High electrical field effects on cell membranes

Electro-dewatering performance of sewage sludge under interrupted pulsating voltage: A comparison between square shape and half-sine shape waveform

Electro-dewatering of sewage sludge with pulsating voltage was conducted under the two different wave shapes (square wave (SQW) and half-sine wave (HSW)) to investigate the influence of wave shape and duty cycle on sludge dewatering performance. The results indicated that, under the same average voltage, the moisture content of dewatered sludge with HSW was 10.3%-35.4% lower than that with SQW, suggesting the better dewatering performance of HSW. The optimal dewatering performance was achieved at duty cycle of 80% for SQW and 60% for SHW. The chemical oxygen demand of filtrate from HSW could be 13% higher than that from SQW, indicating the higher capacity of HSW in breaking sludge cells/floc structure. The applied voltage during electrochemical treatment promoted the hydrolysis of protein in filtrate, and the main components in the electro-dewatered filtrate were fulvic acid- and humic acid-like substances. The specific energy consumption for sludge electro-dewatering were 0.015-0.269 kWh/(kg removed water), and it was almost in linear relationship with duty cycle. By overall considering the energy consumption and electro-dewatering performance, the condition of 60% duty cycle with HSW was obviously better than other conditions, which provides a meaningful guidance for future application of sludge electro-dewatering technology with pulsating voltage.

Controllable and Stable Fusion Strategy on Microfluidics

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

Physical limits on galvanotaxis

Eukaryotic cells can polarize and migrate in response to electric fields via "galvanotaxis," which aids wound healing. Experimental evidence suggests cells sense electric fields via molecules on the cell's surface redistributing via electrophoresis and electroosmosis, though the sensing species has not yet been conclusively identified. We develop a model that links sensor redistribution and galvanotaxis using maximum likelihood estimation. Our model predicts a single universal curve for how galvanotactic directionality depends on field strength. We can collapse measurements of galvanotaxis in keratocytes, neural crest cells, and granulocytes to this curve, suggesting that stochasticity due to the finite number of sensors may limit galvanotactic accuracy. We find cells can achieve experimentally observed directionalities with either a few (∼100) highly polarized sensors or many (∼10^{4}) sensors with an ∼6-10% change in concentration across the cell. We also identify additional signatures of galvanotaxis via sensor redistribution, including the presence of a tradeoff between accuracy and variance in cells being controlled by rapidly switching fields. Our approach shows how the physics of noise at the molecular scale can limit cell-scale galvanotaxis, providing important constraints on sensor properties and allowing for new tests to determine the specific molecules underlying galvanotaxis.

Inactivation of Escherichia coli by dual-functional zerovalent Fe/Al composites in water

A bimetallic Fe/Al disinfection system was developed to examine the feasibility of inactivation of water borne microorganisms. In this study, the effectiveness and mechanisms of the bimetallic Fe/Al system on the inactivation of model bacteria, Escherichia coli (E. coli), were investigated. Results revealed that the Fe/Al system effectively inactivated E. coli to reach nearly 2 logs (99%) removal within 20 min and 4 logs (99.99%) at 24 h, indicating that the Fe/Al composite was able to sustain a long-term disinfection capacity. The inactivation ability resulted from hydroxyl radicals produced by a Fenton reaction through in-situ self-generated Fe²⁺ and H₂O₂ species in the Fe/Al system. In addition to the attack by the radicals, some of E. coli were adsorbed onto the Fe/Al composite (zeta potential of 30-50 mV) as a result of Coulomb interaction. Scanning electron microscope (SEM) images showed that the adsorbed bacteria had damaged pores at the two ends of their rod-like cells. This phenomenon suggested that a micro-electric field between the Fe/Al galvanic couple induced electroporation of the adsorbed E. coli and thus further advanced additional inactivation ability for the bacteria disinfection.

