AC Dielectrophoresis Research Articles

Recent Advances in Dielectrophoretic Manipulation and Separation of Microparticles and Biological Cells.

Dielectrophoresis (DEP) is an advanced microfluidic manipulation technique that is based on the interaction of polarized particles with the spatial gradient of a non-uniform electric field to achieve non-contact and highly selective manipulation of particles. In recent years, DEP has made remarkable progress in the field of microfluidics, and it has gradually transitioned from laboratory-scale research to high-throughput manipulation in practical applications. This paper reviews the recent advances in dielectric manipulation and separation of microparticles and biological cells and discusses in detail the design of chip structures for the two main methods, direct current dielectrophoresis (DC-DEP) and alternating current dielectrophoresis (AC-DEP). The working principles, technical implementation details, and other improved designs of electrode-based and insulator-based chips are summarized. Functional customization of DEP systems with specific capabilities, including separation, capture, purification, aggregation, and assembly of particles and cells, is then performed. The aim of this paper is to provide new ideas for the design of novel DEP micro/nano platforms with the desired high throughput for further development in practical applications.

Rapid and Sensitive Detection by Combining Electric Field Effects and Surface Plasmon Resonance: A Theoretical Study.

Surface plasmon resonance (SPR) has been extensively employed in biological sensing, environmental detection, as well as chemical industry. Nevertheless, the performance possessed by conventional surface plasmon resonance (SPR) biosensors can be further limited by the transport of analyte molecules to the sensing surface, noteworthily when small molecules or low levels of substances are being detected. In this study, a rapid and highly sensitive SPR biosensor is introduced to enhance the ability of the target analytes' collection by integrating AC electroosmosis (ACEO) and dielectrophoresis (DEP). Both the above-mentioned phenomena principally arise from the generation of the AC electric fields. This generation can be tailored by shaping the interdigitated electrodes (IDEs) that also serve as the SPR biomarker sensing area. The effects exerted by different parameters (e.g., the frequency and voltage of the AC electric field as well as microelectrode structures) are considered in the iSPR (interdigitated SPR) biosensor operation, and the iSPR biosensors are optimized with the sensitivity. The results of this study confirm that the iSPR can efficiently concentrate small molecules into the SPR sensing area, such that SPR reactions achieve an order of magnitude increase, and the detection time is shortened. The rapid and sensitive sensor takes on critical significance in the development of on-site diagnostics in a wide variety of human and animal health applications.

Optimization of upstream particle concentration from flow using AC electro-osmosis and dielectrophoresis.

There are many applications where upstream sample processing is required to concentrate dispersed particles in flow; this may be to increase the concentration (e.g., to enhance biosensor accuracy) or to decrease it (e.g., by removing contaminants from flow). The AC electrokinetic phenomenon, dielectrophoresis (DEP), has been used widely for particle trapping for flow, but the magnitude of the force drops reduces rapidly with distance from electrode edges, so that nm-scale particles such as viruses and bacteria are only trapped when near the electrode surface. This limits the usable flow rate in the device and can render the final device unusable for practical applications. Conversely, another electrokinetic phenomenon, AC electro-osmosis (ACEO), can be used to move particles to electrode surfaces but is unable to trap them from flow, limiting their ability for sample cleanup or trap-and-purge concentration. In this paper, we describe the optimization of ACEO electrodes aligned parallel to pressure-driven flow as a precursor/preconditioner to capture particles from a flow stream and concentrate them adjacent to the channel wall to enhance DEP capture. This is shown to be effective at flow rates of up to 0.84 ml min-1. Furthermore, the analysis of the 3D flow structure in the ACEO device by both simulation and confocal microscopy suggests that while the system offers significant benefits, the flow structure in the volume near the channel lid is such that while substantial trapping can occur, particles in this part of the chamber cannot be trapped, independent of the chamber height.

Trapping of a Single Microparticle Using AC Dielectrophoresis Forces in a Microfluidic Chip.

