Electrical stimulation of the nervous system: The principle of safe charge injection with noble metal electrodes

Semiconducting electrodes for neural interfacing: a review

In the past 50 years, the advent of electronic technology to directly interface with neural tissue has transformed the fields of medicine and biology. Devices that restore or even replace impaired bodily functions, such as deep brain stimulators and cochlear implants, have ushered in a new treatment era for previously intractable conditions. Meanwhile, electrodes for recording and stimulating neural activity have allowed researchers to unravel the vast complexities of the human nervous system. Recent advances in semiconducting materials have allowed effective interfaces between electrodes and neuronal tissue through novel devices and structures. Often these are unattainable using conventional metallic electrodes. These have translated into advances in research and treatment. The development of semiconducting materials opens new avenues in neural interfacing. This review considers this emerging class of electrodes and how it can facilitate electrical, optical, and chemical sensing and modulation with high spatial and temporal precision. Semiconducting electrodes have advanced electrically based neural interfacing technologies owing to their unique electrochemical and photo-electrochemical attributes. Key operation modalities, namely sensing and stimulation in electrical, biochemical, and optical domains, are discussed, highlighting their contrast to metallic electrodes from the application and characterization perspective.

Methods of poly(3,4)-ethylenedioxithiophene (PEDOT) electrodeposition on metal electrodes for neural stimulation and recording

Conductive polymers are of great interest in the field of neural electrodes because of their potential to improve the interfacial properties of electrodes. In particular, the conductive polymer poly (3,4)-ethylenedioxithiophene (PEDOT) has been widely studied for neural applications.Objective:This review compares methods for electrodeposition of PEDOT on metal neural electrodes, and analyses the effects of deposition methods on morphology and electrochemical performance.Approach:Electrochemical performances were analysed against several deposition method choices, including deposition charge density and co-ion, and correlations were explained to morphological and structural arguments as well as characterisation methods choices.Main results:Coating thickness and charge storage capacity are positively correlated with PEDOT electrodeposition charge density. We also show that PEDOT coated electrode impedance at 1 kHz, the only consistently reported impedance quantity, is strongly dependent upon electrode radius across a wide range of studies, because PEDOT coatings reduces the reactance of the complex impedance, conferring a more resistive behaviour to electrodes (at 1 kHz) dominated by the solution resistance and electrode geometry. This review also summarises how PEDOT co-ion choice affects coating structure and morphology and shows that co-ions notably influence the charge injection limit but have a limited influence on charge storage capacity and impedance. Finally we discuss the possible influence of characterisation methods to assess the robustness of comparisons between published results using different methods of characterisation.Significance:This review aims to serve as a common basis for researchers working with PEDOT by showing the effects of deposition methods on electrochemical performance, and aims to set a standard for accurate and uniform reporting of methods.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

Electrotactile Feedback Applications for Hand and Arm Interactions: A Systematic Review, Meta-Analysis, and Future Directions

Haptic feedback is critical in a broad range of human-machine/computer-interaction applications. However, the high cost and low portability/wearability of haptic devices remain unresolved issues, severely limiting the adoption of this otherwise promising technology. Electrotactile interfaces have the advantage of being more portable and wearable due to their reduced actuators' size, as well as their lower power consumption and manufacturing cost. The applications of electrotactile feedback have been explored in human-computer interaction and human-machine-interaction for facilitating hand-based interactions in applications, such as prosthetics, virtual reality, robotic teleoperation, surface haptics, portable devices, and rehabilitation. This article presents a technological overview of electrotactile feedback, as well a systematic review and meta-analysis of its applications for hand-based interactions. We discuss the different electrotactile systems according to the type of application. We also discuss over a quantitative congregation of the findings, to offer a high-level overview into the state-of-art and suggest future directions. Electrotactile feedback systems showed increased portability/wearability, and they were successful in rendering and/or augmenting most tactile sensations, eliciting perceptual processes, and improving performance in many scenarios. However, knowledge gaps (e.g., embodiment), technical (e.g., recurrent calibration, electrodes' durability) and methodological (e.g., sample size) drawbacks were detected, which should be addressed in future studies.

SIROF stabilized PEDOT/PSS allows biocompatible and reversible direct current stimulation capable of driving electrotaxis in cells

Electrotaxis is a naturally occurring phenomenon in which ionic gradients dictate the directed migration of cells involved in different biological processes such as wound healing, embryonic development, or cancer metastasis. To investigate these processes, direct current (DC) has been used to generate electric fields capable of eliciting an electrotactic response in cells. However, the need for metallic electrodes to deliver said currents has hindered electrotaxis research and the application of DC stimulation as medical therapy. This study aimed to investigate the capability of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) on sputtered iridium oxide film (SIROF) electrodes to generate stable direct currents. The electrochemical properties of PEDOT/PSS allow ions to be released and reabsorbed depending on the polarity of the current flow. SIROF stabilized PEDOT/PSS electrodes demonstrated exceptional stability in voltage and current controlled DC stimulation for periods of up to 12 hours. These electrodes were capable of directing cell migration of the rat prostate cancer cell line MAT-LyLu in a microfluidic chamber without the need for chemical buffers. This material combination shows excellent promise for accelerating electrotaxis research and facilitating the translation of DC stimulation to medical applications thanks to its biocompatibility, ionic charge injection mechanisms, and recharging capabilities in a biological environment.

