Iterative dipole moment method for calculating dielectrophoretic forces of particle-particle electric field interactions

Dielectrophoresis (DEP) is one of the most popular techniques for bio-particle manipulation in microfluidic systems. Traditional calculation of dielectrophoretic forces of single particle based on the approximation of equivalent dipole moment (EDM) cannot be directly applied on the dense particle interactions in an electrical field. The Maxwell stress tensor (MST) method is strictly accurate in the theory for dielectrophoretic forces of particle interaction, but the cumbersome and complicated numerical computation greatly limits its practical applications. A novel iterative dipole moment (IDM) method is presented in this work for calculating the dielectrophoretic forces of particle-particle interactions. The accuracy, convergence, and simplicity of the IDM are confirmed by a series of examples of two-particle interaction in a DC/AC electrical field. The results indicate that the IDM is able to calculate the DEP particle interaction forces in good agreement with the MST method. The IDM is a purely analytical operation and does not require complicated numerical computation for solving the differential equations of an electrical field while the particle is moving.     

Multiplexing microelectrodes for dielectrophoretic manipulation and electrical impedance measurement of single particles and cells in a microfluidic device

This work presents a microfluidic device, which was patterned with (i) microstructures for hydrodynamic capture of single particles and cells, and (ii) multiplexing microelectrodes for selective release via negative dielectrophoretic (nDEP) forces and electrical impedance measurements of immobilized samples. Computational fluid dynamics (CFD) simulations were performed to investigate the fluidic profiles within the microchannels during the hydrodynamic capture of particles and evaluate the performance of single‐cell immobilization. Results showed uniform distributions of velocities and pressure differences across all eight trapping sites. The hydrodynamic net force and the nDEP force acting on a 6 μm sphere were calculated in a 3D model. Polystyrene beads with difference diameters (6, 8, and 10 μm) and budding yeast cells were employed to verify multiple functions of the microfluidic device, including reliable capture and selective nDEP‐release of particles or cells and sensitive electrical impedance measurements of immobilized samples. The size of immobilized beads and the number of captured yeast cells can be discriminated by analyzing impedance signals at 1 MHz. Results also demonstrated that yeast cells can be immobilized at single‐cell resolution by combining the hydrodynamic capture with impedance measurements and nDEP‐release of unwanted samples. Therefore, the microfluidic device integrated with multiplexing microelectrodes potentially offers a versatile, reliable, and precise platform for single‐cell analysis.     

Simple and cost-effective determination of ciprofloxacin hydrochloride by electrical micro-titration

By employing an electrical micro-titration system, in which a capacitively coupled contactless conductivity detector(C4D) was used to monitor the reaction process in real time, herein a novel method for determining ciprofloxacin hydrochloride(CIPHCl) was developed for the first time. Mode 1: Standard CIPHCl solutions at different concentrations were loaded into reaction cells, respectively, and were titrated with standard Ag+. Upon the titration, the formation of a precipitate alters the number of ions in the solution, raising the change of conductivity, which was monitored by a special C~4 D to construct a titration curve. The endpoint of the titration was located from the peak of the curve. Between the elapsed time and the initial concentration of titrand, a linear relationship was established over the range of2.0–8.0 mmol/L. Mode 2: Standard Fe~(3+) took the place of Ag~+, and was used as titrant to recognize ciprofloxacin contributed to the formation of complexation, which also resulting a change of solution conductivity. Under optimized conditions, a working range of 1.0–5.0 mmol/L CIPHCl was found. Because the reaction solutions were isolated from the working electrodes, this pioneer work shows significant simplicity and cost-effectiveness, by eliminating the requirements for detector exchange/renewal between different measurements, and by involving no auxiliary chemicals. Both of the two approaches were applied successfully to determine CIPHCl in tablet samples. And the results were in good agreement with those obtained by reference method.     

聚二甲基硅氧烷-玻璃微流控芯片制作及高压非接触式电导检测系统的研制

报道了基于边缘加固的PDMS-玻璃芯片制作新方法。在给定条件下,测得芯片的剪切强度为20.4N/cm2,电渗流为2.9×10-4cm2/Vs(RSD5.6%,n=5),方向指向阴极。研制了一套新型高压非接触式电导(HV-CCD)检测系统,详尽地介绍了检测器的接口和信号屏蔽技术。在优化条件下,30s内实现了对K 、Na 、Li 的分离检测,检出限(3×S/N)分别为2.0μmol/L、3.0μmol/L、5.0μmol/L。     

Analysis of Glutamic Acid in Cerebrospinal Fluid by Capillary Electrophoresis with High Frequency Conductivity Detection

Glutamic acid(Glu)is the most important amino acid in the nerve center system1.It isessential to detect Glu in cerebrospinal fluids(CSF),because the level of Glu in CSF hasimplicated several diseases such as metabolic diseases or central nervous systemdisorders2.The measurement of Glu in CSF had been performed by high performanceliquid chromatography(HPLC)and capillary electrophoresis(CE)with inducedfluorescence detection3-7.Since Glu shows no fluorescence,different reagents had beenus…     

