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

The use of microfluidic devices in the field of plasma-liquid interaction can unlock unique possibilities to investigate the effects of plasma-generated reactive species for environmental and biomedical applications. So far, very little simulation work has been performed on microfluidic devices in contact with a plasma source. We report on the modelling and computational simulation of physical and chemical processes taking place in a novel plasma-microfluidic platform. The main production and transport pathways of reactive species both in plasma and liquid are modelled by a novel modelling approach that combines 0D chemical kinetics and 2D transport mechanisms. This combined approach, applicable to systems where the transport of chemical species occurs in unidirectional flows at high Péclet numbers, decreases calculation times considerably compared to regular 2D simulations. It takes advantage of the low computational time of the 0D reaction models while providing spatial information through multiple plug-flow simulations to yield a quasi-2D model. The gas and liquid flow profiles are simulated entirely in 2D, together with the chemical reactions and transport of key chemical species. The model correctly predicts increased transport of hydrogen peroxide into the liquid when the microfluidic opening is placed inside the plasma effluent region, as opposed to inside the plasma region itself. Furthermore, the modelled hydrogen peroxide production and transport in the microfluidic liquid differs by less than 50% compared with experimental results. To explain this discrepancy, the limits of the 0D–2D combined approach are discussed. [ABSTRACT FROM AUTHOR]

This paper discusses a low-cost technology for the fabrication of microfluidic devices on glass substrate using metal embossing technique. The fabrication technique demonstrated is a much simpler approach of embossing on glass using thermo-compression process with a patterned metal layer to define device structure. Well established printed circuit board fabrication photo-process is used to realize the desired planar geometry on metal layer deposited over a glass substrate. The depth of the channel is defined by the thickness of the metal deposited by electro-plating. The embossing technology offers a relatively safer approach conducive to batch processing to enable repeatable, high-yield, low-cost devices fabricated using low-cost equipment. Major challenges of achieving adhesion of the deposited thick nickel layer without peel-off and control of the thermo-compression process to achieve reliable and repeatable embossing without structural distortions were addressed. To prove its suitability for manufacturing, experiments were carried out with full wafer of 6″ × 6″ square glass wafer and optimal process steps for low-cost microfluidic device manufacturing have been well established. [ABSTRACT FROM AUTHOR]

Developing laser fabricating technology to improve the etching rate of microchannels is of great significance for the development of microfluidic devices. In this paper, the laser processing parameters of the fs + ps double-pulse sequence, including temporal delay and pulse energy ratio, have been investigated to optimize the fabrication of microchannels, in order to obtain a high etching rate. As a result, the optimal energy ratio could be obtained at Efs:Eps= 2:1 in negative delays. Furthermore, the etching rate of microchannels was significantly improved by 40.8% compared with those written by fs + fs double-pulse sequence with an energy ratio of 1:1. This improvement is possibly because fs sub-pulse excited seed electron may trigger impact ionization via an efficient energy absorption process within the following ps sub-pulse, this energy accumulation and subsequent thermal diffusion lead to an increase of the modified area. The aforementioned results possibly offer an effective route to improve the etching rate of microchannels which is in favor of accelerating the fabrication of three-dimensional microfluidic devices. [ABSTRACT FROM AUTHOR]

Channel fabrication technology has become increasingly important for microfluidic and nanofluidic devices. In particular, glass channels have high chemical and physical stability, high optical transparency, and ease of surface modification, so that there is increasing interest in glass microfluidic devices for chemical experiments in microfluidics and nanofluidics. For the fabrication of glass channels, especially those with a high aspect ratio (depth/width), lithography using a metal resist and dry etching have mainly been used. However, there are still issues involving the surface roughness of the etched channel and the low etching selectivity. In this study, a microchannel fabrication method with high etching selectivity that produces a smooth etched surface was developed. First, interference during dry etching by remaining Cr particles after the photolithography and Cr etching processes was assumed as the cause of the rough etched surface. Three different dry etching processes were introduced to verify this. In process 1 without removal of the Cr particles, the etched surface was not flat and had a 1 μ m scale roughness. In process 2 where a cleaning process was included and high power etching was conducted, a smooth surface with a 1 nm scale roughness and a faster etching rate of 0.3 μ m min−1 were obtained. For this high-power etching condition, the etching selectivity (fused silica/Cr) was relatively low at approximately 39–43. In process 3 with a cleaning process and low-power etching, although the etching rate was relatively low at 0.1 μ m min−1, a smooth surface with 1 nm scale roughness (10 nm scale roughness deeper than 40 μ m in the depth region) and a much higher etching selectivity of approximately 79–84 were obtained. The dry etching method presented in this study represents a significant contribution to microfluidics/nanofluidics for microchannel/nanochannel fabrication. [ABSTRACT FROM AUTHOR]

Microfluidic actuators based on thermally-induced actuation are gaining intense attraction due to their usage in disease diagnosis and drug release-related devices. These devices use a thermally-expandable polymer called Expancel that expands once its temperature exceeds a particular threshold value. Achieving such devices that are cost-effective and consume low input power is crucial for attaining efficacy. Therefore, the need for a low-energy consuming actuator necessitates the improved configurations of microheaters that provide the required heat. We report a novel topology of a copper-based microheater called square-wave meander, exhibiting a 44% higher output temperature, showing high actuation efficiency, as compared to the conventionally used meander design. The reason for increased temperature with low input energy is attributed to increased resistance by a jagged structure while maintaining the same surface area, i.e. without changing the effective thickness of the microheater. Numerical modeling demonstrates the comparison of temperature and electric potential contours for reported and conventionally used microheaters. We reveal the merit of the reported design by comparing the volumetric thermal strains for both designs. We experimentally demonstrate the increased expansion of 25% for the reported design at the same applied current of 200 mA and faster operation time. Later, we show the microfluidic actuator device integrated into the microheater and poly-dimethylsiloxane-Expancel, controlling the operation/actuation of a fluid through a microchannel. This work might improve the performance of the advanced microfluidic-based drug release and other fluid-based applications. [ABSTRACT FROM AUTHOR]

