Microelectrode Structure Research Articles

Effects of Faradaic currents on AC electroosmotic flows with coplanar symmetric electrodes

Microelectrodes subjected to AC potentials are usually employed to induce electrolyte flow, commonly known as AC electroosmotic (ACEO) flows. In this work, we apply novel theoretical results for analysing the influence of Faradaic reactions on these flows. Specifically, we choose to analyse the electrokinetic flow induced by a pair of coplanar symmetric microelectrodes, as the simplest microelectrode structure used in experiments. The liquid is assumed to be a 1:1 electrolyte with ions of different diffusivities. For simplicity, in the model only the cations are responsible for the Faradaic reactions while the anions do not react at the electrodes. We show how to use the novel model to numerically obtain the electrokinetic flow induced by the pair of electrodes. A remarkable new prediction of the model is that, for certain values of the parameters, fluid flow can occur in opposite direction to that obtained in the absence of Faradaic reactions.

Electric Field Simulation of High-throughput Cell Electrofusion Chip

The electric field profile within a cell-fusion chip is of great significance for cell manipulation and cell-fusion efficiency, which is a main factor considered in chip design. This profile is mainly decided by the channel geometry and microelectrode structure. In the cell-fusion chip, the microelectrode array which was composed of a large number of microelectrodes was used to obtain high cell-fusion efficiency. Its simulation was difficult because there were many electrodes, complex channel geometry and microelectrode structure on this chip. ANSYS software was used in this study to simulate the electric field profile (strength and gradient) in the cell fusion chip. Comparison between different designs, the layout of electrodes was optimized and an interdigital, pectinate, rectangular microelectrode arrays were selected as the main components of the cell-electrofusion chip. In the preliminary experiments on this chip prototype, many plant protoplasts could be fused simultaneously. The fusion efficiency (about 40%) was much larger than those in traditional chemical induced fusion (< 1%), electrofusion (< 10%) and rectangular symmetrical electrodes (< 20%), which validated that this novel chip could be used as a high-throughput cell-fusion tool.

A dielectrophoretic barrier-based microsystem for separation of microparticles

This paper presents a microfluidic system for separation of microparticles based on the use of dielectrophoretic barriers, which are constructed by aligning two layers of microelectrode structure face-to-face on the top and bottom sides of the microchannel. The energized barriers tend to prevent the particles in the flow from passing through. However, particles may penetrate the barriers if a sufficiently high flow rate is used. The flow velocity at which the particles begin to penetrate the barrier is defined as threshold velocity. Different particles are of different threshold velocities so that they can be separated. In this paper, the electrodes are configured with open ends and aligned with a certain angle to the direction of the flow. Polystyrene microbeads of different sizes (i.e., 9.6 and 16 μm in diameter) are studied in the tests. Under the experimental conditions, two particle trajectories are observed: the 9.6 μm beads penetrate the barriers and move straightly toward the fluidic outlet, while the 16 μm beads snake their way along the electrode edges at a relatively low speed. The two subpopulations of particles are separated into spatial distance of ∼10 mm within tens of seconds. The system presents a rapid and dynamic separation process within a continuous flow.

A 3D paired microelectrode array for accumulation and separation of microparticles

This paper presents a microfluidic system comprising a three-dimensional microelectrode structure for accumulation and separation of micro-sized particles based on the combination of negative dielectrophoresis (DEP) and hydrodynamic force. The paired electrode array is constructed by aligning two layers of microelectrode structure face to face on the top and bottom sides of the microchannel. Dielectrophoretic gates are generated between the top and bottom electrodes with high-frequency ac voltage. These gates are designed to study the behavior of microparticles such as polystyrene beads or cells carried by a laminar flow past the electrodes. Depending on the relative strength of the DEP force and hydrodynamic force acting on the particles, the particles can either penetrate the gates or settle by the gates. The threshold velocity at which the particles begin to penetrate the gates depends on a number of parameters such as channel height, particle size, dielectric properties, electrode width and local heating, etc. A wide range of these parameters give rise to approaches of accumulating and separating microparticles. For this purpose, a microfluidic device with the paired microelectrode array sitting on the channel has been designed and fabricated using microfabrication techniques. Polystyrene beads were used to study the performance of the device and particle behavior. Using a proper range of flow rates, particle accumulation and separation are successfully achieved with the microsystem.

A 3-D Microelectrode System for Dielectrophoretic Manipulation of Microparticles

This paper presents a microfluidic system for manipulation and separation of micron-sized particles based on the combined use of negative dielectrophoresis (DEP) and hydrodynamic forces. A 3-D microelectrode structure (so called paired electrode array) are constructed face to face on the top and bottom sides of the microchannel and driven with highfrequency AC voltage to generate dielectrophoretic gates. Depending on the relative strengths of the two forces, particles such as polystyrene beads or cells carrying by a laminar flow can either penetrate the gate or settle there. This gives rise to certain applications including selectively concentrating particles from the flow, separating particles depending on their sizes or dielectric properties, and automatically positioning particles to selective locations. For this purpose, a microfluidic device consisting of the paired electrode array sitting on the channel has been fabricated using microfabrication techniques. Polystyrene beads were used to study the performance of the device. Experimental results including the concentration and separation of particles are presented.

