A multi-channel electroporation microchip for gene transfection in mammalian cells

Bioelectronic tools for understanding the universal language of electrical signaling across species and kingdoms

Modern bioelectronic tools are rapidly advancing to detect electric potentials within networks of electrogenic cells, such as cardiomyocytes, neurons, and pancreatic beta cells. However, it is becoming evident that electrical signaling is not limited to the animal kingdom but may be a universal form of cell-cell communication. In this review, we discuss the existing evidence of, and tools used to collect, subcellular, single-cell and network-level electrical signals across kingdoms, including bacteria, plants, fungi, and even viruses. We discuss how cellular networks employ altered electrical "circuitry" and intercellular mechanisms across kingdoms, and we assess the functionality and scalability of cutting-edge nanobioelectronics to collect electrical signatures regardless of cell size, shape, or function. Researchers today aim to design micro- and nano-topographic structures which harness mechanosensitive membrane and cytoskeletal pathways that enable tight electrical coupling to subcellular compartments within high-throughput recording systems. Finally, we identify gaps in current knowledge of inter-species and inter-kingdom electrical signaling and propose critical milestones needed to create a central theory of electrical signaling across kingdoms. Our discussion demonstrates the need for high resolution, high throughput tools which can probe multiple, diverse cell types at once in their native or experimentally-modeled environments. These advancements will not only reveal the underlying biophysical laws governing the universal language of electrical communication, but can enable bidirectional electrical communication and manipulation of biological systems.

Nanoporous electroporation needle for localized intracellular delivery in deep tissues

The exogenous control of intracellular drug delivery has been shown to improve the overall efficacy of therapies by reducing nonspecific off-target toxicity. However, achieving a precise on-demand dosage of a drug in deep tissues with minimal damage is still a challenge. In this study, we report an electric-pulse-driven nanopore-electroporation (nEP) system for the localized intracellular delivery of a model agent in deep tissues. Compared with conventional bulk electroporation, in vitro nEP achieved better transfection efficiency (>60%) with a high cell recovery rate (>95%) under a nontoxic low electroporation condition (40 V). Furthermore, in vivo nEP using a nanopore needle electrode with a side drug-releasing compartment offered better control over the dosage release, time, and location of propidium iodide, which was used as a model agent for intracellular delivery. In a pilot study using experimental animals, the nEP system exhibited two times higher transfection efficiency of propidium iodide in the thigh muscle tissue, while minimizing tissue damage (<20%) compared to that of bulk electroporation. This tissue-penetrating nEP platform can provide localized, safe, and effective intracellular delivery of diverse therapeutics into deep tissues in a controlled manner.

Microarray Chip and Method for Simultaneous and Highly Consistent Electroporation of Multiple Cells of Different Sizes

Cell electroporation is an important cell manipulation technology to artificially transfer specific extracellular components into cells. However, the consistency of substance transport during the electroporation process is still an issue due to the wide size distribution of the natural cells. In this study, a cell electroporation microfluidic chip based on a microtrap array is proposed. The microtrap structure was optimized for single-cell capture and electric field focusing. The effects of the cell size on the cell electroporation in the microchip were investigated through simulation and experiment methods using the giant unilamellar vesicle as the simplified cell model, and a numerical model of a uniform electric field was used as a comparison. Compared with the uniform electric field, a lower threshold electric field is required to induce electroporation and produces a higher transmembrane voltage on the cell under a specific electric field in the microchip, showing an improvement in cell viability and electroporation efficiency. The larger perforated area produced on the cells in the microchip under a specific electric field allows a higher substance transfer efficiency, and the electroporation results are less affected by the cell size, which is beneficial for improving substance transfer consistency. Furthermore, the relative perforation area increases with the decrease of the cell diameter in the microchip, which is exactly opposite to that in a uniform electric field. By manipulating the electric field applied to the microtrap individually, a consistent proportion of substance transfer during electroporation of cells with different sizes can be achieved.