By providing a facile and scalable alternative to otherwise complex and resource-intensive synthesis of graphene, laser-induced graphene (LIG) is spearheading the translation of graphene-based propositions to deployable technologies for societal benefit. LIG is a versatile and economical synthesis approach which is being used on a variety of substrates and in a multitude of applicationsâ€"including miniaturized sensing systems. One aspect that has not been addressed thoroughly in LIG-based miniaturized sensing systems is its successful integration with microfluidics and its possible use in point-of-care settings. To further diversify the applications of LIG with integrated microfluidics, this work reports on the development of an integrated flexible microfluidics-LIG based electrochemical biosensor. The work describes the methodology to develop a polydimethylsiloxane-LIG scribed polyamide microfluidic device in a leakage-free flexible application. In view of the excellent electrical and electrochemical properties of LIG, such device has been employed for electrochemical biosensing. The biosensing capabilities of the microfluidic device were validated via sensing of cardiac troponin Iâ€"a gold standard cardiac biomarker for early identification of acute myocardial infarction (AMI). The developed biosensor demonstrated a detection and quantification limit of 45.33 pg mlâˆ'1 and 151.10 pg mlâˆ'1 respectively, which are in clinically significant ranges for diagnosis of AMI. The µ -fluidic biosensor was also analyzed for stability and interference with other cardiac biomarkers. The developed integrated µ -fluidic electrochemical biosensor was evaluated for possible point-of-source applications in conjunction with a custom 3D printed peristaltic pump and smartphone-enabled miniaturized potentiostat. [ABSTRACT FROM AUTHOR]

Intelligent responses, transformation and desired delivery of liquids involve the intersection among materials, physics, and chemistry. The precise programmed operation has been found rather significant in many emerging fields, such as microfluidic devices, digital fluids, intelligent matter, responsive liquid machines, interactive display, soft robotic system and drug delivery, etc. This article is dedicated to presenting a comprehensive review of recent advances in the programmable actuation of liquid matter controlled by unique substrate structures and various physical fields. First, the fundamental theories of asymmetric surface microstructure and specifically administrated external effects such as electric, magnetic, acoustic, light, and thermal fields to drive droplets are illustrated. Then, we compare the advantages and disadvantages of these typical technological strategies. Furthermore, the practical multifunctional extension of programmable liquids based on different actuation mechanisms is presented. Finally, we summarize the design schemes of programming units for liquid matter and give perspective on possible future research directions, providing a brief reference for researchers interested in the field. [ABSTRACT FROM AUTHOR]

Microsphere biolasers employing biological materials as their cavity matrix have attracted tremendous research attention due to their potential for bio-integration, cell-tracking and biosensing. Several techniques have been applied for fabricating microsphere biolasers such as emulsion and vacuum freeze-drying but the current technology generally can not control the output size of the laser and therefore hinder them from many applications. In this work, we demonstrate that a low-cost microfluidic device can be very effective in fabricating nearly monodisperse dye-doped protein microspheres with up to 70% of them having the same size. Under optical pumping, these microspheres emit lasing emission with a lasing threshold of ∼1 µ J and a quality factor of ∼2.5 Ă— 103. The lasing mechanism is ascribed to whispering gallery mode. Furthermore, the obtained microlasers can be employed for temperature sensing based on the wavelength shift of lasing mode with increasing temperature. The sensor sensitivity in the measured range of 25 °Câ€"50 °C is about 0.47 nm/°C. More interestingly, microlasers of the same size exhibit a very similar sensing performance which confirm their high reproducibility and reliability. Owing to the biocompatibility and small size, these miniature laser-based sensors can be implantable in skins and tissues for biological studies and medical diagnostic. [ABSTRACT FROM AUTHOR]

Non-invasive contactless simultaneous sensing and heating of individual droplets would allow droplet microfluidics to empower a wide range of applications. However, it is challenging to realize simultaneous sensing and heating of individual droplets as the resonance frequency of the droplet fluid, which is decided by its permittivity, must be known so that energy is only supplied at this frequency for droplet heating with one resonator. To tailor the energy transfer in real-life heating applications, the droplet has to be sensed first to identify its corresponding resonance frequency, which is used to dynamically tune the frequency for supplying the required energy for heating this particular droplet. To achieve this goal, two resonators are needed, with one for sensing and one for heating. Integrating multiple resonators into one typical microfluidic device limits placement of the resonators to be as close as possible, which would raise the concern of crosstalk between them. The crosstalk would result in inaccurate sensing and heating. This study focuses on numerically and experimentally investigating the effect of influencing parameters on the crosstalk between two adjacent resonators with the ultimate goal of providing guidance for multiplexing the resonators in a typical microfluidic device. ANSYS HFSS is used to perform the electromagnetic analysis based on the finite element method. Experimental studies are conducted on a microfluidic chip integrated with two resonators to validate the numerical results. An optimal distance between two resonators is suggested, with the recommendation for the resonator size and heating power towards simultaneous sensing and heating of individual droplets. [ABSTRACT FROM AUTHOR]

We report the fabrication and experimental study of a flexible bi-layer microfluidic device for blood oxygenation, mimicking the thin alveolar exchange barrier constituting a lung. A facile technique is employed to fabricate the device by sandwiching a thin polymeric membrane as the gas exchange layer between two flexible microchannels. A numerical model coupling the mass, momentum, and species transport equations, is used to simulate oxygen diffusion between the blood and oxygen channels across the gas exchange membrane. The oxygen saturation is experimentally measured at different locations in the blood channel along the flow direction and compared against the simulation results, which show a very good agreement. The effect of blood and oxygen flow rates, channel height, and membrane thickness on the variations in oxygen concentration in the blood and oxygen channels and the diffusion membrane are studied. The outcome of the present study may find relevance in the development of organ-on-chip devices for blood oxygenation. [ABSTRACT FROM AUTHOR]

In this work, a two-channel, water-based cooling system was integrated into a polydimethylsiloxane (PDMS)-glass microfluidic device for application in single-cell biological studies. This system is designed to cool living cells to single-digit temperatures in situ, without requiring any features of the electron-beam fabricated master mould to be changed, and without interfering either biologically or optically with the cells themselves. The temperature profile inside the device was mapped using multiple thermocouples mounted inside the device, over time. A parametric study including coolant flow rate, distance between the cooling channel and the fluidic channel, and number of active cooling channels was performed to evaluate the performance of the system. By using ice water as the coolant, we have demonstrated stable on-chip cooling reaching an average temperature of 4.9 °C when operated at a coolant flow rate of 23 ml minâˆ'1 and using two active cooling channels, positioned only 400 µ m away from the cell trapping sites. The maximum observed temperature deviation during an 80 min stability test was ±0.2 °C. We have observed that flowing room temperature culture media through the device with active cooling had no influence on the temperature inside the chip, demonstrating its suitability for use in live cell culture experiments. Finally, we have also demonstrated that the active cooling system successfully decreased the cell metabolism of trapped Escherichia coli resulting in a decreased growth rate of the bacteria. [ABSTRACT FROM AUTHOR]

