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.

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.

This paper presents a comprehensive experimental study and characterization of material and bonding of PDMS based structures to various substrates. A previously published method [1] of bonding is further improved with the inclusion of more substrate material and additional characteristics. Uncured PDMS is used as an adhesive to bond PDMS devices reversibly to various substrates including a number of commonly used substrate materials that are not supported by the widely used plasma treatment method. We have optimized parameters such as PDMS base to curing agent ratio, curing temperature, and PDMS device age to obtain better bond strengths and quality. Bond strengths are presented for semiconductor substrates (silicon, zinc oxide, and silicon dioxide), metals (gold, aluminum), photoresists (SU-8, AZxx) and glass. Silicon based substrates experienced minor amounts of surface residue, but the method is fully reversible for other tested substrates. Bond strengths were measured as maximum endurable pressure between PDMS and substrates. Maximum average bond strengths of more than 0.4 MPa were achieved for substrates with Si-O groups. Other substrates exhibited maximum average bond strengths in the range 0.2–0.3 MPa. Also presented is a method that avoids alignment step for PDMS microfluidic device bonding, named the non-aligned method. This method provides bond strengths of more than 0.1 MPa. Presented methods do not need special equipment or processes such as plasma generators or temperature increases. Biocompatibility tests are performed for materials used in fabrications to ensure applicability in bio-sensing related devices.

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.

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.

SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

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.

An overview is given of research activities in the field of fluid components or systems built with microfabrication technologies. This review focuses on the fluidic behaviour of the various devices, such as valves, pumps and flow sensors as well as the possibilities and pitfalls related to the modelling of these devices using simple flow theory. Finally, a number of microfluidic systems are described and comments on future trends are given.

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.

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.

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%.

While electrospray devices have been used in a variety of applications for decades, they have recently seen a surge in research within the field of electric propulsion. These research efforts have helped significantly improve the understanding of electrospray thruster operation and optimization, however they have primarily been focused on capillary-based, droplet emitting devices due to the more readily available manufacturing techniques. In contrast, ion emitting, porous-media-based electrospray devices are less developed both theoretically and experimentally. Presented here are fabrication methods and thruster characterization results for an entirely conventionally machined, high performance porous-media electrospray thruster. The goal of this work was to explore the performance capabilities of an ion-mode electrospray thruster which could be fabricated and tested rapidly using techniques readily available to virtually any institution, with the hope of enabling more academic and industrial development of this technology. The thruster described here consisted of 576 emitters conventionally machined out of porous borosilicate glass and is able to maintain stable operation up to ± 700 µA of emitted ion current. The overall thruster design is described, and detailed fabrication steps are presented for this device. Additionally, performance characteristics are discussed for both positive and negative ion emission, including I–V curves and direct thrust measurements, as well as measurements of the emitted ion angular, 2D spatial, mass, and energy distributions. Examples of the performance of this device compared to other devices found in the literature are also discussed.

Stencil contact printing is widely used to fabricate conductive patterns, and it is particularly used with solder paste to create interconnections. However, stencil contact printing is becoming inefficient for electronic components owing to the ever decreasing size of the components. An alternative method for fine pattern formation is screen printing, i.e. gap printing with a screen mask, which exploits the thixotropic characteristics of solder paste. Nevertheless, the mesh of the screen mask prevents the paste from permeating, resulting in irregular patterns. To address this issue, we propose gap printing with a mesh-cut screen mask. In this paper, we describe the fabrication procedure of the mask, and demonstrate the effectiveness of the proposed printing in the formation of fine and thick circular patterns; the patterns are shown to have low variations in size compared with conventional printing methods. The proposed method is expected to contribute to the further miniaturisation of electronic devices.

In this study, we have proposed volume-preserving strategies to boost chaoticadvection and improve the mixing efficiency of serpentine micromixers. The proposed strategies revolve around the point that the volume of the micromixer is kept constant during the manipulation. The first strategy involves the utilization of a nozzle-diffuser (ND) shaped microchannel. Using this, the velocity of the fluids fluctuates in an alternating pattern, leading to additional chaotic advection, a decrease in the mixing path, and an increase in the mixing index. The second strategy uses non-aligned inlets to generate swirl inducing effects at the microchannel entrance, where the collision of two fluids generates angular momentum in the flow, providing more chaotic advection. These strategies proved to be effective in boosting the mixing efficiency over wide ranges of Re in which 60% enhancement (from 20.53% to 80.31%) was achieved for Re of 30 by applying an ND shaped microchannel, and 20% enhancement (from 12.71% to 32.21%) was achieved for a critical Re of 15 by applying both of the strategies simultaneously.

