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.

Supercritical carbon dioxide (scCO ₂) is often used to replace harmful solvents and can dissolve a wide range of organic compounds. With a favorable critical point at 31 °C and 7.4 MPa, reaching above the critical point for scCO ₂ is fairly accessible. Because of the compressible nature of scCO ₂ and the large changes of viscosity and density with temperature and pressure, there is a need to determine the behavior of scCO ₂ in microfluidic systems. Here, the influence of how parameters such as flow rate, temperature, pressure, and flow ratio affects the length of parallel flow of water and scCO ₂ and the length of the created CO ₂ segments are investigated and modeled using multivariate data analysis for a 10 mm long double-y channel. The parallel length and segment size were observed in the laminar regime around and above the critical point of CO ₂. The flow ratio between the two fluids together with the flow rate influenced both the parallel length and the segment sizes, and a higher pressure resulted in shorter parallel lengths. Regarding the segment length of CO ₂, longer segments were a result of a higher Weber number for H ₂O together with a higher temperature in the channel.

Polyimide is one of the most popular substrate materials for the microfabrication of flexible electronics, while polydimethylsiloxane (PDMS) is the most widely used stretchable substrate/encapsulant material. These two polymers are essential in fabricating devices for microfluidics, bioelectronics, and the internet of things; bonding these materials together is a crucial challenge. In this work, we employ click chemistry at room temperature to irreversibly bond polyimide and PDMS through thiol-epoxy bonds using two different methods. In the first method, we functionalize the surfaces of the PDMS and polyimide substrates with mercaptosilanes and epoxysilanes, respectively, for the formation of a thiol-epoxy bond in the click reaction. In the second method, we functionalize one or both surfaces with mercaptosilane and introduce an epoxy adhesive layer between the two surfaces. When the surfaces are bonded using the epoxy adhesive without any surface functionalization, an extremely small peel strength (<0.01 N mm ⁻¹) is measured with a peel test, and adhesive failure occurs at the PDMS surface. With surface functionalization, however, remarkably higher peel strengths of ~0.2 N mm ⁻¹ (method 1) and  >0.3 N mm ⁻¹ (method 2) are observed, and failure occurs by tearing of the PDMS layer. We envision that the novel processing route employing click chemistry can be utilized in various cases of stretchable and flexible device fabrication.

The design of MEMS devices employing movable structures is crucially dependant on the mechanical behaviour of the deposited materials. It is therefore important to be able to fully characterize the micromachined films and predict with confidence the mechanical properties of patterned structures. This paper presents a characterization technique that enables the residual stress in MEMS films to be mapped at the wafer level by using microstructures released by surface micromachining. These dedicated MEMS test structures and the associated measurement techniques are used to extract localized information on the strain and Young’s modulus of the film under investigation. The residual stress is then determined by numerically coupling this data with a finite element analysis of the structure. This paper illustrates the measurement routine and demonstrates it with a case study using electrochemically deposited alloys of nickel and iron, particularly prone to develop high levels of residual stress. The results show that the technique enables wafer mapping of film non-uniformities and identifies wafer-to-wafer differences. A comparison between the results obtained from the mapping technique and conventional wafer bow measurements highlights the benefits of using a procedure tailored to films that are non-uniform, patterned and surface-micromachined, as opposed to simple standard stress extraction methods. The presented technique reveals detailed information that is generally unexplored when using conventional stress extraction methods such as wafer bow measurements.

A number of polydimethysiloxane (PDMS) bonding techniques have been reported in the literature over the last several years as the focus on multilayer PDMS microfluidic devices has increased. Oxygen plasma bonding, despite cost, additional fabrication time and inconsistent bonding results, has remained a widely used method for bonding PDMS layers. A comparative study of four rapid, inexpensive alternative PDMS–PDMS bonding approaches was undertaken to determine relative bond strength. These include corona discharge, partial curing, cross-linker variation and uncured PDMS adhesive. Partial curing and uncured PDMS adhesive demonstrated a considerable improvement in bond strength and consistency by retaining average bond strengths of over 600 kPa, which was more than double the average bond strength of oxygen plasma. A description of each technique and their performance relative to oxygen plasma bonding is included.

The development of polymer micromachining technologies that complement traditional silicon approaches has enabled the broadening of microelectromechanical systems (MEMS) applications. Polymeric materials feature a diverse set of properties not present in traditional microfabrication materials. The investigation and development of these materials have opened the door to alternative and potentially more cost effective manufacturing options to produce highly flexible structures and substrates with tailorable bulk and surface properties. As a broad review of the progress of polymers within MEMS, major and recent developments in polymer micromachining are presented here, including deposition, removal, and release techniques for three widely used MEMS polymer materials, namely SU-8, polyimide, and Parylene C. The application of these techniques to create devices having flexible substrates and novel polymer structural elements for biomedical MEMS (bioMEMS) is also reviewed.

