Microfluidic device embedding electrodes for dielectrophoretic manipulation of cells‐A review

Microfluidic device embedding electrodes realizes cell manipulation with the help of dielectrophoresis. Cell manipulation is an important technology for cell sorting and cell population purification. Till now, the theory of dielectrophoresis has been greatly developed. Microfluidic devices with various arrangements of electrodes have been reported from the beginning of the single non‐uniform electric field to the later multiple physical fields. This paper reviews the research status of microfluidic device embedding electrodes for cell manipulation based on dielectrophoresis. Firstly, the working principle of dielectrophoresis is explained. Next, cell manipulation approaches based on dielectrophoresis are introduced. Then, different types of electrode arrangements in the microfluidic device for cell manipulation are discussed, including planar, multilayered and microarray dot electrodes. Finally, the future development trend of the dielectrophoresis with the help of microfluidic devices is prospected. With the rapid development of microfluidic technology, in the near future, high precision, high throughput, high efficiency, multifunctional, portable, economical and practical microfluidic dielectrophoresis will be widely used in the fields of biology, medicine, agriculture and so on.     

Dielectrophoretic manipulation of particle mixtures employing asymmetric insulating posts

A novel scheme for particle separation with insulator‐based dielectrophoresis (iDEP) was developed. This technique offers the capability for an inverted order in particle elution, where larger particles leave the system before smaller particles. Asymmetrically shaped insulating posts, coupled with direct current (DC) biased low‐frequency alternating current (AC) electric potentials, were used to successfully separate a mixture of 500 nm and 1 μm polystyrene particles (size difference of 0.5 μm in diameter). In this separation, the 1 μm particles were eluted first, demonstrating the discriminatory potential of this methodology. To extend this technique to biological samples, a mixture containing Saccharomyces cerevisiae cells (6.3 μm) and 2 μm polystyrene particles was also separated, with the cells being eluted first. The asymmetric posts featured a shorter sharp half and a longer blunt half; this produced an asymmetry in the forces exerted on the particles. The negative DC offset produced a net displacement of the smaller particles toward the upstream direction, while the post asymmetry produced a net displacement of the larger particles toward the downstream direction. This new iDEP approach provides a setup where larger particles are quickly concentrated at the outlet of the post array and can be released first when in a mixture with smaller particles. This new scheme offers an extra set of parameters (alternating current amplitude, DC offset, post asymmetry, and shape) that can be manipulated to obtain a desired separation. This asymmetric post iDEP technique has potential for separations where it is important to quickly elute and enrich larger and more fragile cells in biological samples.     

Dynamic manipulation of modes in an optical waveguide using dielectrophoresis

The emergence of optofluidics has brought a high degree of tuneability and reconfigurability to optical devices. These possibilities are provided by characteristics of fluids including mobility, wide range of index modulation, and abrupt interfaces that can be easily reshaped. In this work, we created a new class of optofluidic waveguides, in which suspended mesoparticles were employed to greatly enhance the flexibility of the system. We demonstrated tuneable quasi single mode waveguides using spatially controllable mesoparticles in optofluidics. The coupling of waveguiding modes into the assembly of mesoparticles produces strong interactions and resonant conditions, which promote the transitions of the waveguiding modes. The modal response of the system depends on the distribution of packed particles above the polymeric rib waveguide which can be readily controlled under the appropriate combination of dielectrophoresis and hydrodynamic forces.     

Review of cell and particle trapping in microfluidic systems

The ability to obtain ideal conditions for well-defined chemical microenvironments and controlled temporal chemical and/or thermal variations holds promise of high-resolution cell response studies, cell-cell interactions or e.g. proliferation conditions for stem cells. It is a major motivation for the rapid increase of lab-on-a-chip based cell biology research. In view of this, new chip-integrated technologies are at an increasing rate being presented to the research community as potential tools to offer spatial control and manipulation of cells in microfluidic systems. This is becoming a key area of interest in the emerging lab-on-a-chip based cell biology research field. This review focuses on the different technical approaches presented to enable trapping of particles and cells in microfluidic system.     

