Biocompatibility and charge injection property of iridium film formed by ion beam assisted deposition

Iridium thin film formed by electron-beam evaporation with simultaneous bombardment of Ar ion beam was evaluated for a stimulating neural electrode. The electrochemical behavior of as-deposited Ir film on Ni-Ti sample was almost identical to bulk Ir by producing much higher open-circuit corrosion potential and much lower anodic current density than the uncoated Ni-Ti in both 1N sulfuric acid and saline solution. The charge injection capability of Ir film was compared with that of Pt electrode currently used mostly as a stimulating neural electrode. The charge density of Pt was small and unchanged with increasing number of activating cycles in 0.1M H(2)SO(4), whilst the Ir film continuously produced increases in charge density. The charge injection density of Ir film in physiological solution was higher for the more activated sample under the identical stimulating condition. Attachment and proliferation with PC12 cells on Ir-coated CP Ti without applying electrical stimulation was similar to the polished CP Ti. A network of neurons and extending axons were formed on Ir film.     

Protein adsorption and peroxidation of rat retinas under stimulation of a neural probe coated with polyaniline

For the purpose of investigating the potential use of conducting polymers, i.e. polyaniline (PANi), as electrode coating material for improving the function of neural probes, a PANi-coated platinum (Pt) electrode was prepared by the in situ polymerization method. Protein adsorption was observed by atomic force microscopy/scanning electron microscopy and sodium dodecyl sulfate polyacrylamide gel electrophoresis, as well as quantification. Peroxidation of rat retinas was evaluated by determination of conjugated dienes and PLOOH, which were quantified by UV–visible spectrophotometer and high-performance liquid chromatography. The stability of PANi coating for 6 months was also estimated with an in vitro electrical stimulation system. This revealed that: (1) PANi with regular and compact nanoparticles 20–40 nm in diameter was successfully polymerized on the uncoated platinum electrode surface; (2) the PANi-coated Pt electrode adsorbed fewer retinal fragments and induced less peroxidation than the uncoated platinum electrode; (3) in contrast to the uncoated platinum electrode, the PANi-coated Pt electrode surface tended to aggregate retinal fragments rather than spread them, which may help to reduce inflammation and scar formation in long-term implantation; (4) the PANi coating exhibited excellent properties in terms of the intactness and the stable nanoparticle morphology after 6 months’ electrical stimulation, while corrosion occurred on the uncoated platinum electrode after 1 month.     

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.     

Highly stable carbon nanotube doped poly(3,4-ethylenedioxythiophene) for chronic neural stimulation

The function and longevity of implantable microelectrodes for chronic neural stimulation depends heavily on the electrode materials, which need to present high charge injection capability and high stability. While conducting polymers have been coated on neural microelectrodes and shown promising properties for chronic stimulation, their practical applications have been limited due to unsatisfying stability. Here, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with pure carbon nanotubes (CNTs) was electrochemically deposited on Pt microelectrodes to evaluate its properties for chronic stimulation. The PEDOT/CNT coated microelectrodes demonstrated much lower impedance than the bare Pt, and the PEDOT/CNT film exhibited excellent stability. For both acute and chronic stimulation tests, there is no significant increase in the impedance of the PEDOT/CNT coated microelectrodes, and none of the PEDOT/CNT films show any cracks or delamination, which have been the limitation for many conducting polymer coatings on neural electrodes. The charge injection limit of the Pt microelectrode was significantly increased to 2.5 mC/cm(2) with the PEDOT/CNT coating. Further in vitro experiments also showed that the PEDOT/CNT coatings are non-toxic and support the growth of neurons. It is expected that this highly stable PEDOT/CNT composite may serve as excellent new material for neural electrodes.     

