistors (MIS-HEMTs) to evaluate the radiation hardness involved with the Silicon Nitride (SiN) passivation/GaN cap interface. The impact of proton irradiation on the static and dynamic current characteristics of devices with and without pre-treatment were analyzed with 5 MeV proton irradiation. In terms of transfer characteristics before and after the proton irradiation, the drain current of the devices without and with pre-treatment were reduced by an increase in sheet and contact resistances after the proton irradiation. In contrast with the static current characteristics, the gate-lag characteristics of the device with pre-treatment were significantly degenerated. In the device with pre-treatment, the hydrogen passivation for surface states of the GaN cap was formed by the pre-treatment and SiN deposition processes. Since the hydrogen passivation was removed by the proton irradiation, the newly created vacancies resulted in the degeneration of gate-lag characteristics. After nine months in an ambient atmosphere, the gate-lag characteristics of the device with pre-treatment were recovered because of the hydrogen recombination. These results demonstrated that the radiation hardness of MIS-HEMTs was affected by the SiN/GaN interface quality. Full article

Droplet microfluidics revolutionizes the way experiments and analyses are conducted in many fields of science, based on decades of basic research. Applied sciences are also impacted, opening new perspectives on how we look at complex matter. In particular, food and nutritional sciences still have many research questions unsolved, and conventional laboratory methods are not always suitable to answer them. In this review, we present how microfluidics have been used in these fields to produce and investigate various droplet-based systems, namely simple and double emulsions, microgels, microparticles, and microcapsules with food-grade compositions. We show that droplet microfluidic devices enable unprecedented control over their production and properties, and can be integrated in lab-on-chip platforms for in situ and time-resolved analyses. This approach is illustrated for on-chip measurements of droplet interfacial properties, droplet–droplet coalescence, phase behavior of biopolymer mixtures, and reaction kinetics related to food digestion and nutrient absorption. As a perspective, we present promising developments in the adjacent fields of biochemistry and microbiology, as well as advanced microfluidics–analytical instrument coupling, all of which could be applied to solve research questions at the interface of food and nutritional sciences. Full article

Flavin adenine dinucleotide (FAD) is a coenzyme and acts as a redox cofactor in metabolic process. Owing to such problems as poor electron transfer properties, unfavorable adsorption, and lack of stability on rigid electrodes, the bio-electrochemical applications of FAD have been limited. Herein, a novel fabrication method was developed for the immobilization process using 2D MXene (Ti₃C₂T_x), which enhanced the redox property of FAD and improved the electro-catalytic reduction of hydrogen peroxide (H₂O₂) in neutral medium. The FAD-immobilized Ti₃C₂T_x electrode (FAD/Ti₃C₂T_x) was studied by UV-Visible and Raman spectroscopies, which confirmed the successful adsorption of FAD on the Ti₃C₂T_x surface. The surface morphology and the elemental composition of Ti₃C₂T_x were investigated by high resolution transmission electron microscopy and the energy dispersive X-ray analysis. The redox property of the FAD/Ti₃C₂T_x modified glassy carbon electrode (FAD/Ti₃C₂T_x/GCE) was highly dependent on pH and exhibited a stable redox peak at −0.455 V in neutral medium. Higher amounts of FAD molecules were loaded onto the 2D MXene (Ti₃C₂T_x)-modified electrode, which was two times higher than the values in the reported work, and the surface coverage (ᴦ_FAD) was 0.8 × 10⁻¹⁰ mol/cm². The FAD/Ti₃C₂T_x modified sensor showed the electrocatalytic reduction of H₂O₂ at −0.47 V, which was 130 mV lower than the bare electrode. The FAD/Ti₃C₂T_x/GCE sensor showed a linear detection of H₂O₂ from 5 nM to 2 µM. The optimization of FAD deposition, amount of Ti₃C₂T_x loading, effect of pH and the interference study with common biochemicals such as glucose, lactose, dopamine (DA), potassium chloride (KCl), ascorbic acid (AA), amino acids, uric acid (UA), oxalic acid (OA), sodium chloride (NaCl) and acetaminophen (PA) have been carried out. The FAD/Ti₃C₂T_x/GCE showed high selectivity and reproducibility. Finally, the FAD/Ti₃C₂T_x modified electrode was successfully applied to detect H₂O₂ in ovarian cancer cell lines. Full article

