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PubMed1979

Saturation of the response to light in Limulus ventral photoreceptor.

1. Limulus ventral photoreceptor cells were voltage-clamped with two intracellular micro-electrodes. The light-induced membrane current was recorded for brief stimuli. From observation of discrete waves (quantum bumps) at low stimulus energies and the early receptor potential at high energies, the stimulus energy was related to the number of rhodopsin molecules photosiomerized. 2. In the dark-adapted cell the log (peak light-induced current) reached almost its maximum value when about 10(3) of the 10(9) rhodopsin molecules in the cell were photoisomerized. 3. The magnitude of the maximum light-induced current was not significantly altered after iontophoresis of EGTA into the cell. This treatment is known to counteract the Ca2+-mediated reduction in sensitivity to light. 4. Current pulses were injected into the unclamped cell during the receptor potential. The form of the voltage deflexion (a step followed by a curve) suggested that the effective electrical equivalent of the cell was a membrane capacitance in parallel with a light-dependent membrane resistance, Rm, and in series with another, light-invariant, resistance, Rs. Rs ranged from 7 to 24 k omega (five cells). 5. During a receptor potential the ratio Rm/Rs was never observed to fall below 1.7 no matter how intense the light flash. Hence, it is concluded that the light-induced current saturated essentially because Rm fell to a minimum value. 6. Charging curves gave a value for the capacitance, and hence the area, of the surface membrane. From this it was estimated that there were 10(5)-10(6) microvilli on each cell. 7. These results show that the light-induced increase in membrane conductance in a dark-adapted cell comes close to its maximum value when the number of photoisomerizations is about 1/1000 the total number of microvilli. We suggest that absorption of a photon by a rhodopsin molecule in a microvillus causes an increase in membrane conductance on parts of the surface membrane beyond that microvillus. 8. In the presence of moderate background illumination the sensitivity to non-saturating superimposed flashes was greatly decreased (e.g. by 10(3) while the saturating light-induced current was only slightly decreased (e.g. by 15%). At higher background intensities the saturating light-induced current was further decreased (e.g., with a background that photoisomerized 10(6.25) molecules per second the saturating light-induced current was reduced by 47%).

Animals↗

PubMed2006

Improved voltage transfer coefficients for nonconductive materials in radiofrequency glow discharge optical emission spectrometry.

In radiofrequency glow discharge emission spectrometry (RF-GDOES), the excitation voltage used to create the plasma is applied to the back or front end of the sample to be analyzed. In this paper we focus on back-applied voltage systems (a configuration that represents about half of the instruments available on the market), and on applied voltage problems (the power coupling efficiency and materials analysis are beyond the scope of this study). In the RF-GDOES of nonconductive samples, a voltage drop develops inside the material. The voltage transfer coefficient is defined as the ratio between the peak voltage in front of the sample (facing the plasma) and the peak voltage applied to the back of the sample. In this work, we show that it is possible to increase the voltage transfer coefficient by increasing the capacitance of the sample. The capacitance of a given nonconductive material depends on its surface, its thickness and its permittivity. Increasing the voltage transfer coefficient permits higher power deposition in the plasma. This study is based on an electrical equivalent circuit for the discharge device, which takes into account the sample and reactor capacitances as well as the voltage probes used for the measurements. This circuit, when modeled by a commercial electrical circuit simulator, gives the voltage transfer coefficient as a function of the sample capacitance. Different approaches to increasing the sample capacitance and their influence on the voltage transfer coefficient are presented and related to the 750.4 nm argon line intensity, which is correlated to the electron density.

Journal Article↗

PubMed1995

Impedance imaging using induced currents.

The principal features of an electrical impedance tomography system using induced currents are described. Images of the distribution of conductivity in a two-dimensional phantom are obtained using an algorithm based on a sensitivity matrix. Results are also presented which demonstrate the separation of conductivity and permittivity images from measurements of the complex peripheral voltage, the formation of unreferenced permittivity images, and the use of capacitively coupled electrodes in a measuring system without direct electrical contact.

Algorithms↗

PubMed2006

Biophysical assessment of atopic dermatitis skin and effects of a moisturizer.