In this paper, we describe a novel fabrication method of a microfluidic integrated surface plasmon resonance (SPR) gold chip based on a (3-mercaptopropyl) trimethoxy silane (MPTMS) self-assembled monolayer. This monolayer was formed at the surface of a microfluidic chip made of polydimethylsiloxane (PDMS). Its presence was confirmed by contact angle and Fourier transform infrared spectroscopy measurements on the modified PDMS surface. A basic, but nevertheless appropriate, 4-channel microfluidic system was made on PDMS and reported on a gold SPR sensor. Sealing tests were carried-out by injecting continuous flows of solutions under gradient pressure up to 1.8 bar. Bonding strength of chemical and corona binding were measured and compared. The test of the integrated microfluidic SPR sensor on an SPR bench validated its functionality and proved as well that no leakage is observed between the different microfluidic channels.

The mechanical properties of SU-8 at microscale were measured under both micropillar compression and nanoindentation on a film on a substrate. To the best of our knowledge, this paper reports the first set of results for microcompression of SU-8 micropillars for measurement of mechanical properties using viscoelastic analysis. The effects of loading rate and micropillar size are examined. It was determined that the SU-8 exhibits viscoelastic properties at room temperature, the time-average Young’s modulus increases in general with the loading rate. The average Young’s modulus determined by compression of a micropillar was 4.1 GPa at a strain rate near 10 ⁻³ s ⁻¹. For nanoindentation on a SU-8 film supported by a silicon substrate, the default output from the nanoindenter for the Young’s modulus was approximately 6.0 GPa with the consideration of elastic–plastic behavior of the SU-8. When the viscoelastic effects were considered, the time-average Young’s modulus at a given strain rate was determined to be near 3.6 GPa, which agrees with the reported values in the literature obtained from tension and bending, and also correlates reasonably well with data from microcompression. This work indicates that viscoelastic analysis is necessary to extract the valid mechanical properties at nano/microscales for SU-8.

Analyte mixing and delivery to a functionalized sensor surface are important to realize several advantages associated with biosensors integrated with microfluidic channels. Here, we present a comparison between a herringbone structure (HBS) and a curved passive mixing structure of their efficiency at facilitating mixing and surface saturation using fluorescein included in one of the inlets of a Y-channel microfluidic device. We performed a large parametric study to assess the effects of varying the height of the microfluidic channel as well as the height, width, and spacing of the passive mixing structures. Scanning confocal microscopy combined with a custom-designed image-analysis procedure were utilized to visualize and quantify the observed changes in efficiency in inducing solute mixing by the different designs. The flow patterns within the channels were found to vary significantly with changes in the geometry of the passive mixing structures, which in turn affected the efficiency of the channel at mixing the fluid and saturating the surface opposite the mixing structures. The solute mixing as a function of the channel length was also determined; an initial slow mixing rate does not always coincide with a low mixing index (MI). We found that the range of MIs for the curved mixing structure 1 cm downstream from the inlet was 0.85–0.99 whilst for our HBS it was 0.74–0.98, depending on the design parameters of the passive mixing structures. Overall, this study shows that the curved passive mixing structure family is more robust in inducing efficient mixing than the HBSs.

In recent decades, micromachined ultrasonic transducers (MUTs) have been investigated as an alternative to conventional piezocomposite ultrasonic transducers, primarily due to the advantages that microelectromechanical systems provide. Miniaturized ultrasonic systems require ultrasonic transducers integrated with complementary metal-oxide-semiconductor circuits. Hence, piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs) have been developed as the most favorable solutions. This paper reviews the basic equations to understand the characteristics of thin-film-based piezoelectric devices and presents recent research on pMUTs, including current approaches and limitations. Methods to improve the coupling coefficient of pMUTs are also investigated, such as device structure, materials, and fabrication techniques. The device structure improvements include multielectrode pMUTs, partially clamped boundary conditions, and 3D pMUTs (curved and domed types), where the latter can provide an electromechanical coupling coefficient of up to 45%. The piezoelectric coefficient ( e ₃₁) can be increased by controlling the crystal texture (seed layer of γ-Al ₂O ₃), using single-crystal (PMN-PT) materials, or control of residual stresses (using SiO ₂ layer). Arrays of pMUTs can be implemented for various applications including intravascular ultrasound, fingerprint sensors, rangefinders in air, and wireless power supply systems. pMUTs are expected to be an ideal solution for applications such as mobile biometric security (fingerprint sensors) and rangefinders due to their superior power efficiency and compact size.

Wide-range humidity sensing and monitoring applications including instrumentation, agriculture, meteorology, biomedicine, and food processing have attracted long-standing interests, where recently substantial progress is made in both sensing-material science and microfabrication technologies to achieve portable, reliable and low-cost humidity sensing instruments. Due to their high sensitivity, enormous miniaturization potential, and well-developed high-volume microfabrication technologies, microelectromechanical systems (MEMS)-based piezoresistive cantilever devices covered by large-surface-area nanostructures of hygroscopic materials offer an ideal platform for highly sensitive humidity detection.