Integrated Cardiomyocyte-Based Biosensing Platform for Electroporation-Triggered Intracellular Recording in Parallel with Delivery Efficiency Evaluation

Electroporation is a proven technique that can record action potential of cardiomyocytes and serve for biomolecular delivery. To ensure high cell viability, micro-nanodevices cooperating with low-voltage electroporation are frequently utilized in research, and the effectiveness of delivery for intracellular access is typically assessed using an optical imaging approach like flow cytometry. However, the efficiency of in situ biomedical studies is hampered by the intricacy of these analytical approaches. Here, we develop an integrated cardiomyocyte-based biosensing platform to effectively record action potential and evaluate the electroporation quality in terms of viability, delivery efficiency, and mortality. The ITO-MEA device of the platform possesses sensing/stimulating electrodes which combines with the self-developed system to achieve intracellular action potential recording and delivery by electroporation trigger. Moreover, the image acquisition processing system analyzes various parameters effectively to assess delivery performance. Therefore, this platform has the potential for drug delivery therapy and pathology research for cardiology.

Effect of Experimental Electrical and Biological Parameters on Gene Transfer by Electroporation: A Systematic Review and Meta-Analysis

The exact mechanisms of nucleic acid (NA) delivery with gene electrotransfer (GET) are still unknown, which represents a limitation for its broader use. Further, not knowing the effects that different experimental electrical and biological parameters have on GET additionally hinders GET optimization, resulting in the majority of research being performed using a trial-and-error approach. To explore the current state of knowledge, we conducted a systematic literature review of GET papers in in vitro conditions and performed meta-analyses of the reported GET efficiency. For now, there is no universal GET strategy that would be appropriate for all experimental aims. Apart from the availability of the required electroporation device and electrodes, the choice of an optimal GET approach depends on parameters such as the electroporation medium; type and origin of cells; and the size, concentration, promoter, and type of the NA to be transfected. Equally important are appropriate controls and the measurement or evaluation of the output pulses to allow a fair and unbiased evaluation of the experimental results. Since many experimental electrical and biological parameters can affect GET, it is important that all used parameters are adequately reported to enable the comparison of results, as well as potentially faster and more efficient experiment planning and optimization.

Laser Machined Fiber-based Microprobe: Application in Microscale Electroporation

Microscale electroporation devices are mostly restricted to in vitro experiments (i.e., microchannel and microcapillary). Novel fiber-based microprobes can enable in vivo microscale electroporation and arbitrarily select the cell groups of interest to electroporate. We developed a flexible, fiber-based microscale electroporation device through a thermal drawing process and femtosecond laser micromachining techniques. The fiber consists of four copper electrodes (80 μm), one microfluidic channel (30 μm), and has an overall diameter of 400 μm. The dimensions of the exposed electrodes and channel were customizable through a delicate femtosecond laser setup. The feasibility of the fiber probe was validated through numerical simulations and in vitro experiments. Successful reversible and irreversible microscale electroporation was observed in a 3D collagen scaffold (seeded with U251 human glioma cells) using fluorescent staining. The ablation regions were estimated by performing the covariance error ellipse method and compared with the numerical simulations. The computational and experimental results of the working fiber-based microprobe suggest the feasibility of in vivo microscale electroporation in space-sensitive areas, such as the deep brain.

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.

Accurate and efficient intracellular delivery biosensing system by nanostrawed electroporation array

Electroporation serves as a powerful technique to introduce the exogenous nucleotides, DNA, RNA, proteins, dyes, and virus particles into cells. Through the effect of high intensity of electric field, the permeability of the cell membrane is instantaneously improved to absorb the exogenous molecules in surrounding medium. To protect the cell viability, ultralow-voltage electroporation techniques are well developed by versatile devices, and delivery efficiency is commonly assessed by label-based analysis by microscope-ImageJ or flow cytometry. However, accuracy and complexity of these analytical strategies still hinder efficient and precise biomedical studies in situ. Here we developed an intracellular delivery biosensing system by nanostrawed electroporation array (NEA) that can efficiently assess the universal electroporation performance by cell viability, delivery efficiency, and cell mortality. The intracellular delivery biosensing system consists of NEA, fluorescent microscope, and automated analysis software. Intracellular delivery biosensing system of electroporation quality is based on the enhanced fluorescent watershed segmentation (enhanced FWS) algorithm, which possessed low deviation (~5%) and significantly shortened the operation time (~8 s/10 images) in contrast to high deviation (~13%) and long operation time (~10 min/10 images) of conventional inaccurate, labor-intensive and time-consuming methods. It is believed that the intracellular delivery biosensing system will be a promising universal platform to assess nanodevice electroporation in the fields of cell biology, biotechnology, and medicine.

