Numerical simulation of AC electrothermal micropump using a fully coupled model

The classic model by Ramos et al. for numerical simulation of alternating current electrothermal (ACET) flow is a decoupled model based on an electrothermal force derived using a linear perturbation method, which is not appropriate for the applications, where Joule heating is large and the effect of temperature rise on material properties cannot be neglected. An electrically–thermally–hydrodynamically coupled (fully coupled) ACET flow model considering variable electrical and thermophysical properties of the fluids with temperature was developed. The model solves AC electrical equations and is based on a more general electrostatic force expression. Comparisons with the classic decoupled model were conducted through the numerical simulations of an ACET micropump with asymmetric electrode pairs. It was found that when temperature rise is small the fully coupled model has the same results with the classic model, and the difference between the two models becomes larger and larger with the increasing temperature. The classic decoupled model underestimates the maximum temperature rise and pumping velocity, since it cannot consider the increase in electrical conductivity and the decrease in viscosity with temperature. The critical frequencies where the lowest velocity occurs or pumping direction reverses are shifted to higher frequencies with the increasing voltage according to the fully coupled model, while are kept unchanged according to the classic model.     

Two-phase AC electrothermal fluidic pumping in a coplanar asymmetric electrode array

The ability to achieve fast fluid flow yet maintain a relatively low temperature rise is important for AC electrothermal (ACET) micropumping, especially in applications such as bioMEMS and lab-on-a-chip systems. In this paper, we propose a two-phase ACET fluidic micropump using a coplanar asymmetric electrode array. The proposed structure applies a two-phase AC voltage, i.e., voltage of phase 0°/180°, to the narrow electrodes while the wide electrodes are at ground potential. Numerical simulation demonstrates that this simple coplanar electrode configuration can achieve at least 25% faster fluid flow rates than using a single AC signal. By selecting certain design parameters, a two-phase ACET structure can achieve up to 50% faster fluid flow rates than a corresponding single-phase structure. The simple two-phase AC signal sources are easily produced by using inverter buffers, which is a considerable improvement compared to the multi-phase AC signals required by other electrokinetic micropumping methods, such as traveling wave structures.     

Configurable AC electroosmotic generated in-plane microvortices and pumping flow in microchannels

This paper proposes the patterned AC electroosmotic flows (AC-EOF) by simply grouping discrete electrodes together to form various electrode configurations for generation of in-plane microvortices with clockwise/counter-clockwise rotation, and pumping flow in a microchannel. The rotational direction of in-plane microvortices and pumping flow direction can be controlled using the same electrode pattern by simple switching of the voltages on the electrodes. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices and pumping flow. The rotational strength of microvortices and flow rates of pumping flows were found to increase with the increase of the applied voltages, and an optimal value was achieved at an appropriately applied frequency. Moreover, the dependency of the applied voltages, frequencies, and the heights of the measured planes on the rotational strength of in-plane microvortices between the interdigitated and discrete electrode configurations were examined. The discrepancy in electrode geometry results in a small performance reduction, whereas it can be compensated for the ability of switching the rotational direction of in-plane microvortices using the same device. The configurable in-plane microvortices and pumping flow in microchannels provide the potential for micromixing applications and for the integration into a lab-on-a-chip system.     

Fabrication and mathematical analysis of an electrochemical microactuator (ECM) using electrodes coated with platinum nano-particles

This paper reports the design, fabrication, modeling, and analysis of an electrochemical microactuator (ECM). The driving mechanism of the ECM is based on the reversible electrolysis process of water. The expansion and shrinkage of gas bubbles generated in a micro electrochemical chamber during a reversible electrolysis process can be used for actuation in microfluidic systems. The fluidic components of the ECM were fabricated on a glass substrate using UV lithography of SU-8. Two electrodes were used, with one Pt black (coated with platinum nanoparticles) as working and auxiliary electrode and the other Ag/AgCl coated as reference. The nano particles coated on the working electrode help to boost the surface-to-volume ratio of the electrode and to obtain higher operation frequency. Pulse electropotentials were supplied for active control of expansion and shrinkage of gas bubbles using reproducible electrochemical reactions. The theoretical volume change rate of gas bubbles was simulated as a function of time using the ideal gas law and compared with the measured volume change. The experimental results show the electrode coated with platinum nanoparticles helped to enhance the reversible electrolysis process significantly. The results also show that the dynamic model can be used to simulate the dynamic behaviors of the ECM actuator. The ECM can be used in microfluidic applications such as pumping or valves.     

