Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22–40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm × 2 cm and 2 cm × 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100–500 μs above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100–500 μs, the frequency over a range of 20–50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.     

Spinal cord direct current stimulation: finite element analysis of the electric field and current density

Applied low-intensity direct current (DC) stimulates and directs axonal growth in models of spinal cord injury (SCI) and may have therapeutic value in humans. Using higher electric strengths will probably increase the beneficial effects, but this faces the risk of tissue damage by electricity or toxic reactions at the electrode–tissue interface. To inform the optimisation of DC-based therapeutics, we developed a finite element model (FEM) of the human cervical spine and calculated the electric fields (EFs) and current densities produced by electrodes of different size, geometry and location. The presence of SCI was also considered. Three disc electrodes placed outside the spine produced low-intensity, uneven EFs, whereas the EFs generated by the same electrodes located epidurally were about three times more intense. Changes in electrical conductivity after SCI had little effect on the EF magnitudes. Uniformly distributed EFs were obtained with five disc electrodes placed around the dura mater, but not with a paddle-type electrode placed in the dorsal epidural space. Replacing the five disc electrodes by a single, large band electrode yielded EFs > 5 mV/mm with relatively low current density (2.5 μA/mm²) applied. With further optimisation, epidural, single-band electrodes might enhance the effectiveness of spinal cord DC stimulation.     

60 Hz ventricular fibrillation thresholds for large-surface-area electrodes

The paper describes a series of animal experiments in which large-surface-area disk electrodes were used to study the current density required for ventricular fibrillation. The electrical currents were introduced to the heart both by applying the electrodes directly to the heart and by applying the electrodes to the surface of the chest near the heart. The electrode areas studied ranged from 1000 to 30 000 mm². The results show that, for large-area electrodes, fibrillation thresholds are determined by current density. The thresholds approach a constant value of 3·5 μA mm⁻²     

Estimating nerve excitation thresholds to cutaneous electrical stimulation by finite element modeling combined with a stochastic branching nerve fiber model

Electrical stimulation of cutaneous tissue through surface electrodes is an often used method for evoking experimental pain. However, at painful intensities both non-nociceptive Aβ-fibers and nociceptive Aδ- and C-fibers may be activated by the electrical stimulation. This study proposes a finite element (FE) model of the extracellular potential and stochastic branching fiber model of the afferent fiber excitation thresholds. The FE model described four horizontal layers; stratum corneum, epidermis, dermis, and hypodermal used to estimate the excitation threshold of Aβ-fibers terminating in dermis and Aδ-fibers terminating in epidermis. The perception thresholds of 11 electrodes with diameters ranging from 0.2 to 20 mm were modeled and assessed on the volar forearm of healthy human volunteers by an adaptive two-alternative forced choice algorithm. The model showed that the magnitude of the current density was highest for smaller electrodes and decreased through the skin. The excitation thresholds of the Aδ-fibers were lower than the excitation thresholds of Aβ-fibers when current was applied through small, but not large electrodes. The experimentally assessed perception threshold followed the lowest excitation threshold of the modeled fibers. The model confirms that preferential excitation of Aδ-fibers may be achieved by small electrode stimulation due to higher current density in the dermoepidermal junction.     

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.     

The Effects of Concentric Ring Electrode Electrical Stimulation on Rat Skin

Surface electrodes are commonly used electrodes clinically, in applications such as functional electrical stimulation for the restoration of motor functions, pain relief, transcutaneous electrical nerve stimulation, electrocardiographic monitoring, defibrillation, surface cardiac pacing, and advanced drug delivery systems. Common to these applications are occasional reports of pain, tissue damage, rash, or burns on the skin at the point where electrodes are placed. In this study, we quantitatively analyzed the effects of acute noninvasive electrical stimulation from concentric ring electrodes (CRE) to determine the maximum safe current limit. We developed a three-dimensional multi-layer model and calculated the temperature profile under the CRE and the corresponding energy density with electrical-thermal coupled field analysis. Infrared thermography was used to measure skin temperature during electrical stimulation to verify the computer simulations. We also performed histological analysis to study cell morphology and characterize any resulting tissue damage. The simulation results are accurate for low energy density distributions. It can also be concluded that as long as the specified energy density applied is kept below 0.92 (A²/cm⁴·s⁻¹), the maximum temperature will remain within the safe limits. Future work should focus on the effects of the electrode paste.     