Electrical Characterization of the Tongue and the Soft Palate Using Lumped-Element Model for Intraoral Neuromodulation

Intraoral functions are results of complex and well-orchestrated sensorimotor loop operations, and therefore vulnerable to small functional or neural defects. To secure the vital intraoral functions, it is important to find a way to favorably intervene the intraoral sensorimotor loop operations. The tongue and the soft palate are heavily associated with intraoral sensorimotor loops, with their dense neural innervations and occupancy of intraoral space. Therefore, electrical stimulation onto the tongue and the soft palate has a great potential to solve the problems in the intraoral functions. However, the electrical interface for both of them have not been characterized yet as a lumped-element model, for designing electrical stimulation and analyzing its effect. In this study, we measured stimulation thresholds to evoke electrotactile feedback and characterized electrical impedance across electrodes using lumped-element models. We found that average perception/discomfort thresholds for the tongue tip, lateral-inferior side of the tongue, and anterolateral side of the soft palate as 0.18/1.31, 0.37/3.99, and 1.19/7.55 mA, respectively. An R-C-R-R-C model represented the electrical interface across the tongue and the soft palate with the highest accuracy. The average component values of the R-C-R-R-C model were found as 2.72kΩ, 45.25nF, 1.27kΩ, 22.09GΩ, and 53.00nF, on average.

Influence of In Vitro Electrical Stimulation on Survival of Spiral Ganglion Neurons

Patients scheduled for cochlear implantation often retain residual hearing in the low frequencies. Unfortunately, some patients lose their residual hearing following implantation and the reasons for this are not well understood. Evidence suggests that electrotoxicity could be one of the factors responsible for this late adverse effect. Therefore, the aim of this study was to investigate the survival of spiral ganglion neurons (SGN) subjected to in vitro electrical stimulation (ES). A stimulation setup was developed to provide defined electrical fields at given points of the chamber. SGN isolated from Sprague Dawley rats (P3-4) were dissociated and cultured in the chamber for 24 h prior to biphasic, pulsed electrical field exposure for another 24 h. The current varied in the range of 0 to 2 mA and the pulse width from 10 to 400 μs. Neurite growth and survival were evaluated with respect to the charge density at the position of the cells. Non-exposed SGN cultures served as control. Charge densities below 2.2 μC·cm^-2·phase^-1 appeared to have no effect on SGN survival and neurite outgrowth. Charge densities above 4.9 μC·cm^-2·phase^-1 were detrimental to almost all cells in culture. After fitting results to a sigmoidal dose response curve, a LD₅₀ of 2.9 μC·cm^-2·phase^-1 was calculated. This screening regarding survival and outgrowth of SGN provides parameters that could be used to further investigate the effect of ES on SGN and to develop possible protection strategies, which could potentially rescue residual hearing in the implanted patients.

Controlling Nerve Growth with an Electric Field Induced Indirectly in Transparent Conductive Substrate Materials

Innovative neurostimulation therapies require improved electrode materials, such as poly(3,4-ethylenedioxythiophene) (PEDOT) polymers or IrO_x mixed ionic-electronic conductors and better understanding of how their electrochemistry influences nerve growth. Amphibian neurons growing on transparent films of electronic (metal) conductors and electronic-ionic conductors (polymers and semiconducting oxides) are monitored. Materials are not connected directly to the power supply, but a dipole is created wirelessly within them by electrodes connected to the culture medium in which they are immersed. Without electrical stimulation neurons grow on gold, platinum, PEDOT-polystyrene sulfonate (PEDOT-PSS), IrO_x , and mixed oxide (Ir-Ti)O_x , but growth is not related to surface texture or hydrophilicity. Stimulation induces a dipole in all conductive materials, but neurons grow differently on electronic conductors and mixed-valence mixed-ionic conductors. Stimulation slows, but steers neurite extension on gold but not on platinum. The rate and direction of neurite growth on PEDOT-PSS resemble that on glass, but on IrO_x and (Ir-Ti)O_x neurites grow faster and in random directions. This suggests electrochemical changes induced in these materials control growth speed and direction selectively. Evidence that the electric dipole induced in conductive material controls nerve growth will impact electrotherapies exploiting wireless stimulation of implanted material arrays, even where transparency is required.

A foldable electrode array for 3D recording of deep-seated abnormal brain cavities

OBJECTIVE

This study describes the design and microfabrication of a foldable thin-film neural implant and investigates its suitability for electrical recording of deep-lying brain cavity walls.

APPROACH

A new type of foldable neural electrode array is presented, which can be inserted through a cannula. The microfabricated electrode is specifically designed for electrical recording of the cavity wall of thalamic lesions resulting from stroke. The proof-of-concept is demonstrated by measurements in rat brain cavities. On implantation, the electrode array unfolds in the brain cavity, contacting the cavity walls and allowing recording at multiple anatomical locations. A three-layer microfabrication process based on UV-lithography and Reactive Ion Etching is described. Electrochemical characterization of the electrode is performed in addition to an in vivo experiment in which the implantation procedure and the unfolding of the electrode are tested and visualized.

