Abstract The escalating demand for miniature components in high-precision industries continues to drive advances in micro-scale manufacturing technologies. Mechanical micro-machining must accommodate an increasingly wide range of materials at small scales, including hard and brittle systems, which present significant challenges in tool development and process optimisation. This work presents a comprehensive review of recent developments and future directions in the mechanical machining of hard and brittle materials, focusing on three material groups, cemented carbides, ceramic composites, and glass ceramics. Key influencing factors are systematically analysed, including size effects, minimum uncut chip thickness, micro-tool design and materials, ductile–brittle cutting regimes, machining strategies, surface integrity, defect formation, and machine-tool platform characteristics. The review emphasises the importance of understanding fundamental micro-milling principles and accurately identifying cutting regime transitions, both of which are critical for advancing the precision micro-machining of hard materials and guiding future research in tool design, process optimisation, and microscale manufacturing science.
Abstract Microextrusion has emerged as a key microforming technology for manufacturing metallic microcomponents with high dimensional accuracy, superior surface quality, and enhanced mechanical performance. This review critically examines recent developments in metallic microextrusion, emphasizing deformation mechanisms, size effects, tribological behavior, microstructural evolution, and process optimization strategies. Particular attention is given to the influence of grain size, dynamic recrystallization, die geometry, lubrication, and thermal assistance on material flow and product quality. Recent advances in finite-element simulation, crystal plasticity modeling, and data-driven optimization techniques are also reviewed. A comparative analysis of experimental and numerical studies reveals that grain refinement, friction management, and temperature-assisted forming significantly improve extrusion performance. Current challenges associated with size effects, tool wear, microstructural heterogeneity, and process scalability are discussed. Finally, future research directions involving multi-physics simulation, digital twins, sustainable lubrication systems, and intelligent process monitoring are proposed for next-generation microforming applications.
Abstract In heat exchangers, particularly those employing thin-walled metallic tubes, surface texturing improves the heat transfer rate ( Q ). This study introduces a novel method for fabricating internal textures on the tubes using electrochemical micromachining. The process employs localized anodic dissolution, ensuring zero tool wear and eliminating residual stresses. A two-stage machining strategy is implemented, first axial channels are produced via electrochemical milling, followed by radial grooves through electrochemical turning on external surface and electrochemical boring on internal surface, respectively, to form micro-pillar-like structures. An electrochemical turn-mill setup is designed and developed for this purpose. The influence of key parameters such as voltage, electrolyte concentration, tool feed rate, and rotational speed of workpiece on groove and channel geometry is examined. Optimization of axial and angular pitch is performed to obtain uniform pillars and minimize unwanted material removal at pillar crown. Multiphysics simulations in 3 dimensions assist in tool design and prediction of the anode profile. Textures with varying pillar heights ( h p ) are fabricated using optimized conditions. Heat transfer increased with Reynolds number ( Re ) for all tube configurations, with externally textured tubes consistently delivering the highest thermal performance over the investigated Re range. Furthermore, wettability analysis demonstrated an h p dependent transition from hydrophobic to hydrophilic behaviour with time indicating a shift from the Cassie–Baxter to the Wenzel wetting regime.
Abstract This study presents a dual-axis acoustically actuated microelectromechanical system (MEMS) mirror enhanced by a parallel double-layer Helmholtz resonator (HR) array. Acoustic actuation is attractive for MEMS mirrors owing to its structural simplicity and external driving capability; however, the limited energy of sound waves complicates the achievement of large-amplitude and multi-axis actuation. Our proposed device integrates a laser-fabricated glass cantilever mirror with a three-dimensional-printed HR array designed to generate two distinct acoustic-resonance peaks. By tuning these peaks to match the mechanical resonant frequencies of the mirror, the acoustic pressure at each axis is efficiently amplified, facilitating simultaneous dual-axis actuation. Experimental results show that the mirror exhibits dual resonant frequencies of approximately 111 and 168 Hz, with the HR array providing acoustic amplification of up to 34 dB, resulting in projected laser patterns orthogonal in two modes. The proposed approach overcomes the limitations of conventional acoustic actuation and provides an effective strategy for enhancing multiaxis MEMS-mirror performance using acoustic metamaterials.
