Low Conductivity Media Research Articles

Cell transport and suspension in high conductivity electrothermal flow with negative dielectrophoresis by immersed boundary-lattice Boltzmann method

The cell transport and suspension using AC electrokinetics is essential for cell patterning and other biomedical applications in microfluidics. To avoid the undue cellular stress and irreversible damage to cells caused by low conductivity media, direct manipulations of cells in physiological solution of high electrical conductivity without dilution becomes significant. The driving mechanism of alternating current electrothermal (ACET) flow makes it attractive for pumping the physiological conductivity solution and transporting cells through the electrohydrodynamic (EHD) force. In addition, negative dielectrophoresis (nDEP) force is induced on a cell when its electrical conductivity is lower than that of solution media. In this paper, the effectiveness of ACET flow and negative DEP force in high conductivity solution is novelly used simultaneously to achieve a successful long-range cell transport and suspension in the microfluidic chamber. An immersed boundary-lattice Boltzmann method (IB-LBM) is developed to investigate the cell transport and suspension mechanism with respect to AC voltage magnitude, electrical conductivities of cell and solution, cell initial position, and cell size. It is found that a sufficient DEP force is indispensable for stabilizing the cell transport process and anchoring cells by overcoming the cell-cell interaction. Based on this, the design of a lab-on-a-chip device to generate a large DEP force is essential for future research to realize an efficient AC electrokinetic-based cell transport and suspension in physiological fluids.

The Integral-Equation Method in Low-Frequency Electrodynamics of Nonhomogeneous Contrast Media

The article analyzes computations of low-frequency electrodynamic fields in a nonhomogeneous conducting medium using the integral equation method. Contrast effects arise when the nonhomogeneities are embedded in a low-conductivity medium. As the conductivity of the enclosing medium decreases, tighter grid spacing is required in the nonhomogeneous region.

Impact of glycerol on Zinc Oxide based thin film transistors with Indium Molybdenum Oxide electrodes

ABSTRACTThin-film transistors (TFT) were fabricated at a room-temperature (RT) with zinc oxide (ZnO) channel and indium molybdenum oxide (IMO) electrodes. To isolate the gate oxide and gate electrode influence on the device performance, common gate configuration on a commercial substrate with thermal SiO2 (100 nm) was selected. A threshold voltage (VTh) of 10 V and ION/IOFF ratio of 1 × 10-5 were obtained. Once the reference data was taken transistors were exposed to glycerol. Temporary changes in device characteristics were observed due to the influence of glycerol, a low conductivity medium. To exclude the possibility of sugar alcohol being the main conductor, measurement on dummy transistor electrode was performed retaining the distance between probes. The TFT device under test revealed ten times higher drain current but also a change in threshold voltage and leakage current. Transistors under glycerol influence were always open with the positive gate bias.

On-chip DNA preconcentration in different media conductivities by electrodeless dielectrophoresis.

Electrodeless dielectrophoresis is the best choice to achieve preconcentration of nanoparticles and biomolecules due to its simple, robust, and easy implementation. We designed a simple chip with microchannels and nano-slits in between and then studied the trapping of DNA in high conductive medium and low conductive medium, corresponding to positive and negative dielectrophoresis (DEP), respectively. It is very important to investigate the trapping in media with different conductivities since one always has to deal with the sample solutions with different conductivities. The trapping process was analyzed by the fluorescent intensity changes. The results showed that DNA could be trapped at the nano-slit in both high and low conductive media in a lower electric field strength (10 V/cm) compared to the existing methods. This is a significant improvement to suppress the Joule heating effect in DEP related experiments. Our work may give insight to researchers for DNA trapping by a simple and low cost device in the Lab-on-a-Chip system.

Real-time dielectrophoretic signaling and image quantification methods for evaluating electrokinetic properties of nanoparticles.