Reagent-based colorimetric analyzers often heat the fluid under analysis for improved reaction kinetics, whilst also aiming to minimize energy use per measurement. Here, a novel method of conserving heat energy on such microfluidic systems is presented. Our design reduces heat transfer to the environment by surrounding the heated optical cell on four sides with integral air pockets, thereby realizing an insulated and suspended bridge structure. Our design was simulated in COMSOL Multiphysics and verified in a polymethyl methacrylate (PMMA) device. We evaluate the effectiveness of the insulated design by comparing it to a non-insulated cell. For temperatures up to 55 °C, the average power consumption was reduced by 49.3% in the simulation and 40.2% in the experiment. The designs were then characterized with the vanadium and Griess reagent assay for nitrate at 35 °C. Nitrate concentrations from 0.25 µM to 50 µM were tested and yielded the expected linear relationship with a limit of detection of 20 nM. We show a reduction in energy consumption from 195 J to 119 J per 10 min measurement using only 4 µL of fluid. Efficient heating on-chip will have broad applicability to numerous colorimetric assays. Full article

Bandwidth is an important parameter for accelerometers, in some cases, even surpassing sensitivity. However, there are few studies focused on the relationship between bandwidth and environmental conditions in practical application of accelerometers. In this paper, we systematically analyze the influence of environment on the bandwidth of accelerometers, obtaining the amplitude–frequency response curves versus damping ratio and properties of materials, wherein temperature and humidity were found as the two dominant factors that influence the bandwidth of accelerometers. Common temperature and humidity variations can result in bandwidth degradation of about 25% according to our theoretical analysis. The finite element method (FEM) is introduced to verify our theoretical analysis, and the accordance of the FEM simulation results and the theoretical results confirmed the validity of our analysis. Furthermore, a modification design is proposed to compensate for the influence of temperature and humidity on the bandwidth of accelerometers. By choosing materials with an appropriate Young’s modulus and coefficient of thermal expansion, the degradation of the bandwidth was substantially diminished by more than one order of magnitude, which can serve as a strong guide for the future realization of accelerometers with a steady and large bandwidth. Full article

Three-dimensional (3D) spintronic devices are attracting significant research interest due to their potential for both fundamental studies and computing applications. However, their implementations face great challenges regarding not only the fabrication of 3D nanomagnets with high quality materials, but also their integration into 2D microelectronic circuits. In this study, we developed a new fabrication process to facilitate the efficient integration of both non-planar 3D geometries and high-quality multi-layered magnetic materials to prototype 3D spintronic devices, as a first step to investigate new physical effects in such systems. Specifically, we exploited 3D nanoprinting, physical vapour deposition and lithographic techniques to realise a 3D nanomagnetic circuit based on a nanobridge geometry, coated with high quality Ta/CoFeB/Ta layers. The successful establishment of this 3D circuit was verified through magnetotransport measurements in combination with micromagnetic simulations and finite element modelling. This fabrication process provides new capabilities for the realisation of a greater variety of 3D nanomagnetic circuits, which will facilitate the understanding and exploitation of 3D spintronic systems. Full article

