Dielectrophoretic cell trapping and parallel one-to-one fusion based on field constriction created by a micro-orifice array

Mechanism study on the influences of buffer osmotic pressure on microfluidic chip-based cell electrofusion

Cell electrofusion is a key process in many research fields, such as genetics, immunology, and cross-breeding. The electrofusion efficiency is highly dependent on the buffer osmotic pressure properties. However, the mechanism by which the buffer osmotic pressure affects cell electrofusion has not been theoretically or numerically understood. In order to explore the mechanism, the microfluidic structure with paired arc micro-cavities was first evaluated based on the numerical analysis of the transmembrane potential and the electroporation induced on biological cells when the electrofusion was performed on this structure. Then, the numerical model was used to analyze the effect of three buffer osmotic pressures on the on-chip electrofusion in terms of membrane tension and cell size. Compared to hypertonic and isotonic buffers, hypotonic buffer not only increased the reversible electroporation area in the cell-cell contact zone by 1.7 times by inducing a higher membrane tension, but also significantly reduced the applied voltage required for cell electroporation by increasing the cell size. Finally, the microfluidic chip with arc micro-cavities was fabricated and tested for electrofusion of SP2/0 cells. The results showed that no cell fusion occurred in the hypertonic buffer. The fusion efficiency in the isotonic buffer was about 7%. In the hypotonic buffer, the fusion efficiency was about 60%, which was significantly higher compared to hypertonic and isotonic buffers. The experimental results were in good agreement with the numerical analysis results.

Controllable and Stable Fusion Strategy on Microfluidics

This paper presents a microfluidic device with 200 cell "cage" structures. Based on this microfluidics device, a new simple and stable electrofusion method was developed. Under hydrodynamic force, the cells moved to the desired "cage" cell capture structure and efficiently formed cell pairs (∼80.0 ± 4.6%). Intimate intercellular connectivity was induced by the precise modulation of hypotonic solution substitution and the microstructure, which ensured no cell movement or displacement during the cell electroporation/electrofusion process. It also guaranteed repeated electroporation occurring in the same contact region and provided a stable cell membrane recombination and a cell fusion microenvironment. When the pulse signal was applied, a high fusion efficiency of ∼88.3 ± 0.6% was demonstrated on the microfluidic device.

Advances in hybridoma preparation using electrofusion technology

As a rapidly developing cell engineering technique, cell electrofusion has been increasingly applied in the field of hybridoma preparation in recent years. However, it is difficult to completely replace the polyethylene glycol-mediated cell fusion using electrofusion due to the high operation requirements, high cost of electrofusion instruments, and lack of prior reference research work. The key elements limiting electrofusion in the field of hybridoma preparation also introduce practical complications, such as the use/choice of electrofusion instruments, setup/optimization of electrical parameters, and precise control of cells. This review summarizes the state of the art of cell electrofusion in hybridoma preparation based on recent published literature, mainly focusing on electrofusion instruments and their components, process control and characterization, and cell treatment. It also provides new information and insightful commentary critically important for further electrofusion development in the field of hybridoma preparation.

A single-cell surgery microfluidic device for transplanting tumor cytoplasm into dendritic cells without nuclei mixing

This study aimed to demonstrate the feasibility of generating tumor cell vaccine models by single-cell surgery in a microfluidic device that integrates one-to-one electrofusion, shear flow reseparation, and on-device culture. The device was microfabricated from polydimethylsiloxane (PDMS) and consisted of microorifices (aperture size: ∼3 μm) for one-to-one fusion, and microcages for on-device culture. Using the device, we could achieve one-to-one electrofusion of leukemic plasmacytoid dendritic cells (DC-like cells) and Jurkat cells with a fusion efficiency of ∼ 80%. Fusion via the narrow microorifices allowed DC-like cells to acquire cytoplasmic contents of the Jurkat cells while preventing nuclei mixing. After fusion, the DC-like cells were selectively reseparated from the Jurkat cells by shear flow application to generate tumor nuclei-free antigen-recipient DC-like (tarDC-like) cells. When cultured as single cells on the device, these cells could survive under gentle medium perfusion with a median survival time of 11.5 h, although a few cells could survive longer than 36 h. Overall, this study demonstrates single-cell surgery in a microfluidic device for potential generation of dendritic cell vaccines which are uncontaminated with tumor nucleic materials. We believe that this study will inspire the generation of safer tumor cell vaccines for cancer immunotherapy.

