Cancellation of cellular responses to nanoelectroporation by reversing the stimulus polarity

The Effects of Bipolar Cancellation Phenomenon on Nano-Electrochemotherapy of Melanoma Tumors: In Vitro and In Vivo Pilot

The phenomenon known as bipolar cancellation is observed when biphasic nanosecond electric field pulses are used, which results in reduced electroporation efficiency when compared to unipolar pulses of the same parameters. Basically, the negative phase of the bipolar pulse diminishes the effect of the positive phase. Our study aimed to investigate how bipolar cancellation affects Ca2+ electrochemotherapy and cellular response under varying electric field intensities and pulse durations (3-7 kV/cm, 100, 300, and 500 ns bipolar 1 MHz repetition frequency pulse bursts, n = 100). As a reference, standard microsecond range parametric protocols were used (100 µs × 8 pulses). We have shown that the cancellation effect is extremely strong when the pulses are closely spaced (1 MHz frequency), which results in a lack of cell membrane permeabilization and consequent failure of electrochemotherapy in vitro. To validate the observations, we have performed a pilot in vivo study where we compared the efficacy of monophasic (5 kV/cm × ↑500 ns × 100) and biphasic sequences (5 kV/cm × ↑500 ns + ↓500 ns × 100) delivered at 1 MHz frequency in the context of Ca2+ electrochemotherapy (B16-F10 cell line, C57BL/6 mice, n = 24). Mice treated with bipolar pulses did not exhibit prolonged survival when compared to the untreated control (tumor-bearing mice); therefore, the bipolar cancellation phenomenon was also occurrent in vivo, significantly impairing electrochemotherapy. At the same time, the efficacy of monophasic nanosecond pulses was comparable to 1.4 kV/cm × 100 µs × 8 pulses sequence, resulting in tumor reduction following the treatment and prolonged survival of the animals.

Modulating Ca2+ influx into adrenal chromaffin cells with short-duration nanosecond electric pulses

Isolated bovine adrenal chromaffin cells exposed to single 2-, 4-, or 5-ns pulses undergo a rapid, transient rise in intracellular Ca2+ mediated by Ca2+ entry via voltage-gated Ca2+ channels (VGCCs), mimicking the activation of these cells in vivo by acetylcholine. However, pulse durations 150 ns or longer elicit larger amplitude and longer-lived Ca2+ responses due to Ca2+ influx via both VGCCs and a yet to be identified plasma membrane pathway(s). To further our understanding of the differential effects of ultrashort versus longer pulse durations on Ca2+ influx, chromaffin cells were loaded with calcium green-1 and exposed to single 3-, 5-, 11-, 25-, or 50-ns pulses applied at their respective Ca2+ activation threshold electric fields. Increasing pulse duration from 3 or 5 ns to only 11 ns was sufficient to elicit increased amplitude and longer-lived Ca2+ responses in the majority of cells, a trend that continued as pulse duration increased to 50 ns. The amplification of Ca2+ responses was not the result of Ca2+ release from intracellular stores and was accompanied by a decreased effectiveness of VGCC inhibitors to block the responses and a reduced reliance on extracellular Na+ and membrane depolarization to evoke the responses. Inhibitors of pannexin channels, P2X receptors, or non-selective cation channels failed to attenuate 50-ns-elicited Ca2+ responses, ruling out these Ca2+-permeable channels as secondary Ca2+ entry pathways. Analytical calculations and numerical modeling suggest that the parameter that best determines the response of chromaffin cells to increasing pulse durations is the time the membrane charges to its peak voltage. These results highlight the pronounced sensitivity of a neuroendocrine cell to pulse durations differing by only tens of nanoseconds, which has important implications for the future development of nanosecond pulse technologies enabling electrostimulation applications for spatially focused and graded in vivo neuromodulation.

Threshold Interphase Delay for Bipolar Pulses to Prevent Cancellation Phenomenon during Electrochemotherapy

Electroporation-based procedures employing nanosecond bipolar pulses are commonly linked to an undesirable phenomenon known as the cancelation effect. The cancellation effect arises when the second pulse partially or completely neutralizes the effects of the first pulse, simultaneously diminishing cells' plasma membrane permeabilization and the overall efficiency of the procedure. Introducing a temporal gap between the positive and negative phases of the bipolar pulses during electroporation procedures may help to overcome the cancellation phenomenon; however, the exact thresholds are not yet known. Therefore, in this work, we have tested the influence of different interphase delay values (from 0 ms to 95 ms) using symmetric bipolar nanoseconds (300 and 500 ns) on cell permeabilization using 10 Hz, 100 Hz, and 1 kHz protocols. As a model mouse hepatoma, the MH-22a cell line was employed. Additionally, we conducted in vitro electrochemotherapy with cisplatin, employing reduced interphase delay values (0 ms and 0.1 ms) at 10 Hz. Cell plasma membrane permeabilization and viability dependence on a variety of bipolar pulsed electric field protocols were characterized. It was shown that it is possible to minimize bipolar cancellation, enabling treatment efficiency comparable to monophasic pulses with identical parameters. At the same time, it was highlighted that bipolar cancellation has a significant influence on permeabilization, while the effects on the outcome of electrochemotherapy are minimal.

Effects of buffer composition and plasmid toxicity on electroporation-based non-viral gene delivery in mammalian cells using bursts of nanosecond and microsecond pulses

Gene electrotransfer (GET) is non-viral gene delivery technique, also known as electroporation-mediated gene delivery or electrotransfection. GET is a method used to introduce foreign genetic material (such as DNA or RNA) into cells by applying external pulsed electric fields (PEFs) to create temporary pores in the cell membrane. This study was undertaken to examine the impact of buffer composition on the efficiency of GET in mammalian cells Also, we specifically compared the effectiveness of high-frequency nanosecond (ns) pulses with standard microsecond (µs) pulses. For the assessment of cell transfection efficiency and viability, flow cytometric analysis, luminescent assays, and measurements of metabolic activity were conducted. The efficiency of electrotransfection was evaluated using two different proteins encoding plasmids (pEGFP-N1 and Luciferase-pcDNA3). The investigation revealed that the composition of the electroporation buffer significantly influences the efficacy of GET in CHO-K1 cell line. The different susceptibility of cell lines to the electric field and the plasmid cytotoxicity were reported. It was also shown that electroporation with nanosecond duration PEF protocols ensured equivalent or even better transfection efficiency than standard µsPEF. Additionally, we successfully performed long-term transfection of the murine 4T1 cell line using high-frequency nanosecond PEFs and confirmed its' applicability in an in vivo model. The findings from the study can be applied to optimize electrotransfection conditions.

Integrated Time Nanosecond Pulse Irreversible Electroporation (INSPIRE): Assessment of Dose, Temperature, and Voltage on Experimental and Clinical Treatment Outcomes

OBJECTIVE

This study sought to investigate a novel strategy using temperature-controlled delivery of nanosecond pulsed electric fields as an alternative to the 50-100 microsecond pulses used for irreversible electroporation.

METHODS

INSPIRE treatments were carried out at two temperatures in 3D tumor models using doses between 0.001 s and 0.1 s. The resulting treatment zones were quantified using viability staining and lethal electric field intensities were determined numerically. Computational modeling was then used to determine parameters necessary for INSPIRE treatments to achieve equivalent treatment zones to clinical electroporation treatments and evaluate the potential for these treatments to induce deleterious thermal damage.