Sheath-assisted focusing of microparticles on lab-on-a-chip platforms

Lab-on-a-chip (LOC) technologies can take advantage of sheath flows for particle/cell focusing before sensing or sorting. The integration of focusing with other microscale manipulation techniques (e.g., sorting) creates a trade-off between the throughput of the device and its performance. Therefore, exploring the effective parameters for cells/particles focusing enables us to improve the desired output of LOC devices. A common configuration for sheath-assisted focusing is Y junctions, which are parametrically studied in this paper. First, a computational model was developed and validated by comparing it with our experimental results. Using COMSOL Multiphysics modeling, the effects of multiple parameters were studied. These parameters include the sheath flow ratio (sheath flow over total flow), width ratio (width of the sheath inlet over the total width), junction angles, and particle size on the focusing width and the distribution of the particles within the focusing region. Then, the numerical data were used to develop two generalized linear models to predict the focusing width of the particles and the standard deviation of the position of the particles. The results showed that the focusing width is greatly impacted by the sheath flow rate ratio. Further, the standard deviation of the position of the particles, which represents the concentration of the particles, is mostly dependent on the flow rate ratio, width ratio, and particle size. Our results provide a better understanding of how the device geometrical and operational factors affect the position of the particles in the development of high-performance on-chip sensing and sorting of both cells and particles.     

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.     

Microfluidics technology for manipulation and analysis of biological cells

Analysis of the profiles and dynamics of molecular components and sub-cellular structures in living cells using microfluidic devices has become a major branch of bioanalytical chemistry during the past decades. Microfluidic systems have shown unique advantages in performing analytical functions such as controlled transportation, immobilization, and manipulation of biological molecules and cells, as well as separation, mixing, and dilution of chemical reagents, which enables the analysis of intracellular parameters and detection of cell metabolites, even on a single-cell level. This article provides an in-depth review on the applications of microfluidic devices for cell-based assays in recent years (2002–2005). Various cell manipulation methods for microfluidic applications, based on magnetic, optical, mechanical, and electrical principles, are described with selected examples of microfluidic devices for cell-based analysis. Microfluidic devices for cell treatment, including cell lysis, cell culture, and cell electroporation, are surveyed and their unique features are introduced. Special attention is devoted to a number of microfluidic devices for cell-based assays, including micro cytometer, microfluidic chemical cytometry, biochemical sensing chip, and whole cell sensing chip.     

Development of a new contactless dielectrophoresis system for active particle manipulation using movable liquid electrodes

This study presents a new DEP manipulation technique using a movable liquid electrode, which allows manipulation of particles by actively controlling the locations of electrodes and applying on–off electric input signals. This DEP system consists of mercury as a movable liquid electrode, indium tin oxide (ITO)‐coated glass, SU‐8‐based microchannels for electrode passages, and a PDMS medium chamber. A simple squeezing method was introduced to build a thin PDMS layer at the bottom of the medium chamber to create a contactless DEP system. To determine the operating conditions, the DEP force and the friction force were analytically compared for a single cell. In addition, an appropriate frequency range for effective DEP manipulation was chosen based on an estimation of the Clausius–Mossotti factor and the effective complex permittivity of the yeast cell using the concentric shell model. With this system, we demonstrated the active manipulation of yeast cells, and measured the collection efficiency and the dielectrophoretic velocity of cells for different AC electric field strengths and applied frequencies. The experimental results showed that the maximum collection efficiency reached was approximately 90%, and the dielectrophoretic velocity increased with increasing frequency and attained the maximum value of 10.85 ± 0.95 μm/s at 100 kHz, above which it decreased.     

Manipulation of self-assembled three-dimensional architecture in reusable acoustofluidic device

Reconstructing of cell architecture plays a vital role in tissue engineering. Recent developments of self-assembling of cells into three-dimensional (3D) matrix pattern using surface acoustic waves have paved a way for a better tissue engineering platform thanks to its unique properties such as nature of noninvasive and noncontact, high biocompatibility, low-power consumption, automation capability, and fast actuation. This article discloses a method to manipulate the orientation and curvature of 3D matrix pattern by redesigning the top wall of microfluidic chamber and the technique to create a 3D longitudinal pattern along preinserted polydimethylsiloxane (PDMS) rods. Experimental results showed a good agreement with model predictions. This research can actively contribute to the development of better organs-on-chips platforms with capability of controlling cell architecture and density. Meanwhile, the 3D longitudinal pattern is suitable for self-assembling of microvasculatures.     