Recent Advances in Microscale Electroporation

Electroporation (EP) is a commonly used strategy to increase cell permeability for intracellular cargo delivery or irreversible cell membrane disruption using electric fields. In recent years, EP performance has been improved by shrinking electrodes and device structures to the microscale. Integration with microfluidics has led to the design of devices performing static EP, where cells are fixed in a defined region, or continuous EP, where cells constantly pass through the device. Each device type performs superior to conventional, macroscale EP devices while providing additional advantages in precision manipulation (static EP) and increased throughput (continuous EP). Microscale EP is gentle on cells and has enabled more sensitive assaying of cells with novel applications. In this Review, we present the physical principles of microscale EP devices and examine design trends in recent years. In addition, we discuss the use of reversible and irreversible EP in the development of therapeutics and analysis of intracellular contents, among other noteworthy applications. This Review aims to inform and encourage scientists and engineers to expand the use of efficient and versatile microscale EP technologies.

Advances and enabling technologies for phase-specific cell cycle synchronisation

Cell cycle synchronisation is the process of isolating cell populations at specific phases of the cell cycle from heterogeneous, asynchronous cell cultures. The process has important implications in targeted gene-editing and drug efficacy of cells and in studying cell cycle events and regulatory mechanisms involved in the cell cycle progression of multiple cell species. Ideally, cell cycle synchrony techniques should be applicable for all cell types, maintain synchrony across multiple cell cycle events, maintain cell viability and be robust against metabolic and physiological perturbations. In this review, we categorize cell cycle synchronisation approaches and discuss their operational principles and performance efficiencies. We highlight the advances and technological development trends from conventional methods to the more recent microfluidics-based systems. Furthermore, we discuss the opportunities and challenges for implementing high throughput cell synchronisation and provide future perspectives on synchronisation platforms, specifically hybrid cell synchrony modalities, to allow the highest level of phase-specific synchrony possible with minimal alterations in diverse types of cell cultures.

Microfluidic Surgery in Single Cells and Multicellular Systems

Microscale surgery on single cells and small organisms has enabled major advances in fundamental biology and in engineering biological systems. Examples of applications range from wound healing and regeneration studies to the generation of hybridoma to produce monoclonal antibodies. Even today, these surgical operations are often performed manually, but they are labor intensive and lack reproducibility. Microfluidics has emerged as a powerful technology to control and manipulate cells and multicellular systems at the micro- and nanoscale with high precision. Here, we review the physical and chemical mechanisms of microscale surgery and the corresponding design principles, applications, and implementations in microfluidic systems. We consider four types of surgical operations: (1) sectioning, which splits a biological entity into multiple parts, (2) ablation, which destroys part of an entity, (3) biopsy, which extracts materials from within a living cell, and (4) fusion, which joins multiple entities into one. For each type of surgery, we summarize the motivating applications and the microfluidic devices developed. Throughout this review, we highlight existing challenges and opportunities. We hope that this review will inspire scientists and engineers to continue to explore and improve microfluidic surgical methods.

A Microfluidic Flip-Chip Combining Hydrodynamic Trapping and Gravitational Sedimentation for Cell Pairing and Fusion

Cancer cell–immune cell hybrids and cancer immunotherapy have attracted much attention in recent years. The design of efficient cell pairing and fusion chips for hybridoma generation has been, subsequently, a subject of great interest. Here, we report a three-layered integrated Microfluidic Flip-Chip (MFC) consisting of a thin through-hole membrane sandwiched between a mirrored array of microfluidic channels and saw-tooth shaped titanium electrodes on the glass. We discuss the design and operation of MFC and show its applicability for cell fusion. The proposed device combines passive hydrodynamic phenomenon and gravitational sedimentation, which allows the transportation and trapping of homotypic and heterotypic cells in large numbers with pairing efficiencies of 75~78% and fusion efficiencies of 73%. Additionally, we also report properties of fused cells from cell biology perspectives, including combined fluorescence-labeled intracellular materials from THP1 and A549, mixed cell morphology, and cell viability. The MFC can be tuned for pairing and fusion of cells with a similar protocol for different cell types. The MFC can be easily disconnected from the test setup for further analysis.

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.

On-site processing of single chromosomal DNA molecules using optically driven microtools on a microfluidic workbench

We developed optically driven microtools for processing single biomolecules using a microfluidic workbench composed of a microfluidic platform that functions under an optical microscope. The optically driven microtools have enzymes immobilized on their surfaces, which catalyze chemical reactions for molecular processing in a confined space. Optical manipulation of the microtools enables them to be integrated with a microfluidic device for controlling the position, orientation, shape of the target sample. Here, we describe the immobilization of enzymes on the surface of microtools, the microfluidics workbench, including its microtool storage and sample positioning functions, and the use of this system for on-site cutting of single chromosomal DNA molecules. We fabricated microtools by UV lithography with SU-8 and selected ozone treatments for immobilizing enzymes. The microfluidic workbench has tool-stock chambers for tool storage and micropillars to trap and extend single chromosomal DNA molecules. The DNA cutting enzymes DNaseI and DNaseII were immobilized on microtools that were manipulated using optical tweezers. The DNaseI tool shows reliable cutting for on-site processing. This pinpoint processing provides an approach for analyzing chromosomal DNA at the single-molecule level. The flexibility of the microtool design allows for processing of various samples, including biomolecules and single cells.