Tuning the transdermal transport by application of external continuous electric field: a coarse-grained molecular dynamics study

The control of skin permeability to specific substances (e.g., medications, vitamins, and nutrients) through stratum corneum is a challenge. Iontophoresis is an option in spite of the lack of a detailed understanding of the underlying molecular mechanism. In the present work, the simulations concerning application of an external continuous electric field to stratum corneum, in a range of low intensity (0-24 mV nm^-1), were carried out using the coarse-grained molecular dynamics approach. Using a set of random seed replicas of the starting configuration, we observed that in the range of electric field intensity of 22-23 mV nm^-1, water-rich lipid vesicles were formed in 20% of cases. Pores appeared in the remaining 80%. We argue that lipids undergo fast re-orientations under electric field inducing mechanical instability, which originates the pores. We presented a simple electrostatic model to interpret the results where the mismatch between electrical permittivities of the membrane and external media and the gradient of the local electric field in the membrane surface, govern the time scales and electric fields for vesicle formation. Our results indicate that just 10% difference between electrical permittivities of the membrane and external media decreases 1/6 the minimal time required for vesicle formation. The minimal electric field required decreases 10 times. The control and tunning of formation of biologically compatible vesicles, capable of transporting substances under low-intensity electric fields, has a promising application in fields such as drug therapy and dermo-cosmetics allowing the use of hydrophilic substances in dermal applications.

Local pulses of electrical potential can induce long-range transient excitations in self-aligned molecular films

Natural liquids can contain self-aligned molecules (such as liquid crystals and biological membranes) which give them unique properties of anisotropic diffusion, coupling between the molecular orientation and flow, etc. Here, we describe the observation of new phenomena in those materials: long-distance transport and molecular orientation waves that are induced by pulses of spatially localized electrical potential. As a result, the morphological properties of the material are significantly altered well beyond the reach of the electrical field. The local dielectric torque-induced reduction of the effective molecular volume and corresponding pressure gradients are in the origin of these phenomena. Our observations are made for electric fields that are an order of magnitude smaller than those present in biological membranes. Thus, this discovery may have important impact on the understanding of the operation of these membranes and on the dynamics of action potential propagation in neural cells. The corresponding possible influence of observed excitation mechanisms on the ionic gates and the role of myelin sheath are discussed.

Mechanism of E. coli Inactivation by Direct-in-liquid Electrical Discharge Plasma in Low Conductivity Solutions

This work investigates and reveals the main mechanism(s) responsible for inactivation of E. coli by in-liquid pulsed electrical discharge plasma in low conductivity solutions. Experiments were designed and performed to explore the effects of plasma-emitted UV light, oxidative radicals, and electric field on E. coli inactivation curves, rate of DNA leakage and visual appearance of the treated microorganisms. Results showed that electric field had the main role in inactivation; scanning electron microscopy images revealed that both plasma and the isolated electric field result in extensive cell wall disruptions. While this damage in the case of plasma treatment was extensive and distributed randomly along the envelope, the electric field-induced damage resulted in disruption primarily at the poles of the bacterial rods. Subsequent experiments conducted with an oxidative radical scavenger suggested that plasma-generated radicals do not contribute directly to the inactivation but assist in cell wall deterioration and extension of the ruptures first generated by the electric field.

Receptor- and store-operated mechanisms of calcium entry during the nanosecond electric pulse-induced cellular response

Nanosecond electric pulses have been shown to open nanopores in the cell plasma membrane by fluorescent imaging of calcium uptake and fluorescent dyes, including propidium (Pr) iodide and YO-PRO-1 (YP₁). Recently, we demonstrated that nsEPs also induce the phosphoinositide intracellular signaling cascade by phosphatidylinositol-4,5-bisphosphate (PIP₂) depletion resulting in physiological responses similar to those observed following stimulation of G_q11-coupled receptors. In this paper, we explore the role of receptor- and store-operated calcium entry (ROCE/SOCE) mechanisms in the observed response of cells to nsEP. We show that addition of the ROCE/SOCE and transient receptor potential channel (TRPC) blocker gadolinium (Gd³⁺, 300 μM) slows PIP2 depletion following 1 and 20 nsEP exposures. Lipid rafts, regions of the plasma membrane rich in PIP₂ and TRPC, are also disrupted by nsEP exposure suggesting that ROCE/SOCE mechanisms are likely impacted. Reducing the expression of stromal interaction molecule 1 (STIM1) protein, a key protein in ROCE and SOCE, in cells exposure to nsEP resulted in a reduction in induced intracellular calcium rise. Additionally, after exposure to 1 and 20 nsEPs (16.2 kV/cm, 5 Hz), intracellular calcium rises were significantly reduced by the addition of GD³⁺ and SKF-96365 (1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy] ethyl-1H-imidazole hydrochloride, 100 μM), a blocker of STIM1 interaction. However, using similar nsEP exposure parameters, SKF-96365 was less effective at reducing YP₁ uptake compared to Gd³⁺. Thus, it is possible that SKF-96365 could block STIM1 interactions within the cell, while Gd³⁺ could acts on TRPC/nanopores from outside of the cell. Our results present evidence of nsEP induces ROCE and SOCE mechanisms and demonstrate that YP₁ and Ca²⁺ cannot be used solely as markers of nsEP-induced nanoporation.