Numerical analysis of non Newtonian fluid flow in a low voltage cascade electroosmotic micropump

In microfluidic devices, many fluids have non-Newtonian behaviors, especially biofluids. The viscosity of these fluids mostly depends on the shear rate. Sometimes the non-Newtonian fluids should be transferred by micropumps in lab-on-chip devices. Previous researchers investigated the flow rate in simple electroosmotic flow micropumps which have a simple channel geometry. In the present study, the effects of non-Newtonian properties of fluid in a low voltage cascade electroosmotic micropump are numerically investigated using the power law model. The micropump is modeled in two dimensional with one symmetric step and has a more complex geometry than previous studies. The numerical results show that, the non-Newtonian behavior of fluid affects flow rate in the micropump. The flow rate decreases if the fluid is dilatant. Also, it increases if the fluid is pseudoplastic. Moreover, the pressure which is needed to stop the electroosmotic flow rate in the micropump is calculated. Results show that, the back pressure has a slight change as the fluid has non-Newtonian behavior.     

A paper-based microfluidic Dot-ELISA system with smartphone for the detection of influenza A

An automated, portable, and integrated paper-based microfluidic system has been developed for influenza A detection with smartphone at point-of-care (POC) settings. The low-cost paper-based microfluidic chip consists of a reagent storage and reaction modules. The storage module, which consists of a couple of reagent chambers with dispensation channels, is responsible for reagent storage and release. The reaction module consists of an absorbent pad and a nitrocellulose (NC) membrane which is functionalized with specific monoclonal antibodies on a test and control spots for immunoassay detection. Microfluidic Dot-ELISA is performed when the dispensed reagent flows through the NC membrane at a controllable speed and reaches the absorbent pad because of the gravity and capillary force without active pumping. A smartphone is used to capture image from the NC membrane with its own camera and process the image with an intelligent algorithm of custom application software which is developed with Java. With a smartphone, the detection result can be displayed and transmitted to other medical agencies if necessary. Experimental results show that, compared with the traditional methods, more convenient and efficient influenza A detection can be achieved with the developed paper-based POC microfluidic chip with the assistance of smartphone.     

Electrochemical detection techniques in micro- and nanofluidic devices

Electrochemical techniques are widely used in microfluidic and nanofluidic devices because they are suitable for miniaturization, have better sensitivity compared to optical detection techniques, and their components can be reliably microfabricated. In addition to the detection and quantification of analytes, electrochemical techniques can be used to monitor processes such as biological cell death and protein/DNA separations/purifications. Such techniques are combined with micro- and nanofluidic devices with point-of-care (POC) applications in mind, where cost, footprint, ease of use, and independence from peripheral equipment are critical for a viable design. A large variety of electrode materials and device configurations have been employed to meet these requirements. This review introduces the reader to the major electrochemical techniques, materials, and fabrication methods for working and reference electrodes, and to surface modifications of electrodes to facilitate electrochemical measurements, in the context of micro- and nanofluidic devices. The continuing development of these techniques holds promise for the next-generation lab-on-a-chip devices, which can realize the goals of this technology such as POC clinical analysis.     

Acoustically Driven Programmable Microfluidics for Biological and Chemical Applications

In summary, we described a novel and unconventional technique to manipulate smallest amounts of liquid on a chip. Using SAW on a piezoelectric substrate, we are able to actuate individual droplets along predetermined trajectories, or induce acoustically driven internal streaming in the fluid. This internal acoustic streaming can efficiently be used to agitate, mix, and stir very small liquid volumes, where the low Reynold's number usually only allows for diffusive mixing. We described several applications of the SAW driven microfluidics, including a nanomixer for microarray applications, a contactless mixer for MTPs, a programmable microfluidic chip for droplet-based assays, and finally a chip performing high resolution microliter PCR. The technique is equally well suited to actuate or agitate small amounts of liquids either in closed volumes or in an open, droplet-based geometry. Each of the approaches has its clear advantages, but also disadvantages. Droplet-based fluidics is certainly well suited to handle smallest amounts of fluids without the risk of cross contamination etc. High temperature processes, however, require additional means like covering the droplet with an oil film. SAW pumping, mixing, and stirring on closed volumes is advantageous over many other pumping schemes as the pumps are easily incorporated into most of the existing microfluidic device or lab-on-a-chip. The combination of the SAW actuated droplet-based fluid handling and SAW driven fluidics in closed volumes opens a wide field of many different applications, a few of which I had the pleasure to present in this article. Many more applications, and many more visualizations of the technology described above can be looked up on Advalytix' website http://www.Advalytix.de.     