Dielectrophoretic Crossover Frequency of Single Particles: Quantifying the Effect of Surface Functional Groups and Electrohydrodynamic Flow Drag Force

We present a comprehensive comparison of dielectrophoretic (DEP) crossover frequency of single particles determined by various experimental methods and theoretical models under the same conditions, and ensure that discrepancy due to uncertain or inconsistent material properties and electrode design can be minimized. Our experiment shows that sulfate- and carboxyl-functionalized particles have higher crossover frequencies than non-functionalized ones, which is attributed to the electric double layer (EDL). To better understand the formation of the EDL, we performed simulations to study the relationship between initial surface charge density, surface ion adsorption, effective surface conductance, and functional groups of both functionalized and nonfunctionalized particles in media with various conductivities. We also conducted detailed simulations to quantify how much error may be introduced if concurrent electrohydrodynamic forces, such as electrothermal and electro-osmotic forces, are not properly avoided during the crossover frequency measurement.

Sequential Cell-Processing System by Integrating Hydrodynamic Purification and Dielectrophoretic Trapping for Analyses of Suspended Cancer Cells

Microfluidic devices employing dielectrophoresis (DEP) have been widely studied and applied in the manipulation and analysis of single cells. However, several pre-processing steps, such as the preparation of purified target samples and buffer exchanges, are necessary to utilize DEP forces for suspended cell samples. In this paper, a sequential cell-processing device, which is composed of pre-processing modules that employ deterministic lateral displacement (DLD) and a single-cell trapping device employing an electroactive microwell array (EMA), is proposed to perform the medium exchange followed by arraying single cells sequentially using DEP. Two original microfluidic devices were efficiently integrated by using the interconnecting substrate containing rubber gaskets that tightly connect the inlet and outlet of each device. Prostate cancer cells (PC3) suspended in phosphate-buffered saline buffer mixed with microbeads were separated and then resuspended into the DEP buffer in the integrated system. Thereafter, purified PC3 cells were trapped in a microwell array by using the positive DEP force. The achieved separation and trapping efficiencies exceeded 94% and 93%, respectively, when using the integrated processing system. This study demonstrates an integrated microfluidic device by processing suspended cell samples, without the requirement of complex preparation steps.

Nuclear transplantation between allogeneic cells through topological reconnection of plasma membrane in a microfluidic system

Previous studies have demonstrated that somatic cells fused with pluripotent stem cells can be reprogrammed on the basis of reprogramming factors acquired from the latter. However, fusion-reprogrammed cells are deemed unsuitable for therapeutic applications mainly because conventional fusion techniques often yield tetraploid fusants that contain exogenous genes acquired from the fusion partners. Here, we present a novel cell-cell topological reconnection technique and demonstrate its application to nuclear transplantation between a somatic cell and a stem cell without nuclei mixing. As a proof of concept, a microfluidic fusion chip embodied with a microslit (4 μm in width) to prevent nuclei mixing was developed and used to perform one-to-one electrofusion of a target somatic cell (Jurkat cell) with an induced pluripotent stem (iPS) cell. To extract its cytoplasm, the target cell was first topologically connected to a sacrificial iPS cell by electrofusion via a microslit, followed by shear flow removal of the latter to obtain a cytoplasm-depleted nucleus of the target cell. Then, to replace the lost cytoplasm, topological reconnection to a second iPS cell was performed similarly by electrofusion, followed by shear flow separation of the target cell to enable it acquire most of the iPS cytoplasm, but without nuclei mixing. Microscopic observation of target cells harvested and cultured post hoc in a microwell confirmed that they manifested cell division. Taken together, these results demonstrate the potential application of the cell-cell topological reconnection technique to somatic cell nuclear transplantation for the generation of autologous pluripotent stem cells.

On the recent developments of insulator-based dielectrophoresis: A review

Insulator-based dielectrophoresis (iDEP), also known as electrodeless DEP, has become a well-known dielectrophoretic technique, no longer viewed as a new methodology. Significant advances on iDEP have been reported during the last 15 years. This review article aims to summarize some of the most important findings on iDEP organized by the type of dielectrophoretic mode: streaming and trapping iDEP. The former is primarily used for particle sorting, while the latter has great capability for particle enrichment. The characteristics of a wide array of devices are discussed for each type of dielectrophoretic mode in order to present an overview of the distinct designs and applications developed with iDEP. A short section on Joule heating effects and electrothermal flow is also included to highlight some of the challenges in the utilization of iDEP systems. The significant progress on iDEP illustrates its potential for a large number of applications, ranging from bioanalysis to clinical and biomedical assessments. The present article discusses the work on iDEP by numerous research groups around the world, with the aim of proving the reader with an overview of the state-of-the-art in iDEP microfluidic systems.

