Journal of Micromechanics and Microengineering

Polydimethylsiloxane (PDMS) elastomers are extensively used for soft lithographic replication of microstructures in microfluidic and micro-engineering applications. Elastomeric microstructures are commonly required to fulfil an explicit mechanical role and accordingly their mechanical properties can critically affect device performance. The mechanical properties of elastomers are known to vary with both curing and operational temperatures. However, even for the elastomer most commonly employed in microfluidic applications, Sylgard 184, only a very limited range of data exists regarding the variation in mechanical properties of bulk PDMS with curing temperature. We report an investigation of the variation in the mechanical properties of bulk Sylgard 184 with curing temperature, over the range 25 °C to 200 °C. PDMS samples for tensile and compressive testing were fabricated according to ASTM standards. Data obtained indicates variation in mechanical properties due to curing temperature for Young's modulus of 1.32–2.97 MPa, ultimate tensile strength of 3.51–7.65 MPa, compressive modulus of 117.8–186.9 MPa and ultimate compressive strength of 28.4–51.7 GPa in a range up to 40% strain and hardness of 44–54 Sh_A.

The need for materials with low density and high strength has drawn a lot of interest from researchers and industry in the last few years. Aluminum 6061 (Al6061) is one of these materials that has the required qualities. Powder-mixed electric discharge machining has become a practical choice for cutting such materials because of its versatile machining capabilities. However, this technique's excessive energy usage and poor cutting efficiency have drawn criticism. Furthermore, there are serious health and environmental risks associated with the typical dielectric (kerosene) used in EDM. Deionized water, a replacement to kerosene, has been used in this work to address the aforementioned problems, improving resource reusability and lowering the dielectric cost. Here, deionized water further makes the operation sustainable and protects the environment from harmful emissions produced during the machining process. Additionally, alumina (Al₂O₃) nano-powder has been mixed in dielectric and used to improve the machining responsiveness. Response surface methodology was used to carry out the investigation. The purpose of this study was to use microscopic analysis to examine the effects on the electrode wear rate (EWR) and accuracy index (AI). Analysis of variance (ANOVA) analyses for both responses revealed that all four parameters are highly significant, with p-values nearly zero (<0.05). Additionally, the coefficient of determination (R²) values for EWR (0.9611) and AI (0.9285) indicate that the proposed models are reliable. The parametric optimization by grey relational analysis (GRA) approach highlighted that the magnitude for EWR and AI is improved by 50.85% and 2.67%, respectively, when optimal condition (I_P: 5 A, S_V: 2 V, S_T: 3 µs, and C_P:1.5 g/100 ml) is set during EDM of Al6061. The proposed EDM model yielded 48.29% and 5.11% better outcomes than the conventional EDM model in terms of EWR and AI, respectively.

Strain sensors have been developed in various fields by converting mechanical deformation into electrical signals. Surface acoustic wave (SAW) devices are beneficial for strain sensing due to their simplicity of fabrication and wireless operation capabilities. In this study, we investigate SAW strain sensors operating at 1.25 $\mathrm{GHz}$. The fabricated SAW resonators using standard photolithography technology are characterized with a custom-made cantilever setup capable of applying defined strain values up to approximately −4000 µε to 4000 µε. From these measurements, a high responsivity even up to this high strain values is demonstrated. We also explore the impact of geometric design parameters on strain-sensing performance. We vary the length of the SAW resonator and observe that the longer the SAW resonator, the more responsive the device gets to strain changes. When the distance between the two reflectors confining the SAW is 2207 $\mu\mathrm{m}$, the responsivity to strain is 114.99 $\mathrm{Hz}\,\mu\epsilon^{-1}$. In summary, this study investigates the feasibility of GHz SAW resonators as high-strain sensors on non-flexible substrates with a custom-built experimental setup, to evaluate their potential for future applications in extreme mechanical environments.

Lift-off process is an alternative to deposition, lithography, and etching of materials. Lift-off is a simple and economical process because it does not require subsequent wet or dry etching. Lifting-off nanometer thick films is a well-developed and repeatable process. However, lifting-off a few micrometer thick films may be challenging. Previously, different techniques were proposed to lift-off micrometer thick films. Herein, a novel method for lifting-off high thickness materials is proposed using a multi-layer AZ 5214E photoresist. The novel method was successful in lifting-off 4 µm thick copper while the copper could even be deposited up to 6 µm with tri-layer AZ 5214E. With four layers of AZ 5214E, the photoresist thickness can be even thicker than 9 µm. As detailed in the study, the photoresist layer thickness can be adjusted by varying the number of layers. This enables the selection of the appropriate number of layers to achieve the desired material thickness. To show the merits of the proposed method, the method is compared to the bi-layer method with AZ 4562 photoresist which is used for lifting-off high thickness materials. In addition to lifting-off thick materials, the proposed method is faster compared to lifting-off using bi-layer AZ 4562. Despite the ability to lift-off thick films, both methods could suffer from lift-off flags if the deposition process is not anisotropic. Solutions to remove the lift-off flags, and reduce the undercut width are demonstrated.