BACKGROUND: The mechanisms of the skin barrier impairment in patients with atopic dermatitis (AD) are still unknown and need further studying. OBJECTIVE: We evaluated the skin of healthy subjects and of patients having atopic dermatitis with an instrument measuring electrical impedance and other noninvasive methods (transepidermal water loss, capacitance) and studied the effects of a new emollient [Proderm (Pro-Q in the USA)]. METHODS: After a 2-week washout period, we treated clinically noneczematous skin on the forearm of 24 patients with AD and assessed the effects with the noninvasive methods. 22 healthy subjects were used as controls. RESULTS: The findings indicate that barrier function and hydration, and certain patterns of electrical impedance of AD skin are abnormal compared with normal skin. Moreover, there was an increase in hydration in patients' skin after treatment and a reversal of certain impedance indices towards normal. CONCLUSIONS: Our findings demonstrate that the moisturizer we used changes some biophysical parameters when applied to atopic skin. In addition, a technique based on electrical impedance seems to give valuable information in atopic skin studies, especially the effects of moisturizers.

Adult↗

PubMed2004

Microporous silicon and biosensor development: structural analysis, electrical characterisation and biocapacity evaluation.

An investigation of the fabrication of microporous silicon (MPS) layers as a material for the development of an electrolyte insulator semiconductor (EIS) capacitance sensor has been performed. The goal was to create a high surface area substrate for the immobilisation of biorecognition elements. Structural analysis of MPS layers as a function of key etch parameters, namely implant type (p or n), implant dose, hydrofluoric acid (HF) etch concentration and current density has been performed using scanning electron microscopy (SEM). It was possible to image porous layers with average pore diameter as low as 4 nm. n-type silicon samples had larger pore networks than p-type samples and reducing the silicon resistivity led to a reduction in the pores per microm2. It was found that increasing the HF etch concentration reduced the average pore diameter and increased the pores per microm2. Increasing the current density at which the etch was performed has the same effect. Understanding the effect of these parameters allows the MPS layer to be tuned to match specifications for optimum biocapacity. Different MPS layers were electrically characterised using capacitance-voltage and capacitance-frequency sweeps, in order to determine the effect of porosity on increases in surface area. The measured capacitance increased with increasing pores per microm2. p-type silicon with a boron implant in the back of the wafer, which had been etched in 25% HF in ethanol at a current density of 75 mA/cm2 yielded the highest capacitance signal per unit area. The effect of porosity and pore size on the biocapacity of the samples was also determined. For avidin immobilisation, with pores sizes above 5 nm, as the porosity increased the biocapacity increased. MPS fabricated in p-type silicon with a front and back implant etched in 25% HF at a current density of 25 mA/cm2 was used for the capacitance detection of synthetic oligonucleotides.

Biosensing Techniques↗

PubMed1995

Electrical properties of extracted rat liver tissue.

We attempted to investigate the process of ischemia-induced disturbances in the rat liver, employing the electrical bio-impedance technique. The electrical bio-impedance was measured continuously over 6 h by the 4-electrode method, at various incubation temperatures, in six liver samples extracted from male Wistar rats. The electrical properties of biological tissues can be expressed in terms of three parameters: extracellular resistance (Re), intracellular resistance (Ri) and cell membrane capacitance (Cm). These three parameters were calculated from the measured values of the electrical impedance by the curve-fitting technique, using a computer program. The Re value increased rapidly after the rat livers were extracted, and then decreased slowly. The Re value reached a peak after about 13 min at 36 degrees C, and then decreased slowly, becoming constant after 3 h. There was a negative correlation between the Tmax of Re (the time when Re reached a maximum) and the incubation temperature (R = -0.973, P < 0.001). The Ri value decreased once in the early stage after extraction, followed by almost no change and then an increase after 4 h at 36 degrees C. The Cm showed a similar pattern of change to the Re value, and a negative correlation was also found between the Tmax of Cm and the incubation temperature (R = -0.969, P < 0.001). The increase in the Re and Cm values, and the decrease in the Ri value for quite long periods after the blood flow has stopped, suggest an increase in the resistance of extracellular fluid due to a decrease in its volume, an increase in cell membrane capacitance due to cell swelling, and a decrease in cellular fluid resistance due to an increase in its volume. The time when the Cm value decreases rapidly after an initial gradual decrease after the peak corresponds well with the time when the Ri value begins to increase, from which it is estimated that cell lysis proceeds and that the flow of extracellular fluid into the cell begins at this time. The findings of this study suggest the possibility of estimating the changes in liver tissue or the tissue structure due to ischemia.

Animals↗

PubMed1994

Genetic transformation of plants by protoplast electroporation.

