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

Particle manipulation under X-force fields

Particle manipulation is a central technique that enhances numerous scientific and medical applications by exploiting micro- and nanoscale control within fluidic environments. In this review, we systematically explore the multifaceted domain of particle manipulation under the influence of various X-force fields, integral to lab-on-a-chip technologies. We dissect the fundamental mechanisms of hydrodynamic, gravitational, optical, magnetic, electrical, and acoustic forces and detail their individual and synergistic applications. In particular, our discourse extends to advanced multi-modal manipulation strategies that harness the combined power of these forces, revealing their enhanced efficiency and precision in complex assays and diagnostic frameworks. The integration of cutting-edge technologies such as artificial intelligence and autonomous systems further enhances the capabilities of these microfluidic platforms, leading to transformative innovations in personalized medicine and point-of-care diagnostics. This review not only highlights current technological advances, but also forecasts the trajectory of future developments, emphasizing the escalating precision and scalability essential for advancing lab-on-a-chip applications.

Electrokinetic flow instabilities in shear thinning fluids with conductivity gradients

Instabilities in the form of periodic or irregular waves at the fluid interface have been demonstrated in microchannel electrokinetic flows with conductivity gradients when the applied electric field is above a threshold value. Most prior studies on electrokinetic instabilities (EKI) are restricted to Newtonian fluids though many of the chemical and biological samples in microfluidic applications exhibit non-Newtonian characteristics. We present in this work an experimental study of the effects of fluid shear thinning on the development of EKI waves through the addition of a small amount of xanthan gum (XG) polymer to both the high- and low-concentration Newtonian buffer solutions. The threshold electric field for the onset of EKI in the XG solution is significantly lower than in the Newtonian solution. However, the propagation speed, amplitude and frequency of EKI waves in the former are all smaller. Increasing the polymer concentration reduces the threshold electric field and as well the critical electric Rayleigh number that considers the fluid property variations in XG solutions. This decreasing trend indicates the enhancing effect of fluid shear thinning on EKI, which is qualitatively consistent with a recent numerical prediction. However, the measured wave properties all follow a non-monotonic trend with XG concentration, different from the continuously decreasing electroosmotic velocity.

Dependence of acoustophoretic aggregation on the impedance of microchannel's walls

BACKGROUND AND OBJECTIVES

Acoustofluidic manipulation of particles and biological cells has been widely applied in various biomedical and engineering applications, including effective separation of cancer cell, point-of-care diagnosis, and cell patterning for tissue engineering. It is often implemented within a polydimethylsiloxane (PDMS) microchannel, where standing surface acoustic waves (SSAW) are generated by sending two counter-propagating ultrasonic waves on a piezoelectric substrate.

METHODS

In this paper, we develop a full cross-sectional model of the acoustofluidic device using finite element method, simulating the wave excitation on the substrate and wave propagation in both the fluid and the microchannel wall. This model allows us to carry out extensive parametric analyses concerning the acoustic properties of the fluid and the microchannel wall, as well as the dimensions of the channel, to explore their influences on the acoustic field, fluid flow and microparticle aggregation.

RESULTS

Our findings demonstrate an order-of-magnitude enhancement in acoustic pressure amplitude and aggregation speed and a reduction in the particle threshold radius to submicron levels, which can be achieved through adjustments to the channel height and the difference in acoustic impedance between the channel wall and the fluid. The optimum channel heights are determined, which depend on the acoustic properties of the channel wall. The particle trajectories, movements along pressure nodal planes, and terminal positions are identified, with relative strength between the radiation force and the streaming force compared in different combinations of parameters.

CONCLUSIONS

This work demonstrates that finetuning the dimensions and acoustic properties of the fluid and microchannel wall in acoustofluidic device can greatly enhance particle aggregation throughput and reduce constraints on particle size. Our findings offer valuable insights into device design and optimization.

Editorial for the Micro/Nanoscale Electrokinetics Section

Electrokinetics is the study of fluid flow and particle motion driven by electricity [...].

Separating the Living from the Dead: An Electrophoretic Approach

Cell viability studies are essential in numerous applications, including drug development, clinical analysis, bioanalytical assessments, food safety, and environmental monitoring. Microfluidic electrokinetic (EK) devices have been proven to be effective platforms to discriminate microorganisms by their viability status. Two decades ago, live and dead Escherichia coli (E. coli) cells were trapped at distinct locations in an insulator-based EK (iEK) device with cylindrical insulating posts. At that time, the discrimination between live and dead cells was attributed to dielectrophoretic effects. This study presents the continuous separation between the live and dead E. coli cells, which was achieved primarily by combining linear and nonlinear electrophoretic effects in an iEK device. First, live and dead E. coli cells were characterized in terms of their electrophoretic migration, and then the properties of both live and dead E. coli cells were input into a mathematical model built using COMSOL Multiphysics software to identify appropriate voltages for performing an iEK separation in a T-cross iEK channel. Subsequently, live and dead cells were successfully separated experimentally in the form of an electropherogram, achieving a separation resolution of 1.87. This study demonstrated that linear and nonlinear electrophoresis phenomena are responsible for the discrimination between live and dead cells under DC electric fields in iEK devices. Continuous electrophoretic assessments, such as the one presented here, can be used to discriminate between distinct types of microorganisms including live and dead cell assessments.

Continuous Focusing of Particles by AC-Electroosmosis and Induced Dipole Interactions

Continuous particle focusing by using microfluidics is an effective method for separating particles, cells, or droplets for analytical purposes. Previously, it was shown that an alternating current across rectangular microchannels with slightly deformed side walls results in vortex flow patterns caused by alternating current electroosmosis (AC-EOF) and could lead to particle focusing. In this work, we explore this mechanism by experimentally studying the particle focusing behavior for various fluid flow velocities through a microchannel. Since it is unlikely that the particles are kept in their focused position solely by convection, a theoretical force balance between the hydrodynamic and the induced dipole force was determined. In our experiments, it was found that there is no substantial effect of the pressure-driven fluid velocity on the particle focusing velocity within the studied range. From the theoretical force balance calculations, it was determined that while the addition of the induced dipole force can still not completely describe the experimentally observed particle focusing, the induced dipole can be strong enough to overcome the hydrodynamic force. Finally, it is hypothesized that under specific circumstances, including a repulsive electrostatic force between a particle and electrode wall can complete the theoretical particle focusing force balance. Alternative phenomena that could also play a role in particle focusing are proposed.