Elucidating the Mechanisms of Two Unique Phenomena Governed by Particle-Particle Interaction under DEP: Tumbling Motion of Pearl Chains and Alignment of Ellipsoidal Particles

Particle-particle interaction plays a crucial role in determining the movement and alignment of particles under dielectrophoresis (DEP). Previous research efforts focus on studying the mechanism governing the alignment of spherical particles with similar sizes in a static condition. Different approaches have been developed to simulate the alignment process of a given number of particles from several up to thousands depending on the applicability of the approaches. However, restricted by the simplification of electric field distribution and use of identical spherical particles, not much new understanding has been gained apart from the most common phenomenon of pearl chain formation. To enhance the understanding of particle-particle interaction, the movement of pearl chains under DEP in a flow condition was studied and a new type of tumbling motion with unknown mechanism was observed. For interactions among non-spherical particles, some preceding works have been done to simulate the alignment of ellipsoidal particles. Yet the modeling results do not match experimental observations. In this paper, the authors applied the newly developed volumetric polarization and integration (VPI) method to elucidate the underlying mechanism for the newly observed movement of pearl chains under DEP in a flow condition and explain the alignment patterns of ellipsoidal particles. The modeling results show satisfactory agreement with experimental observations, which proves the strength of the VPI method in explaining complicated DEP phenomena.

A planar dielectrophoresis-based chip for high-throughput cell pairing

This paper reports the design and fabrication of a planar chip for high-throughput cell trapping and pairing (more than 2400 single cell-cell pairs in a microwell array) in a 1 × 1.5 cm area by positive dielectrophoresis (p-DEP) within only several minutes. The p-DEP was generated by applying an alternating current signal on a novel two-pair interdigitated array (TPIDA) electrode. The TPIDA electrode not only enabled the planar chip to be incorporated with a most often used PDMS microfluidic channel, but also contributed to a high single cell-cell pairing efficiency up to 74.2% by decreasing the induced electric field during consecutive p-DEP trapping of two cell types. Furthermore, the paired cells in each microwell could be "pushed" together into a microbaffle by a liquid flow through a capillary-sized channel, resulting in single cell-cell contact. More importantly, the planar chip could be used repeatedly by a simple water cleaning process. The planar chip offers an effective way for high-throughput single cell-cell pairing, which could provide a facile platform for cell communication and a precise cell pairing step in cell fusion.

High-Density Microfluidic Particle-Cluster-Array Device for Parallel and Dynamic Study of Interaction between Engineered Particles

A high-density and high-performance microfluidic particle-cluster-array device utilizing a novel hydrodynamically tunable pneumatic valve (HTPV) is reported for parallel and dynamic monitoring of the interactions taking place in particle clusters. The key concept involves passive operation of the HTPV through elastic deformation of a thin membrane using only the hydrodynamic force inherent in microchannel flows. This unique feature allows the discrete and high-density (≈30 HTPVs mm^-2 ) arrangement of numerous HTPVs in a microfluidic channel without any pneumatic connection. In addition, the HTPV achieves high-performance clustering (≈92%) of three different particles in an array format through the optimization of key design and operating parameters. Finally, a contamination-free, parallel, and dynamic biochemical analysis strategy is proposed, which employs a simple one-inlet-one-outlet device operated by the effective combination of several techniques, including particle clustering, the interactions between engineered particles, two-phase partitioning and dehydration control of aqueous plugs, and shape/color-based particle identification.

A microfluidic approach towards hybridoma generation for cancer immunotherapy

Dendritic cells/tumor fusions have shown to elicit anti-cancer immunity in different cancer types. However, the application of these vaccines for human cancer immunotherapy are limited by the instable quality and insufficient quanity of fusion cells. We present a cell electrofusion chip fabricated using soft lithography technique, which combines the rapid and precise cell pairing microstructures and the high yield electrofusion micro-electrodes to improve the cell fusion. The design uses hydrodynamic trapping in combination with positive dielectrophoretic force (pDEP) to achieve cell fusion. The chip consists of total 960 pairs of trapping channels, which are capable of pairing and fusing both homogeneous and heterogeneous types of cells. The fused cells can be easily taken out of the chip that makes this device a distinguishable from other designs. We observe pairing efficiency of 68% with fusion efficiency of 64%.