Bond graph modelling of a photovoltaic system feeding an induction motor-pump

Water pumping using induction motors has become one of the most feasible photovoltaic (PV) applications. A bond graph model to enable testing the PV system performance by computer simulation was developed. The PV-powered water pumping system investigated in this paper consists mainly of a PV generator, DC–DC and DC–AC converters, and induction motor-pump. The DC–DC converter control strategy is based on pulse width modulation (PWM). However, the oriented field control is used for the induction machine control. Computer simulations were carried out for maximum power point tracking (MPPT).     

Geometry design of herringbone structures for cancer cell capture in a microfluidic device

Cancer cell detection with high capture efficiency is important for its extensive clinical applications. Herringbone structures in microfluidic devices have been widely adopted to increase the cell capture performance due to its chaotic effect. Given the fact of laminar flow in microfluidic devices, geometry-based optimization acting as a design strategy is effective and can help researchers reduce repetitive trial experiments. In this work, we presented a computational model to track the cell motion and used normalized capture efficiency to evaluate the tumor cell capture performance under various geometry settings. Cell adhesion probability was implemented in the model to consider the nature of ligand–receptor formation and breakage during cell–surface interactions. A facile approach was introduced to determine the two lumped coefficients of cell adhesion probability through two microfluidic experiments. A comprehensive geometric study was then performed by using this model, and results were explained from the fluid dynamics. Although most of the geometric guides agree with the general criterion concluded in the literature, we found herringbone structures with symmetric arms rather than a short arm–long arm ratio of 1/3 are optimal. This difference mainly comes from the fact that our model considers the particulate nature of cells while most studies in the literature optimize the geometry merely relying on mixing effects. Thus, our computational model implemented with cell adhesion probability can serve as a more accurate and reliable approach to optimize microfluidic devices for cancer cell capture.     

Effect of electrode geometry on performance of an EHD thin-film evaporator

This paper presents details of an optimization process of electrode geometry for an electrohydrodynamically (EHD) driven thin-film evaporator. The operation principle of the device is based on the action of the EHD force on the molecules of a dielectric liquid in a highly convergent electric field. The force starts at the end of a pair of electrodes, where the electric field changes from zero far from the electrodes to a finite value in between the electrodes. This force drives the liquid up into the spacing between the electrodes. The electrodes in this study were deposited thinly on a SiO/sub 2//Si wafer, so the liquid could be held within micrometers of thickness over the surface. Since the performance of the device in removing heat from the surface is a function of its pumping head and consequently its electrode geometry, the performances of different electrodes were evaluated by testing twelve sets of electrode pairs with different geometries. Then the optimum electrode design was incorporated into the design of a large size (32/spl times/32 mm/sup 2/) EHD thin-film evaporator. The device was fabricated, and its pumping and heat transfer performances were tested. A pumping head equal to the full height of the electrodes and a heat transfer coefficient of 1.9 W/cm/sup 2/./spl deg/C was achieved using HFE-7100 liquid.     

A positive pressure-driven PDMS pump for fluid handling in microfluidic chips

By utilizing the high gas permeability of polydimethylsiloxane (PDMS), a simple positive pressure-driven pumping method was introduced. The pump was an aerated PDMS with a central channel in it and packing with a transfusion bottle. It could be attached to the inlet of microfluidic chip using a Teflon tube to release the air into the microfluidic system and then to create a positive pressure for driving fluid. In comparison with the degas-based PDMS pump, positive pressure-driven PDMS pump offered increased system flexibility and reduced individual device fabrication complexity due to its independence and versatility. More importantly, it offered the advantages that the PDMS pump could be wrapped in transfusion bottles to meet the readily available requirements, and it also easily assembled, which only required the user use a Teflon tube to connect a PDMS pump and a microfluidic chip. This assembly provided great freedom to meet different pumping requirements. Furthermore, this PDMS pump could offer many possible configures of pumping power by adjusting the geometries of the pump or by combining different pump modules, the adjustment of pumping capacity was investigated. To help design pumps with a suitable pumping performance, the sealing effect, pumping pressure and flow rate were also investigated. The results indicated that the performance of the positive pressure-driven PDMS pump was reliable. Finally, we demonstrated the utility of this pumping method by applying it to a PDMS-based viscometer microfluidic chip.     

