Intraspinal stimulation with a silicon-based 3D chronic microelectrode array for bladder voiding in cats

In vitro biocompatibility evaluation of functional electrically stimulating microelectrodes on primary glia

Neural interfacing devices interact with the central nervous system to alleviate functional deficits arising from disease or injury. This often entails the use of invasive microelectrode implants that elicit inflammatory responses from glial cells and leads to loss of device function. Previous work focused on improving implant biocompatibility by modifying electrode composition; here, we investigated the direct effects of electrical stimulation on glial cells at the electrode interface. A high-throughput in vitro system that assesses primary glial cell response to biphasic stimulation waveforms at 0 mA, 0.15 mA, and 1.5 mA was developed and optimized. Primary mixed glial cell cultures were generated from heterozygous CX3CR-1+/EGFP mice, electrically stimulated for 4 h/day over 3 days using 75 μm platinum-iridium microelectrodes, and biomarker immunofluorescence was measured. Electrodes were then imaged on a scanning electron microscope to assess sustained electrode damage. Fluorescence and electron microscopy analyses suggest varying degrees of localized responses for each biomarker assayed (Hoescht, EGFP, GFAP, and IL-1β), a result that expands on comparable in vivo models. This system allows for the comparison of a breadth of electrical stimulation parameters, and opens another avenue through which neural interfacing device developers can improve biocompatibility and longevity of electrodes in tissue.

Charge Injection Enhancement Comparisons of Iridium Oxide Microelectrodes In Vitro and In Vivo Using a Portable Neurostimulator

Microelectrodes are desired to deliver more charges to neural tissues while under electrochemical safety limits. Applying anodic bias potential during neurostimulation is a known technique for charge enhancement. Here, we investigated the levels of charge enhancement with anodic bias potential in vitro and in vivo using a custom-designed portable neurostimulator. We immersed our custom microelectrode probe in saline and measured voltage transients in response to constant current stimulation with and without a 500 mV anodic bias potential. We then inserted the same microelectrode probe into the primary motor cortex of the rat brain and measured voltage transients with the same electronics. Results showed that the charge injection capacity of the activated iridium oxide microelectrode site (with 2000 μm2 geometric surface areas (GSAs)) increased by the use of the anodic bias potentials in both in vitro and in vivo: from 10 nC/phase to 32 nC/phase for 200 μs pulse widths, and from 2 nC/phase to 8 nC/phase, respectively. Thus, the order of charge injection capacities of the four cases tested in this study is as follows (from the lowest to the highest): in vivo without anodic bias, in vivo with anodic bias, in vitro without anodic bias, and in vitro with anodic bias. This work also validated in vivo use of our new portable neurostimulator which received stimulation waveforms wirelessly.

Spinal stimulation for motor rehabilitation immediately modulates nociceptive transmission

Objective. Spinal cord injury (SCI) often results in debilitating movement impairments and neuropathic pain. Electrical stimulation of spinal neurons holds considerable promise both for enhancing neural transmission in weakened motor pathways and for reducing neural transmission in overactive nociceptive pathways. However, spinal stimulation paradigms currently under development for individuals living with SCI continue overwhelmingly to be developed in the context of motor rehabilitation alone. The objective of this study is to test the hypothesis that motor-targeted spinal stimulation simultaneously modulates spinal nociceptive transmission.Approach. We characterized the neuromodulatory actions of motor-targeted intraspinal microstimulation (ISMS) on the firing dynamics of large populations of discrete nociceptive specific and wide dynamic range (WDR) neurons. Neurons were accessed via dense microelectrode arrays implantedin vivointo lumbar enlargement of rats. Nociceptive and non-nociceptive cutaneous transmission was induced before, during, and after ISMS by mechanically probing the L5 dermatome.Main results. Our primary findings are that (a) sub-motor threshold ISMS delivered to spinal motor pools immediately modulates concurrent nociceptive transmission; (b) the magnitude of anti-nociceptive effects increases with longer durations of ISMS, including robust carryover effects; (c) the majority of all identified nociceptive-specific and WDR neurons exhibit firing rate reductions after only 10 min of ISMS; and (d) ISMS does not increase spinal responsiveness to non-nociceptive cutaneous transmission. These results lead to the conclusion that ISMS parameterized to enhance motor output results in an overall net decrease n spinal nociceptive transmission.Significance. These results suggest that ISMS may hold translational potential for neuropathic pain-related applications and that it may be uniquely suited to delivering multi-modal therapeutic benefits for individuals living with SCI.

A portable neurostimulator circuit with anodic bias enhances stimulation injection capacity

Objective.Electrochemically safe and efficient charge injection for neural stimulation necessitates monitoring of polarization and enhanced charge injection capacity of the stimulating electrodes. In this work, we present improved microstimulation capability by developing a custom-designed multichannel portable neurostimulator with a fully programmable anodic bias circuitry and voltage transient monitoring feature.Approach.We developed a 16-channel multichannel neurostimulator system, compared charge injection capacities as a function of anodic bias potentials, and demonstrated convenient control of the system by a custom-designed user interface allowing bidirectional wireless data transmission of stimulation parameters and recorded voltage transients. Charge injections were conducted in phosphate-buffered saline with silicon-based iridium oxide microelectrodes.Main results.Under charge-balanced 200µs cathodic first pulsing, the charge injection capacities increased proportionally to the level of anodic bias applied, reaching a maximum of ten-fold increase in current intensity from 10µA (100µC cm-2) to 100µA (1000µC cm-2) with a 600 mV anodic bias. Our custom-designed and completely portable 16-channel neurostimulator enabled a significant increase in charge injection capacityin vitro. Significance.Limited charge injection capacity has been a bottleneck in neural stimulation applications, and our system may enable efficacious behavioral animal study involving chronic microstimulation while ensuring electrochemical safety.

