Graphene-based micro-electrodes with reinforced interfaces and tunable porous structures for improved neural recordings

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

Biopotentials, which encompass action potentials and plant-specific biopotentials, play a crucial role in cell-to-cell communication and propagate within ionically conductive bio-tissues. Monitoring electrophysiology signals using electronic devices offers a real-time diagnostic approach for various diseases. The emergence of conducting polymer (CP)-based flexible bioelectronics has revolutionized high-quality signal acquisition by seamlessly interfacing with bio-tissues. In this thesis, we explore CP-based bioelectronics in the form of microneedle (MN) patch and hygroscopic adhesive, and investigate their performance in electrophysiology monitoring. Firstly, we introduce the concept of a CP-based dual-conductive stiff-morphing MN patch (DuoMorph) designed for in planta electrophysiology monitoring. By integrating CP PEDOT:PSS with thermoplastic poly lactic-co-glycolic acid (PLGA), we create a thermos-pressed DuoMorph capable of penetrating poorly conductive epidermal barriers in a minimally invasive manner. The stiffness-morphability of the DuoMorph ensures a low mechanical mismatch with soft plant tissues, effectively suppressing potential foreign body responses. Additionally, the addition of dimethyl sulfoxide (DMSO) further enhances the conductivity of CP. In vitro electrochemical impedance spectroscopy conducted on agarose-based epidermal phantoms and Epipremnum aureum leaves reveals significantly lower bio-electronic interface impedance compared to invasive metallic electrodes. This improvement is attributed to the low charge transfer resistance (Rct) and high double-layer capacitance (Cdl) of PEDOT:PSS. Demonstrations on living Epipremnum aureum plants successfully capture variation potentials induced by mechanical wounding and flame burning. Furthermore, the “lock” effect induced by sap adsorption ensures a better signal-tonoise ratio (SNR) and improved anti-interference capability compared to metallic electrodes. Finally, the DUOMORPH coated with a cation-selective membrane enables real-time in situ ion fluctuation capture, highlighting its potential as a versatile sensing platform for multiple bio-signals. Secondly, inspired by spider silk, we developed an electronically conductive hygroscopic adhesive (HydroAd) that enables a chronic bio-electronic interface. Unlike conventional wet electrodes, which fail due to electrolyte volatilization, and dry electrodes, which provide poor signal quality due to high interface impedance, HydroAd offers significant advantages. HydroAd contains an electrolyte that moistens the stratum corneum, reducing interface impedance to levels lower than those of commercial wet electrodes (CWE). It also maintains a water balance between evaporation and adsorption over long-term period. The incorporation of a crosslinkable charged ionic liquid into CP enhances its electrical properties. Specifically, this modification improves conductivity and increases charge storage and injection capacity by facilitating CP conductive network. The crosslinkable ionic liquid also acts as plasticizer that improves CP’s stretchability. Additionally, the ionic liquid’s hygroscopic nature allows it to function as a superabsorbent, leading to strong electrostatic interactions with water molecules. Gelatin methacryloyl (GelMA) enhances HydroAd's adhesiveness, and by adjusting the composition of the initiator and crosslinker, its adhesiveness can be optimized. Notably, HydroAd maintains a resistance variation of approximately 2.5% and an impedance variation below 10% during cyclic stretching tests. It achieves stable electrical properties and adhesiveness over a record-breaking six-month period. Applications in biopotential monitoring, such as electrocardiogram (ECG), electroencephalogram (EEG), and electrooculogram (EOG), demonstrate that HydroAd offers better signal acquisition quality, with no SNR change observed after a six-month drying process. This highlights its potential as a chronic bio-electronic platform. Finally, we explored the mechanism behind the remarkable water permeability of the HydroAd. Compared to conventional hygroscopic compounds used in electronically conductive adhesives (such as LiCl and uncrosslinked choline-based ionic liquids), HydroAd exhibits even higher water permeability. It surpasses not only conventional encapsulation materials like PDMS (Polydimethylsiloxane), PI (Polyimide), and highly permeable thermoplastic polyurethane (TPU) but also the CWE. Raman spectroscopy and differential scanning calorimetry confirmed that after crosslinking, the charged network within HydroAd modulates the water state, increasing the ratio of intermediate water (loosely bound) to bound water (strongly bound), denoted as the I/B ratio. This adjustment decreases the overall water enthalpy, resulting in improved electrode breathability. On sweaty skin, HydroAd’s high permeability enables rapid water evaporation and adhesion recovery within just 30 minutes. Further investigation revealed that the high I/B ratio plays a crucial role in regulating both electrochemical and mechanical properties. Specifically, it leads to decreased electrical impedance and enhanced adhesion. By triggering the formation of electrochemical interface, the higher content of intermediate water enhances the Cdl while suppressing electrolyte resistance (Rs) and Rct. To demonstrate its practical application, we designed a 10-channel HydroAd electrode array for surface electromyography (sEMG)-based gesture recognition. Remarkably, HydroAd-based gesture recognition achieves an impressive prediction accuracy of over 95%, surpassing both metallic dry electrodes and CWE. Even after a six-month drying process, its performance remains unchanged. This positions HydroAd as an ideal platform for advanced human-machine interaction.

