Faradic resistance of the electrode/electrolyte interface

A new method is used to measure the direct-current (Faradic) resistance of a single electrode/electrolyte interface. The method employs a constant-current pulse and a potential-sensing electrode. By choosing a sufficiently long pulse duration, the voltage between the test and potential-sensing electrode exhibits a three-phase response. In the steady-state phase, the voltage measured is equal to the current flowing through the electrode Faradic resistance and the resistance of the electrolyte between the test and potential-sensing electrode. By measuring this latter resistance with a high-frequency sinusoidal alternating current, the voltage drop in the electrolyte is calculated and subtracted from the voltage measured between the test and potential-sensing electrode, thereby allowing calculation of the Faradic resistance. By plotting the reciprocal of the Faradic resistance against current density and fitting the data points to a third-order polynomial, it is possible to determine the zero-current density (Faradic) resistance. This technique was used to determine the Faradic resistance of electrodes (0·1 cm²) of stainless-steel, platinum, platinum-iridium and rhodium in 0·9 per cent NaCl at 25°C. The zero current Faradic resistance is lowest for platinum (30·3 kΩ), slightly higher for platinum-iridium (47·6kΩ), much higher for rhodium (111 kΩ) and highest for type 316 stainless-steel (345 kΩ). In all cases, the Faradic resistance decreases dramatically with increasing current density.     

Electrical properties of metallic electrodes

The series equivalent resistance R and capacitance C of metal/saline electrode/electrolyte interfaces were measured as a function of frequency (100 Hz–20k Hz) and current density (0·25 to 1000 A m⁻²) for eight typical electrode metals. For each of the metals tested, R decreased and C increased as the current density was increased above a critical value (with the exception of silver and MP35N at frequencies above 1 kHz for which R increased and C decreased slightly). With the exception of copper, the current density linearity limit (for 10 per cent decrease in R or 10 per cent increase in C) increased with increasing frequency and, in most cases, the current density linearity limit for 10 per cent increase in C was slightly less than that for 10 per cent decrease in R. Among the metals tested, copper and aluminium had the lowest current carrying capability and rhodium had the highest current-carrying capability. The current carrying capabilities of 316 SS, platinum, silver and MP35N, were intermediate and similar. With increasing current density, an increase in the electrode/electrolyte capacitance was the most sensitive indicator of the current-carrying linearity limit.     

Thoracic geometry and its relation to electrical current distribution: consequences for electrode placement in electrical impedance cardiography

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10⁻³ (Ωcm)⁻¹ and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.     

Electrosynthesis of poly(3-methylthiophene) films with high strength and stabile conductive properties

The electrochemical polymerization of 3-methylthiophene was significantly facilitated by the presence of freshly distilled boron fluoride-ethyl ether as supporting electrolyte and stainless steel as electrode during the polymerization. The lower oxidation potential of the monomer led to high overall stereoregularity and stacking order of poly(3-methylthiophene) chains. A current density of 1 mA/cm² and a monomer concentration of 35–70 mmol/L turned out to be optimal electrosynthesis conditions. Poly(3-methylthiophene) films had high tensile strength (73 MPa) and flexibility. The high stereoregularity of poly(3-methylthiophene) resulted in high conductivity with improved environmental stability.     

Development of a platinized platinum/iridium electrode for use in vitro

Silver/silver chloride (Ag/AgCl) electrodes possess excellent electrical properties for measuring the electrical activity of gastrointestinal smooth muscle but exert toxic effects on this tissue in vitro. We thus developed a platinum electrode for use in vitro, the construction of these electrodes relying upon the formation of a glass-platinum/iridium seal. The platinum/iridium (Pt/Ir) electrodes were platinized using a current density of 0.45 mA mm⁻². The electrode impedance at 0.01 Hz showed a minimum with platinization current-time products greater than 500 mA s mm⁻². However, deposits in excess of 600 mA s mm⁻² were readily removed by mechanical abrasion and proved unsatisfactory. Optimal platinization was obtained with a deposit of platinum-black corresponding to a current-time product of 550 mA s mm⁻². Optimally-platinized electrodes (geometric surface area 0.11 mm²) had a stable and reproducible potential with a drift of less than 1 μV min⁻¹ and a lower impedance than optimally chlorided silver electrodes (geometric surface area 0.46 mm²) at frequencies higher than 0.25 Hz. The platinized Pt/Ir electrodes were used to record the electrical activity of gastrointestinal smooth muscle in vitro.     

