Characterization of single-cell electroporation by using patch-clamp and fluorescence microscopy

Microfluidic Electroporation Arrays for Investigating Electroporation-Induced Cellular Rupture Dynamics

Electroporation is pivotal in bioelectrochemistry for cellular manipulation, with prominent applications in drug delivery and cell membrane studies. A comprehensive understanding of pore generation requires an in-depth analysis of the critical pore size and the corresponding energy barrier at the onset of cell rupture. However, many studies have been limited to basic models such as artificial membranes or theoretical simulations. Challenging this paradigm, our study pioneers using a microfluidic electroporation chip array. This tool subjects live breast cancer cell species to a diverse spectrum of alternating current electric field conditions, driving electroporation-induced cell rupture. We conclusively determined the rupture voltages across varying applied voltage loading rates, enabling an unprecedented characterization of electric cell rupture dynamics encompassing critical pore radius and energy barrier. Further bolstering our investigation, we probed cells subjected to cholesterol depletion via methyl-β-cyclodextrin and revealed a strong correlation with electroporation. This work not only elucidates the dynamics of electric rupture in live cell membranes but also sets a robust foundation for future explorations into the mechanisms and energetics of live cell electroporation.

Recent advances in microfluidic-based electroporation techniques for cell membranes

Electroporation is a fundamental technique for applications in biotechnology. To date, the ongoing research on cell membrane electroporation has explored its mechanism, principles and potential applications. Therefore, in this review, we first discuss the primary electroporation mechanism to help establish a clear framework. Within the context of its principles, several critical terms are highlighted to present a better understanding of the theory of aqueous pores. Different degrees of electroporation can be used in different applications. Thus, we discuss the electric factors (shock strength, shock duration, and shock frequency) responsible for the degree of electroporation. In addition, finding an effective electroporation detection method is of great significance to optimize electroporation experiments. Accordingly, we summarize several primary electroporation detection methods in the following sections. Finally, given the development of micro- and nano-technology has greatly promoted the innovation of microfluidic-based electroporation devices, we also present the recent advances in microfluidic-based electroporation devices. Also, the challenges and outlook of the electroporation technique for cell membrane electroporation are presented.

Single-cell individualized electroporation with real-time impedance monitoring using a microelectrode array chip

The ability to precisely deliver molecules into single cells while maintaining good cell viability is of great importance to applications in therapeutics, diagnostics, and drug delivery as it is an advancement toward the promise of personalized medicine. This paper reports a single-cell individualized electroporation method with real-time impedance monitoring to improve cell perforation efficiency and cell viability using a microelectrode array chip. The microchip contains a plurality of sextupole-electrode units patterned in an array, which are used to perform in situ electroporation and real-time impedance monitoring on single cells. The dynamic recovery processes of single cells under electroporation are tracked in real time via impedance measurement, which provide detailed transient cell states and facilitate understanding the whole recovery process at the level of single cells. We define single-cell impedance indicators to characterize cell perforation efficiency and cell viability, which are used to optimize electroporation. By applying the proposed electroporation method to different cell lines, including human cancer cell lines and normal human cell lines individually, optimum stimuli are determined for these cells, by which high transfection levels of enhanced green fluorescent protein (EGFP) plasmid into cells are achieved. The results validate the effectiveness of the proposed single-cell individualized electroporation/transfection method and demonstrate promising potential in applications of cell reprogramming, induced pluripotent stem cells, adoptive cell therapy, and intracellular drug delivery technology.

Ultrathin glass fiber microprobe for electroporation of arbitrary selected cell groups

A new type of ultrathin fiber microprobe for selective electroporation is reported. The microprobe is 10 cm long and has a diameter of 350 µm. This microprobe is a low cost tool, which allows electroporation of an arbitrary selected single cell or groups of cells among population with use of a standard microscope and cell culture plates. The microprobe in its basic form contains two metal microelectrodes made of a silver-copper alloy, running along the fiber, each with a diameter of 23 µm. The probe was tested in vitro on a population of normal and cancer cells. Successful targeted electroporation was observed by means of accumulation of trypan blue (TB) dye marker in the cell. The electroporation phenomenon was also verified with propidium iodide and AnnexinV in fluorescent microscopy.

Discontinuous Galerkin Model of Cellular Electroporation

Electroporation (EP) is a phenomenon involving both nonlinear biophysical processes and complex geometries. When exposed to strong electric fields, the formation of pores within a cell membrane increases the membrane permeability. Discontinuous Galerkin (DG) finite element methods can directly enforce these flux jumps across the thin cell membrane interface. We implement a DG finite element method to model the electric field, pore formation, and transmembrane flux of charged solutes during EP. Our model is readily extensible for parallel computation on high performance clusters and agrees with previous reports.

Current Trends of Microfluidic Single-Cell Technologies

The investigation of human disease mechanisms is difficult due to the heterogeneity in gene expression and the physiological state of cells in a given population. In comparison to bulk cell measurements, single-cell measurement technologies can provide a better understanding of the interactions among molecules, organelles, cells, and the microenvironment, which can aid in the development of therapeutics and diagnostic tools. In recent years, single-cell technologies have become increasingly robust and accessible, although limitations exist. In this review, we describe the recent advances in single-cell technologies and their applications in single-cell manipulation, diagnosis, and therapeutics development.

