An investigation of hydrous oxide growth on iridium in base

Activated iridium oxide film (AIROF) electrodes for neural tissue stimulation

OBJECTIVE

Iridium oxide films are commonly used as a high charge-injection electrode material in neural devices. Yet, few studies have performed in-depth assessments of material performance versus film thickness, especially for films grown on three-dimensional (instead of planar) metal surfaces in neutral pH electrolyte solutions. Further, few studies have investigated the driving voltage requirements for constant-current stimulation using activated iridium oxide (AIROF) electrodes, which will be a key constraint for future use in wirelessly powered neural devices.

APPROACH

In this study, iridium microwire probes were activated by repeated potential pulsing in room temperature phosphate buffered saline (pH 7.1-7.3). Electrochemical measurements were recorded in three different electrolyte conditions for probes with different geometric surface areas (GSAs) as the AIROF thickness was increased.

MAIN RESULTS

Maintaining an anodic potential bias during the inter-pulse interval was required for AIROF electrodes to deliver charge levels considered necessary for neural stimulation. Potential pulsing for 100-200 cycles was sufficient to achieve charge injection levels of 2.5 mC cm^-2 (50 nC/phase in a biphasic pulse) in PBS with 2000 µm² iridium probes. Increasing the electrode surface area to 3000 µm² and 4000 µm² significantly increased charge-injection capacity, reduced the driving voltage required to deliver a fixed amount of charge, and reduced polarization of the electrodes during constant-current pulsing.

SIGNIFICANCE

This study establishes methods for choosing an activation protocol and a desired GSA for three-dimensional iridium electrodes suitable for neural tissue insertion and stimulation, and provides guidelines for evaluating electrochemical performance of AIROF using model saline solutions.

Probing the solid-liquid interface with tender x rays: A new ambient-pressure x-ray photoelectron spectroscopy endstation at the Swiss Light Source

A new endstation to perform operando chemical analysis at solid-liquid interfaces by means of ambient pressure x-ray photoelectron spectroscopy (APXPS) is presented. The endstation is located at the Swiss Light Source and can be attached to the soft x-ray in situ spectroscopy beamline (X07DB) for solid-gas type experiments and to a tender x-ray beamline (PHOENIX I) for solid-liquid interface experiments. The setup consists of three interconnected ultrahigh vacuum chambers: one for sample preparation using surface science techniques, the analysis chamber for APXPS experiments, and an entry-lock chamber for sample transfer across the two pressure regimes. The APXPS chamber is designed to study solid-liquid interfaces stabilized by the dip and pull method. Using a three-electrode setup, the potential difference across the solid-electrolyte interface can be controlled, as is demonstrated here using an Ir(001) electrode dipped and pulled from a 0.1M KOH electrolyte. The new endstation is successfully commissioned and will offer unique opportunities for fundamental studies of phenomena that take place at solid-liquid interfaces and that are relevant for fields such as electrochemistry, photochemistry, or biochemistry, to name a few.

Impact of Electrocatalyst Activity and Ion Permeability on Water-Splitting Photoanodes

Electrocatalyst (EC)-modified semiconductor (SC) photoelectrodes are key elements of solar water-splitting systems. The SC|EC interface affects the composite photoelectrode behavior but is poorly understood. We uncover the role of EC activity and SC|EC interface properties using a range of metal (Ni, Fe, Ni-Fe, Co, Ir) oxide or (oxy)hydroxide ECs deposited on model single-crystal n-TiO2 photoanodes. The impedance and photoelectrochemical response of the system was nearly independent of EC oxygen evolution activity if the catalyst was deposited electrochemically as an ion-permeable (oxy)hydroxide or hydrous oxide. When dense oxides (e.g., ion-impermeable) ECs were used, the response depended strongly on the EC. These data demonstrate that the EC and SC interface structures are more important than the EC activity in determining the composite photoanode response, confirming recent SC|EC interface simulations for ion-permeable ECs. These results thus inform the design of high-performance water-oxidizing photoanodes with direct SC|EC interfaces.

Improved Poly(3,4-Ethylenedioxythiophene) (PEDOT) for Neural Stimulation

OBJECTIVE

This study compares the stability of three variations of the conductive polymer poly(3,4-ethylenedioxythiophene) or PEDOT for neural micro-stimulation under both in vitro and in vivo conditions. We examined PEDOT films deposited with counter-ions tetrafluoroborate (TFB) and poly(styrenesulfonate) (PSS), and

PEDOT

PSS combined with carbon nanotubes (CNTs).