Adhesion patterning by a novel air-lock technique enables localization and in-situ real-time imaging of reprogramming events in one-to-one electrofused hybrids

Although fusion of somatic cells with embryonic stem (ES) cells has been shown to induce reprogramming, single-cell level details of the transitory phenotypic changes that occur during fusion-based reprogramming are still lacking. Our group previously reported on the technique of one-to-one electrofusion via micro-slits in a microfluidic platform. In this study, we focused on developing a novel air-lock patterning technique for creating localized adhesion zones around the micro-slits for cell localization and real-time imaging of post fusion events with a single-cell resolution. Mouse embryonic fibroblasts (MEF) were fused individually with mouse ES cells using a polydimethylsiloxane (PDMS) fusion chip consisting of two feeder channels with a separating wall containing an array of micro-slits (slit width ∼3 μm) at a regular spacing. ES cells and MEFs were introduced separately into the channels, juxtaposed on the micro-slits by dielectrophoresis and fused one-to-one by a pulse voltage. To localize fused cells for on-chip culture and time-lapse microscopy, we implemented a two-step approach of air-lock bovine serum albumin patterning and Matrigel coating to create localized adhesion areas around the micro-slits. As a result of time-lapse imaging, we could determine that cell division occurs within 24 h after fusion, much earlier than the 2-3 days reported by earlier studies. Remarkably, Oct4-GFP (Green Fluorescent Protein) was confirmed after 25 h of fusion and thereafter stably expressed by daughter cells of fused cells. Thus, integrated into our high-yield electrofusion platform, the technique of air-lock assisted adhesion patterning enables a single-cell level tracking of fused cells to highlight cell-level dynamics during fusion-based reprogramming.

Optically-Induced Cell Fusion on Cell Pairing Microstructures

Cell fusion is a critical operation for numerous biomedical applications including cell reprogramming, hybridoma formation, cancer immunotherapy, and tissue regeneration. However, unstable cell contact and random cell pairings have limited efficiency and yields when utilizing traditional methods. Furthermore, it is challenging to selectively perform cell fusion within a group of cells. This study reports a new approach called optically-induced cell fusion (OICF), which integrates cell-pairing microstructures with an optically-induced, localized electrical field. By projecting light patterns onto a photoconductive film (hydrogen-rich, amorphous silicon) coated on an indium-tin-oxide (ITO) glass while an alternating current electrical field was applied between two such ITO glass slides, “virtual” electrodes could be generated that could selectively fuse pairing cells. At 10 kHz, a 57% cell paring rate and an 87% fusion efficiency were successfully achieved at a driving voltage of 20  V_pp, suggesting that this new technology could be promising for selective cell fusion within a group of cells.

One-to-one encapsulation based on alternating droplet generation

This paper reports the preparation of encapsulated particles as models of cells using an alternating droplet generation encapsulation method in which the number of particles in a droplet is controlled by a microchannel to achieve one-to-one encapsulation. Using a microchannel in which wettability is treated locally, the fluorescent particles used as models of cells were successfully encapsulated in uniform water-in-oil-in-water (W/O/W) emulsion droplets. Furthermore, 20% of the particle-containing droplets contained one particle. Additionally, when a surfactant with the appropriate properties was used, the fluorescent particles within each inner aqueous droplet were enclosed in the merged droplet by spontaneous droplet coalescence. This one-to-one encapsulation method based on alternating droplet generation could be used for a variety of applications, such as high-throughput single-cell assays, gene transfection into cells or one-to-one cell fusion.

A Cell Electrofusion Chip for Somatic Cells Reprogramming

Cell fusion is a potent approach to explore the mechanisms of somatic cells reprogramming. However, previous fusion methods, such as polyethylene glycol (PEG) mediated cell fusion, are often limited by poor fusion yields. In this study, we developed a simplified cell electrofusion chip, which was based on a micro-cavity/ discrete microelectrode structure to improve the fusion efficiency and to reduce multi-cell electrofusion. Using this chip, we could efficiently fuse NIH3T3 cells and mouse embryonic stem cells (mESCs) to induce somatic cells reprogramming. We also found that fused cells demethylated gradually and 5-hydroxymethylcytosine (5hmC) was involved in the demethylation during the reprogramming. Thus, the cell electrofusion chip would facilitate reprogramming mechanisms research by improving efficiency of cell fusion and reducing workloads.

Effects of ROCK inhibitor Y-27632 on cell fusion through a microslit

We previously reported a direct cytoplasmic transfer method using a microfluidic device, in which cell fusion was induced through a microslit (slit-through-fusion) by the Sendai virus envelope (HVJ-E) to prevent nuclear mixing. However, the method was impractical due to low efficiency of slit-through-fusion formation and insufficient prevention of nuclear mixing. The purpose of this study was to establish an efficient method for inducing slit-through-fusion without nuclear mixing. We hypothesized that modulation of cytoskeletal component can decrease nuclear migration through the microslit considering its functions. Here we report that supplementation with Y-27632, a specific ROCK inhibitor, significantly enhances cell fusion induction and prevention of nuclear mixing. Supplementation with Y-27632 increased the formation of slit-through-fusion efficiency by more than twofold. Disruption of F-actin by Y-27632 prevented nuclear migration between fused cells through the microslit. These two effects of Y-27632 led to promotion of the slit-through-fusion without nuclear mixing with a 16.5-fold higher frequency compared to our previous method (i.e., cell fusion induction by HVJ-E without supplementation with Y-27632). We also confirmed that mitochondria were successfully transferred to the fusion partner under conditions of Y-27632 supplementation. These findings demonstrate the practicality of our cell fusion system in producing direct cytoplasmic transfer between live cells.