RESULTS

Lethal thresholds between 1109 and 709 V/cm were found for nominal 0.01 s treatments with pulses between 350 ns and 2000 ns at physiological temperatures. Further increases in dose resulted in significant decreases in lethal thresholds. Given these experimental results, treatment zones comparable to clinical electroporation are possible by increasing the dose and voltage used with nanosecond duration pulses. Temperature-controlled simulations indicate minimal thermal cell death while achieving equivalent treatment volumes to clinical electroporation.

CONCLUSION

Nanosecond electrical pulses can achieve comparable outcomes to traditional electroporation provided sufficient electrical doses or voltages are applied. The use of temperature-controlled delivery may minimize thermal damage during treatment.

SIGNIFICANCE

Intense muscle stimulation and the need for cardiac gating have limited irreversible electroporation. Nanosecond pulses can alleviate these challenges, but traditionally have produced significantly smaller treatment zones. This study suggests that larger ablation volumes may be possible with the INSPIRE approach and that future in vivo studies are warranted.

Therapeutic perspectives of high pulse repetition rate electroporation

Electroporation, a technique that uses electrical pulses to temporarily or permanently destabilize cell membranes, is increasingly used in cancer treatment, gene therapy, and cardiac tissue ablation. Although the technique is efficient, patients report discomfort and pain. Current strategies that aim to minimize pain and muscle contraction rely on the use of pharmacological agents. Nevertheless, technical improvements might be a valuable tool to minimize adverse events, which occur during the application of standard electroporation protocols. One recent technological strategy involves the use of high pulse repetition rate. The emerging technique, also referred as "high frequency" electroporation, employs short (micro to nanosecond) mono or bipolar pulses at repetition rate ranging from a few kHz to a few MHz. This review provides an overview of the historical background of electric field use and its development in therapies over time. With the aim to understand the rationale for novel electroporation protocols development, we briefly describe the physiological background of neuromuscular stimulation and pain caused by exposure to pulsed electric fields. Then, we summarize the current knowledge on electroporation protocols based on high pulse repetition rates. The advantages and limitations of these protocols are described from the perspective of their therapeutic application.

Negative effects of cancellation during nanosecond range High-Frequency calcium based electrochemotherapy in vitro

Drug delivery using nanosecond pulsed electric fields is a new branch of electroporation-based treatments, which potentially can substitute European standard operating procedures for electrochemotherapy. In this work, for the first time, we characterize the effects of ultra-fast repetition frequency (1-2.5 MHz) nanosecond pulses (5-9 kV/cm, 200 and 400 ns) in the context of nano-electrochemotherapy with calcium. Additionally, we investigate the feasibility of bipolar symmetric (↑200 ns + ↓200 ns) and asymmetric (↑200 ns + ↓400 ns) nanosecond protocols for calcium delivery. The effects of bipolar cancellation and the influence of interphase delay (200 ns) are overviewed. Human lung cancer cell lines A549 and H69AR were used as a model. It was shown that unipolar pulses delivered at high frequency are effective for electrochemotherapy with a significant improvement in efficiency when the delay between separate pulses is reduced. Bipolar symmetric pulses trigger the cancellation phenomenon limiting applications for drug delivery and can be compensated by the asymmetry of the pulse (↑200 ns + ↓400 ns or ↑400 ns + ↓200 ns). The results of this study can be successfully used to derive a new generation of nsPEF protocols for successful electrochemotherapy treatments.

Ultra-Low Intensity Post-Pulse Affects Cellular Responses Caused by Nanosecond Pulsed Electric Fields

High-intensity nanosecond pulse electric fields (nsPEF) can preferentially induce various effects, most notably regulated cell death and tumor elimination. These effects have almost exclusively been shown to be associated with nsPEF waveforms defined by pulse duration, rise time, amplitude (electric field), and pulse number. Other factors, such as low-intensity post-pulse waveform, have been completely overlooked. In this study, we show that post-pulse waveforms can alter the cell responses produced by the primary pulse waveform and can even elicit unique cellular responses, despite the primary pulse waveform being nearly identical. We employed two commonly used pulse generator designs, namely the Blumlein line (BL) and the pulse forming line (PFL), both featuring nearly identical 100 ns pulse durations, to investigate various cellular effects. Although the primary pulse waveforms were nearly identical in electric field and frequency distribution, the post-pulses differed between the two designs. The BL's post-pulse was relatively long-lasting (~50 µs) and had an opposite polarity to the main pulse, whereas the PFL's post-pulse was much shorter (~2 µs) and had the same polarity as the main pulse. Both post-pulse amplitudes were less than 5% of the main pulse, but the different post-pulses caused distinctly different cellular responses. The thresholds for dissipation of the mitochondrial membrane potential, loss of viability, and increase in plasma membrane PI permeability all occurred at lower pulsing numbers for the PFL than the BL, while mitochondrial reactive oxygen species generation occurred at similar pulsing numbers for both pulser designs. The PFL decreased spare respiratory capacity (SRC), whereas the BL increased SRC. Only the PFL caused a biphasic effect on trans-plasma membrane electron transport (tPMET). These studies demonstrate, for the first time, that conditions resulting from low post-pulse intensity charging have a significant impact on cell responses and should be considered when comparing the results from similar pulse waveforms.

Next generation CANCAN focusing for remote stimulation by nanosecond electric pulses

Focusing electric pulse effects away from electrodes is a challenge because the electric field weakens with distance. Previously we introduced a remote focusing method based on bipolar cancellation, a phenomenon of low efficiency of bipolar nanosecond electric pulses (nsEP). Superpositioning two bipolar nsEP into a unipolar pulse canceled bipolar cancellation ("CANCAN" effect), enhancing bioeffects at a distance despite the electric field weakening. Here, we introduce the next generation (NG) CANCAN focusing with unipolar nsEP packets designed to produce bipolar waveforms near electrodes (suppressing electroporation) but not at the remote target. NG-CANCAN was tested in CHO cell monolayers using a quadrupole electrode array and labeling electroporated cells with YO-PRO-1 dye. We routinely achieved 1.5-2 times stronger electroporation in the center of the quadrupole than near electrodes, despite a 3-4-fold field attenuation. With the array lifted 1-2 mm above the monolayer (imitating a 3D treatment), the remote effect was enhanced up to 6-fold. We analyzed the role of nsEP number, amplitude, rotation, and inter-pulse delay, and showed how remote focusing is enhanced when re-created bipolar waveforms exhibit stronger cancellation. Advantages of NG-CANCAN include the exceptional versatility of designing pulse packets and easy remote focusing using an off-the-shelf 4-channel nsEP generator.

