Historical evolution of circuit models for the electrode-electrolyte interface

Electrodes are widely used to measure bioelectric events and to stimulate excitable tissues. In one form or another, electrodes have been around for nearly two centuries; yet our ability to predict their properties is extremely limited, despite considerable research, especially during the last century. This paper chronicles the accumulation of knowledge about the electrode-electrolyte interface as a circuit element. Our understanding of this interface starts with the Helmholtz double layer of charge and progresses through the Warburg and Fricke lowcurrent-density models, which demonstrated that the resistive and capacitive components are polarization elements, the values of which depend on frequency. The discovery by Schwan, showing that the components of the Warburg-Fricke model are current-density dependent, is recounted, along with the discovery of the rectifying properties of the electrode-electrolyte interface and how it was put to practical use. The very high current-density operation of the interface is discussed in terms of gas evolution, arcing, and shock-wave production. Finally the evolution of recording electrodes is traced. Because electrodes can be operated over a very wide range of current density, it is unlikely that a single model can be created for the electrode-electrolyte interface, although over a restricted current-density range such a model may be possible.     

Ag/AgCl electrode assembly for thin smooth muscle electromyography

An electrode assembly for in vivo recording of the electrical activities of thin muscular layers is described. It comprises an active electrode surrounded by a ring, which avoids the recording of interfering signals. An improved technique for chlorinating the silver electrodes is presented: a partial electrolytic removal of an initial thick deposit is performed. This decreases the impedance, and lowers the ageing degradation of the electrodes in vivo. Recordings of extremely low frequency signals are allowed, whereas standard Ag/AgCl electrodes are inefficient when recording signals in the frequency domain under 0·1 Hz. The technical features which are described can be adapted to any Ag/AgCl electrode developed for in vivo measurements.     

Design criteria of neuron/electrode interface. The focused ion beam technology as an analytical method to investigate the effect of electrode surface morphology on neurocompatibility

"Neurocompatibility" is a broad definition which comprises aspects of biocompatibility, chemical and physical surface properties, and biostability of an artificial substrate interfaced with a neural tissue. The main issue coming from the analysis of the state of art of neuroprosthesis and neuron/electrode interfaces is the strong influence of electrode surface morphology on neurocompatibility. Enhanced functions of neurons have been observed on nano-structured materials. This paper proposes the use of focused ion beam (FIB) technology as high precision machining technique to modify the surface morphology of an interface material. By controlling the ion milling in three dimensions, the fabrication of a surface with any predefined morphology becomes possible with nanometric precision. In vitro tests on PC12 cells cultured on surfaces with different morphologies show that the surface morphology influences the cell adhesion. Experimental results suggest an enhancement of the interaction between cells and artificial surfaces at a specific scale (tens of nanometres) which is the typical scale of cellular interaction in the extra-cellular matrix (ECM) of living organisms.     

The physiologically modulated electrode potentials at the depth electrode-brain interface in humans

To study the modulated electrical potential specifically related to the electrode-brain interface (EBI) in deep brain stimulation (DBS) under physiological condition, we quantitatively identified the physiologically modulated electrode potentials by decomposing the local field potentials (LFPs) recorded from 11 patients (18 electrodes in four different brain regions) who underwent DBS, and correlated them with simultaneously recorded physiological signals of blood pressure (BP) and respiration. Results showed that electrode potentials were modulated by BP and respiration and could be detected as a specific component of the compound LFP signals with a mean (+/-S.D.) amplitude of 6.9+/-1.7 microV. The detection rate and amplitude of the modulated electrode potentials were independent from brain regions and neurological disorders. The current approach can be used to study the changes in properties of the EBI under physiological condition and to investigate the effects of the EBI on the 'crossing' current of either the neural signals to be recorded or the electrical pulses for neurostimulation.     

An improved electrode system for recording EEG from the skull

A simple procedure for making electrodes to be implanted in rabbits or rats for obtaining EEG's from the skull is described.     