Abstract We report flexible III-Nitride drum resonators in which actuation and detection are both performed piezoelectrically and integrated monolithically on the same released film. The present devices use either undoped AlGaN/GaN or GaN active layers and combine on-chip piezoelectric actuation with charge-based detection. Simultaneous electrical and optical characterization of the resonance modes yields electromechanical coupling factors up to 4.60 fC/nm and nanometer-scale displacements under sub-volt excitation. We show that AlGaN/GaN heterostructures outperform GaN-only devices and attribute this to the constructive addition of the same-sign AlGaN and GaN piezoelectric coefficients combined with increased geometric stiffness. Optical profilometry and finite-element modelling further identify residual compressive stress as a dominant factor accounting for the higher frequencies of AlGaN/GaN devices relative to GaN. Finally, as a proof of concept, we exploit the resonance frequency-temperature dependence to operate the devices as thermal sensors, achieving a temperature coefficient of frequency (TCF) of ≈ -1500 ppm/°C and a temperature resolution down to ~ 0.015 °C (best device). These results establish integrated piezoelectric III-N resonators as building blocks for flexible sensing, actuation, and operation in harsh environments.
Abstract In engineering, adding lubricating grease at the friction interface offers a promising strategy to simultaneously enhance the electrical output and durability of triboelectric nanogenerators (TENGs). However, current efforts to optimize the electrification performance of grease-lubricated TENGs remain exploratory, lacking quantitative understanding of how structural parameters influence output and durability. In this manuscript, using a rotary freestanding TENG under grease lubrication, we develop two regression-based models: a charge density-dependent contact model and an electrostatic induction model incorporating interfacial temperature rise. Subsequently, numerical algorithms such as the augmented Lagrange method and sparse LU decomposition are used to solve the electromechanical-thermal multiphysics coupled model, and the influence of TENG structural parameters on surface temperature rise and electrification performance under grease lubrication is studied. The results show that the electrostatic force in grease-lubricated conditions can reach 20.3 times that under dry friction, significantly affecting electrification performance; reducing the grating number improves open-circuit voltage but substantially lowers short-circuit current; Increasing the gap ratio enhances short-circuit current, while open-circuit voltage exhibits a non-monotonic trend. Moreover, durability remains unaffected by grating number but declines with increasing gap ratio due to higher interfacial temperature rise and accelerated charge density decay. This study can provide a theoretical basis for the quantitative design of grease-lubricated TENGs with high output and durability.
Abstract Gecko-inspired wedge arrays have been proposed for microfluidic handling of shear-sensitive samples, but the flow and shear characteristics near individual wedge tips under confined laminar conditions remain insufficiently quantified. A polydimethylsiloxane (PDMS) millifluidic channel containing a single row of sharp wedge structures (height 300 μ m, base width 200 μ m, and pitch 400 μ m) is fabricated, and micro-particle image velocimetry is employed to measure the mid-plane velocity and vorticity fields at three water flow rates. The corresponding Reynolds numbers based on the hydraulic diameter are 6.3, 9.5, and 12.6. Under these flow conditions, the estimated elastic deformation of the PDMS wedges is negligible, allowing the structures to be treated as quasi-rigid during the present measurements. Time-averaged velocity fields reveal a persistent low-speed region between adjacent wedges, bounded by higher-speed streams along the wedge sidewalls, while vorticity fields demonstrate tip-scale vortices that feed downstream shear layers. Local in-plane shear rates of 15 – 20 s − 1 at the highest flow rate are estimated from the measured mid-plane velocity gradients, corresponding to order-of-magnitude shear-stress estimates below 0.02 Pa in the low-speed region. These values are not direct measurements of wall shear stress. The results quantify how wedge arrays redistribute local shear by establishing a protected low-shear pathway between neighbouring tips and flanking higher-shear lanes, providing quantitative guidance for the design of wedge-based microchannels for gentle transport and fouling control. Unlike conventional rigid micro-obstacles, however, the compliant nature of PDMS wedge arrays introduces a potential fluid–structure interaction pathway that may become important under stronger hydrodynamic loading. Investigating deformation-mediated flow modulation therefore represents an important direction for future studies.