Recent clinical successes of neuromodulator devices such as cochlear implants, deep brain stimulation (DBS) and spinal cord stimulation have generated a wide interest in exploiting emerging state-of-the-art biomaterials for therapeutic electrical stimulation of neurological diseases and disorders. Some of these applications require localized stimulation using microelectrodes with a charge injection density beyond 0.5 mC/cm2 that exceeds Pt and Pt alloys charge injection limits (0.3 mC/cm2). This is where IrOx, TiN and conducting polymers (CPs) emerge as potential candidates as materials for next generation microelectrodes. Here, we present a systematic in-vitro charge injection study to benchmark stimulation performance of PEDOT:PSS on Au, PEDOT:PSS on Pt, and reference Au and Pt electrodes across 20μm to 2000μm electrode diameters. In our study, higher charge injection capacity was measured for PEDOT:PSS/Pt microelectrodes (20 µm to 150 µm diameter size) which was ~3.4 time larger than Pt and ~9 time larger than Au microelectrodes. However, macroscale PEDOT:PSS/Au electrodes (200 µm to 2000 µm diameter size) displayed higher charge injection capacity compare to other electrode materials. Optimal OCP bias voltage was presented at 0.4 V for both PEDOT:PSS/Au and Pt micro/macro electrodes. Particularly, CICs of microelectrodes experienced ~55% and ~135% enhancement compare to zero bias voltage, resulted in maximum 1.88 mC/cm2 and 0.8 mC/cm2 CIC for PEDOT:PSS/Au and Pt microelectrodes respectively. CICs of PEDOT:PSS/Au micro/macro electrodes boosted ~ 13-14% further at 0.5 V OCP bias voltage by pulse engineering (asymmetric pulse injection). Charge injection capacity introduced as frequency-dependent parameter whereas PEDOT:PSS/Au microdots and macrodots demonstrated 40% and 21% larger CIC by pulse width expansion from 200 µs to 2 ms duration; which highlights the importance of accurate regulation of stimulation methodology parameters such as pulse width to make proper balance between safe (higher CIC) and initiative neural stimulation (higher current and frequency). Finally, Charge storage capacity (CSC) evaluation revealed the largest CSC for Pt micro/macro electrodes compare to other electrode materials, whereas PEDOT:PSS/Au micro/macro electrodes offered higher capability to convert those available stored charges into injected electrical pulse (higher CIC/ CSC ratio). We believe that these results could pave the way toward establishing safe and efficient clinically-relevant stimulation protocol for PEDOT:PSS microelectrodes.