Optimum electrolytic chloriding of silver electrodes

The low frequency impedance of bare silver electrodes in contact with physiological saline was found to exhibit capacitive reactance and by the deposition of chloride the impedance became resistive in nature. It was found that a chloride deposit of 100–500 mA.sec/cm² of electrode area provided the lowest electrode-electrolyte impedance. Prolongation of chloriding beyond this range increased the electrode-electrolyte impedance at all frequencies but did not alter the resistive nature of the impedance. To achieve a chloride deposit which is proportional to the product of mA and sec it was found that a minimum chloriding current density of 5 mA/cm² of electrode area should be used.     

Some studies related to electricity generation from biological fuel cells and galvanic cells,in vitro andin vivo

A biological fuel cell such as an oxygen/glucose cell has been considered to be an ideal power source for implantable cardiac pacemakers and similar devices. This study is mainly concerned with the development of a single oxygen/glucose cell, which can be implanted inside the body fluids with the simultaneous presence of oxidant and fuel(s). Present results ofin vitro experiments with a single oxygen/glucose cell consisting of a platinum black on platinum-mesh electrode (cathode) and a platinum black on porous-graphite electrode (anode) have shown that cell performance was markedly increased after glucose addition. Half-cell testings by means of a potentiostat coupled to a linear sweep generator also demonstrated that the anode current of a platinised graphite electrode after glucose was over 3 mA/cm². It was also found that a steady and continuous power output of 20 μW/cm² could be generated from the above oxygen/glucose cell. When the cell was testedin vivo in the rat. the continuous power output was steady at 3·3 μW/cm². The present study also investigated the possibility of generating electricity from an oxygen concentration cell and an oxygen/hydrogen cell. It was concluded that a platinum black on porous graphite electrode offers promise as a specific electrode for glucose oxidation. An oxygen/hydrogen cell could also provide a long termin vivo power supply provided the Pd-H electrode is properly encapsulated. The significance of the present study in relation to the development of biological power sources is discussed, and the importance of fundamental investigation for the development of a specific electrode is emphasised.     

Design of electrodes and current limits for low frequency electrical impedance tomography of the brain

For the novel application of recording of resistivity changes related to neuronal depolarization in the brain with electrical impedance tomography, optimal recording is with applied currents below 100 Hz, which might cause neural stimulation of skin or underlying brain. The purpose of this work was to develop a method for application of low frequency currents to the scalp, which delivered the maximum current without significant stimulation of skin or underlying brain. We propose a recessed electrode design which enabled current injection with an acceptable skin sensation to be increased from 100 μA using EEG electrodes, to 1 mA in 16 normal volunteers. The effect of current delivered to the brain was assessed with an anatomically realistic finite element model of the adult head. The modelled peak cerebral current density was 0.3 A/m², which was 5 to 25-fold less than the threshold for stimulation of the brain estimated from literature review.     

Another Look at the Relationship of Electrodermal Activity to Electrode Contact Area

The present study was undertaken lo reexamine the hypothesis that the relationship between skin conductance and electrode size is monotonic and linear. Skin conductance activity was recorded from 48 right-handed male subjects using 6 different sixes of electrode collars ranging in exposed surface area from .131 cm² to .786 cm². The dependent measures were skin conductance level (SCL); skin conductance response (SCR) amplitude to a series of 8 loud tones; latency, rise time, and recovery half-time of the first tone elicited response; (he largest self-generated SCR; and the number of nonspecific responses. The results indicated a significant linear relationship between contact area and SCL, stimulus and self-generated SCR amplitude, and the number of nonspecific responses. Latency was not affected by electrode size although the other time-based measures were. Differences in skin conductance activity were found among different palmar recording sites. The observed linear relationship between electrode size and electrodermal measures has implications for current models of electrodermal activity and for the comparison of results across studies in which different electrode contact areas are used.     

Computation of the impedance characteristic of metal electrodes for biological investigations

In a previous study, basic electrochemistry was used to derive analytical expressions for the equivalent network components of the a.c. impedance of biological needle electrodes. In the present paper, numerical parameter values are determined and inserted into these expressions. Matching of computer-calculated characteristics with experimental results has been used to determine certain constants, which are difficult to determine in a more direct way. The electrode impedance at very low frequencies turns out to be almost purely resistive in character and mainly determined by the exchange current density of the metal-electrolyte system. In the intermediate-frequency range the diffusion impedance together with the double-layer capacitance are the decisive factors. The magnitude of the impedance diminishes here with fⁿ, where ?1相似文献   

The Relationship of EDA to Electrode Size

The pari passu relationship between contact area of electrode and SCLs and SCR amplitudes which is predicted on the basis of current electrical models of EDA was investigated in a series of experiments employing a wide range of commonly used contact areas: .017 to .786 cm² (.15 to 1.0 cm diameter). Having systematically eliminated obvious methodological and procedural problems a nonmonotonic relationship remained. On the basis of these findings two recommendations were made: 1) the use of an intermediate size, .5 cm² (.80 cm diameter), of electrode in recording EDA; 2) SCLs and SCR amplitudes should be reported in terms of actual electrode size rather than specific conductance units as previously suggested. No effect of electrode size was found for time-based measures of EDA. Results are discussed in terms of practical implications.     