Since resonant gravimetric sensing is the dominant humidity sensing technique in recent research works, in this paper, resonant actuation principles for microcantilevers (i.e. the dynamic operation mode) are addressed and compared with respect to the quality of the amplitude and phase signals, as required for on-line frequency tracking using a phase-locked loop circuit. Parasitic feedthrough effects are considered between the resonance-mode ( f  ₀) excitation element and the piezoresistive detection circuit, which can lead to a reduction of stop-band attenuation, the generation of a parallel resonance in close vicinity of f  ₀, a hardly detectable 90° phase jump, and a long-term drift of resonance frequency and phase shift. Methods for eliminating these parasitic feedthrough effects have been considered, including de-embedding of the motional signal by later data processing and the integration of a reference cantilever or circuit.

Then, different concepts of environmental sensing using microcantilevers are described, including detection of particulate matter and gas molecules/volatile organic compounds. Depending on the condition of the cantilever during sensing operation, two different modes have been used to sense the target analyte (i.e. static and dynamic modes). In a static operation mode, mass change of the cantilever, surface stress, or swelling of a layer on top related to the uptake and binding of particles or molecules on the cantilever are detectable via a deformation of the cantilever (i.e. by deflection or strain), which can be sensed by an integrated piezoresistive strain gauge. Quasistatic bending of the cantilever as well as frequency down-shift of an excited resonance mode are normally used for detection.

Humidity adsorption/desorption characteristics of such piezoresistive microcantilevers can be modified using hygroscopic layered materials deposited on the cantilever, among which we address metal oxides, ceramics, organics, or organic/inorganic composites. These thin layers comprise preferentially concave or convex nanostructures (e.g. pores, particles, colloids, rods, or fins), which provide a sensing surface of large surface-to-volume ratio and thus a large number of binding sites for highly efficient adhesion of water molecules.

Finally, fabrication processes of integrated piezoresistive microcantilever-based humidity sensors, including micromachining/MEMS technology, integration of nanostructures and their combination with deposited hydrophilic materials are described. Lastly, their humidity sensing performance is compared with competing state-of-the art and advanced MEMS devices, e.g. capacitive micromachined ultrasonic transducers, quartz crystal microbalance, thin-film bulk acoustic resonator, surface acoustic wave resonator and complementary metal oxide semiconductor-MEMS for gravimetric sensing with respect to, e.g. sensitivity, hysteresis, and response time.

Microchannels formed in non-conductive substrates like fused silica, glass and quartz, etc, have wide applications in the field of micro-fluidic and lab-on-chip applications due to their optical transparency, chemical inertness, and biocompatible nature. Electrochemical discharge machining (ECDM) has emerged as a potential low-cost fabrication method to fabricate microfeatures in these materials, compared to conventional laser etching techniques. In this paper, numerical simulation and experimental fabrication of microchannels in a glass substrate using the ECDM based micromilling technique is demonstrated. Stainless steel needle as tool electrode is used in alkaline electrolyte medium. The effects of process parameters viz. tool feed rate, pulse frequency and machining voltage on material removal rate (MRR) and surface roughness (SR) of the microchannels were analysed. The experimental results showed that the MRR and SR increases with an increase in machining voltage and tool feed rate but reduces with an increase in the pulse frequency. Simulations using FEM-based model showed similar trends in MRR with that of experiments. A comparison between the cross-section profiles obtained by the experimental work and predicted profile by the numerical simulation showed some deviation between them due to the Gaussian heat flux assumption in the numerical model. Optical images showed that KOH performance is comparatively better than NaOH with respect to thermal damage and width of cut. Further, multi-objective optimization was performed using utility theory coupled with Taguchi’s method to optimize the process parameters. Moreover, the capability of the ECDM process was demonstrated in fabricating various other micro-features such as sinusoidal channel, letter engraving, etc in a glass substrate, which can be extended to other brittle materials like quartz, fused silica, ceramic, etc.

Shape memory alloys (SMAs) have the potential to be used for a wide variety of microelectromechanical systems (MEMS) applications, providing a unique combination of large deflections and high work output. A major drawback for SMAs in many applications has been the low frequency response, which is typically on the order of 100 Hz or lower, even in microscale SMA actuators. In MEMS applications, the higher surface-to-volume ratios have enabled responses to be improved by an order or magnitude or more. By further shrinking the SMA film/device dimensions, the frequency response may be improved even further. In this paper, we present a new, simplified process for fabricating sputtered, thin film SMA MEMS actuators based on nickel-titanium alloy (NiTi or, aka, NITINOL) that consisted of only one photo step to pattern the actuators using SU8. When heated through its solid–solid phase transition from low-temperature martensite to high-temperature austenite, the NiTi alloy undergoes changes in associated physical properties, such as Young’s modulus, resistivity, and surface roughness, that are critical to controlling MEMS performance. For example, these material property changes allow for the design of active or passive microscale sensors and actuators. In the new process, we are able to fabricate ultrathin films of NiTi with nanoscale thickness, which can be thermally cycled through two stable positions very rapidly, making it an intriguing thermal sensor and actuator material for high frequency applications. Additionally, NiTi can be used as an active thermal switch through resistive (i.e. joule) heating. We demonstrated a greatly improved frequency response of up to 3000 Hz with turn on voltages as low as 0.5 V (corresponding to only 1 mW power consumption) for devices exhibiting microns of cantilever tip deflection over millions of cycles, indicating these new SMA MEMS actuators have potential application for low voltage switching, modulation and tuning.

