Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC-biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC-biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC-field induced-charge electrokinetic (ICEK) phenomenon, which behaves as counter-rotating micro-vortices around the ideally polarizable surfaces of FE. Finite-element simulation is carried out by coupling the dual-frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500 nm) and BSA molecules (∼10 nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous-flow analyte manipulation in modern microfluidic systems.     

Electrokinetic particle entry into microchannels

The fundamental understanding of particle electrokinetics in microchannels is relevant to many applications. To date, however, the majority of previous studies have been limited to particle motion within the area of microchannels. This work presents the first experimental and numerical investigation of electrokinetic particle entry into a microchannel. We find that the particle entry motion can be significantly deviated from the fluid streamline by particle dielectrophoresis at the reservoir-microchannel junction. This negative dielectrophoretic motion is induced by the inherent non-uniform electric field at the junction and is insensitive to the microchannel length. It slows down the entering particles and pushes them toward the center of the microchannel. The consequence is the demonstrated particle deflection, focusing, and trapping phenomena at the reservoir-microchannel junction. Such rich phenomena are studied by tuning the AC component of a DC-biased AC electric field. They are also utilized to implement a selective concentration and continuous separation of particles by size inside the entry reservoir.     

Recent advances in direct current electrokinetic manipulation of particles for microfluidic applications

Microfluidic devices have been extensively used to achieve precise transport and placement of a variety of particles for numerous applications. A range of force fields have thus far been demonstrated to control the motion of particles in microchannels. Among them, electric field‐driven particle manipulation may be the most popular and versatile technique because of its general applicability and adaptability as well as the ease of operation and integration into lab‐on‐a‐chip systems. This article is aimed to review the recent advances in direct current (DC) (and as well DC‐biased alternating current) electrokinetic manipulation of particles for microfluidic applications. The electric voltages are applied through electrodes that are positioned into the distant channel‐end reservoirs for a concurrent transport of the suspending fluid and manipulation of the suspended particles. The focus of this review is upon the cross‐stream nonlinear electrokinetic motions of particles in the linear electroosmotic flow of fluids, which enable the diverse control of particle transport in microchannels via the wall‐induced electrical lift and/or the insulating structure‐induced dielectrophoretic force.     

Hybrid electrokinetic manipulation in high-conductivity media

This study reports a hybrid electrokinetic technique for label-free manipulation of pathogenic bacteria in biological samples toward medical diagnostic applications. While most electrokinetic techniques only function in low-conductivity buffers, hybrid electrokinetics enables effective operation in high-conductivity samples, such as physiological fluids (～1 S m(-1)). The hybrid electrokinetic technique combines short-range electrophoresis and dielectrophoresis, and long-range AC electrothermal flow to improve its effectiveness. The major technical hurdle of electrode instability for manipulating high conductivity samples is tackled by using a Ti-Au-Ti sandwich electrode and a 3-parallel-electrode configuration is designed for continuous isolation of bacteria. The device operates directly with biological samples including urine and buffy coats. We show that pathogenic bacteria and biowarfare agents can be concentrated for over 3 orders of magnitude using hybrid electrokinetics.     

Induction and suppression of cell lysis in an electrokinetic microfluidic system

The ability to strategically induce or suppress cell lysis is critical for many cellular-level diagnostic and therapeutic applications conducted within electrokinetic microfluidic platforms. The chemical and structural integrity of sub-cellular components is important when inducing cell lysis. However, metal electrodes and electrolytes participate in undesirable electrochemical reactions that alter solution composition and potentially damage protein, RNA, and DNA integrity within device microenvironments. For many biomedical applications, cell viability must be maintained even when device-imposed cell-stressing stimuli (e.g., electrochemical reaction byproducts) are present. In this work, we explored a novel and tunable method to accurately induce or suppress device-imposed artifacts on human red blood cell (RBC) lysis in non-uniform AC electric fields. For precise tunability, a dielectric hafnium oxide (HfO₂) layer was used to prevent electron transfer between the electrodes and the electric double layer and thus reduce harmful electrochemical reactions. Additionally, a low concentration of Triton X-100 surfactant was explored as a tool to stabilize cell membrane integrity. The extent of hemolysis was studied as a function of time, electrode configuration (T-shaped and star-shaped), cell position, applied non-uniform AC electric field, with uncoated and HfO₂ coated electrodes (50 nm), and absence and presence of Triton X-100 (70 µM). Tangible outcomes include a parametric analysis relying upon literature and this work to design, tune, and operate electrokinetic microdevices to intentionally induce or suppress cellular lysis without altering intracellular components. Implications are that devices can be engineered to leverage or minimize device-imposed biological artefacts extending the versatility and utility of electrokinetic diagnostics.     

