A novel dry active electrode for EEG recording

The design and testing of a "dry" active electrode for electroencephalographic recording is described. A comparative study between the EEG signals recorded in human volunteers simultaneously with the classical Ag-AgCl and "dry" active electrodes was carried out and the reported preliminary results are consistent with a better performance of these devices over the conventional Ag-AgCl electrodes.     

Efficient electrode spacing for examining spatial organizationduring ventricular fibrillation

Spatial organization has been observed during episodes of ventricular fibrillation (VF) by recording epicardial unipolar electrograms on a grid of electrodes. In such studies, the choice of spacing between electrodes is an important decision, affecting the resolution and the size of the domain to be studied. A basic tenet of sampling theory, the Nyquist criterion, states that an electrode spacing smaller than half the smallest significant wavelength is required to capture the important details of a spatially sampled process. The authors suggest a method to choose a practical interelectrode spacing by examining wavenumber power spectra of high-resolution VF data recorded from a square 11×11 array of electrodes spaced 0.28 mm apart. The plaque was sutured on the epicardium near the left ventricular apex in 7 anesthetized pigs. VF was induced with AC simulation. Unipolar extracellular electrograms were simultaneously recorded from each channel for 2 s after the induction of VF. Each signal was sampled in time at 1000 Hz. Wavenumber power spectra were calculated for 100 ms segments using the zero-delay wavenumber spectrum method, for a total of 140 power spectra. All spectra had dominant peaks at the origin and fell off rapidly with increasing wavenumber (decreasing wavelength). In all the spectra, every wavelength shorter than 1.4 mm contributed insignificant power. Furthermore, in 134 of 140 spectra (96%), insignificant power levels were associated with every wavelength shorter than 2.8 mm. These results suggest that, for unipolar extracellular electrodes, an intersensor spacing on the order of 1 mm is appropriate to study organization during early VF     

Characterization of micromachined spiked biopotential electrodes

We present the characterization of dry spiked biopotential electrodes and test their suitability to be used in anesthesia monitoring systems based on the measurement of electroencephalographic signals. The spiked electrode consists of an array of microneedles penetrating the outer skin layers. We found a significant dependency of the electrode-skin-electrode impedance (ESEI) on the electrode size (i.e., the number of spikes) and the coating material of the spikes. Electrodes larger than 3 x 3 mm2 coated with Ag-AgCl have sufficiently low ESEI to be well suited for electroencephalograph (EEG) recordings. The maximum measured ESEI was 4.24 k(omega) and 87 k(omega), at 1 kHz and 0.6 Hz, respectively. The minimum ESEI was 0.65 k(omega) an 16 k(omega), at the same frequencies. The ESEI of spiked electrodes is stable over an extended period of time. The arithmetic mean of the generated dc offset voltage is 11.8 mV immediately after application on the skin and 9.8 mV after 20-30 min. A spectral study of the generated potential difference revealed that the ac part was unstable at frequencies below approximately 0.8 Hz. Thus, the signal does not interfere with a number of clinical applications using real-time EEG. Comparing raw EEG recordings of the spiked electrode with commercial Zipprep electrodes showed that both signals were similar. Due to the mechanical strength of the silicon microneedles and the fact that neither skin preparation nor electrolytic gel is required, use of the spiked electrode is convenient. The spiked electrode is very comfortable for the patient.     

Selective activation of distant nerve by surface electrode array

Neural prostheses for restoring lost functions can benefit from selective activation of nerves with limited number and density of electrodes. Here, we show by simulations and animal experiments that multipoint simultaneous stimulation with a surface electrode array can selectively activate nerves in a bundle at a desired location in between the array or at a desired depth, which are referred to as lateral or depth-wise gating stimulation, respectively. The stimulation broadly generates action potentials with cathodic source electrodes, and simultaneously blocks unnecessary propagation with downstream anodic gate electrodes. In general, stimulation with a small diameter electrode can affect a nearly hemispherical region, while a large electrode is effective at a more vertically compressed region, i.e., a surface of nerve bundle. The gating stimulation takes advantage of the size effects by utilizing an asymmetrical electrode array. The array of the lateral gating stimulation is designed to have four electrodes; a pair of large source electrodes and a pair of small gate electrodes. The depth-wise gating stimulation array consists of two electrodes; a large gate and small source electrodes. The simulation first demonstrated that appropriate combination of currents at the source and gate electrodes can change recruitment patterns of nerves with lateral or depth-wise selectivity as desired. We then applied the lateral gating stimulation on the rat spinal cords and obtained a preliminary support for the feasibility.     

