Performance of platinum stimulating electrodes mapped on the limitvoltage plane

The corrosion of platinum electrodes in saline is studied by measuring loss of metal resulting from the passage of current and plotting the data on the electrode fimit-voltage plane (ELVP). Some results show that with this form of presentation, the effects of current density, pauses in the current and the presence of blood serum (to imitate extracellular fluid), is not strong. Evidence shows that in these neutral solutions corrosion occurs when the positive voltage excursion is great enough for acid to be evolved locally by the oxidation of water. When electrode voltage is not controlled and it is capacitor-coupled (e.g. in practical stimulators). serum has a beneficial effect on the voltage, tending to reduce corrosion.     

Electrical characteristic of the titanium mesh electrode for transcutaneous intrabody communication to monitor implantable artificial organs

We have developed a tissue-inducing electrode using titanium mesh to obtain mechanically and electrically stable contact with the tissue for a new transcutaneous communication system using the human body as a conductive medium. In this study, we investigated the electrical properties of the titanium mesh electrode by measuring electrode–tissue interface resistance in vivo. The titanium mesh electrode (Hi-Lex Co., Zellez, Hyogo, Japan) consisted of titanium fibers (diameter of 50 μm), and it has an average pore size of 200 μm and 87 % porosity. The titanium mesh electrode has a diameter of 5 mm and thickness of 1.5 mm. Three titanium mesh electrodes were implanted separately into the dorsal region of the rat. We measured the electrode–electrode impedance using an LCR meter for 12 weeks, and we calculated the tissue resistivity and electrode–tissue interface resistance. The electrode–tissue interface resistance of the titanium mesh electrode decreased slightly until the third POD and then continuously increased to 75 Ω. The electrode–tissue interface resistance of the titanium mesh electrode is stable and it has lower electrode–tissue interface resistance than that of a titanium disk electrode. The extracted titanium mesh electrode after 12 weeks implantation was fixed in 10 % buffered formalin solution and stained with hematoxylin-eosin. Light microscopic observation showed that the titanium mesh electrode was filled with connective tissue, inflammatory cells and fibroblasts with some capillaries in the pores of the titanium mesh. The results indicate that the titanium mesh electrode is a promising electrode for the new transcutaneous communication system.     

Lab-on-a-chip sensor for detection of highly electronegative heavy metals by anodic stripping voltammetry

This work describes development of a lab-on-a-chip sensor for electrochemical detection of highly electronegative heavy metals such as manganese and zinc by anodic stripping voltammetry. The sensor consists of a three-electrode system, with a bismuth working electrode, a Ag/AgCl reference electrode, and a Au auxiliary electrode. Hydrolysis at the auxiliary electrode is a critical challenge in such electrochemical sensors as its onset severely limits the ability to detect electronegative metals. The bismuth working electrode is used due to its comparable negative detection window and reduced toxicity with respect to a conventional mercury electrode. Through optimization of the sensor layout and the working electrode surface, effects of hydrolysis were substantially reduced and the potential window was extended to the −0.3 to −1.9 V range (vs. Ag/AgCl reference electrode), which is far more negative than what is possible with conventional Au, Pt, or carbon electrodes. The described lab-on-a-chip sensor for the first time permits reliable and sensitive detection of the highly electronegative manganese. The favorable performance of the bismuth electrode coupled with its environmentally-friendly nature make the described sensor attractive for applications where disposable chips are desirable. With further development and integrated sample preparation, the lab-on-a-chip may be converted into a point-of-care platform for monitoring heavy metals in blood (e.g., assessment of manganese exposure).     

Noise characteristics of stainless-steel surface electrodes

Bioelectric events measured with surface electrodes are subject to noise components which may be significant in comparison with low-level biological signals such as evoked neuroelectric potentials, and myoelectric potentials. In an effort to better understand noise arising from these electrodes, electrode and measurement system noise is modelled. The effect of electrode surface area on electrode impedance and noise is studied using circular stainless-steel electrodes of varying diameters. The main contributions of the work are the development of a model for stainless-steel electrode noise as a function of electrode area, and demonstrating that, for the band-width of interest to evoked neuroelectric and myoelectric signals (8–10 000 Hz), the primary noise components are thermal and amplifier current generated. The magnitudes of both of these depend on the electrode impedance magnitude. Electrode impedance is shown to be a power function of both electrode diameter and frequency, consistent with a capacitive electrode model.     