Control of the Electroporation Efficiency of Nanosecond Pulses by Swinging the Electric Field Vector Direction

Reversing the pulse polarity, i.e., changing the electric field direction by 180°, inhibits electroporation and electrostimulation by nanosecond electric pulses (nsEPs). This feature, known as "bipolar cancellation," enables selective remote targeting with nsEPs and reduces the neuromuscular side effects of ablation therapies. We analyzed the biophysical mechanisms and measured how cancellation weakens and is replaced by facilitation when nsEPs are applied from different directions at angles from 0 to 180°. Monolayers of endothelial cells were electroporated by a train of five pulses (600 ns) or five paired pulses (600 + 600 ns) applied at 1 Hz or 833 kHz. Reversing the electric field in the pairs (180° direction change) caused 2-fold (1 Hz) or 20-fold (833 kHz) weaker electroporation than the train of single nsEPs. Reducing the angle between pulse directions in the pairs weakened cancellation and replaced it with facilitation at angles <160° (1 Hz) and <130° (833 kHz). Facilitation plateaued at about three-fold stronger electroporation compared to single pulses at 90-100° angle for both nsEP frequencies. The profound dependence of the efficiency on the angle enables novel protocols for highly selective focal electroporation at one electrode in a three-electrode array while avoiding effects at the other electrodes. Nanosecond-resolution imaging of cell membrane potential was used to link the selectivity to charging kinetics by co- and counter-directional nsEPs.

The influence of asymmetrical bipolar pulses and interphase intervals on the bipolar cancellation phenomenon in the ovarian cancer cell line

The application of negative polarity electrical pulse (↓) following positive polarity pulses (↑) may induce bipolar cancellation (BPC), a unique physiological response believed to be specific to nanosecond electroporation (nsEP). The literature lacks analysis of bipolar electroporation (BP EP) involving asymmetrical sequences composed of nanosecond and microsecond pulses. Moreover, the impact of interphase interval on BPC caused by such asymmetrical pulse needs consideration. In this study, the authors utilized the ovarian clear carcinoma cell line (OvBH-1) model to investigate the BPC with asymmetrical sequences. Cells were exposed to pulses delivered in 10-pulse bursts but as uni- or bipolar, symmetrical, or asymmetrical sequences with a duration of 600 ns or 10 µs and electric field strength equal to 7.0 or 1.8 kV/cm, respectively. It was shown that the asymmetry of pulses influences BPC. The obtained results have also been investigated in the context of calcium electrochemotherapy. The reduction of cell membrane poration, and cell survival have been observed following Ca²⁺ electrochemotherapy. The effects of interphase delays (1 and 10 µs) on the BPC phenomenon were reported. Our findings show that the BPC phenomenon can be controlled using pulse asymmetry or delay between the positive and negative polarity of the pulse.

A molecular dynamics study of phospholipid membrane electroporation induced by bipolar pulses with different intervals

The mechanism of changes in cell electroporation (EP) during the intervals of bipolar pulses is still unclear, and few studies have investigated the effect of the intervals at the molecular level. In this study, EP induced by bipolar pulses (BP) with different intervals was investigated using all-atom molecular dynamics simulations. Firstly, EP was formed during the positive pulses of 2 ns and 0.5 V nm^-1, then the effects of various intervals of 0, 1, 5, and 10 ns on EP evolution were investigated, and the dynamic changes of different degrees of EP induced by the following negative pulses of 2 ns and 0.5 V nm^-1 were analyzed. The elimination effect of intervals was determined and it was related to the degrees of EP and the time of intervals. At the last moment of the intervals the phospholipid membrane was classified and quantitatively defined in three states according to the degrees of EP, namely, Resealing, Destabilizing and Retaining states. These states appeared due to the combined effect of both the positive pulse and the interval, and the states represent the degrees of EP which had different responses after applying the negative pulse. These results can improve our understanding of the fundamental mechanism of BP-induced EP.

Advances in pulsed electric stimuli as a physical method for treating liquid foods

There is a need for alternative technologies that can deliver safe and nutritious foods at lower costs as compared to conventional processes. Pulsed electric field (PEF) technology has been utilised for a plethora of different applications in the life and physical sciences, such as gene/drug delivery in medicine and extraction of bioactive compounds in food science and technology. PEF technology for treating liquid foods involves engineering principles to develop the equipment, and quantitative biochemistry and microbiology techniques to validate the process. There are numerous challenges to address for its application in liquid foods such as the 5-log pathogen reduction target in food safety, maintaining the food quality, and scale up of this physical approach for industrial integration. Here, we present the engineering principles associated with pulsed electric fields, related inactivation models of microorganisms, electroporation and electropermeabilization theory, to increase the quality and safety of liquid foods; including water, milk, beer, wine, fruit juices, cider, and liquid eggs. Ultimately, we discuss the outlook of the field and emphasise research gaps.

Action spectra and mechanisms of (in) efficiency of bipolar electric pulses at electroporation

The reversal of the electric field direction inhibits various biological effects of nanosecond electric pulses (nsEP). This feature, known as "bipolar cancellation," enables interference targeting of nsEP bioeffects remotely from stimulating electrodes, for prospective applications such as precise cancer ablation and non-invasive deep brain stimulation. This study was undertaken to achieve the maximum cancellation of electroporation, by quantifying the impact of the pulse shape, duration, number, and repetition rate across a broad range of electric field strengths. Monolayers of endothelial cells (BPAE) were electroporated in a non-uniform electric field. Cell membrane permeabilization was quantified by YO-PRO-1 (YP) dye uptake and correlated to local electric field strength. For most conditions tested, adding an opposite polarity phase reduced YP uptake by 50-80 %. The strongest cancellation, which reduced YP uptake by 95-97 %, was accomplished by adding a 50 % second phase to 600-ns pulses delivered at a high repetition rate of 833 kHz. Strobe photography of nanosecond kinetics of membrane potential in single CHO cells revealed the temporal summation of polarization by individual unipolar nsEP applied at sub-MHz rate, leading to enhanced electroporation. In contrast, there was no summation for bipolar pulses, and increasing their repetition rate suppressed electroporation. These new findings are discussed in the context of bipolar cancellation mechanisms and remote focusing applications.

Nanosecond PEF Induces Oxidative Stress and Apoptosis via Proteasomal Activity Inhibition in Gastric Adenocarcinoma Cells with Drug Resistance

Nanosecond (ns) pulsed electric field (PEF) is a technology in which the application of ultra-short electrical pulses can be used to disrupt the barrier function of cell plasma and internal membranes. Disruptions of the membrane integrity cause a substantial imbalance in cell homeostasis in which oxidative stress is a principal component. In the present study, nsPEF-induced oxidative stress was investigated in two gastric adenocarcinoma cell lines (EPG85-257P and EPG85-257RDB) which differ by their sensitivity to daunorubicin. Cells were exposed to 200 pulses of 10 ns duration, with the amplitude and pulse repetition frequency at 1 kHz, with electric field intensity varying from 12.5 to 50 kV/cm. The electroporation buffer contained either 1 mM or 2 mM calcium chloride. CellMask DeepRed visualized cell plasma permeabilization, Fluo-4 was used to visualize internal calcium ions content, and F-actin was labeled with AlexaFluor^®488 for the cytoskeleton. The cellular viability was determined by MTT assay. An alkaline and neutral comet assay was employed to detect apoptotic and necrotic cell death. The luminescent method estimated the modifications in GSSG/GSH redox potential and the imbalance of proteasomal activity (chymotrypsin-, trypsin- and caspase-like). The reactive oxygen species (ROS) level was measured by flow cytometry using dihydroethidium (DHE) dye. Morphological visualization indicated cell shrinkage, affected cell membranes (characteristic bubbles and changed cell shape), and the reorganization of actin fibers with sites of its dense concentration; the effect was more intense with the increasing electric field strength. The most significant decrease in cell viability and GSSG/GSH redox potential was noted at the highest amplitude of 50 kV/cm, and calcium ions amplified this effect. nsPEF, particularly with calcium ions, inhibited proteasomal activities, resulting in increased protein degradation. nsPEF increased the percentage of apoptotic cells and ROS levels. The EPG85-257 RDB cell line, which is resistant to standard chemotherapy, was more sensitive to applied nsPEF protocols. The applied nsPEF method disrupted the metabolism of cancer cells and induced apoptotic cell death. The nsPEF ability to cause apoptosis, oxidative stress, and protein degradation make the nsPEF methodology a suitable alternative to current anticancer pharmacological methods.