Nonlinear Electrophoresis of Microparticles in Shear Thinning Fluids

The nonlinear electric field dependence of particle electrophoresis has been demonstrated to occur in Newtonian fluids for highly charged particles under large electric fields. It has also been predicted to arise from the rheological effects of non-Newtonian fluids even at small electric fields. We present in this work an experimental verification of nonlinear electrophoresis in shear thinning xanthan gum solutions through a straight rectangular microchannel. The addition of polymer into a Newtonian buffer solution is found to change the electric field dependence from linear to superlinear for electroosmotic, electrokinetic, and electrophoretic velocities. The nonlinear index of each of these electrokinetic phenomena increases with the increasing polymer or buffer concentration, among which electrophoresis exhibits the strongest nonlinearity. Both these observed trends are captured by a dimensionless electrokinetic shear thinning number that depends on the power-law index of fluid viscosity and the Debye length.

Charge-Based Separation of Microparticles Using AC Insulator-Based Dielectrophoresis

Surface charge is an important property of particles. It has been utilized to separate particles in microfluidic devices, where dielectrophoresis (DEP) is often the driving force. However, current DEP-based particle separations based on the charge differences work only for particles of similar sizes. They become less effective and may even fail for a mixture of particles differing in both charge and size. We demonstrate that our recently developed AC insulator-based dielectrophoresis (AC iDEP) technique can direct microparticles toward charge-dependent equilibrium positions in a ratchet microchannel. Such charge-based particle separation is controlled by the imposed AC voltage frequency and amplitude but is nearly unaffected by the size of either type of particle in the mixture except for the time required to achieve an effective separation. This AC iDEP technique may potentially be used to focus and separate submicron or even nanoparticles because of its virtually "infinite" channel length.

Entropy generation in a ciliary flow of an Eyring-Powell ternary hybrid nanofluid through a channel with electroosmosis and mixed convection

Drug delivery systems, where the nanofluid flow with electroosmosis and mixed convection can help in efficient and targeted drug delivery to specific cells or organs, could benefit from understanding the behavior of nanofluids in biological systems. In current work, authors have studied the theoretical model of two-dimensional ciliary flow of blood-based (Eyring-Powell) nanofluid model with the insertion of ternary hybrid nanoparticles along with the effects of electroosmosis, magnetohydrodynamics, thermal radiations, and mixed convection. Moreover, the features of entropy generation are also taken into consideration. The system is modeled in a wave frame with the approximations of large wave number and neglecting turbulence effects. The problem is solved numerically by using the shooting method with the assistance of computational software "Mathematica" for solving the governing equation. According to the temperature curves, the temperature will increase as the Hartman number, fluid factor, ohmic heating, and cilia length increase. It is also disclosed that ternary hybrid nanoparticles result in a change in flow rate when other problem parameters are varied, and the same is true for temperature graphs. Engineers and scientists can make better use of nanofluid-based cooling systems in electronics, automobiles, and industrial processes with the aid of the study's findings.

A High-Throughput Circular Tumor Cell Sorting Chip with Trapezoidal Cross Section

Circulating tumor cells are typically found in the peripheral blood of patients, offering a crucial pathway for the early diagnosis and prediction of cancer. Traditional methods for early cancer diagnosis are inefficient and inaccurate, making it difficult to isolate tumor cells from a large number of cells. In this paper, a new spiral microfluidic chip with asymmetric cross-section is proposed for rapid, high-throughput, label-free enrichment of CTCs in peripheral blood. A mold of the desired flow channel structure was prepared and inverted to make a trapezoidal cross-section using a micro-nanotechnology process of 3D printing. After a systematic study of how flow rate, channel width, and particle concentration affect the performance of the device, we utilized the device to simulate cell sorting of 6 μm, 15 μm, and 25 μm PS (Polystyrene) particles, and the separation efficiency and separation purity of 25 μm PS particles reached 98.3% and 96.4%. On this basis, we realize the enrichment of a large number of CTCs in diluted whole blood (5 mL). The results show that the separation efficiency of A549 was 88.9% and the separation purity was 96.4% at a high throughput of 1400 μL/min. In conclusion, we believe that the developed method is relevant for efficient recovery from whole blood and beneficial for future automated clinical analysis.

Constrained Volume Micro- and Nanoparticle Collection Methods in Microfluidic Systems

Particle trapping and enrichment into confined volumes can be useful in particle processing and analysis. This review is an evaluation of the methods used to trap and enrich particles into constrained volumes in microfluidic and nanofluidic systems. These methods include physical, optical, electrical, magnetic, acoustic, and some hybrid techniques, all capable of locally enhancing nano- and microparticle concentrations on a microscale. Some key qualitative and quantitative comparison points are also explored, illustrating the specific applicability and challenges of each method. A few applications of these types of particle trapping are also discussed, including enhancing biological and chemical sensors, particle washing techniques, and fluid medium exchange systems.