Robust three-dimensional nanotube-in-micropillar array electrodes to facilitate size independent electroporation in blood cell therapy

Blood is an attractive carrier for plasmid and RNA-based medicine in cell therapy. Electroporation serves as a favorable delivery tool for simple operation, quick internalization, minimum cell culture involvement, and low contamination risk. However, the delivery outcome of electroporation heavily depends on the treated cells such as their type, size, and orientation to the electric field, not ideal for highly heterogeneous blood samples. Herein, a new electroporation system was developed towards effective transfection to cells in blood regardless of their large diversity. By coupling replica molding and infiltration-coating processes, we successfully configured a three-dimensional electrode comprised of a polymer micropillar array on which carbon nanotubes (CNTs) are partially embedded. During electroporation, cells sag between micropillars and deform to form a conformal contact with their top and side surfaces. The implanted CNTs not only provide a robust conductive coating for polymer micropattern but also have their protruded ends face the cell membrane vertically everywhere with maximum transmembrane potential. Regardless of their largely varied sizes and random dispersion, both individual blood cell type and whole blood samples were effectively transfected with plasmid DNA (85% after 24 h and 95% after 72 h, or 2.5-3.0 folds enhancement). High-dose RNA probes were also introduced, which regulate better the expression levels of exogenous and endogenous genes in blood cells. Besides its promising performance on non-viral delivery routes to cell-related studies and therapy, the involved new fabrication method also provides a convenient and effective way to construct flexible electronics with stable micro/nano features on the surface.

In situ electroporation of mammalian cells through SiO2 thin film capacitive microelectrodes

Electroporation is a widely used non-viral technique for the delivery of molecules, including nucleic acids, into cells. Recently, electronic microsystems that miniaturize the electroporation machinery have been developed as a new tool for genetic manipulation of cells in vitro, by integrating metal microelectrodes in the culture substrate and enabling electroporation in-situ. We report that non-faradic SiO₂ thin film-insulated microelectrodes can be used for reliable and spatially selective in-situ electroporation of mammalian cells. CHO-K1 and SH-SY5Y cell lines and primary neuronal cultures were electroporated by application of short and low amplitude voltage transients leading to cell electroporation by capacitive currents. We demonstrate reliable delivery of DNA plasmids and exogenous gene expression, accompanied by high spatial selectivity and cell viability, even with differentiated neurons. Finally, we show that SiO₂ thin film-insulated microelectrodes support a double and serial transfection of the targeted cells.

Microfluidic Electroporation Coupling Pulses of Nanoseconds and Milliseconds to Facilitate Rapid Uptake and Enhanced Expression of DNA in Cell Therapy

Standard electroporation with pulses in milliseconds has been used as an effective tool to deliver drugs or genetic probes into cells, while irreversible electroporation with nanosecond pulses is explored to alter intracellular activities for pulse-induced apoptosis. A combination treatment, long nanosecond pulses followed by standard millisecond pulses, is adopted in this work to help facilitate DNA plasmids to cross both cell plasma membrane and nuclear membrane quickly to promote the transgene expression level and kinetics in both adherent and suspension cells. Nanosecond pulses with 400–800 ns duration are found effective on disrupting nuclear membrane to advance nuclear delivery of plasmid DNA. The additional microfluidic operation further helps suppress the negative impacts such as Joule heating and gas bubble evolution from common nanosecond pulse treatment that lead to high toxicity and/or ineffective transfection. Having appropriate order and little delay between the two types of treatment with different pulse duration is critical to guarantee the effectiveness: 2 folds or higher transfection efficiency enhancement and rapid transgene expression kinetics of GFP plasmids at no compromise of cell viability. The implementation of this new electroporation approach may benefit many biology studies and clinical practice that needs efficient delivery of exogenous probes.

Flow micropillar array electroporation to enhance size specific transfection to a large population of cells

Despite serving as a popular non-viral delivery approach, electroporation carries several drawbacks in its current configurations. We developed a Flow Micropillar-array Electroporation (FME) system to wisely regulate an important transmembrane-determining factor, namely cell size variations among individual cells, to achieve effective transfection. In FME, cells flow through a slit-type microfluidic channel on which carbon electrodes with well-patterned micropillar array texture are integrated as the top and bottom wall. Gravity helps bring cells to the micropillar array surface so that the permeable area on cells in different size populations is specified by their size regardless their random location fact. Without sacrificing cell viability, we demonstrate this FME concept by delivering DNA plasmids to several mammalian cell lines with obvious transfection enhancement when compared to a commercial system (K562: 3.0 folds; A549: 3.3 folds; HeLa: 1.8 folds, COS7: 1.7 folds; 293T: 2.9 folds; mES: 2.5 folds). Moreover, carbon-based electrodes are less expensive, more durable, and convenient for integration with a microfluidic setup which enables rapid and massive transfection capability that many therapeutic application needs. The success of FME may benefit many emerging biological studies and clinical practice that requires effective transfection to a large population of cells in limited processing time.