Assessment of the humane aspects of electric lancing of whales by measurement of current densities in the brain and heart of dead animals

The potential physiological effects of the electric lance are assessed, as used in Japanese whaling operations. Current densities are measured in the brains and hearts of six whales to which a controlled current of 5 A is applied by two electrodes inserted at various sites in the carcasses. The whales vary in size from 1.8 m (200 kg) to 16 m (40 t). The minimum current density in the brain necessary to cause depolarisation of neurones is estimated to be 10 mA cm⁻² and to cause ventricular fibrillation is estimated to be 0.5 mA cm⁻². No current densities exceeding 4.8 mA cm⁻² are recorded in the brain. Very few recordings of current density from the heart are above 0.5 mA cm⁻², and they occurr only when electrodes are in optimal positions. When electrodes are placed as in whaling operations, no whale over 3 m in length would receive current densities in the heart or brain sufficient to cause permanent dysfunction. It is concluded that electric lancing is ineffective as a secondary method of killing whales and that the current densities recorded could cause pain and suffering to an already distressed animal.     

Controlled metallic nanopillars for low impedance biomedical electrode

Radial metallic nanopillar/nanowire structures can be created by a controlled radiofrequency (RF) plasma processing technique on the surface of certain alloy wires, including important biomedical alloys such as MP35N (Co–Ni–Cr–Mo alloy), platinum–iridium and stainless steel. In electrode applications such as pacemakers or neural stimulators, the increase in surface area in elongated MP35N nanopillars allows for decreased surface impedance and greater current density. However, the nanopillar height on MP35N alloy tends to be self-limiting at ∼1–3 μm. The objective of this study was to further elongate the radial nanopillars so as to reduce electrode impedance for biomedical electrode applications. Intelligent experimental design allowed for efficient investigation of processing parameters, including plasma material, process duration, power, pressure and repetition. It was found that multi-step repeated processing in the parameter-controlled RF environment could increase nanopillar height to ∼10 μm, a 400% improvement, while the RF plasma processing with identical total duration but in a single step did not lead to desired nanopillar elongation. Measurement of electrode impedance in phosphate-buffered saline solution showed an associated decrease to one-fifth of the surface impedance of unprocessed wire for signals below 100 Hz. For the purposes of this study, MP35N and Pt–Ir wires were characterized and demonstrated augmented surface impedance properties which, in combination with superior cell integration, enhanced biomedical electrode performance.     

Comparison of electrical transients and corrosion responses of pulsed MP35N and 316LVM electrodes

Functional neuromuscular stimulation (FNS) is often limited by electrode malfunctions such as corrosion and breakage, particularly for intramuscular and epimysial type electrodes. As a result, the electrochemical charge injection characteristics and corrosion responses of single strand 316LVM stainless steel and MP35N nickel-cobalt alloy electrodes were evaluatedin vitro. For charge balance, capacitor coupled monophasic protocols with varying charge injections were employed. Electrodes were evaluated with either positive-first or negative-first pulses, 60 Hz, 100 μsec pulse duration, and stimulation periods from 100 to 240 hours. Charge injection densities ranged from 20 to 80 μC/cm². For both anodic-first and cathodic-first pulsing, the potential transients for the MP35N electrodes were more extreme than for the 316LVM electrodes over the test period, and increased corrosion was apparent on the MP35N electrodes from both optical and scanning electron microscopy. Therefore, 316LVM, but not MP35N, may be suitable for FNS applications with charge injection densities less than 40 μC/cm².     

Theory and design of capacitor electrodes for chronic stimulation

Conventional metal electrodes generate electrochemical byproducts during stimulation of nerve or muscle. These byproducts may cause tissue damage, especially with the long-term stimulation necessary with neural prosthetic devices. To prevent the possibility of such damage, completely insulated electrodes have been devised which deliver current pulses by capacitive charging of the electrode surface, not involving electrochemical reactions. Anodised discs of porous tantalum, 1·0 mm in diameter and 0·25 mm thick, can deliver 0·5 ms, 5 mA pulses. Such electrodes are available as components of commercial capacitors and are easily adapted for biological use. The design may be optimised by mathematical analysis of an equivalent electrical circuit.In vitro tests demonstrate a clear advantage of these electrodes over capacitively coupled platinum-iridium electrodes in preventing oxidation-reduction reactions. The electrodes are stable on chronic implantation and should provide a safer interface between neural prosthetic devices and human tissue.     