Electrodeposition of anchored polypyrrole film on microelectrodes and stimulation of cultured cardiac myocytes

The electrically conducting polymer polypyrrole (PPy) was electrochemically deposited onto Pt microelectrodes on a polyimide (PI) substrate. Pre-modification of the PI surface with a self-assembled monolayer of octadecyltrichlorosilane-induced anisotropic lateral growth of PPy along the PI surface and enhanced adhesive strength of the PPy film. The lateral growth of PPy film around the electrode anchored the whole film to the substrate. External stimulation of cultured cardiac myocytes was carried out using the PPy-coated microelectrode. The myocytes on the microelectrode substrate were electrically conjugated to form a sheet, and showed synchronized beating upon stimulation. The threshold charge for effective stimulation of a 0.8 cm(2) sheet of myocytes was around 0.2 microC, roughly corresponding to a membrane depolarization of 250 mV.     

Scanning electron microscopy of platinum scala tympani electrodes following chronic stimulation in patients

Platinum (Pt) electrodes from three auditory prostheses (Cochlear Pty Ltd) were examined for evidence of corrosion following implantation periods of up to 1000 days. These devices were used for periods ranging from 1600 to 10,400 h and developed maximum charge densities of 0.257 microC mm-2 geom. per phase. Scanning electron microscopy of the surface of the 66 stimulated electrodes examined showed no evidence of definitive Pt corrosion. Their surface features were essentially identical to control (unstimulated) electrodes. In addition, there was no evidence of any change in the surface morphology of the Silastic carrier adjacent to the stimulating electrodes. These results indicate that Pt is a suitable electrode material for neural prostheses that use relatively large surface area electrodes (0.1-1.0 mm2) and low to moderate charge densities (0.01-0.26 microC mm-2 geom. per phase).     

Controlled metallic nanopillars for low impedance biomedical electrode

Radial metallic nanopillar/nanowire structures can be created by a controlled radiofrequency (RF) plasma processing technique on the surface of certain alloy wires, including important biomedical alloys such as MP35N (Co–Ni–Cr–Mo alloy), platinum–iridium and stainless steel. In electrode applications such as pacemakers or neural stimulators, the increase in surface area in elongated MP35N nanopillars allows for decreased surface impedance and greater current density. However, the nanopillar height on MP35N alloy tends to be self-limiting at ∼1–3 μm. The objective of this study was to further elongate the radial nanopillars so as to reduce electrode impedance for biomedical electrode applications. Intelligent experimental design allowed for efficient investigation of processing parameters, including plasma material, process duration, power, pressure and repetition. It was found that multi-step repeated processing in the parameter-controlled RF environment could increase nanopillar height to ∼10 μm, a 400% improvement, while the RF plasma processing with identical total duration but in a single step did not lead to desired nanopillar elongation. Measurement of electrode impedance in phosphate-buffered saline solution showed an associated decrease to one-fifth of the surface impedance of unprocessed wire for signals below 100 Hz. For the purposes of this study, MP35N and Pt–Ir wires were characterized and demonstrated augmented surface impedance properties which, in combination with superior cell integration, enhanced biomedical electrode performance.     

Surface modification of biomaterials through noble metal ion implantation

Studies are described involving effects of noble-metal ion implantation on corrosion inhibition and charge-injection capabilities of surgical Ti-6A1-4V alloy. A major factor linked to excellent long-term biological performance is resistance to metal-ion release to tissues. The elements most resistant to corrosion in aqueous solutions are the noble metals. Disadvantages include expense and general inadequacy of mechanical properties. However, if small quantities can be used to surface-modify a surgical device in the last stage of manufacture, that device could possess an optimum combination of environmental integrity, biological response, mechanical properties, and charge-injection capability at minimum expense. Results for ion-implanted Ir are presented. Iridium has been described as the most corrosion-resistant element known, and its activated oxide as having the highest charge-injection capability of any material known. Ti-6A1-4V samples, ion implanted with 2.5 and 5.0 atomic % peak-maximum concentrations of Ir, were subjected to corrosion treatments to enrich the surface with Ir. Corrosion potential and cyclic voltammetry measurements indicated enrichment in H2SO4, and continued enrichment in isotonic saline, with corrosion potentials approaching that of pure Ir, and charge densities in isotonic saline exceeding that of pure Ir for the 5.0% peak-max Ir implanted material. X-ray photoelectron spectroscopy confirmed the high levels of Ir surface enrichment.     