MAIN RESULTS

Electrochemical characterization validated the suitability of the electrode for in vivo use. CT imaging confirmed the unfolding of the electrode in the brain cavity and analysis of recorded local field potentials showed the ability to record neural signals of biological origin.

SIGNIFICANCE

The conducted research confirms that it is possible to record neural activity from the inside wall of brain cavities at various anatomical locations after a single implantation procedure. This opens up possibilities towards research of abnormal brain cavities and the clinical conditions associated with them, such as central post-stroke pain.

Novel Neurostimulation of Autonomic Pelvic Nerves Overcomes Bladder-Sphincter Dyssynergia

The disruption of coordination between smooth muscle contraction in the bladder and the relaxation of the external urethral sphincter (EUS) striated muscle is a common issue in dysfunctional bladders. It is a significant challenge to overcome for neuromodulation approaches to restore bladder control. Bladder-sphincter dyssynergia leads to undesirably high bladder pressures, and poor voiding outcomes, which can pose life-threatening secondary complications. Mixed pelvic nerves are potential peripheral targets for stimulation to treat dysfunctional bladders, but typical electrical stimulation of pelvic nerves activates both the parasympathetic efferent pathway to excite the bladder, as well as the sensory afferent pathway that causes unwanted sphincter contractions. Thus, a novel pelvic nerve stimulation paradigm is required. In anesthetized female rats, we combined a low frequency (10 Hz) stimulation to evoke bladder contraction, and a more proximal 20 kHz stimulation of the pelvic nerve to block afferent activation, in order to produce micturition with reduced bladder-sphincter dyssynergia. Increasing the phase width of low frequency stimulation from 150 to 300 μs alone was able to improve voiding outcome significantly. However, low frequency stimulation of pelvic nerves alone evoked short latency (19.9–20.5 ms) dyssynergic EUS responses, which were abolished with a non-reversible proximal central pelvic nerve cut. We demonstrated that a proximal 20 kHz stimulation of pelvic nerves generated brief onset effects at lower current amplitudes, and was able to either partially or fully block the short latency EUS responses depending on the ratio of the blocking to stimulation current. Our results indicate that ratios >10 increased the efficacy of blocking EUS contractions. Importantly, we also demonstrated for the first time that this combined low and high frequency stimulation approach produced graded control of the bladder, while reversibly blocking afferent signals that elicited dyssynergic EUS contractions, thus improving voiding by 40.5 ± 12.3%. Our findings support advancing pelvic nerves as a suitable neuromodulation target for treating bladder dysfunction, and demonstrate the feasibility of an alternative method to non-reversible nerve transection and sub-optimal intermittent stimulation methods to reduce dyssynergia.

Phase reduction of a limit cycle oscillator perturbed by a strong amplitude-modulated high-frequency force

The phase reduction method for a limit cycle oscillator subjected to a strong amplitude-modulated high-frequency force is developed. An equation for the phase dynamics is derived by introducing a new, effective phase response curve. We show that if the effective phase response curve is everywhere positive (negative), then an entrainment of the oscillator to an envelope frequency is possible only when this frequency is higher (lower) than the natural frequency of the oscillator. Also, by using the Pontryagin maximum principle, we have derived an optimal waveform of the perturbation that ensures an entrainment of the oscillator with minimal power. The theoretical results are demonstrated with the Stuart-Landau oscillator and model neurons.

In vivo comparison of the charge densities required to evoke motor responses using novel annular penetrating microelectrodes

Electrodes for cortical stimulation need to deliver current to neural tissue effectively and safely. We have developed electrodes with a novel annular geometry for use in cortical visual prostheses. Here, we explore a critical question on the ideal annulus height to ensure electrical stimulation will be safe and effective. We implanted single electrodes into the motor cortex of anesthetized rats and measured the current required to evoke a motor response to stimulation, and the charge injection capacity (CIC) of the electrodes. We compared platinum iridium (PtIr) electrodes with different annulus heights, with and without a coating of porous titanium nitride (TiN). Threshold charge densities to evoke a motor response ranged from 12 to 36 μC.cm^-2.ph^-1. Electrodes with larger geometric surface areas (GSAs) required higher currents to evoke responses, but lower charge densities. The addition of a porous TiN coating did not significantly influence the current required to evoke a motor response. The CIC of both electrode types was significantly reduced in vivo compared with in vitro measurements. The measured CIC was 72 and 18 μC.cm^-2.ph^-1 for electrodes with and without a TiN coating, respectively. These results support the use of PtIr annular electrodes with annulus heights greater than 100 μm (GSA of 38, 000 μm²). However, if the electrodes are coated with porous TiN the annulus height can be reduced to 40 μm (GSA of 16,000 μm²).