METHODS

For the in vitro stability evaluation, implantable micro-wires were coated with the polymers, placed in a vial containing phosphate buffered saline (PBS) under accelerated aging conditions (60°C), and current pulses were applied. The resulting voltage profile was monitored over time. Following the same polymer deposition protocol, chronic neural micro-probes were modified and implanted in the motor cortex of two rats for the in vivo stability comparison. Similar stimulating current pulses were applied and the output voltage was examined. The electrochemical impedance spectroscopic (EIS) data were also recorded and fit to an equivalent circuit model that incorporates and quantifies the time-dependent polymer degradation and impedance associated with tissue surrounding each micro-electrode site.

RESULTS

Both in vitro and in vivo voltage output profiles show relatively stable behavior for the

PEDOT

TFB modified micro-electrodes compared to the

PEDOT

PSS and CNT:

PEDOT

PSS modified ones. EIS modeling demonstrates that the time-dependent increase in the polymeric resistance is roughly similar to the rise in the respective voltage output in vivo and indicates that the polymeric stability and conductivity, rather than the impedance due to the tissue response, is the primary factor determining the output voltage profile. It was also noted that the number of electrodes showing unit activity post-surgery did not decay for

PEDOT

TFB as was the case for

PEDOT

PSS and CNT:

PEDOT

PSS.

CONCLUSIONS

PEDOT

TFB may be an enabling material for achieving long lasting micro-stimulation and recording.

Improving the performance of poly(3,4-ethylenedioxythiophene) for brain-machine interface applications

Conducting polymers, especially poly(3,4-ethylenedioxythiophene) (PEDOT) based materials, are important for developing highly sensitive and microscale neural probes. In the present work, we show that the conductivity and stability of PEDOT can be significantly increased by switching the widely used counter anion poly(styrenesulfonate) (PSS) to the smaller tetrafluoroborate (TFB) anion during the electrodeposition of the polymer. Time-dependent impedance measurements of polymer modified implantable microwires were conducted in physiological buffer solutions under accelerated aging conditions and the relative stability of PEDOT:PSS and PEDOT:TFB modified microwires was compared over time. This study was also extended to carbon nanotube (CNT) incorporated PEDOT:PSS which, according to some reports, is claimed to enhance the stability and electrical performance of the polymer. However, no noticeable difference was observed between PEDOT:PSS and CNT:PEDOT:PSS in our measurements. At the biologically relevant frequency of 1kHz, PEDOT:TFB modified microwires exhibit approximately one order of magnitude higher conductivity and demonstrate enhanced stability over both PEDOT:PSS and CNT:PEDOT:PSS modified microwires. In addition, PEDOT:TFB is not neurotoxic and we show the proof-of-concept for both in vitro and in vivo neuronal recordings using PEDOT:TFB modified microelectrode arrays and chronic electrodes, respectively. Our findings suggest that PEDOT:TFB is a promising conductive polymer coating for the recording of neural activities.

Neural interface dynamics following insertion of hydrous iridium oxide microelectrode arrays

Studies examining traumatic brain injury have suggested a 'window of opportunity' exists for therapeutic agents to mitigate edema and cellular toxicity effectively. However, successful therapy also relies on identifying the extent of blood-brain barrier disruption, which is associated with excessive extra-cellular concentrations of ions, excitatory amino acids, and serum proteins. The following study investigates the use of pH-selective hydrous iridium oxide microelectrodes to assess trauma following insertion of a neural probe. Electrochemical activation of iridium microelectrode arrays was performed in either acidic (0.5 M H2SO4) or weak basic (0.3 M Na2HPO4, pH=8.56) solutions. Both oxides demonstrated super-Nernstian pH sensitivity (-88.5 mV/pH and -77.1 mV/pH, respectively) with little interference by other cations. Data suggest that acid-grown oxide provides better potential stability than base-grown oxide (sigma=2.8 versus 4.9 mV over 5 hours). Implantation of these electrodes into motor cortex and dorsal striatum revealed significant acidosis during and following insertion. Variability in the spatiotemporal pH profile included micro-scale inhomogeneities along the probe shank and significant differences in the averaged pH response between successive insertions using the same depth and speed. This diagnostic technology has important implications for intervention therapies in order to more effectively treat acute surgical brain trauma.