Microfluidic systems for cell pairing and fusion

Cell fusion has become a routine laboratory technique for generating hybrids with diverse genetic and epigenetic properties, and has been used for many different applications. Here, we describe a microfluidics based cell pairing and fusion method that affords controllable formation of cell pairs and high efficiency fusion. The microfluidic device uses passive hydrodynamics and multistep cell loading procedure to immobilize and pair thousands of cells in a dense array of weir-based traps. Once paired, cells can be fused either using chemical or electrical fusion protocols, and provide twofold to tenfold improvement in fusion yields in comparison to commercial systems. The hybrids can be harvested from the device for culture and further studies.

Distinctive translational and self-rotational motion of lymphoma cells in an optically induced non-rotational alternating current electric field

In this paper, the translational motion and self-rotational behaviors of the Raji cells, a type of B-cell lymphoma cell, in an optically induced, non-rotational, electric field have been characterized by utilizing a digitally programmable and optically activated microfluidics chip with the assistance of an externally applied AC bias potential. The crossover frequency spectrum of the Raji cells was studied by observing the different linear translation responses of these cells to the positive and negative optically induced dielectrophoresis force generated by a projected light pattern. This digitally projected spot served as the virtual electrode to generate an axisymmetric and non-uniform electric field. Then, the membrane capacitance of the Raji cells could be directly measured. Furthermore, Raji cells under this condition also exhibited a self-rotation behavior. The repeatable and controlled self-rotation speeds of the Raji cells to the externally applied frequency and voltage were systematically investigated and characterized via computer-vision algorithms. The self-rotational speed of the Raji cells reached a maximum value at 60 kHz and demonstrated a quadratic relationship with respect to the applied voltage. Furthermore, optically projected patterns of four orthogonal electrodes were also employed as the virtual electrodes to manipulate the Raji cells. These results demonstrated that Raji cells located at the center of the four electrode pattern could not be self-rotated. Instead any Raji cells that deviated from this center area would also self-rotate. Most importantly, the Raji cells did not exhibit the self-rotational behavior after translating and rotating with respect to the center of any two adjacent electrodes. The spatial distributions of the electric field generated by the optically projected spot and the pattern of four electrodes were also modeled using a finite element numerical simulation. These simulations validated that the electric field distributions were non-uniform and non-rotational. Hence, the non-uniform electric field must play a key role in the self-rotation of the Raji cells. As a whole, this study elucidates an optoelectric-coupled microfluidics-based mechanism for cellular translation and self-rotation that can be used to extract the dielectric properties of the cells without using conventional metal-based microelectrodes. This technique may provide a simpler method for label-free identification of cancerous cells with many associated clinical applications.

Deformability-based microfluidic cell pairing and fusion

We present a microfluidic cell pairing device capable of sequential trapping and pairing of hundreds of cells using passive hydrodynamics and flow-induced deformation. We describe the design and operation principles of our device and show its applicability for cell fusion. Using our device, we achieved both homotypic and heterotypic cell pairing, demonstrating efficiencies up to 80%. The platform is compatible with fusion protocols based on biological, chemical and physical stimuli with fusion yields up to 95%. Our device further permits its disconnection from the fluidic hardware enabling its transportation for imaging and culture while maintaining cell registration on chip. Our design principles and cell trapping technique can readily be applied for different cell types and can be extended to trap and fuse multiple (>2) cell partners as demonstrated by our preliminary experiments.

Cell pairing using microwell array electrodes based on dielectrophoresis

We report a simple device with an array of 10,000 (100 × 100) microwells for producing vertical pairs of cells in individual microwells with a rapid manipulation based on positive dielectrophoresis (p-DEP). The areas encircled with micropoles which fabricated from an electrical insulating photosensitive polymer were used as microwells. The width (14 μm) and depth (25 μm) of the individual microwells restricted the size to two vertically aligned cells. The DEP device for the manipulation of cells consisted of a microfluidic channel with an upper indium tin oxide (ITO) electrode and a lower microwell array electrode fabricated on an ITO substrate. Mouse myeloma cells stained in green were trapped within 1 s in the microwells by p-DEP by applying an alternating current voltage between the upper ITO and the lower microwell array electrode. The cells were retained inside the wells even after switching off the voltage and washing with a fluidic flow. Other myeloma cells stained in blue were then trapped in the microwells occupied by the cells stained in green to form the vertical cell pairing in the microwells. Cells stained in different colors were paired within only 1 min and a pairing efficiency of over 50% was achieved.