This article describes an optimized protocol for the electroporation of tobacco mesophyll protoplasts together with notes and data on the effects of various parameters and suggestions for work with protoplasts of other species. In this protocol, electroporation is achieved by means of electrical pulses from a high-voltage, capacitive-discharge unit. Procedures are described for measurement of protoplast viability with Evan's blue, the detection of transient expression of CAT and GUS gene plasmid constructs, and for the recovery of stable transformants based on selection for kanamycin resistance.

Cell Survival↗

PubMed2003

Surface complexation modeling of uranium(VI) sorbed onto lanthanum monophosphate.

Sorption/desorption are basic processes in the field of contaminant transport. In order to develop mechanistically accurate thermodynamic sorption models, the simulation of retention data has to take into account molecular scale informations provided by structural investigations. In this way, the uranyl sorption constants onto lanthanum monophosphate (LaPO(4)) were determined on the basis of a previously published structural investigation. The surface complexation modeling of U(VI) retention onto LaPO(4) has been performed using the constant capacitance model included in the FITEQLv3.2 program. The electrical behavior of the solid surface was investigated using electrophoretic measurements and potentiometric titration experiments. The point of zero charge was found to be 3.5 and surface complexation modeling has made it possible to calculate the surface acidity constants. The fitting procedure was done with respect to the spectroscopic results, which have shown that LaPO(4) presents two kinds of reactive surface sites (lanthanum atoms and phosphate groups). The uranyl sorption edges were determined for two surface coverages: 40 and 20% of the surface sites that are occupied, assuming complete sorption. The modeling of these experimental data was realized by considering two uranyl species ("free" uranyl and uranyl nitrate complex) sorbed only onto phosphate surface groups according to the previously published structural investigation. The obtained sorption constants present similar values for both surface complexes and make it possible to fit both sorption edges: logK(U)=9.4 for z.tbnd;P(OH)(2)+UO(2)(2+)<-->z.tbnd;P(OH)(2)UO(2)(2+) and logK(UN)=9.7 for z.tbnd;P(OH)(2)+UO(2)NO(3)(+)<-->z.tbnd;P(OH)(2)UO(2)NO(3)(+).

Journal Article↗

PubMed2000

The mechanism of detection of air pollution by an ionization chamber.

The mechanism of detection of chemical vapors in air by an ionization chamber supplied by DC and AC voltage has been described. The theoretical explanation is based on numerical solutions of the differential equations of the cylindrical ionization chamber. The current of the ionization chamber operating in the AC regime has two components: a conductive component, caused by the ions drifts, and a capacitive component, caused by the distortion of the electric field. The ionization chamber operating in the DC regime has only the first component; hence the AC supplied chamber has larger response than the DC supplied chamber.

Air Pollutants↗

PubMed1978

Electrical coupling and uncoupling of exocrine acinar cells.

The electrical communication network in the mouse pancreatic acinar tissue has been investigated using simultaneous intracellular recording with two separate microelectrodes and direct microscopical control of the localizations of the microelectrode tips. All cells within one acinus were electrically coupled, and the coupling coefficient (the electrotonic potential change in a cell neighboring to the cell into which current is injected [V2] divided by the electrotonic potential change in the cell of current injection [V1]) between two cells near each other (less than 50 micron) was always close to 1. Cells farther apart (50-100 micron) were, in some cases, coupled; in other cases, there was no coupling at all. Coupling coefficients varied between 0 and 1. There was rarely electrical coupling over distances of more than 110 micron. Using microiontophoretic acetylcholine (ACh) application, it was possible to evoke almost complete electrical uncoupling of two previously coupled pancreatic or lacrimal acinar cells from different acini or within one acinus. The effects were fully and quickly reversible. While the ACh-evoked uncoupling in the pancreas was associated with membrane depolarization, ACh caused hyperpolarization in the lacrimal acinar cells. The uncoupling was associated with a very marked reduction in electrical time constant, indicating a reduction in input capacitance (effective surface cell membrane area). The concentrations of stimulants needed to evoke reduction in pancreatic cell-to-cell coupling were 1 micron for ACh, 0.14 nM for caerulein, and 3 nM for bombesin. These concentrations are smaller than those required to evoke maximal enzyme secretion.

Acetylcholine↗

PubMed1976

Pharmacological basis of the treatment of orthostatic disorders with ergot alkaloids.