Development of a multiple thin-film semimicro DC-probe forintracerebral recordings [during surgery]

A thin-film multiple-electrode probe for measuring de potentials at eight sites with interdistances of 1 mm was constructed for the investigation of slow potential changes in deep regions of the human brain during surgery. The thin-film electrodes had to be placed on curved cylinder-shaped surgical instruments with dimensions of 2 mm diameter and 33 cm length used with the Freiburg stereotactic equipment. Several novel technological steps had to be introduced for the solution of the encountered problems: 1) Structuring of the metal layers on curved substrates was accomplished by using flexible masks. 2) Special feed-through technologies had to be invented in order to obtain reliable connections between the thin-film sensors and the copper wires inside the stereotactic instrument. 3) Thin-film Ag-AgCl electrodes had to be formed in order to obtain satisfying recordings of slow potential changes below 10 Hz. Slow potential changes were recorded from different depths in interdistances of only 1 mm with these new miniaturized thin film Ag-AgCl electrodes and bipolar recordings with an electrode interdistance of only 3 mm showed clearly the appearance of Bereitschaftspotentials.     

Construction and validation of a plunge electrode array for three-dimensional determination of conductivity in the heart.

The heart's response to electrical shock, electrical propagation in sinus rhythm, and the spatiotemporal dynamics of ventricular fibrillation all depend critically on the electrical anisotropy of cardiac tissue. Analysis of the microstructure of the heart predicts that three unique intracellular electrical conductances can be defined at any point in the ventricular wall; however, to date, there has been no experimental confirmation of this concept. We report the design, fabrication, and validation of a novel plunge electrode array capable of addressing this issue. A new technique involving nylon coating of 24G hypodermic needles is performed to achieve nonconductive electrodes that can be combined to give moderate-density multisite intramural measurement of extracellular potential in the heart. Each needle houses 13 silver wires within a total diameter of 0.7 mm, and the combined electrode array gives 137 sites of recording. The ability of the electrode array to accurately assess conductances is validated by mapping the potential field induced by a point current source within baths of saline of varying concentration. A bidomain model of current injection in the heart is then used to test an approximate relationship between the monodomain conductivities measured by the array, and the full set of bidomain conductivities that describe cardiac tissue.     

基于铂黑的微针电极阵列表面修饰方法

由于微针电极阵列尖端直径小,空间分辨率高,可以记录单个神经元的放电活动,已成为神经信号记录的首选.但商用微针电极阵列的阻抗较高,降低电极阻抗有利于提高信噪比,改善记录信号质量.采用超声电镀铂黑的方法对微针电极表面进行修饰.测试结果表明铂黑修饰后的微针电极电化学性能优异,1 kHz处阻抗约为2.5 kΩ,相比裸金电极降低...     

Optimization of cardiac defibrillation by three-dimensional finiteelement modeling of the human thorax

The goal of this study was to determine the optimal electrode placement and size to minimize myocardial damage during defibrillation while rendering refractory a critical mass of cardiac tissue of 100%. For this purpose, the authors developed a 3D finite element model with 55,388 nodes, 50,913 hexahedral elements, and simulated 16 different organs and tissues, as well as the properties of the electrolyte. The model used a nonuniform mesh with an average spatial resolution of 0.8 cm in all three dimensions, To validate this model, the authors measured the voltage across 3-cm² Ag-AgCl electrodes when currents of 5 mA at 50 kHz were injected into a human subject's thorax through the same electrodes. For the same electrode placements and sizes and the same injected current, the finite element analysis produced results in good agreement with the experimental data. For the optimization of defibrillation, the authors tested 12 different electrode placements and seven different electrode sizes. The finite element analyses showed that the anterior-posterior electrode placement and an electrode size of about 90 cm² offered the least chance of potential myocardial damage and required a shock energy of less than 350 J for 5-ms defibrillation pulses to achieve 100% critical mass. For comparison. The average cross-sectional area of the heart is ≈48 cm², about half of the optimal area. A second best electrode placement was with the defibrillation electrodes on the midaxillary lines under the armpits. Although this placement had higher chances of producing cardiac damage, it required less shock energy to achieve 100% critical mass     