Electrochemical oxidation of Microcystis aeruginosa using a Ti/RuO2 anode: contributions of electrochemically generated chlorines and hydrogen peroxide

Electrochemical oxidation was proposed as a promising technology for algal control in drinking water treatment. To be effective, the electrogenerated oxidants should have long half-lives and could continually inhibit the growth of algae. In this study, we used the electrochemical system equipped with a Ti/RuO₂ anode which focus on generating long half-life chlorines and H₂O₂. We explored the impact of electrical field and electrogenerated oxidants on algal inhibition, and we investigated the production of electrogenerated reactive species and their contributions to the inhibition of Microcystis aeruginosa (M. aeruginosa) in simulated surface water with low Cl^- concentrations (< 18 mg/L). We developed a kinetic model to simulates the concentrations of chlorines and H₂O₂. The results showed that electrical field and electrogenerated oxidants were both important contributors to algal inhibition during electrochemical oxidation treatment. The Ti/RuO₂ anode mainly generates chlorines and H₂O₂ from Cl^- and water. During the electrolysis at current density of 20 mA/cm², when initial Cl^- concentrations increased from 0 to 18 mg/L (0-5.07 × 10^-4 mol/L), the chlorines increased from 0 to 3.62 × 10^-6 mol/L, and the H₂O₂ concentration decreased from 3.68 × 10^-6 to 1.15 × 10^-6 mol/L. Our model made decent predictions of other Cl^- concentrations by comparing with experiment data which validated the rationality of this modeling approach. The electrogenerated chlorine species were more effective than H₂O₂ at an initial Cl^- concentration of 18 mg/L.

Effect of drug amlodipine on the charged lipid bilayer cell membranes DMPS and DMPS + DMPC: a molecular dynamics simulation study

In this work, the effects of the anti-hypertensive drug amlodipine in native and PEGylated forms on the malfunctioning of negatively charged lipid bilayer cell membranes constructed from DMPS or DMPS + DMPC were studied by molecular dynamics simulation. The obtained results indicate that amlodipine alone aggregates and as a result its diffusion into the membrane is retarded. In addition, due to their large size aggregates of the drug can damage the cell, rupturing the cell membrane. It is shown that PEGylation of amlodipine prevents this aggregation and facilitates its diffusion into the lipid membrane. The interaction of the drug with negatively charged membranes in the presence of an aqueous solution of NaCl, as the medium, is investigated and its effects on the membrane are considered by evaluating the structural properties of the membrane such as area per lipid, thickness, lipid chain order and electrostatic potential difference between bulk solution and lipid bilayer surface. The effect of these parameters on the diffusion of the drug into the cell is critically examined and discussed.

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

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

Theoretical analysis of transmembrane potential of cells exposed to nanosecond pulsed electric field

PURPOSE

Intracellular electroporation occurs when the cells are exposed to nanosecond pulsed electric field (nsPEF). It is believed the electroporation (formation and extension of pores on the membrane induced by external electric field) is affected significantly by the transmembrane potential. This paper analyzed transmembrane potential induced by nsPEF in the term of pulse frequency spectrum, aiming to provide a theoretical explanation to intracellular bio-effects.

METHODS

Based on the double-shelled spherical cell model, the frequency dependence of transmembrane potential was obtained by solving Laplace's equation, while the time course of transmembrane potential was obtained by a method combined with discrete Fourier transform and Laplace transform. First-order Debye equation was used to describe the dielectric relaxation of the cell medium.

