Actively controlled release of Dexamethasone from neural microelectrodes in a chronic in vivo study

Interfacing with the Brain: How Nanotechnology Can Contribute

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain-machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain-machine interfaces and look forward in discussing perspectives and limitations based on the authors' expertise across a range of complementary disciplines─from neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

Bio-inspired electronics: Soft, biohybrid, and "living" neural interfaces

Neural interface technologies are increasingly evolving towards bio-inspired approaches to enhance integration and long-term functionality. Recent strategies merge soft materials with tissue engineering to realize biologically-active and/or cell-containing living layers at the tissue-device interface that enable seamless biointegration and novel cell-mediated therapeutic opportunities. This review maps the field of bio-inspired electronics and discusses key recent developments in tissue-like and regenerative bioelectronics, from soft biomaterials and surface-functionalized bioactive coatings to cell-containing 'biohybrid' and 'all-living' interfaces. We define and contextualize key terminology in this emerging field and highlight how biological and living components can bridge the gap to clinical translation.

Sapphire-Based Optrode for Low Noise Neural Recording and Optogenetic Manipulation

Electrophysiological recording of neurons in deep brain regions using optogenetic stimulation is a powerful method for understanding and regulating the role of complex neural activity in biological behavior and cognitive function. Optogenetic techniques have significantly advanced neuroscience research by enabling the optical manipulation of neural activities. Because of the significance of the technique, constant advancements in implantable optrodes that integrate optical stimulation with low-noise, large-scale electrophysiological recording are in demand to improve the spatiotemporal resolution for various experimental designs and future clinical applications. However, robust and easy-to-use neural optrodes that integrate neural recording arrays with high-intensity light emitting diodes (LEDs) are still lacking. Here, we propose a neural optrode based on Gallium Nitride (GaN) on sapphire technology, which integrates a high-intensity blue LED with a 5 × 2 recording array monolithically for simultaneous neural recording and optogenetic manipulation. To reduce the noise interference between the recording electrodes and the LED, which is in close physical proximity, three metal grounding interlayers were incorporated within the optrode, and their ability to reduce LED-induced artifacts during neural recording was confirmed through both electromagnetic simulations and experimental demonstrations. The capability of the sapphire optrode to record action potentials has been demonstrated by recording the firing of mitral/tuft cells in the olfactory bulbs of mice in vivo. Additionally, the elevation of action potential firing due to optogenetic stimulation observed using the sapphire probe in medial superior olive (MSO) neurons of the gerbil auditory brainstem confirms the capability of this sapphire optrode to precisely access neural activities in deep brain regions under complex experimental designs.

Recent Progress in Flexible Microelectrode Arrays for Combined Electrophysiological and Electrochemical Sensing

Understanding brain function requires advanced neural probes to monitor electrical and chemical signaling across multiple timescales and brain regions. Microelectrode arrays (MEAs) are widely used to record neurophysiological activity across various depths and brain regions, providing single-unit resolution for extended periods. Recent advancements in flexible MEAs, built on micrometer-thick polymer substrates, have improved integration with brain tissue by mimicking the brain's soft nature, reducing mechanical trauma and inflammation. These flexible, subcellular-scale MEAs can record stable neural signals for months, making them ideal for long-term studies. In addition to electrical recording, MEAs have been functionalized for electrochemical neurotransmitter detection. Electroactive neurotransmitters, such as dopamine, serotonin, and adenosine, can be directly measured via electrochemical methods, particularly on carbon-based surfaces. For non-electroactive neurotransmitters like acetylcholine, glutamate, and γ-aminobutyric acid, alternative strategies, such as enzyme immobilization and aptamer-based recognition, are employed to generate electrochemical signals. This review highlights recent developments in flexible MEA fabrication and functionalization to achieve both electrochemical and electrophysiological recordings, minimizing sensor fowling and brain damage when implanted long-term. It covers multi-time scale neurotransmitter detection, development of conducting polymer and nanomaterial composite coatings to enhance sensitivity, incorporation of enzyme and aptamer-based recognition methods, and the integration of carbon electrodes on flexible MEAs. Finally, it summarizes strategies to acquire electrochemical and electrophysiological measurements from the same device.

Enhancing biocompatibility of the brain-machine interface: A review

In vivo implantation of microelectrodes opens the door to studying neural circuits and restoring damaged neural pathways through direct electrical stimulation and recording. Although some neuroprostheses have achieved clinical success, electrode material properties, inflammatory response, and glial scar formation at the electrode-tissue interfaces affect performance and sustainability. Those challenges can be addressed by improving some of the materials' mechanical, physical, chemical, and electrical properties. This paper reviews materials and designs of current microelectrodes and discusses perspectives to advance neuroprosthetics performance.