Integrated wireless neural interface based on the Utah electrode array

This report presents results from research towards a fully integrated, wireless neural interface consisting of a 100-channel microelectrode array, a custom-designed signal processing and telemetry IC, an inductive power receiving coil, and SMD capacitors. An integration concept for such a device was developed, and the materials and methods used to implement this concept were investigated. We developed a multi-level hybrid assembly process that used the Utah Electrode Array (UEA) as a circuit board. The signal processing IC was flip-chip bonded to the UEA using Au/Sn reflow soldering, and included amplifiers for up to 100 channels, signal processing units, an RF transmitter, and a power receiving and clock recovery module. An under bump metallization (UBM) using potentially biocompatible materials was developed and optimized, which consisted of a sputter deposited Ti/Pt/Au thin film stack with layer thicknesses of 50/150/150 nm, respectively. After flip-chip bonding, an underfiller was applied between the IC and the UEA to improve mechanical stability and prevent fluid ingress in in vivo conditions. A planar power receiving coil fabricated by patterning electroplated gold films on polyimide substrates was connected to the IC by using a custom metallized ceramic spacer and SnCu reflow soldering. The SnCu soldering was also used to assemble SMD capacitors on the UEA. The mechanical properties and stability of the optimized interconnections between the UEA and the IC and SMD components were measured. Measurements included the tape tests to evaluate UBM adhesion, shear testing between the Au/Sn solder bumps and the substrate, and accelerated lifetime testing of the long-term stability for the underfiller material coated with a a-SiC_x:H by PECVD, which was intended as a device encapsulation layer. The materials and processes used to generate the integrated neural interface device were found to yield a robust and reliable integrated package.     

The use of hydrogel as an electrode-skin interface for electrode array FES applications

Functional electrical stimulation is commonly used to restore function in post-stroke patients in upper and lower limb applications. Location of the electrodes can be a problem hence some research groups have begun to experiment with electrode arrays. Electrode arrays are interfaced with a thin continuous hydrogel sheet which is high resistivity to reduce transverse currents between electrodes in the array. Research using electrode arrays has all been conducted in a laboratory environment over short time periods but it is suspected that this approach will not be feasible over longer time periods due to changes in hydrogel resistivity.High resistivity hydrogel samples were tested by leaving them in contact with the skin over a seven day period. The samples became extremely conductive with resistivities reaching around 10–50 Ω m. The effect of these resistivity changes was studied using finite element analysis to solve for the stationary current quasi-static electric field gradient in the tissue. Electrical stimulation efficiency and focality were calculated for both a high and low resistivity electrode–skin interface layer at different tissue depths. The results showed that low resistivity hydrogel produced significant decreases in stimulation efficiency and focality compared to high resistivity hydrogel.     

Linear and nonlinear properties of platinum electrode polarisation II: time domain analysis

The linear and nonlinear transient behaviour of a platinum electrode in physiological saline (0·9% NaCl) was investigated. The interface overpotential (current) in response to a step current (voltage) was monitored over six decades of pulse duration starting at 10 μs, both in the linear range, i.e. where the polarising current and the polarisation voltage are proportional, and in the nonlinear range, where this proportionality is lost. The time invariance of the Pt electrode/physiological saline interface parameters in the linear range and at the onset of nonlinearity over the studied band of frequencies and pulse durations is deduced from the equivalence of experimental results obtained in the frequency and time domains using the appropriate transformation when mapping the two domains. We conclude that the interface can be analysed and modelled in either domain in the linear range and at the onset of nonlinearity.     

Platinum electrode noise in the ENG spectrum

The noise from Pt electrodes has not been measured before. We measured the impedance and noise between pairs of Pt electrodes in cylindrical cells within frequency bands which contain the electroneurogram band (ENG, 500–10 kHz). The results show that the noise from the Pt-saline interface and also the volume conductor are the same as the thermal noise from the real part of those impedances. This means that Pt electrodes are convenient for neural recording because the total source noise from the electrodes and the volume conductor (tissue) can be estimated from the measured total impedance and calculating the thermal noise associated with the resistive part. This differs from Ag/AgCl electrodes for which the interface noise has been reported to be higher than the thermal noise, and therefore calculation of the total noise will be more difficult.     

Miniature calomel electrode for recording DC potential changes accompanying spreading depression in the freely moving rat

A miniaturized calomel electrode for recording DC potential changes in freely moving rats is described. It consists of a glass capillary pipette with a saline + agar bridge, separated from mercury by a layer of calomel + cotton. DC potential changes accompanying cortical spreading depression could be measured for more than one month in the chronic preparation. The calomel electrode is easier to construct and superior in performance to the silver-silver chloride electrodes.     

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.     