Development of a DC-Biased AC-Stimulated Microfluidic Device for the Electrokinetic Separation of Bacterial and Yeast Cells

Electrokinetic (EK) microsystems, which are capable of performing separations without the need for labeling analytes, are a rapidly growing area in microfluidics. The present work demonstrated three distinct binary microbial separations, computationally modeled and experimentally performed, in an insulator-based EK (iEK) system stimulated by DC-biased AC potentials. The separations had an increasing order of difficulty. First, a separation between cells of two distinct domains (Escherichia coli and Saccharomyces cerevisiae) was demonstrated. The second separation was for cells from the same domain but different species (Bacillus subtilis and Bacillus cereus). The last separation included cells from two closely related microbial strains of the same domain and the same species (two distinct S. cerevisiae strains). For each separation, a novel computational model, employing a continuous spatial and temporal function for predicting the particle velocity, was used to predict the retention time (tR,p) of each cell type, which aided the experimentation. All three cases resulted in separation resolution values Rs>1.5, indicating complete separation between the two cell species, with good reproducibility between the experimental repetitions (deviations < 6%) and good agreement (deviations < 18%) between the predicted tR,p and experimental (tR,e) retention time values. This study demonstrated the potential of DC-biased AC iEK systems for performing challenging microbial separations.

Nonlinear electrophoresis of nonspherical particles in a rectangular microchannel

Nonlinear electrophoresis offers advantageous prospects in microfluidic manipulation of particles over linear electrophoresis. Existing theories established for this phenomenon are entirely based on spherical particle models, some of which have been experimentally verified. However, there is no knowledge on if and how the particle shape may affect the nonlinear electrophoretic behavior. This work presents an experimental study of the nonlinear electrophoretic velocities of rigid peanut- and pear-shaped particles in a rectangular microchannel, which are compared with rigid spherical particles of similar diameter and surface charge in terms of the particle slenderness. We observe a decrease in the nonlinear electrophoretic mobility, whereas an increase in the nonlinear index of electric field when the particle slenderness increases from the peanut- to pear-shaped and spherical particles. The values of the nonlinear index for the nonspherical particles are, however, still within the theoretically predicted range for spherical particles. We also observe an enhanced nonlinear electrophoretic behavior in a lower concentration buffer solution regardless of the particle shape.

Advances in Nanoarchitectonics: A Review of "Static" and "Dynamic" Particle Assembly Methods

Particle assembly is a promising technique to create functional materials and devices from nanoscale building blocks. However, the control of particle arrangement and orientation is challenging and requires careful design of the assembly methods and conditions. In this study, the static and dynamic methods of particle assembly are reviewed, focusing on their applications in biomaterial sciences. Static methods rely on the equilibrium interactions between particles and substrates, such as electrostatic, magnetic, or capillary forces. Dynamic methods can be associated with the application of external stimuli, such as electric fields, magnetic fields, light, or sound, to manipulate the particles in a non-equilibrium state. This study discusses the advantages and limitations of such methods as well as nanoarchitectonic principles that guide the formation of desired structures and functions. It also highlights some examples of biomaterials and devices that have been fabricated by particle assembly, such as biosensors, drug delivery systems, tissue engineering scaffolds, and artificial organs. It concludes by outlining the future challenges and opportunities of particle assembly for biomaterial sciences. This review stands as a crucial guide for scholars and professionals in the field, fostering further investigation and innovation. It also highlights the necessity for continuous research to refine these methodologies and devise more efficient techniques for nanomaterial synthesis. The potential ramifications on healthcare and technology are substantial, with implications for drug delivery systems, diagnostic tools, disease treatments, energy storage, environmental science, and electronics.

Particle Size-Dependent Electrophoresis in Polymer Solutions

It has long been known that the electrophoretic velocity of a charged particle is independent of its size under the thin-Debye-layer limit. This so-called Smoluchowski velocity is, however, valid only for Newtonian fluids. A couple of recent theoretical studies predict the rheology-induced particle size dependence of electrophoresis in non-Newtonian fluids. This work presents the first experimental demonstration of such dependence in viscoelastic poly(ethylene oxide) (PEO) solutions. Three different-sized particles are observed to travel at the same electrophoretic velocity in a Newtonian buffer through a rectangular microchannel. In contrast, their measured electrophoretic velocities in the PEO solution exhibit an increasing trend for larger particles, which is consistent with theoretical prediction. This particle size dependence is found to grow with an increasing concentration or length of the PEO polymer. Both trends are attributed to enhanced fluid elasticity, as characterized by the increasing elasticity number.

Assessing the Discriminatory Capabilities of iEK Devices under DC and DC-Biased AC Stimulation Potentials

There is a rising need for rapid and reliable analytical methods for separating microorganisms in clinical and biomedical applications. Microscale-insulator-based electrokinetic (iEK) systems have proven to be robust platforms for assessing a wide variety of microorganisms. Traditionally, iEK systems are usually stimulated with direct-current (DC) potentials. This work presents a comparison between using DC potentials and using DC-biased alternating-current (AC) potentials in iEK systems for the separation of microorganisms. The present study, which includes mathematical modeling and experimentation, compares the separation of bacterial and yeast cells in two distinct modes by using DC and DC-biased AC potentials. The quality of both separations, assessed in terms of separation resolution (Rs), showed a complete separation (Rs = 1.51) with the application of a DC-biased low-frequency AC signal but an incomplete separation (Rs = 0.55) with the application of an RMS-equivalent DC signal. Good reproducibility between experimental repetitions (<10%) was obtained, and good agreement (~18% deviation) was observed between modeling and experimental retention times. The present study demonstrates the potential of extending the limits of iEK systems by employing DC-biased AC potentials to perform discriminatory separations of microorganisms that are difficult to separate with the application of DC potentials.

Three-dimensional lab-on-a-foil device for dielectrophoretic separation of cancer cells

A simple, low-cost, three-dimensional (3D) lab-on-a-foil microfluidic device for dielectrophoretic separation of circulating tumor cells (CTCs) is designed and constructed. Disposable thin films are cut by xurography and microelectrode array are made with rapid inkjet printing. The multilayer device design allows the studying of spatial movements of CTCs and red blood cells (RBCs) under dielectrophoresis (DEP). A numerical simulation was performed to find the optimum driving frequency of RBCs and the crossover frequency for CTCs. At the optimum frequency, RBCs were lifted 120 µm in z-axis direction by DEP force, and CTCs were not affected due to negligible DEP force. By utilizing the displacement difference, the separation of CTCs (modeled with A549 lung carcinoma cells) from RBCs in z-axis direction was achieved. With the nonuniform electric field at optimized driving frequency, the RBCs were trapped in the cavities above the microchannel, whereas the A549 cells were separated with a high capture rate of 86.3% ± 0.2%. The device opens not only the possibility for 3D high-throughput cell separation but also for future developments in 3D cell manipulation through rapid and low-cost fabrication.