Ergot alkaloids increase the tone of isolated canine vein strips dose-dependently in concentrations considerably lower than noradrenaline, but the maximal responses are only about one third of those to noradrenaline. This action can be blocked by prior administration of phentolamine. It is therefore concluded that the long-lasting stimulant action of ergot alkaloids on vascular smooth muscle is mediated mainly by alpha-adrenoceptors. Using the autoperfused hind limb of the cat, it has been shown that dihydroergotamine increases dose-dependently the tone of the capacitance vessels in a manner very similar to electrical stimulation of the sympathetic nerve with increasing frequencies. However, in contrast to sympathetic nerve stimulation, dihydroergotamine elicits only a very weak increase in arteriolar resistance.

Animals↗

PubMed1996

A new and efficient method for gene transfer into mouse FM3A cells using metaphase chromosomes by electroporation.

We introduced chromosome-mediated genes into mouse thymidine kinase-deficient FM3A (FM3Atk-) cells, by electroporation. The effects of some parameters on the electric shock-mediated transfection of FM3Atk- cells were investigated. Gene transfer of mouse L929 metaphase chromosome DNA into FM3Atk- resulted in a maximum frequency of (3.0 +/- 0.3) x 10(-5) at a cell density of 2.0 x 10(8)/ml and chromosome dosage of 5.0 x 10(7) cell equivalents/ml in a buffer containing 0.25 M mannitol, 0.5 mM MgCl2, 0.1mM CaCl2, and 1 mM Tris-HCl (pH 7.1). The highest yield of the transformants was obtained at an electric field strength of 1 kV/cm and a capacitance of 35 microF, with a single exponentially decaying pulse at 0 degrees C was optimal for post-shock incubation after electroporation. The tk gene was detected in the transformants by in situ hybridization analysis.

Animals↗

PubMed1997

Effects of DNA topology on transformation efficiency of Bacillus subtilis ISW1214 by electroporation.

We report an investigation of electrotransformation by three different topological isomers, circular supercoiled (sc DNA), circular relaxed (cr DNA), and linearized (In DNA) forms of the plasmids pUB110 (4.5 kbp) and pBDR331T (12.6 kbp), of a Gram-positive bacterium, Bacillus subtilis ISW1214. Treatment of the sc DNA with calf thymus topoisomerase I removed the superhelicity and the DNA assumed the relaxed circular form. Treatment of sc DNA with restriction endonculease linearized the DNA. The transformation with the sc DNA of pUB110 resulted in the maximum efficiency of (2.6 +/- 0.6) x 10(5) transformants per microgram DNA higher than that (2.0 +/- 0.3) x 10(4) transformants per microgram DNA for the cr DNA, using the DNA concentration of 20 micrograms/ml at an electric field strength of 7 kV/cm and a capacitance of 10 microF with a single decayed pulse. The transformation efficiency (TE) for the ln DNA was zero. The variations of TE for different topological forms of DNA reflected their relative stability in the host cells. The molecular efficiency (ME, transformants per molecule) for sc DNA was nearly one order of magnitude greater for the lower molecular size of pUB110 DNA than that for the higher molecular size of pBDR331T DNA.

Animals↗

PubMedDate not supplied

[Some aspects of animal-to-human approximation of low frequency electromagnetic field exposure conditions].

Appropriateness of representation of a biological object surface as an equipotential surface has been proved for conditions of a quasistatic exposure to EMF of frequencies lower than 1 MHz. The conditions, at which a self capacitance of a biological object is its basic electrical parameter, have been considered. A factor of animal-to-human approximation of low-frequency EMF exposure conditions was estimated on the basis of equal dose loading in biological objects of different geometric sizes.

Animals↗

PubMed1997

A mechanism of skin appendage macropores electroactivation during iontophoresis.

A physical mechanism of activation of skin appendage macropores under the influence of an electric field is considered theoretically. The macropore is considered as a long cylinder tube which is closed and flattened out before the electric field is applied. The charging of the capacitance of the macropore walls is the driving force of electroactivation. During this process the free energy of the system decreases, which is energetically favourable and results in water pulled into the tube and to a gradual opening of the tube. It is shown that consideration of the macropore wall conductance leads to a considerable slowdown of electroactivation. The opening time of a separate macropore is estimated. It is equal to 30 min for a macropore 4 mm in length. The dependence of the surface density of activated macropores on time is calculated theoretically. The obtained theoretical results are in a good agreement with the literature data.