Abstract Droplet microfluidics is transforming biomedical and life sciences by enabling high-throughput, quantitative analysis of molecules and cells while minimizing reagent consumption. Despite these advantages, the widespread adoption of droplet microfluidics in biological and clinical settings has been hindered by complex device fabrication, as well as external pumps and precisely controlled fluidic systems. To address these challenges, we introduce the integrated microfluidic pipette tip (IMPTip), a simple and robust platform for pump-free generation of highly uniform water-in-oil droplets. The IMPTip produces monodisperse droplets with diameters as small as 30 µ m (coefficient of variation <4%) and supports a broad tuneable size range from 30 to 800 µ m through geometry-driven step emulsification. Droplet formation is largely insensitive to flow rate, maintaining uniformity across a wide operating range of 0.2–50 µ l min −1 , thereby eliminating the need for precise flow control. Bypassing the need for external pumps, continuous-phase oil setups, and time-consuming flow rate tuning under a microscope, the IMPTip offers unprecedented simplicity and ease of use for producing uniform droplets. This approach has the potential to significantly broaden access to droplet microfluidics for researchers in biological and clinical fields, streamlining workflows for applications such as digital polymerase chain reaction, drug screening, single-cell analysis, and early disease diagnosis.
The clinical implant of stem cell-derived retinal pigment epithelium (RPE) monolayer for age-related macular degeneration requires rigorous validation of the monolayer's cellular maturity. Conventional bulk molecular assays lack the sensitivity to resolve cellular heterogeneity and are prone to damaging the RPE monolayer. This study developed and validated an automated nanoliter-scale microfluidic platform for single-cell transcriptomic quality control of engineered RPE monolayers. A compact polydimethylsiloxane -based microfluidic platform was developed to generate monodisperse 10 nL RPE cell cDNA droplets and 90 nL quantitative PCR (qPCR) reagent droplets using dual-focused-flow geometries. Deterministic droplet fusion was achieved via direct current electrocoalescence, enabling ∼100% merging efficiency. An auxiliary co-flow spacing mechanism was implemented to prevent secondary coalescence during downstream transport. Automated droplet collection into oil-filled 96-well plates was achieved through a custom two-dimensional gantry system, and droplet volume consistency was verified using a Python-based computer vision algorithm. Biological validation was conducted using H14 human embryonic stem cell-derived RPE cells targeting β-actin and lineage-specific markers MITF1, MITF2, PEDF, and PMEL17. The platform demonstrated stable generation and deterministic merging of nanoliter droplets, yielding uniform 100 nL reaction volumes suitable for qPCR analysis. Automated volumetric verification confirmed high droplet uniformity and reliability. Gene expression analysis revealed robust detection of housekeeping and lineage-specific genes at nanoliter scales. These findings demonstrated the feasibility of performing nanoliter-scale qPCR using the proposed microfluidic workflow. The workflow enabled reliable detection of extremely low quantities of nucleic acid using a widely available commercial qPCR platform, providing a practical and accessible approach for routine laboratory and translational research applications. The system-maintained assay sensitivity while significantly reducing reagent consumption and sample input. This automated microfluidic platform enabled reproducible, sample-efficient, single-cell transcriptomic validation of stem cell-derived RPE monolayers. By integrating droplet generation, deterministic merging, automated collection, and computational verification, the system addressed key limitations of bulk molecular quality control of RPE monolayers. This work established a scalable and cost-effective nanoliter qPCR framework for high-resolution molecular quality assurance of regenerative cell therapies.