Polydimethylsiloxane (PDMS) is a widely used material for microfluidic devices due to its low cost, superior optical properties and fast iterative design process. Its softness however creates challenges for the device design and operation because part of the applied pressures contributes to deform chips instead of controlling the flow. The resulting dynamic behaviour is often ignored in passive microfluidic that focuses on the static behaviour of the chip, however, can cause low accuracy to active microfluidic that actuates flow frequently. Therefore, understanding the dynamic behaviour of microfluidic devices due to material compliance is of fundamental and practical importance. In this study, the microfluidic chip compliance is carefully considered by separating it from the sample tubing compliance. The capacitance is retrieved by assuming a symmetric RC circuit based on the experimentally determined time constant and chip resistance. The experimental capacitance is compared to a theoretical formula for chip designs with different height-to-width ratios and height-to-length ratios and for various fluids. The accuracy is within one order of magnitude that is much closer than previous approximations. [ABSTRACT FROM AUTHOR]

Hot embossing is a microfabrication technique for making microchannels and microfluidic devices. Temperature and pressure along with the material thermos-mechanical properties are the key parameters in controlling the shape of embossed channels. In this paper, experimental and numerical investigations of pressure and temperature effects on channel dimensions are presented. The material used as workpieces is poly methyl methacrylate (PMMA). The depth, upper width and lower width are the main dimensions of microchannel which are studied in this work. Experiments were performed at temperatures of 140 °C, 150 °C, 160 °C, 170 °C and 180 °C and pressures of 235, 295 and 340 kPa. Numerical and experimental results show a good agreement, i.e. 6.7% difference in dimensional length in the worst case and less than 0.3% in the best case. Based on width and depth values obtained via both experiments and simulations an optimum condition of temperature and pressure is presented in this paper for forming of microchannel on PMMA. Based on performed experiments, 150 °C with 295 kPa is found to be the nearest condition to the optimum point. [ABSTRACT FROM AUTHOR]

This work contributes to the development of microfluidics devices in a simpler way and by keeping the cost of microfabrication as low as possible. Implementing methods to develop microfluidic devices and to have control over the channel size rapidly and easily have become a challenge. In the present work, direct laser writing (DLW) is proposed as a manufacturing technique to develop a T-junction microfluidic droplet generator operating in the squeezing regime. Owing to the channel size, DLW depends on both the focused laser spot properties and the interface material refractive indices. The Debyeâ€"Wolf theory behind this technique predicts the focal shift of the laser spot to avoid optical misalignment during the fabrication process. The Debyeâ€"Wolf theory and the COMSOL ray-tracing module are presented as complementary design tools. The flow rates and the channel sizes were investigated under the two-phase COMSOL module. Experimental droplet generation results were assisted with the solution of the Navierâ€"Stokes equation with appropriate boundary conditions. Microdroplet size characterization, as well as simulations, show a linear tendency with the flow rate ratios (FRRs). For FRR ranging from 0.25 to 2, droplet lengths from ∼ 320   ÎĽ m to ∼ 1050   ÎĽ m were observed. This information is confirmed by implementing a digital image processing protocol. The dynamics of droplets formation, strongly influenced by the input flow rate fluctuations caused by the syringes and micropump operation, are presented. This protocol aims the full control of these devices to perform potential applications, such as particle insulators, drug delivery, fluidic microlasers, among others. [ABSTRACT FROM AUTHOR]

Trapping/separating bio-entities via magnetic field gradients created a vast number of possibilities to develop biosensors for the early detection of diseases without the need for expensive equipment or physician/lab technicians. Thus, opening a window for at-home disposable rapid test kits. In the scope of the current work, an innovative and cost-effective technique to form well-organized arrays of Nd–Fe–B patterns was successfully developed. High aspect ratio Nd–Fe–B flakes were synthesized by surfactant-assisted ball milling technique. Nd–Fe–B flakes were distributed and patterned into a PDMS matrix by the aforementioned technique. A microfluidic channel was integrated on the fabricated Nd–Fe–B/PDMS patch with a high magnetic field gradient to form a microfluidic device. Fe nanoparticles, suspended in hexane, were flowed through the microfluidic channel, and trapping of the magnetic nanoparticles was observed. More experiments would be needed to quantitatively study efficiency. Ergo, the microfluidic device with high trapping efficiency was developed. The established technique has the potential to outperform the precedents in trapping efficiency, cost, and ease of production. The developed device could be integrated into disposable test kits for the early detection of various diseases. [ABSTRACT FROM AUTHOR]

Separation and counting of blood cells from whole blood is very significant in measurement and data extraction in health care diagnostics and monitoring health parameters. Many techniques are available for blood cell separation out of which dielectrophoresis (DEP) on lab on chip platform is an emerging technique due to its phenomenal advantages that the force in this technique acts directly on cells based on dielectric properties. In DEP more challenges lie in optimization of the electric field at lower operating voltages as higher voltages damage the biological cells via joule heating and the separation process cannot be accomplished. Due to this optimization of the electric field distribution in the microchannel of the cell separation device is very much important and can be achieved via numerical model or design optimization of electrodes shapes and material. In this work, a microfluidic cell separation device is optimized using the finite element method for the ternary separation of blood cells. The design is realized with two inlets, three outlets, and one main channel having tapered sidewall electrodes for cell separation purpose. Inlet velocities are optimized to achieve improved cell focusing before subjecting cells to separation effects by varying the velocity ratio of main inlet velocity to buffer inlet velocity. Results have shown successful separation of platelets, red blood cells, and white blood cells with 96.7%–100% yield and purity with velocity ratio from 1:4 to 1:5 at electrode excitation voltage of 1.5 V. [ABSTRACT FROM AUTHOR]