Real‐time image signaling and quantification methods are described that allow easy‐to‐use, fast extraction of the electrical properties of nanoparticles. Positive dielectrophoretic (pDEP) collection rate analysis enables the dielectric properties of very small samples of nanoparticles to be accurately quantified. Advancing earlier work involving dual‐cycle pulsed pDEP 1 collection experiments, we report the development of a statistical image quantification method that significantly advances the evaluation of nanoparticle dielectric properties. Compared with traditional methods that require information about the geometry of the electrode array to be entered for semiautomated quantification 2, the new statistical approach described does not require a priori knowledge of device geometry. The efficacy of the statistical method is experimentally demonstrated using 200 nm diameter latex nanospheres, suspended in low conductivity medium, that are attracted by pDEP onto planar castellated electrode arrays with 5‐micron‐sized features. The method is shown to yield estimates for the nanoparticle conductivity and surface conductance, σp=25.8 mS/m and KS=1.29 nS, that concur closely with those obtained using traditional geometric methods previously reported 1. Consequently, the statistical method is accurate, fast, robust, supervisor‐free, and useful for determining nanoparticle electrokinetic parameters.

Cell membrane permeabilization by 12-ns electric pulses: Not a purely dielectric, but a charge-dependent phenomenon

Electric pulses of a few nanoseconds in duration can induce reversible permeabilization of cell membrane and cell death. Whether these effects are caused by ionic or purely dielectric phenomena is still discussed. We address this question by studying the impact of conductivity of the pulsing buffer on the effect of pulses of 12ns and 3.2MV/m on the DC-3F mammalian cell line. When pulses were applied in a high-conductivity medium (1.5S/m), cells experienced both reversible electropermeabilization and cell death. On the contrary, no effect was observed in the low-conductivity medium (0.1S/m). Possible artifacts due to differences in viscosity, temperature increase or electrochemical reactions were excluded. The influence of conductivity reported here suggests that charges still play a role, even for 12-ns pulses. All theoretical models agree with this experimental observation, since all suggest that only high-conductivity medium can induce a transmembrane voltage high enough to induce pore creation, in turn. However, most models fail to describe why pulse accumulation is experimentally required to observe biological effects. They mostly show no increase of permeabilization with accumulation of pulses. Currently, only one model properly describes pulse accumulation by modeling diffusion of the altered membrane regions.

Assessment of cytoplasm conductivity by nanosecond pulsed electric fields.

The aim of this paper is to propose a new method for the better assessment of cytoplasm conductivity, which is critical to the development of electroporation protocols as well as insight into fundamental mechanisms underlying electroporation. For this goal, we propose to use nanosecond electrical pulses to bypass the complication of membrane polarization and a single cell to avoid the complication of the application of the "mixing formulas." Further, by suspending the cell in a low-conductivity medium, it is possible to force most of the sensing current through the cytoplasm for a more direct assessment of its conductivity. For proof of principle, the proposed technique was successfully demonstrated on a Jurkat cell by comparing the measured and modeled currents. The cytoplasm conductivity was best assessed at 0.32 S/m and it is in line with the literature. The cytoplasm conductivity plays a key role in the understanding of the basis mechanism of the electroporation phenomenon, and in particular, a large error in the cytoplasm conductivity determination could result in a correspondingly large error in predicting electroporation. Methods for a good estimation of such parameter become fundamental.

Fluorometric assay to compensate for non-viable cells during electroporation.

A fluorometric assay is described that allows adjustment for non-viable cells that result during electroporation. The technique, unlike others, relies on only one dye, requires a single instrument, and eliminates the need for a separate cell counting step. Murine melanoma (B16-F10) cells were electroporated using electric fields ranging from 400 to 2500 V/cm in the presence of SYTOX(®)-green. Compensation for the fluorescence resulting from non-viable cells was facilitated by a correction curve established by lysing a known number of cells in the presence of SYTOX(®)-green. In uncorrected data, an applied electric field of 2500 V/cm increased dye delivery but also reduced cell viability significantly. Compensating for the fluorescence of non-viable cells showed that changing the field strength to 800 V/cm or 2500 V/cm from 400 V/cm had only marginal effects on membrane pore formation. The fluorometric assay was used to compare electroporation in high conductivity (PBS) and a low conductivity medium (LC-PBS). Statistically significant increases of 10 to 30-fold were observed for cells electroporated at 400 V/cm and 800 V/cm in LC-PBS.