Beam-type piezoelectric energy harvesters (PEHs), particularly those utilizing piezoelectric materials, have garnered considerable attention as efficient devices for converting ambient mechanical vibrations into electrical energy. This comprehensive review article thoroughly examines the mathematical models employed in beam-type PEHs, emphasizing their evolution and limitations. The study also delves into both theoretical and experimental analyses of design configurations, placing a special focus on the impact of geometries on energy harvesting efficiency. In conclusion, the paper explores recent advancements and improvements, along with potential avenues for future research, providing a concise overview tailored for professionals and scholars engaged in this specialized field.

The demand for rapid, high-quality, and controlled mixing at the microscale has led to the development of various types of micromixers. Micromixers are commonly categorised as active, or passive based on whether they utilise external energy to enhance mixing. Passive micromixers utilise a complex geometry to enhance the diffusion coefficient at lower Reynolds numbers and induce chaotic advection at higher Reynolds numbers for effectively mixing fluids without external energy. Active micromixers, on the other hand, achieve precise, fast, and controllable mixing by employing external energy sources such as pressure, electric, magnetic, or acoustic fields. Some active methods such as magnetic field-driven micromixers need fluids with specific properties. Others, such as acoustic field-driven micromixers apply to various types of fluids. Bubbles can be used as membranes or stirrers in microfluidic devices for both passive and active micromixers. They are easy to use, compatible with microfluidic systems, low cost, and effective. Improvements in manufacturing methods, notably, 3D printing have emerged as promising methods for the development of new micromixer designs. In this paper, a wide range of micromixer types is reviewed and the main mechanism for enhanced mixing is investigated. This study aims to guide researchers proposing innovative designs. Furthermore, it is shown that combining different methods can lead to the development of more effective micromixers, promising further advancements in microscale mixing technology.

MEMS environmental sensors, including pressure, gas, and humidity sensors, require protection from mechanical damage, particle exposure, and condensing moisture, while maintaining their ability to exchange gases with the environment. This work introduces a novel packaging approach for MEMS environmental sensors using substrate-embedded filters made from microfine powders through PowderMEMS^® microfabrication technology. The study demonstrates the successful fabrication of gas permeable, functionalized PowderMEMS^® filters on 200 mm Si-wafers for wafer-level packaging of MEMS environmental sensors. Utilizing complete Si-wafers allows for all MEMS sensors on a device wafer to be packaged in a single substrate bonding step, followed by die singulation. The processed wafers are shown to be compatible with high-temperature glass-frit substrate bonding. Alternatively, individual chips with PowderMEMS^® filters can be assembled discretely onto standard semiconductor packages to serve as gas-permeable filters. Successful hydrophobation of the inherently hydrophilic PowderMEMS^® structures by deposition of hydrophobic nanofilms is demonstrated and resistance to water ingress is evaluated by immersion testing. Given that many MEMS gas sensors are cross-reactive to oxidizing gases like ozone, this study also explores the integration of ozone-degrading catalytic powder into the PowderMEMS^® filters. As a proof-of-concept, commercial MEMS ozone sensors are modified with catalytic PowderMEMS^® caps, and successful ozone degradation is demonstrated. While PowderMEMS^® processing is typically conducted on 200 mm Si-wafers, other suitable substrates include glass and (fiber-reinforced) polymers.

We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

Microfluidic devices have been conventionally fabricated using traditional photolithography or through the use of soft lithography both of which require multiple complicated steps and a clean room setup. Xurography is an alternative rapid prototyping method which has been used to fabricate microfluidic devices in less than 20–30 minutes. The method is used to pattern two-dimensional pressure-sensitive adhesives, polymer sheets, and metal films using a cutting plotter and these layers are bonded together using methods including adhesive, thermal, and solvent bonding. This review discusses the working principle of xurography along with a critical analysis of parameters affecting the patterning process, various materials patterned using xurography, and their applications. Xurography can be used in the fabrication of microfluidic devices using four main approaches: making multiple layered devices, fabrication of micromolds, making masks, and integration of electrodes into microfluidic devices. We have also briefly discussed the bonding methods for assembling the two-dimensional patterned layers. Due to its simplicity and the ability to easily integrate multiple materials, xurography is likely to grow in prominence as a method for fabrication of microfluidic devices.