This paper investigates the characteristics of higher-order transverse mode fully clamped membrane resonators that are piezoelectrically transduced as a class of resonators that completely seal the domain above the resonator from the domain below. Such isolation between the top and bottom of a resonator is preferable when encasing the device in a microfluidic cell for liquid-phase sensing measurements, a setup that can be beneficial for sensing applications. Most fully clamped membrane resonators are hampered by low liquid-phase quality factor (Q) and large motional resistance (Rm). This paper describes the design of higher-order transverse (2, 2) mode square membrane and (2, 0) mode circular membrane resonators, and the effect of device scaling on liquid-phase Q and Rm. We show a best-case liquid-phase Rm of 54 kΩ and Q of 270 among the various membrane sizes tested. This level of liquid-phase Q is higher than some contour mode resonators. Compared to other fully clamped membrane resonators in the literature, the results here represent a significant improvement in both Rm and Q, highlighting the potential of these membrane resonators for liquid-phase mass sensing applications. [ABSTRACT FROM AUTHOR]

Flow measurement is an essential requirement in medical, industrial, automotive and environmental applications, Thanks to the rapid development of microelectromechanical system (MEMS) micro-fabrication techniques, silicon-based microflow sensor chips used as the key component of microfluidic control systems have attracted particular attentions due to the miniaturized chip-size, high precision, low power consumption, rapid response and low-cost batch-fabrication capability. Up to present, many different types of silicon microflow sensors have been proposed and developed. This paper provides a review on recently completed works on design, fabrication and properties of the MEMS-based silicon monolithic microflow sensors. Some technical challenges and application outlooks are also discussed in the paper. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) devices offer many benefits in chemistry and biomedicine, enabling precise manipulation of micro-droplets, mixing of liquids by acoustic streaming and pumping of liquids in enclosed channels, while presenting a cost-effective and easy fabrication and integration with electronic devices. In this work, we present microfluidic devices which use graphene-based interdigital transducers (IDTs) to generate SAWs with a frequency of 100 MHz and an amplitude of up to 200 pm, which allow us to manipulate microparticle solutions by acoustic streaming. Due to the negligible mass loading of the piezoelectric surface by graphene, the SAWs generated by these devices have no frequency shift, typically observed when metal IDTs are used. [ABSTRACT FROM AUTHOR]

The advantages of planar Hall effect (PHE) sensors—their thermal stability, very low detection limits, and high sensitivities—have supported a wide range of advanced applications such as nano-Tesla (nT) magnetometers, current sensing, or low magnetic moment detection in lab-on-a-chip devices. In this review we outline the background and implications of these PHE sensors, starting from fundamental physics through their technological evolution over the past few decades. Key parameters affecting the performance of these sensors, including noise from different sources, thermal stability, and magnetoresistance magnitudes are discussed. The progression of sensor geometries and junctions from disk, cross-to-bridge, ring, and ellipse configuration is also reviewed. The logical sequence of these structures from single magnetoresistive layers to bi-, tri-layers, and spin-valves is also covered. Research contributions to the development of these sensors are highlighted with a focus on microfluidics and flexible sensorics. This review serves as a comprehensive resource for scientists who wish to use PHE for fundamental research or to develop new applications and devices. The conclusions from this report will benefit the development, production, and performance evaluation of PHE-based devices and microfluidics, as well as set the stage for future advances. [ABSTRACT FROM AUTHOR]

Packaging is an often overlooked component in microfluidic devices for biomedical implant applications. Robust and reliable connectors to interface microscale and macroscale features are especially critical for chronic implant applications. Existing microfluidic packaging methods are incompatible with emerging polymeric materials designed to enhance device integration with the surrounding tissue. A microfluidic connector scheme was developed to promote compatibility with novel materials and implant applications. The connectors and an adhesive wax were printed on a scaffold via additive manufacturing processes. The low-temperature packaging process entailed bonding the connector to a polymer nanocomposite-based intracortical microfluidic probe using an adhesive wax. The robustness of the packaging was assessed by measuring the tensile and shear bond strengths of the connector-adhesive wax-polymer film interface. After soak testing for 4 weeks, the bond strength continued to exceed the force required to infuse fluids through the microfluidic channel. Further, the shear bond strength exceeded typical probe insertion forces by at least ten-fold. These results support the use of the connector and thermal bonding method as a viable option for chronic implant applications. [ABSTRACT FROM AUTHOR]

In order to achieve high-efficiency and high-precision fabrication of nanochannels in micro/nano-fluidic devices, a new method for integrating nano-imprint lithography manufacturing nano-channels and polydimethylsiloxane (PDMS) microchannels with silicon nitride as a bonding layer is proposed. Nanoimprint technology has the advantages of high-precision replication of the template nano-groove size and the ability to form a large number of channels at one time. By depositing silicon nitride on the surface of the nano-trench to improve its bonding force with PDMS treated with oxygen plasma, the bonding strength can withstand pressures above 0.3 MPa. The accuracy of making nanochannels with a characteristic size of 100 nm in this solution can reach more than 95%. Fluorescence experiments using this device show that the device has better enrichment capacity, and its mechanism can be explained by the size exclusion enrichment effect. [ABSTRACT FROM AUTHOR]

This paper reports an IC-compatible method for fabricating a PDMS-based resistive pulse sensing (RPS) device with embedded nanochannel (nanochannel-RPS) for label-free analysis of biomolecules and bionanoparticles, such as plasmid DNAs and exosomes. Here, a multilayer lithography process was proposed to fabricate the PDMS mold for the microfluidic device, comprising a bridging nanochannel, as the sensing gate. RPS was performed by placing the sensing and excitation electrodes symmetrically upstream and downstream of the sensing gate. In order to reduce the noise level, a reference electrode was designed and placed beside the excitation electrode. To demonstrate the feasibility of the proposed nanochannel-RPS device and sensing system, polystyrene micro- and nanoparticles with diameters of 1 μm and 300 nm were tested by the proposed device with signal-to-noise ratios (SNR) ranging from 9.1–30.5 and 2.2–5.9, respectively. Furthermore, a nanochannel with height of 300 nm was applied for 4 kb plasmid DNA detection, implying the potential of the proposed method for label-free quantification of nanoscale biomolecules. Moreover, HeLa cell exosomes, known as a well-studied subtype of extracellular vesicles, were measured and analyzed by their size distribution. The result of the resistive pulse amplitude corresponded well to that of nanoparticle tracking analysis (NTA). The proposed nanochannel-RPS device and the sensing strategy are not only capable of label-free analysis for nanoscale biomolecules and bionanoparticles, but are also cost-effective for large-scale manufacturing. [ABSTRACT FROM AUTHOR]