Recent advances in PEDOT/PProDOT-derived nano biosensors: engineering nano assemblies for fostering advanced detection platforms for biomolecule detection

With the recent unprecedented emergence of a global pandemic, unknown diseases and new metabolic patterns expressing serious health issues, the requirement to develop new diagnostic tools, therapeutic solutions, and healthcare and environmental monitoring systems are significantly higher in the present situation. Considering that high sensitivity, selectivity, stability and a low limit of detection (LOD) are inevitable requirements for an ideal biosensor, the class of conducting polymers of poly(3,4-ethylenedioxythiophene) (PEDOT) and recently poly(3,4-propylenedioxythiophene) (PProDOT) materials have been demonstrated to be promising candidates for designing sensor devices. Nanostructure engineering of these polymeric materials with tunable surface properties and side chain functionalization to enable sensor probe conjugation combined with signal amplification devices such as OECTs and OFETs can fulfil the requirements of next-generation smart nano-biosensors. In this review, we analyze recent reports on PEDOT/PProDOT nanostructures and nanocomposites for developing nano-biosensors and their application in the detection of different biomarkers, environmental, toxicology, marine and aquatic monitoring, forensic and illicit drug detection, etc. In addition, we discuss the challenges associated with the design of PEDOT/PProDOT nano-biosensors and future perspectives on the exploration of novel sensor platforms, particularly PProDOT derivatives for bioelectronics and novel design strategies for next-generation smart nano-biosensors.

Deciphering platinum dissolution in neural stimulation electrodes: Electrochemistry or biology?

Platinum (Pt) is the metal of choice for electrodes in implantable neural prostheses like the cochlear implants, deep brain stimulating devices, and brain-computer interfacing technologies. However, it is well known since the 1970s that Pt dissolution occurs with electrical stimulation. More recent clinical and in vivo studies have shown signs of corrosion in explanted electrode arrays and the presence of Pt-containing particulates in tissue samples. The process of degradation and release of metallic ions and particles can significantly impact on device performance. Moreover, the effects of Pt dissolution products on tissue health and function are still largely unknown. This is due to the highly complex chemistry underlying the dissolution process and the difficulty in decoupling electrical and chemical effects on biological responses. Understanding the mechanisms and effects of Pt dissolution proves challenging as the dissolution process can be influenced by electrical, chemical, physical, and biological factors, all of them highly variable between experimental settings. By evaluating comprehensive findings on Pt dissolution mechanisms reported in the fuel cell field, this review presents a critical analysis of the possible mechanisms that drive Pt dissolution in neural stimulation in vitro and in vivo. Stimulation parameters, such as aggregate charge, charge density, and electrochemical potential can all impact the levels of dissolved Pt. However, chemical factors such as electrolyte types, dissolved gases, and pH can all influence dissolution, confounding the findings of in vitro studies with multiple variables. Biological factors, such as proteins, have been documented to exhibit a mitigating effect on the dissolution process. Other biological factors like cells and fibro-proliferative responses, such as fibrosis and gliosis, impact on electrode properties and are suspected to impact on Pt dissolution. However, the relationship between electrical properties of stimulating electrodes and Pt dissolution remains contentious. Host responses to Pt degradation products are also controversial due to the unknown chemistry of Pt compounds formed and the lack of understanding of Pt distribution in clinical scenarios. The cytotoxicity of Pt produced via electrical stimulation appears similar to Pt-based compounds, including hexachloroplatinates and chemotherapeutic agents like cisplatin. While the levels of Pt produced under clinical and acute stimulation regimes were typically an order of magnitude lower than toxic concentrations observed in vitro, further research is needed to accurately assess the mass balance and type of Pt produced during long-term stimulation and its impact on tissue response. Finally, approaches to mitigating the dissolution process are reviewed. A wide variety of approaches, including stimulation strategies, coating electrode materials, and surface modification techniques to avoid excess charge during stimulation and minimise tissue response, may ultimately support long-term and safe operation of neural stimulating devices.

Recording Quality Is Systematically Related to Electrode Impedance

Extracellular recordings with planar microelectrodes are the gold standard technique for recording the fast action potentials of neurons in the intact brain. The introduction of microfabrication techniques has revolutionized the in vivo recording of neuronal activity and introduced high-density, multi-electrode arrays that increase the spatial resolution of recordings and the number of neurons that can be simultaneously recorded. Despite these innovations, there is still debate about the ideal electrical transfer characteristics of extracellular electrodes. This uncertainty is partly due to the lack of systematic studies comparing electrodes with different characteristics, particularly for chronically implanted arrays over extended time periods. Here a high-density, flexible, and thin-film array is fabricated and tested, containing four distinct electrode types differing in surface material and surface topology and, thus, impedance. It is found that recording quality is strongly related to electrode impedance with signal amplitude and unit yield negatively correlated to impedance. Electrode impedances are stable for the duration of the experiment (up to 12 weeks) and recording quality does not deteriorate. The findings support the expectation from the theory that recording quality will increase as impedance decreases.