Residual stresses can be advantageously used to permanently preload flexure micro-mechanisms in order to modify their deflection and stiffness. This paper presents a new preloading chevron mechanism (PCM) used to amplify the preloading effect of thin film residual stress. To evaluate the preloading performances of this structure, the deflection characteristics of buckled beams and flexure linear stages preloaded by a PCM is investigated experimentally. All the mechanisms are manufactured from a monocrystalline silicon substrate using deep reactive ion etching and residual stress is provided by wet thermal oxidation. Measurements show that the deflection magnitude of fixed-fixed oxidized silicon buckled beams can be increased by up to 5 times when a PCM is integrated. The flexure linear stages studied in this research are composed of a parallel leaf spring stage connected to two fixed-guided buckled beams preloaded by a PCM. Depending on the beam dimensions, the stage translational stiffness can be set to a specific value. We designed a near-zero positive stiffness linear stage revealing a measured stiffness reduction of 98%, and a bistable linear stage with a constant negative stiffness region. Thanks to the elevated preloading displacement supplied by the PCM, the operating stroke (actuation region where the stiffness remains constant) is relatively large (more than 0.4 mm travel for 2.59 mm leaf spring length). The analytical and numerical models carried out to design the mechanisms are in good agreement with the experimental data. The results show that the fixed frame stiffness has a significant effect on the preloading performances due to the substantial forces exerted by the PCM. Furthermore, the presented preloading concept, modeling and sizing method could be applied to other compliant mechanism designs, scales and materials, enabling applications in microelectromechanical systems and watchmaking.

Wet etching in hydrofluoric acid (HF) is one of the most common routes for the surface texturing of silica, leading to improved optical properties, which find applications in several fields. In this work, wet etching of silica is mediated by the deposition of chemically synthesized gold nanoparticles (NPs) on the substrate. NPs of different sizes are coated on silica and act as a mask when etching using HF. The effect of parameters such as etching time, NP size, and HF concentration on the surface morphologies and transmittance are studied and correlated with the chemical etching mechanism. This work reveals that a proper choice of masking and etching conditions can modulate the optical transmission of silica. Etching leads to the formation of arrays of micron-size elongated pits. The pit width and surface roughness are found to increase with etching time and HF concentration, in turn leading to a decrease in transmittance. The results show that NP masking is an effective way to control silica etching and in turn, modifies the transmittance of the substrate.

This paper proposes a new and efficient micromixer (double-layer vortex split-and-recombine) based on the principles of splitting-recombination and vortex mechanisms, which employs a dual-layer vortex structure for split-composite and chaotic convection. Through numerical simulations and experimental studies of the micromixer, the advantages of its fluid mixing capabilities were outlined. Based on this, the Reynolds number (Re) and the aspect ratio (λ) of the micromixer's channel were coordinated to comprehensively study the mixing performance and pressure loss. The results indicate that the mixing efficiency is optimal when Re = 25–100 and λ = 0.5. The mixing index (ϕ) decreases as the aspect ratio (λ) increases, and the effect of Re on pressure loss also diminishes. At Re = 25–100, the maximum mixing volume flow rate occurs at an aspect ratio of 1, and the mixer with an aspect ratio of 1 demonstrates superior performance in mixing volume flow rate as Re increases, compared to mixers with other aspect ratios. The overall performance index (Φ) of the micromixer increases with the aspect ratio (λ), with relatively better performance at λ = 1.25 and λ = 1.5. Both simulation and experimental results show that the micromixer not only has a simple channel structure and dual-layer stacking, but also exhibits excellent mixing performance, offering significant potential for applications in chemical and biological engineering.

This paper presents a novel design of a micro-electro-mechanical-systems (MEMS) fluxgate sensor featuring double-layer induction coils. We utilize the micro-casting process which comprises two steps of using deep reactive ion etching to etch coil molds and filling the molds with molten Zn–Al alloy to form the coils. This process has the capacity to fabricate intricate three-dimensional coil structures, overcoming the limitations associated with the traditional electroplating process. The parameters of the core and the coils are optimized by Maxwell simulation software. Open-loop test results indicate that the fabricated sensor exhibits a sensitivity of 6091 V T⁻¹, a power consumption of 3.6 mW, a linear range of ± 220 µT, a noise power density of 681 pT/√Hz@1 Hz and a bandwidth from DC to 122 Hz when excited by a 400 kHz sine wave with a root-mean-square value of 30 mA. Our study shows a significant improvement in sensor performance with a sensitivity at least twice that of comparable MEMS fluxgate sensors. This novel structure of double-layer induction coils not only pushes the design boundaries of MEMS fluxgate sensors but also holds promise for precision magnetic sensing.

Currently, micromachined ultrasonic transducers are classified as capacitive micromachined ultrasonic transducer (CMUT) and piezoelectric micromachined ultrasonic transducers (PMUTs). CMUT present higher electromechanical coupling coefficients, high receiving sensitivity, and higher bandwidth, exhibiting superior performance compared to PMUT and their traditional counterparts. Micro-nano materials, with advantages such as high surface area, improved electronic performance, biocompatibility, and easy integration with miniaturization, are widely applied in various fields including electronics, energy, environment protection, and medicine. The combination of CMUT and micro-nano materials has become a hot research topic in the fields of medicine and biochemistry in recent years. Integrating CMUT with micro-nano materials plays an important role in biochemical testing, drug monitoring, and medical diagnosis, promoting the prediction of disease progression and timely implementation of effective measures. This work primarily discusses the integration of CMUT with micro-nano materials, emphasizing that the innovative application of these materials significantly enhances the performance, thereby advancing the development of related technologies.