表面蝶骨电极、T1/T2电极以及毫针蝶骨电极在癫(痫)患者部分剥夺睡眠脑电图中的应用

目的:比较表面蝶骨电极、T1/T2电极与毫针蝶骨电极对发作间期癫(痫)样放电的检出频率及其检出放电的波幅关系.方法:在部分剥夺睡眠的脑电图记录中,先以表面蝶骨电极、T1/T2电极以及10-20系统16道头皮电极记录1.5h,然后以毫针蝶骨电极、T1/T2电极以及10-20系统16道头皮电极记录1.5h.选择发作间期放电主要分布于表面蝶骨电极或毫针蝶骨电极的癫(痫)患者33例,计算前后两段记录中T1/T2电极、表面或毫针蝶骨电极对发作间期放电的检出频率,最大波幅放电在T1/T2电极、表面或毫针蝶骨电极的出现频率,T1/T2电极、表面或毫针蝶骨电极记录到放电的平均波幅.结果:在前后两段记录中,表面蝶骨电极、毫针蝶骨电极对发作间期放电的检出频率(89％、91％)高于T1/T2电极对发作间期放电的检出频率(72％、56％)(P=0.01＜0.05、P＜0.01),最大波幅放电在表面蝶骨电极、毫针蝶骨电极的出现频率(56％、69％)高于在T1/T2电极的出现频率(21％、19％)(P＜0.01、P＜0.01);局限于表面蝶骨电极的放电(13.85％)少于局限于毫针蝶骨电极的放电(27.01％)(P=0.021＜0.05);表面蝶骨电极、毫针蝶骨电极检出发作间期放电的平均波幅(92.83 μV、87.38 μV)与T1/T2电极检出发作间期放电的平均波幅(98.67 μV、87.67 μV)未见差异(P=0.447、P=0.967).结论:对于发作间期放电主要分布于表面蝶骨电极或毫针蝶骨电极的癫(痫)患者,毫针蝶骨电极、表面蝶骨电极对发作间期放电的检出频率高于T1/T2电极对发作间期放电的检出频率,最大波幅放电在毫针蝶骨电极、表面蝶骨电极的出现频率高于在T1/T2电极的出现频率.在部分剥夺睡眠的情况下,表面蝶骨电极、T1/T2电极以及毫针蝶骨电极均能相对一致的检出有无发作间期放电.     

Different storage methods for biopotential skin electrodes (sintermetallic Ag/AgCl) and their influence on the bias potential

Summary The aim of the present study was to investigate the influence of various methods of storage on the Differences in Electrode Potential (DEP). For periods of 22 h each, 10 Ag/AgCl sintered metal biopotential skin electrodes were stored in 4 media: dry, in a 0.9% NaCl solution, in distilled water and in electrode jellies. When storing the electrodes in 0.9% NaCl, distilled water, and electrode jelly all the electrodes of one make were stored together in a container filled with the medium in question, with and without the application of an external short circuit. After storage the electrodes were filled with one of 3 electrode jellies (Beckman Electrode Electrolyte, Parker Spectra 360, Siemens Electrode Gel). The DEPs between each two electrodes were measured 10, 30, 60 and 120 min after filling. The results show no difference for the two makes of electrodes (Beckman and IVM), but clear differences (up to factor 50) were observed in the electrode jellies used. The method of storage has a significant influence on the DEP. Dry storage yielded the lowest values (p<0.001) for all electrodes and electrode jellies (x between 84 and 318 V) and the highest stability. When stored in 0.9% NaCl, distilled water and electrode jelly, both with and without external short circuit, the electrodes showed DEPs in the mV range and drifted strongly during the period of observation, except after short-circuited storage in 0.9% NaCl, when a phase of stability is reached after 60 min.This project was supported by a grant from the österreichischer Fonds zur Förderung der wissenschaftlichen Forschung     

诱发电位仪电极接触电阻检测系统的研制方法

本文研制的电极接触电阻检测系统是诱发电位仪前置放大部分的一个重要系统。该系统运用单片机进行控制，利用数字逻辑电路控制由模拟电子开关和电阻组成的并联电网络，不仅对各通道电极的接触电阻进行自动有效检测，而且对诱发电位测量中特殊的耳电极也能够自动有效检测。     

Effects of ingesting 6% and 12% glucose/electrolyte beverages during prolonged intermittent cycling in the heat

Summary This study compared the effects of ingesting 6% (MC) and 12% (HC) glucose/electrolyte beverages, and a flavored water placebo (P) on markers of fluid absorption, palatability, and physiological function during prolonged intermittent cycling in the heat. On three occasions, 15 trained male cyclists performed two 60 min cycling bouts at 65% (E ₁ and E ₂). A brief exhaustive performance ride (3 min) was completed after E ₁ and E ₂, and after 20 min recovery (P ₁, P ₂, P ₃). Every 20 min, subjects consumed 275 mL of P, MC or HC. The first drink contained 20 mL of D₂O, a tracer of fluid entry into blood plasma. Plasma D₂O accumulation was slower for HC than for P and MC (P<0.001). HC caused more nausea (P<0.01) and fullness (P<0.05) than MC or P, and subjects said they would be less likely to consume HC during training or competition (P<0.10). Sweat rates, HR, T _re, T _sk, , and PV were similar for all drinks. Performance of P ₁, P ₂, P ₃ were not different among drinks. However, four cyclists failed to maintain the prescribed work rate during E ₂ for HC but only one failed for MC and P. These data suggest that the slow absorption of a 12% glucose/electrolyte beverage during prolonged intermittent exercise in the heat may increase the risk of gastrointestinal distress and thereby limit performance.     