Characterization of Cell Membrane Permeability In Vitro Part I: Transport Behavior Induced by Single-Pulse Electric Fields

Most experimental studies of electroporation focus on permeabilization of the outer cell membrane. Some experiments address delivery of ions and molecules into cells that should survive; others focus on efficient killing of the cells with minimal temperature rise. A basic method for quantifying electroporation effectiveness is measuring the membrane's diffusive permeability. More specifically, comparisons of membrane permeability between electroporation protocols often rely on relative fluorescence measurements, which are not able to be directly connected to theoretical calculations and complicate comparisons between studies. Here we present part I of a 2-part study: a research method for quantitatively determining the membrane diffusive permeability for individual cells using fluorescence microscopy. We determine diffusive permeabilities of cell membranes to propidium for electric field pulses with durations of 1 to 1000 μs and strengths of 170 to 400 kV/m and show that diffusive permeabilities can reach 1.3±0.4×10-8 m/s. This leads to a correlation between increased membrane permeability and eventual propidium uptake. We also identify a subpopulation of cells that exhibit a delayed and significant propidium uptake for relatively small single pulses. Our results provide evidence that cells, especially those that uptake propidium more slowly, can achieve large permeabilities with a single electrical pulse that may be quantitatively measured using standard fluorescence microscopy equipment and techniques.

Characterization of Cell Membrane Permeability In Vitro Part II: Computational Model of Electroporation-Mediated Membrane Transport

Electroporation is the process by which applied electric fields generate nanoscale defects in biological membranes to more efficiently deliver drugs and other small molecules into the cells. Due to the complexity of the process, computational models of cellular electroporation are difficult to validate against quantitative molecular uptake data. In part I of this two-part report, we describe a novel method for quantitatively determining cell membrane permeability and molecular membrane transport using fluorescence microscopy. Here, in part II, we use the data from part I to develop a two-stage ordinary differential equation model of cellular electroporation. We fit our model using experimental data from cells immersed in three buffer solutions and exposed to electric field strengths of 170 to 400 kV/m and pulse durations of 1 to 1000 μs. We report that a low-conductivity 4-(2-hydroxyethyl)-1 piperazineethanesulfonic acid buffer enables molecular transport into the cell to increase more rapidly than with phosphate-buffered saline or culture medium-based buffer. For multipulse schemes, our model suggests that the interpulse delay between two opposite polarity electric field pulses does not play an appreciable role in the resultant molecular uptake for delays up to 100 μs. Our model also predicts the per-pulse permeability enhancement decreases as a function of the pulse number. This is the first report of an ordinary differential equation model of electroporation to be validated with quantitative molecular uptake data and consider both membrane permeability and charging.

Comparison between direct and reverse electroporation of cells in situ: a simulation study

The discovery of the human genome has unveiled new fields of genomics, transcriptomics, and proteomics, which has produced paradigm shifts on how to study disease mechanisms, wherein a current central focus is the understanding of how gene signatures and gene networks interact within cells. These gene function studies require manipulating genes either through activation or inhibition, which can be achieved by temporarily permeabilizing the cell membrane through transfection to deliver cDNA or RNAi. An efficient transfection technique is electroporation, which applies an optimized electric pulse to permeabilize the cells of interest. When the molecules are applied on top of seeded cells, it is called “direct” transfection and when the nucleic acids are printed on the substrate and the cells are seeded on top of them, it is termed “reverse” transfection. Direct transfection has been successfully applied in previous studies, whereas reverse transfection has recently gained more attention in the context of high‐throughput experiments. Despite the emerging importance, studies comparing the efficiency of the two methods are lacking. In this study, a model for electroporation of cells in situ is developed to address this deficiency. The results indicate that reverse transfection is less efficient than direct transfection. However, the model also predicts that by increasing the concentration of deliverable molecules by a factor of 2 or increasing the applied voltage by 20%, reverse transfection can be approximately as efficient as direct transfection.

Magneto-elasto-electroporation (MEEP): In-vitro visualization and numerical characteristics

A magnetically controlled elastically driven electroporation phenomenon, or magneto-elasto-electroporation (MEEP), is discovered while studying the interactions between core-shell magnetoelectric nanoparticles (CSMEN) and biological cells in the presence of an a.c. magnetic field. In this paper we report the effect of MEEP observed via a series of in-vitro experiments using core (CoFe2O4)-shell (BaTiO3) structured magnetoelectric nanoparticles and human epithelial cells (HEP2). The cell electroporation phenomenon and its correlation with the magnetic field modulated CSMEN are described in detail. The potential application of CSMEN in electroporation is confirmed by analyzing crystallographic phases, multiferroic properties of the fabricated CSMEN, influences of d.c. and a.c. magnetic fields on the CSMEN and cytotoxicity tests. The mathematical formalism to quantitatively describe the phenomena is also reported. The reported findings provide insights into the underlying MEEP mechanism and demonstrate the utility of CSMEN as an electric pulse-generating nano-probe in electroporation experiments with a potential application toward accurate and efficient targeted cell permeation.