This study investigates the fatigue life of cantilever-style hinges in digital micromirror devices (DMD), which are critical for their optical switching function. The hinges, which facilitate the rotation of reflective mirrors, are subjected to significant mechanical stress due to repetitive flexing, posing a risk of device failure. Prior to the main continuous bending fatigue test, a preliminary resonance-based fatigue experiment was conducted using microscopic laser Doppler vibrometry. The amplitude–frequency response of the cantilever-hinged micromirrors was measured, identifying a resonant frequency through Gaussian curve fitting. Operating at this resonant frequency, the micromirrors achieved stable performance more than 2 × 10¹⁰ oscillation cycles, with frequency stability throughout, affirming the reliability of the hinge structure under low-stress, high-cycle fatigue conditions. To further assess fatigue resistance, atomic force microscopy was subsequently employed to conduct continuous bending fatigue tests, in which cyclic stress loading was applied to induce continuous bending in the cantilever hinges. Force–displacement curves obtained during the loading process were analyzed to determine the deflection and spring constants of the hinges at different stages of bending. Experimental results reveal that after 5 × 10⁵ continuous bending cycles, plastic deformation occurred in the hinges along the bending axis, with the end morphology of the hinge exhibiting a significant residual plastic deformation of 417.18 nm compared to its initial state. This research provides valuable insights into the high-stress, low-cycle fatigue behavior of nanoscale devices, contributing to the understanding and improvement of DMD reliability.

Currently, micromachined ultrasonic transducers are classified as capacitive micromachined ultrasonic transducer (CMUT) and piezoelectric micromachined ultrasonic transducers (PMUTs). CMUT present higher electromechanical coupling coefficients, high receiving sensitivity, and higher bandwidth, exhibiting superior performance compared to PMUT and their traditional counterparts. Micro-nano materials, with advantages such as high surface area, improved electronic performance, biocompatibility, and easy integration with miniaturization, are widely applied in various fields including electronics, energy, environment protection, and medicine. The combination of CMUT and micro-nano materials has become a hot research topic in the fields of medicine and biochemistry in recent years. Integrating CMUT with micro-nano materials plays an important role in biochemical testing, drug monitoring, and medical diagnosis, promoting the prediction of disease progression and timely implementation of effective measures. This work primarily discusses the integration of CMUT with micro-nano materials, emphasizing that the innovative application of these materials significantly enhances the performance, thereby advancing the development of related technologies.

Beam-type piezoelectric energy harvesters (PEHs), particularly those utilizing piezoelectric materials, have garnered considerable attention as efficient devices for converting ambient mechanical vibrations into electrical energy. This comprehensive review article thoroughly examines the mathematical models employed in beam-type PEHs, emphasizing their evolution and limitations. The study also delves into both theoretical and experimental analyses of design configurations, placing a special focus on the impact of geometries on energy harvesting efficiency. In conclusion, the paper explores recent advancements and improvements, along with potential avenues for future research, providing a concise overview tailored for professionals and scholars engaged in this specialized field.

Inspired by the advances in microfabrication of microelectromechanical systems (MEMSs), microphysiological systems (MPSs) capitalized on the fabrication techniques of MEMS technology and pivoted to biomedical applications with select biomaterials and design principles. With the new initiative to refute animal testing and develop valid and reliable alternatives, MPS platforms are in greater demand than ever. This paper will first present the major types of MPSs in the cardiovascular research space, and then review the core design principles of such systems to closely replicate the in vivo physiology. Fabrication methodologies of the platform, as well as technologies that enable patterning and functionalizing scaffolds, and the various sensing modalities that can interface with such MPS platforms, are reviewed and discussed. This review aims to provide a comprehensive picture of cardiac MPSs in which microfluidics play an important role in the design, fabrication, and sensing modalities, and prospects of how this platform can continue to drive further improvements in cardiovascular research and medicine.

The demand for rapid, high-quality, and controlled mixing at the microscale has led to the development of various types of micromixers. Micromixers are commonly categorised as active, or passive based on whether they utilise external energy to enhance mixing. Passive micromixers utilise a complex geometry to enhance the diffusion coefficient at lower Reynolds numbers and induce chaotic advection at higher Reynolds numbers for effectively mixing fluids without external energy. Active micromixers, on the other hand, achieve precise, fast, and controllable mixing by employing external energy sources such as pressure, electric, magnetic, or acoustic fields. Some active methods such as magnetic field-driven micromixers need fluids with specific properties. Others, such as acoustic field-driven micromixers apply to various types of fluids. Bubbles can be used as membranes or stirrers in microfluidic devices for both passive and active micromixers. They are easy to use, compatible with microfluidic systems, low cost, and effective. Improvements in manufacturing methods, notably, 3D printing have emerged as promising methods for the development of new micromixer designs. In this paper, a wide range of micromixer types is reviewed and the main mechanism for enhanced mixing is investigated. This study aims to guide researchers proposing innovative designs. Furthermore, it is shown that combining different methods can lead to the development of more effective micromixers, promising further advancements in microscale mixing technology.