Does the shape of the electric pulse matter in electroporation?

Electric pulses are widely used in biology, medicine, industry, and food processing. Numerous studies indicate that electroporation (EP) is a pulse-dependent process, and the electric pulse shape and duration strongly determine permeabilization efficacy. EP protocols are precisely planned in terms of the size and charge of the molecules, which will be delivered to the cell. In reversible and irreversible EP applications, rectangular or sine, polar or bipolar pulses are commonly used. The usage of pulses of the asymmetric shape is still limited to high voltage and low voltage (HV/LV) sequences in the context of gene delivery, while EP-based applications of ultra-short asymmetric pulses are just starting to emerge. This review emphasizes the importance and role of the pulse shape for membrane permeabilization by EP.

Electroporation and cell killing by milli- to nanosecond pulses and avoiding neuromuscular stimulation in cancer ablation

Ablation therapies aim at eradication of tumors with minimal impact on surrounding healthy tissues. Conventional pulsed electric field (PEF) treatments cause pain and muscle contractions far beyond the ablation area. The ongoing quest is to identify PEF parameters efficient at ablation but not at stimulation. We measured electroporation and cell killing thresholds for 150 ns–1 ms PEF, uni- and bipolar, delivered in 10- to 300-pulse trains at up to 1 MHz rates. Monolayers of murine colon carcinoma cells exposed to PEF were stained with YO-PRO-1 dye to detect electroporation. In 2–4 h, dead cells were labeled with propidium. Electroporation and cell death thresholds determined by matching the stained areas to the electric field intensity were compared to nerve excitation thresholds (Kim et al. in Int J Mol Sci 22(13):7051, 2021). The minimum fourfold ratio of cell killing and stimulation thresholds was achieved with bipolar nanosecond PEF (nsPEF), a sheer benefit over a 500-fold ratio for conventional 100-µs PEF. Increasing the bipolar nsPEF frequency up to 100 kHz within 10-pulse bursts increased ablation thresholds by < 20%. Restricting such bursts to the refractory period after nerve excitation will minimize the number of neuromuscular reactions while maintaining the ablation efficiency and avoiding heating.

An Investigation for Large Volume, Focal Blood-Brain Barrier Disruption with High-Frequency Pulsed Electric Fields

The treatment of CNS disorders suffers from the inability to deliver large therapeutic agents to the brain parenchyma due to protection from the blood-brain barrier (BBB). Herein, we investigated high-frequency pulsed electric field (HF-PEF) therapy of various pulse widths and interphase delays for BBB disruption while selectively minimizing cell ablation. Eighteen male Fisher rats underwent craniectomy procedures and two blunt-tipped electrodes were advanced into the brain for pulsing. BBB disruption was verified with contrast T1W MRI and pathologically with Evans blue dye. High-frequency irreversible electroporation cell death of healthy rodent astrocytes was investigated in vitro using a collagen hydrogel tissue mimic. Numerical analysis was conducted to determine the electric fields in which BBB disruption and cell ablation occur. Differences between the BBB disruption and ablation thresholds for each waveform are as follows: 2-2-2 μs (1028 V/cm), 5-2-5 μs (721 V/cm), 10-1-10 μs (547 V/cm), 2-5-2 μs (1043 V/cm), and 5-5-5 μs (751 V/cm). These data suggest that HF-PEFs can be fine-tuned to modulate the extent of cell death while maximizing peri-ablative BBB disruption. Furthermore, numerical modeling elucidated the diffuse field gradients of a single-needle grounding pad configuration to favor large-volume BBB disruption, while the monopolar probe configuration is more amenable to ablation and reversible electroporation effects.

Electrotherapies for Glioblastoma

Non-thermal, intermediate frequency (100-500 kHz) electrotherapies present a unique therapeutic strategy to treat malignant neoplasms. Here, pulsed electric fields (PEFs) which induce reversible or irreversible electroporation (IRE) and tumour-treating fields (TTFs) are reviewed highlighting the foundations, advances, and considerations of each method when applied to glioblastoma (GBM). Several biological aspects of GBM that contribute to treatment complexity (heterogeneity, recurrence, resistance, and blood-brain barrier(BBB)) and electrophysiological traits which are suggested to promote glioma progression are described. Particularly, the biological responses at the cellular and molecular level to specific parameters of the electrical stimuli are discussed offering ways to compare these parameters despite the lack of a universally adopted physical description. Reviewing the literature, a disconnect is found between electrotherapy techniques and how they target the biological complexities of GBM that make treatment difficult in the first place. An attempt is made to bridge the interdisciplinary gap by mapping biological characteristics to different methods of electrotherapy, suggesting important future research topics and directions in both understanding and treating GBM. To the authors' knowledge, this is the first paper that attempts an in-tandem assessment of the biological effects of different aspects of intermediate frequency electrotherapy methods, thus offering possible strategies toward GBM treatment.

Strobe photography mapping of cell membrane potential with nanosecond resolution

The ability to directly observe membrane potential charging dynamics across a full microscopic field of view is vital for understanding interactions between a biological system and a given electrical stimulus. Accurate empirical knowledge of cell membrane electrodynamics will enable validation of fundamental hypotheses posited by the single shell model, which includes the degree of voltage change across a membrane and cellular sensitivity to external electric field non-uniformity and directionality. To this end, we have developed a high-speed strobe microscopy system with a time resolution of ~ 6 ns that allows us to acquire time-sequential data for temporally repeatable events (non-injurious electrostimulation). The imagery from this system allows for direct comparison of membrane voltage change to both computationally simulated external electric fields and time-dependent membrane charging models. Acquisition of a full microscope field of view enables the selection of data from multiple cell locations experiencing different electrical fields in a single image sequence for analysis. Using this system, more realistic membrane parameters can be estimated from living cells to better inform predictive models. As a proof of concept, we present evidence that within the range of membrane conductivity used in simulation literature, higher values are likely more valid.

Effects of high-frequency nanosecond pulses on prostate cancer cells

Electroporation with pulsed electric fields show a potential to be applied as an experimental focal therapy of tumors. Sub-microsecond regime of electric pulses displays unique electrophysical features operative in cells and membranes. Recently, MHz compression of nanosecond pulses electric fields (nsPEFs) bursts proved to enhance the effectiveness of the therapy. High morbidity of prostate cancer (PCa) and risk of overtreatment associated with this malignancy call for new minimal-invasive treatment alternative. Herein we present the in vitro study for developing applications based on this new technology. In this study, we used flow cytometric analysis, cell viability assay, caspase activity analysis, wound healing assay, confocal microscopy study, and immunofluorescence to investigate the biological effect of high-frequency nsPEFs on PCa cells. Our results show that high-frequency nsPEFs induces the permeabilization and cell death of PCa cells. The cytotoxicity is significantly enhanced in MHz compression of pulses and with the presence of extracellular Ca²⁺. High-frequency nsPEFs trigger changes in PCa cells' cytoskeleton and their mobility. The presented data show a therapeutic potential of high-frequency nsPEFs in a PCa setting. The sub-microsecond regime of pulses can potentially be applied in nanosecond electroporation protocols for PCa treatment.