Pin–Plate Electrode System for Emulsification of a Higher Conductivity Leaky Dielectric Liquid into a Low Conductivity Medium

In this experimental study we propose the use of highly nonuniform electric field to emulsify a leaky dielectric oil into another oil. Specifically, a pin–plate electrode system is used to emulsify castor oil into silicone oil. The method is suitable for the emulsification of a higher conductivity leaky dielectric oil dispersed in a lower conductivity medium and is suitable for an already existing emulsion, unlike electrospray methods. The process is stabilized by charging of the dispersed phase and the associated electrohydrodynamic flow. A balance of electrocoalescence and breakup leads to a stationary drop size distribution. A short process time indicates its suitability for continuous operation.

Copper-immobilized platinum electrocatalyst for the effective reduction of nitrate in a low conductive medium: Mechanism, adsorption thermodynamics and stability

The electrocatalytic reduction of NO3− and its intermediate NO2− in neutral medium was performed at a Cu-immobilized Pt surface. The voltammetric investigations showed that the bare Cu electrode has little effect on nitrate reduction reactions (NRR) whereas an enhanced catalytic effect (i.e. a positive shift of the peak potential and an increased reduction current) was observed when Cu particles were immobilized onto Pt surface. At the Cu–Pt electrode surface, the NRR process was observed to occur via a two-step reduction mechanism with a transfer of 2 and 6 electrons in the first and second steps, respectively. Similar results were obtained by chronoamperometric (CA) studies. Closer NRR mechanistic studies at the as prepared Cu–Pt electrode revealed concentration-dependent kinetics with a “critical” nitrate ion concentration of ca. 0.02M. Moreover, NRR proceeded via a simple adsorption–desorption mechanism following a Langmuir isotherm with an adsorption Gibbs free-energy of ca. −10.16kJmol−1 (1st step) and ca. −10.05kJmol−1 (2nd step). By means of a Pt|Nafion|Cu–Pt type reactor without any supporting electrolyte, bulk electrolysis was performed to identify nitrate reduction products. It was found that after 180min of electrolysis, 51% of NO3− was converted into NO2− intermediate. This percentage decreased to 30% in CO2 buffered conditions. However, when a tri-metallic Pt–Pd–Cu electrode was employed as a cathode, all of the NO2− produced could be successfully converted into NH3 and N2. The electrocatalysis of nitrate ion on Cu–Pt electrode surface showed no apparent surface poisoning as confirmed by its stability after excessive CV runs. This was further supported by surface analysis and morphology of the as-prepared catalyst with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis.

Novel Nanosecond Pulsed Electric Field Device for Noncontact Treatment of Cells in Native Culture Conditions

Nanosecond pulsed electric fields (nsPEFs) have been found to induce either cell death or cellular changes to a wide range of cells. However, for the current way of delivering the electric field to the cell (by electrodes contacting the cell culture), cells need to be placed in low-conductivity media unsuitable for maintaining cell growth. Here, we present a novel device for noncontact exposure of cells to nsPEF. The cells can be treated in their native cell culture medium and culture vessel. A single-turn loop antenna is designed that can contain a cylindrical cell culture vessel. The electric field within the loop is created by magnetic induction and depends on the time derivative of the current through the loop. To generate the required field strength of 10 kV/cm with duration of ~ 6 ns, a fast high-voltage, low-inductance RLC circuit is developed. A fast rising current of 100 A/ns is achieved in this circuit. The developed method is used for the nsPEF treatment of either adherent [Chinese hamster ovary (CHO-K)] or suspension (CHO-S) cells in their native culture medium and vessel. We discovered that nsPEF enhances cell proliferation. This novel effect of nsPEF has significant applicability in improving the yield and lowering the cost of cell cultures and bioreactors.

Scaled Experimental Verification of Single-Well Induction Conductivity Measurement Through Nonmagnetic Casing

The electromagnetic induction measurement of dimensions, integrity, or material properties of conductive objects is especially challenging in the case when the electrical conductivity of these objects spans five to seven orders of magnitude. Such an application is the conductivity measurement of the surrounding rocks from within a metal casing of an oil well. The rock conductivity measurement is required in order to determine and revaluate hydrocarbon-bearing layers. We present the first experimental verification of the single-well through-casing induction measurement on a scaled laboratory model of a borehole lined with a stainless steel or copper casing surrounded with a low-conductive medium. The measurements are in the frequency range of 2-16 kHz (10-80 Hz in case of the actual borehole). We explain the theoretical background of the method, describe the scaled model and experimental procedure, and discuss the experimental results. The measurement results are in agreement with the theoretical predictions (relative error less than 15%), and the equivalence of the results for the steel and copper casings (average relative discrepancy less than 10%) corroborates that the rock conductivity measurement can be corrected for variations in the casing properties (dimensions, electrical conductivity, and magnetic permeability).