Bioelectronic Direct Current Stimulation at the Transition Between Reversible and Irreversible Charge Transfer

Many biological processes rely on endogenous electric fields (EFs), including tissue regeneration, cell development, wound healing, and cancer metastasis. Mimicking these biological EFs by applying external direct current stimulation (DCS) is therefore the key to many new therapeutic strategies. During DCS, the charge transfer from electrode to tissue relies on a combination of reversible and irreversible electrochemical processes, which may generate toxic or bio-altering substances, including metal ions and reactive oxygen species (ROS). Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrodes are emerging as suitable candidates for DCS to improve biocompatibility compared to metals. This work addresses whether PEDOT electrodes can be tailored to favor reversible biocompatible charge transfer. To this end, different PEDOT formulations and their respective back electrodes are studied using cyclic voltammetry, chronopotentiometry, and direct measurements of H2O2 and O2. This combination of electrochemical methods sheds light on the time dynamics of reversible and irreversible charge transfer and the relationship between capacitance and ROS generation. The results presented here show that although all electrode materials investigated generate ROS, the onset of ROS can be delayed by increasing the electrode's capacitance via PEDOT coating, which has implications for future bioelectronic devices that allow longer reversibly driven pulse durations during DCS.

PEDOT/CNT Flexible MEAs Reveal New Insights into the Clock Gene's Role in Dopamine Dynamics

Substantial evidence has shown that the Circadian Locomotor Output Cycles Kaput (Clock) gene is a core transcription factor of circadian rhythms that regulates dopamine (DA) synthesis. To shed light on the mechanism of this interaction, flexible multielectrode arrays (MEAs) are developed that can measure both DA concentrations and electrophysiology chronically. The dual functionality is enabled by conducting polymer PEDOT doped with acid-functionalized carbon nanotubes (CNT). The PEDOT/CNT microelectrode coating maintained stable electrochemical impedance and DA detection by square wave voltammetry for 4 weeks in vitro. When implanted in wild-type (WT) and Clock mutation (MU) mice, MEAs measured tonic DA concentration and extracellular neural activity with high spatial and temporal resolution for 4 weeks. A diurnal change of DA concentration in WT is observed, but not in MU, and a higher basal DA concentration and stronger cocaine-induced DA increase in MU. Meanwhile, striatal neuronal firing rate is found to be positively correlated with DA concentration in both animal groups. These findings offer new insights into DA dynamics in the context of circadian rhythm regulation, and the chronically reliable performance and dual measurement capability of this technology hold great potential for a broad range of neuroscience research.

Fully flexible implantable neural probes for electrophysiology recording and controlled neurochemical modulation

Targeted delivery of neurochemicals and biomolecules for neuromodulation of brain activity is a powerful technique that, in addition to electrical recording and stimulation, enables a more thorough investigation of neural circuit dynamics. We have designed a novel, flexible, implantable neural probe capable of controlled, localized chemical stimulation and electrophysiology recording. The neural probe was implemented using planar micromachining processes on Parylene C, a mechanically flexible, biocompatible substrate. The probe shank features two large microelectrodes (chemical sites) for drug loading and sixteen small microelectrodes for electrophysiology recording to monitor neuronal response to drug release. To reduce the impedance while keeping the size of the microelectrodes small, poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemically coated on recording microelectrodes. In addition, PEDOT doped with mesoporous sulfonated silica nanoparticles (SNPs) was used on chemical sites to achieve controlled, electrically-actuated drug loading and releasing. Different neurotransmitters, including glutamate (Glu) and gamma-aminobutyric acid (GABA), were incorporated into the SNPs and electrically triggered to release repeatedly. An in vitro experiment was conducted to quantify the stimulated release profile by applying a sinusoidal voltage (0.5 V, 2 Hz). The flexible neural probe was implanted in the barrel cortex of the wild-type Sprague Dawley rats. As expected, due to their excitatory and inhibitory effects, Glu and GABA release caused a significant increase and decrease in neural activity, respectively, which was recorded by the recording microelectrodes. This novel flexible neural probe technology, combining on-demand chemical release and high-resolution electrophysiology recording, is an important addition to the neuroscience toolset used to dissect neural circuitry and investigate neural network connectivity.

Drug delivery via a 3D electro-swellable conjugated polymer hydrogel

Spatiotemporal controlled drug delivery minimizes side-effects and enables therapies that require specific dosing patterns. Conjugated polymers (CP) can be used for electrically controlled drug delivery; however so far, most demonstrations were limited to molecules up to 500 Da. Larger molecules could be incorporated only during the CP polymerization and thus limited to a single delivery. This work harnesses the record volume changes of a glycolated polythiophene p(g3T2) for controlled drug delivery. p(g3T2) undergoes reversible volumetric changes of up to 300% during electrochemical doping, forming pores in the nm-size range, resulting in a conducting hydrogel. p(g3T2)-coated 3D carbon sponges enable controlled loading and release of molecules spanning molecular weights of 800-6000 Da, from simple dyes up to the hormone insulin. Molecules are loaded as a combination of electrostatic interactions with the charged polymer backbone and physical entrapment in the porous matrix. Smaller molecules leak out of the polymer while larger ones could not be loaded effectively. Finally, this work shows the temporally patterned release of molecules with molecular weight of 1300 Da and multiple reloading and release cycles without affecting the on/off ratio.

