A cell electrofusion microfluidic device integrated with 3D thin-film microelectrode arrays

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.

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.

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.

Application of Microfluidic Technology in Antibody Screening

Specific antibodies are widely used in the biomedical field. Current screening methods for specific antibodies mainly involve hybridoma technology and antibody engineering techniques. However, these technologies suffer from tedious screening processes, long preparation periods, high costs, low efficiency, and a degree of automation, which have become a bottleneck for the screening of specific antibodies. To overcome these difficulties, microfluidics has been developed as a promising technology for high-throughput screening and high purity of antibody. In this review, we provide an overview of the recent advances in microfluidic applications for specific antibody screening. In particular, hybridoma technology and four antibody engineering techniques (including phage display, single B cell antibody screening, antibody expression, and cell-free protein synthesis) based on microfluidics have been introduced, challenges, and the future outlook of these technologies are also discussed. This article is protected by copyright. All rights reserved.

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.

High-throughput deterministic pairing and coculturing of single cells in a microwell array using combined hydrodynamic and recirculation flow captures

Single-cell level coculture facilitates the study of cellular interactions for uncovering unknown physiological mechanisms, which are crucial for the development of new therapies for diseases. However, efficient approaches for high-throughput deterministic pairing of single cells and traceable coculture remain lacking. In this study, we report a new microfluidic device, which combines hydrodynamic and recirculation flow captures, to achieve high-throughput and deterministic pairing of single cells in a microwell array for traceable coculture. Compared with the existing techniques, the developed device exhibits advantages with regard to pairing efficiency, throughput, determinacy, and traceability. Through repeating a two-step method, which sequentially captures single cells in a meandering channel and a microwell array, cell number and type can be easily controlled. Double and triple single-cell pairings have been demonstrated with an efficiency of 72.2% and 38.0%, respectively. Cellular engulfment using two breast cell lines is investigated on a developed microfluidic chip as a biological case study, in which the morphological characteristics and the incidence rate are analyzed. This research provides an efficient and reliable alternative for the coculture of single cells on the microfluidic platform for various biomedical applications, such as studying cellular engulfment and tumor sphere formation under single-cell pairing condition.

Dielectrophoretic Separation of Particles Using Microfluidic Chip with Composite Three-Dimensional Electrode

Integrating three-dimensional (3D) microelectrodes on microfluidic chips based on polydimethylsiloxane (PDMS) has been a challenge. This paper introduces a composite 3D electrode composed of Ag powder (particle size of 10 nm) and PDMS. Ethyl acetate is added as an auxiliary dispersant during the compounding process. A micromachining technique for processing 3D microelectrodes of any shape and size was developed to allow the electrodes to be firmly bonded to the PDMS chip. Through theoretical calculations, numerical simulations, and experimental verification, the role of the composite 3D microelectrodes in separating polystyrene particles of three different sizes via dielectrophoresis was systematically studied. This microfluidic device separated 20-, 10-, and 5-μm polystyrene particles nondestructively, efficiently, and accurately.

Microfluidic Irreversible Electroporation-A Versatile Tool to Extract Intracellular Contents of Bacteria and Yeast

Exploring the dynamic behavior of cellular metabolism requires a standard laboratory method that guarantees rapid sampling and extraction of the cellular content. We propose a versatile sampling technique applicable to cells with different cell wall and cell membrane properties. The technique is based on irreversible electroporation with simultaneous quenching and extraction by using a microfluidic device. By application of electric pulses in the millisecond range, permanent lethal pores are formed in the cell membrane of Escherichia coli and Saccharomyces cerevisiae, facilitating the release of the cellular contents; here demonstrated by the measurement of glucose-6-phosphate and the activity of the enzyme glucose-6-phosphate dehydrogenase. The successful application of this device was demonstrated by pulsed electric field treatment in a flow-through configuration of the microfluidic chip in combination with sampling, inactivation, and extraction of the intracellular content in a few seconds. Minimum electric field strengths of 10 kV/cm for E. coli and 7.5 kV/cm for yeast S. cerevisiae were required for successful cell lysis. The results are discussed in the context of applications in industrial biotechnology, where metabolomics analyses are important.

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.

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.