Multiphysics modelling of electroporation under uni- or bipolar nanosecond pulse sequences

A nonlinear dispersive multiphysics model of single-cell electroporation was proposed in this paper. The time-domain Debye model was utilised to describe the membrane dispersion while the dynamic pore radius function was deployed to modify the plasma membrane conductivity. The dynamic spatial distributions of the ion concentration were dominated by the Nernst-Planck function. First, a single nanosecond pulsed electric field was applied to verify our model and to explore the effects of dispersion and dynamic pore radius on the redistribution of the electric field. The dispersive membrane was found to increase the transmembrane potential, expedite the electroporation process, and weaken the membrane permeability; however, adding the dynamic pore radius function had the opposite effect on transmembrane potential and membrane permeability. The responses of the cells exposed to unipolar and bipolar nanosecond pulse sequences were subsequently simulated. During the application of unipolar pulse sequences, the pore radius and perforation area showed a step-like accumulation, and significant increases in the perforation area and intracellular ion concentration were observed with higher frequency pulse sequences and wider subpulse intervals. The bipolar cancellation effect was also observed in terms of membrane permeability and pore radius.

Interference targeting of bipolar nanosecond electric pulses for spatially focused electroporation, electrostimulation, and tissue ablation

Stimulation and electroporation by nanosecond electric pulses (nsEP) are distinguished by a phenomenon of bipolar cancellation, which stands for a reduced efficiency of bipolar pulses compared to unipolar ones. When two pairs of stimulating electrodes are arrayed in a quadrupole, bipolar cancellation inhibits nsEP effects near the electrodes, where the electric field is the strongest. Two properly shaped and synchronized bipolar nsEP overlay into a unipolar pulse towards the center of the electrode array, thus canceling the bipolar cancellation (a "CANCAN effect"). High efficiency of the re-created unipolar nsEP outweighs the weakening of the electric field with distance and focuses nsEP effects to the center. In monolayers of CHO, BPAE, and HEK cells, CANCAN effect achieved by the interference of two bipolar nsEP enhanced electroporation up to tenfold, with a peak at the quadrupole center. Introducing a time interval between bipolar nsEP prevented the formation of a unipolar pulse and eliminated the CANCAN effect. Strong electroporation by CANCAN stimuli killed cells over the entire area encompassed by the electrodes, whereas the time-separated pulses caused ablation only in the strongest electric field near the electrodes. The CANCAN approach is promising for uniform tumor ablation and stimulation targeting away from electrodes.

A Theoretical Argument for Extended Interpulse Delays in Therapeutic High-Frequency Irreversible Electroporation Treatments

High-frequency irreversible electroporation (H-FIRE) is a tissue ablation modality employing bursts of electrical pulses in a positive phase-interphase delay (d1)-negative phase-interpulse delay (d2) pattern. Despite accumulating evidence suggesting the significance of these delays, their effects on therapeutic outcomes from clinically-relevant H-FIRE waveforms have not been studied extensively.

OBJECTIVE

We sought to determine whether modifications to the delays within H-FIRE bursts could yield a more desirable clinical outcome in terms of ablation volume versus extent of tissue excitation.

METHODS

We used a modified spatially extended nonlinear node (SENN) nerve fiber model to evaluate excitation thresholds for H-FIRE bursts with varying delays. We then calculated non-thermal tissue ablation, thermal damage, and excitation in a clinically relevant numerical model.

RESULTS

Excitation thresholds were maximized by shortening d1, and extension of d2 up to 1,000 μs increased excitation thresholds by at least 60% versus symmetric bursts. In the ablation model, long interpulse delays lowered the effective frequency of burst waveforms, modulating field redistribution and reducing heat production. Finally, we demonstrate mathematically that variable delays allow for increased voltages and larger ablations with similar extents of excitation as symmetric waveforms.

CONCLUSION

Interphase and interpulse delays play a significant role in outcomes resulting from H-FIRE treatment.

SIGNIFICANCE

Waveforms with short interphase delays (d1) and extended interpulse delays (d2) may improve therapeutic efficacy of H-FIRE as it emerges as a clinical tissue ablation modality.

High-Frequency and High-Voltage Asymmetric Bipolar Pulse Generator for Electroporation Based Technologies and Therapies

Currently, in high-frequency electroporation, much progress has been made but limited to research groups with custom-made laboratory prototype electroporators. According to the review of electroporators and economic evaluations, there is still an area of pulse parameters that needs to be investigated. The development of an asymmetric bipolar pulse generator with a maximum voltage of 4 kV and minimum duration time of a few hundred nanoseconds, would enable in vivo evaluation of biological effects of high-frequency electroporation pulses. Herein, from a series of most commonly used drivers and optical isolations in high-voltage pulse generators the one with optimal characteristics was used. In addition, the circuit topology of the developed device is described in detail. The developed device is able to generate 4 kV pulses, with theoretical 131 A maximal current and 200 ns minimal pulse duration, the maximal pulse repetition rate is 2 MHz and the burst maximal repetition rate is 1 MHz. The device was tested in vivo. The effectiveness of electrochemotherapy of high-frequency electroporation pulses is compared to “classical” electrochemotherapy pulses. In vivo electrochemotherapy with high-frequency electroporation pulses was at least as effective as with “classical” well-established electric pulses, resulting in 86% and 50% complete responses, respectively. In contrast to previous reports, however, muscle contractions were comparable between the two protocols.

A nanosecond pulsed electric field (nsPEF) can affect membrane permeabilization and cellular viability in a 3D spheroids tumor model

Three-dimensional (3D) cellular models represent more realistically the complexity of in vivo tumors compared to 2D cultures. While 3D models were largely used in classical electroporation, the effects of nanosecond pulsed electric field (nsPEF) have been poorly investigated. In this study, we evaluated the biological effects induced by nsPEF on spheroid tumor model derived from the HCT-116 human colorectal carcinoma cell line. By varying the number of pulses (from 1 to 500) and the polarity (unipolar and bipolar), the response of nsPEF exposure (10 ns duration, 50 kV/cm) was assessed either immediately after the application of the pulses or over a period lasting up to 6 days. Membrane permeabilization and cellular death occurred following the application of at least 100 pulses. The extent of the response increased with the number of pulses, with a significant decrease of viability, 24 h post-exposure, when 250 and 500 pulses were applied. The effects were highly reduced when an equivalent number of bipolar pulses were delivered. This reduction was eliminated when a 100 ns interphase interval was introduced into the bipolar pulses. Altogether, our results show that nsPEF effects, previously observed at the single cell level, also occur in more realistic 3D tumor spheroids models.