The burgeoning internet of things and artificial intelligence technologies have prospered a variety of emerging applications. Human–machine interfaces (HMIs), for instance, enables users with intuitive, efficient, and friendly way to interact with machines, capable of instant information acquisition, processing, communication, and feedback, etc. These features require ultra-compact and high-performance transducers, and therefore self-powered sensors have become the key underlying technology for HMI applications. This review focuses on the piezoelectric, triboelectric, and hybrid self-powered sensors with particular attention to their microstructures and fabrication methods, showing that both traditional microfabrication and emerging fabrication methods like three-dimensional (3D) printing, electrospinning, and braiding have contributed to the planar, array, porous, fabric, and composite type self-powered sensors. Moreover, the integration method of piezoelectric and triboelectric sensor arrays is investigated. The crosstalk issue is highlighted, i.e. the signal interference between adjacent sensing units, and current solutions such as array design optimization, signal processing improvement, and material innovation to reduce crosstalk sensitivity have been reviewed through specific examples. Three categories of HMI applications have been outlined, including intelligent interaction, robotics, and human monitoring, with detailed explanations of how the self-powered sensors support these HMI applications. Through discussion of challenges and prospects, it is proposed that further coordinating the design and fabrication of micro devices with HMIs will potentially boost the intelligent application with even higher level of diversification, convenience, and interconnectivity.

Electrode-integrated microfluidic chips play a pivotal role in applying electrochemical impedance spectroscopy (EIS) across various domains. This technology has significantly transformed biomedical research, facilitating progress in drug discovery, diagnostics, and cell analysis. The architecture of these chips integrated with electrodes critically influences the precision and dependability of EIS outcomes. This study developed diverse microfluidic chip designs, including circular, deltoid, and deltoid-like shapes, to explore microenvironmental dynamics on EIS assessments. Moreover, computational fluid dynamics (CFD) was utilized to examine the flow properties within the proposed chip designs by investigating the relationship between pressure and velocities in the microenvironment. The study also assessed the effects of varying flow rates (1, 10, 100 µL) on EIS analysis and the simulation studies. Findings indicated that there were empty spaces in the circular design, which is commonly used, and it was not suitable for EIS experiments. Furthermore, it was noted that even with reduced altitude in the EIS measurement area, the environment remained conducive to more accurate measurements. A flow rate of 10 µL/min was identified as optimal in this research, as it offered the best balance among charge transfer resistance (Rct), capacitance (Q), and open circuit potential (OCP) values, while also minimizing the sample volume which is very important for microfluidic chip design and applications. This study demonstrated a strong interaction between microfluidic chip designs for electrode integration and EIS outcomes. On the other hand, it has yielded a reliable, cost-effective, rapid, practical, reusable, and portable platform after choosing an appropriate architecture for the electrode housing.

Mechanoluminescence (ML) is the emission of photons from materials in response to a mechanical stimulus, with a good correlation between ML intensity and stress amplitude. Depositing a ZnS: Cu layer on the monolithically integrated photodetector surface to construct a stress and light dual-mode sensor. The developed sensor achieves a high-stress sensitivity with a detection limit of 0.525 MPa, a response time under 10 ms, and a minimum resolution of 0.1 MPa. Furthermore, the product is straightforward to manufacture and lightweight. Additionally, the ZnS: Cu layer's excellent light transmission allows for dual-mode stress and light detection, making it ideal for deployment in large industrial machinery. This work not only proposes a novel dual-mode sensor but also provides a promising approach to extending photodetector utility and functionality by applying a film to the photodetector, opening pathways for versatile applications.

Piezoelectric micromachined ultrasonic transducers (PMUTs) play a crucial role in the advancement of portable and wearable ultrasound imaging devices. Matching strategy of conventional thickness mode transducer for acoustic propagation is quite well developed yet. However, PMUTs which employ thin film's flexural mode vibration lack systematic acoustic matching guidelines critical for optimizing performance. This study introduces the first comprehensive design framework for PMUT matching layers, integrating theoretical modeling, finite element method (FEM) analysis and ultrasound characterization as well as phased-array ultrasound imaging validation. Being different from conventional thickness mode devices, the results indicate that a matching layer cannot simultaneously improve PMUT's sensitivity and bandwidth (BW). Employing the matching layer thickness to be a quarter ultrasound wavelength, materials having an acoustic impedance above 1.5 MRayl enhance sensitivity, whereas those with an impedance below 1.5 MRayl improve BW. For our developed PMUT having center frequency of 6.3MHz, a PDMS with an acoustic impedance of 1 MRayl was selected as matching layer, and it increases the PMUT's BW from 20% to 39% while resulting in a 41% reduction in sensitivity. The results align closely with the theoretical predictions. To enhance PMUT's sensitivity, matching materials like polybutadiene and thermoplastic polyurethane are more effective. These guidelines provide a foundation for rapid material selection and dimensional optimization, facilitating the broader application of miniaturized PMUTs in medical imaging and industrial sensing.