Dielectrophoresis study of temporal change in internal conductivity of single CHO cells after electroporation by pulsed electric fields

Applying sufficiently strong pulsed electric fields to a cell can permeabilize the membrane and subsequently affect its dielectric properties. In this study, we employ a microfluidic dielectrophoresis cytometry technique to simultaneously electroporate and measure the time-dependent dielectric response of single Chinese hamster ovary cells. Using experimental measurements along with numerical simulations, we present quantitative results for the changes in the cytoplasm conductivity of single cells within seconds after exposure to 100 μs duration pulsed electric fields with various intensities. It is shown that, for electroporation in a medium with conductivity lower than that of the cell's cytoplasm, the internal conductivity of the cell decreases after the electroporation on a time scale of seconds and stronger pulses cause a larger and more rapid decrease. We also observe that, after the electroporation, the cell's internal conductivity is constrained to a threshold. This implies that the cell prevents some of the ions in its cytoplasm from diffusing through the created pores to the external medium. The temporal change in the dielectric response of each individual cell is continuously monitored over minutes after exposure to pulsed electric fields. A time constant associated with the cell's internal conductivity change is observed, which ranges from seconds to tens of seconds depending on the applied pulse intensity. This experimental observation supports the results of numerical models reported in the literature.

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%.

Application of Vertical Electrodes in Microfluidic Channels for Impedance Analysis

This paper presents a microfluidic device with electroplated vertical electrodes in the side walls for impedance measurement. Based on the proposed device, the impedance of NaCl solutions with different concentrations and polystyrene microspheres with different sizes was measured and analyzed. The electroplating and SU-8-PDMS (SU-8-poly(dimethylsiloxane)) bonding technologies were firstly integrated for the fabrication of the proposed microfluidic device, resulting in a tightly three-dimensional structure for practical application. The magnitude of impedance of the tested solutions in the frequency range of 1 Hz to 100 kHz was analyzed by the Zennium electrochemical workstation. The results show that the newly designed microfluidic device has potential for impedance analysis with the advantages of ease of fabrication and the integration of 3D electrodes in the side walls. The newly designed impedance sensor can distinguish different concentrations of polystyrene microspheres and may have potential for cell counting in biological areas. By integrating with other techniques such as dielectrophoresis (DEP) and biological recognition technology, the proposed device may have potential for the assay to identify foodborne pathogen bacteria.

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.

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.

Dielectrophoresis for bioparticle manipulation

As an ideal method to manipulate biological particles, the dielectrophoresis (DEP) technique has been widely used in clinical diagnosis, disease treatment, drug development, immunoassays, cell sorting, etc. This review summarizes the research in the field of bioparticle manipulation based on DEP techniques. Firstly, the basic principle of DEP and its classical theories are introduced in brief; Secondly, a detailed introduction on the DEP technique used for bioparticle manipulation is presented, in which the applications are classified into five fields: capturing bioparticles to specific regions, focusing bioparticles in the sample, characterizing biomolecular interaction and detecting microorganism, pairing cells for electrofusion and separating different kinds of bioparticles; Thirdly, the effect of DEP on bioparticle viability is analyzed; Finally, the DEP techniques are summarized and future trends in bioparticle manipulation are suggested.

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.

Electroformation and electrofusion of giant vesicles in a microfluidic device

Electroformation and electrofusion of giant vesicles with diameters of 10-20μm have been performed in a microfluidic device with high-density microelectrodes forming the sidewalls of the microchannel. Electroformation of giant vesicles by a solution mixture of phosphatidylcholine (PC) and cholesterol (Chol) with different concentrations under AC electric field was investigated. Under the conditions of 0.5-12mg/mL PC and 0.1-2.4mg/mL Chol, vesicles were electroformed by the AC electric field imposed. About 60% electroformed vesicles were giant (unilamellar) vesicles with diameters 10-20μm. The eletroformed vesicles were collected from the chip, re-suspended in fresh buffer, and then separated by centrifugation to segregate the ones with desired diameters (10-20μm). Electrofusion of the giant vesicles was conducted in the same chip. Vesicles were aligned to form pairs under AC electric field due to positive dielectrophoresis, and the paired vesicles were subsequently fused upon the application of high strength electrical pulses. The alignment and fusion efficiencies were, respectively, about 50% and 20%.

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.