Electrodeposited Iridium Oxide on Platinum Nanocones for Improving Neural Stimulation Microelectrodes

Sustainability inspired fabrication of next generation neurostimulation and cardiac rhythm management electrodes via reactive hierarchical surface restructuring

Over the last two decades, platinum group metals (PGMs) and their alloys have dominated as the materials of choice for electrodes in long-term implantable neurostimulation and cardiac rhythm management devices due to their superior conductivity, mechanical and chemical stability, biocompatibility, corrosion resistance, radiopacity, and electrochemical performance. Despite these benefits, PGM manufacturing processes are extremely costly, complex, and challenging with potential health hazards. Additionally, the volatility in PGM prices and their high supply risk, combined with their scarce concentration of approximately 0.01 ppm in the earth's upper crust and limited mining geographical areas, underscores their classification as critical raw materials, thus, their effective recovery or substitution worldwide is of paramount importance. Since postmortem recovery from deceased patients and/or refining of PGMs that are used in the manufacturing of the electrodes and microelectrode arrays is extremely rare, challenging, and highly costly, therefore, substitution of PGM-based electrodes with other biocompatible materials that can yield electrochemical performance values equal or greater than PGMs is the only viable and sustainable solution to reduce and ultimately substitute the use of PGMs in long-term implantable neurostimulation and cardiac rhythm management devices. In this article, we demonstrate for the first time how the novel technique of "reactive hierarchical surface restructuring" can be utilized on titanium-that is widely used in many non-stimulation medical device and implant applications-to manufacture biocompatible, low-cost, sustainable, and high-performing neurostimulation and cardiac rhythm management electrodes. We have shown how the surface of titanium electrodes with extremely poor electrochemical performance undergoes compositional and topographical transformations that result in electrodes with outstanding electrochemical performance.

Fast Electrodeposition of MXene/PDA Composites for High‐Performance Bioelectronic Interfaces: An In Vitro Evaluation

Bioelectrode is critical to many biomedical researches. However, traditional materials (typically noble metals) and manufacturing techniques limit the large‐scale production of bioelectrodes. Herein, a fast electrochemical approach is proposed to deposit versatile MXene/polydopamine (PDA) composites on a metalized substrate. PDA coating can improve the adhesion between MXene and the substrate, while MXene provides rough surfaces with unique micro/nanostructure and outstanding electrical/optical/thermal performance. The impedance of the as‐prepared bioelectrode at 1 kHz is down to 8.48 Ω cm2. The corresponding cathodic charge storage capacity (CSCc) and charge injection capacity (CIC) are up to ≈250 and 6.59 mC cm−2 respectively, much superior to that of bare Pt and other conventional material‐based electrodes. The MXene/PDA composites also demonstrate robust stability under continuous electrostimulation for 1 × 108 pulse cycles and 1000 CV cycles. Moreover, MXene/PDA composites show a high and rapid photothermal response. Photoelectrochemical activity is also observed with high photocurrent, ≈40 folds larger than that of bare Pt. The utility of this new electrode in ascorbic acid sensing is demonstrated. Excellent biocompatibility is verified via neuron adhesion test and viability assay.

Advancing the Frontiers of Neuroelectrodes: A Paradigm Shift towards Enhanced Biocompatibility and Electrochemical Performance

The aim of this study is the fabrication of unprecedented neuroelectrodes, replete with exceptional biological and electrical attributes. Commencing with the synthesis of polyethylene glycol and polyethyleneimine-modified iron oxide nanoparticles, the grafting of Dimyristoyl phosphatidylcholine was embarked upon to generate DMPC-SPION nanoparticles. Subsequently, the deposition of DMPC-SPIONs onto a nickel-chromium alloy electrode facilitated the inception of an innovative neuroelectrode-DMPC-SPION. A meticulous characterization of DMPC-SPIONs ensued, encompassing zeta potential, infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analyses. Evaluations pertaining to hemolysis and cytotoxicity were conducted to ascertain the biocompatibility and biosafety of DMPC-SPIONs. Ultimately, a comprehensive assessment of the biocompatibility, electrochemical properties, and electrophysiological signal acquisition capabilities of DMPC-SPION neuroelectrodes was undertaken. These findings conclusively affirm the exemplary biocompatibility, electrochemical capabilities, and outstanding capability in recording electrical signals of DMPC-SPION neuroelectrodes, with an astounding 91.4% augmentation in electrode charge and a noteworthy 13% decline in impedance, with peak potentials reaching as high as 171 μV and an impressive signal-to-noise ratio of 15.92. Intriguingly, the novel DMPC-SPION neuroelectrodes herald an innovative pathway towards injury repair as well as the diagnosis and treatment of neurological disorders.