Detection of cell-free DNA (cfDNA) has an impact on DNA analysis in liquid biopsies. However, current strategies to detect cfDNA have limitations that should be overcome, such as having low sensitivity and requiring much time and a specialized instrument. Thus, non-invasive and rapid detection tools are needed for disease prevention and early-stage treatment. Here we developed a device having a microheater integrated with zinc oxide nanowires (microheater-ZnO-NWs) to detect target single-stranded DNAs (ssDNAs) based on DNA probe hybridization. We confirmed experimentally that our device realized in-situ annealed DNA probes by which we subsequently detected target ssDNAs. We envision that this device can be utilized for fundamental studies related to nanobiodevice-based DNA detection. [ABSTRACT FROM AUTHOR]

This paper reports the use of laser ablation for the fabrication of a microfluidic device with integrated electrodes on a printed circuit board (PCB-MEMS). The fabricated novel device has flame retardant (FR)-4 resin as substrate, copper (Cu) as active material, and SU-8 polymer as a structural material. The resolution, repeatability, and simplicity achieved in this approach, along with the low cost of the involved materials and methods, enable an affordable micromachining technique with a rapid fabricationtest cycle to develop active microfluidic systems on a chip. Furthermore, an Electrochemical Impedance Spectroscopy (EIS) analysis was performed at different Chinese Hamster Ovary (CHO) cell concentrations (105 to 108 cfu ml-1) to validate device assembly and functionality. The electrical properties of the system were characterized and discussed through a proposed equivalent circuit model comprising five passive elements. The model describes ohmic and non-ideal capacitive behavior of the cytoplasm and membrane of the CHO cells as well as the suspending medium and electrical connections of the device. Likewise, two characteristic times associated with relaxation processes caused by a change in the path of charge transport through the cells are determined. [ABSTRACT FROM AUTHOR]

In this work, a novel on-chip micro pressure sensor was developed for microfluidic pressure monitoring. Polydimethylsiloxane (PDMS) microfluidic chip contained a working fluid channel with a sealed detection channel beneath it. Any change in pressure in the working fluid channel would change the volume of the detection channel. A mixture of two immiscible fluids was sealed in the detection channel. The pressure of the working fluid can be monitored by measuring the interface displacement of the two fluids in the detection channel. A PDMS film between the working channel and detection channel can avoid cross-contamination between fluids. We acquired a calibration curve of the pressure sensor for measurement and optimized the performance of the sensor through parametric studies. Moreover, two pressure sensors were integrated into a microchip to characterize the pressure drop in the microchannel. The developed pressure sensor is inexpensive and easy to be integrated into microfluidic devices to monitor the flow conditions for cell culture, fluid mixing, and droplet manipulation. [ABSTRACT FROM AUTHOR]

A novel microfluidic device for efficient passive mixing is reported. A series of sharp corner structures designed on side wall of an arc microchannel was utilized to induce three-dimensional vortices, which strongly stretched and folded the interface and significantly increased the interfacial contact area between different fluids. Additionally, the sharp corner structure induced high radial velocity and reduced the diffusion length because of the restricted effect of the geometrical structure. Eventually, the fluid mixing was significantly enhanced, and a mixing efficiency from ∼87% to 92% was achieved at moderate Reynolds number (Re) from 3.0 to 24.2 (10 to 80 μl min−1). The influence of the sharp corner position on the mixing was also investigated by the experiment and the numerical simulation. With single-layer structure and high mixing efficiency, the present device has great potential in the mixing process in variety of lab-chip applications, such as clinic diagnosis and analytic chemistry. [ABSTRACT FROM AUTHOR]

This paper reports on a rapid, lossless process for the bonding of poly(methyl methacrylate) (PMMA) substrates for use in microfluidic devices for blood tests, utilizing H2O plasma, and H2O linked rapid thermal annealing (RTA) treatments. The bonding of PMMA produced with H2O plasma linked with RTA was analyzed, and its effect on the bonding of free radicals was investigated. The PMMA surface treatment was performed at constant RF power and H2O vapor flow but changing plasma treatment times in both processes. The surface modification of the PMMA subjected to the H2O plasma treatment was studied by optical emission spectroscopy, which confirmed that the relative intensity of hydroxyl radicals and oxygen emission peaks. The surface of the PMMA after being subjected to the plasma linked with RTA treatment was also characterized. The free energy increased with the plasma treatment time. The tensile test results revealed that the strength of OH bonds on the PMMA surface increased with a high plasma treatment time of 120 s. After bonding, UV–visible transmission spectrum measurements revealed that the transparency of the microfluidic device increased. We demonstrated the bonding of a microfluidic device with plasma treatment without deformation. This method provides an ideal bonding technique for the manufacture of microfluidic devices with PMMA. This technique has more advantages than other bonding techniques. [ABSTRACT FROM AUTHOR]

In this review, electrochemical sensor arrays were investigated for various chemical/biological sensing applications including proteins, whole cell pathogens, and small molecules. Utilization of multi-electrode arrays is important for monitoring cancer and other diseases by allowing for detection of multiple proteins in a single assay. Various voltammetric, amperometric, and potentiometric techniques have been used for investigating the electrochemical properties of the electrode surfaces. Electrodes modified with different nanomaterials, antibodies, enzymes, aptamers, etc. have been used to improve performance. This review focuses on recent publications that provide insight into the development of sensor arrays based on microfluidic and paper-based devices. We also discuss incorporation of these devices with portable potentiostats and wireless data acquisition technology for simultaneous analysis of multiple targets. [ABSTRACT FROM AUTHOR]

Lab-on-chip (LOC) is recognised as one of the most affordable solutions for integrating electronics and fluidics devices. In this field, bonding plays a vital role because it provides the means for attaching multiple components onto a substrate, transforming them into a microfluidic circuit. Bonding is an integral step, especially when designing a device that is free from leakage and eventual clogging. A comprehensive review of the latest irreversible bonding technologies is discussed in this paper, in which the focus is on the layered microfluidic systems with large sensor arrays. This review covers microfluidic devices fabricated from a rigid-type glass–fibre-printed circuit board and a thermoplastic flexible printed circuit with 186 references whose development date back three decades ago. The bonding techniques are organised into the following four groups: (a) adhesive bonding, (b) thermal and solvent bonding, (c) surface modification and dry bonding and (d) photoresist groups. Other techniques are available beyond these groupings, but they can be classified into the nearest group to facilitate the discussion. This paper will benefit researchers and practitioners aiming to develop polymer-based LOC devices. [ABSTRACT FROM AUTHOR]