RESULTS

Frequency-domain analysis showed that when the electric field frequency was higher than 10⁵ Hz, the transmembrane potential on the organelle membrane (ΔΦ_o) was increasing to exceed the transmembrane potential on the cellular membrane (ΔΦ_c). In the time-domain analysis, transmembrane potentials induced by four nsPEF (short trapezoid, long trapezoid, bipolar and sine shapes) with the same field strength were compared with each other. It showed that ΔΦ_o is obviously larger than ΔΦ_c if the curve of the normalized frequency spectrum of the pulse is more similar with the curve of normalized ΔΦ_o in frequency domain. Pulses with major frequency components higher than 10⁸ Hz lead to both small ΔΦ_o and ΔΦ_c. This may explain why high power pulsed microwave lead to unobvious bio-effects of cells than nsPEF with trapezoid form.

CONCLUSION

Through the pulse frequency spectrum it is clearer to understand the relationship between nsPEF and the transmembrane potential.

Investigating the association between photosynthetic efficiency and generation of biophotoelectricity in autotrophic microbial fuel cells

Microbial fuel cells operating with autotrophic microorganisms are known as biophotovoltaic devices. It represents a great opportunity for environmentally-friendly power generation using the energy of the sunlight. The efficiency of electricity generation in this novel system is however low. This is partially reflected by the poor understanding of the bioelectrochemical mechanisms behind the electron transfer from these microorganisms to the electrode surface. In this work, we propose a combination of electrochemical and fluorescence techniques, giving emphasis to the pulse amplitude modulation fluorescence. The combination of these two techniques allow us to obtain information that can assist in understanding the electrical response obtained from the generation of electricity through the intrinsic properties related to the photosynthetic efficiency that can be obtained from the fluorescence emitted. These were achieved quantitatively by means of observed changes in four photosynthetic parameters with the bioanode generating electricity. These are the maximum quantum yield (Fv/Fm), alpha (α), light saturation coefficient (Ek) and maximum rate of electron transfer (rETRm). The relationship between the increases in the current density collected by the bioanode to the decrease of the rETRm values in the photosynthetic pathway for the two microorganisms was also discussed.

Directed multistep biocatalysis using tailored permeabilized cells

: Recent developments in the field of biocatalysis using permeabilized cells are reviewed here, with a special emphasis on the newly emerging area of multistep biocatalysis using permeabilized cells. New methods of metabolic engineering using in silico network design and new methods of genetic engineering provide the opportunity to design more complex biocatalysts for the synthesis of complex biomolecules. Methods for the permeabilization of cells are thoroughly reviewed. We provide an extended review of useful available databases and bioinformatics tools, particularly for setting up genome-scale reconstructed networks. Examples described include phosphorylated carbohydrates, sugar nucleotides, and polyketides.

A molecular insight into the electro-transfer of small molecules through electropores driven by electric fields

The transport of chemical compounds across the plasma membrane into the cell is relevant for several biological and medical applications. One of the most efficient techniques to enhance this uptake is reversible electroporation. Nevertheless, the detailed molecular mechanism of transport of chemical species (dyes, drugs, genetic materials, …) following the application of electric pulses is not yet fully elucidated. In the past decade, molecular dynamics (MD) simulations have been conducted to model the effect of pulsed electric fields on membranes, describing several aspects of this phenomenon. Here, we first present a comprehensive review of the results obtained so far modeling the electroporation of lipid membranes, then we extend these findings to study the electrotransfer across lipid bilayers subject to microsecond pulsed electric fields of Tat11, a small hydrophilic charged peptide, and of siRNA. We use in particular a MD simulation protocol that allows to characterize the transport of charged species through stable pores. Unexpectedly, our results show that for an electroporated bilayer subject to transmembrane voltages in the order of 500mV, i.e. consistent with experimental conditions, both Tat11 and siRNA can translocate through nanoelectropores within tens of ns. We discuss these results in comparison to experiments in order to rationalize the mechanism of drug uptake by cells. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Perspectives on external electric fields in molecular simulation: progress, prospects and challenges