In recent years, research and development of high-efficiency perovskite solar cells (PSCs) have gained momentum. However, the inherent issues of high-temperature stability and hysteresis have constrained the device from commercial feasibility. Researchers have proposed different electron transport layer (ETL) based PSCs to minimize the aforesaid issues. Recently, reduced cerium oxide/[6,6]-phenyl-C61-butyric acid methyl ester (CeO _x /PCBM ETL) based PSC device is developed with power conversion efficiency (PCE) of 16.85% and improved stability. In the present work, CeO _x /PCBM ETL based PSC device is simulated and calibrated to provide the scope for further improvement in terms of the overall conversion efficiency of the device. The device is further optimized by parametric variation such as doping and thickness of CeO _x /PCBM ETL layers. The optimized device with added carbon nanotubes CNTs (to enhance moisture stability) is employed in the monolithic tandem solar cell, and the efficiency potential of a monolithic, hysteresis and moisture free perovskite/crystalline silicon heterojunction (c-Si HJ) tandem solar cell is investigated. Silicon-based, i.e. hydrogenated p-type microcrystalline silicon oxide ( µc-Si _{1− x} O _x :H) and hydrogenated n-type amorphous silicon tunnel junction (TJ) is used to model the TJ between two diodes. Comprehensive analysis and optimization of the tandem device are done in terms of optical and electrical performance with different thicknesses of perovskite and c-Si. The tandem device proposed in this work yielded a maximum PCE of 23.08%.

This work presents design, fabrication and optimization of methanol concentration and flow channel cross-sectional geometry for enhanced power output in passive micro-direct methanol fuel cells. Passive micro-direct methanol fuel cells are fabricated with flow channels in silicon having both rectangular and trapezoidal cross-sectional geometry for flow of methanol at anode and air at cathode using microelectromechanical systems (MEMS) fabrication technique. The experiments are conducted at 25 °C by feeding methanol with a flow rate of 25 μl min ⁻¹ and supply of air at cathode by air-breathing method. Results show a peak in open circuit voltage and power density at 7 M methanol concentration for passive micro-direct methanol fuel cells having both rectangular and trapezoidal cross-sectional geometry. A study of influence of silicon flow channel cross-sectional geometry on passive micro-direct methanol fuel cell performance shows for the first time that the flow channels with trapezoidal cross-section enhance the power density (6.64 mW cm ⁻²) nearly by a factor of two compared to that of flow channels with rectangular cross-section (3.9 mW cm ⁻²) at 7 M methanol concentration. We believe that, though our results of significant enhancement of power density with trapezoidal fuel flow channels are obtained with micro-direct methanol fuel cells as a platform, they should also be applicable to other proton exchange membrane fuel cells with ethanol or humidified hydrogen as fuel.

Fine manipulation of particles is essential for the analysis of complex samples such as blood or environmental water, where rare particles of interest may be masked by millions of others. Inertial focusing is amongst the most promising techniques for this task, enabling label-free manipulation of particles with sub-micron resolution at very high flow rates. However, the phenomenon still remains difficult to predict due to the focus position shifting in tortuous ways as function of the channel geometry, flow rate and particle size. Here, we present a new line of microfluidics that exploit inertial focusing in high aspect ratio curved (HARC) microchannels and overcome this limitation. Consisting of a single curved channel, HARC systems provide a highly predictable, single focus position near the centre of the inner wall, largely independent of the flow rate and particle size. An explanation of the mechanism of migration and focus of particles, together with its governing equations, is provided based on simulations in COMSOL Multiphysics and experimental results. HARC microchannels built in silicon-glass were used for experimental validation, achieving a high quality, single focus position for a range of microparticles with sizes of 0.7–1 µm and bacterial cells (Escherichia coli). The recovery of 1 µm particles was 99.84% with a factor 4 in concentration. With a stable focus position, we envision that HARC systems will bring the technology closer to implementation in laboratories for analysis of complex fluids with biological particles like cells and organelles.

A lot of mechanical energy is lost in the braking process of automobiles, and it is feasible to harvest the energy and power electronic devices by using the technology of a triboelectric nanogenerator (TENG). In this study, we propose a pulsed freestanding TENG (PF-TENG) with a grid structure to harvest mechanical energy in the braking process, and use electric brushes to achieve a unidirectional pulsed output. We also design a passive power management circuit (P-PMC) to process and store the energy output. First, the general analytical solutions of the open-circuit voltage, short-circuit charge and output capacitance are derived from the theoretical model. According to the simulation results by using the finite element method, it is proved that the output energy of PF-TENG is independent of the grid number and proportional to the rotation speed. Second, the circuit simulation results demonstrate that P-PMC achieves impedance matching with the PF-TENG. The output power can maintain the maximum value under a wide load range and the optimal conversion efficiency is 94%. It is also found that the charging speed increases when the inductance value or the capacitance value decreases. In addition, a large pulse width makes the PF-TENG discharge completely. Finally, it is demonstrated that the PF-TENG can be used as a sensor to detect the wear depth of the brake pad with the open-circuit voltage.