A Microelectrode Array Modified by PtNPs/PB Nanocomposites Used for the Detection and Analysis of Glucose-Sensitive Neurons under Different Blood Glucose States

Hypoglycemia state damages the organism, and glucose-excited and glucose-inhibited neurons from the ventral medial hypothalamus can regulate this state. Therefore, it is crucial to understand the functional mechanism between blood glucose and electrophysiology of glucose-excited and glucose-inhibited neurons. To better detect and analyze this mechanism, a PtNPs/PB nanomaterials modified 32-channel microelectrode array with low impedance (21.91 ± 6.80 kΩ), slight phase delay (-12.7° ± 2.7°), high double layer capacitance (0.606 μF), and biocompatibility was developed to realize in vivo real-time detection of the electrophysiology activities of glucose-excited and glucose-inhibited neurons. The phase-locking level of some glucose-inhibited neurons elevated during fasting (low blood glucose state) and showed theta rhythms after glucose injection (high blood glucose state). With an independent oscillating ability, glucose-inhibited neurons can provide an essential indicator to prevent severe hypoglycemia. The results reveal a mechanism for glucose-sensitive neurons to respond to blood glucose. Some glucose-inhibited neurons can integrate glucose information input and convert it into theta oscillating or phase lock output. It helps in enhancing the interaction between neurons and glucose. Therefore, the research can provide a basis for further controlling blood glucose by modulating the characteristics of neuronal electrophysiology. This helps reduce the damage of organisms under energy-limiting conditions, such as prolonged manned spaceflight or metabolic disorders.

Mechanical Stability of Nano-Coatings on Clinically Applicable Electrodes, Generated by Electrophoretic Deposition

The mechanical stability of implant coatings is crucial for medical approval and transfer to clinical applications. Here, electrophoretic deposition (EPD) is a versatile coating technique, previously shown to cause significant post-surgery impedance reduction of brain stimulation platinum electrodes. However, the mechanical stability of the resulting coating has been rarely systematically investigated. In this work, pulsed-DC EPD of laser-generated platinum nanoparticles (PtNPs) on Pt-based, 3D neural electrodes is performed and the in vitro mechanical stability is examined using agarose gel, adhesive tape, and ultrasonication-based stress tests. EPD-generated coatings are highly stable inside simulated brain environments represented by agarose gel tests as well as after in vivo stimulation experiments. Electrochemical stability of the NP-modified surfaces is tested via cyclic voltammetry and that multiple scans may improve coating stability could be verified, indicated by higher signal stability following highly invasive adhesive tape stress tests. The brain sections post neural stimulation in rats are analyzed via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Measurements reveal higher levels of Pt near the region stimulated with coated electrodes, in comparison to uncoated controls. Even though local concentrations in the vicinity of the implanted electrode are elevated, the total Pt mass found is below systemic toxicologically relevant concentrations.

Slippery Epidural ECoG Electrode for High-Performance Neural Recording and Interface

Chronic implantation of an epidural Electrocorticography (ECoG) electrode produces thickening of the dura mater and proliferation of the fibrosis around the interface sites, which is a significant concern for chronic neural ECoG recording applications used to monitor various neurodegenerative diseases. This study describes a new approach to developing a slippery liquid-infused porous surface (SLIPS) on the flexible ECoG electrode for a chronic neural interface with the advantage of increased cell adhesion. In the demonstration, the electrode was fabricated on the polyimide (PI) substrate, and platinum (Pt)-gray was used for creating the porous nanocone structure for infusing the silicone oil. The combination of nanocone and the infused slippery oil layer created the SLIPS coating, which has a low impedance (4.68 kΩ) level favourable for neural recording applications. The electrochemical impedance spectroscopy and equivalent circuit modelling also showed the effect of the coating on the recording site. The cytotoxicity study demonstrated that the coating does not have any cytotoxic potentiality; hence, it is biocompatible for human implantation. The in vivo (acute recording) neural recording on the rat model also confirmed that the noise level could be reduced significantly (nearly 50%) and is helpful for chronic ECoG recording for more extended neural signal recording applications.