In this review, the application of a wide variety of external electric fields in molecular simulation shall be discussed, including time-varying and electromagnetic, as well as the utility and potential impact and prospects for exploitation of such simulations for real-world and industrial end use. In particular, non-equilibrium molecular dynamics will be discussed, as well as challenges in addressing adequate thermostatting and scaling field amplitudes to more experimentally relevant levels. Attention shall be devoted to recent progress and advances in external fields in ab initio molecular simulation and dynamics, as well as elusive challenges thereof (and, to some extent, for molecular dynamics from empirical potentials), such as timescales required to observe low-frequency and intensity field effects. The challenge of deterministic molecular dynamics in external fields in sampling phase space shall be discussed, along with prospects for application of fields in enhanced-sampling simulations. Finally, the application of external electric fields to a wide variety of aqueous, nanoscale and biological systems will be discussed, often motivated by the possibility of exploitation in real-world applications, which serve to underpin our molecular-level understanding of field effects in terms of microscopic mechanisms, and possibly with a view to control thereof.

Membrane disorder and phospholipid scrambling in electropermeabilized and viable cells

Membrane electropermeabilization relies on the transient permeabilization of the plasma membrane of cells submitted to electric pulses. This method is widely used in cell biology and medicine due to its efficiency to transfer molecules while limiting loss of cell viability. However, very little is known about the consequences of membrane electropermeabilization at the molecular and cellular levels. Progress in the knowledge of the involved mechanisms is a biophysical challenge. As a transient loss of membrane cohesion is associated with membrane permeabilization, our main objective was to detect and visualize at the single-cell level the incidence of phospholipid scrambling and changes in membrane order. We performed studies using fluorescence microscopy with C6-NBD-PC and FM1-43 to monitor phospholipid scrambling and membrane order of mammalian cells. Millisecond permeabilizing pulses induced membrane disorganization by increasing the translocation of phosphatidylcholines according to an ATP-independent process. The pulses induced the formation of long-lived permeant structures that were present during membrane resealing, but were not associated with phosphatidylcholine internalization. These pulses resulted in a rapid phospholipid flip/flop within less than 1s and were exclusively restricted to the regions of the permeabilized membrane. Under such electrical conditions, phosphatidylserine externalization was not detected. Moreover, this electrically-mediated membrane disorganization was not correlated with loss of cell viability. Our results could support the existence of direct interactions between the movement of membrane zwitterionic phospholipids and the electric field.

Transient oscillation of shape and membrane conductivity changes by field pulse-induced electroporation in nano-sized phospholipid vesicles

The results of electrooptical and conductometrical measurements on unilamellar lipid vesicles (of mean radius a = 90 nm), filled with 0.2 M NaCl solution, suspended in 0.33 M sucrose solution of 0.2 mM NaCl, and exposed to a stepwise decaying electric field (time constant τE = 154 μs) in the range 10 ≤ E0 (kV cm(-1)) ≤ 90, are analyzed in terms of cyclic changes in vesicle shape and vesicle membrane conductivity. The two peaks in the dichroitic turbidity relaxations reflect two cycles of rapid membrane electroporation and slower resealing of long-lived electropores. The field-induced changes reflect structural transitions between closed (C) and porated (P) membrane states, qualified by pores of type P1 and of type P2, respectively. The transient change in the membrane conductivity and the transient shape oscillation are based on changes in the pore density of the (larger) P2-pores along a hysteresis cycle. The P2-pore formation leads to transient net ion flows across the P2-pores and to transient changes in the membrane field. The kinetic data are numerically processed in terms of coupled structural relaxation modes. Using the torus-hole pore model, the mean inner pore radii are estimated to be r1 = 0.38 (±0.05) nm and r2 = 1.7 (±0.1) nm, respectively. The observation of a transient oscillation of membrane electroporation and of shape changes in a longer lasting external field pulse is suggestive of potential resonance enhancement, for instance, of electro-uptake by, and of electro-release of biogenic molecules from, biological cells in trains of long-lasting low-intensity voltage pulses.

1.5 V battery driven reduced graphene oxide-silver nanostructure coated carbon foam (rGO-Ag-CF) for the purification of drinking water

A porous carbon foam (CF) electrode modified with a reduced graphene oxide-Ag (rGO-Ag) nanocomposite has been fabricated to purify water. It can perform as an antibacterial device by killing pathogenic microbes with the aid of a 1.5 V battery, with very little power consumption. The device is recycled ten times with good performance for long term usage. It is shown that the device may be implemented as a fast water purifier to deactivate the pathogens in drinking water.

