Irreversible Electrical Breakdown Research Articles

Confocal microscopy observations of electrical pre-breakdown of bi-layer elastomer dielectrics

At high electric fields, the electrical energy stored in a soft elastomer dielectric can be comparable to the mechanical deformation energy it produces. This has led to the development of a class of electrically controlled, large strain dielectric elastomer actuators for soft robotics and energy harvesting devices. At large electric fields, the electro-mechanically induced deformation can lead to pseudo-periodic surface morphological instabilities which then grow with increasing field into stable pre-breakdown defects prior to final, irreversible electrical breakdown. Under these extremes of combined large electrical and mechanical deformations, the morphological evolution of the pre-breakdown defects has not hitherto been reported. In contrast to the filamentary breakdown of much stiffer dielectrics, fluorescence confocal microscopy reveals an array of defects that evolve through a complex, reversible series of morphologies, transitioning from axi-symmetric “pits” to “crack-like” shapes that can “twist” and deflect, and finally open to form an array of holes. The observations suggest that the transitions, from axi-symmetric pits to flat, slit-like defects and then to an array of holes, are geometric instabilities. The implications for using a soft elastomer layer to increase the dielectric breakdown of a stiffer dielectric are discussed.

MODELLING AND ANALYSIS OF ELECTROPORATION PARAMETERS OF THE MEMBRANE OF A BIOLOGICAL CELL IN A VARIED INTENSITY PULSED ELECTRIC FIELD

Context. The problem of constructing electroporation models for membranes of biological cells by the methods of nonlinear approximation using the experimental dependences of their specific electric conductivity on intensity pulsed electric field was solved in the paper.Objective is a construction of models, which adequately describe the experimentally obtained nonlinear effects of the conductivity of the cell, including reversible electroporation, irreversible electrical breakdown or local reversible electrical breakdown of membranes at the fusion of two contacting cells.Method. Polynomials of 8–10 degrees are chosen as the functions that modelling the experimental ones and the criteria for estimating the parameters of electroporation are the coordinates of the local extrema of their curvature and inflexion points that characterize the specified state of the cell membrane at current field intensity. The approximation problem was solved by the least squares method. The calculation of the estimate of the polynomials coefficients was carried out by the Gaussian elimination – the forward and reverse moves were realized. It is possible to search for extrema of the obtained polynomials of high degrees by specifying a calculation error. The root-mean-square error of the approximation is used for finding the degree of the polynomial. The current curvature of the polynomial is counted by calculating the first and second order derivatives of conductivity. The values of the curvature, which obtained via these methods, make possible to determine the inflexion points of the curve for purpose to determine breakdown of a cell membrane.Results. Applied software was developed, polynomial models of the conductivity of a biological cell in a varied intensity pulsed electric field were constructed and their quantitative mathematical analysis was carried out by using this software. All these calculations are proved by graphs, some of which can be viewed on an enlarged scale.Conclusions. The parameters of electroporation of a biological cell membrane obtained by analysing the curvature function of polynomial models are calculated. The developed analytical methods and software for determining the parameters of electroporation allow us to recommend them for use in practice in calculating the numerical values of the field intensity and conductivity at which specific electroporation regimes of the biological cell membrane are provided.

Isoprenoid lipid chains increase membrane resistance to pore formation

The process of electroporation of bilayer lipid membranes made of diphytanoylphosphatidylcholine (DPhPC) has been studied. DPhPC combines a polar part typical for lipids of eukaryotic cells and branched isoprenoid chains forming the hydrophobic “tails” of archaea lipids. From the experimental dependence of the average lifetime on the magnitude of the applied transmembrane potential difference we evaluated line tension of the pore edge in DPhPC bilayers (Γ) and the diffusion coefficient of the defect in the space of the pore radii (D). Line tension Γ was 5.5 ± 0.2 pN and diffusion coefficient D, (6 ± 2) × 10−19 m2/s. Comparison of these values with the values typical for eukaryotic lipids suggests that the critical radius of the pore, further growth of which results in an irreversible electrical breakdown of the lipid bilayer, is less in the case of DPhPC than for conventional membranes of phosphatidylcholines, but the probability of the defect emergence and the increase of its radius is also lower for the DPhPC membranes. Apparently, these peculiarities can be accounted for by high hydrophobicity of the branched isoprenoid chains of DPhPC.