Relating Epicardial to Body Surface Potential Distributions by Means of Transfer Coefficients Based on Geometry Measurements

Although it has been known throughout this century that a complex sequence of electrical events is produced on the body surface by the electrophysiological properties of the heart, the question of how well these body surface events can be explained mathematically on the basis of experimental measurements of cardiac geometry and electrical activity remains unanswered. Recent advances in experimental capabilities have made possible the near simultaneous measurement of both cardiac epicardial and corresponding body surface potential distributions from in vivo animal preparations using chronically implanted electrodes to keep the volume conductor intact. This report provides a method for finding transfer coefficients that relate the epicardial and body surface potential distributions to each other. The method is based on knowing the geometric location of each electrode, and on having enough electrodes to establish the geometric shape and the potential distribution of closed epicardial and body surfaces. However, the method does not require that either the heart or body surfaces have any special shape, such as that of a sphere, or that any electrical quantities, such as voltage gradients, be known in addition to the potentials. The use of potential distributions to represent heart electrical activity is advantageous since such distributions can be directly measured experimentally, without a transformation to any other form, such as multiple current-generating dipoles, being required. This report includes a statement of the underlying integral equations, the procedure. for finding the equations' coefficients from geometry measurements, some considerations for computer algorithms, and an example.     

A simulation study evaluating the performance of high-density electrode arrays on myocardial tissue

Multielectrode arrays used to detect cellular activation have become so dense (electrodes per square millimeter) as to jeopardize the basic assumptions of activation mapping; namely, that electrodes are points adequately separated as to not interfere with the tissue or each other. This paper directly tests these assumptions for high-density electrode arrays. Using a finite element model with modified Fitzhugh-Nagumo kinetics, we represent electrodes as isopotential surfaces of varying widths and spacing ratio (SR) (center-to-center spacing divided by electrode width). We examine the signal strength and ability of a single electrode to detect activation due to a passing wavefront. We find that high-density arrays do not cause significant wavefront curvature or alter activation timing in the underlying tissue. Relationships between signal strength, cross talk, and array design are explained by the interaction of the propagating wavefront and induced sources on the isopotential electrodes. Sensitivity analysis shows that these results may be generalized to a wide range of physiologically relevant designs and applications. We conclude that electrode array designs in which electrode spacing greatly exceeds electrode diameter are overly conservative and that arrays with a SR of less than 2.0 may perform successfully in electrophysiological studies.     

Ceramic-based multisite electrode arrays for chronic single-neuron recording

A method is described for the manufacture of a microelectrode array for chronic, multichannel, single neuron recording. The ceramic-based, multisite electrode array has four recording sites patterned onto a ceramic shaft the size of a single typical microwire electrode. The sites and connecting wires are applied to the ceramic substrate using a reverse photolithographic procedure. Recording sites (22 x 80 microm) are separated by 200 microm along the shaft. A layer of alumina insulation is applied over the whole array (exclusive of recording sites) by ion-beam assisted deposition. These arrays were capable of recording single neuron activity from each of their recording sites for at least three weeks during chronic implantation in the somatosensory cortex of rats, and several sites had recordings that lasted for more than 8 weeks. The vertical arrangement of the recording sites on these electrodes is ideal for simultaneously recording across the different layers of brain areas such as the cerebral cortex and hippocampus in chronic preparations.     

An Analysis of Transfer Coefficients Calculated Directly from Epicardial and Body Surface Potential Measurements in the Intact Dog

This study dealt with the question of how to estimate body surface potentials from epicardial potential distributions in intact dogs; in particular, the study considered the feasibility of obtaining transfer coefficients directly from sequences of epicardial and body surface measurements of ventricular excitation and repolarization potential distributions, rather than from measurements of the geometry of the volume conductor. The transfer coefficients were calculated from the measured potentials via the mathematical method of using a Bayes estimator. The merit of this approach was that it offered the possibility of accurately representing the characteristics of the volume conductor without directly measuring either the volume conductor's geometry or its inhomogeneities. The experimental protocol made use of measurements from two dogs. Data from the first dog were used to obtain two sets of transfer coefficients, one by the Bayes method as applied to measured sequences of epicardial and body surface potentials, and the other by a method based on the geometric position of each epicardial and body surface electrode. These two sets of transfer coefficients were found to be similar in pattern and value size. Additionally, results of forward simulations in the first dog, based on the measured epicardial potentials and each set of transfer coefficients, were compared to the measured body potentials. The results showed that the simulated potentials were closer to the measured potentials when the transfer coefficients obtained from the potentials were used, rather than when the geometric coefficients were used.     