A potential low cost and environmentally friendly supercapacitor has been prepared from Palm Kernel Shell Biochar (PKSB). In this study, physical and electrochemical properties of raw, activated and chemical treated (potassium hydroxide (KOH)) as supercapacitors such as high carbon content, high charge storage capacity and stable were evaluated. For physical analyses, the scanning electron microscopy (SEM) was used to study the surface morphology and surface area and porosity were measured using Brunaurer-Emmert-Teller (BET). The chemical treated PKSB shows the highest surface area values of 55.15 m2/g as compared to raw and activated samples with surface area are 0.17 m2/g and 19.32 m2/g, respectively. This is verified by in enhancement of capacitance achieved from 1.76 × 10−3 Fg-1 for the activated biochar and 1.87 × 10−6 Fg-1 for untreated PKSB showed by Raman spectroscopy. This enhancement reflected the charge storage capacity is attributed to the creation of broad distribution in pore size and a larger surface area. In addition, this phenomenon also supported by the electrochemical profiles through cyclic voltammogram (CV) measured by Potentiostat-Gavanostat (EIS). CV of the treated PKSB gave better square shape than the activated and raw biochar samples. These characterizations conclude that the raw palm kernel biochar need further treatment to become supercapacitor electrodes to replace activated carbon.

Hierarchically porous ceria with tunable pore structure from particle-stabilized foams

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The active sites of the UEA are coated with thin films of either platinum (Pt) or iridium oxide (IrOx). Pt and IrOx have attracted attention as a stimulating or recording material due to their ability to transfer between ionic and electronic current and to resist corrosion. The physical, mechanical, chemical, electrical and optical properties of thin films depend on the method and deposition parameters used to deposit the films. In this work, surface morphology, impedance and charge capacity of Pt and sputtered iridium oxide film (SIROF) were investigated and compared with each other. UEAs with similar electrode area and shape were employed in this study. DC sputtering was used to deposit Pt films and pulsed-dc reactive sputtering was used to deposit SIROF. The electrodes coated with SIROF and Pt were characterized by scanning electron microcopy, cyclic voltammetry, electrochemical impedance spectroscopy and potential transient measurements to measure charge injection capacity (CIC). SIROF and Pt selectively deposited on the electrode tip had dendrite and granular microstructure, respectively. The CIC of unbiased SIROF and Pt was 2 and 0.3 mC cm−2, respectively. The average impedance at 1 kHz, of SIROF and Pt electrodes, was 6 kΩ and 125 kΩ, respectively. Low impedance and high CIC make SIROF promising stimulation/recording material for neural prosthetic applications.

Multihollow nanocomposite microspheres with tunable pore structures by templating Pickering double emulsions

Enhancing the performance of quasi-solid-state flexible supercapacitors with Ag and MnO2 co-decorated carbon nanofibrous electrodes

Objective. In many applications, multielectrode arrays employed as neural implants require a high density and a high number of electrodes to precisely record and stimulate the activity of the nervous system while preserving the overall size of the array. Approach. Here we present a multilayer and three-dimensional (3D) electrode array, together with its manufacturing method, enabling a higher electrode density and a more efficient signal transduction with the biological tissue. Main results. The 3D structure of the electrode array allows for a multilayer placement of the interconnects within a flexible substrate, it narrows the probe size per the same number of electrodes, and it maintains the electrode contacts at the same level within the tissue. In addition, it augments the electrode surface area, leading to a lower electrochemical impedance and a higher charge storage capacity. To characterize the recordings capabilities of the multilayer 3D electrodes, we measured visually evoked cortical potentials in mice and analysed the evolution of the peak prominences and latencies according to different light intensities and recording depths within the brain. The resulting signal-to-noise ratio is improved compared to flat electrodes. Finally, the 3D electrodes have been imaged inside a clarified mouse brain using a light-sheet microscope to visualize their integrity within the tissue. Significance. The multilayer 3D electrodes have proved to be a valid technology to ensure tissue proximity and higher recording/stimulating efficiencies while enabling higher electrode density and reducing the probe size.

Mesoporous silica nanoparticles with tunable pore structures have been successfully fabricated by simply tuning the ethanol amounts in the precursor.