Lift at low Reynolds number

Lift forces are widespread in hydrodynamics. These are typically observed for big and fast objects and are often associated with a combination of fluid inertia (i.e. large Reynolds numbers) and specific symmetry-breaking mechanisms. In contrast, the properties of viscosity-dominated (i.e. low Reynolds numbers) flows make it more difficult for such lift forces to emerge. However, the inclusion of boundary effects qualitatively changes this picture. Indeed, in the context of soft and biological matter, recent studies have revealed the emergence of novel lift forces generated by boundary softness, flow gradients and/or surface charges. The aim of the present review is to gather and analyse this corpus of literature, in order to identify and unify the questioning within the associated communities, and pave the way towards future research.

Fluid Elasticity-Enhanced Insulator-Based Dielectrophoresis for Sheath-Free Particle Focusing in Very Dilute Polymer Solutions

Focusing particles into a narrow stream is usually a necessary step in microfluidic flow cytometry and particle sorting. We demonstrate that the addition of a small amount of poly(ethylene oxide) (PEO) polymer into a buffer solution can reduce by almost 1 order of magnitude the threshold DC electric field for single-line dielectrophoretic focusing of particles in a constricted microchannel. The particle focusing effectiveness of this fluid elasticity-enhanced insulator-based dielectrophoresis (E-iDEP) in very dilute PEO solutions gets enhanced with the increase of the PEO molecular weight and particle size. These two trends are consistent with a theoretical analysis that accounts for the fluid elasticity effects on the electrokinetic and dielectrophoretic particle motions. Surprisingly, the particle-focusing effectiveness of E-iDEP is observed to first increase and then decrease with an increase in the PEO concentration.

Directed colloidal assembly and banding via DC electrokinetics

Manipulating the transport and assembly of colloidal particles to form segregated bands or ordered supracolloidal structures plays an important role in many aspects of science and technology, from understanding the origin of life to synthesizing new materials for next-generation manufacturing, electronics, and therapeutics. One commonly used method to direct colloidal transport and assembly is the application of electric fields, either AC or DC, due to its feasibility. However, as colloidal segregation and assembly both require active redistribution of colloidal particles across multiple length scales, it is not apparent at first sight how a DC electric field, either externally applied or internally induced, can lead to colloidal structuring. In this Perspective, we briefly review and highlight recent advances and standing challenges in colloidal transport and assembly enabled by DC electrokinetics.

How Do Colloidal Nanoparticles Move in a Solution under an Electric Field?: In Situ Light Scattering Analysis

Understanding the dynamics of colloidal nanoparticles (NPs) in a solution is the key to assembling them into solids through a solution process such as electrophoretic deposition. In this study, newly developed in situ analysis with light scattering is used to examine NP dynamics induced by a non-uniform electric field. We reveal that the symmetric directions of moving NP aggregates are due to dielectrophoresis between the cylindrical electrodes, while the actual NP deposition is based on the charge of NPs (electrophoresis). Over time, the symmetry of the dynamics becomes less evident, inducing feeble deposition as the less-ordered dynamics become stronger. Eventually, two separate deposition mechanisms emerge as the deposition rate decreases with the change in the NP dynamics. Furthermore, we identify the vortex-like NP motion between the electrodes. These in situ analyses provide insights into the electrophoretic deposition mechanism and NP behavior in a solution under an electric field for fine film construction.

Nonlinear electrophoresis of dielectric particles in Newtonian fluids

In classical electrokinetics, the electrophoretic velocity of a dielectric particle is a linear function of the applied electric field. Theoretical studies have predicted the onset of nonlinear electrophoresis at high electric fields because of the nonuniform surface conduction over the curved particle. However, experimental studies have been left behind and are insufficient for a fundamental understanding of the parametric effects on nonlinear electrophoresis. We present in this work a systematic experimental study of the effects of buffer concentration, particle size, and particle zeta potential on the electrophoretic velocity of polystyrene particles in a straight rectangular microchannel for electric fields of up to 3 kV/cm. The measured nonlinear electrophoretic particle velocity is found to exhibit a 2(±0.5)-order dependence on the applied electric field, which appears to be within the theoretically predicted 3- and 3/2-order dependences for low and high electric fields, respectively. Moreover, the obtained nonlinear electrophoretic particle mobility increases with decreasing buffer concentration (for the same particle) and particle size (for particles with similar zeta potentials) or increasing particle zeta potential (for particles with similar sizes). These observations are all consistent with the theoretical predictions for high electric fields.

Microfluidic Applications of Artificial Cilia: Recent Progress, Demonstration, and Future Perspectives

Artificial cilia-based microfluidics is a promising alternative in lab-on-a-chip applications which provides an efficient way to manipulate fluid flow in a microfluidic environment with high precision. Additionally, it can induce favorable local flows toward practical biomedical applications. The endowment of artificial cilia with their anatomy and capabilities such as mixing, pumping, transporting, and sensing lead to advance next-generation applications including precision medicine, digital nanofluidics, and lab-on-chip systems. This review summarizes the importance and significance of the artificial cilia, delineates the recent progress in artificial cilia-based microfluidics toward microfluidic application, and provides future perspectives. The presented knowledge and insights are envisaged to pave the way for innovative advances for the research communities in miniaturization.