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.     

New system for neonatal hearing screening based on auditory steady state responses

Carbonized rubber electrodes were tested extensively when they were first developed 30 years ago, but modern carbonized rubber electrodes have not received the type of scrutiny that the first electrodes received. Modern electrodes differ from the original electrodes in that they come with a self-adhesive electrode gel called hydrogel as part of their composition. The present study was undertaken to examine the current distribution and impedance characteristics of five brands of carbonized rubber electrodes and to examine the current distribution between electrodes during electrical stimulation in six subjects. Several different electrode sizes were tested between 3 and 10 cm. The current flow between the electrodes was determined by measuring the voltage across the skin on human subjects in 15 discrete locations between the electrodes. Blood flow was also measured between the electrodes with a laser Doppler flow meter to assess the physiological effect of current distribution on the skin at several skin temperatures. The results of these studies showed that at low currents, such as is used in TENS, very little current is actually applied through the skin due to the high impedance of the electrodes. At current levels normally used for electrical stimulation for functional movement, while current flow is better in most electrodes, it is very uneven, resulting in high current density in the centre of the electrodes and a fall off of at least 50% in current intensity at the edges of the electrode. There was very little difference in current density between small and large electrodes due to the high current density in the centre. Skin blood flow altered the movement of current between the electrodes and also may contribute to electrode performance. The implication of these studies is that electrode design needs to be altered for better current distribution, especially at low stimulation currents.     

Current distribution under electrodes in relation to stimulation current and skin blood flow: are modern electrodes really providing the current distribution during stimulation we believe they are?

Carbonized rubber electrodes were tested extensively when they were first developed 30 years ago, but modern carbonized rubber electrodes have not received the type of scrutiny that the first electrodes received. Modern electrodes differ from the original electrodes in that they come with a self-adhesive electrode gel called hydrogel as part of their composition. The present study was undertaken to examine the current distribution and impedance characteristics of five brands of carbonized rubber electrodes and to examine the current distribution between electrodes during electrical stimulation in six subjects. Several different electrode sizes were tested between 3 and 10 cm. The current flow between the electrodes was determined by measuring the voltage across the skin on human subjects in 15 discrete locations between the electrodes. Blood flow was also measured between the electrodes with a laser Doppler flow meter to assess the physiological effect of current distribution on the skin at several skin temperatures. The results of these studies showed that at low currents, such as is used in TENS, very little current is actually applied through the skin due to the high impedance of the electrodes. At current levels normally used for electrical stimulation for functional movement, while current flow is better in most electrodes, it is very uneven, resulting in high current density in the centre of the electrodes and a fall off of at least 50% in current intensity at the edges of the electrode. There was very little difference in current density between small and large electrodes due to the high current density in the centre. Skin blood flow altered the movement of current between the electrodes and also may contribute to electrode performance. The implication of these studies is that electrode design needs to be altered for better current distribution, especially at low stimulation currents.     

Histologic and physiologic evaluation of electrically stimulated peripheral nerve: Considerations for the selection of parameters

Helical electrodes were implanted around the left and right common peroneal nerves of cats. Three weeks after implantation one nerve was stimulated for 4–16 hours using charge-balanced, biphasic, constant current pulses. Compound action potentials (CAP) evoked by the stimulus were recorded from over the cauda equina before, during and after the stimulation. Light and electron microscopy evaluations were conducted at various times following the stimulation. The mere presence of the electrode invariably resulted in thickened epineurium and in some cases increased peripheral endoneurial connective tissue beneath the electrodes. Physiologic changes during stimulation included elevation of the electrical threshold of the large axons in the nerve. This was reversed within one week after stimulation at a frequency of 20 Hz, but often was not reversed following stimulation at 50–100 Hz. Continuous stimulation at 50 Hz for 8–16 hours at 400 μA or more resulted in neural damage characterized by endoneurial edema beginning within 48 hours after stimulation, and early axonal degeneration (EAD) of the large myelinated fibers, beginning by 1 week after stimulation. Neural damage due to electrical stimulation was decreased or abolished by reduction of the duration of stimulation, by stimulating at 20 Hz (vs. 50 Hz) or by use of an intermittent duty cycle. These results demonstrate that axons in peripheral nerves can be irreversely damaged by 8–16 hours of continuous stimulation at 50 Hz. However, the extent to which these axons may subsequently regenerate is uncertain. Therefore, protocols for functional electrical stimulation in human patients probably should be evaluated individually in animal studies.     

