The transition from field emission to collisional space-charge limited current with nonzero initial velocity

Multiple electron emission mechanisms often contribute in electron devices, motivating theoretical studies characterizing the transitions between them. Previous studies unified thermionic and field emission, defined by the Richardson-Laue-Dushman (RLD) and Fowler-Nordheim (FN) equations, respectively, with the Child-Langmuir (CL) law for vacuum space-charge limited current (SCLC); another study unified FN and CL with the Mott-Gurney (MG) law for collisional SCLC. However, thermionic emission, which introduces a nonzero injection velocity, may also occur in gas, motivating this analysis to unify RLD, FN, CL, and MG. We exactly calculate the current density as a function of applied voltage over a range of injection velocity (i.e., temperature), mobility, and gap distance. This exact solution approaches RLD, FN, and generalized CL (GCL) and MG (GMG) for nonzero injection velocity under appropriate limits. For nonzero initial velocity, GMG approaches zero for sufficiently small applied voltage and mobility, making these gaps always space-charge limited by either GMG at low voltage or GCL at high voltage. The third-order nexus between FN, GMG, and GCL changes negligibly from the zero initial velocity calculation over ten orders of magnitude of applied voltage. These results provide a closed form solution for GMG and guidance on thermionic emission in a collisional gap.

Selective Electroporation of Tumor Cells Under AC Radiofrequency Stimulation - A Numerical Study

Self-consistent evaluations of membrane electroporation along with local heating in single spherical cells arising from external AC radiofrequency electrical stimulation have been carried out. The present numerical study seeks to determine whether healthy and malignant cells exhibit separate electroporative responses with regards to operating frequency. It is shown that cells of Burkitt's lymphoma would respond to frequencies >4.5 MHz, while normal B-cells would have negligible porative effects in that higher frequency range. Similarly, a frequency separation between the response of healthy T-cells and malignant species is predicted with a threshold of about 4 MHz for cancer cells. The present simulation technique is general and so would be able to ascertain the beneficial frequency range for different cell types. The demonstration of higher frequencies to induce poration in malignant cells, while having minimal affecting healthy ones, suggests the possibility of selective electrical targeting for tumor treatments and protocols. It also opens the doorway for tabulating selectivity enhancement regimes as a guide for parameter selection towards more effective treatments while minimizing deleterious effects on healthy cells and tissues.

Space-charge-limited current density for nonplanar diodes with monoenergetic emission using Lie-point symmetries

Understanding space-charge-limited current density (SCLCD) is fundamentally and practically important for characterizing many high-power and high-current vacuum devices. Despite this, no analytic equations for SCLCD with nonzero monoenergetic initial velocity have been derived for nonplanar diodes from first principles. Obtaining analytic equations for SCLCD for nonplanar geometries is often complicated by the nonlinearity of the problem and over constrained boundary conditions. In this Letter, we use the canonical coordinates obtained by identifying Lie-point symmetries to linearize the governing differential equations to derive SCLCD for any orthogonal diode. Using this method, we derive exact analytic equations for SCLCD with a monoenergetic injection velocity for one-dimensional cylindrical, spherical, tip-to-tip (t-t), and tip-to-plate (t-p) diodes. We specifically demonstrate that the correction factor from zero initial velocity to monoenergetic emission depends only on the initial kinetic and electric potential energies and not on the diode geometry and that SCLCD is universal when plotted as a function of the canonical gap size. We also show that SCLCD for a t-p diode is a factor of four larger than a t-t diode independent of injection velocity. The results reduce to previously derived results for zero initial velocity using variational calculus and conformal mapping.

Infrared Laser-Based Single Cell Permeabilization by Plasma Membrane Temperature Gradients

Single cell microinjection provides precise tuning of the volume and timing of delivery into the treated cells; however, it also introduces workflow complexity that requires highly skilled operators and specialized equipment. Laser-based microinjection provides an alternative method for targeting a single cell using a common laser and a workflow that may be readily standardized. This paper presents experiments using a 1550 nm, 100 fs pulse duration laser with a repetition rate of 20 ns for laser-based microinjection and calculations of the hypothesized physical mechanism responsible for the experimentally observed permeabilization. Chinese Hamster Ovarian (CHO) cells exposed to this laser underwent propidium iodide uptake, demonstrating the potential for selective cell permeabilization. The agreement between the experimental conditions and the electropermeabilization threshold based on estimated changes in the transmembrane potential induced by a laser-induced plasma membrane temperature gradient, even without accounting for enhancement due to traditional electroporation, strengthens the hypothesis of this mechanism for the experimental observations. Compared to standard 800 nm lasers, 1550 nm fs lasers may ultimately provide a lower cost microinjection method that readily interfaces with a microscope and is agnostic to operator skill, while inducing fewer deleterious effects (e.g., temperature rise, shockwaves, and cavitation bubbles).