Long term performance of porous platinum coated neural electrodes

Over the last several years, there has been a growing interest in neural implants for the study and diagnostics of neurological disorders as well as for the symptomatic treatment of central nervous system related diseases. One of the major challenges is the trade-off between small electrode sizes for high selectivity between single neurons and large electrode-tissue interface areas for excellent stimulation and recording properties. This paper presents an approach of increasing the real surface area of the electrodes by creating a surface microstructure. Two major novelties let this work stand out from existing approaches which mainly make use of porous coatings such as platinum black or iridium oxide, or Poly(3,4-ethylenedioxythiophene) (PEDOT). Roughening is carried out by a dry etching process on the silicon electrode core before being coated by a sputtered platinum layer, eliminating complicated deposition processes as for the materials described above. The technology is compatible with any commonly used coating material. In addition, the surface roughening is compatible with high aspect ratio penetrating electrode arrays such as the well-established Utah electrode array, whose unique geometry presents a challenge in the surface modification of active electrode sites. The dry etching process is well characterized and yields a high controllability of pore size and depth. This paper confirms the superior electrochemical properties including impedance, charge injection capacity, and charge storage capacity of surface engineered electrode arrays compared to conventional arrays over a period of 12 weeks. Furthermore, mechanical stability of the modified electrodes was tested by implantation in the brain of a recently deceased rat. In conclusion, the larger interface surface of the electrodes does not only decrease the impedance which should lead to enhanced Signal to noise ratio (SNR) for recording purposes, but also yields higher charge injection capacities, which improve the stimulation characteristics of the implants.     

Glucose biosensor based on the immobilization of glucose oxidase on electrochemically synthesized polypyrrole-poly(vinyl sulphonate) composite film by cross-linking with glutaraldehyde

Abstract In this study, a novel amperometric glucose biosensor was developed by immobilizing glucose oxidase (GOX) by cross-linking via glutaraldehyde on electrochemically polymerized polypyrrole-poly(vinyl sulphonate) (PPy-PVS) films on the surface of a platinum (Pt) electrode. Electropolymerization of pyrrole and poly(vinyl sulphonate) on the Pt surface was carried out with an electrochemical cell containing pyrrole and poly(vinyl sulphonate) by cyclic voltammetry between -1.0 and +?2.0 V (vs.Ag/AgCl) at a scan rate of 50 mV/s upon the Pt electrode. The amperometric determination was based on the electrochemical detection of H(2)O(2) generated in enzymatic reaction of glucose. Determination of glucose was carried out by the oxidation of enzymatically produced H(2)O(2) at 0.4 V vs. Ag/AgCl. The effects of pH and temperature were investigated and optimum parameters were found to be 7.5 and 65°C, respectively. The effect of working potential was investigated and optimum potential was determined to be 0.4 V. The operational stability of the enzyme electrode was also studied. The response of the PPy/PVS-GOX glucose biosensor exhibited good reproducibility with a relative standard deviation (RSD) of 2.48%. The glucose biosensor retained 63% of initial activity after 93 days when stored in 0.1 M phosphate buffer solution of pH 7.5 at 4°C. With the low operating potential, the biosensor demonstrated little interference from the possible interferants.     