Electrical properties of new tin dioxide varistor ceramics at high currents

New dense SnO 2 -based varistor ceramics with high nonlinear current–voltage characteristics (nonlinearity coefficients are of approximately 50) in a system of SnO 2 –CoO–Nb 2 O 5 –Cr 2 O 3 –Y 2 O 3 –SrO–MgO are reported. The current–voltage behaviour at high currents is studied by using exponential voltage pulses. The obtained SnO 2 varistor ceramics exhibit low grain resistivity values of 0.23–0.64 ohm cm. To date, such values are the lowest known for SnO 2 varistors, and are closely approaching the grain resistivity of the ZnO varistor. The current–voltage characteristics of the obtained SnO 2 -based varistor materials are reproducible in a wide current range from 10 −11 to approximately 10 4 A cm −2 . The minimum current density and the minimum electric field necessary to cause the irreversible electrical breakdown are measured. It is established that a decrease in the grain resistivity leads to an increase in the minimum current density necessary for irreversible electrical breakdown to occur.

Mechanisms and application of irreversible electrical breakdown of steep pulse in tumor therapy

Irreversible electrical breakdown (IREB) of steep pulse is a new physical method for tumor therapy. Because of the distinct mechanism, it can destroy tumor tissue effectively with non- thermal effect, and high safety. IREB can be performed easily and quickly, and may be a promising additional method for com-bined therapy of tumor. Key words: Electroporation; Neoplasms; Therapy

Effect of voltage on the conduction of a high-density polyethylene/carbon black composite at the NTC region

The time dependences of current and Joule heating under various voltages are studied to high-density polyethylene (HDPE) filled with carbon black (CB) of 0.082 in volume at an ambient temperature of 158 °C located at the region of negative temperature coefficient (NTC) of resistivity. The composite shows either a transition from the intrinsic to the steady conductions or an irreversible electrical breakdown depending on the applied voltage. The intrinsic conduction in the absence of Joule heating is explained combining two electron tunneling processes while the electric field induced NTC effect is related to the production of additional conducting pathways resulted from cold-emission.

Experimental Studies on Killing and Inhibiting Effects of Steep Pulsed Electric Field (SPEF) to Target Cancer Cell and Solid Tumor

Electroporation (i.e., reversible electrical breakdown) has been used for biomedicine such as electroporation therapy, transdermal drug delivery, cellular drug carrier systems, and gene transfer therapy. However, little research on the biological effect of irreversible electrical breakdown has been published. In this paper, the killing and inhibiting effects of pulsed electric field, whose rising edge is 100 ns level (we call it steep pulsed electric field, SPEF), to target cancer cell and solid tumor were successfully observed. The SPEF consists of two parts: the rising process containing ample high-frequency components, and the falling process containing lots of low-frequency components. Cell exposed to the pulse presented multiple changes: destructed membrane, swollen nucleus, disintegrated karyotheca, and cavitated, swollen mitochondria in histological and ultrastructure observations. SPEF blocked cell transition from G0/G1 phase to S+G2/M phase in flow cytometry analysis, and tumor growth retardation was observed in tumor-bearing mice. These experimental results show SPEF have a certain function to generate irreversible electrical breakdown in tumor cells, and might provide a new promising therapeutic approach for tumor treatment.

Theory of electroporation of planar bilayer membranes: predictions of the aqueous area, change in capacitance, and pore-pore separation

A large increase in the transmembrane voltage, U(t), of a fluid bilayer membrane is believed to result in the occurrence of temporary aqueous pathways ("pores") across the membrane. The number, size, and evolution dynamics of these pores are expected to be crucial to the transport of water-soluble species ranging from small ions to macromolecules such as proteins and nucleic acids. In this paper we use a transient aqueous pore theory to estimate the fraction of the membrane area, Fw(t), which is temporarily occupied by water-filled pores for short square, exponential, and bipolar square pulses. For short pulses, "reversible electrical breakdown" occurs when the transmembrane voltage reaches about 1 V, and Fw(t) is predicted to rise rapidly, but always to be less than 10(-3). The conductance of a large number of pores causes reversible electrical breakdown and prevents a significantly larger U from being reached. Despite the large dielectric constant of water, for reversible electroporation the associated change in membrane capacitance, delta C, due to the pores is predicted to be small. Moreover, for a flat membrane the minimum value of the mean pore-pore separation is large, about 60 times the minimum pore radius. In flat membranes, pores are predicted to repel, but the opposite is expected for curved cell membranes, allowing the possibility of coalescence in cell membranes. For some moderate values of U, rupture (irreversible electrical breakdown) occurs, as one or more supracritical pores expand to the membrane boundary and the entire membrane area becomes aqueous. In all cases it is found that a quantitative description of electroporation requires that a pore size distribution, rather than a single size pore.