Nonlinear transmission line implemented as a combined pulse forming line and high-power microwave source

Nonlinear transmission lines (NLTLs) are typically driven by pulse forming lines (PFLs) or Marx generators to generate high repetition rate, high power microwaves (HPMs) with fewer auxiliary systems than conventional sources. This paper reports the development of an even more compact HPM system that utilizes a composite-based hybrid NLTL as the PFL and HPM generator in a single device. We designed the following three different combinations of nickel zinc ferrite (NZF) and barium strontium titanate (BST) inclusion volume loads in a polydimethylsiloxane host material to provide magnetic field dependent permeability and electric field dependent permittivity, respectively: 25% NZF, 10% BST/15% NZF, and 15% BST/10% NZF. By constructing the NLTL in a coaxial geometry, this device uses the capacitance and length of the NLTL to generate a fast rise-time high voltage pulse with microwave oscillations that occurred both during and after the pulse after exceeding a threshold charging voltage. The output frequency of the NLTLs ranged from 950 MHz to 2.2 GHz during the pulse for all volume loadings and was 1 GHz after the pulse for the 10% BST/15% NZF and 15% BST/10% NZF volume loadings. The oscillations generated after the pulse were much higher in amplitude and achieved 160 kW at a 15 kV charging voltage for the 15% BST/10% NZF composite-based NLTL.

Electrical stimulation of whole blood for growth factor release and potential clinical implications

Clinicians have increasingly applied platelet-rich plasma (PRP) for wound healing treatments. Topical treatments commonly require biochemical agents such as bovine thrombin to activate PRP ex vivo for clotting and growth factor release to facilitate healing upon application to the wound of interest. Recent studies have explored electrical stimulation as an alternative to bovine thrombin for PRP activation due to the former's cost, workflow complexity and potentially significant side effects; however, both approaches require separating the PRP from whole blood (WB) prior to activation. Eliminating the separation (typically centrifugation) step would reduce the cost and duration of the clinical procedure, which may be critical in trauma and surgical applications. We hypothesize that electric pulses (EPs) can release growth factors from WB, as they do from PRP, without requiring centrifugation of WB into PRP. A pilot study for two donors demonstrates the potential for EP stimulated growth factor release from WB. This motivates future experiments assessing EP parameter optimization for WB activation and in vivo studies to determine the clinical benefits for topical treatments and, especially, for injections in orthopedic applications that already utilize non-treated/non-activated WB.

Nanosecond electric pulses rapidly enhance the inactivation of Gram-negative bacteria using Gram-positive antibiotics

Physically disrupting microorganism membranes to enable antibiotics to overcome resistance mechanisms that inhibit or excrete antibiotics has great potential for reducing antibiotic doses and rendering resistance mechanisms inert. We demonstrate the synergistic inactivation of a Gram-positive (Staphylococcus aureus) and two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria by combining 222 30 kV/cm electric pulses (EPs) or 500 20 kV/cm EPs with 300-ns EP duration with various antibiotics with different mechanisms of action is demonstrated. Doses of antibiotics that produced no inactivation in 10 min of exposure in solution with bacteria induced several log reductions under the influence of nanosecond EPs. Combining 2 μg/L or 20 μg/mL of rifampicin with the 30 kV/cm EPs enhanced Staphylococcus aureus inactivation compared with EPs alone, while only a few of the other combinations demonstrated improvement. Combining 2 μg/L or 20 μg/mL of mupirocin or rifampicin with either EP train enhanced E. coli inactivation compared with EPs alone. Combining 2 μg/L or 20 μg/mL of erythromycin or vancomycin with the 30 kV/cm EPs enhanced E. coli inactivation compared with EPs alone. These results indicate that EPs can make Gram-positive antibiotics efficient for inactivating Gram-negative bacteria with future studies required to optimize EP parameters for other antibiotics and Gram-negative bacteria.