Stable, long-term single-neuronal recording from the rat spinal cord with flexible carbon nanotube fiber electrodes

Objective. Flexible implantable electrodes enable months-long stable recording of single-unit signals from rat brains. Despite extensive efforts in the development of flexible probes for brain recording, thus far there are no conclusions on their application in long-term single neuronal recording from the spinal cord which is more mechanically active. To this end, we realized the chronic recording of single-unit signals from the spinal cord of freely-moving rats using flexible carbon nanotube fiber (CNTF) electrodes. Approach. We developed flexible CNTF electrodes for intraspinal recording. Continuous in vivo impedance monitoring and histology studies were conducted to explore the critical factors determining the longevity of the recording, as well as to illustrate the evolution of the electrode-tissue interface. Gait analysis were performed to evaluate the biosafety of the chronic intraspinal implantation of the CNTF electrodes. Main results. By increasing the insulation thickness of the CNTF electrodes, single-unit signals were continuously recorded from the spinal cord of freely-moving rats without electrode repositioning for 3-4 months. Single neuronal and local field potential activities in response to somatic mechanical stimulation were successfully recorded from the spinal dorsal horns. Histological data demonstrated the ability of the CNTF microelectrodes to form an improved intraspinal interfaces with greatly reduced gliosis compared to their stiff metal counterparts. Continuous impedance monitoring suggested that the longevity of the intraspinal recording with CNTF electrodes was determined by the insulation durability. Gait analysis showed that the chronic presence of the CNTF electrodes caused no noticeable locomotor deficits in rats. Significance. It was found that the chronic recording from the spinal cord faces more stringent requirements on the electrode structural durability than recording from the brain. The stable, long-term intraspinal recording provides unique capabilities for studying the physiological functions of the spinal cord relating to motor, sensation, and autonomic control in both health and disease.

A Simple Table-Top Technique for Multi-Signal Pseudo-Extracellular Recording

Validation of neural probe performance often includes implantation in live animals, to assess ability to detect and distinguish signals generated by individual neurons. While this method is informative, an effective in vitro alternative would streamline device development and improve ethical considerations by reducing the use of animals in the validation of neural recording devices. Here, we describe a simple system using ball electrodes to apply multiple neural waveforms to phosphate buffered saline, which are simultaneously recorded by a microelectrode probe. Using this technique, our neural probe was able to detect and distinguish spikes from multiple units of roughly physiological amplitudes (~100 microvolts peak to peak), indicating promise as an in vitro alternative to animal testing for initial validation of neural recording devices.

A Diagnostic Circuit for Crosstalk Detection in Microelectrode Arrays

Current leakage between channels in microelectrode arrays is a sign of device failure and can lead to shorting of neural signals. The purpose of this project is to detect crosstalk between 32 channels of electrodes. We designed an embedded crosstalk detection system that can stimulate each electrode individually with a constant-current pulse and record voltage transients of the stimulated and adjacent electrodes to generate a matrix of crosstalk values. Charge injection in a phosphate buffered saline solution was used to check the condition of each electrode. A semi-wet condition was then used to determine the percent crosstalk between the channels. The analysis showed that there was minimal crosstalk between the electrodes, except for a known physical defect on the probe. The measurement technique enabled by the electronics circuit has the potential to be used in functional testing and screening of implantable devices.

Maximizing Charge Injection Limits of Iridium Oxide Electrodes with a Programmable Anodic Bias Circuit

Efficacious stimulation of neural tissues requires high charge injection capacity while minimizing electrode polarization. Applying anodic bias on certain electrode materials is a way to enhance charge injection both in vitro and in vivo. We developed an embedded neurostimulator system that enabled a digital control of user-defined bias levels, without requiring a potentiometer or external voltage source. Comparison of charge injection with and without anodic-bias, as well as at different bias potentials were conducted in phosphate-buffered saline with Blackrock iridium oxide microelectrodes. Results showed that a nine-fold increase in current intensity and charge injection capacity, was achieved with a 0.7 V anodic bias and within electrochemically safe limits.

Unprotected sidewalls of implantable silicon-based neural probes and conformal coating as a solution

Silicon-based implantable neural devices have great translational potential as a means to deliver various treatments for neurological disorders. However, they are currently held back by uncertain longevity following chronic exposure to body fluids. Conventional deposition techniques cover only the horizontal surfaces which contain active electronics, electrode sites, and conducting traces. As a result, a vast majority of today's silicon devices leave their vertical sidewalls exposed without protection. In this work, we investigated two batch-process silicon dioxide deposition methods separately and in combination: atomic layer deposition and inductively-coupled plasma chemical vapor deposition. We then utilized a rapid soak test involving potassium hydroxide to evaluate the coverage quality of each protection strategy. Focused ion beam cross sectioning, scanning electron microscopy, and 3D extrapolation enabled us to characterize and quantify the effectiveness of the deposition methods. Results showed that bare silicon sidewalls suffered the most dissolution whereas ALD silicon dioxide provided the best protection, demonstrating its effectiveness as a promising batch process technique to mitigate silicon sidewall corrosion in chronic applications.