3D Numerical Study of the Electrokinetic Motion of a Microparticle Adsorbed at a Horizontal Oil/Water Interface in an Infinite Domain

This work builds a three-dimensional (3D) simulation model and studies the electrokinetic velocity of a microparticle adsorbed at a horizontal oil/water interface in an infinite domain. The effects of the interface zeta potentials, the electric field, the oil dynamic viscosity, and the contact angle between the particle and the oil/water interface are investigated in detail. The results show that in an infinite oil/water interface system, both the negatively charged mobile oil/water interface and the negatively charged particle adsorbed to it move toward the positive electrode of the DC electric field, and the particle velocity increases along with the contact angle, the electric field strength, and the absolute values of negative zeta potential of both the particle and the oil/water interface. When the oil/water interface is positively charged with a relatively small zeta potential, the negatively charged microparticle also moves in the opposite direction of the electric field. The larger the oil dynamic viscosity, the smaller the electrokinetic velocity of the microparticle at the interface. Additionally, the numerical simulation results are compared with the reported experiment results under the same conditions, and they have good agreement.

Hand-Powered Inertial Microfluidic Syringe-Tip Centrifuge

Conventional sample preparation techniques require bulky and expensive instruments and are not compatible with next-generation point-of-care diagnostic testing. Here, we report a manually operated syringe-tip inertial microfluidic centrifuge (named i-centrifuge) for high-flow-rate (up to 16 mL/min) cell concentration and experimentally demonstrate its working mechanism and performance. Low-cost polymer films and double-sided tape were used through a rapid nonclean-room process of laser cutting and lamination bonding to construct the key components of the i-centrifuge, which consists of a syringe-tip flow stabilizer and a four-channel paralleled inertial microfluidic concentrator. The unstable liquid flow generated by the manual syringe was regulated and stabilized with the flow stabilizer to power inertial focusing in a four-channel paralleled concentrator. Finally, we successfully used our i-centrifuge for manually operated cell concentration. This i-centrifuge offers the advantages of low device cost, simple hand-powered operation, high-flow-rate processing, and portable device volume. Therefore, it holds potential as a low-cost, portable sample preparation tool for point-of-care diagnostic testing.

Microscale electrokinetic-based analysis of intact cells and viruses

Miniaturized electrokinetic methods have proven to be robust platforms for the analysis and assessment of intact microorganisms, offering short response times and higher integration than their bench-scale counterparts. The present review article discusses three types of electrokinetic-based methodologies: electromigration or motion-based techniques, electrode-based electrokinetics, and insulator-based electrokinetics. The fundamentals of each type of methodology are discussed and relevant examples from recent reports are examined, to provide the reader with an overview of the state-of-the-art on the latest advancements on the analysis of intact cells and viruses with microscale electrokinetic techniques. The concluding remarks discuss the potential applications and future directions.

Emerging on-chip electrokinetic based technologies for purification of circulating cancer biomarkers towards liquid biopsy: A review

Early detection of cancer can significantly reduce mortality and save lives. However, the current cancer diagnosis is highly dependent on costly, complex, and invasive procedures. Thus, a great deal of effort has been devoted to exploring new technologies based on liquid biopsy. Since liquid biopsy relies on detection of circulating biomarkers from biofluids, it is critical to isolate highly purified cancer-related biomarkers, including circulating tumor cells (CTCs), cell-free nucleic acids (cell-free DNA and cell-free RNA), small extracellular vesicles (exosomes), and proteins. The current clinical purification techniques are facing a number of drawbacks including low purity, long processing time, high cost, and difficulties in standardization. Here, we review a promising solution, on-chip electrokinetic-based methods, that have the advantage of small sample volume requirement, minimal damage to the biomarkers, rapid, and label-free criteria. We have also discussed the existing challenges of current on-chip electrokinetic technologies and suggested potential solutions that may be worthy of future studies.

Effect of skimming layer in an electroosmotically driven viscoelastic fluid flow over charge modulated walls

We report a numerical study on the effect of the skimming layer in an EOF of Oldroyd-B fluid over charge modulated walls. Three types of flow conditions were identified on the basis of the relative thickness of the skimming layer and the electrical double layer. We observe maximum slip velocity magnitude when the skimming layer thickness is very less than the thickness of the electrical double layer. For higher skimming layer thickness compared to the thickness of electrical double layer, slip velocity magnitude attenuates, and the polymeric stress inside the skimming layer becomes zero. Enhanced fluid elasticity generates asymmetric flow structures inside the microchannel, which can also be achieved by imposing an asymmetric surface charge along the channel walls. Our present analysis highlights the complex flow dynamics of the EOF of biofluids/polymeric fluids with a near-wall region depleted of macro-molecules.

The latest advances on nonlinear insulator-based electrokinetic microsystems under direct current and low-frequency alternating current fields: a review

This review article presents an overview of the evolution of the field of insulator-based dielectrophoresis (iDEP); in particular, it focuses on insulator-based electrokinetic (iEK) systems stimulated with direct current and low-frequency(< 1 kHz) AC electric fields. The article covers the surge of iDEP as a research field where many different device designs were developed, from microchannels with arrays of insulating posts to devices with curved walls and nano- and micropipettes. All of these systems allowed for the manipulation and separation of a wide array of particles, ranging from macromolecules to microorganisms, including clinical and biomedical applications. Recent experimental reports, supported by important theoretical studies in the field of physics and colloids, brought attention to the effects of electrophoresis of the second kind in these systems. These recent findings suggest that DEP is not the main force behind particle trapping, as it was believed for the last two decades. This new research suggests that particle trapping, under DC and low-frequency AC potentials, mainly results from a balance between electroosmotic and electrophoretic effects (linear and nonlinear); although DEP is present in these systems, it is not a dominant force. Considering these recent studies, it is proposed to rename this field from DC-iDEP to DC-iEK (and low-frequency AC-iDEP to low-frequency AC-iEK). Whereas much research is still needed, this is an exciting time in the field of microscale EK systems, as these new findings seem to explain the challenges with modeling particle migration and trapping in iEK devices, and provide perhaps a better understanding of the mechanisms behind particle trapping.