Precise trap and manipulation of individual cells is a prerequisite for single-cell analysis, which has a wide range of applications in biology, chemistry, medicine, and materials. Herein, a microfluidic trapping system with a 3D electrode based on AC dielectrophoresis (DEP) technology is proposed, which can achieve the precise trapping and release of specific microparticles. The 3D electrode consists of four rectangular stereoscopic electrodes with an acute angle near the trapping chamber. It is made of Ag-PDMS material, and is the same height as the channel, which ensures the uniform DEP force will be received in the whole channel space, ensuring a better trapping effect can be achieved. The numerical simulation was conducted in terms of electrode height, angle, and channel width. Based on the simulation results, an optimal chip structure was obtained. Then, the polystyrene particles with different diameters were used as the samples to verify the effectiveness of the designed trapping system. The findings of this research will contribute to the application of cell trapping and manipulation, as well as single-cell analysis.

Alternating-current nonlinear electrokinetics in microfluidic insulator-decorated bipolar electrochemistry

We proposed herein a unique method of insulator-decorated bipolar electrochemistry (IDBE), for realizing large-scale separation of bioparticles in microchannels driven by AC dielectrophoresis (DEP). In IDBE, a pair of planar driving electrodes is placed at the bottom of channel sidewalls, between which an array of the rectangular floating electrode (FE) strips without external Ohmic contact are evenly spaced along transversal direction, and a series of insulating dielectric blocks are periodically deposited above all the inter-electrode gaps and in full contact with the channel bottom surface. By creating local field maximum and minimum at multiple sites, IDBE extends well the actuating range of DEP force field from the immediate vicinity of electrode tips in traditional bipolar electrochemistry to current fluid bulk. Considering DEP force plays the dominant role around 1 MHz, we utilize Lagrange particle tracing algorithm to calculate motion trajectories of incoming samples for testing the feasibility of microchip in continuous separation of live and dead yeast cells. By applying suitable voltage parameters, highly efficient DEP sorting is theoretically achievable under a moderate inlet flow rate, where most of the viable yeasts are trapped by positive-DEP to sharp dielectric edges, while all the incoming nonviable yeasts are repelled by negative-DEP to the top surface of both FE and insulating block to form multiple thin beams co-flowing into the channel outlet. The microfluidic device exploiting insulators on bipolar FE effectively expands the actuating range of nonlinear electrodynamics and provides invaluable guidelines for developing flexible electrokinetic frameworks in modern microfluidic systems.

Numerical characterization of inter-core coalescence by AC dielectrophoresis in double-emulsion droplets.

The utilization of an alternating current electric field provides a good means to achieve controlled coalescence between paired inner cores encapsulated in water-in-oil-in-water double-emulsion (DE) droplets. Although previous studies have experimentally determined the conditions under which inter-core electrokinetic fusion occurs, the transient interfacial dielectrophoretic (DEP) dynamics key to understand the underlying fluid mechanics is still unclear from a physical point of view. By coupling DEP motion of two-phase flow to phase-field formulation, bulk-coupled numerical simulations are conducted to characterize the spatial-temporal evolution of the surface charge wave and the resulting nonlinear electrical force induced at both the core/shell and medium/shell oil/water interfaces. The effect of interfacial charge relaxation and droplet geometry on inter-core attractive dipolar interaction is investigated within a wide parametric space, and four distinct device operation modes, including normal inter-core fusion, shell elongation, partial core leakage, and complete core release, are well distinguished from one another by flow regime argumentation. Our results herein reveal for the first time the hitherto unknown transient electrohydrodynamic fluid motion of DE droplet driven by Maxwell-Wagner structural polarization. The dynamic simulation method proposed in present study points out an effective outlet to predict the nonlinear electrokinetic behavior of multicore DE droplets for realizing a more controlled triggering of microscale reactions for a wide range of applications in drug discovery, skin care, and food industry.

Sub-100 nm Nanoparticle Upconcentration in Flow by Dielectrophoretic Forces.

