Sequential Field-Flow Cell Separation Method in a Dielectrophoretic Chip With 3-D Electrodes

This paper presents a sequential dielectrophoretic field-flow separation method for particle populations using a chip with a 3-D electrode structure. A unique characteristic of our chip is that the walls of the microfluidic channels also constitute the device's electrodes. This property confers the opportunity to use the electrodes' shape to generate not only the electric field gradient required for dielectrophoretic force but also a fluid velocity gradient. This interesting combination gives rise to a new solution for the dielectrophoretic separation of two particle populations. The proposed sequential field-flow separation method consists of four steps. First, the microchannel is filled with the mixture of the two populations of particle. Second, the particle populations are trapped in different locations of the microfluidic channels. The population, which exhibits positive dielectrophoresis (DEP), is trapped in the area where the distance between the electrodes is the minimum, while the other population that exhibits negative DEP is trapped in locations of maximum distance between electrodes. In the next step, increasing the flow in the microchannels will result in an increased hydrodynamic force that sweeps the cell population trapped by positive DEP out of the chip. In the last step, the electric field is removed, and the second population is swept out and collected at the outlet. For theoretical and experimental exemplification of the separation method, a population of viable and nonviable yeast cells was considered.     

Dielectrophoresis-based microfluidic platform to sort micro-particles in continuous flow

Non-invasive separation of particles with different sizes and sensitivities has been a challenge and interest for point-of-care diagnostics and personalized treatment. Dielectrophoresis is widely known as a powerful technique to sort the particles and (most importantly to) distinguish cells and monitor their state without the need for biochemical tags. In this paper, a dielectrophoresis-based microchannel design is proposed which allows for continuous particle sorting and separation under the applied AC field. It is also practical to implement the platform for monitoring cell behavior irregularities caused by certain diseases toward diagnosis and treatment. In this regard, the device employs dielectrophoretic (DEP) force exerted on the particles by only two electrodes with oblique arrangement in the channel. The electrodes are arranged with a bevel angle to the fluid flow direction but they are not parallel and therefore a gradually decreasing electric field is achieved along the channel’s width. As a result, the dielectrophoretic force, acting on the particles of different sizes, would also gradually decrease along channels width which renders the necessary distinguishing lateral displacements of particles for separation. Therefore, the particles with different sizes can be sorted in a continuous-flow regime and be received at multiple outlet reservoirs with no need to turn the electric field on/off. The presented device is fabricated and evaluated in the experiment to prove its feasibility. Afterward, using numerical simulations, we investigate the optimum design parameters in the presented device to enhance device efficiency for separating particles with different size ranges.

     

Stepwise gray-scale light-induced electric field gradient for passive and continuous separation of microparticles

This article presents a gray-scale light-induced dielectrophoresis (GS-LIDEP) method that induces the lateral displacements normal to the through-flow for continuous and passive separation of microparticles. In general, DEP force only can affect the particles within very local areas due to the electric field is exponentially decayed by the distance away from the electrodes. Unlike with conventional LIDEP, a broad-ranged electrical field gradient can easily be created by GS pattern illumination, which induces DEP forces with two directions for continuous separation of particles to their specific sub-channels. Candia albicans were effectively guided to the specific outlet with the efficiency of 90% to increase the concentration of the sample below the flow rate of 0.6?μl/min. 2 and 10?μm polystyrene particles can also be passively and well separated using the multi-step GS pattern through positive and negative DEP forces, respectively, under an applied voltage of 36?V_p–p at the frequency of 10?kHz. GS-LIDEP generated a wide-ranged DEP force that is capable of working on the entire area of the microchannel, and thus the mix of particles can be passively and continuously separated toward the opposite directions by the both positive and negative GS-LIDEP forces. This simple, low cost, and flexible separation/manipulation platform could be very promising for many applications, such as in-field detections/pretreatments.     