Predicting patterns of epicardial potentials during ventricularfibrillation

Ventricular fibrillation (VF) is a fatal cardiac arrhythmia, characterized by uncoordinated propagation of activation wavefronts in the ventricular myocardium. Short-term predictions of epicardial potential fields during VF in pigs were attempted using linear techniques, and prediction accuracy was measured at various stages during sustained episodes. VF was induced in five pigs via premature electrical stimulation. Unipolar electrograms were recorded from an epicardial array of 506 electrodes in a 22×23 array with 1-mm spacing. Optimal spatial basis functions (modes) and time-varying weighting coefficients were found using the Karhunen-Loeve decomposition. Linear autoregressive (AR) models incorporating the dynamics of only a few spatial modes led to predicted patterns that were qualitatively similar to observed patterns. Predictions were made 0.256 s into the future, based on 0.768 s of past data, over an area of approximately 5 cm² on the ventricular epicardium. The mean squared error of predictions varied from as much as 1.23 to as little as 0.14, normalized to the variance of the actual data. Inconsistency in long-term forecasts is partly due to the limitations of linear AR models. Changes in predictability, however, were consistent. Predictability varied inversely with spatial complexity, as measured by the mean squared error of a five-mode approximation. Predictability also increased significantly during the first minute of VF     

Motion Artifact from Spot and Band Electrodes During Impedance Cardiography

We have modified impedance cardiography for monitoring cardiac output during stress tests. Employing an off-line microcomputer, our instrument ensemble averaged impedance signals to minimize the effect of motion artifacts. We proposed a new four-spot electrode array and replaced the usual encircling band electrode array with it. We tested ten normal subjects and compared the signal-to-noise ratio (SNR) from our spot electrode array to that from a typical band electrode array at rest and during four levels of exercise on a treadmill. The average of the sighal-to-noise ratios for ten subjects from our spot electrode array was 13.6-45.5 percent larger than that from the band electrode array at rest and during four levels of exercise. Thus, it is desirable to replace band electrodes with spot electrodes in impedance cardiography for exercising subjects.     

STUDY OF FEASIBILITY OF USING Ag-AgCl ELECTRODE AS REFERENCE ELECTRODE OF THE ISFET

The preparation,principle and measurement results of the complex ISFET withAg-AgCl reference electrode are presented in this paper.Through experiment and theory weendeavor to show the feasibility of using the Ag-AgCl electrode which is without solution contactas reference electrode and the way of overcoming its instability;it gives a rational explanationfor phenomenon of the ion sensitive field effect transistor which does not conform to Nernstianresponse,when we measured cations of Na~+, Ca~(++),etc.with the Ag-AgCl reference electrode.     

用银-氯化银作ISFET参比电极的可行性研究

本文叙述了用银-氯化银作参比电极的复合离子敏感场效应晶体管的制备、原理和测量结果。通过实验和理论分析,提出了用无液接的银-氯化银电极作参比电极的可行性和克服其不稳定性的途径;并对用银-氯化银参比电极测量Na+,Ca++等阳离子时,敏感场效应晶体管不符合能斯特响应的现象进行了解释。     

An Efficient and Fair Strategy for Radio Resources Allocation in Multi-radio Wireless Mesh Networks