Microdosimetry applied to nanosecond pulsed electric fields: a comparison on a single cell between real and ideal waveforms

A microdosimetric analysis using ideal and real pulses was carried out in this paper. To perform this goal, authors employed an algorithm developed recently for nsPEF based on Laplace's equation and able to take into account cell compartment dispersivity. A comparison between biphasic real and ideal waveforms was carried out. The ideal pulse induced the highest pore density efficiency, hence evidencing that a device optimization to avoid waveform degradation and losses has a fundamental impact on the performances of the delivered pulses at the single cell level.

Experimental characterization and numerical modeling of tissue electrical conductivity during pulsed electric fields for irreversible electroporation treatment planning

Irreversible electroporation is a new technique to kill cells in targeted tissue, such as tumors, through a nonthermal mechanism using electric pulses to irrecoverably disrupt the cell membrane. Treatment effects relate to the tissue electric field distribution, which can be predicted with numerical modeling for therapy planning. Pulse effects will change the cell and tissue properties through thermal and electroporation (EP)-based processes. This investigation characterizes these changes by measuring the electrical conductivity and temperature of ex vivo renal porcine tissue within a single pulse and for a 200 pulse protocol. These changes are incorporated into an equivalent circuit model for cells and tissue with a variable EP-based resistance, providing a potential method to estimate conductivity as a function of electric field and pulse length for other tissues. Finally, a numerical model using a human kidney volumetric mesh evaluated how treatment predictions vary when EP- and temperature-based electrical conductivity changes are incorporated. We conclude that significant changes in predicted outcomes will occur when the experimental results are applied to the numerical model, where the direction and degree of change varies with the electric field considered.

Electroporetic transfection of pepper protoplasts with plant potyviruses

Potyviruses are a persistent threat to bell pepper (Capsicum annuum L.) production worldwide. Much effort has been expended to study the resistance response of pepper cultivars at whole plant levels but with only limited effort at the cellular level using protoplasts. A pepper protoplast isolation procedure is available but an inoculation procedure is needed that provides consistent and highly efficient infection. An electroporation-based procedure for inoculation of potyviruses was developed using a base procedure developed for Cucumber mosaic virus (CMV). The final parameters identified for efficient potyvirus infection of pepper protoplasts involves two 25ms pulses, 200V each pulse with a 10s interval between pulses. Depending on the method of detection, e.g., ELISA versus RT-PCR, potyvirus RNA inoculum ranged from 10 to 40μg with infection detection occurring with samples of 50,000-100,000 protoplasts.

A patch clamp study on the electro-permeabilization of higher plant cells: Supra-physiological voltages induce a high-conductance, K+ selective state of the plasma membrane

Permeabilization of biological membranes by pulsed electric fields ("electroporation") is frequently used as a tool in biotechnology. However, the electrical properties of cellular membranes at supra-physiological voltages are still a topic of intensive research efforts. Here, the patch clamp technique in the whole cell and the outside out configuration was employed to monitor current-voltage relations of protoplasts derived from the tobacco culture cell line "Bright yellow-2". Cells were exposed to a sequence of voltage pulses including supra-physiological voltages. A transition from a low-conductance (~0.1 nS/pF) to a high-conductance state (~5 nS/pF) was observed when the membrane was either hyperpolarized or depolarized beyond threshold values of around -250 to -300 mV and +200 to +250 mV, respectively. Current-voltage curves obtained with ramp protocols revealed that the electro-permeabilized membrane was 5-10 times more permeable to K+ than to gluconate. The K+ channel blocker tetraethylammonium (25 mM) did not affect currents elicited by 10 ms-pulses, suggesting that the electro-permeabilization was not caused by a non-physiological activation of K+ channels. Supra-physiological voltage pulses even reduced "regular" K+ channel activity, probably due to an increase of cytosolic Ca2+ that is known to inhibit outward-rectifying K+ channels in Bright yellow-2 cells. Our data are consistent with a reversible formation of aqueous membrane pores at supra-physiological voltages.