基于介电泳的电极阵列电场仿真研究

介电泳方法被广泛地应用于微纳颗粒的分离和操纵中,实现介电泳操作的关键是设计满足所需电场分布的电极阵列.针对目前在微电极阵列设计中尚缺乏简单有效的电场解析方法的现状,提出一种基于格林公式的电极阵列电场的解析方法.首先介绍了传统介电泳和行波介电泳的概念和计算模型,分析了介电泳过程与电极上所施加的交变电压的频率和幅度的关系,然后在确立电极电势的边界条件的基础上,采用基于格林公式的电场解析方法,建立了非均匀电场的解析模型,得出不同条件下的电极阵列电场分布的仿真结果,最后利用FEMLAB有限元仿真软件对解析模型进行了对比仿真, 验证了该解析模型的可行性.基于格林公式的电场解析求解方法能够有效地提高电极阵列设计中的针对性以及缩短电极设计的时间.     

基于微环形阵列电极的细胞分离富集芯片的设计与实验研究

根据介电泳操作原理,设计了微环形阵列电极结构,建立了细胞分离富集芯片模型,采用COMSOL软件分析微环形阵列电极的电场分布和介电泳力方向并确定了最大和最小电场强度的位置,利用ITO玻璃和PDMS制备了细胞分离富集芯片.通过酵母菌细胞的介电泳富集实验和酵母菌细胞与聚苯乙烯小球的分离富集实验,明确了酵母菌细胞的临界频率,实现了酵母菌细胞和聚苯乙烯小球的分离富集.结果显示,在溶液电导率为60μs/cm,交流信号电压为8Vp-p时,酵母菌细胞在1kHz~45kHz频率范围内做负介电泳运动并富集在环形内部,45kHz为酵母菌细胞的临界频率,在45kHz~10MHz频率范围内做正介电泳运动并富集在环形边缘;1.5MHz时聚苯乙烯小球做负介电泳运动并富集在环形内部,富集倍数达到11.66.     

A micro shear stress sensor based on laterally aligned carbon nanotubes

This paper reports the development of a micro thermal shear stress sensor that utilizes multiwalled carbon nanotubes as the sensing element. The sensor was fabricated by laterally aligning randomly distributed nanotubes into a 360 μm long and 90 μm wide conductive trace between two triangular shaped micro electrodes through the use of a high frequency AC electric field. During operation, the aligned nanotubes are electrically heated to an elevated temperature and surface shear stress is measured indirectly by the amount of convective heat transfer from the heated nanotubes to the surrounding fluid flow.The nanotube alignment process was primarily controlled by three different phenomena: dielectrophoresis, joule heating, and Brownian motion. Numerical simulations, together with experimental verifications, indicated that a successful alignment could only be realized if: (1) the dielectrophoretic force was positive, (2) the electro-thermal force was also positive, and (3) the dielectrophoretic force was high enough to overcome Brownian motion. The aligned nanotube trace has a room-temperature resistance of 580 Ω, which corresponds to a conductivity of 2.7 × 10⁴ S/m. The absolute temperature coefficient of resistivity ranges from 0.01 to 0.04% °C⁻¹. This is about one order of magnitude smaller than the highly doped polysilicon sensing material used in the MEMS micro shear stress sensor. The shear stress sensitivity of the nanotube trace operated at a 3% overheat ratio is found to follow the theoretical sensor power ∝ (shear stress)^1/3 relationship, provided the shear stress level is higher than 0.34 mPa. This result confirms the feasibility of using aligned multi-walled carbon nanotubes as a thermal shear stress sensing material.     

介电泳芯片的研制及不同细胞介电分离的应用

制备了包括指状交叉、城墙状和梯形的微电极阵列芯片装置.并用这些芯片探索了生物细胞的介电响应.另外观察了酵母和鸡血红细胞的迁移、旋转和融合以及几种细胞收集图片.发现了两种细胞的正、负介电泳现象,确定了这两种细胞的分离条件.讨论了两种细胞正、负介电泳的原因.利用同一芯片在相同的条件下一种细胞移向强场区(正介电泳),另一种细胞移向弱场区(负介电泳).因此可用同一芯片分离不同的细胞.有望建立一种非接触式细胞分离技术,而且在分离过程中不需要添加任何试剂.     

