Journal of Micromechanics and Microengineering

Blood plasma separation is vital in the field of diagnostics and health care. Due to the inherent advantages obtained in the transition to microscale, the recent trend in these fields is a rapid shift towards the miniaturization of complex macro processes. Plasma separation in microdevices is one such process which has received extensive attention from researchers globally. Blood plasma separation techniques based on microfluidic platforms can be broadly classified into two categories. While active techniques utilize external force fields for separation, the passive techniques are dependent on biophysical effects, cell behavior, hydrodynamic forces and channel geometry for blood plasma separation. In general, passive separation methods are favored in comparison to active methods because they tend to avoid design complexities and are relatively easy to integrate with biosensors; additionally they are cost effective. Here we review passive separation techniques demonstrating separation and blood behavior at microscale. We present an extensive review of relevant biophysical laws, along with experimental details of various passive separation techniques and devices exploiting these physical effects. The relative performances, and the advantages and disadvantages of microdevices discussed in the literature, are compared and future challenges are brought about.

Molybdenum and molybdenum nitride thin films are presented, which are deposited by reactive dc magnetron sputtering. The influence of deposition parameters, especially the amount of nitrogen during film synthesization, to mechanical and electrical properties is investigated. The crystallographic phase and lattice constants are determined by x-ray diffraction analyses. Further information on the microstructure as well as on the biaxial film stress are gained from techniques such as transmission electron microscopy, scanning electron microscopy and the wafer bow. Furthermore, the film resistivity and the temperature coefficient of resistance are measured by the van der Pauw technique starting from room temperature up to 300 °C. Independent of the investigated physical quantity, a dominant dependence on the sputtering gas nitrogen content is observed compared to other deposition parameters such as the plasma power or the sputtering gas pressure in the deposition chamber.

In this work, design and experiments on various blood plasma microdevices based on hydrodynamic flow separation techniques is carried out. We study their performance as a function of dependent governing parameters such as flow rate, feed hematocrit, and microchannel geometry. This work focuses on understanding separation phenomena in simple geometries; subsequently, individual simple geometrical parameters and biophysical effects are combined to fabricate hybridized designs, resulting in higher separation efficiencies. The distinctive features of our microfluidic devices are that they employ elevated dimensions (of the order of hundreds of microns), and thereby can be operated continuously over sufficient duration without clogging, while simplicity of fabrication makes them cost effective. The microdevices have been experimentally demonstrated over the entire range of hematocrit (i.e. from Hct 7% to Hct 45%). A high separation efficiency of about (78.34  ±  2.7)% with pure blood is achieved in our best hybrid design. We believe that the theory and experimental results presented in this study will aid designers and researchers working in the field of blood plasma separation microdevices.

A novel combination of ground-based and flight tests was employed to examine the reliability of capacitive radio-frequency microelectromechanical switches for use in space applications. Laboratory tests were initially conducted to examine the thermomechanical effects of packaging and space-like thermal stresses on the pull-in voltage of the devices; during this process it was observed that operational stability is highly dependent on the geometrical design of the switch and this must be taken in to account during the design stage. To further expose the switches to acceleration levels experienced during a space mission, they were launched on board a sounding rocket and then subjected to free-fall from a height of over 1.3 km with a resulting impact of over 3500 g. Post launch analysis indicates that the switches are remarkably resilient to high levels of acceleration. Some evidence is also present to indicate that time-dependent strain relaxation in die attach epoxy materials may contribute to minor variations in device shape and performance.

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed.

The first option consists in a double layer of silicon nitride with a relatively high deposition rate of ~100 nm min ⁻¹ and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm ⁻¹, yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Micro rotating-structure manufacture has been largely carried out using a variety of processing methods. However, it is difficult to achieve the high-precise machining of the micro rotating structure due to the material constraints and the geometrical limitation of the free-form profile. This paper discusses the fabrication of micro-rotating structure using micro reciprocated wire electrical discharge machining (wire-EDM). To ensure machining accuracy, a gravity takeup for maintaining constant wire tension and a unit for suppressing the wire vibration are designed and fabricated, respectively. Besides, the effects of processing parameters on rotating groove width are investigated by conducting preliminary experiments, showing that high open voltage, discharge capacitance and revolving speed, and low discharge resistance and wire tension cause larger rotating groove width. Nevertheless, the rotating groove width firstly decreases with the reference voltage and feed rate increase in the initial stage and then increases when applying higher values. Based on the preliminary experiments, the proper processing parameters are determined and are used for machining micro bellows core-mould. Ultimately, the micro bellows core-mould is successfully machined by micro reciprocated wire-EDM, exhibiting high machining accuracy, excellent geometric shape and uniformity. Specifically, the machining errors along the axial direction and the radial direction of the micro bellows core-mould are less than 1.4 µm and 2.3 µm, respectively.