Spatiotemporal pH dynamics following insertion of neural microelectrode arrays

Insertion trauma is a critical issue when assessing intracortical electrophysiological and neurochemical recordings. Previous reports document a wide variety of insertion techniques with speeds ranging from 10 microm/s to 10 m/s. We hypothesize that insertion speed has an effect on tissue trauma induced by implantation of a neural probe. In order to monitor the neural interface during and after probe insertion, we have developed a silicon-substrate array with hydrous iridium oxide microelectrodes for potentiometric recording of extracellular pH (pH(e)), a measure of brain homeostasis. Microelectrode sites were sensitive to pH in the super-Nernstian range (-85.9 mV/pH unit) and selective over other analytes including ascorbic acid, Na(+), K(+), Ca(2+), and Mg(2+). Following insertion, arrays recorded either triphasic or biphasic pH(e) responses, with a greater degree of prolonged acidosis for insertions at 50 microm/s than at 0.5 mm/s or 1.0 mm/s (p<0.05). Spatiotemporal analysis of the recordings also revealed micro-scale variability in the pH(e) response along the array, even when using the same insertion technique. Implants with more intense acidosis were often associated histologically with blood along the probe tract. The potentiometric microsensor array has implications not only as a useful tool to measure extracellular pH, but also as a feedback tool for delivery of pharmacological agents to treat surgical brain trauma.

Chronic neural recordings using silicon microelectrode arrays electrochemically deposited with a poly(3,4-ethylenedioxythiophene) (PEDOT) film

Conductive polymer coatings can be used to modify traditional electrode recording sites with the intent of improving the long-term performance of cortical microelectrodes. Conductive polymers can drastically decrease recording site impedance, which in turn is hypothesized to reduce thermal noise and signal loss through shunt pathways. Moreover, conductive polymers can be seeded with agents aimed at promoting neural growth toward the recording sites or minimizing the inherent immune response. The end goal of these efforts is to generate an ideal long-term interface between the recording electrode and surrounding tissue. The goal of this study was to refine a method to electrochemically deposit surfactant-templated ordered poly(3,4-ethylenedioxythiophene) (PEDOT) films on the recording sites of standard 'Michigan' probes and to evaluate the efficacy of these modified sites in recording chronic neural activity. PEDOT-coated site performance was compared to control sites over a six-week evaluation period in terms of impedance spectroscopy, signal-to-noise ratio, number of viable unit potentials recorded and local field potential recordings. PEDOT sites were found to outperform control sites with respect to signal-to-noise ratio and number of viable unit potentials. The benefit of reduced initial impedance, however, was mitigated by the impedance contribution of typical silicon electrode encapsulation. Coating sites with PEDOT also reduced the amount of low-frequency drift evident in local field potential recordings. These findings indicate that electrode sites electrochemically deposited with PEDOT films are suitable for recording neural activity in vivo for extended periods. This study also provided a unique opportunity to monitor how neural recording characteristics develop over the six weeks following implantation.

Use of micromachined probes for the recording of cardiac electrograms in isolated heart tissues

Micromachined probes, with iridium (Ir) microelectrodes on silicon shanks, were evaluated to assess their suitability for cardiac electrogram recording. The electrochemical activation (anodic oxidation) procedure for the circular Ir microelectrode was investigated using the square wave potential according to the electrode size, number of cycles, and cathodic-anodic potential level of the square wave. Increase in the charge storage capacity was pronounced either in smaller electrodes or with higher potential level of the square wave. The electrode impedance reduced in a similar manner with increasing number of cycle irrespective of the electrode size. With either lower potential level (-0.70/+0.60 V) or smaller number of cycle (200 cycles) than those for the activation of stimulating electrode, the likelihood of overactivation of the recording microelectrode can be minimized. These anodic IrOx film (AIROF) microelectrodes were used for the recording of extracellular electrograms in two different ex vivo cardiac tissue preparations. A single-shank microprobe was applied to the left ventricle of a mouse heart. Both the spontaneous and paced transmural responses propagating between epicardium and endocardium were obtained. Longitudinal cardiac wavefronts propagating along the rabbit papillary muscle were also recorded with a unique multiple-shank design. The measured mean amplitude and the propagation velocity of the extracellular voltage were 12.2 +/- 1.8 mV and 58.9 +/- 2.2 cm/s, respectively (n = 27). These microprobes with precisely defined electrode spacing make a useful tool for the spatial and temporal mapping of electrical properties in isolated heart tissues ex vivo.