A practical technique for measuring human biofluid conductivity using high gain-frequency characteristics

Currently, the study of ion composition and performance in human biofluids plays an important role in biomedical engineering research and technology. This field may become universal for human diagnostics; it allows early detection of different diseases in humans by measuring changes in ion behaviour in human biofluids. Practical experiments were conducted to analyse the liquid composite electrolyte conductivity in an alternating electric current field. These experiments allow the contribution of separate types of ions to the overall conductivity to be estimated. The method of estimating the concentration of active ions contained in biofluids is also introduced; it illustrates the possibility of performing qualitative and quantitative analysis over a wide range of concentrations and compositions. The authors present a procedure to determine the concentration of active liquid ions based on conductivity gain-frequency characteristic curve tracing. The experimental results validate the practical use of the proposed method. The results of this research are promising, and further investigation is required to further improve the method.

Targeted tissue ablation with nanosecond pulses

In-vivo porcine studies on the effect of nanosecond high voltage pulses on liver tissue have shown that cell death can be induced in well-defined tissue volumes without damaging collagen-predominant structures. Comparison of the experimental results with the results of a three-dimensional finite element model allowed us to determine the threshold electric field for cell death. For 30, 100 nanosecond long pulses this was found to be in the range from 12 to 15 kV/cm. Modelling of the temperature distribution in the tissue using Pennes' bioheat equation showed that the lethal effect of nanosecond pulses on cells is non-thermal. Muscle contractions, generally caused by high voltage pulses, were significantly reduced for the 100 nanosecond pulses compared to microsecond long pulses. The results of these studies indicate that high voltage nanosecond pulses reliably kill normal liver cells in vivo and therefore may be useful for liver tumor treatments.

Microdosimetry for nanosecond pulsed electric field applications: a parametric study for a single cell

A microdosimetric study of nanosecond pulsed electric fields, including dielectric dispersivity of cell compartments, is proposed in our paper. A quasi-static solution based on the Laplace equation was adapted to wideband signals and used to address the problem of electric field estimation at cellular level. The electric solution was coupled with an asymptotic electroporation model able to predict membrane pore density. An initial result of our paper is the relevance of the dielectric dispersivity, providing evidence that both the transmembrane potential and the pore density are strongly influenced by the choice of modeling used. We note the crucial role played by the dielectric properties of the membrane that can greatly impact on the poration of the cell. This can partly explain the selective action reported on cancerous cells in mixed populations, if one considers that tumor cells may present different dielectric responses. Moreover, these kinds of studies can be useful to determine the appropriate setting of nsPEF generators as well as for the design and optimization of new-generation devices.

Transmembrane potential measurements on plant cells using the voltage-sensitive dye ANNINE-6

The charging of the plasma membrane is a necessary condition for the generation of an electric-field-induced permeability increase of the plasmalemma, which is usually explained by the creation and the growth of aqueous pores. For cells suspended in physiological buffers, the time domain of membrane charging is in the submicrosecond range. Systematic measurements using Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) protoplasts stained with the fast voltage-sensitive fluorescence dye ANNINE-6 have been performed using a pulsed laser fluorescence microscopy setup with a time resolution of 5 ns. A clear saturation of the membrane voltage could be measured, caused by a strong membrane permeability increase, commonly explained by enhanced pore formation, which prevents further membrane charging by external electric field exposure. The field strength dependence of the protoplast's transmembrane potential V (M) shows strong asymmetric saturation characteristics due to the high resting potential of the plants plasmalemma. At the pole of the hyperpolarized hemisphere of the cell, saturation starts at an external field strength of 0.3 kV/cm, resulting in a measured transmembrane voltage shift of ∆V(M) = -150 mV, while on the cathodic (depolarized) cell pole, the threshold for enhanced pore formation is reached at a field strength of approximately 1.0 kV/cm and ∆V(M) = 450 mV, respectively. From this asymmetry of the measured maximum membrane voltage shifts, the resting potential of BY-2 protoplasts at the given experimental conditions can be determined to V(R) = -150 mV. Consequently, a strong membrane permeability increase occurs when the membrane voltage diverges |V(M)| = 300 mV from the resting potential of the protoplast. The largest membrane voltage change at a given external electric field occurs at the cell poles. The azimuthal dependence of the transmembrane potential, measured in angular intervals of 10° along the circumference of the cell, shows a flattening and a slight decrease at higher fields at the pole region due to enhanced pore formation. Additionally, at the hyperpolarized cell pole, a polarization reversal could be observed at an external field range around 1.0 kV/cm. This behavior might be attributed to a fast charge transfer through the membrane at the hyperpolarized pole, e.g., by voltage-gated channels.