In vivo comparison of the charge densities required to evoke motor responses using novel annular penetrating microelectrodes

Electrodes for cortical stimulation need to deliver current to neural tissue effectively and safely. We have developed electrodes with a novel annular geometry for use in cortical visual prostheses. Here, we explore a critical question on the ideal annulus height to ensure electrical stimulation will be safe and effective. We implanted single electrodes into the motor cortex of anesthetized rats and measured the current required to evoke a motor response to stimulation, and the charge injection capacity (CIC) of the electrodes. We compared platinum iridium (PtIr) electrodes with different annulus heights, with and without a coating of porous titanium nitride (TiN). Threshold charge densities to evoke a motor response ranged from 12 to 36 μC.cm^-2.ph^-1. Electrodes with larger geometric surface areas (GSAs) required higher currents to evoke responses, but lower charge densities. The addition of a porous TiN coating did not significantly influence the current required to evoke a motor response. The CIC of both electrode types was significantly reduced in vivo compared with in vitro measurements. The measured CIC was 72 and 18 μC.cm^-2.ph^-1 for electrodes with and without a TiN coating, respectively. These results support the use of PtIr annular electrodes with annulus heights greater than 100 μm (GSA of 38, 000 μm²). However, if the electrodes are coated with porous TiN the annulus height can be reduced to 40 μm (GSA of 16,000 μm²).

Biphasic monopolar electrical stimulation induces rapid and directed galvanotaxis in adult subependymal neural precursors

Introduction

Following injury such as stroke, adult mammalian subependymal neural precursor cells (NPCs) are induced to proliferate and migrate toward the lesion site where they differentiate into neural cells, albeit with limited efficacy. We are interested in enhancing this migratory ability of NPCs with the long-term goal of promoting neural repair. Herein we build on our previous studies demonstrating that direct current electric fields (DCEFs) promote rapid and cathode-directed migration of undifferentiated adult NPCs (but not differentiated phenotypes) - a phenomenon known as galvanotaxis. While galvanotaxis represents a promising strategy to promote NPC recruitment to lesion sites, stimulation of neural tissue with DCEFs is not a clinically-viable strategy due to the associated accumulation of charge and toxic byproducts. Balanced biphasic waveforms prevent the accumulation of charge and thus are outside of the limitations of DCEFs. In this study, we investigated the effects of balanced biphasic electrical stimulation on the migratory behaviour of undifferentiated subependymal NPCs and their differentiated progeny.

Methods

NPCs were isolated from the subependymal zone of adult mouse brains and cultured in a NPC colony-forming assay to form neurospheres. Neurospheres were plated onto galvanotaxis chambers in conditions that either promoted maintenance in an undifferentiated state or promoted differentiation into mature phenotypes. Chambers containing cells were then time-lapse imaged in the presence of either biphasic monopolar, or biphasic bipolar electrical stimulation, or in the complete absence of electrical stimulation. Single cell migration was subsequently tracked and the cells’ magnitude of velocity, directedness and tortuosity were quantified.

Results

We demonstrate, for the first time, the use of balanced biphasic electric fields to induce galvanotaxis of NPCs. Undifferentiated adult mouse subependymal NPCs exposed to biphasic monopolar stimulation undergo rapid and directed migration toward the cathode. In contrast, both biphasic bipolar stimulation and the lack of electrical stimulation produced non-directed migration of NPCs. Notably, NPCs induced to differentiate into mature phenotypes prior to exposure to electrical stimulation do not migrate in the presence or absence of biphasic stimulation.

Conclusion

We purport that balanced biphasic stimulation represents a clinically-viable technique for mobilizing NPCs that may be integrated into strategies for promoting endogenous neurorepair.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0049-6) contains supplementary material, which is available to authorized users.

Photovoltaic Retinal Prosthesis with High Pixel Density

Retinal degenerative diseases lead to blindness due to loss of the "image capturing" photoreceptors, while neurons in the "image processing" inner retinal layers are relatively well preserved. Electronic retinal prostheses seek to restore sight by electrically stimulating surviving neurons. Most implants are powered through inductive coils, requiring complex surgical methods to implant the coil-decoder-cable-array systems, which deliver energy to stimulating electrodes via intraocular cables. We present a photovoltaic subretinal prosthesis, in which silicon photodiodes in each pixel receive power and data directly through pulsed near-infrared illumination and electrically stimulate neurons. Stimulation was produced in normal and degenerate rat retinas, with pulse durations from 0.5 to 4 ms, and threshold peak irradiances from 0.2 to 10 mW/mm(2), two orders of magnitude below the ocular safety limit. Neural responses were elicited by illuminating a single 70 μm bipolar pixel, demonstrating the possibility of a fully-integrated photovoltaic retinal prosthesis with high pixel density.

Safety of multi-channel stimulation implants: a single blocking capacitor per channel is not sufficient after single-fault failure

One reason given for placing capacitors in series with stimulation electrodes is that they prevent direct current flow and therefore tissue damage under fault conditions. We show that this is not true for multiplexed multi-channel stimulators with one capacitor per channel. A test bench of two stimulation channels, two stimulation tripoles and a saline bath was used to measure the direct current flowing through the electrodes under two different single fault conditions. The electrodes were passively discharged between stimulation pulses. For the particular condition used (16 mA, 1 ms stimulation pulse at 20 Hz with electrodes placed 5 cm apart), the current ranged from 38 to 326 μA depending on the type of fault. The variation of the fault current with time, stimulation amplitude, stimulation frequency and distance between the electrodes is given. Possible additional methods to improve safety are discussed.