Controllable cell manipulation in a microfluidic pipette-tip design using capacitive coupling of electric fields

Systems designed toward cell manipulation by electric fields are inherently challenged by energy dissipation along the electrode-electrolyte interface. A promising remedy is the introduction of high-k electrode passivation, enabling efficient capacitive coupling of electric fields into biological samples. We present the implementation of this strategy in a reusable pipette tip design featuring a 10 μl chamber volume for life science applications. Prototype validation and comparison to conductive gold-coated electrodes reveal a consistent and controllable biological effect that significantly increases the reproducibility of lysis events. The system provides precise descriptions of HEK-293 lysis dependency to variables such as field strength, frequency, and conductivity. Over 80% of cells were reversibly electroporated with minimal electrical lysis over a broad range of field settings. Successful transfection requires exponential decay pulses and showcases how modulating capacitive coupling can advance our understanding of fundamental mechanics in the field of electroporation.

A flow through device for simultaneous dielectrophoretic cell trapping and AC electroporation

Isolation of cells and their transfection in a controlled manner is an integral step in cell biotechnology. Electric field approaches such as dielectrophoresis (DEP) offers a more viable method for targeted immobilization of cells without any labels. For transfection of cells to incorporate exogenous materials, electrical methods such as electroporation, are preferred over chemical and viral delivery methods since they minimally affect cell viability and can target many types. However prior approaches to both methods required multiple excitation sources, an AC source for DEP-based trapping and another DC source for electroporation. In this paper, we present a first of its kind flow through lab-on-chip platform using a single AC excitation source for combined trapping using negative dielectrophoresis (nDEP) and AC electroporation. Use of AC fields for electroporation eliminates the unwanted side effects of electrolysis or joule heating at electrodes compared to DC electroporation. Adjusting the flow rate and the electrical parameters of the incident AC field precisely controls the operation (trap, trap with electroporation and release). The platform has been validated through trapping and simultaneous transfection of HEK-293 embryonic kidney cells with a plasmid vector containing a fluorescent protein tag. Numerical scaling analysis is provided that indicates promise for individual cell trapping and electroporation using low voltage AC fields.

Calibration of on-chip cell electroporation by a pseudo-volumetric uptake model

Most conventional methods for assessing uptake of exogenous molecules and nanomaterials into cells use the projected two-dimensional (2D) area of uptake intensity into individual cells. However, since most cells have a three-dimensional (3D) spherical shape, volumetric uptake cannot be quantified accurately using 2D area analysis. Here, we present a method for calibrating the electroporative uptake intensity of small molecules by using a novel predictable spherical volume (PSV) model, which is more accurate and quantitative than previous methods. As a proof-of-concept, we visualized the electroporative uptake of propidium iodide (PI) into mammalian cells in a single rectangular polydimethylsiloxane (PDMS) microfluidic channel, often used for direct observation of on-chip cell electroporation. Our PSV method yielded more accurate results than conventional methods and faithfully reflected volumetric changes in uptake intensity, even those due to microflow. We believe that this approach can be potentially beneficial for screening the electroporative uptake efficiency of cell-membrane impermeable nanodrugs, such as functional nanoparticles incorporated with a small drug capable of slowly diffusing inside cells.

Electroporation for nanomedicine: a review

Here, we present a review of recent advances in electroporation for the delivery of nanomedicine as intracellular carriers by electroporation (NICE) in a drug format with functional nanoparticles.