Nanocone-Array-Based Platinum-Iridium Oxide Neural Microelectrodes: Structure, Electrochemistry, Durability and Biocompatibility Study

Neural interfaces provide a window for bio-signal modulation and recording with the assistance of neural microelectrodes. However, shrinking the size of electrodes results in high electrochemical impedance and low capacitance, thus limiting the stimulation/recording efficiency. In order to achieve critical stability and low power consumption, here, nanocone-shaped platinum (Pt) with an extensive surface area is proposed as an adhesive layer on a bare Pt substrate, followed by the deposition of a thin layer of iridium oxide (IrO_x) to fabricate high-performance nanocone-array-based Pt-IrO_x neural microelectrodes (200 μm in diameter). A uniform nanocone-shaped Pt with significant roughness is created via controlling the ratio of NH₄⁺ and Pt⁴⁺ ions in the electrolyte, which can be widely applicable for batch production on multichannel flexible microelectrode arrays (fMEAs) and various substrates with different dimensions. The Pt-IrO_x nanocomposite-coated microelectrode presents a significantly low impedance down to 0.72 ± 0.04 Ω cm² at 1 kHz (reduction of ~92.95%). The cathodic charge storage capacity (CSC_c) and charge injection capacity (CIC) reaches up to 52.44 ± 2.53 mC cm^-2 and 4.39 ± 0.36 mC cm^-2, respectively. Moreover, superior chronic stability and biocompatibility are also observed. The modified microelectrodes significantly enhance the adhesion of microglia, the major immune cells in the central nervous system. Therefore, such a coating strategy presents great potential for biomedical and other practical applications.

Femtosecond laser hierarchical surface restructuring for next generation neural interfacing electrodes and microelectrode arrays

Long-term implantable neural interfacing devices are able to diagnose, monitor, and treat many cardiac, neurological, retinal and hearing disorders through nerve stimulation, as well as sensing and recording electrical signals to and from neural tissue. To improve specificity, functionality, and performance of these devices, the electrodes and microelectrode arrays-that are the basis of most emerging devices-must be further miniaturized and must possess exceptional electrochemical performance and charge exchange characteristics with neural tissue. In this report, we show for the first time that the electrochemical performance of femtosecond-laser hierarchically-restructured electrodes can be tuned to yield unprecedented performance values that significantly exceed those reported in the literature, e.g. charge storage capacity and specific capacitance were shown to have improved by two orders of magnitude and over 700-fold, respectively, compared to un-restructured electrodes. Additionally, correlation amongst laser parameters, electrochemical performance and surface parameters of the electrodes was established, and while performance metrics exhibit a relatively consistent increasing behavior with laser parameters, surface parameters tend to follow a less predictable trend negating a direct relationship between these surface parameters and performance. To answer the question of what drives such performance and tunability, and whether the widely adopted reasoning of increased surface area and roughening of the electrodes are the key contributors to the observed increase in performance, cross-sectional analysis of the electrodes using focused ion beam shows, for the first time, the existence of subsurface features that may have contributed to the observed electrochemical performance enhancements. This report is the first time that such performance enhancement and tunability are reported for femtosecond-laser hierarchically-restructured electrodes for neural interfacing applications.

Electrodeposited Platinum Iridium Enables Microstimulation With Carbon Fiber Electrodes

Ultrasmall microelectrode arrays have the potential to improve the spatial resolution of microstimulation. Carbon fiber (CF) microelectrodes with cross-sections of less than 8 μm have been demonstrated to penetrate cortical tissue and evoke minimal scarring in chronic implant tests. In this study, we investigate the stability and performance of neural stimulation electrodes comprised of electrodeposited platinum-iridium (PtIr) on carbon fibers. We conducted pulse testing and characterized charge injection in vitro and recorded voltage transients in vitro and in vivo. Standard electrochemical measurements (impedance spectroscopy and cyclic voltammetry) and visual inspection (scanning electron microscopy) were used to assess changes due to pulsing. Similar to other studies, the application of pulses caused a decrease in impedance and a reduction in voltage transients, but analysis of the impedance data suggests that these changes are due to surface modification and not permanent changes to the electrode. Comparison of scanning electron microscope images before and after pulse testing confirmed electrode stability.