Controllable in-situ cell electroporation with cell positioning and impedance monitoring using micro electrode array

This paper reports a novel microarray chip for in-situ, real-time and selective electroporation on individual cells integrated with cell positioning and impedance monitoring. An array of quadrupole-electrode units (termed positioning electrodes) and pairs of planar center electrodes located at the centers of each quadrupole-electrode unit were fabricated on the chip. The positioning electrodes are used to trap and position living cells onto the center electrodes based on negative dielectrophoresis (nDEP). The center electrodes are used for in-situ cell electroporation, and also used to measure cell impedance for monitoring cellular dynamics in real time. Controllably selective electroporation and electrical measurement on the cells in array are realized. We present an evidence of selective electroporation through use of fluorescent dyes. Subsequently we use in-situ and real-time impedance measurement to monitor the process, which demonstrates the dynamic behavior of the cell electroporation. Finally, we show the use of this device to perform successful transfection onto individual HeLa cells with vector DNA encoding a green fluorescent.

Protein-fluctuation-induced water-pore formation in ion channel voltage-sensor translocation across a lipid bilayer membrane

We have applied a combined fluorescence microscopy and single-ion-channel electric current recording approach, correlating with molecular dynamics (MD) simulations, to study the mechanism of voltage-sensor domain translocation across a lipid bilayer. We use the colicin Ia ion channel as a model system, and our experimental and simulation results show the following: (1) The open-close activity of an activated colicin Ia is not necessarily sensitive to the amplitude of the applied cross-membrane voltage when the cross-membrane voltage is around the resting potential of excitable membranes; and (2) there is a significant probability that the activation of colicin Ia occurs by forming a transient and fluctuating water pore of ∼15 Å diameter in the lipid bilayer membrane. The location of the water-pore formation is nonrandom and highly specific, right at the insertion site of colicin Ia charged residues in the lipid bilayer membrane, and the formation is intrinsically associated with the polypeptide conformational fluctuations and solvation dynamics. Our results suggest an interesting mechanistic pathway for voltage-sensitive ion channel activation, and specifically for translocation of charged polypeptide chains across the lipid membrane under a transmembrane electric field: the charged polypeptide domain facilitates the formation of hydrophilic water pore in the membrane and diffuses through the hydrophilic pathway across the membrane; i.e., the charged polypeptide chain can cross a lipid membrane without entering into the hydrophobic core of the lipid membrane but entirely through the aqueous and hydrophilic environment to achieve a cross-membrane translocation. This mechanism sheds light on the intensive and fundamental debate on how a hydrophilic and charged peptide domain diffuses across the biologically inaccessible high-energy barrier of the hydrophobic core of a lipid bilayer: The peptide domain does not need to cross the hydrophobic core to move across a lipid bilayer.

Mechanisms of transfer of bioactive molecules through the cell membrane by electroporation

A short review of biophysical mechanisms for electrotransfer of bioactive molecules through the cell membrane by using electroporation is presented. The concept of transient hydrophilic aqueous pores and membrane electroporation mechanisms of single cells and cells in suspension models are analyzed. Alongside the theoretical approach, some peculiarities of drug and gene electrotransfer into cells and applications in clinical trials are discussed.

Model of pore formation in a single cell in a flow-through channel with micro-electrodes

Microfluidic channels with embedded micro-electrodes are of growing use in devices that aim to electroporate single cells. In this article we present an analysis of pore evolution in a single cell passing by two planar electrodes that are separated by a nano-gap. The cell experiences an electric field that changes in time, as it goes over the electrodes in the channel. The nano-gap between the electrodes enhances the electric field's strength in the micro-channel, thus enabling the use of low potential difference between the electrodes. By computing the electric field on the surface of the cell we can calculate the pore density, as predicted by the model described by Krassowska and Filev (Biophys. J. 92(2):404-417, 2007). The simulation presented in this article is a useful tool for planning and executing experiments of single-cell electroporation in flow-through devices. We demonstrate how different parameters, such as cell size and the size of the gap between the electrodes, change the pore density and show how electroporation between micro-electrodes on the same plane is different from conventional electroporation between facing electrodes.

Electrical impedance tomographic imaging of a single cell electroporation

A living cell placed in a high strength electric field, can undergo a process known as electroporation. It is believed that during electroporation nano-scale defects (pores) occur in the membrane of the cell, causing dramatic changes to the permeability of its membrane. Electroporation is an important technique in biotechnology and medicine and numerous methods are being developed to improve the understanding and use of the technology. We propose to extend the toolbox available for studying electroporation by generating impedance distribution images of the cell as it undergoes electroporation using Electrical Impedance Tomography (EIT). To investigate the feasibility of this concept, we develop a mathematical model of the process of electroporation in a single cell and of EIT of the process and show simulation results of a computer-based finite element model (FEM). Our work is an attempt to develop a new imaging tool for visualizing electroporation in a single cell, offering a different temporal and spatial resolution compared to the state of the art, which includes bulk measurements of electrical properties during single cell electroporation, patch clamp and voltage clamp measurement in single cells and optical imaging with colorimetric dyes during single cell electroporation. This paper is a preliminary theoretic feasibility study.