Fluid rheological effects on streaming dielectrophoresis in a post-array microchannel

Recent studies have demonstrated the strong influences of fluid rheological properties on insulator-based dielectrophoresis (iDEP) in single-constriction microchannels. However, it is yet to be understood how iDEP in non-Newtonian fluids depends on the geometry of insulating structures. We report in this work an experimental study of fluid rheological effects on streaming DEP in a post-array microchannel that presents multiple contractions and expansions. The iDEP focusing and trapping of particles in a viscoelastic polyethylene oxide solution are comparable to those in a Newtonian buffer, which is consistent with the observations in a single-constriction microchannel. Similarly, the insignificant iDEP effects in a shear-thinning xanthan gum solution also agree with those in the single-constriction channel except that gel-like structures are observed to only form in the post-array microchannel under large DC electric fields. In contrast, the iDEP effects in both viscoelastic and shear-thinning polyacrylamide solution are significantly weaker than in the single-constriction channel. Moreover, instabilities occur in the electroosmotic flow and appear to be only dependent on the DC electric field. These phenomena may be associated with the dynamics of polymers as they are electrokinetically advected around and through the posts.

Low-cost polymer-film spiral inertial microfluidic device for label-free separation of malignant tumor cells

We developed a low-cost polymer-film spiral inertial microfluidic device for the effective size-dependent separation of malignant tumor cells. The device was fabricated in polymer films by rapid laser cutting and chemical bonding. After fabricating the prototype device, the separation performance of our device was evaluated using particles and cells. The effects of operational flow rate, cell diameter, and cell concentration on the separation performance were explored. Our device successfully separated tumor cells from polydisperse white blood cells according to their different migration modes and lateral positions. Then, the separation of rare cells was carried out using the high-concentration lysed blood spiked with 200 tumor cells. Experimental results showed that 83.90% of the tumor cells could be recovered, while 99.87% of white blood cells could be removed. We successfully employed our device for processing clinical pleural effusion samples from patients with advanced metastatic breast cancer. Malignant tumor cells with an average purity of 2.37% could be effectively enriched, improving downstream diagnostic accuracy. Our device offers the advantages of label-free operation, low cost, and fast fabrication, thus being a potential tool for effective cell separation.

Secondary-flow-aided single-train elastic-inertial focusing in low elasticity viscoelastic fluids

Elastic-inertial focusing has attracted increasing interest in recent years due to the three-dimensional (3D) single-train focusing ability it offers. However, multi-train focusing, instead of single-train focusing, was observed in viscoelastic fluids with low elasticity as a result of the competition between inertia effect and viscoelasticity effect. To address this issue, we employed the secondary flow to facilitate single-train elastic-inertial focusing in low elasticity viscoelastic fluids. A three-section contraction-expansion channel was designed to induce the secondary flow to pinch the multiplex focusing trains into a single one exactly at the channel centerline. After demonstrating the focusing process and mechanism in our device, we systematically explored and discussed the effects of particle diameter, operational flow rate, polymer concentration, and channel dimension on particle focusing performances. Our device enables single-train focusing of particles in viscoelastic fluids with low elasticity, and offers advantages of planar single-layer structure, and sheathless, external-field free operation.

Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes.

Review of nonlinear electrokinetic flows in insulator-based dielectrophoresis: From induced charge to Joule heating effects

Insulator-based dielectrophoresis (iDEP) has been increasingly used for particle manipulation in various microfluidic applications. It exploits insulating structures to constrict and/or curve electric field lines to generate field gradients for particle dielectrophoresis. However, the presence of these insulators, especially those with sharp edges, causes two nonlinear electrokinetic flows, which, if sufficiently strong, may disturb the otherwise linear electrokinetic motion of particles and affect the iDEP performance. One is induced charge electroosmotic (ICEO) flow because of the polarization of the insulators, and the other is electrothermal flow because of the amplified Joule heating in the fluid around the insulators. Both flows vary nonlinearly with the applied electric field (either DC or AC) and exhibit in the form of fluid vortices, which have been utilized to promote some applications while being suppressed in others. The effectiveness of iDEP benefits from a comprehensive understanding of the nonlinear electrokinetic flows, which is complicated by the involvement of the entire iDEP device into electric polarization and thermal diffusion. This article is aimed to review the works on both the fundamentals and applications of ICEO and electrothermal flows in iDEP microdevices. A personal perspective of some future research directions in the field is also given.

On the potential of microscale electrokinetic cascade devices

Phages used for phage therapy of multidrug resistant bacteria must be highly purified prior to use. There are limited purification approaches that are broadly applicable to many phage types. Electrokinetics has shown great potential to manipulate phages, but obstructions from the cell debris produced during phage propagation can severely diminish the capacity of an electrokinetic device to concentrate and purify phage samples. A multipart insulator-based electrokinetic device is proposed here to remove the larger, undesirable components of mixtures from phage preparations while transferring the freshly purified and concentrated sample to a second stage for downstream analysis. By combining the large debris prescreen and analysis stages in a streamlined system, this approach simultaneously reduces the impact of clogging and minimizes the sample loss observed during manual transferring of purified samples. Polystyrene particles were used to demonstrate a diminished sample loss of approximately one order of magnitude when using the cascade device as opposed to a manual transfer scheme. The purification and concentration of three different phage samples were demonstrated using the first stage of the cascade device as a prescreen. This design provides a simple method of purifying and concentrating valuable samples from a complex mixture that might impede separation capacity in a single channel.