Biocompatible reference electrodes to enhance chronic electrochemical signal fidelity in vivo

In vivo electrochemistry is a vital tool of neuroscience that allows for the detection, identification, and quantification of neurotransmitters, their metabolites, and other important analytes. One important goal of in vivo electrochemistry is a better understanding of progressive neurological disorders (e.g., Parkinson's disease). A complete understanding of such disorders can only be achieved through a combination of acute (i.e., minutes to hours) and chronic (i.e., days or longer) experimentation. Chronic studies are more challenging because they require prolonged implantation of electrodes, which elicits an immune response, leading to glial encapsulation of the electrodes and altered electrode performance (i.e., biofouling). Biofouling leads to increased electrode impedance and reference electrode polarization, both of which diminish the selectivity and sensitivity of in vivo electrochemical measurements. The increased impedance factor has been successfully mitigated previously with the use of a counter electrode, but the challenge of reference electrode polarization remains. The commonly used Ag/AgCl reference electrode lacks the long-term potential stability in vivo required for chronic measurements. In addition, the cytotoxicity of Ag/AgCl adversely affects animal experimentation and prohibits implantation in humans, hindering translational research progress. Thus, a move toward biocompatible reference electrodes with superior chronic potential stability is necessary. Two qualifying materials, iridium oxide and boron-doped diamond, are introduced and discussed in terms of their electrochemical properties, biocompatibilities, fabrication methods, and applications. In vivo electrochemistry continues to advance toward more chronic experimentation in both animal models and humans, necessitating the utilization of biocompatible reference electrodes that should provide superior potential stability and allow for unprecedented chronic signal fidelity when used with a counter electrode for impedance mitigation.

Superior galvanostatic electrochemical deposition of platinum nanograss provides high performance planar microelectrodes forin vitroneural recording

Objective.Platinum nanograss (Ptng) has been demonstrated as an excellent coating to increase the electrode roughness and reduce the impedance of microelectrodes for neural recording. However, the optimisation of the original potentiostatic electrochemical deposition (PSED) method has been performed by the original group only and noin vitrovalidation of functionality was reported.Approach.This study firstly reinvestigates the use of the PSED method for Ptng coating at different charge densities which highlights non-uniformities in the edges of the microelectrodes for increasing deposition charge densities, leading to a decreased impedance which is in fact an artefact. We then introduce a novel Ptng fabrication method of galvanostatic electrochemical deposition (GSED).Main results.We demonstrate that the GSED deposition method also significantly reduces the electrode impedance, raises the charge storage capacity and provides a significantly more planar electrode surface in comparison to the PSED method with negligible edge effects. In addition, we demonstrate how high-quality neural recordings were performed, for the first time, using the Ptng GSED deposition microelectrodes from human hNT neurons and how spiking and bursting were observed.Significance.Thus, the GSED Ptng deposition method presented here provides an alternative method of microelectrode fabrication for neural applications with excellent impedance and planarity of surface.

Advanced Metallic and Polymeric Coatings for Neural Interfacing: Structures, Properties and Tissue Responses

Neural electrodes are essential for nerve signal recording, neurostimulation, neuroprosthetics and neuroregeneration, which are critical for the advancement of brain science and the establishment of the next-generation brain-electronic interface, central nerve system therapeutics and artificial intelligence. However, the existing neural electrodes suffer from drawbacks such as foreign body responses, low sensitivity and limited functionalities. In order to overcome the drawbacks, efforts have been made to create new constructions and configurations of neural electrodes from soft materials, but it is also more practical and economic to improve the functionalities of the existing neural electrodes via surface coatings. In this article, recently reported surface coatings for neural electrodes are carefully categorized and analyzed. The coatings are classified into different categories based on their chemical compositions, i.e., metals, metal oxides, carbons, conducting polymers and hydrogels. The characteristic microstructures, electrochemical properties and fabrication methods of the coatings are comprehensively presented, and their structure-property correlations are discussed. Special focus is given to the biocompatibilities of the coatings, including their foreign-body response, cell affinity, and long-term stability during implantation. This review article can provide useful and sophisticated insights into the functional design, material selection and structural configuration for the next-generation multifunctional coatings of neural electrodes.