Size Specific Transfection to Mammalian Cells by Micropillar Array Electroporation

Electroporation serves as a promising non-viral gene delivery approach, while its current configuration carries several drawbacks associated with high-voltage electrical pulses and heterogeneous treatment on individual cells. Here we developed a new micropillar array electroporation (MAE) platform to advance the electroporation-based delivery of DNA and RNA probes into mammalian cells. By introducing well-patterned micropillar array texture on the electrode surface, the number of pillars each cell faces varies with its plasma membrane surface area, despite their large population and random locations. In this way, cell size specific electroporation is conveniently carried out, contributing to a 2.5~3 fold increase on plasmid DNA transfection and an additional 10–55% transgene knockdown with siRNA probes, respectively. The delivery efficiency varies with the number and size of micropillars as well as their pattern density. As MAE works like many single cell electroporation are carried out in parallel, the electrophysiology response of individual cells is representative, which has potentials to facilitate the tedious, cell-specific protocol screening process in current bulk electroporation (i.e., electroporation to a large population of cells). Its success might promote the wide adoption of electroporation as a safe and effective non-viral gene delivery approach needed in many biological research and clinical treatments.

Gold nanoparticles electroporation enhanced polyplex delivery to mammalian cells

Nonviral methods have been explored as the replacement of viral systems for their low toxicity and immunogenicity. However, they have yet to reach levels competitive to their viral counterparts. In this paper, we combined physical and chemical methods to improve the performance of polyplex delivery of DNA and small interfering RNA. Specifically, gold nanoparticles (AuNPs) were used to carry polyplex (a chemical approach) while electroporation (a physical approach) was applied for fast and direct cytosolic delivery. In this hybrid approach, cationic polymer molecules condense and/or protect genetic probes as usual while AuNPs help fix polycations to reduce their cytotoxicity and promote the transfection efficiency of electroporation. AuNPs of various sizes were first coated with polyethylenimine, which were further conjugated with DNA plasmids or small interfering RNA molecules to form AuNPs-polyplex. The hybrid nanoparticles were then mixed with cells and introduced into cell cytosol by electroporation. The delivery efficiency was evaluated with both model anchor cells (i.e., NIH/3T3) and suspension cells (i.e., K562), together with their impact on cell viability. We found that AuNP-polyplex showed 1.5∼2 folds improvement on the transfection efficiency with no significant increase of toxicity when compared to free plasmid delivery by electroporation alone. Such a combination of physical and chemical delivery concept may stimulate further exploration in the delivery of various therapeutic materials for both in vitro and in vivo applications.

Continuous cell electroporation for efficient DNA and siRNA delivery based on laminar microfluidic chips

Electroporation is a high-efficiency and low-toxicity physical gene transfer method. Traditional electroporation is limited to only low volume cell samples. Here we present a continuous cell electroporation method based on commonly used microfluidic chip fabrication technology. Using easily fabricated PDMS microfluidic chip, syringe pumps, and pulse generator, we show efficient delivery of both DNA and siRNA into different cell lines. We describe the protocol of chip fabrication, apparatus setup, and cell electroporation assay. Typically, the fabrication of the devices takes 1 or 2 days and the continuous electroporation assay takes 1 h.

Microfluidic electroporation for cellular analysis and delivery

Electroporation is a simple yet powerful technique for breaching the cell membrane barrier. The applications of electroporation can be generally divided into two categories: the release of intracellular proteins, nucleic acids and other metabolites for analysis and the delivery of exogenous reagents such as genes, drugs and nanoparticles with therapeutic purposes or for cellular manipulation. In this review, we go over the basic physics associated with cell electroporation and highlight recent technological advances on microfluidic platforms for conducting electroporation. Within the context of its working mechanism, we summarize the accumulated knowledge on how the parameters of electroporation affect its performance for various tasks. We discuss various strategies and designs for conducting electroporation at the microscale and then focus on analysis of intracellular contents and delivery of exogenous agents as two major applications of the technique. Finally, an outlook for future applications of microfluidic electroporation in increasingly diverse utilities is presented.

Sequential multi-molecule delivery using vortex-assisted electroporation

We developed an on-chip microscale electroporation system that enables sequential delivery of multiple molecules with precise and independent dosage controllability into pre-selected identical populations of target cells. The ability to trap cells with uniform size distribution contributed to enhanced molecular delivery efficiency and cell viability. Additionally, the system provides real-time monitoring ability of the entire delivery process, allowing timely and independent modification of cell- and molecule-specific electroporation parameters. The precisely controlled amount of inherently membrane-impermeant molecules was transferred into human cancer cells by varying electric field strengths and molecule injection durations. The proposed microfluidic electroporation system's improved viability and comparable gene transfection efficiency to that of commercial systems suggest that the current system has great potential to expand the research fields that on-chip electroporation techniques can be used in.