This paper reports on a glass reflow process and its applications to microsystems. Glass compounded silicon structures are achieved using a silicon mold under a high temperature environment, a long process time and assistance of enhancement of the surface wettability. Three applications employing the glass reflow process have been proposed and investigated. Firstly, the silicon through-wafer interconnects, embedded inside the Tempax glass, have been successfully demonstrated and show a resistance of about 10 Ω per feed-through. Secondly, a thick glass layer for thermal isolations is reported. The compounded glass can thermally isolate the heated silicon with a temperature difference of more than 100 °C when the temperature of the silicon part is 140 °C at an input power of 200 mW. Additionally, the silicon micro-heater is also evaluated for its reliability. Lastly, glass capillaries and pillars with and without liquid penetration have been proposed for obtaining the high resolution light field information. The optical windows integrated with an image sensor for an optical modulator are clearly demonstrated and a light modulation effect dependent on liquid penetration is observed.

We characterise computationally and experimentally a three-dimensional (3D) microfluidic passive mixer for various Reynolds numbers ranging from 1 to 100, corresponding to primary flow rates of 10–870 µl min ⁻¹. The 3D mixing channel is composed of multiple curved segments: circular arcs situated in the substrate plane and curved nozzle/diffuser elements normal to the substrate plane. Numerical simulation provides a detailed understanding of the mixing mechanism resulting from the geometrical topology of the mixer. These Comsol software-based simulations reveal the development of two secondary flows perpendicular to the primary flow: a swirling flow resulting from tangential injection of the flow into the nozzle holes and Dean vortices present in the circular arcs. These phenomena are particularly important at a Reynolds number larger than 30, where mixing occurs by chaotic advection. Experimentally, the 3D mixer is fabricated in a monolithic glass substrate by powder blasting machining, exploiting eroding powder beams at various angles of impact with respect to the substrate plane. Experimental mixing was characterised using two coloured dyes, showing nearly perfect mixing for a microfluidic footprint of the order of a few mm ², in good agreement with the simulations.

In this paper, an automatic-recovery inertial switch is presented which for the first time adopts gallium–indium (EGaIn) as the switching metal droplet. The device consists of a glass substrate with patterned sensing electrodes, a PDMS microfluidic chip with microchannels and microvalves and a metal droplet. Here, we used EGaIn as the conductive element of the switch because it has several advantages compared with other conductive materials such as water or mercury. Specifically, the proposed device has the ability to automatically recover and can be used repeatedly. In the initial off-state, the droplet is stored in the reservoir. During the working state, the metal droplet passes through the channel and connects the sensing electrodes when the acceleration exceeds the designed threshold level. After that, the EGaIn will return to its original position by a subtle use of its structural characteristics.

The miniaturization of the 3D Rogowski current transducer down to the micro-scale is essential for device integration and expansion of its application scope, particularly for ‘lab-on-a-chip’ systems. However, fabrication of 3D miniaturized Rogowski coils remains challenging as most relative methods still rely on the 2D micromachining process. In this paper, a miniaturized Rogowski coil current transducer was fabricated using an improved femtosecond laser wet etching technology and a metal microsolidification process, in which a metal alloy with a relatively high melting point was used and a robust but simple packaging structure based on a conical electrode was developed. The results show that the miniaturized Rogowski coil current transducer reveals a response time of less than 1 ns, high sensitivity and good detection capability for high-frequency electrical signals. The miniaturized Rogowski coil can easily be integrated into functional microsystems and will be widely applicable for high-frequency electric signal detection and circuit protection.