Analyte mixing and delivery to a functionalized sensor surface are important to realize several advantages associated with biosensors integrated with microfluidic channels. Here, we present a comparison between a herringbone structure (HBS) and a curved passive mixing structure of their efficiency at facilitating mixing and surface saturation using fluorescein included in one of the inlets of a Y-channel microfluidic device. We performed a large parametric study to assess the effects of varying the height of the microfluidic channel as well as the height, width, and spacing of the passive mixing structures. Scanning confocal microscopy combined with a custom-designed image-analysis procedure were utilized to visualize and quantify the observed changes in efficiency in inducing solute mixing by the different designs. The flow patterns within the channels were found to vary significantly with changes in the geometry of the passive mixing structures, which in turn affected the efficiency of the channel at mixing the fluid and saturating the surface opposite the mixing structures. The solute mixing as a function of the channel length was also determined; an initial slow mixing rate does not always coincide with a low mixing index (MI). We found that the range of MIs for the curved mixing structure 1 cm downstream from the inlet was 0.85–0.99 whilst for our HBS it was 0.74–0.98, depending on the design parameters of the passive mixing structures. Overall, this study shows that the curved passive mixing structure family is more robust in inducing efficient mixing than the HBSs.

In this study, an image inspection method was introduced to two-arm Archimedean spiral antenna patterns to quantify and qualify their in-line integrity, which was linked to their off-line electrical characteristics in terms of the capacitance values through machine learning. The pattern was intentionally deteriorated in shape to imitate potential fabrication variations existing in the microelectronic production line, and six physical features including the inner line edge roughness (LER), outer LER, integrated LER, inner arm length, outer arm length, and arm area were collected. Two groups of training and testing samples were simulated and fabricated. Based on Gaussian process regression with the covariance function in the form of a squared exponential, a model was developed to predict the capacitance values from the performances of the six features. The accuracy of the developed model was evaluated using the coefficient of determination and root-mean-square error. The results indicate that the developed model is capable of predicting the off-line electrical characteristics of microelectronic components based on their in-line pattern integrities. Advanced studies also reveal that although all LER values and arm lengths contribute to the electrical characteristics, the arm area is decisive.

This paper develops a novel processing method of plasma electrolytic oxidation-assisted micromilling (PEOAM). Electrolyte solutions with KH₂PO₄, NaAlO₂, or Na₂SiO₃ as the main component are designed, and three types of oxide films are grown on the surface of a Ti6Al4V alloy in situ by means of plasma electrolytic oxidation. The morphology and composition of the oxide films are characterized by scanning electron microscope and energy dispersive spectrum. Additionally, the cutting force and surface roughness of PEOAM are measured by dynamometer and white light interferometer, respectively. A comparison between PEOAM and conventional micromilling in terms of cutting force, tool wear, chips, and surface roughness is conducted, with results showing that oxide films with about 20 μm thickness are loose and porous, their hardness decreasing to a minimum of 1.12 GPa, which corresponds to 23.3% of the original hardness value. At the axial cutting depth of 18 μm, compared to the F_x, F_y, and F_z values of the Ti6Al4V alloy substrate, the average milling forces of the NaAlO₂ oxide film are the most significantly reduced (35.1%, 15.7%, and 94.8% of the original values, respectively). At the axial cutting depth of 25 μm, the surface roughness (R_a) value of PEOAM is reduced by 0.08–0.12 μm. Consequently, under the same cutting parameters, PEOAM can effectively reduce the cutting force, prolong the service life of the tool, and improve surface quality.

Applying an external electric field over a polarizable electrode or object within microchannels can induce an electric double layer (EDL) around channel walls and create induced-charge electrokinetics (ICEK) within channels. The primary consequence of the induced charge is the generation of micro-vortices around the polarizable electrode or object, presenting great potential for various microfluidic applications. This review presents the advances in theoretical, numerical and experimental studies on the physics and applications of ICEK within microfluidics. In particular, the characteristics and performance of ICEK-based microfluidic components in active micromixers, micropumps, and microvalves are critically reviewed, followed by discussing the applications of ICEK in electrophoresis and particle/cell manipulation within microfluidics. Furthermore, the opportunities and challenges of ICEK-based microfluidic devices are highlighted. This work facilitates recognizing deliverable ICEK-based microfluidic technologies with unprecedented functionality for the next generation of biomedical applications with predictable manufacturability and functionality.

The development of microelectrode arrays (MEAs) along with complementary advances in electronics, mechanics and software to connect with these arrays has led to the in vitro interfacing and benchtop electrophysiological models of several electrically active cells such as neurons and cardiomyocytes proving vital models and testing of human disease conditions in a dish/on a chip. This topical review deals with the micro/nanofabrication technology development of Microelectrodes Arrays from early silicon based developments to today’s additive manufacturing technologies that have been employed to address bio-micro-electro-mechanical systems tool development in this space. Specifically 2D and 3D MEAs technologies have been reviewed in this paper along with a broad overview of some of the biological applications using these devices that are advancing the very state of biomedical research.