Method for electric parametric characterization and optimization of electroporation on a chip

We have developed a rapid method to optimize the electric parameters of cell electroporation. In our design, a pair of ring-dot formatted electrodes was used to generate a radial distribution of electric field from the center to the periphery. Varied electric field intensity was acquired in different annulus when an electric pulse was applied. Cells were cultured on the microchips for adherent cell electroporation and in situ observation. The electroporation parameters of electric field intensity were explored and evaluated in terms of cell viability and transfection efficiency. The optimization was performed in consideration of both cell viability, which was investigated to decrease as electric field increases, and the transfection rate, which normally increases at stronger electric field. The electroporation characteristics HEK-293A and Hela cells were investigated, and the optimum parameters were obtained. Verified by a commercial electroporation system as well as self-made microchips endowed the optimization with wider meaning. At last, as applications, we acquired the optimal electroporation pulse intensity of Neuro-2A cells and a type of primary cell (human umbilical vein endothelial cell, HUVEC) by one time electroporation using the proposed method.

Flow-through comb electroporation device for delivery of macromolecules

We present a microfluidic electroporation device with a comb electrode layout fabricated in polydimethylsiloxane (PMDS) and glass. Characterization experiments with HeLa cells and fluorescent dextran show efficient delivery (∼95%) with low toxicity (cell viability ∼85%) as well as rapid pore closure after electroporation. The activity of delivered molecules is also verified by silencing RNA (siRNA) studies that demonstrate gene knockdown in GFP expressing cells. This simple, scalable approach to microfluidic, flow-through electroporation could facilitate the integration of electroporation modules within cell analysis devices that perform multiple operations.

Micro-/nanofluidics based cell electroporation

Non-viral gene delivery has been extensively explored as the replacement for viral systems. Among various non-viral approaches, electroporation has gained increasing attention because of its easy operation and no restrictions on probe or cell type. Several effective systems are now available on the market with reasonably good gene delivery performance. To facilitate broader biological and medical applications, micro-/nanofluidics based technologies were introduced in cell electroporation during the past two decades and their advances are summarized in this perspective. Compared to the commercially available bulk electroporation systems, they offer several advantages, namely, (1) sufficiently high pulse strength generated by a very low potential difference, (2) conveniently concentrating, trapping, and regulating the position and concentration of cells and probes, (3) real-time monitoring the intracellular trafficking at single cell level, and (4) flexibility on cells to be transfected (from single cell to large scale cell population). Some of the micro-devices focus on cell lysis or fusion as well as the analysis of cellular properties or intracellular contents, while others are designed for gene transfection. The uptake of small molecules (e.g., dyes), DNA plasmids, interfering RNAs, and nanoparticles has been broadly examined on different types of mammalian cells, yeast, and bacteria. A great deal of progress has been made with a variety of new micro-/nanofluidic designs to address challenges such as electrochemical reactions including water electrolysis, gas bubble formation, waste of expensive reagents, poor cell viability, low transfection efficacy, higher throughput, and control of transfection dosage and uniformity. Future research needs required to advance micro-/nanofluidics based cell electroporation for broad life science and medical applications are discussed.

Seeing the electroporative uptake of cell-membrane impermeable fluorescent molecules and nanoparticles

This paper presents direct visualization of uptake directionality for cell-membrane impermeant fluorescent molecules and fluorescence-doped nanoparticles at a single-cell level during electroporation. To observe directly the uptake direction, we used microchannel-type electroporation that can generate a relatively symmetric and uniform electric field. For all the image frames during electroporation, fluorescence intensities that occurred at cell membranes in both uptake directions toward the electrodes have been sequentially recorded and quantitatively analyzed pixel by pixel. In our experiments, we found that fluorescent molecules, even not labeled to target biomolecules, had their own uptake direction with different intensities. It is also observed that the uptake intensity toward the cell membrane had a maximal value at a certain electric voltage, not at the highest value of voltages applied. The results also imply that the uptake direction of fluorescence-doped nanoparticles can be determined by a net surface charge of uptake materials and sizes in the electroporative environments. In summary, we performed a quantitative screening and direct visualization of uptake directionality for a set of fluorescent molecules and fluorescence-doped nanoparticles using electric-pulsation. Taking a closer look at the uptake direction of exogenous materials will help researchers to understand an unknown uptake phenomenon in which way foreign materials are inclined to move, and furthermore to design functional nanoparticles for electroporative gene delivery.