The development of a multichannel electrode array for retinal prostheses

The development of a multielectrode array is the key issue for retinal prostheses. We developed a 10 × 10 platinum electrode array that consists of an 8-μm polyimide layer sandwiched between 5-μm polymonochloro-para-xylylene (parylene-C) layers. Each electrode was formed as a 30-μm-high bump by Pt/Au double-layer electroplating. We estimated the charge delivery capability (CDC) of the electrode by measuring the CDCs of two-channel electrode arrays. The dimensions of each electrode of the two-channel array were the same as those of each electrode formed on the 10 × 10 array. The results suggest that for cathodic-first (CF) pulses, 80% of electrodes surpassed our development target of 318 μC/cm², which corresponds to the charge density of pulses of 500 μs duration and 200 μA amplitude for a 200-μm-diameter planar electrode.     

Identification of mimotopes for the H4 minor histocompatibility antigen

The H4 minor histocompatibility antigen (HA) of mice includes a single immunogenic peptide presented by H-2Kb molecules that stimulates skin allograft rejection and is immunodominant in the stimulation of cytolytic T lymphocytes (CTL) specific for multiple minor HA. We have identified H4 mimotopes that are recognized by the H4-specific M9 CTL clone through the use of a random peptide library comprised of bacterial clones expressing an inducible fusion protein tailed with the octamer sequence SXIXFXXL. Eight discrete mimotopes were identified that sensitized RMA-S cells for lysis by M9 CTL down to concentrations of 10(-11) M. Comparable reactivity was observed with a short-term, H4- specific CTL line indicating that the mimotopes were not solely specific for the selecting M9 clone. All mimotopes included Gly at p2 and either Val or Ile at p4, suggesting a requirement for a hydrophobic residue with specific conformation. All mimotopes included either Arg or His at p7, implicating a requirement for a specific positively charged amino acid at that position. The sixth position was more variable with four of eight mimotopes having a Val residue with single mimotopes including alternative amino acids, the majority of which were hydrophobic. Analysis of mimotopes for hydrophobicity and charge by reverse-phase HPLC and capillary electrophoresis respectively indicated that (i) mimotopes with Val at both p4 and p6 were hydrophobically similar (but not identical) to the natural H4 peptide, and (ii) a S --> E substitution at p1 resulted in a peptide (EGIVFVRL) with charge characteristics equivalent to those of the natural H4 peptide.      

Neurotrophin-eluting hydrogel coatings for neural stimulating electrodes

Improved sensory and motor prostheses for the central nervous system will require large numbers of electrodes with low electrical thresholds for neural excitation. With the eventual goal of reducing stimulation thresholds, we have investigated the use of biodegradable, neurotrophin-eluting hydrogels (i.e., poly(ethylene glycol)-poly(lactic acid), PEGPLA) as a means of attracting neurites to the surface of stimulating electrodes. PEGPLA hydrogels with release rates ranging from 1.5 to 3 weeks were synthesized. These hydrogels were applied to multielectrode arrays with sputtered iridium oxide charge-injection sites. The coatings had little impact on the iridium oxide electrochemical properties, including charge storage capacity, impedance, and voltage transients during current pulsing. Additionally, we quantitatively examined the ability of neurotrophin-eluting, PEGPLA hydrogels to promote neurite extension in vitro using a PC12 cell culture model. Hydrogels released neurotrophin (nerve growth factor, NGF) for at least 1 week, with neurite extension near that of an NGF positive control and much higher than extension seen from sham, bovine serum albumin-releasing boluses, and a negative control. These results show that neurotrophin-eluting hydrogels can be applied to multielectrode arrays, and suggest a method to improve neuron-electrode proximity, which could result in lowered electrical stimulation thresholds. Reduced thresholds support the creation of smaller electrode structures and high density electrode prostheses, greatly enhancing prosthesis control and function.     