PEDOT‐Modified Microelectrodes. Preparation, Characterisation and Analytical Performances

AbstractPoly(3,4‐ethylenedioxythiophene) (PEDOT) modified microelectrodes were prepared by electropolymerisation of the relevant monomer from CH3CN or H2O solutions. The electrochemical behaviour of the obtained coatings was investigated by cyclic voltammetry in both organic and aqueous media. The anodic responses obtained for a typical benchmark analyte such as ascorbic acid was used to test the different coatings; calibration curves were built up in order to evaluate the repeatability of the response and the reproducibility of the prepared sensing system. Moreover, the ability of the modified microelectrodes to work in low conductivity media was studied, and exploratory tests in dense food matrix was performed.

Study of Corrosion Inhibition Processes of Copper in a Low Conductivity Medium by a Selected Thiol Compound

not Available.

Theoretical and Experimental Analysis of Electroporated Membrane Conductance in Cell Suspension

An intense electric field can be applied to increase the membrane conductance G(m) and consequently, the conductivity of cell suspension. This phenomenon is called electroporation. This mechanism is used in a wide range of medical applications, genetic engineering, and therapies. Conductivity measurements of cell suspensions were carried out during application of electric fields from 40 to 165 kV/m. Experimental results were analyzed with two electroporation models: the asymptotic electroporation model was used to estimate G(m) at the beginning and at the end of electric field pulse, and the extended Kinosita electroporation model to increase G(m) linearly in time. The maximum G(m) was 1-7 × 10(4) S/m(2), and the critical angle (when the G(m) is insignificant) was 50°-65°. In addition, the sensitivity of electroporated membrane conductance to extracellular and cytoplasmatic conductivity and cell radius has been studied. This study showed that external conductivity and cell radius are important parameters affecting the pore-opening phenomenon. However, if the cell radius is larger than 7 μm in low conductivity medium, the cell dimensions are not so important.

Peak heights and peak widths at half-height in square wave voltammetry without and with ohmic potential drop for reversible and irreversible systems

The behavior of voltammograms in square wave voltammetry in the presence of a significant ohmic potential drop for reversible and irreversible simple electrode reactions has been investigated. It is shown that the effects of ohmic potential drop can be drastically reduced by choosing adequately the square wave amplitude. Zone diagrams are drawn to define the conditions for which peak currents and peak widths at half-height can be expressed by simple formulas without considering the occurrence of ohmic potential drop. It follows that square wave voltammetry can be properly implemented in low-conductive media or in molten salt media where high concentrations of depolarizer are used.

Dielectric spectroscopy of poly(butylene succinate-co -butylene adipate) films

The results of dielectric studies in semicrystalline poly (butylene succinate-co-butylene adipate) (PBSA) by means of dielectric relaxation spectroscopy from 163 to 353 K and in the frequency range of 0.056–17,800 Hz are reported. The subglass β process is found to follow an Arrhenius temperature dependence, whereas the segmental α process follows a Vogel–Fulcher–Tammann temperature dependence. The higher temperature Maxwell–Wagner–Sillars (MWS) process was studied by applying the Bruggeman-Hanai equation, which helped to understand the underlying relaxation dynamics. The MWS process originates from the charge accumulation at the interface between the low-conductivity continuous medium and the high-conductivity dispersed particles. A three-phase model in which the medium comprises the crystalline and rigid amorphous phases (RAPs) and the particle comprises solely the mobile amorphous phases (MAPs) is required to explain the particle fraction variation with temperature. With increasing temperature, the MAP fraction increases at the expense of the RAP fraction within the continuous medium. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

Focus: Chromatography and cell patterning

AbstractCover illustration:Focus: Chromatography and cell patterning. In this issue of BTJ Giorgio Carta and Theresa Bankston present two papers on self‐association chromatography. DOI: http://dx.doi.org/10.1002/biot.201000221, DOI: http://dx.doi.org/10.1002/biot.201000222. The cover image shows negative dielectrophoretic cell patterning of HepG2 human liver cells labeled with fluorescent dye DiO (green) on a chip. Puttaswamy et al. show enhanced cell viability and adhesion using a low conductivity medium. DOI: http://dx.doi.org/10.1002/biot.201000194.