Probing enzymatic activity inside single cells

We report a novel approach for determining the enzymatic activity within a single suspended cell. Using a steady-state microfluidic delivery device and timed exposure to the pore-forming agent digitonin, we controlled the plasma membrane permeation of individual NG108-15 cells. Mildly permeabilized cells (~100 pores) were exposed to a series of concentrations of fluorescein diphosphate (FDP), a fluorogenic alkaline phosphatase substrate, with and without levamisole, an alkaline phosphatase inhibitor. We generated quantitative estimates for intracellular enzyme activity and were able to construct both dose-response and dose-inhibition curves at the single-cell level, resulting in an apparent Michaelis contant Km of 15.3 μM ± 1.02 (mean ± standard error of the mean (SEM), n = 16) and an inhibition constant Ki of 0.59 mM ± 0.07 (mean ± SEM, n = 14). Enzymatic activity could be monitored just 40 s after permeabilization, and five point dose-inhibition curves could be obtained within 150 s. This rapid approach offers a new methodology for characterizing enzyme activity within single cells.

Nanofountain probe electroporation (NFP-E) of single cells

The ability to precisely deliver molecules into single cells is of great interest to biotechnology researchers for advancing applications in therapeutics, diagnostics, and drug delivery toward the promise of personalized medicine. The use of bulk electroporation techniques for cell transfection has increased significantly in the past decade, but the technique is nonspecific and requires high voltage, resulting in variable efficiency and low cell viability. We have developed a new tool for electroporation using nanofountain probe (NFP) technology, which can deliver molecules into cells in a manner that is highly efficient and gentler to cells than bulk electroporation or microinjection. Here we demonstrate NFP electroporation (NFP-E) of single HeLa cells within a population by transfecting them with fluorescently labeled dextran and imaging the cells to evaluate the transfection efficiency and cell viability. Our theoretical analysis of the mechanism of NFP-E reveals that application of the voltage creates a localized electric field between the NFP cantilever tip and the region of the cell membrane in contact with the tip. Therefore, NFP-E can deliver molecules to a target cell with minimal effect of the electric potential on the cell. Our experiments on HeLa cells confirm that NFP-E offers single cell selectivity, high transfection efficiency (>95%), qualitative dosage control, and very high viability (92%) of transfected cells.

Single-cell juxtacellular transfection and recording technique

Genetic modifications and pharmacological studies enable the analysis of protein function in living cells. While many of these studies investigate the effect of proteins by bulk administration or withdrawal of the protein in complex cellular networks, understanding the more subtle mechanisms of protein function requires fine-tuned changes on a single-cell level without affecting the balance of the system. In order to analyse the consequences of protein modification at the single-cell level, we have developed a single-cell transfection method in the loose patch configuration, which allows juxtacellular recordings of neuronal cells prior to juxtacellular transfection. CA1 pyramidal neurons were selected based on morphological and electrophysiological criteria. Using a patch clamp amplifier which allows sensitive recordings of action currents in the loose seal mode as well as electroporation with high-voltage electrical stimulation the identified neurons were transfected with a combination of specific nucleotides, e.g. siRNA and a plasmid coding for GFP for later cell retrieval. Two days after transfection, whole-cell patch clamp recordings of transfected cells were performed to analyse electrophysiological properties. Action potential firing and synaptic transmission of single electroporated CA1 pyramidal cells were comparable to untransfected cells. Our study presents a method which enables identification of neurons by juxtacellular recording prior to single-cell juxtacellular transfection, allowing subsequent analysis of morphological and electrophysiological parameters several days after the genetic modification.

Electroporation followed by electrochemical measurement of quantal transmitter release from single cells using a patterned microelectrode

An electrochemical microelectrode located immediately adjacent to a single neuroendocrine cell can record spikes of amperometric current that result from exocytosis of oxidizable transmitter from individual vesicles, i.e., quantal exocytosis. Here, we report the development of an efficient method where the same electrochemical microelectrode is used to electropermeabilize an adjacent chromaffin cell and then measure the consequent quantal catecholamine release using amperometry. Trains of voltage pulses, 5-7 V in amplitude and 0.1-0.2 ms in duration, were used to reliably trigger release from cells using gold electrodes. Amperometric spikes induced by electropermeabilization had similar areas, peak heights and durations as amperometric spikes elicited by depolarizing high K(+) solutions, therefore release occurs from individual secretory granules. Uptake of trypan blue stain into cells demonstrated that the plasma membrane is permeabilized by the voltage stimulus. Voltage pulses did not degrade the electrochemical sensitivity of the electrodes assayed using a test analyte. Surprisingly, robust quantal release was elicited upon electroporation in the absence of Ca(2+) in the bath solution (0 Ca(2+)/5 mM EGTA). In contrast, electropermeabilization-induced transmitter release required Cl(-) in the bath solution in that bracketed experiments demonstrated a steep dependence of the rate of electropermeabilization-induced transmitter release on [Cl(-)] between 2 and 32 mM. Using the same electrochemical electrode to electroporate and record quantal release of catecholamines from an individual chromaffin cell allows precise timing of the stimulus, stimulation of a single cell at a time, and can be used to load membrane-impermeant substances into a cell.