Analysis of electrostimulation and electroporation by high repetition rate bursts of nanosecond stimuli

Exposures to short-duration, strong electric field pulses have been utilized for stimulation, ablation, and the delivery of molecules into cells. Ultrashort, nanosecond duration pulses have shown unique benefits, but they require higher field strengths. One way to overcome this requirement is to use trains of nanosecond pulses with high repetition rates, up to the MHz range. Here we present a theoretical model to describe the effects of pulse trains on the plasma membrane and intracellular membranes modeled as resistively charged capacitors. We derive the induced membrane potential and the stimulation threshold as functions of pulse number, pulse duration, and repetition rate. This derivation provides a straightforward method to calculate the membrane charging time constant from experimental data. The derived excitation threshold agrees with nerve stimulation experiments, indicating that nanosecond pulses are not more effective than longer pulses in charging nerve fibers. The derived excitation threshold does not, however, correctly predict the nanosecond stimulation of cardiomyocytes. We show that a better agreement is possible if multiple charging time constants are considered. Finally, we expand the model to intracellular membranes and show that pulse trains do not lead to charge buildup, but can create significant oscillations of the intracellular membrane potential.

2-ns Electrostimulation of Ca2+ Influx into Chromaffin Cells: Rapid Modulation by Field Reversal

Cellular effects of nanosecond-pulsed electric field exposures can be attenuated by an electric field reversal, a phenomenon called bipolar pulse cancellation. Our investigations of this phenomenon in neuroendocrine adrenal chromaffin cells show that a single 2-ns, 16 MV/m unipolar pulse elicited a rapid, transient rise in intracellular Ca²⁺ levels due to Ca²⁺ influx through voltage-gated calcium channels. The response was eliminated by a 2-ns bipolar pulse with positive and negative phases of equal duration and amplitude and fully restored (unipolar-equivalent response) when the delay between each phase of the bipolar pulse was 30 ns. Longer interphase intervals evoked Ca²⁺ responses that were greater in magnitude than those evoked by a unipolar pulse (stimulation). Cancellation was also observed when the amplitude of the second (negative) phase of the bipolar pulse was half that of the first (positive) phase but progressively lost as the amplitude of the second phase was incrementally increased above that of the first phase. When the amplitude of the second phase was twice that of the first phase, there was stimulation. By comparing the experimental results for each manipulation of the bipolar pulse waveform with analytical calculations of capacitive membrane charging/discharging, also known as accelerated membrane discharge mechanism, we show that the transition from cancellation to unipolar-equivalent stimulation broadly agrees with this model. Taken as a whole, our results demonstrate that electrostimulation of adrenal chromaffin cells with ultrashort pulses can be modulated with interphase intervals of tens of nanoseconds, a prediction of the accelerated membrane discharge mechanism not previously observed in other bipolar pulse cancellation studies. Such modulation of Ca²⁺ responses in a neural-type cell is promising for the potential use of nanosecond bipolar pulse technologies for remote electrostimulation applications for neuromodulation.

Quadrupoles for Remote Electrostimulation Incorporating Bipolar Cancellation

Introduction: A method that utilizes nanosecond bipolar cancellation (BPC) near a quadrupole electrodes to suppress a biological response but cancels the distal BPC at the quadrupole center, i.e., cancellation of cancellation (CANCAN), may allow for a remote focused stimulation at the quadrupole center. Objectives: The primary object of this study was to outline the requirement of the CANCAN implementation and select an effective quadrupole configuration. Results: We have studied three quadrupole electrode configurations, a rod quadrupole, a plate quadrupole (Plate-Q), and a resistor quadrupole. The pulse shapes of electric fields include monophasic pulses, cancellation pulses, and additive pulses. The Plate-Q appears the best for CANCAN as it shows the highest percentage of cancellation pulses among all pulse shapes, allowing for the best spatial focus. Conclusion: For the region of interest characterized in the Plate-Q configuration, the maximum magnitude of bipolar field is twice as that of the unipolar field, which allows for the CANCAN demonstration that involves membrane electropermeabilization.

Dye Transport through Bilayers Agrees with Lipid Electropore Molecular Dynamics

Although transport of molecules into cells via electroporation is a common biomedical procedure, its protocols are often based on trial and error. Despite a long history of theoretical effort, the underlying mechanisms of cell membrane electroporation are not sufficiently elucidated, in part, because of the number of independent fitting parameters needed to link theory to experiment. Here, we ask if the electroporation behavior of a reduced cell membrane is consistent with time-resolved, atomistic, molecular dynamics (MD) simulations of phospholipid bilayers responding to electric fields. To avoid solvent and tension effects, giant unilamellar vesicles (GUVs) were used, and transport kinetics were measured by the entry of the impermeant fluorescent dye calcein. Because the timescale of electrical pulses needed to restructure bilayers into pores is much shorter than the time resolution of current techniques for membrane transport kinetics measurements, the lifetimes of lipid bilayer electropores were measured using systematic variation of the initial MD simulation conditions, whereas GUV transport kinetics were detected in response to a nanosecond timescale variation in the applied electric pulse lifetimes and interpulse intervals. Molecular transport after GUV permeabilization induced by multiple pulses is additive for interpulse intervals as short as 50 ns but not 5-ns intervals, consistent with the 10-50-ns lifetimes of electropores in MD simulations. Although the results were mostly consistent between GUV and MD simulations, the kinetics of ultrashort, electric-field-induced permeabilization of GUVs were significantly different from published results in cells exposed to ultrashort (6 and 2 ns) electric fields, suggesting that cellular electroporation involves additional structures and processes.

High-voltage 10 ns delayed paired or bipolar pulses for in vitro bioelectric experiments

Recent studies proved that classical bio-effects induced by nanosecond pulsed electric field (nsPEF) can be reduced by the delivery of a negative polarity pulse generated immediately after a positive polarity pulse. This phenomenon is known as "bipolar cancellation" and it was reported for a wide range of bipolar pulses with pulse duration from 2 ns to 900 ns. On the contrary, paired pulses, i.e., two identical pulses with the same polarity, increased traditional nsPEF outcomes. Herein, we propose a novel robust and flexible generator, based on the frozen-wave concept, able to produce a broad range of pulses with the duration of 10 ns and delay between 17 and 360 ns. Numerical simulations and experimental measurements were performed to fully characterize the proposed generator. YO-PRO^TM-1 uptake was investigated in the U87-MG human glioblastoma cell line as a marker of membrane permeabilization in response to 10 ns, 11.5MV/m nsPEF. Our results showed that bipolar cancellation occurred for delays of 0-30 ns and decreased as a function of the interphase interval. In addition, we observed that cellular response following the application of paired nsPEF was more than two-fold compared to the unipolar pulse response and was independent from the interphase interval.

Pulsed radiofrequency for chronic pain: In vitro evidence of an electroporation mediated calcium uptake

Pulsed radiofrequency (PRF) treatments for chronic pain consist in the delivery of a train of sinusoidal electric bursts to the targeted nerve. Despite numerous clinical evidence of its efficiency, the mechanism of action of PRF remains unclear. Since most of the reported biological effects of PRF can be initiated by a calcium influx into the neurons, we hypothesized that PRF may induce a mild electroporation effect causing a calcium uptake. To test this hypothesis, HEK-293 cells were exposed to PRF bursts and cytosolic calcium and Yo-Pro-1 uptake were monitored. After a single burst, calcium peaks were observed for electric fields above 480 V/cm while the uptake of Yo-pro-1 was insignificant. After a train of 120 bursts, the electric fields required to induce a calcium and Yo-pro-1 uptake decreased to 330 V/cm and 880 V/cm respectively. Calcium peaks were not detected when cells were treated in calcium free media. The temperature increase during the treatments was lower than 5 °C in all cases. Finally, the cell response for different burst frequencies and extracellular media conductivities correlated with the induced transmembrane voltage calculated with a numerical model. Our results support the hypothesis of an electroporation mediated calcium influx.