Variability of Electrode Positions Using Electrode Caps

We investigated the variability of electrode positions for a multi-channel, custom electrode cap placed onto participants’ heads without taking scalp measurements. The electrode positions were digitized in a three-dimensional space for 10 young adult participants on three separate occasions. Positional variability was determined for 15 selected electrodes within the three-dimensional preauricular-nasion (PAN) coordinate system and from this system, angular coordinate variability was also determined. The standard deviations of the 15 selected electrodes ranged from 3.0 to 12.7 mm in the PAN system. These data resulted in a variability of 2.0° to 10.4° among the angular coordinates. The measurements indicated slightly greater variability of electrode positions compared to studies when electrodes were placed using scalp measurements. The implication of this study is that the use of electrode caps may not be appropriate when electroencephalographic (EEG) or evoked potential (EP) techniques depend on accurate electrode placement. Additionally, if a longitudinal study is performed, electrode locations should be checked to ensure that they conform with previous sessions.     

Evidence of Potential Averaging over the Finite Surface of a Bioelectric Surface Electrode

Most bioelectric signals are not only functions of time but also exhibit a variation in spatial distribution. Surface EMG signals are often “summarized” by a large electrode. The effect of such an electrode is interpreted as averaging the potential at the surface of the skin beneath the electrode. We first introduce an electrical equivalent model to delineate this principle of averaging. Next, in a realistic finite element model of EMG generation, two outcome variables are evaluated to assess the validity of the averaging principle. One is the change in voltage distribution in the volume conductor after electrode application. The other is the change in voltage across the high impedance double layer between tissue and electrode. We found that the principle of averaging is valid, once the impedance of the double layer is sufficiently high. The simulations also revealed that skin conductivity plays a role. High-density surface EMG provided experimental evidence consistent with the simulation results. A grid with 120 small electrodes was placed over the thenar muscles of the hand. Electrical nerve stimulation assured a reproducible compound muscle response. The averaged grid response was compared with a single electrode matching the surface of the high-density electrodes. The experimental results showed relatively small errors indicating that averaging of the surface potential by the electrode is a valid principle under most practical conditions.     

Optimisation of transcutaneous cardiac pacing by three-dimensional finite element modelling of the human thorax

The goal of the study is to determine by finite element analysis (FE) the optimal electrode placement, size and electrolyte resistivity that minimise the pain experienced by patients during successful transcutaneous cardiac pacing (TCP). The three-dimensional FE model generated for this purpose has 55 388 nodes, 50 913 hexahedral elements and simulated 16 different organs and tissues, as well as the properties of the electrolyte. The model uses a non-uniform mesh with an average spatial resolution of 0.8 cm in all three dimensions. To validate this model, the voltage across 3 cm² Ag−AgCl electrodes is measured when currents of 5 mA at 50 kHz are injected into a subject's thorax through the same electrodes. For the same electrode placements and sizes and the same injected current, the FE analysis produced results in good agreement with the experimental data. The optimisation analysis tested seven different electrode placements, five different electrode sizes and six different electrolyte resistivities. The analysis indicates that the anteriorposterior electrode placement, electrode sizes of about 90 cm² and electrolytes with resistivity of about 800 ω·cm yield the most uniform current distribution through the skin, thus having the best chances to minimise the pain delivered to the patient during successful TCP. The anterior-anterior electrode placement is the second most efficient.     