A double-layer flexible sensor array with an active thermal insulation method for fluid wall shear stress was fabricated in this study and evaluated experimentally. Static calibration of the sensor was studied in both wind and water tunnels and the experimental results reveal that the sensor’s static performance was improved by active thermal insulation. Compared to single-layer methods, the static sensitivity of the proposed double-layer sensor is increased by approximately 48% in a wind tunnel and 13% in a water tunnel. Additionally, consistent deviations in the static calibration coefficients of sensors with different basic parameters were clearly compensated in the wind tunnel. The calibration coefficient deviation of the sensors was reduced from 57% in single-layer mode to 5% in double-layer mode.

We developed a lead zirconate titanate (PZT) thin film actuator integrated with buried piezoresistors for the dynamic and static deformation sensing of a PZT MEMS actuator. We demonstrated the fabrication of sol-gel deposited PZT thin film devices combined with buried piezoresistors and proved, for the first time, the process compatibility of these materials. Dopant concentration measured by secondary ion mass spectrometry (SIMS) analysis confirms that the piezoresistor was successfully buried into the device. Motion detection of the fabricated MEMS cantilever actuated by the PZT thin film was successful and consistent with optical measurement as well as design values. From these results, we can conclude that our PZT actuator and piezoresistive sensors can be monolithically integrated. The fabrication process developed here can be used for high-stability piezoelectric MEMS actuators with feed-back control of position.

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.

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.

Scalable and cost-efficient transfer of nanomaterials and microstructures from their original fabrication substrate to a new host substrate is a key challenge for realizing heterogeneously integrated functional systems, such as sensors, photonics, and electronics. Here we demonstrate a high-throughput and versatile integration method utilizing conventional wire bonding tools to transfer-print carbon nanotubes (CNTs) and silicon microstructures. Standard ball stitch wire bonding cycles were used as scalable and high-speed pick-and-place operations to realize the material transfer. Our experimental results demonstrated successful transfer printing of single-walled CNTs (100 m-diameter patches) from their growth substrate to polydimethylsiloxane, parylene, or Au/parylene electrode substrates, and realization of field emission cathodes made of CNTs on a silicon substrate. Field emission measurements manifested excellent emission performance of the CNT electrodes. Further, we demonstrated the utility of a high-speed wire bonder for transfer printing of silicon microstructures (60 m 60 m 20 m) from the original silicon on insulator substrate to a new host substrate. The achieved placement accuracy of the CNT patches and silicon microstructures on the target substrates were within 4 m. These results show the potential of using established and extremely cost-efficient semiconductor wire bonding infrastructure for transfer printing of nanomaterials and microstructures to realize integrated microsystems and flexible electronics.

In this paper we report on the fabrication of bistable micro electromechanical systems (MEMS) membranes, which have diameters in the range of 600–800 µm, a total thickness of 3.13 µm and feature integrated low power piezoelectric transducers based on aluminium nitride. To estimate the impact of the membrane asymmetry due to the integrated piezoelectric transducers, an asymmetric constant in the potential energy calculation of the bistable system is introduced, thus enabling a proper theoretical prediction of the membrane behaviour. To switch between the two bistable ground states, rectangular pulses with frequencies in the range of 50–100 kHz and a peak-to-peak voltage of 30 V _pp are applied. Two different actuation schemes were investigated, whereas one shows positive and the other negative pulse amplitudes. With a Laser-Doppler Vibrometer the velocity of the membranes during the bistable switching process is measured and integrated over time to calculate the membrane displacement in the centre. FFT (fast Fourier transform) spectra of an applied broadband white noise signal were determined in both ground states and showed a strongly decreased dominant resonance frequency in the lower ground state. The results also showed, that the asymmetry of the system causes different switching behaviours for each bistable ground state, whereas it requires less energy to switch from the lower to the upper ground state. Furthermore, it was demonstrated that a minimum of two pulses are needed for switching when using positive rectangular pulses of 30 V _pp in contrast to four when applying negative pulses. The pulse frequency causing switching was in the range of 60–110 kHz, strongly depending on the geometry and applied signal scheme. Additionally, a positive voltage offset applied to the pulse signal characteristics resulted in both a wider range of frequencies suitable for switching and in a decrease of the dominant resonance frequency, which is also beneficial for the switching process and indicates the potential for efficient switching of bistable MEMS membranes.