Nanosecond pulsed electric field induced proliferation and differentiation of osteoblasts and myoblasts

Low-intensity electric fields can induce changes in cell differentiation and cytoskeletal stresses that facilitate manipulation of osteoblasts and mesenchymal stem cells; however, the application times (tens of minutes) are of the order of physiological mechanisms, which can complicate treatment consistency. Intense nanosecond pulsed electric fields (nsPEFs) can overcome these challenges by inducing similar stresses on shorter timescales while additionally inducing plasma membrane nanoporation, ion transport and intracellular structure manipulation. This paper shows that treating myoblasts and osteoblasts with five 300 ns PEFs with intensities from 1.5 to 25 kV cm^-1 increased proliferation and differentiation. While nsPEFs above 5 kV cm^-1 decreased myoblast population growth, 10 and 20 kV cm^-1 trains increased myoblast population by approximately fivefold 48 h after exposure when all cell densities were set to the same level after exposure. Three trials of the PEF-treated osteoblasts showed that PEF trains between 2.5 and 10 kV cm^-1 induced the greatest population growth compared to the control 48 h after treatment. Trains of nsPEFs between 1.5 and 5 kV cm^-1 induced the most nodule formation in osteoblasts, indicating bone formation. These results demonstrate the potential utility for nsPEFs to rapidly modulate stem cells for proliferation and differentiation and motivate future experiments to optimize PEF parameters for in vivo applications.

The Transition to Paschen's Law for Microscale Gas Breakdown at Subatmospheric Pressure

The decrease in electronic device size necessitates greater understanding of gas breakdown and electron emission at microscale to optimize performance. While traditional breakdown theory using Paschen’s law (PL), driven by Townsend avalanche, fails for gap distance d \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lesssim $$\end{document}≲ 15 μm, recent studies have derived analytic equations for breakdown voltage when field emission and Townsend avalanche drive breakdown. This study derives a new analytic equation that predicts breakdown voltage V_B within 4% of the exact numerical results of a previously derived theory and new experimental results at subatmospheric pressure for gap distances from 1–25 μm. At atmospheric pressure, V_B transitions to PL near the product of pressure and gap distance, pd, corresponding to the Paschen minimum; at lower pressures, the transition to PL occurs to the left of the minimum. We further show that the work function plays a major role in determining which side of the Paschen minimum V_B transitions to PL as pressure approaches atmospheric pressure while field enhancement and the secondary emission coefficient play smaller roles. These results indicate that appropriate combinations of these parameters cause V_B to transition to PL to the left of the Paschen minimum, which would yield an extended plateau similar to some microscale gas breakdown experimental observations.

Synergistic bacterial inactivation by combining antibiotics with nanosecond electric pulses

Antibiotic resistance mechanisms render current antibiotics ineffective, requiring higher concentrations of existing drugs or the development of more powerful drugs for infection treatment. This study demonstrates the synergistic inactivation of a gram-positive (Staphylococcus aureus) and a gram-negative (Escherichia coli) bacteria by combining either tobramycin or rifampicin with 300-ns electric pulses (EPs). For EPs depositing the same total energy density into the sample with no drug, higher electric fields induced greater inactivation, indicating a threshold for irreversible electroporation at these fields and membrane recovery in between lower intensity EPs. Synergistic inactivation generally increased with increasing drug concentration up to 20 μg/mL compared to strictly EP treatment. Combining even 1/20 of the clinical dose of tobramycin with a train of EPs induced between 2.5 and 3.5 log inactivation after only 10 min of exposure compared to hours to induce inactivation with a clinical dose with no EPs. Similarly, combining a train of EPs with a clinically relevant dose of rifampicin induced 7 to 9 log inactivation over the same time of exposure. These results indicate the promise of combining EPs with antibiotics to rapidly inactivate antibiotic-resistant bacteria in localized treatment areas.

Design, characterization and experimental validation of a compact, flexible pulsed power architecture for ex vivo platelet activation

Electric pulses can induce various changes in cell dynamics and properties depending upon pulse parameters; however, pulsed power generators for in vitro and ex vivo applications may have little to no flexibility in changing the pulse duration, rise- and fall-times, or pulse shape. We outline a compact pulsed power architecture that operates from hundreds of nanoseconds (with the potential for modification to tens of nanoseconds) to tens of microseconds by modifying a Marx topology via controlling switch sequences and voltages into each capacitor stage. We demonstrate that this device can deliver pulses to both low conductivity buffers, like standard pulsed power supplies used for electroporation, and higher conductivity solutions, such as blood and platelet rich plasma. We further test the effectiveness of this pulse generator for biomedical applications by successfully activating platelets ex vivo with 400 ns and 600 ns electric pulses. This novel bioelectrics platform may provide researchers with unprecedented flexibility to explore a wide range of pulse parameters that may induce phenomena ranging from intracellular to plasma membrane manipulation.