This paper introduces a low-cost, rapid, simple method for fabricating 3D-printed microfluidic chips with inspiration from LEGO® bricks to accommodate modular microfluidic platforms. A fused filament fabrication 3D printer was operated and modified to print a single 400µm layer of thermoplastic material onto a polymethyl methacrylate substrate, resulting in a very smooth and transparent microchannel by eliminating the limitations of this additive manufacturing. Specific male/female chip-to-world and chip-to-chip connectors have been developed to implement the device's modularity. In addition, various post-processing procedures were considered because of the inherent surface roughness of the fused filament fabrication method. Furthermore, several bonding methods have been tested and utilized to attach different layers of microfluidic chips. Several microfluidic components were designed and fabricated, including T-shaped and cross-shaped microchannels, zig-zag pattern micromixers, and four oval-shaped parallel microchannel. The chips' validity was examined by injecting food dye into a group of modules to observe the chips' leakage and fluid circulation behavior. At last, a fluorescent test was implemented to observe the mixing efficiency of the micromixer chip. The proposed fabrication method, from materials' cost, accessibility, and commercialized perspectives, offers high throughput process. In other words, it could be fabricated and implemented in most lab environments with limited facilities and budget without the necessity for expensive equipment. All microfluidic chips in this work have been designed using a modular concept, a relatively new approach in microfluidics that allows users to reconfigure connections and microfluidic components to obtain the desired system.

Transfer printing is a micro or nanofabrication technology used to transfer a thin film coated on a stamp to a substrate. The printing process requires a large adhesion force between the thin film and the substrate. In this study, we investigated the transfer printing of gold (Au) thin films via atomic diffusion bonding. An Au thin film coated on micro/nanoridges of a (h-) polydimethylsiloxane (PDMS) stamp was contacted with another Au thin film previously coated on a substrate. The two films strongly bonded via the interdiffusion of Au atoms, and the Au thin film on the stamp was transfer-printed with micro/nanopatterns onto the substrate under atmospheric conditions. The patterns have widths of 100 µm, 150 µm, and 150 nm and thicknesses of 30, 50, and 100 nm. We investigated the effects of the air-exposure time and stamp (indentation) modulus on the transfer-printing of line-patterned Au thin films. We found that longer air exposure increased the water contact angle and surface roughness of the Au thin films, indicating that air exposure induces contaminations and promotes water absorption, thereby decreasing a transfer rate of thin films. Both the Au-coated stamp and the substrate must be stored in a low-humidity environment before transfer printing. The modulus of the (h-)PDMS stamp was controlled by varying the ratio of the prepolymer and curing agent. A stamp with a lower modulus improved adhesiveness between the stamp and the substrate and provided a large contact area, thereby increasing the transfer rate and reducing the processing time. We successfully transfer-printed Au thin films with 150-nm-wide line patterns.

Electrode-integrated microfluidic chips play a pivotal role in applying electrochemical impedance spectroscopy (EIS) across various domains. This technology has significantly transformed biomedical research, facilitating progress in drug discovery, diagnostics, and cell analysis. The architecture of these chips integrated with electrodes critically influences the precision and dependability of EIS outcomes. This study developed diverse microfluidic chip designs, including circular, deltoid, and deltoid-like shapes, to explore microenvironmental dynamics on EIS assessments. Moreover, computational fluid dynamics (CFD) was utilized to examine the flow properties within the proposed chip designs by investigating the relationship between pressure and velocities in the microenvironment. The study also assessed the effects of varying flow rates (1, 10, 100 µL) on EIS analysis and the simulation studies. Findings indicated that there were empty spaces in the circular design, which is commonly used, and it was not suitable for EIS experiments. Furthermore, it was noted that even with reduced altitude in the EIS measurement area, the environment remained conducive to more accurate measurements. A flow rate of 10 µL/min was identified as optimal in this research, as it offered the best balance among charge transfer resistance (Rct), capacitance (Q), and open circuit potential (OCP) values, while also minimizing the sample volume which is very important for microfluidic chip design and applications. This study demonstrated a strong interaction between microfluidic chip designs for electrode integration and EIS outcomes. On the other hand, it has yielded a reliable, cost-effective, rapid, practical, reusable, and portable platform after choosing an appropriate architecture for the electrode housing.

The need for materials with low density and high strength has drawn a lot of interest from researchers and industry in the last few years. Aluminum 6061 (Al6061) is one of these materials that has the required qualities. Powder-mixed electric discharge machining has become a practical choice for cutting such materials because of its versatile machining capabilities. However, this technique's excessive energy usage and poor cutting efficiency have drawn criticism. Furthermore, there are serious health and environmental risks associated with the typical dielectric (kerosene) used in EDM. Deionized water, a replacement to kerosene, has been used in this work to address the aforementioned problems, improving resource reusability and lowering the dielectric cost. Here, deionized water further makes the operation sustainable and protects the environment from harmful emissions produced during the machining process. Additionally, alumina (Al₂O₃) nano-powder has been mixed in dielectric and used to improve the machining responsiveness. Response surface methodology was used to carry out the investigation. The purpose of this study was to use microscopic analysis to examine the effects on the electrode wear rate (EWR) and accuracy index (AI). Analysis of variance (ANOVA) analyses for both responses revealed that all four parameters are highly significant, with p-values nearly zero (<0.05). Additionally, the coefficient of determination (R²) values for EWR (0.9611) and AI (0.9285) indicate that the proposed models are reliable. The parametric optimization by grey relational analysis (GRA) approach highlighted that the magnitude for EWR and AI is improved by 50.85% and 2.67%, respectively, when optimal condition (I_P: 5 A, S_V: 2 V, S_T: 3 µs, and C_P:1.5 g/100 ml) is set during EDM of Al6061. The proposed EDM model yielded 48.29% and 5.11% better outcomes than the conventional EDM model in terms of EWR and AI, respectively.