Insulator-based dielectrophoretic focusing and trapping of particles in non-Newtonian fluids

Insulator-based dielectrophoretic (iDEP) microdevices have been limited to work with Newtonian fluids. We report an experimental study of the fluid rheological effects on iDEP focusing and trapping of polystyrene particles in polyethylene oxide, xanthan gum, and polyacrylamide solutions through a constricted microchannel. Particle focusing and trapping in the mildly viscoelastic polyethylene oxide solution are slightly weaker than in the Newtonian buffer. They are, however, significantly improved in the strongly viscoelastic and shear thinning polyacrylamide solution. These observed particle focusing behaviors exhibit a similar trend with respect to electric field, consistent with a revised theoretical analysis for iDEP focusing in non-Newtonian fluids. No apparent focusing of particles is achieved in the xanthan gum solution, though the iDEP trapping can take place under a much larger electric field than the other fluids. This is attributed to the strong shear thinning-induced influences on both the electroosmotic flow and electrokinetic/dielectrophoretic motions.

AC Insulator-Based Dielectrophoretic Focusing of Particles and Cells in an "Infinite" Microchannel

It is often necessary to prefocus particles and cells into a tight stream for subsequent separation and/or analysis in microfluidic devices. A DC electric field has been widely used for particle and cell focusing in insulator-based dielectrophoretic (iDEP) microdevices, where a large field magnitude, a high constriction ratio, and/or a long microchannel are usually required to enhance the iDEP effect. We demonstrate, in this work, an AC iDEP focusing technique, which utilizes a low-frequency AC electric field to generate both an oscillatory electrokinetic flow of the particle/cell suspension and a field direction-independent dielectrophoretic force for particle/cell focusing in a virtually "infinite" microchannel. We also develop a theoretical analysis to evaluate this focusing in terms of the AC voltage frequency, amplitude, and particle size, which are each validated through both experimental demonstration and numerical simulation. The effectiveness of AC iDEP focusing increases with the second order of electric field magnitude, superior to DC iDEP focusing with only a first-order dependence. This feature and the "infinite" channel length together remove the necessity of large electric field and/or small constriction in DC iDEP focusing of small particles.

Diffusiophoresis of a highly charged soft particle normal to a conducting plane

Diffusiophoresis of a soft particle in electrolyte solutions normal to a conducting solid plane is investigated theoretically in this study, focusing on the highly charged particle in particular. A pseudo-spectral method based on Chebyshev polynomial is adopted to solve the resultant governing electrokinetic equations. It was found, among other things, that the closer the soft particle is to the plane, the faster it moves in general, provided only the chemiphoresis component of the diffusiophoresis is involved, i.e., no diffusion potential is present. The presence of the conducting plane is found to have three effects upon the particle motion nearby: the geometric boundary confinement effect, the electrostatic mirror-image force analog effect, and the hydrodynamic retarding effect. The enhancement of the double layer polarization by the first two effects leads to the seeming intriguing observation mentioned above. The particle always moves away from the plane in chemiphoresis. If a diffusion potential is present, however, then it is possible to drive the particle toward the plane. The results have potential applications in drug delivery.

Microscale nonlinear electrokinetics for the analysis of cellular materials in clinical applications: a review

This review article presents a discussion of some of the latest advancements in the field of microscale electrokinetics for the analysis of cells and subcellular materials in clinical applications. The introduction presents an overview on the use of electric fields, i.e., electrokinetics, in microfluidics devices and discusses the potential of electrokinetic-based methods for the analysis of liquid biopsies in clinical and point-of-care applications. This is followed by four comprehensive sections that present some of the newest findings on the analysis of circulating tumor cells, blood (red blood cells, white blood cells, and platelets), stem cells, and subcellular particles (extracellular vesicles and mitochondria). The valuable contributions discussed here (with 131 references) were mainly published during the last 3 to 4 years, providing the reader with an overview of the state-of-the-art in the use of microscale electrokinetic methods in clinical analysis. Finally, the conclusions summarize the main advancements and discuss the future prospects.

Analysis of microorganisms with nonlinear electrokinetic microsystems

Nonlinear electrokinetics (EK), specifically electrophoresis of the second kind, dielectrophoresis (DEP) and electrorotation (EROT), have gained significant interest recently for their flexibility and labeless discriminant manner of operation. The current applications of these technologies are a clear advancement from what they were when first discovered, but also still show strong signs of future growth. The present review article presents a discussion of the current uses of microscale nonlinear EK technologies as analytical, sensing, and purification tools for microorganisms. The discussion is focused on some of the latest discoveries with various nonlinear EK microfluidic techniques, such as DEP particle trapping and EROT for particle assessments, for the analysis of microorganisms ranging from viruses to parasites. Along the way, special focus was given to key research articles from within the past two years to provide the most up-to-date knowledge on the current state-of-the-art within the field of microscale EK, and from there, an outlook on where the future of the field is headed is also included.

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.

Inertial microfluidics: Recent advances

Inertial microfluidics has attracted significant attentions in last decade due to its superior advantages of high throughput, label- and external field-free operation, simplicity, and low cost. A wide variety of channel geometry designs were demonstrated for focusing, concentrating, isolating, or separating of various bioparticles such as blood components, circulating tumor cells, bacteria, and microalgae. In this review, we first briefly introduce the physics of inertial migration and Dean flow for allowing the readers with diverse backgrounds to have a better understanding of the fundamental mechanisms of inertial microfluidics. Then, we present a comprehensive review of the recent advances and applications of inertial microfluidic devices according to different channel geometries ranging from straight channels, curved channels to contraction-expansion-array channels. Finally, the challenges and future perspective of inertial microfluidics are discussed. Owing to its superior benefit for particle manipulation, the inertial microfluidics will play a more important role in biology and medicine applications.