Continuous organelle separation in an insulator-based dielectrophoretic device

Heterogeneity in organelle size has been associated with devastating human maladies such as neurodegenerative diseases or cancer. Therefore, assessing the size-based subpopulation of organelles is imperative to understand the biomolecular foundations of these diseases. Here, we demonstrated a ratchet migration mechanism using insulator-based dielectrophoresis in conjunction with a continuous flow component that allows the size-based separation of submicrometer particles. The ratchet mechanism was realized in a microfluidic device exhibiting an array of insulating posts, tailoring electrokinetic and dielectrophoretic transport. A numerical model was developed to elucidate the particle migration and the size-based separation in various conditions. Experimentally, the size-based separation of a mixture of polystyrene beads (0.28 and 0.87 $\mu$m) was accomplished demonstrating good agreement with the numerical model. Furthermore, the size-based separation of mitochondria was investigated using a mitochondria mixture isolated from HepG2 cells and HepG2 cells carrying the gene Mfn-1 knocked out, indicating distinct size-related migration behavior. With the presented continuous flow separation device, larger amounts of fractionated organelles can be collected in the future allowing access to the biomolecular signature of mitochondria subpopulations differing in size.     

The surface conductance of red blood cells and platelets is modulated by the cell membrane potential

Dielectrophoretic analysis of cell electrical properties via the Clausius–Mossotti model has been widely used to estimate values of the membrane conductance, membrane capacitance and cytoplasm conductivity of cells. However, although the latter two values produced by this method compare well to those acquired by other electrophysiological methods, the membrane conductance is often substantially larger than that acquired by methods such as patch clamp. In this paper, the electrical properties of red blood cells (RBC) are analysed at two conductivities and following membrane-altering treatments, to develop a mathematical model of membrane conductance. Results suggest that the RBC “membrane conductance” term is primarily dominated by surface conduction, comprising an element related to medium conductivity augmented by conduction in the electrical double layer, which is in turn altered by the cell membrane potential. Validation of the relationship between membrane potential and membrane conductance was performed using platelets, where a similar relationship was observed. This sheds new light on the origin and significance of the membrane conductance term and explains for the first time phenomena of alterations in the parameter counter to changes in membrane potential or cytoplasm conductivity.     

Parallelized continuous flow dielectrophoretic separation of DNA

Numerous microfluidic separation applications have been shown in the past years providing a fast analysis of biological samples like DNA or proteins. Microfluidic separation based on dielectrophoresis (DEP), that is the migration of a polarizable object in an inhomogeneous electric field, provides numerous advantages. However, the main drawback of DEP separation devices is that they are not sufficient for large-scale sample purification due to the lack of high sample throughput. In this work, we present for the first time a microfluidic device with two parallelized dielectrophoretic separations of (biological) samples smaller than 1 µm. The separation is carried out by means of insulator-based DEP, that is an insulating ridge reduced the flow through height and thus created a nanoslit at which the selective DEP forces occur. The device consists of a cross injector, two parallel operation regions and separate harvesting reservoirs where the samples are collected. Each DEP operation region contains an insulating ridge. We successfully demonstrate the separation of 100 and 40 nm beads and 10 and 5 kbp DNA with a separation purity of more than 80%. This states the proof-of-concept for up-scaling of dielectrophoretic separation by parallelization. As the present technique is virtually label-free, it offers a fast purification, for example in the production of gene vaccines.     

微流控芯片非接触电导法测定感冒药中盐酸伪麻黄碱和氢溴酸右美沙芬

研究了用微流控芯片非接触式电导法分离检测感冒药日夜百服咛片中的两种主要成分盐酸伪麻黄碱和氢溴酸右美沙芬的方法。优化条件为:缓冲液20 mmol/L Tris 20 mmol/L H3BO3(pH=8.0);进样电压300 V;进样时间10.0 s;分离电压3.0 kV。非接触电导检测器激发电压60 V(Vp-p),频率60 kHz。两种成分的线性范围分别为20~1000 mg/L和10~1000 mg/L;检出限分别为10和5.0 mg/L;样品回收率分别为99.3%和97.6%。     

Electrodeless dielectrophoresis: Impact of geometry and material on obstacle polarization

Insulator‐based (electrodeless) dielectrophoresis (iDEP) is a promising particle manipulation technique, based on movement of matter in inhomogeneous fields. The inhomogeneity of the field arises because the excitatory field distorts at obstacles (posts). This effect is caused by accumulation of polarization charges at material interfaces. In this study, we utilize a multipole expansion method to investigate the influence of geometry and material on field distortion of posts with arbitrary cross‐sections in homogeneous electric fields applied perpendicular to the longitudinal axis of the post. The post then develops a multipole parallel or anti parallel to the excitatory field. The multipoles intensity is defined by the post's structure and material properties and directly influences the DEP particle trapping potential. We analyzed posts with circular and rhombus‐shaped cross‐sections with different cross‐sectional width‐to‐height ratios and permittivities for their polarization intensity, multipole position, and their particle trapping behavior. A trade‐off between high maximum field gradient and high coverage range of the gradient is presented, which is determined by the sharpness of the post's edges. We contribute to the overall understanding of the post polarization mechanism and expect that the results presented will help optimizing the structure of microchannels with arrays of posts for electrodeless DEP application.