Use of irreversible electrical breakdown of lipid bilayers for the study of interaction of membranes with surface active molecules

Lipid bilayers were formed in the presence of different macromolecules and high electrical fields were used to induce mechanical rupture of the membranes. The kinetics of pore formation during irreversible breakdown was studied as a function of the macromolecules. We observed that macromolecules having a strong binding affinity to the membrane alter the time course of pore formation significantly. We propose this method as a simple test for adsorption of macromolecules to membranes.

Spatially resolved electrical measurements of SiO2 gate oxides using atomic force microscopy

Using a modified atomic force microscope (AFM) with a conducting cantilever, we have investigated the dielectric strength of SiO2 gate oxide films. This has been achieved by spatially resolving the prebreakdown tunneling current flowing between the silicon substrate and tip. During AFM imaging a voltage ramp was applied to the tip at each image point so as to determine the local threshold voltage required to generate a small tunneling current in the oxide, without causing an irreversible electrical breakdown. For an oxide 12-nm thick this voltage was found to vary by more than a factor of 2.7 over an area of 0.14 μm2, with a maximum value of 40.5 V. This suggests that the breakdown strength of conventional metal-oxide-silicon capacitors may not be limited by the intrinsic dielectric strength of the oxide, but by imperfections or nonuniformities in the Si/SiO2 structure. By preventing irreversible oxide breakdown during scanning, we can image the dielectric properties of oxide films with a lateral resolution better than 20 nm.

Kinetics of pore size during irreversible electrical breakdown of lipid bilayer membranes

The kinetics of pore formation followed by mechanical rupture of lipid bilayer membranes were investigated in detail by using the charge-pulse method. Membranes of various compositions were charged to a sufficiently high voltage to induce mechanical breakdown. The subsequent decrease of membrane voltage was used to calculate the conductance. During mechanical breakdown, which was probably caused by the widening of one single pore, the membrane conductance was a linear and not exponential function of time after the initial starting process. In a large number of experiments using various lipids and electrolytes, the characteristic opening process of the pore turned out to be independent of the actual membrane potential and electrolyte concentration. Our theoretical analysis of the pore formation suggested that the voltage-induced irreversible breakdown is due to a decrease in edge energy when the pore had formed. After initiation of the pore, the electrical contribution to surface tension is negligible. The time course of the increase of pore size shows that our model of the irreversible breakdown is in good agreement with mechanical properties of membranes reported elsewhere.

The shape of lipid molecules and monolayer membrane fusion

The contact between two bilayer membranes results in their monolayer fusion comprising the formation of a trilaminar structure (a single bilayer connected to two bilayers over the whole perimeter) in the contact region. The time required for monolayer fusion was measured and irreversible electrical breakdown was studied for membranes of different compositions. A theoretical model of the monolayer fusion is suggested to explain the results. It assumes that the structural reorganization underlying the process involves the formation of a stalk between bilayers as a result of local bending of the interacting monolayers. This structural reorganization is similar to the hydrophilic pore formation in a bilayer under irreversible breakdown. However, the directions of the monolayer bending are different in the two processes and, therefore, the bending energies depend oppositely on the effective shape of lipid molecules. Theoretical predictions agree well with experimental data. The applicability of the suggested mechanism to biomembrane fusion is discussed.

An investigation of electrical breakdown of bimolecular lipid membranes

The investigation is concerned with the irreversible electrical breakdown of bimolecular lipid membranes, depending on the velocity of linear voltage scanning. It was found that the membrane breakdown potential depended on the velocity of electric field variation. For instance, at voltage scanning velocities of up to 0.1 V/s, the rupture of membrane from glycerol monooleate occurs at 0.20–0.25 V and, at velocities higher than 1 V/s, at 0.5–0.6 V. Then the film breakdown depending on lipid phase transition was studied. At high velocities of imposed voltage scanning, the disruption of the bimolecular lipid membranes was shown not to depend on their phase states; at the same time, at low velocities, one could note a slight difference in the stability of the films at temperatures higher and lower than those of the phase transition. Whereas transition from gel to liquid-crystalline state involves transition from an ordered to a less ordered membrane structure with a sharp increase in the number of defects in the membrane, the authors, conclude that the film breakdown in the second case occurs by the ‘defect’ mechanism suggested earlier. It was also assumed that, in certain cases involving low velocities of voltage scanning, membrane breakdown may occur because of variation in the interfacial tension and in the contact angle between the film and torus. Possible mechanisms of the membrane irreversible electrical breakdown at high velocities of voltage variation are discussed. It was shown that breakdown should occur as a result of membrane compression in an electric field by a mechanism previously examined. The elastic moduli of a number of membranes were calculated by the breakdown criterion suggested earlier. They were found to coincide with the results of other investigators and, depending on the type of lipid, to equal 10 5–10 6 Pa.