Beam-type piezoelectric energy harvesters (PEHs), particularly those utilizing piezoelectric materials, have garnered considerable attention as efficient devices for converting ambient mechanical vibrations into electrical energy. This comprehensive review article thoroughly examines the mathematical models employed in beam-type PEHs, emphasizing their evolution and limitations. The study also delves into both theoretical and experimental analyses of design configurations, placing a special focus on the impact of geometries on energy harvesting efficiency. In conclusion, the paper explores recent advancements and improvements, along with potential avenues for future research, providing a concise overview tailored for professionals and scholars engaged in this specialized field.

Strain sensors have been developed in various fields by converting mechanical deformation into electrical signals. Surface acoustic wave (SAW) devices are beneficial for strain sensing due to their simplicity of fabrication and wireless operation capabilities. In this study, we investigate SAW strain sensors operating at 1.25 $\mathrm{GHz}$. The fabricated SAW resonators using standard photolithography technology are characterized with a custom-made cantilever setup capable of applying defined strain values up to approximately −4000 µε to 4000 µε. From these measurements, a high responsivity even up to this high strain values is demonstrated. We also explore the impact of geometric design parameters on strain-sensing performance. We vary the length of the SAW resonator and observe that the longer the SAW resonator, the more responsive the device gets to strain changes. When the distance between the two reflectors confining the SAW is 2207 $\mu\mathrm{m}$, the responsivity to strain is 114.99 $\mathrm{Hz}\,\mu\epsilon^{-1}$. In summary, this study investigates the feasibility of GHz SAW resonators as high-strain sensors on non-flexible substrates with a custom-built experimental setup, to evaluate their potential for future applications in extreme mechanical environments.

Lift-off process is an alternative to deposition, lithography, and etching of materials. Lift-off is a simple and economical process because it does not require subsequent wet or dry etching. Lifting-off nanometer thick films is a well-developed and repeatable process. However, lifting-off a few micrometer thick films may be challenging. Previously, different techniques were proposed to lift-off micrometer thick films. Herein, a novel method for lifting-off high thickness materials is proposed using a multi-layer AZ 5214E photoresist. The novel method was successful in lifting-off 4 µm thick copper while the copper could even be deposited up to 6 µm with tri-layer AZ 5214E. With four layers of AZ 5214E, the photoresist thickness can be even thicker than 9 µm. As detailed in the study, the photoresist layer thickness can be adjusted by varying the number of layers. This enables the selection of the appropriate number of layers to achieve the desired material thickness. To show the merits of the proposed method, the method is compared to the bi-layer method with AZ 4562 photoresist which is used for lifting-off high thickness materials. In addition to lifting-off thick materials, the proposed method is faster compared to lifting-off using bi-layer AZ 4562. Despite the ability to lift-off thick films, both methods could suffer from lift-off flags if the deposition process is not anisotropic. Solutions to remove the lift-off flags, and reduce the undercut width are demonstrated.

In this work, we report a method that enables a standard electrostatic microelectromechanical system (MEMS) device to perform complex sensing functionalities, such as detecting the presence of helium without a sensing material or a conditioning circuit. Helium is a noble, odorless, non-reactive gas that is very challenging to detect. It is used in critical applications such as storing nuclear fuel waste inside a dry cask. In these applications, its leakage from the dry cask may indicate the cask's safe operation's degradation. A departure from the common practice of exciting the MEMS around its mechanical resonance, the method is based on exciting the MEMS around its electrical resonance circuit. This method shows that the tiny difference between the air dielectric constant (1.000 59) and helium (1.000 067) corresponding to only a few Femtofarad level capacitances produces a 25 mV difference without a conditioning circuit. Simulation results confirmed those findings and explored the sensor response at different operation conditions. This method eliminates the need for a heated microstructure and the need for absorption material. This method is not limited to gas sensing. It can be applied to other sensing mechanisms, such as acceleration and pressure measurements, and eliminate the complex circuit to read small capacitance in these applications.