Optical monitoring of neural networks evoked by focal electrical stimulation on microelectrode arrays using FM dyes

Patch–clamping or microelectrode arrays (MEA) are conventional methods to monitor the electrical activity in biological neural networks in vitro. Despite the effectiveness of these techniques, there are disadvantages including the limited number of electrodes and the predetermined location of electrodes in MEAs. In particular, these drawbacks raise a difficulty in monitoring a number of neurons outnumbering the electrodes. Here, we propose an optical technique to determine the effective range of focal electrical stimulation using FM dyes in neural networks grown on planar-type MEAs. After 3 weeks in culture, electrical stimulation was delivered to neural networks via an underlying electrode in the presence of FM dyes. The stimulation induced the internalization of the dye into the neurons around the stimulating electrodes. Fluorescent images of dye distribution successfully showed the effects of focal stimulation. A range of stimulus amplitudes and frequencies were examined to collect fluorescence images. FM-dye uptake after electrical stimulation resulted in the labeling of cells up to approximately 300 μm away from the stimulating electrode. Fluorescence intensity increased proportionally to stimulation amplitude. Tetrodotoxin was shown to inhibit the labeling of neurons except those located immediately adjacent (within 40 μm) from the stimulating electrode. In the presence of AMPA and NMDA receptors antagonists, the FM-dye labeling appeared within 80 μm from the electrode, indicating directly evoked neural networks via blocking of glutamatergic synaptic transmission. These results showed that FM dyes can be a useful tool for monitoring activity-dependent synaptic events and determining the effect of focal stimulation in cultured neural networks.     

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.     

Fastin vivo measurements of local tissue impedances using needle electrodes

The objective of the research is to show an in vivo, fast method of measurement of local tissue bio-impedance in the beta dispersion region (0–200 kHz). A needle electrode is used for the purpose. The performances with respect to circuits, electrodes, measurement area and electrical representations are evaluated. A measurement example is shown, and the electrical representations are discussed and compared using it. The method discussed, although invasive, is considered to be useful for local tissue diagnoses concerning structures and physiological functions.     

Bursts of high-frequency synchronized electrical activity in the dog neocortex during food-related operant conditioning

Bursts of high-frequency (HF, 80–90 Hz, 70–80 μV) oscillations in the electrical activity (EA, 1–200 Hz) of the dog neocortex were studied during operant conditioning. These bursts of HF oscillations appeared in the EA of interstimulus intervals at the generalization stage on a background of dominant oscillations of lower frequency and amplitude (10–40 μV). Use of a new strategy for primary analysis of EA production (specifically, a coefficient of inhomogeneity) allowed amplitude-frequency inhomogeneity of the EA to be estimated, with isolation of bursts of HF oscillations. Use of an original nonharmonic analysis, consisting of expansion of EA waves into a system of half-waves which were used to construct distribution maps, revealed the regional properties of bursts of HF oscillations. The results of these investigations supplement previous data obtained using other methodological approaches (Fourier transformation and spectral density factor analysis). The properties of bursts of HF oscillations observed here provide evidence for the differential involvement of cortical areas (even close-lying areas separated by distances of 3–5 mm) in the spatial-temporal organization of potentials typical of this conditioning paradigm. Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 47, No. 5, pp. 828–838, September–October, 1997.     

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.     