An intrafascicular electrode for recording of action potentials in peripheral nerves

We are developing a new type of bipolar recording electrode intended for implantation within individual fascicles of mammalian peripheral nerves. In the experiments reported here we used electrodes fabricated from 25 μm diameter Pt wire, 50 μm 90% Pt-10% Ir wire and 7 μm carbon fibers. The electrodes were implanted in the sciatic nerves of rats and in the ulnar nerves of cats. The signal-to-noise ratio of recorded activity induced by nonnoxious mechanical stimulation of the skin and joints was studied as a function of the type of electrode material used, the amount of insulation removed from the recording zone, and the longitudinal separation of the recording zones of bipolar electrode pairs. Both acute and short term (two day) chronic experiments were performed. The results indicate that a bipolar electrode made from Teflon^™-insulated, 25 μm diameter, 90% Pt-10% Ir wire, having a 1–2 mm long recording zone, can be used for recording of peripheral nerve activity when implanted with one wire inside the fascicle and the other lead level with the first lead, but outside the fascicle. No insulating cuff needs to be placed around the nerve trunk.     

An amperometric enzyme electrode for creatine determination prepared by the immobilization of creatinase and sarcosine oxidase in poly(vinylferrocenium)

A new enzyme electrode for the determination of creatine was developed by immobilizing creatinase (CI) and sarcosine oxidase (SO). The enzymes were co-immobilized in a poly(vinylferrocenium) matrix onto the surface of a platinum working electrode. Crosslinking with glutaraldehyte (GA) and bovine serum albumin (BSA) was selected as the best immobilization method for the enzymatic system. Determination of creatine was performed by the oxidation of enzymatically generated H2O2 at + 0.7 V vs. Ag/AgCl. The linear working range of the electrode was 2.0 x 10(-5) - 3.2 x 10(-4) M and the response time was about 50 s. The effects of pH, temperature, enzyme ratio and buffer concentration were investigated and optimum parameters were found to be 7.5, 37 degrees C, 2.5:1 (CI:SO) and 0.05 M, respectively. The stability and reproducibility of the enzyme electrode have been also studied.     

Visuotopic mapping through a multichannel stimulating implant in primate V1

We report on our efforts to establish an animal model for the development and testing of a cortical visual prostheses. One-hundred-fifty-two electrodes were implanted in the primary visual cortex of a rhesus monkey. The electrodes were made from iridium with an activated iridium oxide film, which has a large charge capacity for a given surface area, and insulated with parylene-C. One-hundred-fourteen electrodes were functional after implantation. The activity of small (2-3) neuronal clusters was first recorded to map the visually responsive region corresponding to each electrode. The animal was then trained in a memory (delayed) saccade task, first with a visual target, then to a target defined by direct cortical stimulation with coordinates specified by the stimulating electrode's mapped receptive field. The SD of saccade endpoints was approximately 2.5 larger for electrically stimulated versus visual saccades; nevertheless, when trial-to-trial scatter was averaged out, the correlation between saccade end points and receptive field locations was highly significant and approached unity after several months of training. Five electrodes were left unused until the monkey was fully trained; when these were introduced, the receptive field-saccade correlations were high on the first day of use (R = 0.85, P = 0.03 for angle, R = 0.98, P < 0.001 for eccentricity), indicating that the monkey had not learned to perform the task empirically by memorizing reward zones. The results of this experiment suggest the potential for rigorous behavioral testing of cortical visual prostheses in the macaque.     

In vivo studies of polypyrrole/peptide coated neural probes

Neural probes are micromachined multichannel electrode arrays that facilitate the functional stimulation and recording of neurons in the peripheral and central nervous system. For long-term implantations, surface modification is necessary for maintaining the stable connection between electrodes and neurons. The conductive polymer polypyrrole (PPy) and synthetic peptide DCDPGYIGSR were co-deposited on the electrode surface by electrochemical polymerization. The stability of PPy/DCDPGYIGSR coatings was tested in soaking experiments. It was found that the peptide was entrapped in the PPy film and did not diffuse away within 7 weeks of soaking in DI water. Coated probes were implanted in guinea pig brain for periods of 1, 2 and 3 weeks. Recording tests were performed and the impedance was monitored. The explanted probes and tissue were examined by immunocytochemical studies. Significantly more neurofilament positive staining was found on the coated electrode which indicated that the coatings had established strong connections with the neuronal structure in vivo. Good recordings were obtained from the coated sites that had neurons attached. First week tissue sections had no significant gliosis. In week 2, a layer of non-neuronal tissue consisting of mostly meningeal fibroblasts and ECM protein including at least fibronectin was formed around the probe tracks of both coated and uncoated probes. Astrocytes started to form a loosely organized layer by the end of the third week.     