Abstract Microtextured hydrophobic surfaces can significantly reduce interfacial friction and hydrodynamic drag by trapping stable air pockets (Cassie–Baxter state) between surface features and the liquid. In this study, we present a low-cost and scalable method to fabricate microtextured polydimethylsiloxane (PDMS) surfaces via replication of aluminum molds produced by a simple two-step wet etching process, consisting of anisotropic etching followed by isotropic etching. The resulting PDMS surfaces exhibited irregular microstructures with surface roughness ranging from 4.3 to 5.1 µ m, high apparent contact angles up to 136.7°, and air-pocket coverage of up to 30.5%, enabling stable interfacial slip. Friction and drag characteristics were systematically evaluated using a custom dual-beam leaf flexure system and a rotating propeller setup. Compared with smooth PDMS, the microtextured surfaces achieved up to 70% reduction in friction at the solid–solid interface, 32% reduction at the solid–liquid interface, and 20% reduction in underwater drag. The drag-reduction performance remained stable over 120 h of continuous operation. These results demonstrate that microtextured surfaces reduce hydrodynamic resistance through air-layer-induced slip and delayed flow separation, providing a practical strategy for scalable drag-reducing surfaces.
Abstract Nanopocket membranes featuring tapered, conical pores that narrow from top to bottom enabling size-selective particle capture and release, but conventional designs exceed 5 µ m in thickness and require high operating pressures. Here we report a four-step process for fabricating parylene-C thin films with regularly spaced nanopockets, each containing a nanopore. Pores were defined by i-line photolithography, with exposure dose optimized to produce tapered sidewalls and complete photoresist clearing; a second parylene deposition narrowed the openings from ∼900 nm to below 170 nm, forming the nanopocket geometry. Reactive ion etching with O 2 /CHF 3 opened the base while suppressing nanoforest formation. Surface grafting of poly(L-lysine)-poly(ethylene glycol) (PLL–PEG) further reduced the effective pore diameter. Atomic force microscopy measured ∼30 nm brush lengths, consistent with experimentally derived h`ydraulic permeability estimates. Performance was validated in a tangential-flow microfluidic platform using bead mixtures (50 nm–10 µ m), demonstrating passage of small particles, capture of mid-sized particles, and exclusion of large ones. Selective capture and release of liposomes and extracellular vesicles, with simultaneous passage of fluorescent protein, illustrate the platform’s utility for gentle, size-based isolation of nanoparticles.
Abstract Metallic microcomponents can be reliably prepared by microelectroforming using sandwich-structured silver-polydimethylsiloxane (Ag-PDMS) composite molds at low cost. This paper presents the forming mechanism of metallic microcomponents fabricated by microelectroforming using these molds. The results show that Ag microparticles distributed in the molds act like isolated electrodes, and nickel atoms nucleate around them and form tree-ring layers during microelectroforming. Compared with conventional molds consisting of an Ag substrate and photoresist microstructures and with Ag-coated PDMS molds, the sandwich-structured Ag-PDMS composite molds can be reliably reused several times. The microelectroforming process of nickel microcomponents using Ag-PDMS molds is studied by COMSOL numerical simulation software. The simulation results show that the Ag microparticles act like isolated electrodes during the microelectroforming process, efficiently improving the homogeneity of current density distribution in the microcavities of these molds.