A High-Power Dielectric Biconical Antenna for Treatment of Subcutaneous Targets

A dielectric biconical antenna (DiBiCA) for radiating subnanosecond pulses to treat subcutaneous tissue was designed, constructed, and tested. It is composed of a conical wave launcher and truncated conical emitter. In between, there is a short cylinder that provides a space for a ring terminating resistor. The material of the antenna has a dielectric constant of 28, so its size is small (length: 7 cm and aperture diameter: 2.2 cm). It was housed in an oil container to withstand high voltages and avoid surface flashover. The radiated electric field, measured in water, increased as the input voltage increased up to 30 kV but leveled off for higher voltages up to 50 kV, presumably because of losses in the antenna dielectric. The maximum field was 1.5 kV/cm for a depth of 5 mm and 1.0 kV/cm for a depth of 20 mm. Although the dielectric loss mechanism remains to be investigated, the antenna can be useful for noninvasive delivery of subnanosecond pulses to induce biological responses on subcutaneous targets. The DiBiCA radiated pulses were shown to change the viabilities of dendritic cells and macrophages for 10-min exposure. Bioelectromagnetics. 2020;41:413-424. © 2020 Bioelectromagnetics Society.

Development of transmembrane potential in concentric spherical, confocal spheroidal, and bispherical vesicles subjected to nanosecond-pulse electric field

Electroporation of concentric compound spherical and confocal spheroidal as well as eccentric compound spherical vesicles, considered to be good models for corresponding nucleate cells, are investigated with an emphasis on their response to nanosecond pulse electric field (nsPEF). Analytical models are developed for the estimation of the transmembrane potential (TMP) across the bilayers of the inner and the outer vesicles and finite-element simulations are also carried out for the eccentric case. Our calculations show that with an increase in the aspect ratio, while the TMP decreases when nsPEF is used, it increases for confocal spheroids when the pulse width is greater than the membrane charging time, leading to fully charged vesicles. Bipolar pulses are shown to effectively control the TMP for a desired time period in the nsPEF regime, and a fast decay of the TMP to zero can be achieved by judicious use of pulse polarity. The external conductivity is found to significantly influence the TMP in nsPEF, unlike millisecond pulses where its effect is insignificant. Additionally the critical electric field required to induce a TMP of 1 V at the inner vesicle is presented for different pulse widths, rise time, as well as membrane capacitance, and the TMP of the outer vesicle is found to be within limits of reversible poration. It is found that the maximum TMP has a roughly linear dependence on the outer aspect ratio of the vesicle. We also introduce a new method to obtain the particular solution to the Laplace equation for bispherical system, and it is validated with finite-element simulations. Our study on nsPEF electroporation of bispherical vesicles shows that the north pole TMP is typically greater than the south pole, thereby suggesting the typical pathway a charged species might take inside an eccentric nucleate cell under electroporation.

Probing Nanoelectroporation and Resealing of the Cell Membrane by the Entry of Ca2+ and Ba2+ Ions

The principal bioeffect of the nanosecond pulsed electric field (nsPEF) is a lasting cell membrane permeabilization, which is often attributed to the formation of nanometer-sized pores. Such pores may be too small for detection by the uptake of fluorescent dyes. We tested if Ca²⁺, Cd²⁺, Zn²⁺, and Ba²⁺ ions can be used as nanoporation markers. Time-lapse imaging was performed in CHO, BPAE, and HEK cells loaded with Fluo-4, Calbryte, or Fluo-8 dyes. Ca²⁺ and Ba²⁺ did not change fluorescence in intact cells, whereas their entry after nsPEF increased fluorescence within <1 ms. The threshold for one 300-ns pulse was at 1.5–2 kV/cm, much lower than >7 kV/cm for the formation of larger pores that admitted YO-PRO-1, TO-PRO-3, or propidium dye into the cells. Ba²⁺ entry caused a gradual emission rise, which reached a stable level in 2 min or, with more intense nsPEF, kept rising steadily for at least 30 min. Ca²⁺ entry could elicit calcium-induced calcium release (CICR) followed by Ca²⁺ removal from the cytosol, which markedly affected the time course, polarity, amplitude, and the dose-dependence of fluorescence change. Both Ca²⁺ and Ba²⁺ proved as sensitive nanoporation markers, with Ba²⁺ being more reliable for monitoring membrane damage and resealing.

Interpretation of the molecular mechanism of the electroporation induced by symmetrical bipolar picosecond pulse trains

Picosecond pulse trains (psPTs) are emerging as a new characteristic diagnostic and therapeutic tool in biomedical fields. To specifically determine the stimulus provided to cells, in this article, we use a molecular dynamics (MD) model to show the molecular mechanisms of electroporation induced by symmetrical bipolar psPTs and predict a bipolar cancellation for the studied picosecond pulses. Electric field conditions that do not cause electroporation reveal that the interfacial water molecules continuously flip and redirect as the applied bipolar psPT reverses, and the molecules cannot keep moving in one direction or leave the lipid-water interface. Based on our simulation results, we determine the threshold for electroporation with symmetrical bipolar psPTs. For a fixed electric field intensity, a lower repetition frequency leads to more rapid electroporation. For a fixed repetition frequency, a higher electric field intensity leads to more rapid electroporation. We found that the water dipole relaxation time decreases as the electric field magnitude increases. Additionally, the influences of the symmetrical bipolar psPT intensity and frequency on the pore formation time are presented. Discrete nanoscale pores can form with the applied psPT at terahertz (THz) repetition frequency. When the psPT amplitude increases or the frequency decreases, the number of water bridges will increase. Moreover, for the first time, the molecular mechanism of bipolar cancellation for the studied picosecond pulse is discussed preliminarily. Our results indicate that the influence of the unipolar picosecond pulse on the interfacial water dipoles will accumulate in one direction, but the bipolar picosecond pulse does not cause this effect.

Cancellation effect is present in high-frequency reversible and irreversible electroporation

It was recently suggested that applying high-frequency short biphasic pulses (HF-IRE) reduces pain and muscle contractions in electrochemotherapy and irreversible ablation treatments; however, higher amplitudes with HF-IRE pulses are required to achieve a similar effect as with monophasic pulses. HF-IRE pulses are in the range of a microseconds, thus, the so-called cancellation effect could be responsible for the need to apply pulses of higher amplitudes. In cancellation effect, the effect of first pulse is reduced by the second pulse of opposite polarity. We evaluated cancellation effect with high-frequency biphasic pulses on CHO-K1 in different electroporation buffers. We applied eight bursts of 1-10 µs long pulses with inter-phase delays of 0.5 µs - 10 ms and evaluated membrane permeability and cell survival. In permeability experiments, cancellation effect was not observed in low-conductivity buffer. Cancellation effect was, however, observed in treatments with high-frequency biphasic pulses looking at survival in all of the tested electroporation buffers. In general, cancellation effect depended on inter-phase delay as well as on pulse duration, i.e. longer pulses and longer interphase delay cause less pronounced cancellation effect. Cancellation effect could be partially explained by the assisted discharge and not by the hyperpolarization by the chloride channels.