Microfluidic Production of Autofluorescent BSA Hydrogel Microspheres and Their Sequential Trapping for Fluorescence-Based On-Chip Permanganate Sensing

Microfabrication technologies have extensively advanced over the past decades, realizing a variety of well-designed compact devices for material synthesis, separation, analysis, monitoring, sensing, and so on. The performance of such devices has been undoubtedly improved, while it is still challenging to build up a platform by rationally combining multiple processes toward practical demands which become more diverse and complicated. Here, we present a simple and effective microfluidic system to produce and immobilize a well-defined functional material for on-chip permanganate (MnO4-) sensing. A droplet-based microfluidic approach that can continuously produce monodispersed droplets in a water-in-oil system is employed to prepare highly uniform microspheres (average size: 102 μm, coefficient of variation: 3.7%) composed of bovine serum albumin (BSA) hydrogel with autofluorescence properties in the presence of glutaraldehyde (GA). Each BSA hydrogel microsphere is subsequently immobilized in a microchannel with a hydrodynamic trapping structure to serve as an independent fluorescence unit. Various anions such as Cl-, NO3-, PO43-, Br-, BrO3-, ClO4-, SCN-, HCO3-, and MnO4- are individually flowed into the microchannel, resulting in significant fluorescence quenching only in the case of MnO4-. Linear correlation is confirmed at an MnO4- concentration from 20 to 80 μM, and a limit of detection is estimated to be 1.7 μM. Furthermore, we demonstrate the simultaneous immobilization of two kinds of different microspheres in parallel microchannels, pure BSA hydrogel microspheres and BSA hydrogel microspheres containing rhodamine B molecules, making it possible to acquire two fluorescence signals (green and yellow). The present microfluidics-based combined approach will be useful to record a fingerprint of complicated samples for sensing/identification purposes by flexibly designing the size and composition of the BSA hydrogel microspheres, immobilizing them in a desired manner and obtaining a specific pattern.

Electrokinetic characterization of synthetic protein nanoparticles

The application of nanoparticle in medicine is promising for the treatment of a wide variety of diseases. However, the slow progress in the field has resulted in relatively few therapies being translated into the clinic. Anisotropic synthetic protein nanoparticles (ASPNPs) show potential as a next-generation drug-delivery technology, due to their biocompatibility, biodegradability, and functionality. Even though ASPNPs have the potential to be used in a variety of applications, such as in the treatment of glioblastoma, there is currently no high-throughput technology for the processing of these particles. Insulator-based electrokinetics employ microfluidics devices that rely on electrokinetic principles to manipulate micro- and nanoparticles. These miniaturized devices can selectively trap and enrich nanoparticles based on their material characteristics, and subsequently release them, which allows for particle sorting and processing. In this study, we use insulator-based electrokinetic (EK) microdevices to characterize ASPNPs. We found that anisotropy strongly influences electrokinetic particle behavior by comparing compositionally identical anisotropic and non-anisotropic SPNPs. Additionally, we were able to estimate the empirical electrokinetic equilibrium parameter (eE EEC) for all SPNPs. This particle-dependent parameter can allow for the design of various separation and purification processes. These results show how promising the insulator-based EK microdevices are for the analysis and purification of clinically relevant SPNPs.

Joule heating-enabled electrothermal enrichment of nanoparticles in insulator-based dielectrophoretic microdevices

Insulator-based dielectrophoresis (iDEP) exploits the electric field gradients formed around insulating structures to manipulate particles for diverse microfluidic applications. Compared to the traditional electrode-based dielectrophoresis, iDEP microdevices have the advantages of easy fabrication, free of water electrolysis, and robust structure, etc. However, the presence of in-channel insulators may cause thermal effects because of the locally amplified Joule heating of the fluid. The resulting electrothermal flow circulations are exploited in this work to trap and concentrate nanoscale particles (of 100 nm diameter and less) in a ratchet-based iDEP microdevice. Such Joule heating-enabled electrothermal enrichment of nanoparticles are found to grow with the increase of alternating current or direct current electric field. It also becomes more effective for larger particles and in a microchannel with symmetric ratchets. Moreover, a depth-averaged numerical model is developed to understand and simulate the various parametric effects, which is found to predict the experimental observations with a good agreement.

Protein Dielectrophoresis: I. Status of Experiments and an Empirical Theory

The dielectrophoresis (DEP) data reported in the literature since 1994 for 22 different globular proteins is examined in detail. Apart from three cases, all of the reported protein DEP experiments employed a gradient field factor ∇Em2 that is much smaller (in some instances by many orders of magnitude) than the ~4 1021 V2/m3 required, according to current DEP theory, to overcome the dispersive forces associated with Brownian motion. This failing results from the macroscopic Clausius-Mossotti (CM) factor being restricted to the range 1.0 > CM > -0.5. Current DEP theory precludes the protein's permanent dipole moment (rather than the induced moment) from contributing to the DEP force. Based on the magnitude of the β-dispersion exhibited by globular proteins in the frequency range 1 kHz-50 MHz, an empirically derived molecular version of CM is obtained. This factor varies greatly in magnitude from protein to protein (e.g., ~37,000 for carboxypeptidase; ~190 for phospholipase) and when incorporated into the basic expression for the DEP force brings most of the reported protein DEP above the minimum required to overcome dispersive Brownian thermal effects. We believe this empirically-derived finding validates the theories currently being advanced by Matyushov and co-workers.

Passive Dielectrophoretic Focusing of Particles and Cells in Ratchet Microchannels

Focusing particles into a tight stream is critical for many microfluidic particle-handling devices such as flow cytometers and particle sorters. This work presents a fundamental study of the passive focusing of polystyrene particles in ratchet microchannels via direct current dielectrophoresis (DC DEP). We demonstrate using both experiments and simulation that particles achieve better focusing in a symmetric ratchet microchannel than in an asymmetric one, regardless of the particle movement direction in the latter. The particle focusing ratio, which is defined as the microchannel width over the particle stream width, is found to increase with an increase in particle size or electric field in the symmetric ratchet microchannel. Moreover, it exhibits an almost linear correlation with the number of ratchets, which can be explained by a theoretical formula that is obtained from a scaling analysis. In addition, we have demonstrated a DC dielectrophoretic focusing of yeast cells in the symmetric ratchet microchannel with minimal impact on the cell viability.