Microfluidic-based biosensors toward point-of-care detection of nucleic acids and proteins

This article reviews state-of-the-art microfluidic biosensors of nucleic acids and proteins for point-of-care (POC) diagnostics. Microfluidics is capable of analyzing small sample volumes (10⁻⁹–10⁻¹⁸ l) and minimizing costly reagent consumption as well as automating sample preparation and reducing processing time. The merger of microfluidics and advanced biosensor technologies offers new promises for POC diagnostics, including high-throughput analysis, portability and disposability. However, this merger also imposes technological challenges on biosensors, such as high sensitivity and selectivity requirements with sample volumes orders of magnitude smaller than those of conventional practices, false response errors due to non-specific adsorption, and integrability with other necessary modules. There have been many prior review articles on microfluidic-based biosensors, and this review focuses on the recent progress in last 5 years. Herein, we review general technologies of DNA and protein biosensors. Then, recent advances on the coupling of the biosensors to microfluidics are highlighted. Finally, we discuss the key challenges and potential solutions for transforming microfluidic biosensors into POC diagnostic applications.     

Performance improvement of an AC electroosmotic micropump by hydrophobic surface modification

This paper describes the improvement of bi-directional micropump velocity by deposition of a hydrophobic nanocomposite monolayer. A polymer base nanocomposite coating consisting of a homogeneous mixture of silicon nanoparticles in polydimethylsiloxane (PDMS) is used to improve the hydrophobicity of the micropump surfaces. For hydrophobic nature of PDMS and the monolayer coating with nanoscale surface roughness, the hydrophilic surface of a biased AC electroosmotic micropump will transform to a hydrophobic surface. In our previous research the applied AC voltage, frequency, channel dimension, and electrode width were optimized (Islam and Reyna, Electrophoresis 33(7), 2012). Based on the prior results obtained for the biased AC electroosmotic micropump, the pumping velocity was 300 micron/s in 100-μm channel thickness for applied voltage of 4.4 V at 1 kHz frequency. Here in this work, improvement of the micropump velocity is investigated through a surface modification process. The highest velocity of 450 micron/s is observed by modifying the surface characteristics. This paper will also discuss the synthesis process and characteristics of the polymer base nanocomposite monolayer. In addition to hydrophobicity improvement, adding a thin nanocomposite monolayer will physically separate the electrodes from the pumping liquid, thus eliminating their reaction, which is usually observed due to the application of voltage. As a result, higher voltages can be applied to the electrodes and higher pumping rates are achievable.     

Novel systems for configurable AC electroosmotic pumping

In this paper we present a novel method of creating and using geometric asymmetries for AC electroosmotic pumping. The method relies on grouping similar electrodes together in terms of applied voltage, in order to create configurable asymmetries in periodic electrode arrays, which induce a net pumping AC electroosmotic velocity. Using a numerical model for a system designed by applying the described method, it is demonstrated that by varying the degree of asymmetry it is possible to control the direction of the pumping velocity at a given voltage by simple switching of the voltages on the electrodes.     

Effects of geometry factors on microvortices evolution in confined square microcavities

Recently, microcavities have become a central feature of diverse microfluidic devices for many biological applications. Thus, the flow and transport phenomena in microcavities characterized by microvortices have received increasing research attention. It is important to understand thoroughly the geometry factors on the flow behaviors in microcavities. In an effort to provide a design guideline for optimizing the microcavity configuration and better utilizing microvortices for different applications, we investigated quantitatively the liquid flow characteristics in different square microcavities located on one side of a main straight microchannel by using both microparticle image velocimetry (micro-PIV) and numerical simulation. The influences of the inlet Reynolds numbers (with relatively wider values Re?=?1–400) and the hydraulic diameter of the main microchannel (D_H?=?100, 133 μm) on the evolution of microvortices in different square microcavities (100, 200, 400 and 800 μm) were studied. The evolution and characteristic of the microvortices were investigated in detail. Moreover, the critical Reynolds numbers for the emergence of microvortices and the transformation of flow patterns in different microcavities were determined. The results will provide a useful guideline for the design of microcavity-featured microfluidic devices and their applications.     