Reactive Amine Functionalized Microelectrode Arrays Provide Short-Term Benefit but Long-Term Detriment to In Vivo Recording Performance

Intracortical microelectrode arrays (MEAs) are used for recording neural signals. However, indwelling devices result in chronic neuroinflammation, which leads to decreased recording performance through degradation of the device and surrounding tissue. Coating the MEAs with bioactive molecules is being explored to mitigate neuroinflammation. Such approaches often require an intermediate functionalization step such as (3-aminopropyl)triethoxysilane (APTES), which serves as a linker. However, the standalone effect of this intermediate step has not been previously characterized. Here, we investigated the effect of coating MEAs with APTES by comparing APTES-coated to uncoated controls in vivo and ex vivo. First, we measured water contact angles between silicon uncoated and APTES-coated substrates to verify the hydrophilic characteristics of the APTES coating. Next, we implanted MEAs in the motor cortex (M1) of Sprague-Dawley rats with uncoated or APTES-coated devices. We assessed changes in the electrochemical impedance and neural recording performance over a chronic implantation period of 16 weeks. Additionally, histology and bulk gene expression were analyzed to understand further the reactive tissue changes arising from the coating. Results showed that APTES increased the hydrophilicity of the devices and decreased electrochemical impedance at 1 kHz. APTES coatings proved detrimental to the recording performance, as shown by a constant decay up to 16 weeks postimplantation. Bulk gene analysis showed differential changes in gene expression between groups that were inconclusive with regard to the long-term effect on neuronal tissue. Together, these results suggest that APTES coatings are ultimately detrimental to chronic neural recordings. Furthermore, interpretations of studies using APTES as a functionalization step should consider the potential consequences if the final functionalization step is incomplete.

Tuning Microelectrodes' Impedance to Improve Fast Ripples Recording

Epilepsy is a chronic neurological disorder characterized by recurrent seizures resulting from abnormal neuronal hyperexcitability. In the case of pharmacoresistant epilepsy requiring resection surgery, the identification of the Epileptogenic Zone (EZ) is critical. Fast Ripples (FRs; 200-600 Hz) are one of the promising biomarkers that can aid in EZ delineation. However, recording FRs requires physically small electrodes. These microelectrodes suffer from high impedance, which significantly impacts FRs' observability and detection. In this study, we investigated the potential of a conductive polymer coating to enhance FR observability. We employed biophysical modeling to compare two types of microelectrodes: Gold (Au) and Au coated with the conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (Au/PEDOT:PSS). These electrodes were then implanted into the CA1 hippocampal neural network of epileptic mice to record FRs during epileptogenesis. The results showed that the polymer-coated electrodes had a two-order lower impedance as well as a higher transfer function amplitude and cut-off frequency. Consequently, FRs recorded with the PEDOT:PSS-coated microelectrode yielded significantly higher signal energy compared to the uncoated one. The PEDOT:PSS coating improved the observability of the recorded FRs and thus their detection. This work paves the way for the development of signal-specific microelectrode designs that allow for better targeting of pathological biomarkers.

Implantable bioelectrodes: challenges, strategies, and future directions

Implantable bioelectrodes for regulating and monitoring biological behaviors have become indispensable medical devices in modern healthcare, alleviating pathological symptoms such as epilepsy and arrhythmia, and assisting in reversing conditions such as deafness and blindness. In recent years, developments in the fields of materials science and biomedical engineering have contributed to advances in research on implantable bioelectrodes. However, the foreign body reaction (FBR) is still a major constraint for the long-term application of electrodes. In this paper, four types of commonly used implantable bioelectrodes are reviewed, concentrating on their background, development, and a series of complications caused by FBR after long-term implantation. Strategies for resisting FBRs are then devised in terms of physics, chemistry, and nanotechnology. We analyze the major trends in the future development of implantable bioelectrodes and outline some promising research to optimize the long-term operational stability of electrodes. Although current implantable bioelectrodes have been able to achieve good biocompatibility, low impedance, and low mechanical mismatch and trauma, these devices still face the challenge of FBR. Resistance to FBR is still the key for the long-term effectiveness of bioelectrodes, and a better understanding of the mechanisms of FBR, as well as miniaturization, long-term passivation, and coupling with gene therapy may be the way forward for the next generation of implantable bioelectrodes.

Recent Advances in Bioelectronics for Localized Drug Delivery

The last decade has witnessed remarkable advancements in bioelectronics, ushering in a new era of wearable and implantable devices for drug delivery. By utilizing miniaturized system design and/or flexible materials, bioelectronics illustrates ideal integration with target organs and tissues, making them ideal platforms for localized drug delivery. Furthermore, the development of electrically assisted drug delivery systems has enhanced the efficiency and safety of therapeutic administration, particularly for the macromolecules that encounter additional challenges in penetrating biological barriers. In this review, a concise overview of recent progress in bioelectronic devices for in vivo localized drug delivery, with highlights on the latest trends in device design, working principles, and their corresponding functionalities, is provided. The reported systems based on their targeted delivery locations as wearable systems, ingestible systems, and implantable systems are categorized. Each category is introduced in detail by highlighting the special requirements for devices and the corresponding solutions. The remaining challenges in this field and future directions are also discussed.