Comparison of spherical and non-spherical particles in microchannels under dielectrophoretic force

This paper focuses on the computational and experimental study of dielectrophoretic (DEP) force based manipulation of spherical and non-spherical particles by taking into consideration of both electrokinetic effects and particle hydrodynamics. The model is first validated with conventional dipole moment theory. The movements of a spherical polystyrene particle and a rod-shape particle under a non-uniform electric field created by a pair of non-symmetrical electrodes in a microfluidic channel are studied, and a good agreement between the simulation and experimental results is obtained. Both experimental and simulation results reveal that the rod-shape particle experiences larger DEP force and moves faster than spherical particle with a similar mass. It was also interestingly found that the shape-dependent DEP force distribution on the microscale rod particle results in its unique behavior, which cannot be captured by traditional DEP theory.     

Dielectrophoresis in aqueous suspension: impact of electrode configuration

Dielectrophoresis (DEP) allows to moving neutral or charged particles in liquids by supplying a non-uniform electric field. When using alternating current and insulated electrodes, this is possible in conducting media such as aqueous solutions. However, relatively high field strength is required that is discussed to induce also an undesired Joule heating effect. In this paper, we demonstrate boundary conditions for avoiding this side effect and suggest a novel design of an interdigitated electrode (IDE) configuration to reduce the power consumption. Numerical simulation using OpenFOAM demonstrated that, when replacing conventional plate IDE by cylindrical micro-IDE in microchannel systems, the dielectrophoretic force field, i.e., the electric field gradient squared, becomes stronger and more homogeneously distributed along the electrodes array. Also the resulting particle DEP velocities were highest for the cylindrical IDE. The simulations were experimentally confirmed by measuring velocity of resin particle located at the subsurface of demineralized water. Surprisingly the fluid flow induced by electrothermal effect turned out to be negligible in microchannels when compared to the DEP effect and becomes dominant only for distances between particle and IDE larger than 6,000 μm. The well-agreed experimental and simulation results allow for predicting particle motion. This can be expected to pave the way for designing DEP microchannel separators with high throughput and low energy consumption.     

Reachability and controllability of a particle in a dielectrophoretic system

A dielectrophoretic (DEP) force is a result of the interaction between a nonuniform electric field and a polarizable particle. As the electric field is dominant at the micro/nano scale, this force can be effectively used to manipulate and control particles on this scale. We consider the motion of a particle on an invariant line with the suspending medium being a fluid with a low Reynolds number. This DEP system has two states and two parameters: the two states are indicative of the particle’s position and the induced dipole moment and the two parameters are α and c which depend upon the electric properties of the particle and the medium. The system is described by a set of ordinary differential equations with a quadratic term in the control variable (control being the applied voltage on the electrodes which induces the electric field) making the system non-affine in control. In the existing literature, the controllability studies of the DEP system have been restricted to reachability issues in the context of the time-optimal control problem. Here we present a comprehensive study of reachability, accessibility and controllability.     

Studies of particle holding,separating, and focusing using convergent electrodes in microsorters

This paper presents numerical simulation results on the efficacy of dielectrophoretic (DEP) convergent electrodes in a particle sorter. DEP forces created by non-uniform electric fields are used as holding forces to trap and select the particles from a mixture of many samples, as well as confining forces to focus the particles into a single particle stream in the microsorter for further analysis. The key mechanism of the sorter that can hold particles against destabilizing fluid flows is investigated in this study. A barrier is found at X/L=0.84 and Y=0 in the present DEP sorter. By comparing the DEP and hydrodynamic forces at the barrier, one can determine the release velocity when the zero-net-force condition ceases to exist and the particles start to be released.     

Manipulation of ZnO nanostructures using dielectrophoretic effect

In this paper, the dielectrophoretic manipulation of the nanostructured zinc oxide (ZnO) with microfabricated electrodes and electrode arrays had been studied. The nanorod-like ZnO prepared by the chemical solution growth, with the length of 10 μm, was used as the manipulation target. The electrodes and electrode arrays were prepared by standard IC process. The SEM pictures have been used to examine and evaluate the manipulation results. The influences of the pattern of electrodes, the applied frequency, the concentration and the applied voltage on the dielectrophoretic manipulation effect have been investigated to research the manipulation of particles by dielectrophoresis. We succeeded in manipulating ZnO particles along the electric field and depositing them across the gaps between two electrodes by modulating different factors. It is concluded that the nanostructured ZnO can be manipulated by dielectrophoresis and both the positive dielectrophoretic effect and the negative dielectrophoretic effect can be observed. This manipulation technique is potential for lots of application such as the construction of micro/nano sensors and the nanoelectronic devices.     