Finite Element Analysis of the Current-Density and Electric Field Generated by Metal Microelectrodes

Electrical stimulation via implanted microelectrodes permits excitation of small, highly localized populations of neurons, and allows access to features of neuronal organization that are not accessible with larger electrodes implanted on the surface of the brain or spinal cord. As a result there are a wide range of potential applications for the use of microelectrodes in neural engineering. However, little is known about the current-density and electric field generated by microelectrodes. The objectives of this project were to answer three fundamental questions regarding electrical stimulation with metal microelectrodes using geometrically and electrically accurate finite elements models. First, what is the spatial distribution of the current density over the surface of the electrode? Second, how do alterations in the electrode geometry effect neural excitation? Third, under what conditions can an electrode of finite size be modeled as a point source? Analysis of the models showed that the current density was concentrated at the tip of the microelectrode and at the electrode–insulation interface. Changing the surface area of the electrode, radius of curvature of the electrode tip, or applying a resistive coating to the electrode surface altered the current-density distribution on the surface of the electrode. Changes in the electrode geometry had little effect on neural excitation patterns, and modeling the electric field generated by sharply tipped microelectrodes using a theoretical point source was valid for distances >～ 50μm from the electrode tip. The results of this study suggest that a nearly uniform current-density distribution along the surface of the electrode can be achieved using a relatively large surface area electrode μ00--1000μm²), with a relatively blunt tip (3–6 μm radius of curvature), in combination with～mμ～ 1μm) moderately resistive ～( ～ 50Ωm). © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 0270Dh, 8780-y, 8710+e     

Effect of electrode parameters on the bandwidth of the surface e.m.g. power-density spectrum

The effect of the electrode configuration (unipolar against bipolar), the electrode separation, the electrode position and the electrode contact area on the bandwidth of the power density spectrum of the surface e.m.g. has been studied. The dependence of the half-power point bandwidthB on the interelectrode distanced can be described by the regression function B_Hz=810/d_mm+58. The unipolar electrode configuration yields an e.m.g. signal with the smallest bandwidth. Neither a variation of the electrode position nor a variation of the electrode contact area has a significant effect on the bandwidth of the e.m.g.     

Insulated electrocardiogram electrodes

An integrated system consisting of an insulated electrode and an impedance transformer has been fabricated, which can be used for the acquisition of electrocardiographic data. The electrode itself consists of a thin layer of dielectric material which has been deposited onto a silicon substrate. The impedance transformer is a Fairchild μA 740 operational amplifier used in the unity-gain configuration. The input impedance of the impedance transformer is at least 100 MΩ and its output impedance is about 5 Ω. Both electrode and impedance transformer are contained in a plastic housing which is identical to that used with the NASA Apollo-type electrode. The lower cutoff frequency of the electrode system is between 0·01 and 1·0 Hz, depending on the dielectric used and its thickness. Clinical-quality electrocardiograms have been obtained with these electrodes.     

Electrode adsorption method for determination of enzymatic activity

The paper describes an electrical method for the determination of enzymatic activity. It is based on the fact that certain peptides, which are hydrolysed by enzymes, adsorb on a metal electrode in such a way that the double-layer capacitance of the electrode is changed. The two parts of the hydrolysed peptide do not have this property. The capacitance change depends on the concentration of the peptide and can be used to measure it. We describe more specifically the use of the tripeptide Bz-Phe-Val-Arg-pNA, which is a substrate for enzymes like thrombin and trypsin. Some different ways to use the electrode method are described. The determination of antithrombin activity in plasma and whole blood is taken as a practical example. The necessary instrumentation is briefly described with special attention to an automated equipment, which controls the experimental conditions and reduces errors due to the operator. Experimental results from studies of the frequency dispersion of the electrode impedance are presented. A model for the influence of the adsorbed peptides on the electrical properties of the electrode is derived. Finally, some general remarks are made about the applicability of the electrode adsorption method.     

The iridium/iridium oxide electrode to in vivo measurement of oesophageal and gastric pH

An iridium-iridium oxide electrode for in vivo pH measurement of the distal oesophagus is described. It is small and flexible so it is well accepted by patients for long-term pH monitoring. The electrode also offers the possibility of including another electrode and/or a pressure sensor for simultaneous detection of two or more parameters—gastric pH or oesophageal pressure for example. A clinical study was performed on 15 healthy volunteers and the results were checked on a DGC Nova 4/s computer, showing less than 0.005% of tension in mV out of the established range in a period of 24 h pH monitoring. The volunteers' results were compared with the normal values obtained by DeMeester using a glass electrode on a similar, healthy group of American patients and no significant differences were observed.