Diverse wetting contrasts of binary wettability patterns in nature have inspired a versatile platform for microfluidics manipulation. To date, nevertheless, building a binary wettability patterned micro/nano structure with highly ordered configuration has inevitably brought wettability contamination challenges in interface control. Herein, this work studied a facile approach to fabricate permanent and contamination-free microstructure with high contrast wettability by means of adding a small amount of surfactant to the photoresist, rather than through any surface modification of the substrate. Accordingly, we focused on a systematic research of the wettability contamination prevention, binary wettability interfacial pattern control, and the dynamic electrowetting performance. The results demonstrated the proposed fabrication of binary wettability patterned microstructure without reactive-ion etching would be advantageous for contamination-free, permanent, simple, reliable and precise process, which could enable the microfluidic electrowetting display highly stable and controllable, thus it is expected to suit fields all the way from microfluidic-based fundamental studies to materials processing engineering and device applications. [ABSTRACT FROM AUTHOR]

Two orthogonal standing acoustic waves, generated by piezoelectric excitation, can form a two-dimensional pressure field in microfluidic devices. A phase difference of the excitation waves can be employed to rotate spherical µm-sized silica particles by a torque mediated through the viscous boundary δ around the particle. The measurement of the rotational rate is, so far, limited to high-speed cameras and their frame rate, and gets increasingly difficult when the sphere gets smaller. We report here a new method for measuring the rotational rate of µm sized spherical particles. We utilize an optical trap with high-speed position detection to overcome the frame rate limitation of wide field image recording. The power spectrum of an optically trapped, rotating particle reveals additional peaks corresponding to the rotational frequencies—compared to a non-rotating particle. We validate our method at low rotational rates against high-speed video observation. To demonstrate the potential of this method we addressed a recent controversy about the rotation of particles with a relatively large viscous boundary layer δ. We measured steady-state rotational rates up to 229 Hz (13.8 × 103 rpm) for a particle with a radius R ≈ δ. Recent numerical research suggests that in this regime the existing theoretical approach (valid for) overpredicts the steady-state rotational rate by a factor of 10. With our new method we also confirm the numerical results experimentally. [ABSTRACT FROM AUTHOR]

Flexible and tunable devices have, for decades, been the subject of many research areas, including, integrated systems, flexible electronics and photonics, which are calling for advanced properties of materials. Metasurfaces with designable optical properties have now been intensively studied for flexible photonic devices. Enabled by microelectromechanical systems (MEMS) and microfluidic technologies, reconfigurable metasurfaces base on mechanical actuations offer new approaches for flexible and tunable devices. In this view, we introduce metasurfaces based on MEMS actuations, droplets arrays, flexible substrates and microfluidic systems together with their applications on flexible and tunable devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An effective development platform for custom lab-on-a-chip and lab-on-a-foil solutions has been regarded as a missing element for wider adoption of microfluidic technologies in everyday life. We have used a direct laser structuring device combined with CAD-CAM software and have developed an efficient, fast, and precise procedure for rapid prototyping of widely accessible contemporary materials utilized for flexible microfluidics. Utilization of an ultra-short pulsed laser has enabled us to predict and control the laser ablation process on thin low-temperature co-fired ceramics and Kapton foil. We have demonstrated an agreement between the theoretic predictions and experimental data on ablation rates, paving the way to a fully predictive manufacturing process. Our procedure enables an independent control of microfluidic channel shape, depth, and lateral dimensions down to 10 µm, while maintaining an exceptional process quality achieved by a parametrically optimized laser output at single-picosecond pulse durations. Our novel rapid prototyping solution features short turnover times, minimum material waste, no chemical procedures, and a single-step process free of heat-effects. We demonstrate the improvements to the structuring process on a known difficult-to-manufacture herringbone mixer structures inside a micro-mixer chip application. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, a microfluidic sensor structure has been proposed for operating frequency reconfiguration purpose. The structure has been designed by using split ring resonator architecture with a transmission line. 0%, 20, 40%, 60%, 80% and 100% ethanol volume content in the ethanol-water mixture have been used as samples to understand sensing characteristic of the proposed structure. After that five sensor structure with different dimension coefficients (1.4, 1.2, 1, 0.8 and 0.6) are designed and manufactured. The ethanol-water mixture samples has been applied to the sensor structure and it is observed that there is a linear relation operating frequency band and the dimension coefficient. The study has been realized both numerically and experimentally and the results are in a good agreement. The proposed structure is a good candidate for microfluidic sensing applications and operating frequency band of the structure can be easily reconfigured any necessary and desired range with respect to electrical characteristics of the samples. [ABSTRACT FROM AUTHOR]

Microfluidic devices present new opportunities for a wide range of applicationsin which the temperature control of nanoliter droplets is required. Due to the inherent size of the employed miniature volumes, several challenges exist, related to temperature measurements. In this work, we present a non-contact and label-free dielectric-based temperature measurement technique. A capacitive sensor transduces the temperature-dependent dielectric properties into a capacitive value. With a post-processing tuned matching network, the reflection coefficient of the capacitive sensor is minimized at a specific frequency, directly related to the sample temperature. The system is calibrated via a water-only calibration method, using two calibration values. A measurement accuracy of ± 0.24 °C is achieved, with an offset of −0.19 °C, over a temperature range from 20 °C to 55 °C. The potential for simultaneous use of the proposed sensor as a microwave heater, gives us a dual-purpose device that minimizes the footprint of the chip, and thus its cost, while also reducing the number of electrical connections to the chip. [ABSTRACT FROM AUTHOR]