Monitoring Cortical Response and Electrode-Retina Impedance Under Epiretinal Stimulation in Rats

Retinal prosthesis can restore partial vision in patients with retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Epiretinal prosthesis is one of three therapeutic approaches, which received regulatory approval several years ago. The thresholds of an epiretinal stimulation is partly determined by the size of the physical gap between the electrode and the retina after implantation. Precise positioning of epiretinal stimulating electrode array is still a challenging task. In this study, we demonstrate an approach to positioning epiretinal prostheses for an optimal response at the cortical output by monitoring both the impedance at the electrode-retina interface and the evoked-potential at the cortical level. We implanted a single-channel electrode on the epiretinal surface in adult rats, acutely, guided by both the impedance at the electrode-retina interface and by electrically evoked potentials (EEPs) in the visual cortex during retinal stimulation. We observe that impedance monotonously increases with decreasing electrode-retina distance, but that the strongest cortical responses were achieved at intermediate impedance levels. When the electrode penetrates the retina, the impedance keeps increasing. The effect of stimulation on the retina changes from epiretinal paradigm to intra-retinal paradigm and a decrease in cortical activation is observed. It is found that high impedance is not always favorable to elicit best cortical responses. Histopathological results showed that the electrode was placed at the intra-retinal space at high impedance value. These results show that monitoring impedance at the electrode-retina interface is necessary but not sufficient in obtaining strong evoked-potentials at the cortical level. Monitoring the cortical EEPs together with the impedance can improve the safety of implantation as well as efficacy of stimulation in the next generation of retinal implants.

Electrode Materials for Chronic Electrical Microstimulation

Electrical microstimulation has enabled partial restoration of vision, hearing, movement, somatosensation, as well as improving organ functions by electrically modulating neural activities. However, chronic microstimulation is faced with numerous challenges. The implantation of an electrode array into the neural tissue triggers an inflammatory response, which can be exacerbated by the delivery of electrical currents. Meanwhile, prolonged stimulation may lead to electrode material degradation., which can be accelerated by the hostile inflammatory environment. Both material degradation and adverse tissue reactions can compromise stimulation performance over time. For stable chronic electrical stimulation, an ideal microelectrode must present 1) high charge injection limit, to efficiently deliver charge without exceeding safety limits for both tissue and electrodes, 2) small size, to gain high spatial selectivity, 3) excellent biocompatibility that ensures tissue health immediately next to the device, and 4) stable in vivo electrochemical properties over the application period. In this review, the challenges in chronic microstimulation are described in detail. To aid material scientists interested in neural stimulation research, the in vitro and in vivo testing methods are introduced for assessing stimulation functionality and longevity and a detailed overview of recent advances in electrode material research and device fabrication for improving chronic microstimulation performance is provided.

A Review: Electrode and Packaging Materials for Neurophysiology Recording Implants

To date, a wide variety of neural tissue implants have been developed for neurophysiology recording from living tissues. An ideal neural implant should minimize the damage to the tissue and perform reliably and accurately for long periods of time. Therefore, the materials utilized to fabricate the neural recording implants become a critical factor. The materials of these devices could be classified into two broad categories: electrode materials as well as packaging and substrate materials. In this review, inorganic (metals and semiconductors), organic (conducting polymers), and carbon-based (graphene and carbon nanostructures) electrode materials are reviewed individually in terms of various neural recording devices that are reported in recent years. Properties of these materials, including electrical properties, mechanical properties, stability, biodegradability/bioresorbability, biocompatibility, and optical properties, and their critical importance to neural recording quality and device capabilities, are discussed. For the packaging and substrate materials, different material properties are desired for the chronic implantation of devices in the complex environment of the body, such as biocompatibility and moisture and gas hermeticity. This review summarizes common solid and soft packaging materials used in a variety of neural interface electrode designs, as well as their packaging performances. Besides, several biopolymers typically applied over the electrode package to reinforce the mechanical rigidity of devices during insertion, or to reduce the immune response and inflammation at the device-tissue interfaces are highlighted. Finally, a benchmark analysis of the discussed materials and an outlook of the future research trends are concluded.