Dielectrophoretic separation of carbon nanotubes and polystyrene microparticles

The separation of multi-walled carbon nanotubes (MWCNTs) and polystyrene microparticles using a dielectrophoresis (DEP) system is presented. The DEP system consists of arrays of parallel microelectrodes patterned on a glass substrate. The performance of the system is evaluated by means of numerical simulations. The MWCNTs demonstrate a positive DEP behaviour and can be trapped at the regions of high electric field. However, the polystyrene microparticles demonstrate a negative DEP behaviour at a certain range of frequencies and migrate to the regions of low electric field. Experiments are performed on the microparticles at the frequencies between 100 Hz and 1 MHz to estimate their crossover frequency and select the range of separation frequencies. Further, experiments are conducted at the obtained range of separation frequencies to separate the MWCNTs and polystyrene microparticles.     

Development of a μ-concentrator Using Dielectrophoretic Forces

We present the simulation, development and experimental validation of a μ-concentrator based on dielectrophoresis, DEP.In a first step dielectrophoretic force fields of various electrodes are computed and compared. The simulation results for various electrode dimensions may serve as a general design rule for DEP devices. Favorable electrode designs were realized in gold on glass substrates. The performance of the DEP chips is validated by concentration of E.-Coli bacteria, a separation efficiency of 99.93% was achieved. Furthermore, we outline how the combination of forced convection and DEP allows for bacteria separation at increased flow rates.     

Analytical formulation of electric field and dielectrophoretic force for moving dielectrophoresis using Fourier series

Electrokinetics manipulation and separation of living cells employing microfluidic devices require good knowledge of the strength and distribution of electric field in such devices. AC dielectrophoresis is performed by generating non-uniform electric field using microsize electrodes. Among the several applications of dielectrophoretic phenomenon, this present study considers the recently introduced phenomenon of moving dielectrophoresis. An analytical solution using Fourier series is presented for the electric field distribution and dielectrophoretic force generated inside a microchannel. The potential at the upper part of the microchannel has been found by solving the governing equation of the electric potential with specific boundary conditions. The solutions for the electric field and dielectrophoretic force show excellent agreement with the numerical results. Microdevices were fabricated and experiments were carried out with living cells confirming and validating the analytical solutions.     

Particle focusing in a contactless dielectrophoretic microfluidic chip with insulating structures

The focusing of biological and synthetic particles in microfluidic devices is a crucial step for the construction of many microstructured materials as well as for medical applications. The present study examines the feasibility of using contactless dielectrophoresis (cDEP) in an insulator-based dielectrophoretic (iDEP) microdevice to effectively focus particles. Particles 10?μm in diameter were introduced into the microchannel and pre-confined hydrodynamically by funnel-shaped insulating structures near the inlet. The particles were repelled toward the center of the microchannel by the negative DEP forces generated by the insulating structures. The microchip was fabricated based on the concept of cDEP. The electric field in the main microchannel was generated using electrodes inserted into two conductive micro-reservoirs, which were separated from the main microchannel by 20-μm-thick insulating barriers made of polydimethylsiloxane (PDMS). The impedance spectrum of the thin insulating PDMS barrier was measured to investigate its capacitive behavior. Experiments employing polystyrene particles were conducted to demonstrate the feasibility of the proposed microdevice. Results show that the particle focusing performance increased with increasing frequency of the applied AC voltage due to the reduced impedance of PDMS barriers at high frequencies. When the frequency was above 800?kHz, most particles were focused into a single file. The smallest width of focused particles distributed at the outlet was about 13.1?μm at a frequency of 1?MHz. Experimental results also show that the particle focusing performance improved with increasing applied electric field strength and decreasing inlet flow rate. The usage of the cDEP technique makes the proposed microchip mechanically robust and chemically inert.     