MEMS environmental sensors, including pressure, gas, and humidity sensors, require protection from mechanical damage, particle exposure, and condensing moisture, while maintaining their ability to exchange gases with the environment. This work introduces a novel packaging approach for MEMS environmental sensors using substrate-embedded filters made from microfine powders through PowderMEMS^® microfabrication technology. The study demonstrates the successful fabrication of gas permeable, functionalized PowderMEMS^® filters on 200 mm Si-wafers for wafer-level packaging of MEMS environmental sensors. Utilizing complete Si-wafers allows for all MEMS sensors on a device wafer to be packaged in a single substrate bonding step, followed by die singulation. The processed wafers are shown to be compatible with high-temperature glass-frit substrate bonding. Alternatively, individual chips with PowderMEMS^® filters can be assembled discretely onto standard semiconductor packages to serve as gas-permeable filters. Successful hydrophobation of the inherently hydrophilic PowderMEMS^® structures by deposition of hydrophobic nanofilms is demonstrated and resistance to water ingress is evaluated by immersion testing. Given that many MEMS gas sensors are cross-reactive to oxidizing gases like ozone, this study also explores the integration of ozone-degrading catalytic powder into the PowderMEMS^® filters. As a proof-of-concept, commercial MEMS ozone sensors are modified with catalytic PowderMEMS^® caps, and successful ozone degradation is demonstrated. While PowderMEMS^® processing is typically conducted on 200 mm Si-wafers, other suitable substrates include glass and (fiber-reinforced) polymers.

Kirigami design principles have been widely applied to develop thermally responsive shape-morphing devices across various scales. However, the multi-degree-of-freedom (multi-DoF) morphing capabilities of microelectromechanical-systems (MEMS)-scale kirigami devices under localized Joule heating remain largely unexplored. This paper presents a quadrant kirigami-type electrothermal MEMS actuator with multi-DoF morphing capabilities, demonstrating both one-dimensional piston motion and two-dimensional (2D) tilting motion. The actuator, fabricated using surface and bulk micromachining techniques, features four independent electrical circuits, creating four electrothermally separated quadrants within the MEMS-scale kirigami structure. The temperature distribution within each quadrant of the actuator, subjected to localized Joule heating, was experimentally examined using thermography, revealing distinct temperature contrasts. The actuator demonstrated multi-DoF morphing capabilities, achieving a 2D tilting angular displacement of approximately 10° at 70 mW and a vertical piston displacement of 1.5 mm at 135 mW. These experimental results validate that the electrothermal quadrant design, leveraging localized Joule heating, enhances the DoF morphing capabilities of kirigami-type electrothermal MEMS actuators.

Residual stresses can be advantageously used to permanently preload flexure micro-mechanisms in order to modify their deflection and stiffness. This paper presents a new preloading chevron mechanism (PCM) used to amplify the preloading effect of thin film residual stress. To evaluate the preloading performances of this structure, the deflection characteristics of buckled beams and flexure linear stages preloaded by a PCM is investigated experimentally. All the mechanisms are manufactured from a monocrystalline silicon substrate using deep reactive ion etching and residual stress is provided by wet thermal oxidation. Measurements show that the deflection magnitude of fixed-fixed oxidized silicon buckled beams can be increased by up to 5 times when a PCM is integrated. The flexure linear stages studied in this research are composed of a parallel leaf spring stage connected to two fixed-guided buckled beams preloaded by a PCM. Depending on the beam dimensions, the stage translational stiffness can be set to a specific value. We designed a near-zero positive stiffness linear stage revealing a measured stiffness reduction of 98%, and a bistable linear stage with a constant negative stiffness region. Thanks to the elevated preloading displacement supplied by the PCM, the operating stroke (actuation region where the stiffness remains constant) is relatively large (more than 0.4 mm travel for 2.59 mm leaf spring length). The analytical and numerical models carried out to design the mechanisms are in good agreement with the experimental data. The results show that the fixed frame stiffness has a significant effect on the preloading performances due to the substantial forces exerted by the PCM. Furthermore, the presented preloading concept, modeling and sizing method could be applied to other compliant mechanism designs, scales and materials, enabling applications in microelectromechanical systems and watchmaking.

Microlens arrays (MLAs) play a prominent role and are essential components in the fabrication of 3D imaging and optical systems. Traditional MLA patterns have limited applications, which prompts the demand for novel MLA patterns for advanced applications. The fundamental objective of this study is to demonstrate different types of MLA fabrication process within our department, employing custom-developed equipment for industrial applications. The three distinctive MLA patterns that were designed and developed are symmetrical, non-symmetrical and step-layer pattern MLAs. MATLAB and Auto CAD 2D software were used to design the MLA patterns, and they were developed on a 4 inch silicon wafer using maskless exposure lithography and thermal reflow techniques. The fabricated master mold acts as a template for replicating pattern molds on a glass wafer. Photoresist ma-p-1275 G and polydimethylsiloxane resin were used for pattern development on silicon wafer and glass wafer, respectively. The fabricated MLA patterns were characterized with an optical microscope and scanning electron microscopy. The fabricated MLA pattern replicas offer durability, longevity, reusability and scalability for large-scale productions. The current study offers a comprehensive approach to in-house production on different types of MLA pattern fabrications.