Delivery of molecules into cells using localized single cell electroporation on ITO micro-electrode based transparent chip

Single cell electroporation is one of the nonviral method which successfully allows transfection of exogenous macromolecules into individual living cell. We present localized cell membrane electroporation at single-cell level by using indium tin oxide (ITO) based transparent micro-electrodes chip with inverted microscope. A focused ion beam (FIB) technique has been successfully deployed to fabricate transparent ITO micro-electrodes with submicron gaps, which can generate more intense electric field to produce very localized cell membrane electroporation. In our approach, we have successfully achieved 0.93 μm or smaller electroporation region on the cell surface to inject PI (Propidium Iodide) dye into the cell with 60 % cell viability. This experiments successfully demonstrate the cell self-recover process from the injected PI dye intensity variation. Our localized cell membrane electroporation technique (LSCMEP) not only generates reversible electroporation process but also it provides a clear optical path for potentially monitoring/tracking of drugs to deliver in single cell level.

Single cell electroporation using microfluidic devices

Electroporation is a powerful technique to increase the permeability of cell membranes and subsequently introduce foreign materials into cells. Pores are created in the cell membrane upon application of an electric field (kV/cm). Most applications employ bulk electroporation, at the scale of 1 mL of cells (ca. one million cells). However, recent progresses have shown the interest to miniaturize the technique to a single cell. Single cell electroporation is achieved either using microelectrodes which are placed in close vicinity to one cell, or in a microfluidic format. We focus here on this second approach, where individual cells are trapped in micrometer-size structures within a microchip, exposed in situ to a high electric field and loaded with either a dye (proof-of-principle experiments) or a plasmid. Specifically, we present one device that includes an array of independent electroporation sites for customized and successive poration of nine cells. The different steps of the single cell electroporation protocol are detailed including cell sample preparation, cell trapping, actual cell poration and on-chip detection of pore formation. Electroporation is illustrated here with the transport of dyes through the plasma membrane, the transfection of cells with GFP-encoding plasmids, and the study of the ERK1 signaling pathway using a GFP-ERK1 protein construct expressed by the cells after their transfection with the corresponding plasmid. This last example highlights the power of microfluidics with the implementation of various steps of a process (cell poration, culture, imaging) performed at the single cell level, on a single device.

A simple and fast microfluidic approach of same-single-cell analysis (SASCA) for the study of multidrug resistance modulation in cancer cells

Due to the cellular heterogeneity in multidrug resistance (MDR) cell populations, positive drug effects on the modulation of MDR can be obscured in conventional methods, especially when only a small number of cells are available. To address cellular variations among different MDR cells, we report a new microfluidic approach to study drug effect on MDR modulation, by investigating drug accumulation of daunorubicin in MDR leukemia cells. We have demonstrated that the new approach of same-single-cell analysis by accumulation (denoted as SASCA-A) is not only superior to different-single-cell analysis, but also has key advantages over our previous approach of same-single-cell analysis. First, SASCA-A is much simpler as it does not require multiple cycles of drug uptake and drug efflux. Second, it is faster, only taking about one fourth of the time used in the previous approach. Third, it provides a more 'identical' and reliable control because it compares the time points just before MDR modulator tests. To help understand the dynamics of drug accumulation in MDR cells, we also developed a mathematical model to describe the kinetics of drug accumulation conducted in individual cells. The SASCA-A method will benefit drug resistance research in minor cell subpopulations (e.g., cancer "stem" cells) because this method requires only a small number of cells in identifying the MDR reversal effect.

A lipocentric view of peptide-induced pores

Although lipid membranes serve as effective sealing barriers for the passage of most polar solutes, nonmediated leakage is not completely improbable. A high activation energy normally keeps unassisted bilayer permeation at a very low frequency, but lipids are able to self-organize as pores even in peptide-free and protein-free membranes. The probability of leakage phenomena increases under conditions such as phase coexistence, external stress or perturbation associated to binding of nonlipidic molecules. Here, we argue that pore formation can be viewed as an intrinsic property of lipid bilayers, with strong similarities in the structure and mechanism between pores formed with participation of peptides, lipidic pores induced by different types of stress, and spontaneous transient bilayer defects driven by thermal fluctuations. Within such a lipocentric framework, amphipathic peptides are best described as pore-inducing rather than pore-forming elements. Active peptides bound to membranes can be understood as a source of internal surface tension which facilitates pore formation by diminishing the high activation energy barrier. This first or immediate action of the peptide has some resemblance to catalysis. However, the presence of membrane-active peptides has the additional effect of displacing the equilibrium towards the pore-open state, which is then maintained over long times, and reducing the size of initial individual pores. Thus, pore-inducing peptides, regardless of their sequence and oligomeric organization, can be assigned a double role of increasing the probability of pore formation in membranes to high levels as well as stabilizing these pores after they appear.