Electrically Controlled Vasodilator Delivery from PEDOT/Silica Nanoparticle Modulates Vessel Diameter in Mouse Brain

Vascular damage and reduced tissue perfusion are expected to majorly contribute to the loss of neurons or neural signals around implanted electrodes. However, there are limited methods of controlling the vascular dynamics in tissues surrounding these implants. This work utilizes conducting polymer poly(ethylenedioxythiophene) and sulfonated silica nanoparticle composite (PEDOT/SNP) to load and release a vasodilator, sodium nitroprusside, to controllably dilate the vasculature around carbon fiber electrodes (CFEs) implanted in the mouse cortex. The vasodilator release is triggered via electrical stimulation and the amount of release increases with increasing electrical pulses. The vascular dynamics are monitored in real-time using two-photon microscopy, with changes in vessel diameters quantified before, during, and after the release of the vasodilator into the tissues. This work observes significant increases in vessel diameters when the vasodilator is electrically triggered to release, and differential effects of the drug release on vessels of different sizes. In conclusion, the use of nanoparticle reservoirs in conducting polymer-based drug delivery platforms enables the controlled delivery of vasodilator into the implant environment, effectively altering the local vascular dynamics on demand. With further optimization, this technology could be a powerful tool to improve the neural electrode-tissue interface and study neurovascular coupling.

Biohybrid neural interfaces: improving the biological integration of neural implants

Implantable neural interfaces (NIs) have emerged in the clinic as outstanding tools for the management of a variety of neurological conditions caused by trauma or disease. However, the foreign body reaction triggered upon implantation remains one of the major challenges hindering the safety and longevity of NIs. The integration of tools and principles from biomaterial design and tissue engineering has been investigated as a promising strategy to develop NIs with enhanced functionality and performance. In this Feature Article, we highlight the main bioengineering approaches for the development of biohybrid NIs with an emphasis on relevant device design criteria. Technical and scientific challenges associated with the fabrication and functional assessment of technologies composed of both artificial and biological components are discussed. Lastly, we provide future perspectives related to engineering, regulatory, and neuroethical challenges to be addressed towards the realisation of the promise of biohybrid neurotechnology.

Selective Immunosuppression Targeting the NLRP3 Inflammasome Mitigates the Foreign Body Response to Implanted Biomaterials While Preserving Angiogenesis

Medical devices are a mainstay of the healthcare industry, providing clinicians with innovative tools to diagnose, monitor, and treat a range of medical conditions. For implantable devices, it is widely regarded that chronic inflammation during the foreign body response (FBR) is detrimental to device performance, but also required for tissue regeneration and host integration. Current strategies to mitigate the FBR rely on broad acting anti-inflammatory drugs, most commonly, dexamethasone (DEX), which can inhibit angiogenesis and compromise long-term device function. This study challenges prevailing assumptions by suggesting that FBR inflammation is multifaceted, and selectively targeting its individual pathways can stop implant fibrosis while preserving beneficial repair pathways linked to improved device performance. MCC950, an anti-inflammatory drug that selectively inhibits the NLRP3 inflammasome, targets pathological inflammation without compromising global immune function. The effects of MCC950 and DEX on the FBR are compared using implanted polycaprolactone (PCL) scaffolds. The results demonstrate that both DEX and MCC950 halt immune cell recruitment and cytokine release, leading to reduced FBR. However, MCC950 achieves this while supporting capillary growth and enhancing tissue angiogenesis. These findings support selective immunosuppression approaches as a potential future direction for treating the FBR and enhancing the longevity and safety of implantable devices.

Advancing the interfacing performances of chronically implantable neural probes in the era of CMOS neuroelectronics

Tissue penetrating microelectrode neural probes can record electrophysiological brain signals at resolutions down to single neurons, making them invaluable tools for neuroscience research and Brain-Computer-Interfaces (BCIs). The known gradual decrease of their electrical interfacing performances in chronic settings, however, remains a major challenge. A key factor leading to such decay is Foreign Body Reaction (FBR), which is the cascade of biological responses that occurs in the brain in the presence of a tissue damaging artificial device. Interestingly, the recent adoption of Complementary Metal Oxide Semiconductor (CMOS) technology to realize implantable neural probes capable of monitoring hundreds to thousands of neurons simultaneously, may open new opportunities to face the FBR challenge. Indeed, this shift from passive Micro Electro-Mechanical Systems (MEMS) to active CMOS neural probe technologies creates important, yet unexplored, opportunities to tune probe features such as the mechanical properties of the probe, its layout, size, and surface physicochemical properties, to minimize tissue damage and consequently FBR. Here, we will first review relevant literature on FBR to provide a better understanding of the processes and sources underlying this tissue response. Methods to assess FBR will be described, including conventional approaches based on the imaging of biomarkers, and more recent transcriptomics technologies. Then, we will consider emerging opportunities offered by the features of CMOS probes. Finally, we will describe a prototypical neural probe that may meet the needs for advancing clinical BCIs, and we propose axial insertion force as a potential metric to assess the influence of probe features on acute tissue damage and to control the implantation procedure to minimize iatrogenic injury and subsequent FBR.