Miniaturized platforms for the detection of single-nucleotide polymorphisms

Conventional methods for detecting single-nucleotide polymorphisms (SNPs), the most common form of genetic variation in human beings, are mostly limited by their analysis time and throughputs. In contrast, advances in microfabrication technology have led to the development of miniaturized platforms that can potentially provide rapid high-throughput analysis at small sample volumes. This review highlights some of the recent developments in the miniaturization of SNP detection platforms, including microarray-based, bead-based microfluidic and microelectrophoresis-based platforms. Particular attention is paid to their ease of fabrication, analysis time, and level of throughput.     

Reorientation of microfluidic channel enables versatile dielectrophoretic platforms for cell manipulations

Dielectrophoresis is a versatile tool for the sorting, immobilization, and characterization of cells in microfluidic systems. The performance of dielectrophoretic systems strongly relies on the configuration of microelectrodes, which produce a nonuniform electric field. However, once fabricated, the microelectrodes cannot be reconfigured to change the characteristics of the system. Here, we show that the reorientation of the microfluidic channel with respect to the microelectrodes can be readily utilized to alter the characteristics of the system. This enables us to change the location and density of immobilized viable cells across the channel, release viable cells along customized numbers of streams within the channel, change the deflection pattern of nonviable cells along the channel, and improve the sorting of viable and nonviable cells in terms of flow throughput and efficiency of the system. We demonstrate that the reorientation of the microfluidic channel is an effective tool to create versatile dielectrophoretic platforms using the same microelectrode design.     

A survey of electrokinetically-driven microfluidics for cancer cells manipulation

Cancer is one of the leading causes of annual deaths worldwide, accounting for nearly 10 million deaths each year. Metastasis, the process by which cancer spreads across the patient's body, is the main cause of death in cancer patients. Because the rising trend observed in statistics of new cancer cases and cancer-related deaths does not allow for an optimistic viewpoint on the future—in relation to this terrible disease—the scientific community has sought methods to enable early detection of cancer and prevent the apparition of metastatic tumors. One such method is known as liquid biopsy, wherein a sample is taken from a bodily fluid and analyzed for the presence of CTCs or other cancer biomarkers (e.g., growth factors). With this objective, interest is growing by year in electrokinetically-driven microfluidics applied for the concentration, capture, filtration, transportation, and characterization of CTCs. Electrokinetic techniques—electrophoresis, dielectrophoresis, electrorotation, and electrothermal and EOF—have great potential for miniaturization and integration with electronic instrumentation for the development of point-of-care devices, which can become a tool for early cancer diagnostics and for the design of personalized therapeutics. In this contribution, we review the state of the art of electrokinetically-driven microfluidics for cancer cells manipulation.     

Continuous flow microfluidic cell inactivation with the use of insulating micropillars for multiple electroporation zones

Electroporation is a powerful tool for inactivating cells and transfecting biological cells and has applications in biology, genetic engineering, medicine, environment, and many others. We report a new continuous flow device embedded with insulating micropillars to achieve better performance of cell inactivation. The use of micropillars creates multiple electroporation zones with enhanced local electric field strengths. Using a model solution of Saccharomyces cerevisiae, we examined the inactivation performance of the device under various applied electric voltages and flow rates. Results from the numerical simulations and experiments showed that even with an induced transmembrane potential of 0.58 V, close to 63% of cell inactivation was achieved at a flow rate of 2.5 mL/h. This was higher than the 24% cell inactivation observed for a reference device without micropillars that was subjected to the same conditions.     

A device for investigation of natural cell mobility and deformability

A microfluidic device made of polydimethylsiloxane was developed for continuous evaluation of natural migration mobility of many eukaryotic cells in relaxed and deformed state. The device was fabricated by standard photolithography and soft lithography techniques using the SU-8 3010 negative photoresist on a glass wafer as the master mold. The simple flow-free device exploits the chemotactic movement of cells through a set of mechanical barriers in the direction of concentration gradients of attractants. The barriers are formed by arrays of circular cross-section pillars with decreasing spacing 7, 5, and 3 μm. To pass through the obstacles, the cells are deformed and change their cytoskeletal architecture. The instantaneous migration velocities of cells are monitored in a time-lapse setup of the scanning confocal microscope. Thus, the cellular deformability and migratory activity can easily be evaluated. The functionality of the device was tested with model HeLa cells stably transfected with fluorescent Premo FUCCI Cell Cycle Sensor. The designed device has the potential to be implemented for testing the tendency of patients’ tumors to metastasis.     