Abstract polymethyl methacrylate (PMMA) has been widely used in microfluidics, with the advantages of low cost, easy to process, and optical clarity. PMMA (usually in the form of a plate) can be processed with laser ablation, mechanical milling, hot embossing, and other approaches to form the microstructures and then bonded with another layer of PMMA plate to seal the channel. In the past decade, hundreds of studies have been reported on PMMA-based microfluidics. However, one key piece of information is usually neglected, that is, the PMMA plates used in these studies were fabricated either by casting or extrusion, but this difference is usually overlooked. In this study, we compared the cast and extruded PMMA plates used in microfluidics from various aspects, including optical transparency, surface finishing after laser ablation, and contact angle recovery after surface treatment. After oxygen plasma treatment, the contact angle after recovery is 21° for extruded PMMA and 44° for cast PMMA. PMMA shrinks isotropically in both directions by around 1.8%, while the extruded PMMA plate shrinks anisotropically, with 0.4% shrinkage in one direction and 1.5% in another direction. The surface finishing after laser ablation on cast and extruded PMMA also shows significant differences. We believe this study could be helpful in choosing cast or extruded PMMA plates for PMMA-based microfluidics.
Abstract The calculated impedance spectra agree well with 2D finite-element simulations, while the remaining frequency deviations are attributed to spatial Fourier truncation and local fringing fields near electrode corners. Comparative analysis with a bare-bottom structure shows that the floating Mo layer plays a critical role in improving the effective electromechanical coupling by reorienting the electric field through the piezoelectric thickness. A systematic parametric study further reveals the trade-off among coupling enhancement, mechanical mass loading, and phase velocity. Dispersion analysis indicates that a bottom Mo thickness of 160 nm brings the quasi-symmetric branch close to a zero-group-velocity condition at the Brillouin-zone boundary, thereby improving lateral acoustic confinement. These results establish the proposed model as an efficient design tool for super-high-frequency Lamb wave resonators and provide practical guidelines for floating-electrode composite acoustic devices.
Abstract The thickness of oxide film on zirconium alloy cladding tube is a critical indicator for assessing a nuclear power plant’s operational status. Eddy current sensors (ECSs) are commonly employed to accurately measure this oxide film thickness. However, in practical poolside inspection, the ECS must operate submerged at depths of tens of meters and is connected to signal processing system through long transmission cables, which causes the signal to be heavily contaminated with noise. To address this challenge, for the first time, we report a microfabricated multilayer ECS with noise suppression capability. The 55 µ m-thick sensor comprises four separate coils on different layers yet interconnected by a ground port. This ground port is microfabricated by a step-tapered process to accomplish interlayer connectivity and process simplification. This sensor has 12 different operating modes, since each selected coil for exciting electromagnetic field corresponds to the other 3 pickup coils. The 12 modes provide multiple independent measurement channels for oxide film thickness evaluation, improving redundancy, reliability, and measurement accuracy. When connected to a 30 m transmission cable, the microfabricated sensor exhibits a root-mean-square (RMS)noise voltage of 1.640 mV, which is only approximately one-fourth that of a traditional wound coil-based sensor with equivalent structure. This noise suppression characteristic is primarily attributed to the sensor’s high DC resistance of the microfabricated sensor, which limits the noise currents induced on the coaxial cable shield by electromagnetic interference. The high resistance is due to the thin copper film (1 µ m-thick) in microfabrication process. These 12 calibration curves of the sensor for oxide film thickness measurement all exhibit excellent linearity ( R 2 > 0.98). After multi-mode fusion, the maximum mean absolute error and RMS error across all measurements are 1.2 µ m and 1.4 µ m, respectively. The maximum absolute error is 2.1 µ m, with a mean resolution reaching 2.28 µ m.
Abstract Structure-process co-optimization is essential for sensor design, enabling simultaneous satisfaction of performance specifications and fabrication constraints while reducing development time and experimental cost. However, strong coupling and nonlinear interactions among design parameters make multi-objective optimization highly challenging. This work proposes a data-efficient machine-learning-based structure-process co-optimization framework for the automated synthesis of manufacturable Pirani vacuum sensors. Gaussian process regression serves as a surrogate model to predict key performance metrics, while particle swarm optimization explores the high-dimensional design space. Critical process limitations, including the deep reactive ion etching aspect ratio and buried-oxide-to-heater thickness ratio, are explicitly incorporated to ensure manufacturability. By flexibly configuring optimization objectives, the framework supports scenario-oriented designs, including high-vacuum detection, wide-range pressure sensing, and compact low-power operation. With limited technology computer-aided design simulations, the method rapidly converges to optimized structures that simultaneously satisfy performance targets and fabrication limits, providing a data-efficient and manufacturable solution for application-specific Pirani vacuum sensor design.
