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The application of the method of multipole images using re-expansion techniques to the general case of intersecting conducting spheres under an external potential is described. The method is applicable for cases of electrically floating, grounded, or energized conducting spheres that intersect each other and the numerical results for equal radii agree with the case of completely overlapped spheres and that of touching spheres.

The electric field behavior, in particular the field intensification at a contact point, is very important in complex dielectric systems with gaseous or vacuum insulation. The paper describes the electric field behavior at and near a contact point in various arrangements with a zero contact angle when volume conductivity is present in the solid dielectric. Contact conditions are separated into line, point, and surface contact. The effect of volume conductivity is investigated analytically, and numerically by using the boundary element method. The electric field behavior near a contact point principally depends on the absolute value of complex relative permittivity, and volume conductivity usually promotes the field intensification. In the arrangements of point contact or line contact, the position of peak electric field shifts from a contact point when the volume conductivity is higher than a certain value, while in the arrangement of surface contact, the position is usually more or less remote from the contact point, whether volume conductivity is present or not.

This article presents the analysis of the electric field and dielectrophoretic force on spherical particles with a surface film. In the analysis, we use the method of images with the fundamental solutions given for the various types of particles composed of a core and a surface film. The concept of the apparent conductivity is introduced to clarify the difference between the particle types. The analysis shows that the response of a particle with a surface film to an external field is unique and generally cannot be obtained by replacing the particle with a homogeneous particle. Using numerical examples, we compare the field and force characteristics between the particles that are of different types but exhibit an identical response to a uniform external field. The electric field and force are found smaller on the conductor-core particle but greater on the dielectric-core particle compared with the particle without any surface film. We have calculated the electric field and force by using the boundary element method in which a surface film is treated as a zero-thickness medium. The propriety of such treatment of a surface film depends not only on the film properties but also on the external field. [ABSTRACT FROM AUTHOR]

This article presents an analytical method for calculating electric field and dielectrophoretic force in three-dimensional arrangements of spherical particles. The analytical method is based on the method of images that utilizes the multipole re-expansion and the fundamental solutions for several arrangements of a multipole. It is capable of calculating electric field for various conditions of particles and energization. The method needs much less memory than the already proposed one. The calculation results show that force on a dielectric particle chain in a dielectric fluid depends on the number of particles and the chain direction. However, the maximal attractive and repulsive forces reach their saturation values at about 32 and 12 particles, respectively. When the lower particle of a two-particle chain is in contact with a plate electrode, the dielectrophoretic force on the chain becomes higher on the whole, and it always attracts the chain to the electrode. As a result, the particle chain is stabilized for a wider range of the angle between the chain and the applied field. Neglecting the interaction between the electrodes and particles usually gives adequate accuracy in the force calculation, unless the electrodes are not very close to particles. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The authors present the use of electric-field constriction created by a microfabricated structure to realise high-yield electrofusion of biological cells. The method uses an orifice on an electrically insulating wall (orifice plate) whose diameter is as small as that of the cells. Owing to the field constriction created by the orifice, we can induce the controlled magnitude of membrane voltage selectively around the contact point, regardless of the cell size. The field constriction also ensures 1:1 fusion even when more than two cells are forming a chain at the orifice. A device for electrofusion has been made with a standard SU-8 lithography and PDMS molding, and real-time observation of the electrofusion process is made. Experiments using plant protoplasts or mammalian cells show that the process is highly reproducible, and the yield higher than 90% is achieved. [ABSTRACT FROM AUTHOR]

The estimation of the membrane voltage and the polarisation factor of biological cells provide a base for the study of bio-manipulation techniques, such as dielectrophoresis, electroporation or electrofusion. To model a biological cell, an ellipsoidal particle with an insulating membrane is sometimes employed, but due to the limitation of the confocal nature of the coordinate system, the membrane thickness is assumed to vary with the position, despite the fact that the lipid bilayer membrane has a uniform thickness. The authors present a method to rigorously treat the uniform-thickness condition in a system having an axial symmetry. The method is based on the harmonic expansion of the field, to include the condition of the uniform-membrane thickness as a series expansion of the geometrical factor, and to solve the field problem as an interaction of the harmonic components. The conventional variable thickness model has been identified as being equivalent to neglecting the harmonic interactions in the uniform-thickness model. Numerical calculations are done of the membrane voltage and the polarisation factor, and it has been found that the discrepancy between the proposed rigorous model and the conventional variable thickness model becomes significant when field deformation is large due to the high axial ratio of the ellipsoid. [ABSTRACT FROM AUTHOR]

This paper presents results of calculations of the electric field and dielectrophoretic force on a dielectric particle chain suspended in a host liquid lying between parallel-plate electrodes. The method of calculation is based on the method of multipole images using the multipole re-expansion technique. We have investigated the effect of the particle permittivity, the tilt angle (between the chain and the applied field) and the chain arrangement on the electric field and force. The results show that the electric field intensification rises in accordance with the increase in the ratio of the particle-to-liquid permittivity, Γε. The electric field at the contact point between the particles decreases with increasing tilt angle, while the maximal field at the contact point between the particles and the plate electrodes is almost unchanged. The maximal field can be approximated by a simple formula, which is a quadratic function of Γε. The dielectrophoretic force depends significantly on the distance from other particles or an electrode. However, for the tilt angles in this paper, the horizontal force on the upper particle of the chain always has the direction opposite to the shear direction. The maximal horizontal force of a chain varies proportional to (Γε − 1)1.7 if the particles in the chain are still in contact with each other. The approximated force, based on the force on an isolated chain, has been compared with our calculation results. The comparison shows that no approximation model agrees well with our results throughout the range of permittivity ratios. [ABSTRACT FROM AUTHOR]