Cancellation of nerve excitation by the reversal of nanosecond stimulus polarity and its relevance to the gating time of sodium channels

The initiation of action potentials (APs) by membrane depolarization occurs after a brief vulnerability period, during which excitation can be abolished by the reversal of the stimulus polarity. This vulnerability period is determined by the time needed for gating of voltage-gated sodium channels (VGSC). We compared nerve excitation by ultra-short uni- and bipolar stimuli to define the time frame of bipolar cancellation and of AP initiation. Propagating APs in isolated frog sciatic nerve were elicited by cathodic pulses (200 ns-300 µs), followed by an anodic (canceling) pulse of the same duration after a 0-200-µs delay. We found that the earliest and the latest boundaries for opening the critical number of VGSC needed to initiate AP are, respectively, between 11 and 20 µs and between 100 and 200 µs after the onset of depolarization. Stronger depolarization accelerated AP initiation, apparently due to faster VGSC opening, but not beyond the 11-µs limit. Bipolar cancellation was augmented by reducing pulse duration, shortening the delay between pulses, decreasing the amplitude of the cathodic pulse, and increasing the amplitude of the anodic one. Some of these characteristics contrasted the bipolar cancellation of cell membrane electroporation (Pakhomov et al. in Bioelectrochemistry 122:123-133, 2018; Gianulis et al. in Bioelectrochemistry 119:10-19, 2017), suggesting different mechanisms. The ratio of nerve excitation thresholds for a unipolar cathodic pulse and a symmetrical bipolar pulse increased as a power function as the pulse duration decreased, in remarkable agreement with the predictions of SENN model of nerve excitation (Reilly and Diamant in Health Phys 83(3):356-365, 2002).

Selective distant electrostimulation by synchronized bipolar nanosecond pulses

A unique aspect of electrostimulation (ES) with nanosecond electric pulses (nsEP) is the inhibition of effects when the polarity is reversed. This bipolar cancellation feature makes bipolar nsEP less efficient at biostimulation than unipolar nsEP. We propose to minimize stimulation near pulse-delivering electrodes by applying bipolar nsEP, whereas the superposition of two phase-shifted bipolar nsEP from two independent sources yields a biologically-effective unipolar pulse remotely. This is accomplished by electrical compensation of all nsEP phases except the first one, resulting in the restoration of stimulation efficiency due to cancellation of bipolar cancellation (CANCAN-ES). We experimentally proved the CANCAN-ES paradigm by measuring YO-PRO-1 dye uptake in CHO-K1 cells which were permeabilized by multiphasic nsEP (600 ns per phase) from two generators; these nsEP were synchronized either to overlap into a unipolar pulse remotely from electrodes (CANCAN), or not to overlap (control). Enhancement of YO-PRO-1 entry due to CANCAN was observed in all sets of experiments and reached ~3-fold in the center of the gap between electrodes, exactly where the unipolar pulse was formed, and equaled the degree of bipolar cancellation. CANCAN-ES is promising for non-invasive deep tissue stimulation, either alone or combined with other remote stimulation techniques to improve targeting.

Excitation and electroporation by MHz bursts of nanosecond stimuli

Intense nanosecond pulsed electric field (nsPEF) is a novel modality for cell activation and nanoelectroporation. Applications of nsPEF in research and therapy are hindered by a high electric field requirement, typically from 1 to over 50 kV/cm to elicit any bioeffects. We show how this requirement can be overcome by engaging temporal summation when pulses are compressed into high-rate bursts (up to several MHz). This approach was tested for excitation of ventricular cardiomyocytes and peripheral nerve fibers; for membrane electroporation of cardiomyocytes, CHO, and HEK cells; and for killing EL-4 cells. MHz compression of nsPEF bursts (100-1000 pulses) enables excitation at only 0.01-0.15 kV/cm and electroporation already at 0.4-0.6 kV/cm. Clear separation of excitation and electroporation thresholds allows for multiple excitation cycles without membrane disruption. The efficiency of nsPEF bursts increases with the duty cycle (by increasing either pulse duration or repetition rate) and with increasing the total time "on" (by increasing either pulse duration or number). For some endpoints, the efficiency of nsPEF bursts matches a single "long" pulse whose amplitude and duration equal the time-average amplitude and duration of the bursts. For other endpoints this rule is not valid, presumably because of nsPEF-specific bioeffects and/or possible modification of targets already during the burst. MHz compression of nsPEF bursts is a universal and efficient way to lower excitation thresholds and facilitate electroporation.

Stimulation or Cancellation of Ca2+ Influx by Bipolar Nanosecond Pulsed Electric Fields in Adrenal Chromaffin Cells Can Be Achieved by Tuning Pulse Waveform

Exposing adrenal chromaffin cells to single 150 to 400 ns electric pulses triggers a rise in intracellular Ca²⁺ ([Ca²⁺]_i) that is due to Ca²⁺ influx through voltage-gated Ca²⁺ channels (VGCC) and plasma membrane electropores. Immediate delivery of a second pulse of the opposite polarity in which the duration and amplitude were the same as the first pulse (a symmetrical bipolar pulse) or greater than the first pulse (an asymmetrical bipolar pulse) had a stimulatory effect, evoking larger Ca²⁺ responses than the corresponding unipolar pulse. Progressively decreasing the amplitude of the opposite polarity pulse while also increasing its duration converted stimulation to attenuation, which reached a maximum of 43% when the positive phase was 150 ns at 3.1 kV/cm, and the negative phase was 800 ns at 0.2 kV/cm. When VGCCs were blocked, Ca²⁺ responses evoked by asymmetrical and even symmetrical bipolar pulses were significantly reduced relative to those evoked by the corresponding unipolar pulse under the same conditions, indicating that attenuation involved mainly the portion of Ca²⁺ influx attributable to membrane electropermeabilization. Thus, by tuning the shape of the bipolar pulse, Ca²⁺ entry into chromaffin cells through electropores could be attenuated while preserving Ca²⁺ influx through VGCCs.

Electrofusion by a bipolar pulsed electric field: Increased cell fusion efficiency for monoclonal antibody production

The excessive cell death rate caused by electrofusion with unipolar pulses (UPs) has been a bottleneck to increasing cell fusion efficiency in monoclonal antibody technology. Several studies have confirmed that compared with UPs, bipolar pulses (BPs) with microsecond pulse widths can increase electropermeabilization while reducing cell death. Given these characteristics, BPs were used to increase cell fusion efficiency in this study. Cell staining and hybridoma culture experiments were performed using SP2/0 mouse myeloma cells and lymphocytes. Based on the equal energy principle, UPs and BPs were delivered to electrodes at a distance of 3.81 mm, with electric field intensities ranging from 2 kV/cm to 3 kV/cm and pulse duration of 40 μs for the UPs and 20-20 μs for the BPs. The results of cell staining experiments showed that cell fusion efficiency was 3-fold greater with BPs than with UPs. Similarly, the results of the hybridoma culture experiments showed that the hybridoma yields were 0.26‰ and 0.23‰ (2.5 kV/cm and 3 kV/cm, respectively) in the UP groups and increased to 0.46‰ and 0.35‰ in the BP groups. Taken together, the results show that the efficiency of heterologous cell fusion can be greatly increased if BPs are used instead of the commonly applied UPs. This study may provide a promising method for monoclonal antibody technology.

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.