Fast Polymerization of Polydopamine Based on Titanium Dioxide for High-Performance Flexible Electrodes

Dopamine (DA) and its derivatives are promising for the fabrication of functional films and devices with excellent conductivity and long-term stability; nevertheless its polymerization process is typically prolonged. We have proposed the accelerated deposition process using ultraviolet (UV) irradiation with the existence of nanotitanium dioxide (nano-TiO₂) in order to realize the rapid and stable synthesis of polydopamine (PDA) films. The in situ deposition process of nanostructured coatings such as platinum nanowire (PtNW) was also proposed by reducing the time of polymerization process to less than 1 h. It also increased the platinum (Pt) chelating rate with PDA, which was about 12 times faster than the traditional photo-oxidation method. Compared with the electrodes of the same size based on Ti/Pt sputtering, the impedance of the proposed PDA/TiO₂/PtNW coated electrode was as low as 0.0968 ± 0.0054 kΩ at 1 kHz (reduction of 99.74%). An extremely high cathodic charge storage capacity (CSC_c) up to 234.4 ± 3.16 mC cm^-2 was also observed, which was about 106.5 and 1.6 times higher than that of Ti/Pt and PDA/PtNW electrodes, respectively. In addition to that, significant photocurrent polarization responses were presented for PDA/TiO₂/PtNW electrodes with a stable current of -136.1 μA, exhibiting excellent charge transfer and UV absorption capacities. This co-deposition method has demonstrated great potential to speed up the polymerization process and enhance the electrical performance for flexible electrodes.

High-performance Nanocrystal Platinum on Hierarchical Microelectrode for Biochemical Sensing

A high-performance hierarchical microelectrode array was reported, which significantly enlarged the surface area and reduced edge effect of the unevenly distributed electric field, using novel nanocrystal platinum (Pt) surface modification and hierarchical topologies. A hierarchical three-dimensional (3D) nanostructured Pt was introduced on the surface of microelectrodes (diameter Φ=10 pm, and seed pad Φ=200 μm) to reduce the impedance by ~91.2% down to 2.63 kn at 1kHz, and increase the cathodic charge storage capacity (CSCc) by ~60 times up to 217.9 mC.cm^-2 compared with that of bare Pt substrate, demonstrating superior electrochemical performances to the traditional modified electrodes. In addition, the hierarchical microelectrodes exhibited great stability and linearity for glucose sensing with a detection limit of ~1.5 μM, which showed potential applications for neural interface and biochemical sensing.

3D Nano-crystal Platinum for High-performance Neural Electrode

A novel three-dimensional (3D) nano-crystal platinum (Pt) was introduced as a high-performance layer on the Pt substrate to provide extremely large surface, which significantly reduced electrical impedance by ˜ and increased the cathodic charge storage capacity (CSCc) up to ˜100 mC\textbullet(27 times) compared with that of bare Pt electrodes. In addition, the 3D nano-crystal Pt coating exhibited great mechanical and electrochemical stability as well as biocompatibility, which was critical and beneficial for effective neural stimulation/recording electrodes for practical applications.

Enhanced Mechanical and Electrochemical Performances of Silica-Based Coatings Obtained by Electrophoretic Deposition

To solve the existing problems of silicon dioxide (SiO₂) coating fabricated by the sol-gel method, such as complicated process, long production cycle, uncontrollable quality, etc., an improved electrophoretic deposition (EPD) combined with the sintering process was proposed to prepare SiO₂ coating on a dark nickel (Ni)-coated Q235 steel substrate. Silica sol was prepared by basic catalysis, containing silica of ∼130 g L^-1 with viscosities below 4 mPa s. Silica sol powder was characterized by the differential thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. EPD was applied to prepare SiO₂ coating on the Ni adhesive layer, followed by the sintering process to improve the compactness. In addition, the effects of EPD and sintering parameters were also evaluated. Potentiodynamic polarization and electrochemical impedance spectra were utilized to assess the corrosion behavior of the coating. The results showed that the EPD coating demonstrated excellent wear resistance when deposited at 15 V for 40 s and sintered at 400 °C for 45 min, exhibiting ∼6 μm thickness and a compact morphology. It also showed superior corrosion resistance with i_corr of 1.02 × 10^-7 A cm^-2, which was 2 orders of magnitude lower than that of dip-coating. Combining the EPD and sintering processes could shorten the fabrication period of SiO₂ inorganic coating and also improve the mechanical and corrosion properties, providing guidance for inorganic ceramic fabrication and showing potential for practical applications.