Interfacial nanoarchitectonics for molecular manipulation and molecular machine operation

The nanoarchitectonics concept enables us to produce functional systems and materials from nanoscale units through nanotechnological approaches together with the processes including chemical syntheses, atom/molecule manipulations, self-assemblies, self-organizations, field-induced material regulations, and bio-related processes. Especially, manipulations of molecules (molecular machines) and sophisticated organization would be attractive targets in interfacial nanoarchitectonics. In this short review, we introduce several typical examples on manipulations of functional molecules and molecular machines at interfacial media. The examples are classified roughly according to driving forces of manipulations; (i) manipulations through chemical reactions and interactions; (ii) light-driven manipulations; (iii) electrically controlled manipulations; (iv) mechanical manipulations. Future possibilities of molecular manipulations at interfaces such as usages in biological systems are discussed in perspective section.     

ProteinShop: A tool for interactive protein manipulation and steering

We describe ProteinShop, a new visualization tool that streamlines and simplifies the process of determining optimal protein folds. ProteinShop may be used at different stages of a protein structure prediction process. First, it can create protein configurations containing secondary structures specified by the user. Second, it can interactively manipulate protein fragments to achieve desired folds by adjusting the dihedral angles of selected coil regions using an Inverse Kinematics method. Last, it serves as a visual framework to monitor and steer a protein structure prediction process that may be running on a remote machine. ProteinShop was used to create initial configurations for a protein structure prediction method developed by a team that competed in CASP5. ProteinShop's use accelerated the process of generating initial configurations, reducing the time required from days to hours. This paper describes the structure of ProteinShop and discusses its main features.     

Acoustofluidics 3: Continuum mechanics for ultrasonic particle manipulation

Although ultrasonic particle manipulation is performed in fluids, the adjacent solid material and its modelling play an important part in the design of such devices, as the impedance difference between liquids and solids is not very large. The modelling of the solid is done in the framework of linear elastodynamics, which is described by the displacement field, the strain field and the stress field. These quantities are related by kinematical relations, local linear momentum conservation and constitutive laws. Also material damping is important and can be modelled in the framework of linear viscoelasticity. Because of the finite size of the devices, resonant modes are important to analyse. In addition some of the elements behave as mechanical structures obeying specific equations that take into account the structural boundaries. A practical example is given, which shows the full complexity of device modelling, which is preferably done using the Finite Element Method.     

Dielectrophoretic manipulation and separation of microparticles using curved microelectrodes

This paper presents the development and experimental analysis of a dielectrophoresis (DEP) system, which is used for the manipulation and separation of microparticles in liquid flow. The system is composed of arrays of microelectrodes integrated to a microchannel. Novel curved microelectrodes are symmetrically placed with respect to the centre of the microchannel with a minimum gap of 40 μm. Computational fluid dynamics method is utilised to characterise the DEP field and predict the dynamics of particles. The performance of the system is assessed with microspheres of 1, 5 and 12 μm diameters. When a high‐frequency potential is applied to microelectrodes a spatially varying electric field is induced in the microchannel, which creates the DEP force. Negative‐DEP behaviour is observed with particles being repelled from the microelectrodes. The particles of different dimensions experience different DEP forces and thus settle to separate equilibrium zones across the microchannel. Experiments demonstrate the capability of the system as a field flow fraction tool for sorting microparticles according to their dimensions and dielectric properties.     

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.     

微流控液滴技术及其应用的研究进展

微液滴具有体积小、比表面积大,速度快、通量高,大小均匀、体系封闭,内部稳定等特性,在药物控释、病毒检测、颗粒材料合成、催化剂等领域中均有重要应用.微流控技术的发展为微液滴生成中实现尺寸规格、结构形貌和功能特性等的可控设计和精确操控提供了全新平台.本文概述了微流控液滴技术的基本原理、液滴生成方式及其基本操控,比较分析了微液滴的传统制备法与微流控合成法的异同,介绍了近年来微流控液滴技术在功能材料合成、生物医学和食品加工等领域中的研究新进展,探讨并展望了微流控液滴技术的潜在价值和未来发展方向.