Micro/nano-electroforming has received considerable interest from various industry sectors as an advanced micro-manufacturing technique that offers high dimensional precision and replication accuracy. When tight feature tolerances and miniaturized geometries are required, electroforming provides unique advantages and cost-effective characteristics for fabrication. This paper firstly reviews the historical development of micro-electroforming, particularly within the past two decades. The fundamentals of electroforming and relevant applications are firstly discussed, and the common requirements are then proposed. Based on these requirements, we have focused on the processes of micro-electroforming from the ultraviolet lithography, electroplating, and moulding process to electroforming process characterization and optimization, bath compositions, agitation, and some hybrid processes. Progress from electrochemical micro/nano-manufacturing processes, such as 3D printing by electrodeposition and electrochemical wet stamping, is also included. The eventual nanocomposite electroforming is highlighted from the perspectives of the formation mechanism, deposition properties and relevant applications. Finally, a conclusion and future perspectives are presented. This review will demonstrate how the micro/nano-electroforming process can be further developed to meet the requirements of new product development from precision optics, micro/nano-moulding, high-performance coatings and future micro/nano-manufacturing.

Micro-electromechanical-system (MEMS) based actuators, which transduce certain domains of energy into mechanical movements in the microscopic scale, are increasingly contributing to the areas of biomedical engineering and healthcare applications. They are enabling new functionalities in biomedical devices through their unique miniaturized features. An effective selection of a particular actuator, among a wide range of actuator types available in the MEMS field, needs to be made through the assessment of many factors involved in both the actuator itself and the target application. This paper presents an overview of the state-of-the-art MEMS actuators that have been developed for biomedical applications. The actuation methods, working principle, and imperative features of these actuators are discussed along with their specific applications. An emphasis of this review is placed on temperature-responsive, electromagnetic, piezoelectric, and fluid-driven actuators towards various application areas including lab-on-a-chip, drug delivery systems, cardiac devices and surgical tools. It also highlights the key issues of MEMS actuators in light of biomedical applications.

Micro- and nano-electromechanical systems (M/NEMS) have demonstrated outstanding sensing capabilities down to the yoctogram ( ) scale in vacuum environment and cryogenic temperatures. In order to bring such extraordinary resolution levels into the study of biological processes, suspended microchannel resonators (SMRs) have been developed. SMRs are hollow devices allowing for fluidic confinement inside the body of the resonator, which can thus be kept in dry environment or encapsulated in vacuum. Analyte binding and flow-through experiments can be performed, these latter enabling single-cell analysis. In this paper, we survey the progress of over the past 20 years in the field of SMRs. We review the main fabrication, transduction and packaging strategies. We also provide an insight into the working principle of the sensors and their applications to microfluidics and biology.

Analyte mixing and delivery to a functionalized sensor surface are important to realize several advantages associated with biosensors integrated with microfluidic channels. Here, we present a comparison between a herringbone structure (HBS) and a curved passive mixing structure of their efficiency at facilitating mixing and surface saturation using fluorescein included in one of the inlets of a Y-channel microfluidic device. We performed a large parametric study to assess the effects of varying the height of the microfluidic channel as well as the height, width, and spacing of the passive mixing structures. Scanning confocal microscopy combined with a custom-designed image-analysis procedure were utilized to visualize and quantify the observed changes in efficiency in inducing solute mixing by the different designs. The flow patterns within the channels were found to vary significantly with changes in the geometry of the passive mixing structures, which in turn affected the efficiency of the channel at mixing the fluid and saturating the surface opposite the mixing structures. The solute mixing as a function of the channel length was also determined; an initial slow mixing rate does not always coincide with a low mixing index (MI). We found that the range of MIs for the curved mixing structure 1 cm downstream from the inlet was 0.85–0.99 whilst for our HBS it was 0.74–0.98, depending on the design parameters of the passive mixing structures. Overall, this study shows that the curved passive mixing structure family is more robust in inducing efficient mixing than the HBSs.

In this paper, we describe a novel fabrication method of a microfluidic integrated surface plasmon resonance (SPR) gold chip based on a (3-mercaptopropyl) trimethoxy silane (MPTMS) self-assembled monolayer. This monolayer was formed at the surface of a microfluidic chip made of polydimethylsiloxane (PDMS). Its presence was confirmed by contact angle and Fourier transform infrared spectroscopy measurements on the modified PDMS surface. A basic, but nevertheless appropriate, 4-channel microfluidic system was made on PDMS and reported on a gold SPR sensor. Sealing tests were carried-out by injecting continuous flows of solutions under gradient pressure up to 1.8 bar. Bonding strength of chemical and corona binding were measured and compared. The test of the integrated microfluidic SPR sensor on an SPR bench validated its functionality and proved as well that no leakage is observed between the different microfluidic channels.