Malaria is a disease that affects millions of people worldwide, particularly in developing countries. The development of accurate and efficient methods for the detection of malaria-infected cells is crucial for effective disease management and control. This paper presents the electrical impedance spectroscopy (EIS) of normal and malaria-infected red blood cells. An EIS microfluidic device, comprising a microchannel and a pair of coplanar electrodes, was fabricated for single-cell measurements in a continuous manner. Based on the EIS results, the aim of this work is to discriminate Plasmodium falciparum -infected red blood cells from the normal ones. Different from typical impedance spectroscopy, our measurement was performed for the cells in a low-conductivity medium in a frequency range between 50 kHz and 800 kHz. Numerical simulation was utilized to study the suitability parameters of the microchannel and electrodes for the EIS experiment over the measurement frequencies. The measurement results have shown that by using the low-conductivity medium, we could focus on the change in the conductance caused by the presence of a cell in the sensing electrode gap. The results indicated a distinct frequency spectrum of the conductance between the normal and infected red blood cells, which can be further used for the detection of the disease.

Malaria is a tropical disease caused by parasites in the genus Plasmodium, which still presents 241 million cases and nearly 627,000 deaths recently. In this work, we used the dielectrophoresis (DEP) to characterize red blood cells in a microchannel. The purpose of this work is to determine the difference between the normal and the malaria-infected cells based on the DEP characteristics. The samples were infected cells and normal red blood cells, which were either prepared in culture or obtained from volunteers. Diamond-shaped and curved micropillars were used to create different degrees of DEP in the gap between them. The DEP crossover frequencies were observed with the diamond-shaped micropillars. The cell velocity under negative dielectrophoresis (nDEP) at a low frequency was examined with the curved micropillars. The measured lower crossover frequencies were remarkably different between the malaria-infected cells and the normal cells, whereas the higher crossover frequencies were similar among the samples. The velocity under nDEP was lower for the infected cells than the normal cells. The results imply that the malaria infection significantly decreases the capacitance but increases the conductance of the cell membrane, whereas a change in cytoplasmic conductivity may occur in a later stage of infection., (© 2023 Wiley-VCH GmbH.)

Malaria is a serious disease caused by Plasmodium parasites that infect red blood cells (RBCs). This paper presents the continuous separation of malaria-infected RBCs (iRBCs) from normal blood cells. The proposed method employed the discrete dielectrophoresis (DEP) in a microfluidic device with interdigitated electrodes. Our aim is to treat a sample having high concentration of cells to realize high throughput and to prevent the clogging of the microchannel with the use of the discrete DEP. The discrete DEP force for deflecting cells in the device was controlled by adjusting the magnitude, frequency, and duty cycle of the applied voltage. The effectiveness of the proposed method was demonstrated by separating the malaria-infected cells in samples having a cell concentration of 10 6 cells/µl. From experimental results, we determined the enrichment that is needed to enhance the detection in the case of low parasitemia. The enrichment of the infected cells at the device output was 3000 times as high as that of the input containing 1 infected cell to 10 6 normal cells. Therefore, the proposed method is highly effective and can significantly facilitate the detection of the infected cells for the identification of Malaria patients., (© 2022 Wiley-VCH GmbH.)

This paper presents the application of the discrete dielectrophoretic force to separate polystyrene particles from red blood cells. The separation process employs a simple microfluidic device that is composed of interdigitated electrodes and a microchannel. The discrete dielectrophoretic force is generated by adjusting the duty cycle of the applied voltage. The electrodes make a tilt angle with the microchannel to change the moving direction of the red blood cells. By adjusting the voltage magnitude and duty cycle, we investigate the deflection of red blood cells and the variation of cell velocity along electrode edge under positive dielectrophoresis. The experiments with polystyrene particles show that the enrichment of the particles is greater than 150 times. The maximum separation efficiency is 97% for particle-to-cell number ratio equal to 1:2000 in the sample having high cell concentration. Using the appropriate applied voltage magnitude and duty cycle, the discrete dielectrophoretic force can prevent the clogging of microchannel while successfully separating the particles from the cells with high enrichment and efficiency. The proposed principle can be readily applied to dielectrophoresis-based devices for biomedical sample preparation or diagnosis such as the separation of rare or infected cells from a blood sample., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this paper, we present a novel electrofusion device that enables massive parallelism, using an electrically insulating sheet having a two-dimensional micro-orifice array. The sheet is sandwiched by a pair of micro-chambers with immersed electrodes, and each chamber is filled with the suspensions of the two types of cells to be fused. Dielectrophoresis, assisted by sedimentation, is used to position the cells in the upper chamber down onto the orifices, then the device is flipped over to position the cells on the other side, so that cell pairs making contact in the orifice are formed. When a pulse voltage is applied to the electrodes, most voltage drop occurs around the orifice and impressed on the cell membrane in the orifice. This makes possible the application of size-independent voltage to fuse two cells in contact at all orifices exclusively in 1:1 manner. In the experiment, cytoplasm of one of the cells is stained with a fluorescence dye, and the transfer of the fluorescence to the other cell is used as the indication of fusion events. The two-dimensional orifice arrangement at the pitch of 50 μm realizes simultaneous fusion of 6 × 10³ cells on a 4 mm diameter chip, and the fusion yield of 78-90% is achieved for various sizes and types of cells., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)