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Objective:Conventional cochlear implants provide patients who are deaf with hearing via electrical intracochlear stimulations. Stimulation electrodes are inserted into the cochlea through a cochleostomy or round window membrane (RWM) approach. However, these methods might induce cochlear ossificans and loss of residual hearing by damaging inner ear structures. To avoid an invasive electrode insertion, we developed a novel bone-guided extracochlear implant that stimulated the auditory nerves between the cochlear bones and the RWM to prevent cochlea damage. Power consumption plays an important role in wireless implantable electronic devices. Therefore, we aimed to investigate the effects of different electrodes on the stimulating threshold currents of the auditory nerve and the power consumption of bone-guided extracochlear implants using a commercial stimulator.Materials and Methods:Inert aurum (Au) electrodes were compared with biocompatible platinum (Pt) and iridium oxide (IrOx) electrodes in practical implantable applications. IrOx electrodes were used for their high-charge storage capacity, low impedance, and biocompatibility. The electrodes were fabricated via sputtering and were experimentally characterized with cyclic voltammetry and then examined using in vivo tests.Results:Based on electrical auditory brainstem responses, IrOx electrodes yielded lower acoustic nerve-stimulating threshold currents (132 μA) compared with Au electrodes (204 μA). IrOx electrodes also had a lower acoustic nerve stimulating threshold current (132 μA) compared with Pt electrodes (168 μA).Conclusion:As expected, IrOx electrodes were beneficial in the development of multielectrode bone-guided extracochlear implants, with the lowest acoustic nerve-stimulating threshold and current consumptions compared with Au and Pt electrodes.

Conducting polymers have been extensively reported as promising coating materials for applications involving interactions with electrically excitable tissues. Specifically, metal electrodes functionalized with conducting polymer coatings have been employed as biointerfaces presenting tailored properties to promote electrode integration as well as chronic functionality. Currently, polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) represent the most extensively studied conducting polymers, exhibiting favourable electrochemical properties and biocompatibility. In this paper, we study electrodeposited poly(3,4-ethylenedioxypyrrole) (PEDOP), a conducting polymer which is structurally related to both PEDOT and PPy, and is expected to outperform its “parent” polymers in terms of electrochemical properties and biocompatibility. The performance of PEDOP doped with chloride/phosphate, p-toluenesulfonate or polystyrene sulfonate was subsequently investigated to assess the efficacy of these ionic dopants in promoting electrochemical stability and neural cytocompatibility. Electrodeposited PEDOP films exhibited a high charge storage capacity (50.07 ± 6.96 mC cm−2), charge injection capacity (203 ± 24 μC cm−2) and substantial stability (performance loss of 0.49 ± 0.06% after 100 000 stimulation pulses). Furthermore, PEDOP films promoted enhanced neuron outgrowth and viability relative to control substrates. In particular, PEDOP/PTS was shown to increase the average neurite length by a factor of three when compared with cells cultured on bare Pt control substrates. Consequently, due to its favourable electrochemical characteristics together with high neural cytocompatibility, PEDOP can be indicated as a promising alternative to PPy and PEDOT in the field of neural science.

Hydrogels featuring tunable interconnected pore structures have garnered significant attention in applications requiring controlled mass transport, due to their efficient and adjustable transport properties. However, developing robust fabrication strategies to achieve stable and precisely tailored pore structures remains a significant challenge. Here, a salt ion‐induced phase separation method is presented for the fabrication of polyzwitterionic hydrogels with interconnected pore structures. By leveraging salt ions to modulate polymer chain hydration, this method triggers polyzwitterionic chain aggregation, enabling the formation of interconnected pores. Notably, pore structure regulation is facilely achieved by adjusting salt ion concentration during hydrogel polymerization. When applied to solar steam generation, evaporators based on polyzwitterionic hydrogels with optimized interconnected pore structures demonstrate high‐efficiency mass transport capabilities, exhibiting accelerated evaporation rates of ≈1.950 kg m −2 h −1 . Moreover, these hydrogels demonstrate remarkable stability, sustaining a consistent performance for over 12 h in high‐salinity brine (10 wt.% NaCl). This study provides a practical and scalable method for engineering hydrogel pore structures, offering broad applicability in addressing diverse mass transport challenges across energy, environmental, and biomedical domains.

Fabrication and evaluation of molecularly imprinted multi-hollow microspheres adsorbents with tunable inner pore structures derived from templating Pickering double emulsions