Kinetics of NF-κB nucleocytoplasmic transport probed by single-cell screening without imaging

Transport of protein and RNA cargoes between the nucleus and cytoplasm (nucleocytoplasmic transport) is vital for a variety of cellular functions. The studies of kinetics involved in such processes have been hindered by the lack of quantitative tools for measurement of the nuclear and cytosolic fractions of an intracellular protein at the single cell level for a cell population. In this report, we describe using a novel method, microfluidic electroporative flow cytometry, to study kinetics of nucleocytoplasmic transport of an important transcription factor NF-κB. With data collected from single cells, we quantitatively characterize the population-averaged kinetic parameters such as the rate constants and apparent activation barrier for NF-κB transport. Our data demonstrate that NF-κB nucleocytoplasmic transport fits first-order kinetics very well and is a fairly reversible process governed by equilibrium thermodynamics.

Distinguishing splanchnic nerve and chromaffin cell stimulation in mouse adrenal slices with fast-scan cyclic voltammetry

Electrical stimulation is an indispensible tool in studying electrically excitable tissues in neurobiology and neuroendocrinology. In this work, the consequences of high-intensity electrical stimulation on the release of catecholamines from adrenal gland slices were examined with fast-scan cyclic voltammetry at carbon fiber microelectrodes. A biphasic signal, consisting of a fast and slow phase, was observed when electrical stimulations typically used in tissue slices (10 Hz, 350 μA biphasic, 2.0 ms/phase pulse width) were applied to bipolar tungsten-stimulating electrodes. This signal was found to be stimulation dependent, and the slow phase of the signal was abolished when smaller (≤250 μA) and shorter (1 ms/phase) stimulations were used. The slow phase of the biphasic signal was found to be tetrodotoxin and hexamethonium independent, while the fast phase was greatly reduced using these pharmacological agents. Two different types of calcium responses were observed, where the fast phase was abolished by perfusion with a low-calcium buffer while both the fast and slow phases could be modulated when Ca²(+) was completely excluded from the solution using EGTA. Perfusion with nifedipine resulted in the reduction of the slow catecholamine release to 29% of the original signal, while the fast phase was only decreased to 74% of predrug values. From these results, it was determined that high-intensity stimulations of the adrenal medulla result in depolarizing not only the splanchnic nerves, but also the chromaffin cells themselves resulting in a biphasic catecholamine release.

Single-cell electroporation

Single-cell electroporation (SCEP) is a relatively new technique that has emerged in the last decade or so for single-cell studies. When a large enough electric field is applied to a single cell, transient nano-pores form in the cell membrane allowing molecules to be transported into and out of the cell. Unlike bulk electroporation, in which a homogenous electric field is applied to a suspension of cells, in SCEP an electric field is created locally near a single cell. Today, single-cell-level studies are at the frontier of biochemical research, and SCEP is a promising tool in such studies. In this review, we discuss pore formation based on theoretical and experimental approaches. Current SCEP techniques using microelectrodes, micropipettes, electrolyte-filled capillaries, and microfabricated devices are all thoroughly discussed for adherent and suspended cells. SCEP has been applied in in-vivo and in-vitro studies for delivery of cell-impermeant molecules such as drugs, DNA, and siRNA, and for morphological observations.

Long-term, multisite, parallel, in-cell recording and stimulation by an array of extracellular microelectrodes

Here we report on the development of a novel neuroelectronic interface consisting of an array of noninvasive gold-mushroom-shaped microelectrodes (gMmicroEs) that practically provide intracellular recordings and stimulation of many individual neurons, while the electrodes maintain an extracellular position. The development of this interface allows simultaneous, multisite, long-term recordings of action potentials and subthreshold potentials with matching quality and signal-to-noise ratio of conventional intracellular sharp glass microelectrodes or patch electrodes. We refer to the novel approach as "in-cell recording and stimulation by extracellular electrodes" to differentiate it from the classical intracellular recording and stimulation methods. This novel technique is expected to revolutionize the analysis of neuronal networks in relations to learning, information storage and can be used to develop novel drugs as well as high fidelity neural prosthetics and brain-machine systems.

Preclinical analysis of irreversible electroporation on rat liver tissues using a microfabricated electroporator

A microfabricated electroporator (MFE) for the irreversible electroporation (IRE) of tissues has been developed by miniaturizing a clinical electroporator with a two-needle array while keeping the same electric field strength distribution. Since IRE was brought to special attention as one of the local tissue ablation techniques to treat tumors, many preclinical studies have been conducted to investigate the efficacy of IRE on animal tissues. However, some technical difficulties have been frequently encountered due to the macroscale dimension of clinical electroporators, particularly in experiments on small animal models such as the mouse or rat. Here, the MFE was proposed to solve the associated problems, resulting in time- and cost-effective experimental procedures. With the developed MFE, the effect of IRE on rat liver tissues was analyzed with time by immunohistological stainings and electrical measurement, and the experimental results were compared with those operated with the corresponding real-scale clinical electroporator.