By clarifying the effect of surface conductivity and adhesivity on the electrostatic manipulation condition, a dielectric particle made of any material can be manipulated with surface conductivity. The manipulation system consists of three elements: a conductive probe as a manipulator, a conductive plate as a substrate, and a dielectric particle as the target object for manipulation. The particle can be successfully picked up/placed if a rectangular pulse voltage is applied between the probe and the plate. Four kinds of particle materials are used in the experiment: silica, soda-lime glass, polymethyl methacrylate coated by conductive polymer, and polystyrene coated by surfactant. The radius of each particle is 15 μm. A first-order resistor-capacitor (RC) circuit model is adopted to describe the effect of surface conductivity and adhesivity on the manipulation condition. The manipulation system is modeled as a series circuit consisting of a resistor and a capacitor by considering the surface conductivity. A detachment voltage is defined as the capacitance voltage to detach the particle adhered to the plate or probe. Parameters of the RC model, surface resistance, surface capacitance and detachment voltage are identified by a simulation and measurements. To verify the RC model, the particle’s behavior is observed by a high-speed camera, and the electrical current is measured by an electrometer. A manipulation experiment is demonstrated to show the effectiveness of the model. The particle reaction is observed for each duration and magnitude of the pulse voltage for the manipulation. The optimum pulse voltage for successful manipulation is determined by the parameters of the RC model as the standard. This knowledge is expected to expand the possibility of micro-fabrication technology.

Microfluidic systems fabricated in polydimethylsiloxane (PDMS) enable a broad variety of applications and are widespread in the field of Lab-on-a-Chip. Here we demonstrate semi-contact-writing, a novel method for fabrication of polymer based molds for casting microfluidic PDMS chips in a highly flexible, time and cost-efficient manner. The method is related to direct-writing of an aqueous polymer solution on a planar glass substrate and substitutes conventional, time- and cost-consuming UV-lithography. This technique facilitates on-demand prototyping in a low-cost manner and is therefore ideally suited for rapid chip layout iterations. No cleanroom facilities and less expertise are required. Fabrication time from scratch to ready-to-use PDMS-chip is less than 5 h. This polymer writing method enables structure widths down to 140 μm and controllable structure heights ranging from 5.5 μm for writing single layers up to 98 μm by stacking. As a unique property, freely selectable height variations across a substrate can be achieved by application of local stacking. Furthermore, the molds exhibit low surface roughness ( R _a   =  24 nm, R _RMS  =  28 nm) and high fidelity edge sharpness. We validated the method by fabrication of molds to cast PDMS chips for droplet based flow-through PCR with single-cell sensitivity.

We report the fabrication of nanopores in a silicon oxynitride (SiON) membrane by heating a silicon rich-silicon nitride (SiRN) membrane with a gold nanoparticle array deposited on its surface. The gold nanoparticle array was realized by photolithography and the membrane by wet-etching. The entire process is wafer scale. Nanopore through-holes of an average diameter of 150 nm were produced in a ~22 nm thick membrane. Membranes and nanopores were characterized by atomic force microscopy, scanning transmission electron microscopy, and x-ray photo-electron spectrocopy.

Magnetofluidic spreading (MFS) is a phenomenon in which a uniform magnetic field is used to induce spreading of a ferrofluid core cladded by diamagnetic fluidic streams in a three-stream channel. Applications of MFS include micromixing, cell sorting and novel microfluidic lab-on-a-chip design. However, the relative importance of the parameters which govern MFS is still unclear, leading to non-optimal control of MFS. Hence, in this work, the effect of various key parameters on MFS was experimentally and numerically studied. Our multi-physics model, which combines magnetic and fluidic analysis, showed excellent agreement between theory and experiment. It was found that spreading was mainly due to cross-sectional convection induced by magnetic forces, and can be enhanced by tuning various parameters. Smaller flow rate ratio, higher magnetic field, higher core stream or lower cladding stream dynamic viscosity, and larger magnetic particle size can increase MFS. These results can be used to tune magnetofluidic spreading in microchannels.