Microfluidic approaches for gene delivery and gene therapy

Recent advances in microfluidics have created new and exciting prospects for gene delivery and therapy. The micro-scaled environment within microfluidic systems enables precise control and optimization of multiple processes and techniques used in gene transfection and the production of gene and drug transporters. Traditional non-viral gene transfection methods, such as electroporation, microinjection and optical gene transfection, are improved from the use of innovative microfluidic systems. Additionally, microfluidic systems have also made the production of many viral and non-viral vectors controlled, automated, and reproducible. In summary, the development and application of microfluidic systems are producing increased efficiency in gene delivery and promise improved gene therapy results.

Partial transfection of cells using laminar flows in microchannels

This manuscript describes a convenient method for partial transfection using a Y-shaped microchannel polydimethylsiloxane (PDMS)-glass chip and on-chip cationic lipid-mediated transfection. Enhanced green fluorescent protein genes (pEGFP-N2) were introduced into the COS-7 cells cultured in half of the channel, while red fluorescent protein genes (pDsRed-N1) were introduced into the cells cultured in another half of the channel. This on-chip partial transfection technique provides an avenue for the spatial control of transfection. It is possible to use this technique to perform parallel transfection on chips in order to study cell behaviors under two or more gene transfections in the same culture.

A laminar flow electroporation system for efficient DNA and siRNA delivery

By introducing a hydrodynamic mechanism into a microfluidics-based electroporation system, we developed a novel laminar flow electroporation system with high performance. The laminar buffer flow implemented in the system separated the cell suspension flow from the electrodes, thereby excluding many unfavorable effects due to electrode reaction during electroporation, such as hydrolysis, bubble formation, pH change, and heating. Compared to conventional microfluidic electroporation systems, these improvements significantly enhanced transfection efficiency and cell viability. Furthermore, successful electrotransfection of plasmid DNA and, more importantly, synthetic siRNA, was demonstrated in several hard-to-transfect cell types using this system.

Role of weakly polarized nanoparticles in electroporation

In this study, we present a role for weakly polarized nanoparticles as engineered gene transporters that can enhance electromediated gene delivery. To validate this proof-of-concept, fluorescent poly(ethylene glycol) (PEG)-coated silica nanoparticles (SiNPs) with opposite polarities, SiNPs(RITC)-PEG/PTMA(+) and SiNPs(RITC)-PEG/PMP(-), are used. To investigate the electroporative uptake direction of the nanoparticles-gene complex, we employ microscale electroporation to generate more symmetric and uniform electric fields. The effect of the polarity of the nanoparticles on EGFP gene transfection efficiency in HeLa cells is measured by flow cytometry analysis. The results show that, compared to cationic nanoparticles, anionic nanoparticles have potential as electromediated gene transporters at a low gene concentration. Furthermore, we believe that this finding can be useful for developing a platform that enables electroporation-based gene/drug delivery associated with functional nanoparticles.

An efficient and high-throughput electroporation microchip applicable for siRNA delivery

Here we report a novel electroporation microchip with great performance and compatibility with the standard multi-well plate used in biological research. The novel annular interdigitated electrode design makes it possible to achieve efficient cell transfection as high as 90% under low-strength electrical pulses, thereby circumventing the many adverse effects of conventional cuvette-type and previously reported microchip-based electroporation devices. Using this system, we demonstrated substantially improved cell transfection efficacy and viability in cultured and primary cells, for both plasmid and synthetic siRNA. Improvements of this system open new opportunities for high-throughput applications of siRNA technology in basic and biomedical research.

Targeted nanoparticles enhanced flow electroporation of antisense oligonucleotides in leukemia cells

Liposome nanoparticles (LNs) with a targeting ligand were used in a semi-continuous flow electroporation (SFE) device to enhance in vitro delivery of exogenous oligonucleotides (ODN). Nanoparticles comprising transferrin-targeted lipoplex encapsulating ODN G3139 were mixed with K562 cells (a chronic myeloid leukemia cell line) and incubated for half an hour to accomplish nanoparticle binding. The mixture was then flowed through a SFE channel where electric pulses were given. Better ODN delivery efficiency was achieved with an increase of ∼24% to the case in combination of non-targeted LNs and SFE, and ∼60% to the case using targeted LNs alone, respectively. The MTS assay results confirmed cell viability greater than 75%.