Microfluidic electroporation of tumor and blood cells: observation of nucleus expansion and implications on selective analysis and purging of circulating tumor cells

Circulating tumor cells (CTCs) refer to cells that detach from a primary tumor, circulate in the blood stream, and may settle down at a secondary site and form metastases. The detection and characterization of CTCs are clinically useful for diagnosis and prognosis purposes. However, there has been very little work on purging CTCs from the blood. In this study, we systematically studied electroporation of tumor and blood cells in the context of selective purging and analysis of CTCs, using M109 and mouse blood cells as models. Electroporation is a simple and effective method for disruption of the cell membrane by applying an external electric field. We applied a microfluidic flow-through electroporation to process cells with various electroporation durations and field intensities. With duration of 100-300 ms, we found that the thresholds for electroporation-induced lysis started at 300-400 V cm(-1) for M109, 400-500 V cm(-1) for white blood cells and 1100-1200 V cm(-1) for red blood cells. Due to the substantial difference, we demonstrated the selective electroporation of tumor cells among blood cells and the scale-up of the flow-through electroporation devices for processing samples of millilitre volumes. Using Coherent Anti-stokes Raman Scattering (CARS) and fluorescence microscopy tools, we observed the dramatic increase in the size of the nucleus of a tumor cell in response to the applied field. We suggest that the nucleus expansion is a newly discovered mechanism responsible for rapid tumor cell death resulted from electroporation.

Control of the release of freely diffusing molecules in single-cell electroporation

Single-cell electroporation using an electrolyte-filled capillary is an emerging technique for transient pore formation in adherent cells. Because adherent cells do not have a simple and consistent shape and because the electric field emanating from the tip of the capillary is inhomogeneous, the Schwan equation based on spherical cells in homogeneous electrical fields does not apply. We sought to determine experimental and cell parameters that influence the outcome of a single-cell electroporation experiment. A549 cells were exposed to the thiol-reactive dye Thioglo-1, leading to green fluorescence from intracellular thiol adducts. Electroporation causes a decrease with time of the intracellular fluorescence intensity of Thioglo-1-loaded cells from diffusive loss of thiol adducts. The transient curves thus obtained are well-described by a simple model originally developed by Puc et al. We find that the final fluorescence following electroporation is related to the capillary tip-to-cell distance and cell size (specifically, 2(A/pi)(1/2) where A is the area of the cell's image in pixels. This quantity is the diameter if the image is a circle). In separate experiments, the relationship obtained can be used to control the final fluorescence following electroporation by adjusting the tip-to-cell distance based on cell size. The relationship was applied successfully to A549 as well as DU 145 and PC-3 cells. Finally, F-tests show that the variability in the final fluorescence (following electroporation) is decreased when the tip-to-cell distance is controlled according to the derived relationship in comparison to experiments in which the tip-cell distance is a constant irrespective of cell size.

Electronic interfacing with living cells

The direct interfacing of living cells with inorganic electronic materials, components or systems has led to the development of two broad categories of devices that can (1) transduce biochemical signals generated by biological components into electrical signals and (2) transduce electronically generated signals into biochemical signals. The first category of devices permits the monitoring of living cells, the second, enables control of cellular processes. This review will survey this exciting area with emphasis on the fundamental issues and obstacles faced by researchers. Devices and applications that use both prokaryotic (microbial) and eukaryotic (mammalian) cells will be covered. Individual devices described include microbial biofuel cells that produce electricity, bioelectrical reactors that enable electronic control of cellular metabolism, living cell biosensors for the detection of chemicals and devices that permit monitoring and control of mammalian physiology.

Extracellular Stimulation of Mammalian Neurons Through Repetitive Activation of Na+ Channels by Weak Capacitive Currents on a Silicon Chip

Reliable extracellular stimulation of neuronal activity is the prerequisite for electrical interfacing of cultured networks and brain slices, as well as for neural implants. Safe stimulation must be achieved without damage to the cells. With respect to a future application of highly integrated semiconductor chips, we present an electrophysiological study of capacitive stimulation of mammalian cells in the geometry of adhesion on an insulated titanium dioxide/silicon electrode. We used HEK293 cells with overexpressed NaV1.4 channels and neurons from rat hippocampus. Weak biphasic stimuli of falling and rising voltage ramps were applied in the absence of Faradaic current and electroporation. We recorded the response of the intra- and extracellular voltage and evaluated the concomitant polarization of the attached and free cell membranes. Falling ramps efficiently depolarized the central area of the attached membrane. A transient sodium inward current was activated that gave rise to a weak depolarization of the cell on the order of 1 mV. The depolarization could be enhanced step by step by a train of biphasic stimuli until self-excitation of sodium channels set in. We applied the same protocol to cultured rat neurons and found that pulse trains of weak capacitive stimuli were able to elicit action potentials. Our results provide a basis for safe extracellular stimulation not only for cultured neurons on insulated semiconductor electrodes, but also more generally for metal electrodes in cell culture and brain tissue.

Nonlinear current response of micro electroporation and resealing dynamics for human cancer cells

This paper presents a novel method to measure the dynamic process of membrane permeability during electroporation (EP) on microchips for human cancer cells. Micro EP chips with three-dimensional gold electrodes accommodating a single cell in between were fabricated with a modified electroplating process. Electrochemical impedance spectroscopy (EIS) was carried out with an electrochemistry analyzer on micro EP chips and a nonlinear equivalent circuit model was proposed to describe the dynamic response of the whole system. Using such a method, micro EP current was isolated from undesired leakage current to study the corresponding electroporation dynamics under different input voltages. In addition, cell membrane recovery dynamics after electroporation was also studied and the resealing time constants were determined for different pulse treatments.