Owing to its small size, reliability, fast response to pH changes, durability and its easy storage, the Ir/IrO₂ electrode is ideal for long-term pH monitoring of the upper gastro-intestinal tract.     

The impedance of a spherical monopolar electrode

The impedance of a monopolar electrode immersed in an environmental volume conductor consists of two parts; the impedance of the active electrode-electrolyte interface, and the resistance of the environmental conductor. Two studies were carried out to quantitate these components. First, impedance-frequency data were collected for five spherical stainless-steel electrodes (ranging from 0.473 to 1.11 cm in diameter) immersed in 0.9% saline (ρ=70 Ω-cm). Impedance measurements were made from 100 Hz to 100 kHz and two sets of data were obtained; one before and one after each electrode was polished with fine emery paper. At low frequency, the measured impedances were high and varied with electrode surface preparation. However, above a transition frequency, the impedances were resistive, independent of the electrode surface preparation, and equal to ρ/2πd as predicted from the theory. This study indicates that the low frequency impedance of a monopolar electrode is dominated by the impedance of the electrode-electrolyte interface. Above a transition frequency, the resistance of the environmental conductor dominates, the value of this resistance depending on the electrode geometry and the resistivity (ρ) of the environmental conductor. A second study was conducted, to examine the effect of the distance to the indifferent electrode. A frequency (100 kHz) above the transition frequency was used and impedance data were collected for various distances between the monopolar and indifferent electrodes. The measured resistance increased asymptotically as the distance between the electrodes was increased. When the indifferent electrode diameter was at least 10 times the diameter of the spherical monopolar electrode, the measured resistance was within 5% of the value predicted for an indifferent electrode at infinity.     

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.     

Catheter tip pH electrodes for continuous intravascular recording

A small, flexible catheter-tip pH electrode has been developed for continuous measurement of intravascular pH in man and animals. The electrode utilises a pH sensitive polymer membrane rather than pH glass. The electrode passes through the lumen of a conventional cardiac catheter, by which it may be introduced to any desired site in the circulation. The electrode has no cross-sensitivity to other ions, to osmolality or oxygen tension. It is insensitive to flow and pressure. The response time is less than 300 ms, and the mean drift in vivo is 0.01 pH units/h. Continuous recordings of intravascular pH in animals have been made for up to Th. The electrodes can be sterilised, and have been used in man.     

Electroretinography in dogs using a fiber electrode prototype

We compared two electroretinography (ERG) electrodes in dogs using ERG standards of the International Society for Clinical Electrophysiology of Vision (ISCEV). Ten healthy Yorkshire terrier dogs (mean age, 2.80 ± 1.42 years; 6 females) weighing 5.20 ± 1.56 kg were evaluated using an ERG system for veterinary use. Dark- and light-adapted ERG responses were recorded using an ERG-Jet electrode and a fiber electrode prototype. The examinations were performed during 2 visits, 3 weeks apart. Both electrodes (ERG-Jet or fiber prototype) were used on each animal and the first eye to be recorded (OD × OS) was selected randomly. Three weeks later the examination was repeated on the same animal switching the type of electrode to be used that day and the first eye to be examined. The magnitude and waveform quality obtained with the two electrode types were similar for all ERG responses. ERG amplitudes and implicit times obtained from dogs using the fiber electrode prototype were comparable to those obtained with the ERG-Jet electrode for rod, maximal rod-cone summed, cone, and 30-Hz flicker responses. The fiber electrode prototype is a low-cost device, available as an alternative instrument for clinical veterinary ERG recording for retinal function assessment.     