AC electrokinetic pumping on symmetric electrode arrays

AC electro-osmotic (ACEO) pumping is experimentally demonstrated on a symmetric gold electrode array. Using asymmetric connection of electrodes to the applied AC voltage, spatial asymmetry along the array is created, which produces unidirectional flow of electrolyte. An aqueous solution of 100 μM KCl is selected as the pumping fluid. The liquid velocity obtained as a function of voltage and frequency is compared to that generated using travelling-wave electroosmosis (TWEO) with the same electrode array. The expected velocities from the linear electrokinetic models of ACEO and TWEO are computed numerically. The comparison shows that TWEO generates greater velocity amplitudes and the streamlines are smoother than those generated by ACEO.     

Characterization and modeling of a microfluidic dielectrophoresis filter for biological species

Microfabricated interdigitated electrode array is a convenient form of electrode geometry for dielectrophoretic trapping of particles and biological entities such as cells and bacteria within microfluidic biochips. We present experimental results and finite element modeling of the holding forces for both positive and negative dielectrophoretic traps on microfabricated interdigitated electrodes within a microfluidic biochip fabricated in silicon with a 12-/spl mu/m-deep chamber. Anodic bonding was used to close the channels with a glass cover. An Experimental protocol was then used to measure the voltages necessary to capture different particles (polystyrene beads, yeast cells, spores and bacteria) against destabilizing fluid flows at a given frequency. The experimental results and those from modeling are found to be in close agreement, validating our ability to model the dielectrophoretic filter for bacteria, spores, yeast cells, and polystyrene beads. This knowledge can be very useful in designing and operating a dielectrophoretic barrier or filter to sort and select particles entering the microfluidic devices for further analysis.     

Experiments on opto-electrically generated microfluidic vortices

Strong microfluidic vortices are generated when a near-infrared (1,064 nm) laser beam is focused within a microchannel and an alternating current (AC) electric field is simultaneously applied. The electric field is generated from a parallel-plate, indium tin oxide (ITO) electrodes separated by 50 μm. We present the first μ-PIV analysis of the flow structure of such vortices. The vortices exhibit a sink-type behavior in the plane normal to the electric field and the flow speeds are characterized as a function of the electric field strength and biasing AC signal frequency. At a constant AC frequency of 100 kHz, the fluid velocity increases as the square of the electric field strength. At constant electric field strength fluid velocity does not change appreciably in the 30–50 kHz range and it decreases at larger frequencies (>1 MHz) until at approximately 5 MHz when Brownian motion dominates the movement of the 300 nm μ-PIV tracer particles. Presence of strongly focused laser beams in an interdigitated-electrode configuration can also lead to strong microfluidic vortices. When the center of the illumination is focused in the middle of an electrode strip, particles experiencing negative dielectrophoresis are carried towards the illumination and aggregate in this area.     

基于微流控芯片的尿酸和抗坏血酸并行电化学检测研究

尿样和血样中的尿酸水平是诊断痛风等疾病的重要指标,传统检测方法存在时间长、需要其他辅助试剂等不足,开展了在微流控芯片上对尿酸进行电化学检测的研究：以碳纳米管修饰的丝网印刷电极片为检测单元,构建了PDMS微流控芯片。以微流控芯片为平台,采用微分脉冲伏安法（DPV）对尿酸和抗坏血酸分别进行检测,确定其原始峰值电位。最后,实验测试和分析比较了不同浓度尿酸和抗坏血酸对电化学检测信号的影响。研究结果表明：用DPV法分别检测尿酸和抗坏血酸,测得峰电流与样品浓度均有较好的线性度;从并行检测信号中能够分辨出尿酸和抗坏血酸的氧化峰位;尿酸和抗坏血酸对彼此的DPV峰位无明显影响,但对DPV电流峰值有一定抑制作用。