A Load and Interference aware Resource Allocation strategy (LIRA) is proposed for multi-radio Wireless Mesh Networks (WMNs), combining multiple mechanisms that efficiently optimise radio resources (rate, power and channel) to guarantee max–min fair capacity to every aggregating Mesh Access Point (MAP). LIRA is composed of a rate adaptation and power control mechanism, sensitive to the fat-tree traffic specificities of WMNs, using the highest bit rates at MAP gateways and using, for the ramified links, the minimum ones that satisfy their capacity needs. This enables to efficiently reduce the transmitted power and interference, advantageous for channel reutilisation. LIRA also integrates a load and interference aware channel assignment mechanism, allowing the simultaneous operation of all links without interference. When this is not achievable, two auxiliary mechanisms of channel sharing and interference-free channel reuse can be sub-sequentially used, reducing the capacity of certain MAPs to guarantee fairness to all nodes. LIRA’s gateway flow-control mechanism guarantees that all MAPs respect the allocated capacity, guaranteeing that every MAP is able to operate at its max–min fair capacity. The performance of LIRA is evaluated through simulation, considering IEEE 802.11a. For a classical hexagonal deployment of 19 MAPs with an Internet gateway, it is shown how with only 5 channels LIRA guarantees to every MAP a max–min fair capacity of 3.2 Mbit/s, without packet loss, and delay below 6 ms. It guarantees a max–min fair throughput to every MAP, having a capacity usage efficiency of 66.7 %, an energy efficiency of 26.5 Mbit/J and spectrum efficiency of 0.58 bit/s/Hz. For a more challenging scenario with 27 MAPs and 4 gateways, it is shown how LIRA uses its mechanisms in heterogeneous conditions to also guarantee max–min fair throughput to every MAP, between 5 and 11 Mbit/s, without packet loss, and a delay below 12 ms. Any system improvement will enable to reach higher WMN performance levels using the proposed strategy.     

The nerve-electrode interface of the cochlear implant: currentspread

One of the fundamental facets of the cochlear implant that must be understood to predict accurately the effect of an electrical stimulus on the auditory nerve is the nerve-electrode interface. One aspect of this interface is the degree to which current delivered by an electrode spreads to neurons distant from it. This paper reports a direct mapping of this current spread using recordings from single units from the cat auditory nerve. Large variations were seen in the degree to which the different units are selective in responding to electrodes at different positions within the scala tympani. Three types of units could be identified based on the selectiveness of their response to the different electrodes in a linear array. The first type of unit exhibited a gradual increase in threshold as the stimulating site was moved from more apical to more basal locations within the scala tympani. The second type of unit exhibited a sharp local minimum, with rapid increases in threshold in excess of 6 dB/mm in the vicinity of the minimum. At electrode sites distant from the local minima the rate of change of the threshold approached that of the first type of units. The final type of unit also demonstrated a gradual change in threshold with changing electrode position, however, two local minima, one apical and one basal, could be identified. These three types are hypothesized to correspond to units which originate apical to the electrode array, along the electrode array and basal to the electrode array     

16-Channel Integrated Potentiostat for Distributed Neurochemical Sensing

We present the architecture and VLSI circuit implementation of a BiCMOS potentiostat bank for monitoring neurotransmitter concentration on a screen-printed carbon electrode array. The potentiostat performs simultaneous acquisition of bidirectional reduction-oxidation currents proportional to neurotransmitter concentration on 16 independent channels at controlled redox potentials. Programmable current gain control yields over 100-dB cross-scale dynamic range with 46-pA input-referred rms noise over 12-kHz bandwidth. The cutoff frequency of a second-order log-domain anti-aliasing filter ranges from 50 Hz to 400 kHz. Track-and-hold current integration is triggered at the sampling rate between dc and 200 kHz. A 2.25-mmtimes2.25-mm prototype was fabricated in a 1.2-mum VLSI technology and dissipates 12.5 mW. Chronoamperometry dopamine concentration measurements results are given. Other types of neurotransmitters can be selected by adjusting the redox potential on the electrodes and the surface properties of the sensor coating     

Feasibility of an electrode-reservoir device for transdermal drug delivery by noninvasive skin electroporation

Electrical creation of aqueous pathways across the skin's outer layer [stratum corneum (SC)] provides an approach to transdermal delivery of medium-size water-soluble compounds. However, nerve stimulation should be avoided. Here, we show that a microstructured electrode array can significantly confine the electric field to the nerve-free SC. The prototype electrode-reservoir device (ERD) contains field-confining electrodes and a fluorescent drug surrogate [sulphorhodamine (SR)]. In vivo human experiments at the forearm with approximately rectangular voltage pulses up to 500 V and 1-ms duration cause electroporation as measured by skin resistance change but only rarely caused sensation. Human skin in vitro experiments with such pulses up to 300 V transported SR across the SC. Our results are supported by a model's prediction of the field in the ERD and nearby tissue.