A novel type of microfluidic absorbance cell is presented here that inlays black poly(methyl methacrylate) (PMMA) into a clear PMMA substrate to realize an isolated optical channel with microlitre volumes. Optical measurements are frequently performed on microfluidic devices, offering effective, quick, and robust chemical analysis capabilities on small amounts of sample. Many lab-on-chip systems utilize micrometer-sized channels to analyze liquid samples via light-absorbance measurements, but this requires sophisticated coordination of light through a small cross-section, often requiring collimating and beam-steering optics. Here, we detail the fabrication process to realize long path length absorbance cells based on a simple hybrid-material approach. A z-shape microchannel structure crosses a clear-black interface at both ends of the absorbance cell, thereby creating integral optical windows that permit light coupling into a microchannel completely embedded in black PMMA. Furthermore, we have integrated v-groove prisms on either side of the microfluidic channel. The prisms enabled seamless integration with printed circuit boards and permit the optical elements to be located off-chip without use of epoxies or adhesives. Three path lengths, 10.4, 25.4, and 50.4 mm, were created and used to characterize the novel cell design using typical colorimetric measurements for nitrite and phosphate. We compare the attenuation coefficient measured by our optical cells with the literature, showing excellent agreement across nutrient concentrations from 50 nM–50 μM. The measurements were performed with well-known reagent-based methods, namely the Griess assay for nitrite and the molybdovanadophosphoric acid or the ‘yellow method’ for phosphate. The longest 50.4 mm path length cell had a limit-of-detection of 6 nM for nitrite and 40 nM for phosphate, using less than 12 μl of fluid. The inlaid fabrication method described permits robust and high-performance optical measurements with broad applicability for in situ marine sensors and for numerous lab-on-chip sensors based on colorimetric assays. One such application is shown whereby two inlaid absorbance cells are integrated with four microfluidic check valves to realize a complete lab-on-chip nitrite sensor.

Traditionally, polymeric microcantilevers are assembled by a multitude of process steps comprising liquid spin-coated photoresists and rigid substrate materials. Polymer microcantilevers presented in this work rely instead on commercially available dry film photoresists and allowed an omittance of multiple fabrication steps. Thin, 5 μm thick dry film photoresists are thermally laminated onto prepatterned silicon substrates that contain AFM compatible probe bodies. Partially suspended dry film resists are formed between these probe bodies, which are patterned to yield microcantilevers using conventional photolithography protocols. A limited amount of thermal cycling is required, and sacrificial probe-release layers are omitted as microcantilevers form directly through resist development. Even 1 mm long polymeric cantilevers were fabricated this way with superior in-plane alignment. The general effects of post-exposure bake (PEB) and hardbake protocols on cantilever deflection are discussed. Generally, higher PEB temperatures limit out-of-plane cantilever bending. Hardbake improved vertical alignment only of high-PEB temperature cantilevers, while surprisingly worsening the alignment of low-PEB temperature cantilevers. The mechanism behind the latter is likely explained by complex interactions between the resist and the substrate related to differences in thermal expansion, heat conduction, as well as resist cross-linking gradients. We present furthermore multilayer structures of dry film resists, specifically cylindrical dry film resist pillars on the polymer cantilever, as well as the integration of metal structures onto the polymer cantilever, which should enable in future integrated piezoresistive deflection readout for various sensing applications. Finally, cantilever spring constants were determined by measuring force–displacement curves with an advanced cantilever calibration device, allowing also the determination of both, dry film resist cantilever density and Young’s modulus.

Vibrational modes of higher order in micromachined resonators exhibit low damping in liquid environments, which facilitates accurate sensing even in highly viscous liquids. A steady increment in mode order, however, results in sound dissipation effects at a critical mode number n _crit, which drastically increases damping in the system. Basic understanding in the emerging of sound dissipation in micromachined resonators is therefore of utmost importance, when an application of higher mode orders is targeted. For that reason, we experimentally investigated in this paper the appearance of sound dissipation in higher order non-conventional vibrational modes in MEMS plate resonators in liquids. The results are compared to those of an analytical model and of finite element method analyses. Micromechanical piezoelectric resonators were fabricated and characterized in sample fluids with a dynamic viscosity μ _fluid ranging from 1 to 5 mPa s and density values ρ _fluid ranging from 0.774 up to 0.835 kg l ⁻¹. Quality factors up to 333 are obtained for the eighth mode order in model solution with a dynamic viscosity of 1 mPa s. By monitoring the resonance and damping characteristics as a function of mode order, sound dissipation effects occur, observed by the detection of increased damping, starting at mode number n = 8, which is in good agreement to the predictions of an analytical model and to finite element method simulations. At the critical mode number n _crit, a reduction in quality factor up to 50% is measured. The results show a direct correlation of n _crit and the density of the fluid, which agrees to theory. The lowest value of 8 for n _crit is obtained in a sample liquid with the lowest density value of 0.774 kg l ⁻¹, followed by n _crit = 9 in a sample liquid with ρ _fluid = 0.782 kg l ⁻¹ and n _crit = 10 in a sample liquid with ρ _fluid = 0.835 kg l ⁻¹. These findings are of particular interest for sensing applications in low dense liquids, as sound dissipation effects emerge even at lower mode numbers.