How Epicardial Electrodes Influence the Transmembrane Potential During a Strong Shock

This paper analyzes a possible artifact that may corrupt experiments studying defibrillation of the heart. Our hypothesis is that surface recording electrodes can influence the transmembrane potential during a shock. In the vicinity of an electrode, current leaves the intracellular space to take advantage of the low resistance of the extracellular path, thereby depolarizing the tissue. We calculate the transmembrane potential induced around a circular electrode when exposed to a uniform electric field. The bidomain model represents the electrical behavior of the cardiac tissue, and we account for electrode polarization impedance. Our results show that adjacent regions of depolarization and hyperpolarization exist around the electrode, and that the induced depolarization is greater than 100 mV for a 0.5 mm radius silver–silver chloride electrode in a 500 V/m electric field. We conclude that surface electrodes may produce artifacts during experiments designed to study defibrillation-strength electrical shocks. © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 8719Hh, 8780-y     

Ring electrode for radio-frequency heating of the cornea: Modelling and<Emphasis Type="Italic">in vitro</Emphasis> experiments

Radio-frequency thermokeratoplasty (RF-TKP) is a technique used to reshape the cornea curvature by means of thermal lesions using radio-frequency currents. This curvature change allows refractive disorders such as hyperopia to be corrected. A new electrode with ring geometry is proposed for RF-TKP. It was designed to create a single thermal lesion with a full-circle shape. Finite element models were developed, and the temperature distributions in the cornea were analysed for different ring electrode characteristics. The computer results indicated that the maximum temperature in the cornea was located in the vicinity of the ring electrode outer perimeter, and that the lesions had a semi-torus shape. The results also indicated that the electrode thickness, electrode radius and electrode thermal conductivity had a significant influence on the temperature distributions. In addition,in vitro experiments were performed on rabbit eyes. At 5 W power, the lesions were fully circular. Some lesions showed non-uniform characteristics along their circular path. Lesion depth depended on heating duration (60% of corneal thickness for 20s, and 30% for 10s). The results suggest that the critical shrinkage temperature (55–63°C) was reached at the central stroma and along the entire circular path in all the cases.     

Electrophysiological basis of mono-phasic action potential recordings

Monophasic action potentials (MAPs)_have been recorded for over a century, however, the exact mechanism responsible for their genesis has yet to be elucidated fully. The goal of the paper is to examine the physical basis of MAP recordings. MAP recordings are simulated by modelling a three-dimensional block of cardiac tissue. The effect of the MAP electrode is modelled by introducing a large, non-specific leakage conductance to the small region under the electrode. From the spread of the electrical activity, the equivalent extracellular current flow can be efficiently determined. These computed current sources are then input into a boundary element model of the tissue to determine the surface potentials. Finally, differences in surface potentials are used to compute waveforms that closely resemble MAP recordings. By varying model parameters, the mechanisms responsible for the MAP are determined, and a theory is put forward that can account for all observations. It is hypothesised that the leakage current causes the formation of a double-layer potential with a strength equal to the difference in transmembrane voltage between the regions under the electrode and those outside the electrode, leading to a recorded potential that mimics the transmembrane voltage outside the electrode region, although offset. Based on experimental MAP recordings, an equivalent leakage channel with a conductance of 0.1 mS cm⁻² and a reversal potential of −43 mV is introduced by the electrode.     

Wafer-scale fabrication of penetrating neural microelectrode arrays

The success achieved with implantable neural interfaces has motivated the development of novel architectures of electrode arrays and the improvement of device performance. The Utah electrode array (UEA) is one example of such a device. The unique architecture of the UEA enables single-unit recording with high spatial and temporal resolution. Although the UEA has been commercialized and been used extensively in neuroscience and clinical research, the current processes used to fabricate UEA’s impose limitations in the tolerances of the electrode array geometry. Further, existing fabrication costs have led to the need to develop less costly but higher precision batch fabrication processes. This paper presents a wafer-scale fabrication method for the UEA that enables both lower costs and faster production. More importantly, the wafer-scale fabrication significantly improves the quality and tolerances of the electrode array and allow better controllability in the electrode geometry. A comparison between the geometrical and electrical characteristics of the wafer-scale and conventional array-scale processed UEA’s is presented.