Electrode recovery potential

In some instances the same electrodes are used for stimulation and then for recording a bioelectric event immediately after the stimulus. However, after the current pulse there remains an electrode potential that decays quasiexponentially. We have designated this falling potential the electrode-recovery potential. This study investigated the recovery potentials of single electrodes of rhodium, stainless steel, platinum and platinum-iridium in contact with 0.9% saline at room temperature (25°C) over a current density ranging from 0.1 to 100 mA/cm² using a constant-current pulse. In all cases, with increasing current density, there was a decrease in the time for the electrode potential to fall to one half of the immediate post-stimulus value. Above about 20 mA/cm² the decrease in recovery time was smooth with increasing current density. Below 20 mA/cm², the recovery time was slightly irregular. The shortest recovery times were for platinum and platinum-iridium. The largest decrease in recovery time with increasing current density was for stainless steel, which decreased 10 fold from 0.1 to 100 mA/cm². The recovery time for rhodium decreased about three-and-one half fold over the same current density range. It was found that the waveform of the recovery potential is not a simple exponential because the Warburg and Faradic components of the electrode-electrolyte interface are current-density dependent. In general, for all current densities studied (0.1–100 mA/cm²), there was a sudden initial fall in electrode potential with cessation of current flow, followed by a very gradual nonexponential decrease in potential.     

Biocompatibility considerations at stimulating electrode interfaces

The choice of biocompatible stimulating electrodes for various biomedical applications varies with the type of electrode-tissue interface, biomolecules present, electrolyte background, preparation of electrode, interfacial potential, current density, electrode material, porosity, geometry, and inflammatory response. Illustrative examples are given to demonstrate the importance of these parameters. Topics discussed are: A) DC electrodes applied to partially keratinized epithelial membranes; B) Variation of the electrical impedance and biocompatibility of stimulating electrodes with electrode potential and surrounding pH; C) Influence of electrode geometry, porosity and pore size on biocompatibility; D) Body defense mechanisms at the sites of implantable stimulating electrodes; E) Thrombus formation at stimulating electrode interfaces and F) Sterilization of electrodes to ensure biocompatibility. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an implied or actual endorsement of such products by the Department of Health and Human Services.     

Impedance-gradient electrode reduces skin irritation induced by transthoracic defibrillation

A new type of disposable external defibrillation electrode has been developed to reduce the skin irritation commonly associated with defibrillation and synchronised cardioversion. This design employs an impedance gradient to reduce the proportion of current delivered to the electrode periphery. The temperature distribution under the new electrode was compared with that of four other types of commercially available electrodes after repeated high-energy biphasic defibrillation discharges to domestic swine. Skin temperature distributions were acquired using non-invasive thermography. Measurements of the maximum temperature rise at each electrode site, taken 3.6s after the fifth defibrillation discharge, demonstrated that the new impedance-gradient electrode produced 50–60% less skin heating than two of the three uniform-impedance electrode designs. Histological examination of erythematous sites excised 24h after defibrillation quantified the associated skin damage using a scoring protocol developed for this study. In contrast to previous studies, histological examinations demonstrated second-degree skin burns following defibrillation. The new electrode design, however, induced 44–46% less skin damage than two of the traditional uniform-impedance electrodes.     

Neuromuscular stimulation selectivity of multiple-contact nerve cuff electrode arrays

The feasibility of an electrical stimulation method selectively for activating skeletal muscles innervated by a common peripheral nerve trunk has been investigated. The method utilises ‘snugly’ fitting nerve cuffs that incorporate an array of 12 electrodes. These electrodes have been tested as four longitudinally aligned tripoles (located 90° apart on the cuff inner surface). In acute experiments on rabbit sciatic nerves, we have found that tripolar stimulation with this implant system is in general highly selective. ‘Field steering’, wherein a subthreshold transverse current is used in combination with a longitudinal tripolar current, tends to increase the selectivity of stimulation. On a rabbit sciatic nerve, a combination of adjacent longitudinal tripoles of the 12 electrode array generally yields a stimulation performance similar to that which would be expected if a 24 electrode array is used. This system may find a use in functional neuromuscular stimulation applications which require highly selective control over multiple muscles.     

Calculations of temperature rise produced in body tissue by a spherical electrode

Two estimates of temperature rise produced in body tissue when a spherical electrode passes current have been calculated. The estimates bracket the expected temperature rise. Time-transient and steady-state results have been obtained. The effects of heat transfer through the highly conductive metal electrode and irreversible. Faradaic reactions have been considered. The calculations indicate that electrodes smaller than about 2 μm in radius produce a peak temperature rise of about 1°C when driven by typical square current pulses of 25 μA intensity and 200 μs duration. The results are presented in a graphic form allowing for quick estimation of the expected peak temperature rise around electrodes of a specific radius, which are driven with a pulse of known current density and duration.