Separated interface nerve electrode prevents direct current induced nerve damage

Direct current, DC, can be used to quickly and reversibly block activity in excitable tissue, or to quickly and reversibly increase or decrease the natural excitability of a neuronal population. However, the practical use of DC to control neuronal activity has been extremely limited due to the rapid tissue damage caused by its use. We show that a separated interface nerve electrode, SINE, is a much safer method to deliver DC to excitable tissue and may be valuable as a laboratory research tool or potentially for clinical treatment of disease.

Tremor entrainment by patterned low-frequency stimulation

High-frequency test stimulation for tremor suppression is a standard procedure for functional target localization during deep brain stimulation. This method does not work in cases where tremor vanishes intraoperatively, for example, due to general anaesthesia or due to an insertional effect. To overcome this difficulty, we developed a stimulation technique that effectively evokes tremor in a well-defined and quantifiable manner. For this, we used patterned low-frequency stimulation (PLFS), i.e. brief high-frequency pulse trains administered at pulse rates similar to neurons' preferred burst frequency. Unlike periodic single-pulse stimulation, PLFS enables one to convey effective and considerably greater integral charge densities without violation of safety requirements. In a computational investigation of an oscillatory neuronal network temporarily rendered inactive, we found that PLFS evokes synchronized activity, phase locked to the stimulus. While a stronger increase in the amount of synchrony in the neuronal population requires higher stimulus intensities, the portion of synchronously active neurons nevertheless becomes strongly phase locked to PLFS already at weak stimulus intensities. The phase entrainment effect of PLFS turned out to be robust against variations in the stimulation frequency, whereas enhancement of synchrony required precisely tuned stimulation frequencies. We applied PLFS to a patient with spinocerebellar ataxia type 2 (SCA2) with pronounced tremor that disappeared intraoperatively under general anaesthesia. In accordance with our computational results, PLFS evoked tremor, phase locked to the stimulus. In particular, weak PLFS caused low-amplitude, but strongly phase-locked tremor. PLFS test stimulations provided the only functional information about target localization. Optimal target point selection was confirmed by excellent post-operative tremor suppression.

An Optically Powered Single-Channel Stimulation Implant as Test System for Chronic Biocompatibility and Biostability of Miniaturized Retinal Vision Prostheses

A microsystem based microimplant with an optically powered single-channel stimulator was designed and developed as test system for an epi-retinal vision implant. Biostability of the hybrid assembly and the encapsulation materials were evaluated in pilot experiments in chronic implantations in a cat animal model. The implant was fabricated on a flexible polyimide substrate with integrated platinum electrode, interconnection lines, and contact pads for hybrid integration of electronic components. The receiver part was realized with four photodiodes connected in series. A parylene C coating was deposited on the electronic components as insulation layer. Silicone rubber was used to encapsulate the electronics in the shape of an artificial intraocular lens to allow proper implantation in the eye. Pilot experiments showed the biostability of the encapsulation approach and full electric functionality of the microimplant to generate stimulation currents over the implantation period of three months in two cats. In one cat, electrical stimulation of the retina evoked neuronal responses in the visual cortex and indicated the feasibility of the system approach for chronic use.

Optoelectronic retinal prosthesis: system design and performance

The design of high-resolution retinal prostheses presents many unique engineering and biological challenges. Ever smaller electrodes must inject enough charge to stimulate nerve cells, within electrochemically safe voltage limits. Stimulation sites should be placed within an electrode diameter from the target cells to prevent 'blurring' and minimize current. Signals must be delivered wirelessly from an external source to a large number of electrodes, and visual information should, ideally, maintain its natural link to eye movements. Finally, a good system must have a wide range of stimulation currents, external control of image processing and the option of either anodic-first or cathodic-first pulses. This paper discusses these challenges and presents solutions to them for a system based on a photodiode array implant. Video frames are processed and imaged onto the retinal implant by a head-mounted near-to-eye projection system operating at near-infrared wavelengths. Photodiodes convert light into pulsed electric current, with charge injection maximized by applying a common biphasic bias waveform. The resulting prosthesis will provide stimulation with a frame rate of up to 50 Hz in a central 10 degrees visual field, with a full 30 degrees field accessible via eye movements. Pixel sizes are scalable from 100 to 25 microm, corresponding to 640-10,000 pixels on an implant 3 mm in diameter.

Electrical stimulation protocols for hippocampal synaptic plasticity and neuronal hyper-excitability: are they effective or relevant?