Pre-Polarization Resistance of the Skin as Determined by the Single Square Voltage Pulse Method

Use of a new microcomputer-based instrumentation system to record and analyze current waveforms produced by applying square voltage pulses of 0.5-3.5 V to the skin revealed that peak currents are considerably higher, and corresponding pre-polarization resistance values considerably lower, than previously supposed. However, in keeping with previous observations, we confirmed that the skin is an Ohm's Law conductor in its pre-polarization state and that the pre-polarization resistance of the skin is resolvable into two components, a localized resistance beneath each electrode and a non-localized (body core) resistance. In addition, a three-electrode method was devised to measure the localized resistance at any selected location (target site). Simple equivalent circuit simulations of the initial section of current waveforms are used to assess the degree of instrument-related peak current underestimation inherent even in this system. This preliminary study indicates the need for reassessment of the roles played by the various body tissues, particularly the epidermis, in determining the pre-polarization electrical character of the skin.     

Ultrastructure of the early human feto-maternal interface co-cultured in vitro

BACKGROUND: The study was designed to investigate the ultrastructural features of the early human feto-maternal interface when generated by in-vitro co-culture, and compare these with findings reported previously from human pregnancies. METHODS: Placental villi and decidua parietalis tissues from 8-12 week pregnancies were co-cultured in vitro over a 4-day period. The co-incubations were ended at 24 h intervals and processed for electron microscopical studies, and for immunocytochemistry using anti-cytokeratin antibody (CAM 5.2) for trophoblast. RESULTS: Loss of the syncytium at points of contact with the decidual stroma, cytotrophoblast column formation, differentiation and invasion of extravillous trophoblast (EVT) cells into the decidual stroma over the 4-day period of co-culture were observed. Cellular components, such as actin filaments, microtubules, glycogen granules and lamellipodic processes found in EVT cells were consistent with active cellular locomotion. CONCLUSIONS: These ultrastructural studies emphasize the usefulness of this model in investigating the formation of the feto-maternal interface of human pregnancy. The recruitment of cytotrophoblast to the syncytium by a process involving fusion of the intervening plasma membranes, and the migration of EVT cells causing little or no damage to the surrounding decidual cells, resemble in-vivo data.     

The CXCL12/CXCR4 axis is involved in the maintenance of Th2 bias at the maternal/fetal interface in early human pregnancy

The regulatory mechanism of Th2 bias at the maternal/fetal interface remains unclear. In this study, we characterized cytokine production in decidual stromal cells (DSCs), decidual immune cells (DICs) and embryo-derived trophoblast cells, and investigated the regulation of CXCL12/CXCR4 interaction on Th2 bias at the maternal/fetal interface in early human pregnancy. We found differential production of Th1-type and Th2-type cytokines by trophoblasts, DSCs and DICs. The secretion of these cytokines varied in different cell cocultures, conduced to Th2 bias. Flow cytometry showed that coculture of trophoblasts with DSCs and DICs significantly increased IL-4 and IL-10 production in trophoblasts, and IL-10 production in DSCs. However, the coculture of trophoblasts with DSCs and DICs significantly increased interferon (IFN)-γ expression in DSCs, and tumor-necrosis factor (TNF)-α expression in DICs. No change was seen in Th1-type cytokine production in trophoblasts, and in Th2-type cytokine production in DICs in all cocultures. Furthermore, pre-treatment with anti-CXCR4 neutralizing antibody upregulated the production of the Th1-type cytokines IFN-γ and TNF-α, and downregulated the production of the Th2-type cytokines IL-4 and IL-10, in trophoblasts, DSCs, DICs or their cocultures. Interestingly, rhCXCL12 inhibited production of the Th1-type cytokine TNF-α and enhanced the expression of the Th2-type cytokines such as IL-4 and IL-10 in DICs; this effect was abrogated by anti-CXCR4 antibody. Our present study has elucidated the individual contributions of component cells to the shaping of Th2 bias, and uncovered a complicated cross-talk via the CXCL12/CXCR4 signal at the maternal/fetal interface in early human pregnancy.