Enhanced cell viability and cell adhesion using low conductivity medium for negative dielectrophoretic cell patterning

Negative dielectrophoretic (n-DEP) cell manipulation is an efficient way to pattern human liver cells on micro-electrode arrays. Maintaining cell viability is an important objective for this approach. This study investigates the effect of low conductivity medium and the optimally designed microchip on cell viability and cell adhesion. To explore the influence of conductivity on cell viability and cell adhesion, we have used earlier reported dielectrophoresis (DEP) buffer with a conductivity of 10.2 mS/m and three formulated media with conductivity of 9.02 (M1), 8.14 (M2), 9.55 (M3) mS/m. The earlier reported isotonic sucrose/dextrose buffer (DEP buffer) used for DEP manipulation has the drawback of poor cell adhesion and cell viability. A microchip prototype with well-defined positioning of titanium electrode arrays was designed and fabricated on a glass substrate. The gap between the radial electrodes was accurately determined to achieve good cell patterning performance. Parameters such as dimension of positioning electrode, amplitude, and frequency of voltage signal were investigated to optimize the performance of the microchip.

In this issue: Biotechnology Journal 10/2010

AbstractSelf‐association chromatographyBankston and Carta, Biotechnol. J. 2010, 5, 1028–1039Bankston and Carta, Biotechnol. J. 2010, 5, 1040–1049Apolipoprotein A‐IMilano (apoA‐IM) is a major component of high density lipoprotein (HDL) involved in cholesterol transport. When oxidized, apoA‐IM consists of a homodimer and reversibly self‐associates to higher order oligomers. It is not only a potential therapeutic protein, but also a useful model to study associating proteins. Giorgio Carta and Theresa Bankston from the University of Virginia (Charlottesville, VA, USA) have studied the separation behavior during anion exchange chromatography in two consecutive papers of this issue. While one explains the effects of urea and salt concentration on the binding characteristics, the other studies the kinetics of adsorption on anion exchangers with different pore sizes. By developing a mechanistic model of the process, they can predict separation of the protein. These models are of special importance to develop separation processes for industrial applications.Cell patterningPuttaswamy et al., Biotechnol. J. 2010, 5, 1005–1015Negative dielectrophoretic (n‐DEP) cell manipulation is an efficient way to pattern human liver cells on micro‐electrode arrays. It employs electric fields that move the cells to a desired place because they are less polarizable than the surrounding medium. Excessive charging of the cellular membrane, the non‐physiological medium and shear stress due to the liquid flow might compromise the cell viability. Researchers from Taiwan have now developed a low conductivity medium containing essential components of DMEM medium at low concentration, suitable for DEP‐based cell manipulation at low voltage and frequency. Here the cells show high viability and cell adhesion. They developed a microchip prototype with a well‐defined positioning of titanium electrode arrays on a glass substrate. These cells on the chip now mimic the lobular morphology of real liver tissue (see cover image).Green tea metabolismSchantz et al., Biotechnol. J. 2010, 5, 1050–1059It is well know that green tea polyphenols have a beneficial health effect, which is primarily accounted to the presence of catechins. However, there is not much known on their degradation in the intestine. Therefore authors from Kaiserlautern, Germany, have studied the microbial metabolism and chemical stability of these polyphenols using a well‐established ex vivo model. Fresh ileal fluids from probands were incubated for 24 h under anaerobic conditions with different catechins. Then, the metabolites were analysed by HPLC‐photodiode array detection and tandem mass spectrometry. Interestingly the same metabolites were detected, although the microbiota were substantially different. The authors concluded that microbiota‐dependent liberation of the metabolites occurs before these compounds reach the colon.