AC electrokinetics of conducting microparticles: A review

This paper reviews both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte. Applied AC electric fields interact with the induced charge in the electrical double layer at the metal particle–electrolyte interface. In general, particle motion is governed by both the electric field interacting with the induced dipole on the particle and also the induced-charge electro-osmotic (ICEO) flow around the particle. The importance of the RC time for charging the double layer is highlighted. Experimental measurements of the AC electrokinetic behaviour of conducting particles (dielectrophoresis, electro-rotation and electro-orientation) are compared with theory, providing a comprehensive review of the relative importance of particle motion due to forces on the induced dipole compared with motion arising from induced-charge electro-osmotic flow. In addition, the electric-field driven assembly of conducting particles is reviewed in relation to their AC electrokinetic properties and behaviour.     

Model based design of a microfluidic mixer driven by induced charge electroosmosis

Mixing chemical or biological samples with reagents for chemical analysis is one of the most time consuming operations on microfluidic platforms. This is primarily due to the low rate of diffusive transport in liquid systems. Additionally, much research has focused on detection, rather than sample preparation. In response, we describe a mixer for microfluidic sample preparation based on the electrokinetic phenomenon of induced-charge-electroosmosis (ICEO). ICEO creates microvortices within a fluidic channel by application of alternating current (AC) electric fields. The microvortices are driven by electrostatic forces acting on the ionic charge induced by the field near polarizable materials. By enabling mixing to be turned on or off within a channel of fixed volume, these electronically controlled mixers prevent sample dilution-a common problem with other strategies. A three-dimensional model based on the finite volume method was developed to calculate the electric field, fluid flow, and mass transport in a multi-species liquid. After preliminary experiments, the model was used to rapidly prototype a wide range of designs. A new microfabrication process was developed for devices with vertical sidewalls having conductive metal coatings and embedded electrodes. Mixing experiments were carried out in the devices and the results were compared to the model.     

An Ohmic model for electrokinetic flows of binary asymmetric electrolytes

The transport of electrolytes in electric fields is a ubiquitous phenomenon commonly harnessed in microfluidics. A classic leaky dielectric model for flow generated by electric fields accurately predicts electrohydrodynamic transport phenomenon but is valid for millimeter-scale and larger flows and at relatively low ionic strength. Here, we derive and use a modified version of this model to sub-millimeter scales more relevant to microfluidics, where diffusive transport of charged species becomes non-negligible. We formulate a general equation set, the modified Ohmic model, applicable to the transport of binary, asymmetric electrolytes. We leverage this model to describe a variety of microfluidic electrokinetic systems, including DC electroosmosis, alternating current electrokinetics (ACEK) and induced-charge electroosmosis (ICEO), thus highlighting some unifying principles of these flows.     

ac electroosmosis in rectangular microchannels

Motivated by the growing interest in ac electroosmosis as a reliable no moving parts strategy to control fluid motion in microfluidic devices for biomedical applications, such as lab-on-a-chip, we study transient and steady-state electrokinetic phenomena (electroosmosis and streaming currents) in infinitely extended rectangular charged microchannels. With the aid of Fourier series and Laplace transforms we provide a general formal solution of the problem, which is used to study the time-dependent response to sudden ac applied voltage differences in case of finite electric double layer. The Debye-Huckel approximation has been adopted to allow for an algebraic solution of the Poisson-Boltzmann problem in Fourier space. We obtain the expressions of flow velocity profiles, flow rates, streaming currents, as well as expressions of the complex hydraulic and electrokinetic conductances. We analyze in detail the dependence of the electrokinetic conductance on the extension of linear dimensions relative to the Debye length, with an eye on finite electric double layer effects.     