This paper presents a novel microfluidic chip for upconcentration of sub–100 nm nanoparticles in a flow using electrical forces generated by a DC or AC field. Two electrode designs were optimized using COMSOL Multiphysics and tested using particles with sizes as low as 47 nm. We show how inclined electrodes with a zig-zag three-tooth configuration in a channel of 20 µm width are the ones generating the highest gradient and therefore the largest force. The design, based on AC dielectrophoresis, was shown to upconcentrate sub–100 nm particles by a factor of 11 using a flow rate of 2–25 µL/h. We present theoretical and experimental results and discuss how the chip design can easily be massively parallelized in order to increase throughput by a factor of at least 1250.

Versatile microparticle propulsion system by light-guided dielectrophoresis: Proposed method and theoretical calculation

Developing materials for active matter that can efficiently respond to external stimuli with designed multifold mechanical motions remains a major challenge, and overcoming this will greatly propel the advancement of micromachines and microrobots toward unprecedented biomedical, electronic, and particle-separation applications. Here, we propose an innovative working mechanism that allows multifold-translational-motion control of semiconductor microentities by AC dielectrophoresis with simple visible-light stimulation. We study the dielectrophoresis forces on semiconducting particles of various geometries in aqueous suspension by modeling with the consideration of both the Maxwell–Wagner relaxation and the electrical-double-layer-charging effect. With the obtained understanding, we rationally design a manipulation system that can versatilely transport semiconductors and orient them toward desired directions simultaneously by tuning the light intensity in an electric field. This research could provide insights toward developing a new class of micromachines with rarely found control flexibility and precision and offer a new route toward separation and purification of optoelectric microparticles of different geometries.

Highly Sensitive and Stable NO2 Gas Sensors Based on SWNTs With Exceptional Recovery Time

Room temperature operable and highly sensitive NO2 gas sensors are fabricated based on (i) random and (ii) aligned networks of single-walled carbon nanotubes (SWNTs). The fabricated sensors are very sensitive, stable, and show shorter recovery time in the presence of UV light. Also, the variation of the response and recovery with network density is analyzed. The thin film resistor (TFR) of random network is fabricated by a reliable, cost-effective, and reproducible vacuum filtration method. The aligned network is fabricated using AC di-electrophoresis (DEP) technique. Electrodes spacing is optimized to avoid the chaining effect of aligned and bridged SWNTs between the gold electrode pair to enhance the stability and sensitivity of the sensor. Both the sensors based on random and aligned networks of SWNTs is tested with NO2 at room temperature. It is found that the sensor made of the aligned network shows ~3.5 times more sensitivity as compared to the random networks gas sensor but recovery time increases. It is also observed that sensors fabricated by TFR and aligned network techniques are stable and having less than 0.02 % and 0.15 % change in resistance with baseline, respectively. The TFR gas sensors fabricated using as prepared (AP) and purified and low functionality (P2) SWNTs show higher stability but less sensitive compared to the aligned network. The measured complete recovery time of sensors based on random and aligned SWNTs are 50 sec and 124 sec, respectively, for 0.5 ppm NO2. It is also observed that as the network density decreases response improves but the recovery time increases.

Nanoscale assembly of optoelectronic CdTe microwires using AC dielectrophoresis

This paper describes the effect of varying experimental parameters on the morphology and optoelectronic response of cadmium telluride quantum dot (CdTe QD) microwires formed using alternating current dielectrophoresis (AC-DEP). The thioglyocolic acid (TGA) capped CdTe QDs were assembled at room temperature across interdigitated electrodes (IDEs) under an applied electric field while simultaneously allowing solvent evaporation. The number of iterative rehydration steps required for bridging the microelectrode gaps of sizes 50–150 μm was found to depend on the electric field strength, particle concentration and electrode gap size. The frequency of the AC field applied, particle concentration and capping ligand were further optimized to regulate the fluorescence intensity, current density and chemical sensing behavior of the microassemblies. The overall process was found to be simple, rapid and scalable, and potentially holds promise for energy and sensing applications.