Long-term potentiation (LTP) of synaptic transmission is a widely accepted model that attempts to link synaptic plasticity with memory. LTP models are also now used in order to test how a variety of neurological disorders might affect synaptic plasticity. Interestingly, electrical stimulation protocols that induce LTP appear to display different efficiencies and importantly, some may not be as physiologically relevant as others. In spite of advancements in our understanding of these differences, many types of LTP inducing protocols are still widely used. In addition, in some cases electrical stimulation leads to normal biological phenomena, such as putative memory encoding and in other cases electrical stimulation triggers pathological phenomena, such as epileptic seizures. Kindling, a model of epileptogenesis involving repeated electrical stimulation, leads to seizure activity and has also been thought of, and studied as, a form of long-term neural plasticity and memory. Furthermore, some investigators now use electrical stimulation in order to reduce aspects of seizure activity. In this review, we compare in vitro and in vivo electrical stimulation protocols employed in the hippocampal formation that are utilized in models of synaptic plasticity or neuronal hyperexcitability. Here the effectiveness and physiological relevance of these electrical stimulation protocols are examined in situations involving memory encoding (e.g., LTP/LTD) and epileptiform activity.

Potential-biased, asymmetric waveforms for charge-injection with activated iridium oxide (AIROF) neural stimulation electrodes

The use of potential biasing and biphasic, asymmetric current pulse waveforms to maximize the charge-injection capacity of activated iridium oxide (AIROF) microelectrodes used for neural stimulation is described. The waveforms retain overall zero net charge for the biphasic pulse, but employ an asymmetry in the current and pulse widths of each phase, with the second phase delivered at a lower current density for a longer period of time than the leading phase. This strategy minimizes polarization of the AIROF by the charge-balancing second phase and permits the use of a more positive anodic bias for cathodal-first pulsing or a more negative cathodic bias for anodal-first pulsing to maximize charge injection. Using 0.4-ms cathodal-first pulses, a maximum charge-injection capacity of 3.3 mC/cm2 was obtained with an 0.6-V bias (versus Ag/AgCl) and a pulse asymmetry of 1:8 in the cathodal and anodal pulse widths. For anodal-first pulsing, a maximum charge capacity of 9.6 mC/cm2 was obtained with an asymmetry of 1:3 at an 0.1-V bias. These measurements were made in vitro in carbonate-buffered saline using microelectrodes with a 2000 microm2 surface area.

Laminar Variation in Threshold for Detection of Electrical Excitation of Striate Cortex by Macaques

Macaques were trained to signal their detection of electrical stimulation applied by a movable microelectrode to perifoveal striate cortex. Trains of ≤100 cathodal, 0.2-ms, constant current pulses were delivered at 50 or 100 Hz. The minimum current that could be reliably detected was measured at successive depths along radial electrode penetrations through the cortex. The lowest detection thresholds were routinely encountered when the stimulation was applied to layer 3, particularly just at the juncture between layers 3 and 4A. On the average, there was a twofold variation in threshold along the penetrations, with the highest intracortical thresholds being in layers 4C and 6. Variations as high as 20-fold were obtained in some individual penetrations, whereas relatively little change was observed in others. The minimum detectable current was 1 μA at a site in layer 3, i.e., 10–100 times lower than that for surface stimulation. Because macaques, as do human subjects, find electrical stimulation of striate cortex to be highly similar at all loci (a phosphene in the human case), it is puzzling as to how such uniformity of effect evolves from the exceedingly intricate circuitry available to the effective stimuli. It is hypothesized that the stimulus captures the most excitable elements, which then suppress other functional moieties, producing only the luminance of the phosphene. Lowest thresholds presumably are encountered when the electrode lies among these excitable elements that can, with higher currents, be stimulated directly from some distance or indirectly by the horizontal bands of myelinated axons, the stria of Baillarger.

Psychophysics of Electrical Stimulation of Striate Cortex in Macaques

Macaques indicated their detection of onset or alteration of 0.2-ms pulses applied in various configurations through electrodes implanted in striate cortex. When microelectrodes were introduced and left in place, the threshold for detection of 100-Hz pulses nearly doubled within 24 h. However, for chronically implanted platinum-alloy macroelectrodes detection thresholds usually remained stable for many months, independently of location within striate cortex or its immediately subjacent white matter. Thresholds were unaffected by the visual conditions, such as light versus darkness, or movement of the eyes; but in one animal blind after acute glaucoma thresholds for loci in striate cortex were permanently decreased by about 50%. Learning to respond to electrical stimulation of the optic tract produced no tendency to respond to such stimulation of striate cortex. Onset of stimulation at a given locus could be detected even in the face of continuous supraliminal stimulation at four surrounding loci on a 3-mm radius. The surround stimulation did alter the threshold of the central locus, but such stimuli could not summate if they were subliminal by some 10%. Cessation of stimulation that had been continuing for 1 min to 1 h could be detected if it were being applied at a level 20–75% above that needed for detection of stimulus onset. Continuous stimulation had a pronounced “priming” effect, in that modulation of frequency or intensity of such stimulation by as little as 5% could be detected (e.g., 20 μA in a background of 500 μA, or <2-ms interpulse interval with pulses at 50 Hz). Using pulses inserted in various phase relations to ongoing pulses at 2–5 Hz, it could be determined that stimulus pulses were surrounded by a strong facilitatory period for about 30 ms, which was then replaced by refractoriness. Given the congruence of macaque and human visual anatomy and psychophysics, these results further encourage efforts to develop a cortical prosthesis for the blind.