Sputtered iridium oxide films for neural stimulation electrodes

Sputtered iridium oxide films (SIROFs) deposited by DC reactive sputtering from an iridium metal target have been characterized in vitro for their potential as neural recording and stimulation electrodes. SIROFs were deposited over gold metallization on flexible multielectrode arrays fabricated on thin (15 microm) polyimide substrates. SIROF thickness and electrode areas of 200-1300 nm and 1960-125,600 microm(2), respectively, were investigated. The charge-injection capacities of the SIROFs were evaluated in an inorganic interstitial fluid model in response to charge-balanced, cathodal-first current pulses. Charge injection capacities were measured as a function of cathodal pulse width (0.2-1 ms) and potential bias in the interpulse period (0.0 to 0.7 V vs. Ag|AgCl). Depending on the pulse parameters and electrode area, charge-injection capacities ranged from 1-9 mC/cm(2), comparable with activated iridium oxide films (AIROFs) pulsed under similar conditions. Other parameters relevant to the use of SIROF on nerve electrodes, including the thickness dependence of impedance (0.05-10(5) Hz) and the current necessary to maintain a bias in the interpulse region were also determined.     

Substrate dependent stability of conducting polymer coatings on medical electrodes

Conducting polymer (CP) coatings on medical electrodes have the potential to provide superior performance when compared to conventional metallic electrodes, but their stability is strongly dependant on the substrate properties. The aim of this study was to examine the effect of laser roughening of underlying platinum (Pt) electrode surfaces on the mechanical, electrical and biological performance of CP coatings. In addition, the impact of dopant type on electrical performance and stability was assessed. The CP poly(ethylene dioxythiophene) (PEDOT) was coated on Pt microelectrode arrays, with three conventional dopant ions. The in?vitro electrical characteristics were assessed by cyclic voltammetry and biphasic stimulation. Results showed that laser roughening of the underlying substrate did not affect the charge injection limit of the coated material, but significantly improved the passive stability and chronic stimulation lifetime without failure of the coating. Accelerated material ageing and long-term biphasic stimulus studies determined that some PEDOT variants experienced delamination within as little as 10 days when the underlying Pt was smooth, but laser roughening to produce a surface index of 2.5 improved stability, such that more than 1.3 billion stimulation cycles could be applied without evidence of failure. PEDOT doped with paratoluene sulfonate (PEDOT/pTS) was found to be the most stable CP on roughened Pt, and presented a surface topography which encouraged neural cell attachment.     

H.M. revisits the Tower of Hanoi Puzzle

To address the controversy of whether an intact procedural memory system alone can support the learning of the recursive strategy for solving the Tower of Hanoi Puzzle, the authors tested 2 amnesic patients, H.M. and P.N. Contrary to the report of N. J. Cohen, H. Eichenbaum, B. S. Deacedo, and S. Corkin (1985), both patients failed to master the recursive strategy under the active-interaction condition. In contrast, normal control participants were able to master the strategy under identical testing conditions. The failure of H.M. and P.N. could not be attributed to the differences between the original and current testing conditions. In addition, neither patient showed frontal lobe dysfunction or impairment in procedural memory. Together with evidence provided by theoretical analyses of this puzzle as well as studies on normal participants, the authors conclude that declarative memory plays a vital role in the acquisition of the recursive strategy for solving the Tower of Hanoi Puzzle.