Post-breakdown field emission behavior of a planar-structured single SWNT bundle in air

In this paper, we report the field emission (FE) behavior in air of a single-walled carbon nanotube (SWCNT) bundle after breakdown. Our samples have the planar structure on the SiO2/Si substrate of one breaking-tip of a single SWCNT bundle horizontally pointing to the sidewall of an electrode with a nano-sized gap, which were prepared using microfabrication techniques, AC dielectrophoresis (DEP) and current-induced breakdown in sequence. The current–voltage relationship was monitored under 0–100 V scanning DC voltage. The results show that the planar-structured SWCNT bundle in air presents a significantly different FE performance from the conventional vertical structured carbon nanotube (CNTs) FE devices in vacuum. Similarly, the former FE current also exhibits several interval phenomena instead of monotonically exponentially rising (Di Bartolomeo et al 2007 Carbon 45 2957–71), while the structural damage of CNT tips result from oxidation instead of defects largening as for the latter. This research is conducive to further knowledge of possible electrical behavior after CNT breakdown for the failure effect during the fabrication and operation of CNT-based electronic/sensing devices and systems.

Flexible electrical aptasensor using dielectrophoretic assembly of graphene oxide and its subsequent reduction for cardiac biomarker detection

Cardiac troponin T (cTnT) is considered a clinical standard for its high specificity and sensitivity when diagnosing acute myocardial infarction; however, most studies on the electrical sensors of cardiac troponin biomarkers have focused on cTnI rather than cTnT. This study presents label-free, low-cost, transparent, and flexible aptamer-based immunosensors for the electrical detection of cTnT using reduced graphene oxide (rGO) sheets. GO was first deposited by AC dielectrophoresis between two predefined source and drain electrodes on a 3-aminopropyltriethoxylsilane-modified polyethylene terephthalate substrate. The GO was then reduced using hydrazine vapour without damaging the substrate, resulting in uniform, controlled, and stable deposition of rGO sheets, and demonstrating more stability than those directly deposited by dielectrophoresis. Amine-modified single-strand DNA aptamers against cTnT were immobilized onto the rGO channels. The relative resistance change of this sensor owing to the attachment of cTnT was quantified as the cTnT concentration decreased from 10 ng/mL to 1 pg/mL in phosphate buffered saline (PBS) and 10-fold diluted human serum in PBS, with the limits of detection being 1.2 pg/mL and 1.7 pg/mL, respectively, which is sufficiently sensitive for clinical applications. High-yield and rapid fabrication of the present rGO sensors will have significant influences on scaled-up fabrication of graphene-based sensors.

Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC Electrokinetics.

Alternating current (AC) electrokinetics is a facile way of patterning colloidal particles into advanced structures. We demonstrate the combined use of AC dielectrophoresis (AC-DEP) and AC electrohydrodynamics (AC-EHD) in a microwell electrode geometry for size-tunable assembly of gold nanoparticles (AuNPs) into one-dimensional microwires and two-dimensional films. The AC-DEP force scales with both particle size and field frequency, whereas the AC-EHD force depends only on the field frequency. So, a critical particle diameter ( dc) exists, below which the EHD phenomenon becomes more important and beyond which the DEP force is dominating. We performed theoretical and experimental studies to determine " dc" and how it gets affected by operating parameters like field frequency, voltage, particle number, electrolyte concentration, electrode size, and geometry. Our results show that the morphologies of the colloidal structures transition from films to microwires as the NP diameters vary from nanometers (< dc) to microns (> dc), and no assembly takes place at intermediate sizes (∼ dc). While the film formation is governed purely by surface EHD flows, microwire synthesis is a result of EHD-assisted DEP phenomenon. Also, a minimum particle number, a low salt concentration, and an optimum frequency range is required to initiate assembly.

Numerical Study of Enhancement of Positive Dielectrophoresis Particle Trapping in Electrode-Multilayered Microfluidic Device.