Electrical stimulation of excitable tissue: design of efficacious and safe protocols

The physical basis for electrical stimulation of excitable tissue, as used by electrophysiological researchers and clinicians in functional electrical stimulation, is presented with emphasis on the fundamental mechanisms of charge injection at the electrode/tissue interface. Faradaic and non-Faradaic charge transfer mechanisms are presented and contrasted. An electrical model of the electrode/tissue interface is given. The physical basis for the origin of electrode potentials is given. Various methods of controlling charge delivery during pulsing are presented. Electrochemical reversibility is discussed. Commonly used electrode materials and stimulation protocols are reviewed in terms of stimulation efficacy and safety. Principles of stimulation of excitable tissue are reviewed with emphasis on efficacy and safety. Mechanisms of damage to tissue and the electrode are reviewed.

The effects of electrode material, charge density and stimulation duration on the safety of high-frequency stimulation of the subthalamic nucleus in rats

High-frequency stimulation (HFS) of deep brain structures is a powerful therapeutic tool for the treatment of various movement disorders in patients. However, the pathophysiological mechanisms of this therapeutic approach on basal ganglia network function are still largely unknown. Hitherto, experimental studies have focused on short-term stimulation. Since patients receive HFS for many years, animal studies which reproduce the conditions of long-term stimulation will be necessary to accurately investigate the effects of HFS. However, stimulation parameters of acute HFS cannot be easily transferred to long-term conditions. Accordingly, for this purpose we studied the influence of different charge densities (0, 3, 6.5, 13 and 26 microC/cm2/phase) and duration (4 h or 3 days) of subthalamic nucleus (STN)-HFS using stainless-steel and platinum-iridium (Pt/Ir) electrodes on neuronal tissue damage in rats. Our data demonstrate the advantage of Pt/Ir over stainless-steel electrodes when used in short-term HFS (frequency 130 Hz, pulse width 60 micros) and indicate that HFS using Pt/Ir-electrodes pulsed with 3 microC/cm2/phase over 3 days did not produce any relevant tissue damage in the STN.

Retinal prosthesis for the blind

Most of current concepts for a visual prosthesis are based on neuronal electrical stimulation at different locations along the visual pathways within the central nervous system. The different designs of visual prostheses are named according to their locations (i.e., cortical, optic nerve, subretinal, and epiretinal). Visual loss caused by outer retinal degeneration in diseases such as retinitis pigmentosa or age-related macular degeneration can be reversed by electrical stimulation of the retina or the optic nerve (retinal or optic nerve prostheses, respectively). On the other hand, visual loss caused by inner or whole thickness retinal diseases, eye loss, optic nerve diseases (tumors, ischemia, inflammatory processes etc.), or diseases of the central nervous system (not including diseases of the primary and secondary visual cortices) can be reversed by a cortical visual prosthesis. The intent of this article is to provide an overview of current and future concepts of retinal and optic nerve prostheses. This article will begin with general considerations that are related to all or most of visual prostheses and then concentrate on the retinal and optic nerve designs. The authors believe that the field has grown beyond the scope of a single article so cortical prostheses will be described only because of their direct effect on the concept and technical development of the other prostheses, and this will be done in a more general and historic perspective.

Chronic neural stimulation with thin-film, iridium oxide electrodes

Experiments were conducted to assess the effect of chronic stimulation on the electrical properties of the electrode-tissue system, as measured using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Silicon, micromachined probes with multiple iridium oxide stimulating electrodes (400-1600 micron 2) were implanted in guinea pig cortex. A 10-17 day post-operative recovery period was followed by five days of monopolar stimulation, two hours/electrode each day using biphasic, constant current stimulation (5-100 microA, 100 microseconds/phase). EIS and CV data were taken before and after stimulation. The post-stimulation impedance [at mid-range frequencies (100 Hz-100 kHz)] consistently and significantly decreased relative to prestimulation levels. Impedance magnitude increased permanently at low frequencies (< 100 Hz), correlating to a change in the charge storage capacity (the area under a cyclic voltammagram). Impedance magnitude significantly increased during the recovery period, though this increase could be mostly reversed by applying small currents. A mathematical model of the electrode-tissue system impedance was used to analyze in vivo behavior. The data and modeling results shows that applying charge to the electrode can consistently reduce the impedance of the electrode-tissue system. Analysis of explanted probes suggests that the interaction between the tissue and electrode is dependent on whether chronic pulses were applied. It is hypothesized that the interface between the tissue and metal is altered by current pulsing, resulting in a temporary impedance shift.

Electrical stimulation of neural tissue to evoke behavioral responses

This review yields numerous conclusions. (1) Both unit recording and behavioral studies find that current activates neurons (i.e., cell bodies and axons) directly according to the square of the distance between the electrode and the neuron, and that the excitability of neurons can vary between 100 and 4000 microA/mm2 using a 0.2-ms cathodal pulse duration. (2) Currents as low as 10 microA, which is considered within the range of currents typically used during micro-stimulation, activate from a few tenths to several thousands of cell bodies in the cat motor cortex directly depending on their excitability; this indicates that even low currents activate more than a few neurons. (3) Electrode tip size has no effect on the current density--or effect current spread--at far field, but tip size limits the current-density generated at near field. (4) To minimize neuronal damage, the electrode should be discharged after each pulse and the pulse duration should not exceed the chronaxie of the stimulated tissue. (5) The amount of current needed to evoke behavioral responses depends not only on the excitability of the stimulated substrate but also on the type of behavior being studied.