Pro-myelinating clemastine administration improves recording performance of chronically implanted microelectrodes and nearby neuronal health

Intracortical microelectrodes have become a useful tool in neuroprosthetic applications in the clinic and to understand neurological disorders in basic neurosciences. Many of these brain-machine interface technology applications require successful long-term implantation with high stability and sensitivity. However, the intrinsic tissue reaction caused by implantation remains a major failure mechanism causing loss of recorded signal quality over time. Oligodendrocytes remain an underappreciated intervention target to improve chronic recording performance. These cells can accelerate action potential propagation and provides direct metabolic support for neuronal health and functionality. However, implantation injury causes oligodendrocyte degeneration and leads to progressive demyelination in surrounding brain tissue. Previous work highlighted that healthy oligodendrocytes are necessary for greater electrophysiological recording performance and the prevention of neuronal silencing around implanted microelectrodes over the chronic implantation period. Thus, we hypothesize that enhancing oligodendrocyte activity with a pharmaceutical drug, Clemastine, will prevent the chronic decline of microelectrode recording performance. Electrophysiological evaluation showed that the promyelination Clemastine treatment significantly elevated the signal detectability and quality, rescued the loss of multi-unit activity, and increased functional interlaminar connectivity over 16-weeks of implantation. Additionally, post-mortem immunohistochemistry showed that increased oligodendrocyte density and myelination coincided with increased survival of both excitatory and inhibitory neurons near the implant. Overall, we showed a positive relationship between enhanced oligodendrocyte activity and neuronal health and functionality near the chronically implanted microelectrode. This study shows that therapeutic strategy that enhance oligodendrocyte activity is effective for integrating the functional device interface with brain tissue over chronic implantation period.

Implantable polypyrrole bioelectrodes inducing anti-inflammatory macrophage polarization for long-term in vivo signal recording

Bioelectrodes are critical components of implantable electronic devices that enable precise electrical signal transmission in close contact with living tissues. However, their in vivo performance is often compromised by inflammatory tissue reactions mainly induced by macrophages. Hence, we aimed to develop implantable bioelectrodes with high performance and high biocompatibility by actively modulating the inflammatory response of macrophages. Consequently, we fabricated heparin-doped polypyrrole electrodes (PPy/Hep) and immobilized anti-inflammatory cytokines (interleukin-4 [IL-4]) via non-covalent interactions. IL-4 immobilization did not alter the electrochemical performance of the original PPy/Hep electrodes. In vitro primary macrophage culture revealed that IL-4-immobilized PPy/Hep electrodes induced anti-inflammatory polarization of macrophages, similar to the soluble IL-4 control. In vivo subcutaneous implantation indicated that IL-4 immobilization on PPy/Hep promoted the anti-inflammatory polarization of host macrophages and significantly mitigated scarring around the implanted electrodes. In addition, high-sensitivity electrocardiogram signals were recorded from the implanted IL-4-immobilized PPy/Hep electrodes and compared to bare gold and PPy/Hep electrodes, which were maintained for up to 15 days post-implantation. This simple and effective surface modification strategy for developing immune-compatible bioelectrodes will facilitate the development of various electronic medical devices that require high sensitivities and long-term stabilities. STATEMENT OF SIGNIFICANCE: To fabricate highly immunocompatible conductive polymer-based implantable electrodes with high performance and stability in vivo, we introduced the anti-inflammatory activity to PPy/Hep electrodes by immobilizing IL-4 via non-covalent surface modification. IL-4-immobilized PPy/Hep could significantly mitigate inflammatory responses and scarring around implants by skewing macrophages to an anti-inflammatory phenotype. The IL-4-immobilized PPy/Hep electrodes could successfully record in vivo electrocardiogram signals for up to 15 days with no substantial sensitivity loss, retaining their superior sensitivity compared to bare gold and pristine PPy/Hep electrodes. Our simple and effective surface modification strategy for developing immune-compatible bioelectrodes will facilitate the development of various electronic medical devices that require high sensitivities and long-term stabilities, such as neural electrode arrays, biosensors, and cochlear electrodes.

Generation of direct current electrical fields as regenerative therapy for spinal cord injury: A review

Electrical stimulation (ES) shows promise as a therapy to promote recovery and regeneration after spinal cord injury. ES therapy establishes beneficial electric fields (EFs) and has been investigated in numerous studies, which date back nearly a century. In this review, we discuss the various engineering approaches available to generate regenerative EFs through direct current electrical stimulation and very low frequency electrical stimulation. We highlight the electrode-tissue interface, which is important for the appropriate choice of electrode material and stimulator circuitry. We discuss how to best estimate and control the generated field, which is an important measure for comparability of studies. Finally, we assess the methods used in these studies to measure functional recovery after the injury and treatment. This work reviews studies in the field of ES therapy with the goal of supporting decisions regarding best stimulation strategy and recovery assessment for future work.