Rapid prototyping of polydimethylsiloxane (PDMS) microchips using electrohydrodynamic jet printing: Application to electrokinetic assays

Polydimethylsiloxane (PDMS) based microfluidic devices have found increasing utility for electrophoretic and electrokinetic assays because of their ease of fabrication using replica molding. However, the fabrication of high-resolution molds for replica molding still requires the resource-intensive and time-consuming photolithography process, which precludes quick design iterations and device optimization. We here demonstrate a low-cost, rapid microfabrication process, based on electrohydrodynamic jet printing (EJP), for fabricating non-sacrificial master molds for replica molding of PDMS microfluidic devices. The method is based on the precise deposition of an electrically stretched polymeric solution of polycaprolactone in acetic acid on a silicon wafer placed on a computer-controlled motion stage. This process offers the high-resolution (order 10 $\mu$m) capability of photolithography and rapid prototyping capability of inkjet printing to print high-resolution templates for elastomeric microfluidic devices within a few minutes. Through proper selection of the operating parameters such as solution flow rate, applied electric field, and stage speed, we demonstrate microfabrication of intricate master molds and corresponding PDMS microfluidic devices for electrokinetic applications. We demonstrate the utility of the fabricated PDMS microchips for nonlinear electrokinetic processes such as electrokinetic instability and controlled sample splitting in ITP. The ability to rapid prototype customized reusable master molds with order 10 $\mu$m resolution within a few minutes can help in designing and optimizing microfluidic devices for various electrokinetic applications.     

Effect of direct current dielectrophoresis on the trajectory of a non-conducting colloidal sphere in a bent pore

Dielectrophoresis has shown a wide range of applications in microfluidic devices. Force approximations utilizing the point-dipole method in dielectrophoresis have provided convenient predictions for particle motion by neglecting interactions between the particle and its surrounding electric and flow fields. The validity of this approach, however, is unclear when the particle size is comparable to the characteristic length of the channel and when the particle is in close proximity to the channel wall. To address this issue, we apply an accurate numerical approach based on the boundary-element method (BEM) to solve the coupled electric field, flow, and particle motion. This method can handle much closer particle-wall distances than the other numerical approaches such as the finite-element method. Using the BEM and integrating the Maxwell stress tensor, we simulate an electrokinetic, spherical particle moving within a bent cylindrical pore to investigate how the dielectrophoretic force affects the particle's trajectory. In the simulation, both the particle and the channel wall are non-conducting, and the electric double layers adjacent to the solid surfaces are assumed to be thin with respect to the particle radius and particle-wall gap. The results show that as the particle comes close to the wall, its finite size has an increasingly important effect on its own transient motion and the point-dipole approximation may lead to significant error.     

AC electrokinetic isolation and detection of extracellular vesicles from dental pulp stem cells: Theoretical simulation incorporating fluid mechanics

Extracellular vesicles (EVs) are cell-derived nanoscale vesicles involved in intracellular communication and the transportation of biomarkers. EVs released by mesenchymal stem cells have been recently reported to play a role in cell-free therapy of many diseases. However, the demand for better research tools to replace the tedious conventional methods used to study EVs is getting stronger. EVs' manipulation using alternating current (AC) electrokinetic forces in a microfluidic device has appeared to be a reliable and sensitive diagnosis and trapping technique. Given that different AC electrokinetic forces may contribute to the overall motion of particles and fluids in a microfluidic device, EVs' electrokinetic trapping must be examined considering all dominant forces involved depending on the experimental conditions. In this paper, AC electrokinetic trapping of EVs using an interdigitated electrode arrays is investigated. A 2D numerical simulation incorporating the two significant AC electrokinetic phenomena (Dielectrophoresis and AC electroosmosis) has been performed. Theoretical predictions are then compared with experimental results and allow for a plausible explanation of observations inconsistent with DEP theory. It is demonstrated that the inconsistencies can be attributed to a significant extent to the contribution of the AC electroosmotic effect.     