An easy protocol for on-chip transfection of COS-7 cells with a cationic lipid-based reagent

The microchip has become a miniaturized and integrated platform for cell research and gene analysis. Traditional biological experiments are being performed on chips with greater frequency. Transfection, which is the process of introducing foreign DNA (or RNA) into cultured mammalian cells, is an important tool for basic and applied life science research. Therefore, on-chip transfection is a critical requirement for a fully comprehensive lab-on-chip system. In this study, a microchip made of polydimethylsiloxane (PDMS) and glass was fabricated for continuous cell culture and transfection. Enhanced green fluorescent protein genes (pEGFP) were introduced into COS-7 cells using a cationic lipid-based reagent, Lipofectamine 2000. The optimal concentration of Lipofectamine 2000 was also studied. This on-chip transfection protocol with cationic lipid-based reagent is expected to add to the emerging use of microchip-based life science and drug discovery research.

Semicontinuous flow electroporation chip for high-throughput transfection on mammalian cells

We have recently developed a semicontinuous flow electroporation (SFE) device for in vitro DNA delivery. Cells mixed with plasmid DNA continuously flowed through a serpentine channel, the side walls of which also serving as electrodes. With the use of pWizGFP plasmid and K562 cells as a model system, SFE showed better transgene expression (10-15%) compared to a commercial electroporation system. Quantitative results via MTS assay also revealed a 50% or higher cell viability. Similar observations were also found with pWizGFP transfection to mouse embryonic stem cells. Such improvements were attributed to less gas formation and Joule heating in SFE.

Microscale electroporation: challenges and perspectives for clinical applications

Microscale engineering plays a significant role in developing tools for biological applications by miniaturizing devices and providing controllable microenvironments for in vitro cell research. Miniaturized devices offer numerous benefits in comparison to their macroscale counterparts, such as lower use of expensive reagents, biomimetic environments, and the ability to manipulate single cells. Microscale electroporation is one of the main beneficiaries of microscale engineering as it provides spatial and temporal control of various electrical parameters. Microscale electroporation devices can be used to reduce limitations associated with the conventional electroporation approaches such as variations in the local pH, electric field distortion, sample contamination, and the difficulties in transfecting and maintaining the viability of desired cell types. Here, we present an overview of recent advances of the microscale electroporation methods and their applications in biology, as well as current challenges for its use for clinical applications. We categorize microscale electroporation into microchannel and microcapillary electroporation. Microchannel-based electroporation can be used for transfecting cells within microchannels under dynamic flow conditions in a controlled and high-throughput fashion. In contrast, microcapillary-based electroporation can be used for transfecting cells within controlled reaction chambers under static flow conditions. Using these categories we examine the use of microscale electroporation for clinical applications related to HIV-1, stem cells, cancer and other diseases and discuss the challenges in further advancing this technology for use in clinical medicine and biology.

Effects of cell orientation and electric field frequency on the transmembrane potential induced in ellipsoidal cells

The transmembrane potential (Deltaphi) induced by external electric fields is important both in biotech applications and in new medical therapies. We analyzed the effects of AC field frequency and cell orientation for cells of a general ellipsoidal shape. Simplified equations were derived for the membrane surface points where the maximum Deltaphi is induced. The theoretical results were confirmed in experiments with three-axial chicken red blood cells (a:b:c=6.66 microm:4.17 microm:1.43 microm). Propidium iodide (PI) staining and cell lysis were detected after an AC electropermeabilization (EP) pulse. The critical field strength for both effects increased when the shorter axis of a cell was parallel to the field, as well as at higher field frequency and for shorter pulse durations. Nevertheless, data analysis based on our theoretical description revealed that the Deltaphi required is lower for the shorter axis, i.e. for smaller membrane curvatures. The critical Deltaphi was independent of the field frequency for a given axis, i.e. the field strength had to be increased with frequency to compensate for the membrane dispersion effect. Comparison of the critical field strengths of PI staining in a linear field aligned along semi-axis a (142 kV m(-1)) and a field rotating in the a-b plane (115 kV m(-1)) revealed the higher EP efficiency of rotating fields.

An impulsive, electropulsation-driven backflow in microchannels during electroporation

We describe for the first time an impulsive, electropulsation-driven backflow in microchannels for on-chip cell electroporation.