Evidently, microfluidic devices are proven to be one of the most effective and powerful tools for manipulating, preparing, functionalizing and producing new generation nanoparticles and nanocomposites. Their benefits include low solution/sample feeding, excellent handling of reagents, exceptional control of size and composition, compactness, easy to process with rapid thermal management and cost-effectiveness. Such advantages have led to the endorsement of nano-microscale fabrication methods to develop highly controllable and reproducible minuscule devices. This work aims to design and develop a microscale-based temperature control device with added features like low-cost, portability, miniaturized, easy-to-use, minuscule reaction volume and point-of-source system for the synthesis of nanoparticles. The device incorporates many features such as real-time data access with a GUI interface with a smartphone open-source app for Bluetooth and Database cloud for an Internet of Things module. The portable thermal device is then calibrated and is capable of achieving a maximum temperature of 250 °C in 25 min. The fabricated device is harnessed for the synthesis of manganese oxide (MnO2) nanoparticles. The synthesized nanoparticles were subjected to various characterization techniques like SEM and XPS to analyze the surface morphology. To test the applicability, as a proof of concept, the synthesized nanoparticles were tested for electrochemical sensing of hydrogen peroxide and dopamine. Overall, the portable device can be utilized for carrying out diverse temperature-controlled reactions in a microfluidic system in a user-friendly and automated manner. [ABSTRACT FROM AUTHOR]

Powered microvalves are necessary for a variety of microfluidic applications such as on-demand droplet generators and droplet capture systems. Currently, a central challenge for these microvalve systems is the miniaturization of bulky power sources and control components, for example, air compressors, hydraulic pumps, solenoid valves, and regulators. In this paper, we propose a polydimethylsiloxane (PDMS)-based microvalve integrated with an on-chip power source, an electro-conjugate fluid (ECF) micropump. The assembled PDMS membrane of the device deforms and then blockades the port in the microchannel via fluidic pressure generated by the ECF micropump with the application of high DC voltage. Following finite element method simulations, we utilized 15 electrode pairs for the on-chip ECF micropump. By combining a MEMS process and the bonding process, we successfully realized the designed device and proceeded to evaluation of its performance characteristics. First, we evaluated the performance of the ECF micropump, which showed a maximum output pressure and a flow rate of 49.7 kPa, 87.3 mm3 s−1, respectively, at an applied voltage of 2.0 kV. Second, we found that the minimum sealing pressure of the microvalve was 10 kPa without any load condition. Third, we investigated the cracking pressures, which were 20 kPa and 50 kPa at the applied voltages of 1.5 kV and 2.0 kV to the ECF micropump, respectively. This study experimentally demonstrated the feasibility of the proposed microvalve device and its potential to be integrated into other microfluidic devices for precision control of liquid volumes. [ABSTRACT FROM AUTHOR]

A fully developed electromagnetohydrodynamic (EMHD) flow through a microchannel with patterned hydrodynamic slippage on the channel wall is studied. The flow is driven by the Lorentz force which originates from the interaction between an externally imposed lateral electric field and a perpendicular magnetic field. The governing equations for the velocity with patterned slip boundary conditions are solved analytically by perturbation techniques under the assumption of small Reynolds number Re. In addition, the numerical solutions for the velocity are obtained by using the finite-difference method, and they are found to be in good agreement with the analytical solutions within admissible parameter range. The effects of different parameters on the velocity and volume flow rate due to patterned hydrodynamic slippage are discussed in detail, including wave-number K, Hartmann number Ha, amplitude δ of the patterned slip length, and normalized electric field strength S. The results show that patterned slippage over microchannel walls can induce transverse flows, which will increase the mixing rates in microfluidic devices. In addition, we also find that precise flow control can be achieved by controlling the magnetic flux and the wave-number and also by well choosing the electric field intensity. Our analysis can be used for designing the efficient micro-fluidic mixers. [ABSTRACT FROM AUTHOR]

The use of contactless magnetic forces meets numerous needs in microelectromechanical systems (MEMS) or microfluidic devices. In this view, heterogeneous materials integrating magnetic nanostructures within a non-magnetic matrix such as polymer can produce local variations of magnetic field, at the sub-micrometer scale. Here we report on the synthesis of magnetic composites using electrospun nanofilaments and a polydimethylsiloxane (PDMS) matrix. Varying the precursor nature and heat treatment conditions, we obtained single phase filaments of Fe, FeNi, and MFe2O4 (M = Co, Fe, Ni). Thanks to a fine investigation of their structure and morphology, it was possible to measure from magnetically-soft (µ0HC ⩽ 5 mT) to relatively hard (µ0HC up to 93 mT, MR/MS up to 0.5) behaviors. The common one-dimensional shape of these filaments leads to an anisotropic magnetic response. This can be exploited to achieve self-organization of the filaments in arrays within the non-magnetic matrix. We show the first step towards the development of magnetically anisotropic membranes of PDMS with 0.23 wt% Fe filaments. These composite materials are promising for implementing magnetic functions in microsystems while circumventing complex micro-fabrication steps. [ABSTRACT FROM AUTHOR]

The effective gas diffusivity, porous structure, and tortuosity factor of catalyst layers used in proton exchange membrane fuel cells were evaluated using a microfluidic device. Sufficient gas transport properties of the catalyst layers are a key factor for achieving high-performance catalyst layers and fuel cells. In the present study, catalyst layers with different thicknesses and different carbon supports were evaluated. Stand-alone carbon black and multi-walled carbon nanotubes were blended into the catalyst layers as the support. The all-carbon-black-based catalyst layer contained some volume of isolated pores and some amount of microcracks, which depended on its thickness. The tortuosity factor was evaluated considering the effects of the isolated pores and microcracks. However, the tortuosity factor of the all-carbon-black-based catalyst layer was larger than the well-known Bruggeman-correlated tortuosity factor. When carbon nanotubes were blended into the catalyst layer, the tortuosity factor was drastically decreased to less than one-half that of the carbon-black-based catalyst layers. A change in the number of straight pores formed by the fibrous support and variation of the ionomer distribution can affect the tortuosity factor. [ABSTRACT FROM AUTHOR]