Line-focusing compound silicon x-ray lenses with structure heights exceeding 300 μm were fabricated using deep reactive ion etching. To ensure profile uniformity over the full height, a new strategy was developed in which the perimeter of the structures was defined by trenches of constant width. The remaining sacrificial material inside the lens cavities was removed by etching through the silicon wafer. Since the wafers become fragile after through-etching, they were then adhesively bonded to a carrier wafer. Individual chips were separated using laser micro machining and the 3D shape of fabricated lenses was thoroughly characterized by a variety of means. Optical testing using synchrotron radiation with a photon energy of 56 keV yielded a 300 μm wide beam with a waist of 980 nm (full width at half maximum) at a focal length of 1.3 m. Optical aberrations are discussed in the context of the shape analysis, where a slight bowing of the lens sidewalls and an insufficiently uniform apex region are identified as resolution-limiting factors. Despite these, the proposed fabrication route proved a viable approach for producing x-ray lenses with large structure heights and provides the means to improve the resolution and capabilities of modern x-ray techniques such as x-ray microscopy and 3D x-ray diffraction.

This review systematically addresses the micromachining technologies used for the fabrication of high-performance radio-frequency (RF) passives that can be integrated into low-cost complementary metal-oxide semiconductor (CMOS)-grade (i.e. low-resistivity) silicon wafers. With the development of various kinds of post-CMOS-compatible microelectromechanical systems (MEMS) processes, 3D structural inductors/transformers, variable capacitors, tunable resonators and band-pass/low-pass filters can be compatibly integrated into active integrated circuits to form monolithic RF system-on-chips. By using MEMS processes, including substrate modifying/suspending and LIGA-like metal electroplating, both the highly lossy substrate effect and the resistive loss can be largely eliminated and depressed, thereby meeting the high-performance requirements of telecommunication applications.

Recently, embedded systems and wireless sensor nodes have been gaining importance. For operating these devices several vibration-based energy harvesters have been successfully developed and reported, such as piezoelectric, electromagnetic, and electrostatic energy harvesters (EEHs). This paper presents the state-of-the-art in the field of vibration-based EEHs. Mainly, two types of EEHs, electret-free and electret-based, are reported in the literature. The developed EEHs are mostly of the centimeter scale. These energy harvesters, with resonant frequencies ranging from 2 Hz to 1.7 kHz, when subjected to excitation on the order of 0.25 g to 14.2 g, generate power that ranges from 0.46 nW to 2.1 mW.

In situ stiffness adjustment in microelectromechanical systems is used in a variety of applications such as radio-frequency mechanical filters, energy harvesters, atomic force microscopy, vibration detection sensors. In this review we provide designers with an overview of existing stiffness adjustment methods, their working principle, and possible adjustment range. The concepts are categorized according to their physical working principle. It is concluded that the electrostatic adjustment principle is the most applied method, and narrow to wide ranges in stiffness can be achieved. But in order to obtain a wide range in stiffness change, large, complex devices were designed. Mechanical stiffness adjustment is found to be a space-effective way of obtaining wide changes in stiffness, but these methods are often discrete and require large tuning voltages. Stiffness adjustment through stressing effects or change in Young’s modulus was used only for narrow ranges. The change in second moment of inertia was used for stiffness adjustment in the intermediate range.

This paper reviews mass spectrometers based on micro-electro-mechanical systems (MEMS) technology. The MEMS approach to integration is first briefly described, and the difficulties of miniaturizing mass spectrometers are outlined. MEMS components for ionization and mass filtering are then reviewed, together with additional components for ion detection, vacuum pressure measurement and pumping. Mass spectrometer systems containing MEMS sub-components are then described, applications for miniaturized and portable systems are discussed, and challenges and opportunities are presented.

Bone is an active organ with the capacity of continuous remodeling throughout adult life. In view of the fact that the current gold standard to assess bone remodeling, bone mineral density, suffers from certain limitations, newer techniques are being developed. Currently enzyme-linked immunosorbent assay is commonly used to assess bone turnover markers; the technique, however, is expensive, time consuming and needs trained personnel. Thus, there is a growing demand to fabricate different types of biosensors to provide low cost miniaturized platforms to assess the bone remodeling process more accurately.

This review focuses on the latest advancements in the field of bone biosensing technologies. Its results might help provide possible solutions for translation of this technology for point-of-care diagnostic applications.