Enhancement of positive dielectrophoresis (pDEP) particle trapping by a co-occurring fluid flow under an ac electric field in an electrode-multilayered microfluidic device is investigated by three-dimensional particle-fluid flow simulations. The particle motion near one cross section of the microfluidic device is simulated under a zero flow condition by the Eulerian-Lagrangian method incorporating the ac electrothermal effect, thermal buoyancy, and dielectrophoresis. The mean trapping rate under the steady state Rm is evaluated from the simulated number of trapped particles Ntrap for 54 cases with four parameters: electrode excitation pattern, medium conductivity σ, applied voltage ϕe, and the real part of the Clausius-Mossotti factor Re[K(ω)]. The simulated pDEP velocity in the upper part of the flow channel is validated by an experiment using cell suspension and is fitted so that the non-dimensional velocity error is within 15% of a typical velocity of pDEP. The mean trapping rate Rm is greatly increased by the fluid flow only in the high conductivity and high voltage cases. Regardless of the electrode excitation pattern, Rm increased almost proportionally to the inflow rate into the capture region, where the pDEP force is effective. From a fitted equation of the results, the increase of Rm when Re[K(ω)] = 0.1 to 0.5 is found to be about 20% to 30% of the number of particles transported into the capture regions. The results quantify the enhancement of pDEP trapping by the fluid flow occurring under practical conditions in the device.

Tunable Assembly of Superfine Single-Walled Carbon Nanotube Fibers

We have developed a method of spinning superfine single-walled carbon nanotube (SWNT) fibers to facilitate their assembly in 2D counter microelectrode array via alternating current (AC) dielectrophoresis (DEP) by using clean, combination of ionic surfactants, and stable SWNT aqueous high-quality solutions. Our method can attract metallic SWNTs (M_SWNTs) toward a counter microelectrode array, resulting in an opposite movement of semiconducting SWNTs (S_SWNTs) and leaving S_SWNTs in the suspension by taking advantages of the differences in the relative dielectric constants of M_SWNTs and S_SWNTs with respect to the suspension and substrate-bent AC DEP structure. Our method can also control the stretching or relaxed extent and diameters of the suspended SWNT fibers by tuning the bending degree of a flexible substrate, AC frequency, peak value of AC voltage, and time. The effectiveness of the method is proven by the electrical measurement result‚ scanning electron microscopy image, atomic force microscope image and Raman spectroscopy on test samples.

Single-Walled Carbon-Nanotube-Based Chemocapacitive Sensors with Molecular Receptors for Selective Detection of Chemical Warfare Agents

Single-walled carbon nanotubes (SWCNTs) were fabricated using AC dielectrophoresis into chemocapacitive sensors, and molecular receptors were applied for the selective detection of several chemical warfare agents (CWAs). The selective responses toward nerve simulants (G and V), choking and blister agents as well as a pesticide were investigated with specific receptor molecules that were either covalently functionalized or noncovalently coated onto the surfaces of the SWCNTs. The SWCNT-based chemocapacitive sensors showed reproducibility and sensitivity to 200 ppb for several target molecules. The fabricated sensor arrays were assessed for the selective detection of six different CWAs, and the principal component analysis demonstrated their specificity. VX, a real nerve agent, was tested on the fabricated SWCNT-based chemocapacitive sensor coated with a thiourea-functionalized siloxane polymer, and the successful detection of VX at 100 ppb confirmed that our SWCNT-based sensors are suitable for practical a...

Expanding the flexibility of dynamics simulation on different size particle-particle interactions by dielectrophoresis.

In this paper, we perform flexible and reliable dynamics simulations on different sizes of two or more particles' interactive motions, where they encounter positive or negative dielectrophoresis (DEP) forces. The particles with identical or non-identical size are in close proximity suspended freely in a solution under a homogeneous electric field. According to the description of classic dipole moment, DEP forces make the particles form a straight chain. Therefore, dynamics simulation based on Newton's laws is utilized to understand AC DEP phenomena among multiple particles. To solve the relevant governing equations, Stokes drag and repulsive forces (including wall and particles) are combined with DEP forces to obtain the trajectories of particles. Results show that particles with the same sign of the Clausius-Mossotti (CM) factor revolve clockwise or counterclockwise to attract each other parallel to the electric field direction. Conversely, the particle chain is perpendicular to the field. This programmable advantage is of great benefit to the study of three or four particle motions. Meanwhile, the pearl chain consisting of three or four particles is related not only to an individual CM factor but also to initial spatial configuration. Both the cluster and short chain are dependent on symmetry between the geometric distribution and electric field, while it implies different size particles easily cause the chain structure with less time.