Electrical characteristics of conductive yarns and textile electrodes for medical applications

Clothing with conductive textiles for health care applications has in the last decade been of an upcoming research interest. An advantage with the technique is its suitability in distributed and home health care. The present study investigates the electrical properties of conductive yarns and textile electrodes in contact with human skin, thus representing a real ECG-registration situation. The yarn measurements showed a pure resistive characteristic proportional to the length. The electrodes made of pure stainless steel (electrode A) and 20% stainless steel/80% polyester (electrode B) showed acceptable stability of electrode potentials, the stability of A was better than that of B. The electrode made of silver plated copper (electrode C) was less stable. The electrode impedance was lower for electrodes A and B than that for electrode C. From an electrical properties point of view we recommend to use electrodes of type A to be used in intelligent textile medical applications.     

Deposit free electrolytic brain lesions: Methodologic and histologic observations

The morphologic features of deposit free electrolytic brain lesions made by passing cathodal current through stainless steel electrodes or anodal current through platinum-iridium electrodes were studied. Both procedures produced lesions which resemble those made with radio frequency current in that they are composed of a central cavity surrounded by a narrow annulus of pathologic tissue and are equally effective in grey and white matter. It was found that problems of current blockage due to gas formation and of irregular lesion configuration can be minimized by increasing the electrode tip exposure and decreasing the intensity of the current. Lesions made with a platinum anode were more uniformly of smooth, spherical shape than were cathodal lesions. Reliable relationships between coulombs of current and lesion volumes were demonstrated for both cathodal and anodal deposit-free procedures.     

Generation of unidirectionally propagating action potentials using a monopolar electrode cuff

Unidirectionally propagating action potentials, which can be used to implement transmission failure on peripheral nerve through “collision block,” have been generated electrically on cat myelinated peripheral nerve using a monopolar electrode cuff with the conductor positioned closest to the “arrest” end of the cuff. A single cathode located at least 5 mm from the arrest end resulted in unidirectional propagation with minimal current and charge injection. The range of stimulus current values that produced unidirectional propagation increased with increases in longitudinal asymmetry of cathode placement over the range of asymmetries tested (1.7: 1 to 7: 1). The stimulus current pulse that minimized charge injection was quasitrapezoidal in shape with a plateau pulse width of approximately 350 μsec and an exponential trailing phase having a fall time (90%-10%) of approximately 600 μsec. These parameters were found to be independent of cuff geometry. Arrest efficiency was not degraded using a cuff of sufficient internal diameter to prevent nerve compression in chronic implantation. The critical current density within the extracellular space of the electrode cuff required to produce conduction failure at the arrest end was estimated to be 0.47±0.08 mA/mm².     

Poly(3,4-ethylenedioxythiophene)/multiwall carbon nanotube composite coatings for improving the stability of microelectrodes in neural prostheses applications

With the purpose of improving the stability of microelectrodes under continuous high charge density stimulation, which is required for neural prostheses applications such as visual prostheses, multiwall carbon nanotube (MWCNT)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) composite films were coated onto a platinum microelectrode by electrochemical polymerization. Galvanostatically polymerized PEDOT/MWCNT films demonstrated superior characteristics compared to polystyrene sulfonate doping and potentiostatic polymerization, including a three-dimensional cone morphology and enhanced electrochemical performance (the safe charge injection limit reached 6.2 mC cm^?2 for cathodic-first pulses). Most important of all, the improved stability of the coatings has been revealed through stimulation for 96 h using 3.0 mC cm^?2 current pulses in bicarbonate- and phosphate-buffered saline solution. Cell assays revealed that PEDOT/MWCNT films could promote the adhesion and neurite outgrowth of rat pheochromocytoma cells. Finally, platinum wires coated with PEDOT/MWCNT films were implanted into rat cortex for 6 weeks for histological evaluation. Glial fibrillary acidic protein and neuronal nuclei staining revealed that the films elicit a lower tissue response compared to platinum implants. These results suggest that the galvanostatically polymerized PEDOT/MWCNT films can improve the stability of stimulation microelectrodes and that PEDOT/MWCNT is an excellent candidate material for electrode coating for neural prostheses applications.