We have developed a semi-analytical method based on an extended polynomial approach to model the vibration characteristics of annular microelectromechanical systems (MEMSs) resonators with axisymmetric inner-region (partial) metallization. Due to variations in electrode distribution and electrical boundary conditions, the studied structure is divided into two distinct regions. The model incorporates specific window functions and expands the mechanical displacements and electrical potentials of each region into polynomial series to solve for wave propagation across the entire structure, capturing the differences between the regions. The results obtained using the proposed method through modal and harmonic analyses are validated against existing results from the literature. To evaluate this approach's practical outcomes, the study examines how resonant frequencies, antiresonant frequencies, and dynamic electromechanical coupling coefficients ( kd) depend on the metallization rate. The findings reveal that kd can decrease to zero or increase by up to a factor of 8.47 compared to complete metallization depending on the electrode distribution. Detailed analyses of dispersion curves, impedance response patterns, and mechanical displacement field profiles further support these results, providing comprehensive insights into the electrical and mechanical behaviors of annular piezoelectric plates. These results provide valuable insights for optimizing the design of annular piezoelectric resonators, particularly for MEMS applications.
Abstract Robust millimeter-scale mechanisms have the potential to enable a new generation of end effectors for minimally invasive procedures, aiding surgeons in performing complex tasks with improved precision, repeatability, and dexterity. Despite ongoing advances in fabrication methods, the manufacturing and actuation of complex articulated mechanisms at the millimeter scale involves significant challenges, particularly for integration and assembly. Here, we present the third generation of a complex optoelectromechanical device capable of precisely steering a fiber-delivered surgical laser along two axes of motion. The electromagnetically-driven end effector has been fully redesigned with an emphasis on manufacturability and enabling future high-volume production. The device is 3.8 mm in diameter (4 mm with housing), 10 mm in length, and features ±15 degrees range of motion in both axes with a bandwidth of > 230 Hz. We anticipate this type of end effector to have applications in tissue ablation and excision in endoscopic procedures throughout the gastrointestinal tract.
This paper presents a novel micro-electromechanical system piezoresistive pressure sensor designed for a low-pressure range of 0-2 kPa. The sensor features an innovative square silicon diaphragm incorporating beam-island structures on the front surface and beams on the back surface. This structural configuration effectively concentrates stress in the piezoresistive regions while enhancing local stiffness, thereby achieving high sensitivity and excellent linearity. Finite element method (FEM) simulations were performed to analyze the stress distribution and deflection characteristics of the diaphragm. To efficiently optimize the diaphragm structure, a deep learning-assisted optimization framework combining a multilayer perceptron (MLP) surrogate model with particle swarm optimization (PSO) was developed. The trained MLP captures the nonlinear relationship between structural parameters and sensor performance, while PSO enables continuous and global exploration of the design space to identify optimal configurations. This MLP-PSO framework significantly improves design efficiency and flexibility compared with conventional FEM-based parametric scanning, enabling sensitivity maximization while maintaining low nonlinearity. FEM simulations of the optimized designs were conducted for square diaphragm sizes ranging from 3500 to 3900 mu m, demonstrating consistently high sensitivity and low nonlinearity across the design space. For a diaphragm size of 3795 mu m with a thickness of 16.2 mu m, the proposed sensor achieves a voltage sensitivity of 25.25 mV V-1 kPa-1 and a nonlinearity of 0.12% FS.