Electroporation and electrofusion in field-tailored microstructures

Bringing foreign substances into cells is a basic process in cell engineering. So called reversible breakdown of the cell membrane by electrical pulses opens up transient pores on the membrane through which molecules can diffuse into, or contacting cells can fuse. Using micro-fabricated structures with the dimension smaller than that of cells, the field pattern can be designed and tailored, that enables the handling of single cells, as well as the control over the location and the magnitude of the membrane voltage to achieve low invasive high-yield poration or fusion. This paper reviews some of our research, including gene transfection, electrofusion and cytoplasmic transplant.

Probing cell-cell communication with microfluidic devices

Intercellular communication is a mechanism that regulates critical events during embryogenesis and coordinates signalling within differentiated tissues, such as the nervous and cardiovascular systems. To perform specialized activities, these tissues utilize the rapid exchange of signals among networks that, while are composed of different cell types, are nevertheless functionally coupled. Errors in cellular communication can lead to varied deleterious effects such as degenerative and autoimmune diseases. However, the intercellular communication network is extremely complex in multicellular organisms making isolation of the functional unit and study of basic mechanisms technically challenging. New experimental methods to examine mechanisms of intercellular communication among cultured cells could provide insight into physiological and pathological processes alike. Recent developments in microfluidic technology allow miniaturized and integrated devices to perform intercellular communication experiments on-chip. Microfluidics have many advantages, including the ability to replicate in vitro the chemical, mechanical, and physical cellular microenvironment of tissues with precise spatial and temporal control combined with dynamic characterization, high throughput, scalability and reproducibility. In this Focus article, we highlight some of the recent work and advances in the application of microfluidics to the study of mammalian intercellular communication with particular emphasis on cell contact and soluble factor mediated communication. In addition, we provide some insights into likely direction of the future developments in this field.

A cell electrofusion microfluidic chip using discrete coplanar vertical sidewall microelectrodes

A novel cell electrofusion microfluidic chip using discrete coplanar vertical sidewall electrodes has been designed, fabricated, and tested. The device contains a serpentine-shaped microchannel with 22 500 pairs of vertical sidewall microelectrodes patterned on two opposing vertical sidewalls of the microchannel. The adjacent microelectrodes on each sidewall are separated by coplanar SiO(2) -Polysilicon-SiO(2) /silicon. This design of coplanar discrete vertical sidewall electrodes eliminates the "dead area" present in previous designs using continuous three-dimensional (3D) protruding sidewall electrodes, and generates uniform electric field along the height of the microchannel, leading to a lower voltage required for cell fusion compared to designs using 2D thin-film electrodes. This device is tested to fuse NIH3T3 cells under a low voltage (∼9 V). Almost 100% cells are aligned to the edge of the discrete microelectrodes, and cell-cell pairing efficiency reaches 70%. The electrofusion efficiency is above 40% of the total cells loaded into the device, which is much higher than traditional fusion methods and existing microfluidic devices using continuous 3D protruding sidewall microelectrodes.

Circulating tumors cells as biomarkers: progress toward biomarker qualification

Personalized cancer medicine requires the development of tumor-specific biomarkers to optimize selection of targeted therapies and to better assess response to therapy. Current efforts in several tumor types have shown that patients in whom circulating tumor cells (CTCs) are detected have an inferior prognosis relative to those in whom CTCs are not detected and that the elimination or decrease of CTCs following treatment is associated with improved clinical outcomes. Technological advances in the detection, isolation, capture, and characterization of CTCs from phlebotomy samples obtained in a routine clinical practice setting have enabled the evaluation of different CTC biomarkers. Unmet needs in cancer diagnosis and treatment where CTC biomarkers have been studied include determining prognosis, assessing the effects of treatment, and as a source of tumor for the biologic identification and characterization of determinants to predict sensitivity to one form of treatment versus another and to understand mechanisms of treatment resistance.At present, there is no single definition of a CTC and no single CTC "biomarker." Rather, multiple assays (tests) are in development for CTC biomarkers. However, before the role of any biomarker in medical decision making can be determined, it is essential that the assays used to measure the biomarker are analytically validated in a sequence of trials to generate the evidence to support the biomarker's use in the given context of use. It is against this background that this review focuses on the process of developing CTC biomarker assays, with the objective of outlining the necessary steps to qualify specific CTC tests for medical decision making in clinical practice or drug development. The potential for point-of-care tests is clear.