Administration, Cutaneous↗

PubMed1997

Electrical impedance measurements of root canal length.

Electronic methods are now widely used during endodontic treatment for the assessment of root canal length. These commonly measure the electrical resistance or impedance between the root canal and the buccal mucosa. A number of studies have been undertaken to determine the accuracy of commercially available instruments. The aims of this investigation were to determine the electrical impedance characteristics of the root canal and periapical tissues in vivo, measure the changes relative to the distance of an endodontic instrument from the apical constriction and propose an equivalent circuit modelling the periapical tissues. The length of the root canals of 20 previously untreated teeth were determined using radiographic and electronic methods. Minimal canal preparation was carried out and measurements were made with a size 10 K-Flex file. A microprocessor-controlled LCR analyser was used to measure the electrical impedance characteristics of each root canal. The instrument measured the series and parallel resistive (RS, RP) and capacitance (CS, CP) component of the tissues at two test frequencies, 100 Hz and 1 kHz. Measurements were made for each root canal when the diagnostic file was placed at the apical constriction and repeated when the file was withdrawn to -0.5, -1.0, -1.5, -2.0 and -5.0 mm from the foramen. Readings were taken for each canal after the canal had been dried with paper points, and flooded first with deionised water and then with sodium hypochlorite. The root canals were then prepared, cleaned and obturated using standard endodontic procedures. The LCR analyser selected the series resistance component as the major measurement parameter. There was a clear increase in series resistance (RS) with increasing distance from the radiographic apex for dry canals and those containing deionised water and sodium hypochlorite. The mean resistance for dry canals was markedly higher than for those containing fluid, ranging from 22.19 k omega to 92.07 k omega in comparison with 9.32 k omega to 12.10 k omega for deionised water and from 7.46 k omega to 8.92 k omega for canals containing sodium hypochlorite. There was a marked change in the series and parallel resistive component with distance from the apex, suggesting a complex relationship between the impedance of the smear layer and bulk dentine. The impedance characteristics of a root canal were a complex electrical network comprising resistive and capacitive series and parallel elements.

Dental Pulp Cavity↗

PubMed1985

Electromagnetic fields in biological studies.

For biological or cellular experiments using electromagnetic fields, it is essential that the parameters defining the field be carefully specified if the results are to be meaningful and are to be compared with the same experiment conducted in a different laboratory. The interaction of living systems with electric and magnetic fields can come only through forces exerted on the charges on the system. If the charges are stationary, the only origin of the force is the electric field. This electric field may be established by charge distributions, as in "capacitive plate" experiments, or by time-varying magnetic fields. A geometry commonly used to produce time-varying magnetic fields consists of a pair of coaxial coils, each of equal radius and separated by a distance often equal to the radius. The electric field induced by a varying current in such a pair of coils varies both in space and in time. The electric field strength is zero on the axis of symmetry, and increases to a maximum near the radius of the coils. The strength is proportional to the time rate of change of the current in the coil, which depends not only on the amplitude and shape of the voltage pulse applied to the coil but also on the resistance and inductance of the coil. The purpose of this article is to describe how these important physical parameters may be determined for both geometries.

Animals↗

PubMed1991

Computer-aided design of two-dimensional electric-type hyperthermia applicators using the finite-difference time-domain method.

A hyperthermia applicator design tool consisting of a finite-difference time-domain (FDTD) technique in combination with a graphical display of electric fields and normalized linear temperature rise is described. This technique calculates, rather than assumes, antenna current distributions; it includes mutual interactions between the body and the applicator, and it calculates driving-point impedance and power delivered to the applicator. Results show that the fundamental limitation of 2-D electric-type applicators is overheating of the fat by normal components of the electric field, which exist because of near fields and capacitive coupling with the muscle. Two factors which contribute to the capacitance are the muscle conductivity and the small antenna size in air. Two examples of applicators designed to avoid fat overheating are described: a 27-MHz segmented dipole for heating large tumors to 7 cm depth, and a 100-MHz dipole for small tumors to 5 cm depth. The first uses a water bolus, and the second uses a water bolus with low-permittivity strips to reduce normal fields at the antenna ends. The results of this study describe fundamental limitations of electric field applicators, and illustrate the use of a powerful applicator design tool that allows rapid evaluation of a wide range of ideas for applicators which would require months and years to test experimentally.

Algorithms↗