The need for accessible and inexpensive microfluidic devices requires new manufacturing methods and materials as a replacement for traditional soft lithography and polydimethylsiloxane (PDMS). Recently, with the advent of modern additive manufacturing (AM) techniques, 3D printing has attracted attention for its use in the fabrication of microfluidic devices and due to its automated, assembly-free 3D fabrication, rapidly decreasing cost, and fast-improving resolution and throughput. Here, fused filament fabrication (FFF) 3D printing was used to create microfluidic micromixers and enhance the mixing process, which has been identified as a challenge in microfluidic devices. A design of experiment (DoE) was performed on the effects of studied parameters in devices that were printed by FFF. The results of the colorimetric approach showed the effects of different parameters on the mixing process and on the enhancement of the mixing performance in printed devices. The presence of the geometrical features on the microchannels can act as ridges due to the nature of the FFF process. In comparison to passive and active methods, no complexity was added in the fabrication process, and the ridges are an inherent property of the FFF process. Full article

The manufacturing of inserts for micro injection moulding made of mortar material is presented in this work. The fabrication of the mortar insert described in this publication relied on a versatile and relatively fast rapid prototyping process based on soft tooling. The mortar insert has a QR code with micro features on its surface, which was replicated in acrylonitrile butadiene styrene (ABS) polymer by the micro injection moulding process. With this approach, it is possible to fabricate hard inserts for micro injection moulding purposes that are able to compete with conventional-made inserts made of tool steel. Full article

This work presents an embedding process for ultrathin silicon chips in mechanically flexible solder mask resist and their electrical contacting by inkjet printing. Photosensitive solder mask resist is applied by conformal spray coating onto epoxy bonded ultrathin chips with a daisy chain layout. The contact pads are opened by photolithography using UV direct light exposure. Circular and rectangular openings of 90 µm and 130 µm diameter, respectively, edge length are realized. Commercial inks containing nanoparticular silver and gold are inkjet printed to form conductive tracks between daisy chain structures. Different numbers of ink layers are applied. The track resistances are characterized by needle probing. Silver ink shows low resistances only for multiple layers and 90 µm openings, while gold ink exhibits low resistances in the single-digit Ω-range for minimum two printed layers. Full article

As an important nonlinear optical material, potassium dihydrogen phosphate (KDP) crystal is used in high-power laser beams as the core element of inertial confinement fusion. It is the most general method of single point diamond fly-cutting (SPDF) to produce high precision and crack-free KDP surfaces. Nevertheless, the cutting mechanism of such material remains unclear, and therefore needs further analysis. Firstly, the stress field, cutting force and cutting temperature under different working conditions are calculated by a KDP crystal cutting simulation model. Then, the rules and the cause of change and interaction mechanisms of force and temperature are analyzed by comparing the measurement experiments with simulations. Furthermore, the causes of chip formation and micro-cracks on the machined surface are analyzed based on thermo-mechanical coupling and chip morphology. The conclusion can be deduced: Although the temperature has not reached the phase transition temperature during the finishing process, under high cutting speeds and large unformed chip thickness, such as semi-finishing and roughing, the temperature can reach up to 180 °C or higher, and KDP crystals are very likely to phase transition—chip morphology also verifies this phenomenon. Full article

Capillary self-alignment technique can achieve highly accurate and fast alignment of micro components. Capillary self-alignment technique relies on the confinement of liquid droplets at receptor sites where hydrophobic–hydrophilic patterns are widely used. This paper reports a low-cost microsecond pulse laser micromachining method for fabrication of super hydrophilic–super hydrophobic grooves as receptor sites for capillary self-alignment of microfibers. We investigated the influence of major manufacturing parameters on groove sizes and wetting properties. The effects of the width (20 µm–100 µm) and depth (8 µm–36 µm) of the groove on the volume of water droplet contained inside the groove were also investigated. We show that by altering scanning speed, using a de-focused laser beam, we can modify the wetting properties of the microgrooves from 10° to 120° in terms of the contact angle. We demonstrated that different types of microfibers including natural and artificial microfibers can self-align to the size matching super hydrophilic–super hydrophobic microgrooves. The results show that super hydrophilic–super hydrophobic microgrooves have great potential in microfiber micromanipulation applications such as natural microfiber categorization, fiber-based microsensor construction, and fiber-enforced material development. Full article