High electric field effects on gigahertz dielectric properties of water measured with microwave microfluidic devices

Silicon microstrip line devices with 260 nm planar microfluidic channels are fabricated and used to investigate water dielectric saturation effects. Microwave scattering parameter measurements are conducted from 1 to 16 GHz under different uniform dc electric fields. When the applied dc field is increased to approximately 1 MV/cm, the measured transmission coefficient S(21) is increased up to 18 dB, which indicates a large change in water dielectric properties. Extracted water permittivity (epsilon=epsilon'+jepsilon") shows that epsilon' and epsilon" are changed up to 70% and 50%, respectively.

Gadolinium blocks membrane permeabilization induced by nanosecond electric pulses and reduces cell death

It has been widely accepted that nanosecond electric pulses (nsEP) are distinguished from micro- and millisecond duration pulses by their ability to cause intracellular effects and cell death with reduced effects on the cell plasma membrane. However, we found that nsEP-induced cell death is most likely mediated by the plasma membrane disruption. We showed that nsEP can cause long-lasting (minutes) increase in plasma membrane electrical conductance and disrupt electrolyte balance, followed by water uptake, cell swelling and blebbing. These effects of plasma membrane permeabilization could be blocked by Gd(3+) in a dose-dependent manner, with a threshold at sub-micromolar concentrations. Consequently, Gd(3+) protected cells from nsEP-induced cell death, thereby pointing to plasma membrane permeabilization as a likely primary mechanism of lethal cell damage.

Kinetics, statistics, and energetics of lipid membrane electroporation studied by molecular dynamics simulations

Membrane electroporation is the method to directly transfer bioactive substances such as drugs and genes into living cells, as well as preceding electrofusion. Although much information on the microscopic mechanism has been obtained both from experiment and simulation, the existence and nature of possible intermediates is still unclear. To elucidate intermediates of electropore formation by direct comparison with measured prepore formation kinetics, we have carried out 49 atomistic electroporation simulations on a palmitoyl-oleoyl-phosphatidylcholine bilayer for electric field strengths between 0.04 and 0.7 V/nm. A statistical theory is developed to facilitate direct comparison of experimental (macroscopic) prepore formation kinetics with the (single event) preporation times derived from the simulations, which also allows us to extract an effective number of lipids involved in each pore formation event. A linear dependency of the activation energy for prepore formation on the applied field is seen, with quantitative agreement between experiment and simulation. The distribution of preporation times suggests a four-state pore formation model. The model involves a first intermediate characterized by a differential tilt of the polar lipid headgroups on both leaflets, and a second intermediate (prepore), where a polar chain across the bilayer is formed by 3-4 lipid headgroups and several water molecules, thereby providing a microscopic explanation for the polarizable volume derived previously from the measured kinetics. An average pore radius of 0.47 +/- 0.15 nm is seen, in favorable agreement with conductance measurements and electrooptical experiments of lipid vesicles.

AFM study on the electric-field effects on supported bilayer lipid membranes

Electric-field induced changes in structure and conductivity of supported bilayer lipid membranes (SLM) have been studied at submicroscopic resolution using atomic force microscopy and electrochemical impedance spectroscopy. The SLMs are formed on gold surfaces modified with mixed self-assembled monolayers of a cholesterol-tether and 6-mercaptohexanol. At applied potentials of < or =-0.25 V versus standard hydrogen electrode, the conductance of the SLM increases and membrane areas of <150 nm in size are found to elevate from the surface up to 15 nm in height. To estimate the electric field experienced by the lipid membrane, electrowetting has been used to determine the point of zero charge of a 6-mercaptohexanol-modified surface (0.19 +/- 0.13 V versus standard hydrogen electrode). The effects of electric fields on the structure and conductance of supported membranes are discussed.