In this work, we propose a novel approach to design robust microfluidic devices with integrated plasmonic transducers allowing portability, reduced analysis time through dynamic measurements and high sensitivity. Specifically, the strategy we apply involves two steps: (i) the controlled deposition of gold bipyramidal nanoparticles (AuBPs) onto a functionalized solid glass substrate and (ii) the integration of the as-fabricated plasmonic substrate into a polydimethylsiloxane (PDMS) microfluidic circuit. The localized surface plasmon resonance (LSPR) sensitivity of the plasmonic-microfluidic device was evaluated by monitoring the optical responses at refractive index changes, proving a bulk sensitivity of 243 nm RIU−1 for the longitudinal LSPR band of isolated AuBPs and 150 nm RIU−1 for the band assigned to end-to-end linked nanoparticles. A strong electric field generated in the gaps between AuBPs—due to the generation of the so-called extrinsic 'hot-spots'—was subsequently proved by the volumetric surface enhanced Raman scattering (SERS) detection of molecules in continuous flow conditions by loading the analyte into the microfluidic channel via a syringe pump. In conclusion, our miniaturized portable microfluidic system aims to detect and identify, in real-time with high accuracy, analyte molecules in laminal flow, thus providing a groundwork for further complex biosensing applications. [ABSTRACT FROM AUTHOR]

We introduce a multi-step microfluidic reactor for the synthesis of hybrid nanoparticles. As part of this study, nanoparticles composed of chitosan and iron-oxide are synthesized at room temperature by forming sequential droplets of reagents in the microreactor followed by merging and mixing them in a step by step process. The obtained nanoparticles were characterized by transmission electron microscopy, Fourier transform infrared spectrometry, and energy dispersive x-ray analysis. Results were compared with nanoparticles of the same composition synthesized with batch-wise conventional techniques. As a result, the obtained nanoparticles showed better size distribution. This microfluidic device can be used for the synthesis of other types of nanoparticles that require multi-step procedures. [ABSTRACT FROM AUTHOR]

The controlled biofabrication of stable, aligned collagen hydrogels within microfluidic devices is critically important to the design of more physiologically accurate, longer-cultured on-chip models of tissue and organs. To address this goal, collagen-alginate microgels were formed in a microfluidic channel by calcium crosslinking of a flowing collagen-alginate solution through a cross-channel chitosan membrane spanning a pore allowing ion diffusion but not convection. The gels formed within seconds as isolated islands in a single channel, and their growth was self-limiting. Total gel thickness was controlled by altering the concentration of calcium and collagen-alginate flow rate to reach an equilibrium of calcium diffusion and solution convection at the gel boundary, for a desired thickness of 30–200 μm. Additionally, less calcium and higher flow produced greater compression of the gel, with regions farther from the pore compressing more. An aligned, stable collagen network was demonstrated by collagen birefringence, circumferential texture orientation, and little change in gel dimensions with de-chelation of calcium from alginate by prolonged flow of EDTA in the channel. Resultant gels were most stable and only slightly asymmetric when formed from solutions containing 8 mg ml−1 collagen. Diffusion of 4 kDa and 70 kDa fluorescently-labeled dextran indicated size-dependent diffusion across the gel, and accessibility of the construct to appropriately-sized bioactive molecules. This work demonstrates the physicochemical parameter control of collagen gel formation in microfluidic devices, with utility toward on-chip models of dense extracellular matrix invasion, cancer growth and drug delivery to cells within dense extracellular matrix bodies. [ABSTRACT FROM AUTHOR]

Active particles encapsulated in a droplet are called active compound particles. In addition to the activity induced fluid flows, these multiphase structures may experience imposed flows in the process of generating and manipulating them in microfluidic devices or in their natural habitats. Therefore, we investigate the deformation dynamics and stability of a drop encapsulating a model microswimmer, a spherical squirmer (shaker) concentrically placed in a drop, when subjected to an imposed shear flow. Using Lamb's general solution, Stokes equations are solved to determine the flow field and consequently the shape of the droplet interface. Shear flow always deforms the drop and the flow induced by the active particle may enhance or impede this deformation based on its orientation. The extent of deformation also depends on (i) the size of the active particle and the droplet, (ii) the viscosity of the droplet fluid and the fluid in which it is dispersed, (iii) the orientation of the active particle, and (iv) the relative strength of activity and imposed shear. Finally we find that the presence of an imposed vorticity field has a stabilizing effect on the active compound particle as it rotates the encapsulated swimmer and the drop undergoes a time periodic deformation. [ABSTRACT FROM AUTHOR]

Multilayer interconnections are needed for microdevices with a large number of independent electrodes. A multi-level photolithographic process is commonly employed to provide multilayer interconnections in integrated circuit devices, but it is often too expensive for large-area or disposable devices frequently needed for microfluidics. The printed circuit board (PCB) can provide multilayer interconnection at low cost, but its rough topography poses a challenge for small droplets to slide over. Here we report a low-cost fabrication of low-topography multilayer interconnects by selective and controlled anodization of thin-film metal layers. The process utilizes anodization of metal (tantalum in this paper) or, more specifically, repetitions of a partial anodization to form insulation layers between conductive layers and a full anodization to form isolating regions between electrodes, replacing the usual process of depositing, planarizing, and etching insulation layers. After verifying the electric connections and insulations as intended, the developed method is applied to electrowetting-on-dielectric (EWOD), whose complex microfluidic products are currently built on PCB or thin-film transistor substrates. To demonstrate the utility, we fabricate a three metal-layer EWOD device with steps (surface topography) less than 1 micrometer (vs. > 10 micrometers of PCB EWOD devices) and confirm basic digital microfluidic operations. [ABSTRACT FROM AUTHOR]

Micro-dimples are a specific class of surface textures that are ubiquitous in a wide range of applications where functionalities such as wetting, hydrodynamic drag reduction, and sliding friction reduction are needed. However, fabrication of the micro-dimples through the existing manufacturing techniques involves the use of complex equipment and requires relatively long cycle times. Consequently, we report here proof-of-concept of a method that enables in fabricating micro-dimples in a facile and fast manner. The industrial sieve wire masks were employed to deform metallic sheets at discrete locations by the cold rolling process. The geometry of projections on the wire masks allowed the formation of ellipsoid shaped micro-dimples in two distinct patterns on metallic sheets. The finite element modelling of process-material interactions was accurately predicted the formation of micro-dimples under different reduction ratios. We also demonstrate that the micro-dimples can be easily replicated on polymer surfaces using textured metallic moulds, which facilitates low-cost mass production in microfluidic applications. The presence of micro-dimples on polymer surfaces has led to an increase in water contact angles and resulted in a substantial drag reduction in microchannels. From the tribological viewpoint, the obtained patterns and surface area fractions of the micro-dimples are attractive. These findings provide a promising way to create textured surfaces in an economical manner, with functionalities suitable for microfluidic and tribological devices. [ABSTRACT FROM AUTHOR]