Intravascular electric impedance spectroscopy of atherosclerotic lesions using a new impedance catheter system

Newer techniques are required to identify atherosclerotic lesions that are prone to rupture. Electric impedance spectroscopy (EIS) can characterize biological tissues by measuring the electrical impedance over a frequency range. We tested a newly designed intravascular impedance catheter (IC) by measuring the impedance of different stages of atherosclerosis induced in an animal rabbit model. Six female New Zealand White rabbits were fed for 17 weeks with a 5% cholesterol-enriched diet to induce early forms of atherosclerotic plaques. All aortas were prepared from the aortic arch to the renal arteries and segments of 5-10 mm were marked by ink spots. A balloon catheter system with an integrated polyimide-based microelectrode structure was introduced into the aorta and the impedance was measured at each spot by using an impedance analyzer. The impedance was measured at frequencies of 1 kHz and 10 kHz and compared with the corresponding histomorphometric data of each aortic segment.Forty-four aortic segments without plaques and 48 segments with evolving atherosclerotic lesions could be exactly matched by the histomorphometric analysis. In normal aortic segments (P0) the change of the magnitude of impedance at 1 kHz and at 10 kHz (|Z|(1 kHz) - |Z|(10 kHz), = ICF) was 208.5 +/- 357.6 Omega. In the area of aortic segments with a plaque smaller than that of the aortic wall diameter (PI), the ICF was 137.7 +/- 192.8 Omega. (P 0 vs. P I; p = 0.52), whereas in aortic segments with plaque formations larger than the aortic wall (PII) the ICF was significantly lower -22.2 +/- 259.9 Omega. (P0 vs. PII; p = 0.002). Intravascular EIS could be successfully performed by using a newly designed microelectrode integrated onto a conventional coronary balloon catheter. In this experimental animal model atherosclerotic aortic lesions showed significantly higher ICF in comparison to the normal aortic tissue.

Numerical simulation of travelling wave induced electrothermal fluid flow

Many microdevices for manipulating particles and cells use electric fields to produce a motive force on the particles. The movement of particles in non-uniform electric fields is called dielectrophoresis, and the usual method of applying this effect is to pass the particle suspension over a microelectrode structure. If the suspension has a noticeable conductivity, one important side effect is that the electric field drives a substantial conduction current through the fluid, causing localized Joule-heating. The resulting thermal gradient produces local conductivity and permittivity changes in the fluid. Dielectrophoretic forces acting upon these pockets of fluid will then produce motion of both the fluid and the particles.This paper presents a numerical solution of the electrical force and the resulting electrothermal driven fluid flow on a travelling wave structure. This common electrode geometry consists of interdigitated electrodes laid down in a long array, with the phase of the applied potential shifted by 90° on each subsequent electrode. The resulting travelling electric field was simulated and the thermal field and electrical body force on the fluid calculated, for devices constructed from two typical materials: silicon and glass. The electrothermal fluid flow in the electrolyte over the electrode array was then numerically simulated.The model predicts that the thermal field depends on the conductivity and applied voltage, but more importantly on the geometry of the system and the material used in the construction of the device. The velocity of the fluid flow depends critically on the same parameters, with slight differences in the thermal field for glass and silicon leading to diametrically opposite flow direction with respect to the travelling field for the two materials. In addition, the imposition of slight external temperature gradients is shown to have a large effect on the fluid flow in the device, under certain conditions leading to a reversal of the fluid flow direction.

Analysis of dielectrophoretic electrode arrays for nanoparticle manipulation

A typical microelectrode structure, interdigitated bar electrodes, has been developed for the selective manipulation and separation of nanoparticles using traveling field dielectrophoresis effects under AC electrokinetics. This paper presents meshless numerical solutions of traveling wave dielectrophoresis for such interdigitated electrode array energized with four-phase signal. This meshless method is based on a truncated 2D Taylor series expansion with the first three orders together with a weighted least-square approximation procedure. Dielectrophoretic forces and traveling wave forces are calculated and their applications for nanoparticle manipulation and separation are discussed.

Manipulation of bioparticles using traveling wave dielectrophoresis: numerical approach

A typical microelectrode structure, interdigitated bar electrodes, has been developed for the selective manipulation and separation of bioparticles using traveling field dielectrophoresis effects under AC electrokinetics. This paper presents meshless numerical solutions of traveling wave dielectrophoresis for such interdigitated electrode array energized with 4-phase signal. This meshless method is based on a truncated 2D Taylor series expansion with the first three orders together with a weighted least-square approximation procedure. Dielectrophoretic forces and traveling wave forces are calculated and their applications for bioparticle manipulation and separation are discussed.