Simultaneous Presentation of Dexamethasone and Nerve Growth Factor via Layered Carbon Nanotubes and Polypyrrole to Interface Neural Cells

The implantation of neural electrodes usually induces acute and chronic inflammation, which can result in the formation of glial scars encapsulating the implanted electrodes and the loss of neurons near the active electrode sites. Local presentation of anti-inflammatory drugs or neural protective factors has been evidenced as an effective strategy to modulate inflammatory responses and promote electrode-neuron integration. Here, a novel delivery system for the simultaneous presentation of both anti-inflammatory drugs (dexamethasone, Dex) and nerve-growth-promoting factors (nerve growth factor, NGF) from the electrode sites was developed via layer-structured carbon nanotubes and conductive polymers. The modified electrodes exhibited higher charge storage capacitance and lower electrochemical impedance compared to unmodified electrodes and electrodes coated with polypyrrole/Dex. Dex released from the functional coating under controlled electrochemical stimulation was able to inhibit the lipopolysaccharide-induced secretion or mRNA transcription of inflammatory cytokines, including nitric oxide, TNF-α, and IL-6 in RAW264.7 cells, and control the activation of cultured astrocytes. In addition, the functional coatings did not show a toxic effect on PC12 cells and primary neural cells but exhibited promoted activities on the adhesion, growth, and neurite extension of neural cells.

Chemically revised conducting polymers with inflammation resistance for intimate bioelectronic electrocoupling

Conducting polymers offer attractive mixed ionic-electronic conductivity, tunable interfacial barrier with metal, tissue matchable softness, and versatile chemical functionalization, making them robust to bridge the gap between brain tissue and electronic circuits. This review focuses on chemically revised conducting polymers, combined with their superior and controllable electrochemical performance, to fabricate long-term bioelectronic implants, addressing chronic immune responses, weak neuron attraction, and long-term electrocommunication instability challenges. Moreover, the promising progress of zwitterionic conducting polymers in bioelectronic implants (≥4 weeks stable implantation) is highlighted, followed by a comment on their current evolution toward selective neural coupling and reimplantable function. Finally, a critical forward look at the future of zwitterionic conducting polymers for in vivo bioelectronic devices is provided.

Emerging Penetrating Neural Electrodes: In Pursuit of Large Scale and Longevity

Penetrating neural electrodes provide a powerful approach to decipher brain circuitry by allowing for time-resolved electrical detections of individual action potentials. This unique capability has contributed tremendously to basic and translational neuroscience, enabling both fundamental understandings of brain functions and applications of human prosthetic devices that restore crucial sensations and movements. However, conventional approaches are limited by the scarce number of available sensing channels and compromised efficacy over long-term implantations. Recording longevity and scalability have become the most sought-after improvements in emerging technologies. In this review, we discuss the technological advances in the past 5-10 years that have enabled larger-scale, more detailed, and longer-lasting recordings of neural circuits at work than ever before. We present snapshots of the latest advances in penetration electrode technology, showcase their applications in animal models and humans, and outline the underlying design principles and considerations to fuel future technological development.

PEDOT:PSS-Based Bioelectrodes for Multifunctional Drug Release and Electric Cell-Substrate Impedance Sensing

Electric cell-substrate impedance sensing (ECIS) is an innovative approach for the label-free and real-time detection of cell morphology, growth, and apoptosis, thereby playing an essential role as both a viable alternative and valuable complement to conventional biochemical/pharmaceutical analysis in the field of diagnostics. Constant improvements are naturally sought to further improve the effective range and reliability of this technology. In this study, we developed poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) conducting polymer (CP)-based bioelectrodes integrated into homemade ECIS cell-culture chamber slides for the simultaneous drug release and real-time biosensing of cancer cell viability under drug treatment. The CP comprised tailored PEDOT:PSS, poly(ethylene oxide) (PEO), and (3-glycidyloxypropyl)trimethoxysilane (GOPS) capable of encapsulating antitumor chemotherapeutic agents such as doxorubicin (DOX), docetaxel (DTX), and a DOX/DTX combination. This device can reliably monitor impedance signal changes correlated with cell viability on chips generated by cell adhesion onto a predetermined CP-based working electrode while simultaneously exhibiting excellent properties for both drug encapsulation and on-demand release from another CP-based counter electrode under electrical stimulation (ES) operation. Cyclic voltammetry curves and surface profile data of different CP-based coatings (without or with drugs) were used to analyze the changes in charge capacity and thickness, respectively, thereby further revealing the correlation between their drug-releasing performance under ES operation (determined using ultraviolet-visible (UV-vis) spectroscopy). Finally, antitumor drug screening tests (DOX, DTX, and DOX/DTX combination) were performed on MCF-7 and HeLa cells using our developed CP-based ECIS chip system to monitor the impedance signal changes and their related cell viability results.

Multilayer Arrays for Neurotechnology Applications (MANTA): Chronically Stable Thin-Film Intracortical Implants

Flexible implantable neurointerfaces show great promise in addressing one of the major challenges of implantable neurotechnology, namely the loss of signal connected to unfavorable probe tissue interaction. The authors here show how multilayer polyimide probes allow high-density intracortical recordings to be combined with a reliable long-term stable tissue interface, thereby progressing toward chronic stability of implantable neurotechnology. The probes could record 10-60 single units over 5 months with a consistent peak-to-peak voltage at dimensions that ensure robust handling and insulation longevity. Probes that remain in intimate contact with the signaling tissue over months to years are a game changer for neuroscience and, importantly, open up for broader clinical translation of systems relying on neurotechnology to interface the human brain.