Joule heating effects on electrokinetic flows with conductivity gradients

Instability occurs in the electrokinetic flow of fluids with conductivity and/or permittivity gradients if the applied electric field is beyond a critical value. Understanding such an electrokinetic instability is significant for both improved transport (via the suppressed instability) and enhanced mixing (via the promoted instability) of liquid samples in microfluidic applications. This work presents the first study of Joule heating effects on electrokinetic microchannel flows with conductivity gradients using a combined experimental and numerical method. The experimentally observed flow patterns and measured critical electric fields under Joule heating effects to different extents are reasonably predicted by a depth-averaged numerical model. It is found that Joule heating increases the critical electric field for the onset of electrokinetic instability because the induced fluid temperature rise and in turn the fluid property change (primarily the decreased permittivity) lead to a smaller electric Rayleigh number.     

Electrokinetics with blood

Microfluidics based lab‐on‐a‐chip technology holds tremendous promises towards point‐of‐care diagnosis of diseases as well as for developing engineered devices aimed towards replicating the intrinsic functionalities of human bodies as mediated by blood vessel mimicking circulatory networks. While the analysis of transport of blood including its unique cellular constituents has remained to be the focus of many reported studies, a progressive interest on understanding the interplay between electric field and blood flow dynamics has paved a new way towards further developments from scientific engineering as well as clinical viewpoint. Here, we briefly outline the interconnection between electrokinetics and blood flow through micro‐capillaries, in an effort to address several challenging propositions in a wide variety of applications encompassing biophysical transport to medical diagnostics. We first present the fundamentals of interaction of electric field with cellular components. In conjunction with the unique rheological features of blood, we show that this interaction may turn out to be compelling for the use of electric fields for transporting blood samples through microfluidic conduits. We discuss the perspectives of both direct current and alternating current electrokinetics in the context of blood flow. In addition, we provide a brief outline of the concerned theoretical developments. We also bring out the relevant biophysical perspectives and focus on applications such as blood plasma separation and separation of circulatory tumor cells. Finally, we attempt to provide a futuristic outlook and envisage the potential of combining electrokinetics with blood microcirculation towards developing futuristic biomimetic microdevices that can replicate a novel control mechanism over micro‐circulatory transport in the entire connective network of human bodies. This may effectively pave the way towards the realization of a next‐generation medical simulation device, significantly advanced from what is available under the ambit of the state of art technology in the field.     

Numerical study of a novel induced-charge electrokinetic micro-mixer

A novel micro-mixer based on the induced-charge electrokinetic motion of an electrically conducting particle is proposed and numerically demonstrated in this paper. For most microfluidic applications, it is desired to mix different streams of solutions rapidly in a continuous flow mode. Therefore, in this work, we consider a mixing chamber containing an electrically conducting particle and the mixing chamber is located in the middle of a microchannel. Vortices are generated around the electrically conducting particle in an aqueous solution due to the interaction of the applied electric field and the induced surface charge on the particle. These vortices will enhance significantly the mixing of different solutions around the particle. The effectiveness of mixing the two streams entering the mixing chamber is numerically studied as functions of the applied electric field. Excellent mixing can be achieved in this system under two perpendicularly applied electric fields. The proposed micro-mixer is simple and easy to be fabricated for lab-on-a-chip applications.     

Trapping of DNA by dielectrophoresis

Under suitable conditions, a DNA molecule in solution will develop a strong electric dipole moment. This induced dipole allows the molecule to be manipulated with field gradients, in a phenomenon known as dielectrophoresis (DEP). Pure dielectrophoretic motion of DNA requires alternate current (AC) electric fields to suppress the electrophoretic effect of the molecules net charge. In this paper, we present two methods for measuring the efficiency of DEP for trapping DNA molecules as well as a set of quantitative measurements of the effects of strand length, buffer composition, and frequency of the applied electric field. A simple configuration of electrodes in combination with a microfluidic flow chamber is shown to increase the concentration of DNA in solution by at least 60-fold. These results should prove useful in designing practical microfluidic devices employing this phenomenon either for separation or concentration of DNA.     