This paper reports on frequency tunable MEMS magnetoelectric (ME) sensors. Different designs are studied in respect to ME voltage coefficient and frequency tunability. Compared to state-of-the-art ME sensors, the presented ME resonators display a highly reversible and linear frequency tuning, enabled by applying a DC voltage to piezoelectric actuators. A frequency shift of up to 0.2 Hz V ⁻¹ is demonstrated for a sensor with a limit of detection of 128 pT/Hz ^0.5 at resonance frequency of 13 kHz. This sensor type is of particular interest for vector field sensors and sensor arrays in bio-magnetic applications, where sensors with either identical resonance frequencies or precisely defined frequency spacing are required.

Suspended microfluidic resonators enable detection of fluid density and viscosity with high sensitivity. Here, a two-legged suspended microchannel resonator that probes pico-litres of liquid is presented. The higher resonant modes (flexural and torsional) were explored for increased sensitivity and resolution. Unlike other reported microchannel resonators, this device showed an increase in the quality factor with resonant frequency value. The performance of the resonator was tested by filling the channel with three liquids, one at a time, over a density range of 779−1110 kg m ⁻³ and a viscosity range of 0.89−16.2 mPa s. The highest resolution obtained was 0.011% change in density. Measurements with torsional mode showed an improvement of about six times in sensitivity and about fifteen times in resolution compared to the first flexural mode. When the empty channel was filled with liquids of different viscosity, the quality factor of the first flexural mode remained overall constant with a variation below 3.3% between the fluids, and confirming the inherent property of suspended microchannel resonators. However, it significantly decreased for second flexural and torsional modes. No noticeable difference was observed in the quality factor between different liquid viscosities for all modes.

Light patterned electrical fields have been widely used for the manipulation of microparticles, from cells to microscopic electronic components. In this work, we explore a novel electromechanical phenomenon for particle focusing and sorting where the electrical field patterns are shaped by a combination of the light patterned photoconductor and the channel geometry. This effect results from the combination of particle polarisation described by the Clausius–Mossotti relation and the engineering of large electric gradients produced by choosing the channels height to suit the size of the particles being manipulated. The matched geometry increases the distortion of the field created by a combination of the illuminated photoconductor and the particles themselves and hence the non-uniformity of the field they experience. We demonstrate a new channel integration strategy which allows the creation of precisely defined channel structures in the OET device. By defining channels in photoresist sandwiched between upper and lower ITO coated glass substrates we produce robust channels of well controlled height tailored to the particle. Uniquely, the top substrate is attached before photolithographically defining the channels. We demonstrate versatile control using this effect with dynamically reconfigurable light patterns allowing the retention against flow, focusing and sorting of micro particles within the channels. Contrary to traditional designs, this channel integrated device allows patterned micro channels to be used in conjunction with conductive top and bottom electrodes producing optimal conditions for the dielectrophoretic manipulation as demonstrated by the rapid flow (up to 5 mm s ⁻¹) in which the particles can be focused.

The purpose of this work was to demonstrate the technical feasibility for the fabrication of microgrooves or micropits on dental implants or dental implant abutment surfaces using a novel fabrication method derived from common UV-lithographic microfabrication. Instead of using a flat and rigid chromium/glass mask to structure a photoresist layer on a small cylindrical part, a flexible chromium-coated polymer mask was introduced into the lithographic setup. Through an elastic deformation of the polymer mask, it was possible to achieve lateral resolutions as small as 1.5 µm on small cylinders and to structure conical parts. By subsequent controlled under-etching of the structured photoresist layer, microgrooves of different cross-sectional geometries can be generated and applied to the implant or implant abutment surface. Such structures can be used for contact guidance of human gingival fibroblasts or endothelia cells to enhance the wound healing process and the overall soft-tissue integration.

Magnetic fluids have been around since the 1940s. They come in different forms: magnetorheological fluids (MR fluids) and ferrofluids. MR fluids characterise themselves by having a large change in viscosity under the influence of a magnetic field. Ferrofluids have a significantly smaller change in viscosity, however ferrofluids are colloidal suspensions. After their discovery many applications followed, such as the MR clutch, magnetic damper and bearing applications, in which the fluids are subjected to ultra high shear rates. Little information is available on what happens to the rheological properties under these conditions. In general, the characteristics determined at lower shear rates are extrapolated and used to design new devices. Magnetic fluids have potential in the high tech and high precision applications and their properties need to be known in particular at shear rates around 10 ⁶ s ⁻¹. Commercially available magnetorheometers are not able to measure these fluids at ultra high shear rates and are limited to 10 ⁵ s ⁻¹. Therefore a new magnetorheometer is required to measure ultra high shear rates. In this paper the physical limitations of current measuring principles are analysed and a concept is designed for ultra high shear rate rheometry in combination with a magnetic field. A prototype is fabricated and the techniques used are described. The prototype is tested and compared to a state of the art commercial rheometer. The test results of the prototype rheometer for magnetic fluids show its capability to measure fluids to a range of 10 ⁴ s ⁻¹– s ⁻¹ and the capability to measure the magnetorheological effect of magnetic fluids.