Droplet creation using liquid dielectrophoresis

Dielectrophoresis (DEP) is defined as polarizable particles moving into regions of higher electric field intensity. In liquid DEP (LDEP), a dielectric liquid tends to flow toward regions of high electric field intensity under a non-uniform electric field. This work presents a theoretical model of LDEP based on parallel electrodes. The LDEP force is derived using the lump parameter electromechanical method. The relationship between the minimum actuation voltage and the electrode width is investigated experimentally and theoretically. We also propose a method for creating a 20 nl droplet of deionized water using LDEP. The creation of a water droplet containing 15 μm polystyrene beads is placed at the desired location from a continuous flow driven by LDEP using the developed method.     

Continuous size-based focusing and bifurcating microparticle streams using a negative dielectrophoretic system

Dielectrophoresis (DEP) is an electrokinetic phenomenon which is used for manipulating micro- and nanoparticles in micron-sized devices with high sensitivity. In recent years, electrode-based DEP by patterning narrow oblique electrodes in microchannels has been used for particle manipulation. In this theoretic study, a microchannel with triangular electrodes is presented and a detailed comparison with oblique electrodes is made. For each shape, the behavior of particles is compared for three different configurations of applied voltages. Electric field, resultant DEP force, and particle trajectories for configurations are computed by means of Rayan native code. The separation efficiency of the two systems is assessed and compared afterward. The results demonstrate higher lateral DEP force, responsible for particle separation, distributed wider across the channel width for triangular shape electrodes in comparison with the oblique ones. The proposed electrode shape also shows the ability of particle separation by attracting negative DEP particles to or propelling them from the flow centerline, according to the configuration of applied voltages. A major deficiency of the oblique electrodes, which is the streamwise variation of the lateral DEP force direction near the electrodes, is also eliminated in the proposed electrode shape. In addition, with a proper voltages configuration, the triangular electrodes require lower voltages for particle focusing in comparison with the oblique ones.     

Particle streaming and separation using dielectrophoresis through discrete periodic microelectrode array

This study presents a particle manipulation and separation technique based on dielectrophoresis principle by employing an array of isosceles triangular microelectrodes on the bottom plate and a continuous electrode on the top plate. These electrodes generate non-uniform electric fields transversely across the microchannel. The particles within the flowing fluid experience a dielectrophoretic force perpendicular to the fluid flow direction due to the non-uniform electric fields. The isosceles triangular microelectrodes were designed to continuously exert a small dielectrophoretic force on the particles. Particles experiencing a larger dielectrophoretic force would move further in the perpendicular direction to the fluid flow as they traveled past each microelectrode. Polystyrene microspheres were used as the model particles, with particles of ∅20 μm employed for studying the basic characteristics of this technique. Particle separation was subsequently demonstrated on ∅10 and ∅15 μm microspheres. Using an applied sinusoidal voltage of 20 V_pp and frequency of 1 MHz, a mean separation distance of 0.765 mm between them was achieved at a flow rate of 3 μl/min (~1 mm/s), an important consideration for high throughput separation capability in a micro-scale technology device. This unique isosceles triangular microelectrodes design allows heterogeneous particle populations to be separated into multiple streams in a single continuous operation.     

Control of the microparticle position in the channel based on dielectrophoresis

We report here the control of the microparticles position within fluid flow based on its size by using dielectrophoresis (DEP) with a microelectrode array consisted of rectangular features with the different size of width and gap. 3 μm- and 10 μm-diameter particles were introduced into the channel with 300 μm height at 30 μl/min. An AC electric field (20 V peak–peak and 2 MHz) was then applied to microelectrode arrays to form dielectrophoretic fluid cage, resulting in a formation of flow paths with low electric fields on the arrays. The microparticles separately flow in line streams along the paths formed between the rectangular features of the arrays, the 3 μm-diameter particles mainly flow through the narrow path and 10 μm-diameter particles through the wide path. These results indicated that positions of two types of microparticles in the fluidic channel were easily separated and controlled using the n-DEP.