Electrokinetic techniques applied to electrochemical DNA biosensors

Electrokinetic techniques are contact-free methods currently used in many applications, where precise handling of biological entities, such as cells, bacteria or nucleic acids, is needed. These techniques are based on the effect of electric fields on molecules suspended in a fluid, and the corresponding induced motion, which can be tuned according to some known physical laws and observed behaviours. Increasing interest on the application of such strategies in order to improve the detection of DNA strands has appeared during the recent decades. Classical electrode-based DNA electrochemical biosensors with combined electrokinetic techniques present the advantage of being able to improve the working electrode's bioactive part during their fabrication and also the hybridization yield during the sensor detection phase. This can be achieved by selectively manipulating, driving and directing the molecules towards the electrodes increasing the speed and yield of the floating DNA strands attached to them. On the other hand, this technique can be also used in order to make biosensors reusable, or reconfigurable, by simply inverting its working principle and pulling DNA strands away from the electrodes. Finally, the combination of these techniques with nanostructures, such as nanopores or nanochannels, has recently boosted the appearance of new types of electrochemical sensors that exploit the time-varying position of DNA strands in order to continuously scan these molecules and to detect their properties. This review gives an insight into the main forces involved in DNA electrokinetics and discusses the state of the art and uses of these techniques in recent years.     

Dynamic analyte introduction and focusing in plastic microfluidic devices for proteomic analysis

Isoelectric focusing (IEF) separations, in general, involve the use of the entire channel filled with a solution mixture containing protein/peptide analytes and carrier ampholytes for the creation of a pH gradient. Thus, the preparative capabilities of IEF are inherently greater than most microfluidics-based electrokinetic separation techniques. To further increase sample loading and therefore the concentrations of focused analytes, a dynamic approach, which is based on electrokinetic injection of proteins/peptides from solution reservoirs, is demonstrated in this study. The proteins/peptides continuously migrate into the plastic microchannel and encounter a pH gradient established by carrier ampholytes originally present in the channel for focusing and separation. Dynamic sample introduction and analyte focusing in plastic microfluidic devices can be directly controlled by various electrokinetic conditions, including the injection time and the applied electric field strength. Differences in the sample loading are contributed by electrokinetic injection bias and are affected by the individual analyte's electrophoretic mobility. Under the influence of 30 min electrokinetic injection at constant electric field strength of 500 V/cm, the sample loading is enhanced by approximately 10-100 fold in comparison with conventional IEF.     

Continuous and reversible mixing or demixing of nanoparticles by dielectrophoresis

Mixing and demixing (separation) are essential tasks in microfluidic devices, which seem to be contrary in nature. Accordingly, completely different strategies and devices are usually employed for their realization. We here present a microfluidic device which is capable of performing both these tasks as it can be operated in either mixing or demixing mode. The mixing and demixing processes are reversible and are accomplished by continuous operation of the device. An asymmetric S-shaped ridge extends over the full width of a microfluidic channel (200 μm) creating a constriction of 620 nm in height with an aspect ratio of 1 : 500. Appropriate AC and DC voltages generate electrodeless dielectrophoresis at the constriction as well as (linear) electrokinetic driving forces along the channel. These de/mixing parameters can be adapted in real time in such a way that continuous separation and mixing efficiencies of 85-100% can be achieved. As a proof of concept we demonstrate continuous mixing and demixing of polystyrene nanoparticles (20 and 100 nm). The experimental results are complemented by numerical simulations illustrating the particles' motion under the influence of the electrokinetic effects and thermal noise (diffusion). The monolithic one-step fabrication process by soft lithography (with PDMS in our case) will make integration and combination of several mixing and demixing functions into a more complex lab-on-a-chip device possible.