Stimulation with chronically implanted microelectrodes in the cochlear nucleus of the cat: histologic and physiologic effects

The effects of several hours of continuous electrical stimulation in the cats' cochlear nucleus with chronically implanted activated iridium microelectrodes was investigated from the changes in the evoked response near the inferior colliculus and also by histologic evaluation of the stimulated tissue. The stimulating microelectrodes had geometric surface areas of 75-500 microns2. They were pulsed continuously for 4 h, at a pulse repetition rate of 200 Hz, using charge-balanced pulse pairs. The charge per phase was 1.8 or 3.6 nC/ph. The animals were sacrificed for histologic evaluation 2 h, or several days later. The only remarkable histologic change resulting from the 4 h of stimulation was some aggregation of lymphocytes at the site of stimulation. However, depression of the electrical excitability of neurons near the sites often persisted for several days after 4 h of stimulation at 3.6 nC/phase. The charge per phase of the stimulus pulse pair was correlated strongly with the depression of excitability, and there was a weaker correlation between the depression and the amplitude of the first phase of voltage transient induced across the electrode-tissue interface. The charge density, calculated from the geometric surface area of the stimulating electrodes, was poorly correlated with the severity of the depression. The findings suggest a means of detecting impending stimulation-induced neural damage while it is still reversible.

In vitro measurement and characterization of current density profiles produced by non-recessed, simple recessed, and radially varying recessed stimulating electrodes

Potential fields induced by nonrecessed, simple recessed, and radially varying recessed electrode designs were measured in vitro. Comparison of experimental results with theoretical analyses substantiated the experimental measurement technique and emphasized the importance of considering both nonuniform charge injection and surface electrochemistry when designing implantable stimulating electrodes. Radially varying recesses produced uniform charge injection at the electrode surface and at the aperture-tissue interface. In general, the radially varying recessed electrodes provided a combination of uniform charge injection and flexibility in design and fabrication that warrants their incorporation into all appropriate planar stimulating electrode designs.

Neurophysiological and technical considerations for the design of an implantable phrenic nerve stimulator

Sequential stimulation during one muscle contraction of several compartments of a motor nerve, using multiple-electrodes, allows individual nerve-muscle compartments to be stimulated at fairly low frequencies. This provides time for recovery even during muscle contraction. However, the whole muscle is stimulated at near to its optimum fusion frequency, which provides smooth muscle contraction. This stimulation system imitates the natural activation of skeletal muscle. The new phrenic nerve stimulator described utilises the principle of sequential motor nerve stimulation. It also incorporates a sigh function. The sigh current recruits additional axons at certain intervals and thus creates and keeps available a reserve of conditioned muscle. Clinical advantages result: the conditioning phase after the beginning of long-term phrenic nerve stimulation for electroventilation is shortened and muscle fatigue is delayed. A need of increase of gas exchange can be answered by increasing tidal volume instead of respiration rate alone.

Neuronal activity evoked by chronically implanted intracortical microelectrodes

The averaged evoked compound action potentials (AECAPs) were recorded from the ipsilateral pyramidal tract of awake, unrestrained cats before, during, and after continuous electrical stimulation of the cerebral cortex via chronically implanted activated iridium or platinum-30% iridium (Pt30%Ir) microelectrodes. After stimulating 24 h at 20 pulses per second (pps), using charge-balanced, 200-microseconds pulse pairs of 40 to 80 microA (400 to 800 microC/cm2, 8 to 16 nC/phase (ph), 2 to 4 A/cm2), there was a transient elevation of the threshold of the early (direct) and of the alte (transynaptic) components of the AECAP. After cessation of continuous stimulation at 80 microA, the threshold of the early component of the AECAP remained elevated for as long as 24 h and the late component as long as 4 days, indicating significant but reversible depression of the electrical excitability of cortical neurons close to the microelectrodes. In three cats stimulated 23 h/day for 1 week, the AECAP also recovered to their prestimulus threshold. In contrast, pulsing for 24 h at 320 microA (3200 microC/cm2, 64 nC/ph, 16 A/cm2) produced marked elevation of the threshold of the AECAPs which was not reversed by 7 to 12 days after termination of intracortical stimulation. The electrical excitability of neurons adjacent to (unpulsed) microelectrodes 2 mm from the pulsed electrode was not affected. The observations reported here, in conjunction with the histologic results reported in the companion paper, indicate that both the Pt30%Ir and the iridium microelectrodes can be operated safely at currents to at least 80 microA, charge/ph of 16 A/cm2, and a charge density of 800 microC/cm2 X ph. However, on the basis of the electrophysiologic criteria, both types appear to be unsafe when pulsed at 320 microA (64 nC/ph, 3200 microC/cm2 X ph, 16 A/cm2).