The Influence of the Asymmetric Metal Electrodes on the Self-assembly of Single-walled Carbon Nanotubes by AC Dielectrophoresis

There are two procedures in the single-walled carbon nanotubes (SWCNTs) DEP (dielectrophoresis) assembly process, including the DEP and suspension removal. This paper mainly pays attention to the effects of the suspension removal procedure on the assembly efficiency. We found that the interaction energy between the SWCNTs and electrode metal materials plays a major role in preventing the arriving SWCNTs from getting off the electrodes when the suspension is blown-off, which will impact the DEP assembly yield of SWCNTs further. Three kinds of metal materials having various interaction energy with SWCNTs with the order of Ti>Hf>Al were chosen as the electrode materials and then the SWCNTs DEP assembly experiments were carried out. Our results show that the assembly yields exhibit the accordant order with that of the interaction energy of Ti>Hf>Al, which demonstrates our deduction.

Numerical Study of Particle-Fluid Flow Under AC Electrokinetics in Electrode-Multilayered Microfluidic Device.

A particle-fluid flow under alternating current (ac) electrokinetics was numerically simulated to investigate the three-dimensional (3-D) particle motion in a complex electric field of a high conductivity medium generated by an electrode-multilayered microfluidic device. The simulation model coupling thermal-fluid-electrical and dispersed particle problems incorporates three ac electrokinetics (ACEK) phenomena, namely, the ac electrothermal effect (ACET), thermal buoyancy (TB), and dielectrophoresis (DEP). The electrode-multilayered microfluidic device was fabricated with 40 electrodes exposed at the flow channel sidewalls in five cross sections. The governing equations of the simulation model are solved by the Eulerian-Lagrangian method with finite volume discretization. Fluid flow simulations in three cases with or without consideration of ACET and TB are performed to clarify the contributions of these phenomena. The fluid flow is found to be composed of short-range vortices due to ACET and long-range circulation due to TB based on the features of the electrode-multilayered microfluidic device. The 3-D particle trajectory influenced by the fluid flow is compared with four values of the real part of the Clausius-Mossotti (CM) factor to evaluate the DEP phenomenon. The simulation model is validated by experiments using a cell suspension. The pattern of cell trajectories in the upper part of the flow channel measured by particle tracking velocimetry agrees with the simulated pattern. By comparison of the simulation and experiment, it is found that the cells moving straight away from the electrode on the focal plane are decelerated within the region of 60μm from the electrode by positive-DEP with [Formula: see text]. Furthermore, the 3-D DEP-effective region and the ACET and TB dominant regions for the cells are predicted by evaluating the particle-fluid relative velocity due to DEP force with [Formula: see text]. Consequently, the flow mechanism and dominant region of each ACEK phenomenon in the device are clarified from the 3-D simulation validated by the experiments.

Electric-field driven assembly of live bacterial cell microarrays for rapid phenotypic assessment and cell viability testing

Microarray technology to isolate living cells using external fields is a facile way to do phenotypic analysis at the cellular level. We have used alternating current dielectrophoresis (AC-DEP) to drive the assembly of live pathogenic Salmonella typhi (S.typhi) and Escherichia coli (E.coli) bacteria into miniaturized single cell microarrays. The effects of voltage and frequency were optimized to identify the conditions for maximum cell capture which gave an entrapment efficiency of 90% in 60 min. The chip was used for calibration-free estimation of cellular loads in binary mixtures and further applied for rapid and enhanced testing of cell viability in the presence of drug via impedance spectroscopy. Our results using a model antimicrobial sushi peptide showed that the cell viability could be tested down to 5 μg/mL drug concentration under an hour, thus establishing the utility of our system for ultrafast and sensitive detection.