Plasma membrane temperature gradients and multiple cell permeabilization induced by low peak power density femtosecond lasers

Calculations indicate that selectively heating the extracellular media induces membrane temperature gradients that combine with electric fields and a temperature-induced reduction in the electropermeabilization threshold to potentially facilitate exogenous molecular delivery. Experiments by a wide-field, pulsed femtosecond laser with peak power density far below typical single cell optical delivery systems confirmed this hypothesis. Operating this laser in continuous wave mode at the same average power permeabilized many fewer cells, suggesting that bulk heating alone is insufficient and temperature gradients are crucial for permeabilization. This work suggests promising opportunities for a high throughput, low cost, contactless method for laser mediated exogenous molecule delivery without the complex optics of typical single cell optoinjection, for potential integration into microscope imaging and microfluidic systems.

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Femtosecond lasers can transfect a single cell through optoporation.

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Multiple cells are transfected with much lower power densities at 1550 nm.

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Calculations show that temperature gradients contribute to the mechanism.

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These mechanisms resemble those also involved in microwave biological interactions.

Ultrashort electric pulse induced changes in cellular dielectric properties

The interaction of nanosecond duration pulsed electric fields (nsPEFs) with biological cells, and the models describing this behavior, depend critically on the electrical properties of the cells being pulsed. Here, we used time domain dielectric spectroscopy to measure the dielectric properties of Jurkat cells, a malignant human T-cell line, before and after exposure to five 10ns, 150kV/cm electrical pulses. The cytoplasm and nucleoplasm conductivities decreased dramatically following pulsing, corresponding to previously observed rises in cell suspension conductivity. This suggests that electropermeabilization occurred, resulting in ion transport from the cell's interior to the exterior. A delayed decrease in cell membrane conductivity after the nsPEFs possibly suggests long-term ion channel damage or use dependence due to repeated membrane charging and discharging. This data could be used in models describing the phenomena at work.

Nanosecond electric pulses penetrate the nucleus and enhance speckle formation

Nanosecond electric pulses generate nanopores in the interior membranes of cells and modulate cellular functions. Here, we used confocal microscopy and flow cytometry to observe Smith antigen antibody (Y12) binding to nuclear speckles, known as small nuclear ribonucleoprotein particles (snRNPs) or intrachromatin granule clusters (IGCs), in Jurkat cells following one or five 10ns, 150kV/cm pulses. Using confocal microscopy and flow cytometry, we observed changes in nuclear speckle labeling that suggested a disruption of pre-messenger RNA splicing mechanisms. Pulse exposure increased the nuclear speckled substructures by approximately 2.5-fold above basal levels while the propidium iodide (PI) uptake in pulsed cells was unchanged. The resulting nuclear speckle changes were also cell cycle dependent. These findings suggest that 10ns pulses directly influenced nuclear processes, such as the changes in the nuclear RNA-protein complexes.

Leukemic cell intracellular responses to nanosecond electric fields

Intense, nanosecond (ns) pulsed electric fields (PEFs) are known to affect the intracellular structures of cells. The probability of preferentially inducing subcellular effects increases with decreasing pulse length while effects on the plasma membrane are diminished. This has been demonstrated by applying electrical pulses of 60 and 10 ns duration with electric field intensities of up to 6.5 MV/m to HL-60 cells. Using confocal microscopy, PEF-induced changes in the integrity of the plasma membrane and nucleus were measured by recording fluorescence changes with propidium iodide (PI) and acridine orange (AO), respectively. Results suggest that high voltage, nsPEFs target the nucleus and modify cellular functions while plasma membrane effects are delayed and become smaller as pulse duration is shortened. Cell viability was not affected by these pulses. In spite of the high pulsed electric fields, thermal effects can be neglected because of the ultrashort pulse duration. The results suggest application of this ultrashort pulse technology to modulate nuclear structure and function for potential therapeutic benefit.