Micro/Nano Technologies for High-Density Retinal Implant

During the past decades, there have been leaps in the development of micro/nano retinal implant technologies, which is one of the emerging applications in neural interfaces to restore vision. However, higher feedthroughs within a limited space are needed for more complex electronic systems and precise neural modulations. Active implantable medical electronics are required to have good electrical and mechanical properties, such as being small, light, and biocompatible, and with low power consumption and minimal immunological reactions during long-term implantation. For this purpose, high-density implantable packaging and flexible microelectrode arrays (fMEAs) as well as high-performance coating materials for retinal stimulation are crucial to achieve high resolution. In this review, we mainly focus on the considerations of the high-feedthrough encapsulation of implantable biomedical components to prolong working life, and fMEAs for different implant sites to deliver electrical stimulation to targeted retinal neuron cells. In addition, the functional electrode materials to achieve superior stimulation efficiency are also reviewed. The existing challenge and future research directions of micro/nano technologies for retinal implant are briefly discussed at the end of the review.

Patterned iridium oxide film as neural electrode interface: Biocompatibility and improved neurite outgrowth with electrical stimulation

Iridium (Ir) thin film was deposited on patterned titanium substrate by direct-current (DC) magnetron sputtering, and then activated in sulfuric acid (H₂SO₄) through repetitive potential sweeps to form iridium oxide (IrO_x) as neural electrode interface. The resultant IrO_x film showed a porous and open morphology with aligned microstructure, exhibited superior electrochemical performance and excellent stability. The IrO_x film supported neural stem cells (NSCs) attachment, proliferation and improved processes without causing toxicity. The patterned IrO_x films offered a unique system to investigate the synergistic effects of topographical cue and electrical stimulation on neurite outgrowth. Electrical stimulation, when applied through patterned IrO_x films, was found to further increase the neurite extension of neuron-like cells and significantly reorient the neurite alignment towards to the direction of stimulation. These results indicate that IrO_x film, as electrode-tissue interface is highly stable and biocompatible with excellent electrochemical properties.

Fabrication of Iridium Oxide/Platinum Composite Film on Titanium Substrate for High-Performance Neurostimulation Electrodes

Electrode materials for neural stimulation have been widely investigated for implantable devices. Among them, iridium and iridium oxide are attractive materials for bio-interface applications due to their desirable stability, electrochemical performance, and biocompatibility. In this study, iridium oxide/platinum (IrOx/Pt) composite films were successfully fabricated on titanium substrates by chemical bath deposition and these films are expected to be used as biocompatible stimulation electrodes. We modified the film compositions to optimize the performances. In addition, these IrOx/Pt composite films were characterized before and after annealing by SEM and XRD. We also identified the hydrophilicity of these iridium oxide/platinum composite films by measuring contact angles. Finally, the charge storage capacities of these iridium oxide/platinum composite films were evaluated by an electrochemical workstation. As a result, the charge storage capacities of the iridium oxide/platinum composite films are largely increased, and this leads to a very efficient neurostimulation electrode. Additionally, we successfully demonstrated the chemical bath deposition of IrOx film on the surface of the bullet-shaped titanium microelectrode.

High-performance iridium oxide/platinum nano-leaf composite coatings on microelectrodes for neural stimulation/recording

A novel platinum (Pt) nano-leaf interfacial layer between the Pt substrate and iridium oxide (IrO_x) coating, significantly reduced impedance and increased cathodic charge storage capacity (CSC_c) for neural microelectrodes. Combining the advantages of both materials, herein a lower impedance and higher CSC_c of IrO_x/Pt nano-leaf composite coatings than each simple coating was fabricated with nanoscale roughness. Furthermore, it demonstrated excellent mechanical and electrochemical stability, which was beneficial for effective neural stimulation/recording in various applications.