Nanopipette delivery of individual molecules to cellular compartments for single-molecule fluorescence tracking

We have developed a new method, using a nanopipette, for controlled voltage-driven delivery of individual fluorescently labeled probe molecules to the plasma membrane which we used for single-molecule fluorescence tracking (SMT). The advantages of the method are 1), application of the probe to predefined regions on the membrane; 2), release of only one or a few molecules onto the cell surface; 3), when combined with total internal reflection fluorescence microscopy, very low background due to unbound molecules; and 4), the ability to first optimize the experiment and then repeat it on the same cell. We validated the method by performing an SMT study of the diffusion of individual membrane glycoproteins labeled with Atto 647-wheat germ agglutin in different surface domains of boar spermatozoa. We found little deviation from Brownian diffusion with a mean diffusion coefficient of 0.79 +/- 0.04 microm(2)/s in the acrosomal region and 0.10 +/- 0.02 microm(2)/s in the postacrosomal region; this difference probably reflects different membrane structures. We also showed that we can analyze diffusional properties of different subregions of the cell membrane and probe for the presence of diffusion barriers. It should be straightforward to extend this new method to other probes and cells, and it can be used as a new tool to investigate the cell membrane.

Experimentally determining the iR drop in solution at carbon fiber microelectrodes with current interruption and application to single-cell electroporation

Single-cell electroporation uses microelectrodes, capillaries, or micropipets positioned near single, adherent cells to increase transiently the membrane permeability of the cell. The increased permeability permits, for example, transfection without chemical reagents. When using microelectrodes to apply an electric field to the cell, there is a question of how much voltage to apply. Unlike in bulk electroporation, where hundreds of volts may be applied between electrodes, a rather small voltage is applied to a microelectrode in single-cell electroporation. In the single-cell experiment with microelectrodes, a substantial fraction of the voltage is lost at the interface and does not therefore exist in solution. This problem is the same as the classical electrochemist's problem of knowing the "iR" drop in solution and correcting for it to obtain true interfacial potential differences. Therefore, we have used current interruption to determine the iR drop in solution near microcylinder electrodes. As the field is inhomogeneous, calculations are required to understand the field distribution. Results of the current interruption are validated by comparing two independent measurements of the resistance in solution: one value results from the measured iR drop in conjunction with the known applied current. The other value results from a measured solution conductivity and a computed cell constant. We find substantial agreement in the range of resistances from about 2 to 50 kOmega, but not at higher resistances. We propose a simple, four-step plan that takes a few minutes to calculate the approximate current required to electroporate a cell with an electrode of a particular size, shape, and distance from the cell. We validate the approach with electroporation of single A549 cells.

Single-cell electroporation arrays with real-time monitoring and feedback control

Rapid well-controlled intracellular delivery of drug compounds, RNA, or DNA into a cell--without permanent damage to the cell--is a pervasive challenge in basic cell biology research, drug discovery, and gene delivery. To address this challenge, we have developed a bench-top system comprised of a control interface, that mates to disposable 96-well-formatted microfluidic devices, enabling the individual manipulation, electroporation and real-time monitoring of each cell in suspension. This is the first demonstrated real-time feedback-controlled electroporation of an array of single-cells. Our computer program automatically detects electroporation events and subsequently releases the electric field, precluding continued field-induced damage of the cell, to allow for membrane resealing. Using this novel set-up, we demonstrate the reliable electroporation of an array (n = 15) of individual cells in suspension, using low applied electric fields (<1 V) and the rapid and localized intracellular delivery of otherwise impermeable compounds (Calcein and Orange Green Dextran). Such multiplexed electrical and optical measurements as a function of time are not attainable with typical electroporation setups. This system, which mounts on an inverted microscope, obviates many issues typically associated with prototypical microfluidic chip setups and, more importantly, offers well-controlled and reproducible parallel pressure and electrical application to individual cells for repeatability.

Numerical calculations of single-cell electroporation with an electrolyte-filled capillary

An electric field is focused on one cell in single-cell electroporation. This enables selective electroporation treatment of the targeted cell without affecting its neighbors. While factors that lead to membrane permeation are the same as in bulk electroporation, quantitative description of the single-cell experiments is more complicated. This is due to the fact that the potential distribution cannot be solved analytically. We present single-cell electroporation with an electrolyte-filled capillary modeled with a finite element method. Potential is calculated in the capillary, the solution surrounding the cell, and the cell. The model enables calculation of the transmembrane potential and the fraction of the cell membrane that is above the critical electroporation potential. Electroporation at several cell-to-tip distances of human lung carcinoma cells (A549) stained with ThioGlo-1 demonstrated membrane permeation at distances shorter than approximately 7.0 microm. This agrees well with the model's prediction that a critical transmembrane potential of 250 mV is achieved when the capillary is approximately 6.5 microm or closer to the cell. Simulations predict that at short cell-to-tip distances, the transmembrane potential increases significantly while the total area of the cell above the critical potential increases only moderately.