Highly controlled electrofusion of individually selected cells in dielectrophoretic field cages

The prospect of novel therapeutic approaches has renewed the current interest in the fusion of rare cells, like stem cells or primary immune cells. While conventional techniques are only capable of mass fusion, lab-on-a-chip systems often still lack an acceptable method for making the cells available after processing. Here, we present a microfluidic approach for electrofusion on the single-cell level that offers high control over the cells both before and after fusion. For cell pairing and fusion, we employed dielectrophoresis and AC voltage pulses, respectively. Each cell has been characterized and selected before they were paired, fused and released from the fluidic system for subsequent analysis and cultivation. The successful experimental evaluation of our system was further corroborated by numerical simulations. We obtained fusion efficiencies of more than 30% for individual pairs of mouse myeloma and B cell blasts and showed the proliferating ability of the hybrid cells 3 d after fusion. Since aggregates of more than two cells can be fused, the technique could also be developed further for generating giant cells for low-noise electrophysiology in the context of semi-automated pharmaceutical screening procedures.

A high-throughput dielectrophoresis-based cell electrofusion microfluidic device

A high-throughput cell electrofusion microfluidic chip has been designed, fabricated on a silicon-on-insulator wafer and tested for in vitro cell fusion under a low applied voltage. The developed chip consists of six individual straight microchannels with a 40-μm thickness conductive highly doped Si layer as the microchannel wall. In each microchannel, there are 75 pairs of counter protruding microelectrodes, between which the cell electrofusion is performed. The entire highly doped Si layer is covered by a 2-μm thickness aluminum film to maintain a consistent electric field between different protruding microelectrode pairs. A 150-nm thickness SiO₂ film is subsequently deposited on the top face of each protruding microelectrode for better biocompatibility. Owing to the short distance between two counter protruding microelectrodes, a high electric field can be generated for cell electrofusion with a low voltage imposed across the electrodes. Both mammalian cells and plant protoplasts were used to test the cell electrofusion. About 42-68% cells were aligned to form cell-cell pairs by the dielectrophoretic force. After cell alignment, cell pairs were fused to form hybrid cells under the control of cell electroporation and electrofusion signals. The averaged fusion efficiency in the paired cells is above 40% (the highest was about 60%), which is much higher than the traditional polyethylene glycol method (<5%) and traditional electrofusion methods (∼12%). An individual cell electrofusion process could be completed within 10 min, indicating a capability of high throughput.

Cellular dielectrophoresis: applications to the characterization, manipulation, separation and patterning of cells

Over the past decade, dielectrophoresis (DEP) has evolved into a powerful, robust and flexible method for cellular characterization, manipulation, separation and cell patterning. It is a field with widely varying disciplines, as it is quite common to see DEP integrated with a host of applications including microfluidics, impedance spectroscopy, tissue engineering, real-time PCR, immunoassays, stem-cell characterization, gene transfection and electroporation, just to name a few. The field is finally at the point where analytical and numerical polarization models can be used to adequately describe and characterize the dielectrophoretic behavior of cells, and there is ever increasing evidence demonstrating that electric fields can safely be used to manipulate cells without harm. As such, DEP is slowly making its way into the biological sciences. Today, DEP is being used to manipulate individual cells to specific regions of space for single-cell assays. DEP is able to separate rare cells from a heterogeneous cell suspension, where isolated cells can then be characterized and dynamically studied using nothing more than electric fields. However, there is need for a critical report to integrate the many new features of DEP for cellular applications. Here, a review of the basic theory and current applications of DEP, specifically for cells, is presented.

Direct current insulator-based dielectrophoretic characterization of erythrocytes: ABO-Rh human blood typing

A microfluidic platform developed for quantifying the dependence of erythrocyte (red blood cell, RBC) responses by ABO-Rh blood type via direct current insulator dielectrophoresis (DC-iDEP) is presented. The PDMS DC-iDEP device utilized a 400 x 170 μm² rectangular insulating obstacle embedded in a 1.46-cm long, 200-μm wide inlet channel to create spatial non-uniformities in direct current (DC) electric field density realized by separation into four outlet channels. The DC-iDEP flow behaviors were investigated for all eight blood types (A+, A-, B+, B-, AB+, AB-, O+, O-) in the human ABO-Rh blood typing system. Three independent donors of each blood type, same donor reproducibility, different conductivity buffers (0.52-9.1 mS/cm), and DC electric fields (17.1-68.5 V/cm) were tested to investigate separation dependencies. The data analysis was conducted from image intensity profiles across inlet and outlet channels in the device. Individual channel fractions suggest that the dielectrophoretic force experienced by the cells is dependent on erythrocyte antigen expression. Two different statistical analysis methods were conducted to determine how distinguishable a single blood type was from the others. Results indicate that channel fraction distributions differ by ABO-Rh blood types suggesting that antigens present on the erythrocyte membrane polarize differently in DC-iDEP fields. Under optimized conductivity and field conditions, certain blind blood samples could be sorted with low misclassification rates.

Preface to special topic: dielectrophoresis

This Special Topic section is on dielectrophoresis, a growing area of widespread interest and relevance to the microfluidics and nanofluidics community.