Experimental Investigation on Neural Cell Survival after Dielectrophoretic Trapping

Negative dielectrophoretic forces can effectively be used to trap cortical rat neurons. The creation of dielectrophoretic forces requires electric fields of high non-uniformity. High electric field strengths, however, can cause excessive membrane potentials by which cells may unrecoverably be changed or it may lead to cell death. In a previous study it was found that cells trapped at 3 Vtt/14 MHz did not change morphologically as compared to cells that were not exposed to the electric field. This study investigates the viability of fetal cortical rat neurons after being trapped by negative dielectrophoretic forces at frequencies up to 1 MHz. A planar quadrupole micro-electrode structure was used for the creation of a non-uniform electric field. The sinusoidal input signal was varied in amplitude (3 and 5 Vtt) and frequency (10 kHz-1 MHz). The results presented in this paper show that the viability of dielectrophoretically trapped postnatal cortical rat cells was greatly frequency dependent. To preserve viability frequencies above 100 kHz (at 3 Vtt) or 1 MHz (5 Vtt) must be used.

Thermal conductivity measurements of thin-film resist

In electron-beam and photolithography, local heating can change the resist sensitivity and lead to variations in significant critical dimension. Existing models suffer from the lack of experimental data for the thermal properties of the polymer resist films. We present the measurements of both out-of-plane and in-plane thermal conductivity of thin resist films following different exposure conditions. An optical thermoreflectance technique was used to characterize out-of-plane thermal conductivity; the out-of-plane thermal conductivity of exposed SPR™-700 resist increases as a function of exposure dose. We also designed and fabricated a free-standing micro-electrode structure for measuring the in-plane thermal conductivity and results for poly(methylmethacrylate) films were obtained, indicating that, unlike polyimide films, there is no appreciable anisotropic behavior.

Dielectrophoretic trapping of dissociated fetal cortical rat neurons.

Recording and stimulating neuronal activity at multiple sites can be realized with planar microelectrode arrays. Efficient use of such arrays requires each site to be covered by at least one neuron. By application of dielectrophoresis (DEP), neurons can be trapped onto these sites. This study investigates negative dielectrophoretic trapping of fetal cortical rat neurons. A planar quadrupole microelectrode structure was used for the creation of a nonuniform electric field. The field was varied in amplitude (1, 3, and 5 V) and frequency (10 kHz-50 MHz). Experimental results were compared with a theoretical model to investigate the yield (the number of neurons trapped in the center of the electrode structure) with respect to time, amplitude and frequency of the field. The yield was a function of time(1/3) according to theory. However, unlike the model predicted, an amplitude-dependent frequency behavior was present and unexpected peaks occurred in the DEP-spectra above 1 MHz. Gain/phase measurements showed a rather unpredictable behavior of the electrode plate above 1 MHz, and temperature measurement showed that heating of the medium influenced the trapping effect, especially for larger amplitudes and higher frequencies.

Electromanipulation and separation of cells using travelling electric fields

A multi-layer micro-electrode structure has been developed for the selective manipulation and separation of bioparticles using travelling field dielectrophoresis effects. An important feature is that, in the separation process, the selected particles move in a stationary supporting fluid. Stationary suspensions of viable and non-viable yeast cells were used as a model system to demonstrate the general application of this device for the selective retention or transport of bioparticles in suspended mixtures. The efficiency of this process depends on the dielectric properties of the particles and their suspending medium, and is a sensitive function of the frequency of the travelling field. Apart from their use as particle separators, such micro-electrode devices are also envisaged to form integral components in the development of `biofactory on a chip' technology.

A multisensor system for beer flavour monitoring using an array of conducting polymers and predictive classifiers

This paper describes the results of a three year project, jointly funded by the UK government and industry, to develop a multisensor system capable of discriminating between the aromas of different beers. The system consists of an array of up to 24 conducting polymer sensors (thin films electrodeposited onto a microelectrode structure). The conducting polymers provide the active layer in these conductometric odour sensors and respond differentially to the headspaces of beers and lagers. The interface circuitry and signal conditioning have been designed and realized in custom PCBs housed in a Eurorack-based multisensor system. A comprehensive suite of software modules has been developed to automate the sampling system and process the sensor array data. The output from the polymer array is pre-processed using a variety of algorithms (e.g., fractional change in conductance, normalized relative response) and then classified using a statistical (chemometric fingerprinting technique) or neural predictive classifier (multi-layer perceptron using back-propagation learning). The odour-sensing system can distinguish subtle taints, e.g., 0.5 ppm of diacetyl in an ethanol solution with only nine different varieties of conducting polymers.