Advanced Temporally-Spatially Precise Technologies for On-Demand Neurological Disorder Intervention

Temporal-spatial precision has attracted increasing attention for the clinical intervention of neurological disorders (NDs) to mitigate adverse effects of traditional treatments and achieve point-of-care medicine. Inspiring steps forward in this field have been witnessed in recent years, giving the credit to multi-discipline efforts from neurobiology, bioengineering, chemical materials, artificial intelligence, and so on, exhibiting valuable clinical translation potential. In this review, the latest progress in advanced temporally-spatially precise clinical intervention is highlighted, including localized parenchyma drug delivery, precise neuromodulation, as well as biological signal detection to trigger closed-loop control. Their clinical potential in both central and peripheral nervous systems is illustrated meticulously related to typical diseases. The challenges relative to biosafety and scaled production as well as their future perspectives are also discussed in detail. Notably, these intelligent temporally-spatially precision intervention systems could lead the frontier in the near future, demonstrating significant clinical value to support billions of patients plagued with NDs.

Revolutionizing Drug Delivery and Therapeutics: The Biomedical Applications of Conductive Polymers and Composites-Based Systems

The first conductive polymers (CPs) were developed during the 1970s as a unique class of organic substances with properties that are electrically and optically comparable to those of inorganic semiconductors and metals while also exhibiting the desirable traits of conventional polymers. CPs have become a subject of intensive research due to their exceptional qualities, such as high mechanical and optical properties, tunable electrical characteristics, ease of synthesis and fabrication, and higher environmental stability than traditional inorganic materials. Although conducting polymers have several limitations in their pure state, coupling with other materials helps overcome these drawbacks. Owing to the fact that various types of tissues are responsive to stimuli and electrical fields has made these smart biomaterials attractive for a range of medical and biological applications. For various applications, including the delivery of drugs, biosensors, biomedical implants, and tissue engineering, electrical CPs and composites have attracted significant interest in both research and industry. These bimodalities can be programmed to respond to both internal and external stimuli. Additionally, these smart biomaterials have the ability to deliver drugs in various concentrations and at an extensive range. This review briefly discusses the commonly used CPs, composites, and their synthesis processes. Further highlights the importance of these materials in drug delivery along with their applicability in various delivery systems.

Biopolymeric Coatings for Local Release of Therapeutics from Biomedical Implants

The deployment of structures that enable localized release of bioactive molecules can result in more efficacious treatment of disease and better integration of implantable bionic devices. The strategic design of a biopolymeric coating can be used to engineer the optimal release profile depending on the task at hand. As illustrative examples, here advances in delivery of drugs from bone, brain, ocular, and cardiovascular implants are reviewed. These areas are focused to highlight that both hard and soft tissue implants can benefit from controlled localized delivery. The composition of biopolymers used to achieve appropriate delivery to the selected tissue types, and their corresponding outcomes are brought to the fore. To conclude, key factors in designing drug-loaded biopolymeric coatings for biomedical implants are highlighted.

Electroresponsive and pH-Sensitive Hydrogel as Carrier for Controlled Chloramphenicol Release

Multiresponsive hydrogels, which are smart soft materials that respond to more than one external stimulus, have emerged as powerful tools for biomedical applications, such as drug delivery. Within this context and with the aim of eliminating the systematic administration of antibiotics, special attention is being paid to the development of systems for controlled delivery of antibiotic for topical treatment of bacterial infections. In this work, an electro-chemo responsive hydrogel able to release chloramphenicol (CAM), a broad spectrum antibiotic also used for anticancer therapy, is proposed. This has been prepared by grafting poly(acrylic acid) (PAA) to sodium alginate (Alg) and in situ encapsulation of poly(3,4-ethylenedioxythiophene) nanoparticles loaded with CAM (PEDOT/CAM NPs), which were obtained by emulsion polymerization. Although the response to electrical stimuli of PEDOT was the main control for the release of CAM from PEDOT/CAM NPs, the release by passive diffusion had a relatively important contribution. Conversely, the passive release of antibiotic from the whole engineered hydrogel system, Alg-g-PAA/PEDOT/CAM, was negligible, whereas significant release was achieved under electrostimulation in an acid environment. Bacterial tests and assays with cancer cells demonstrated that the biological activity of CAM remained after release by electrical stimulation. Notably, the successful dual-response of the developed hydrogel to electrical stimuli and pH changes evidence the great prospect of this smart material in the biomedical field, as a tool to fight against bacterial infections and to provide local cancer treatment.

Conductive polymers and composites-based systems: An incipient stride in drug delivery and therapeutics realm

Novel therapies and drug delivery systems (DDS) emphasis on localized, personalized, triggered, and regulated drug administration have heavily implicated electrically responsive DDS. An ideal DDS must deliver drugs to the target region at therapeutically effective concentrations to elicit a pharmacological response, resulting in better prophylaxis of the disease and the treatment. Biodegradable polymers are frequently employed for in-vivo long-term release; however, dose dumping can be anticipated. As a result, current DDSs can be tagged as dubbed "Smart Biomaterials" since they only focus on an on-demand cargo release in response to a trigger or stimulation. These organic materials have been recognized for their metal-like conductivity, as well as their mechanical stability and ease of production. These biomaterials can be programmed to respond to both internal and external stimuli. External pulsed triggers are required for extrinsic stimuli-responsive materials, whereas intrinsic stimuli-responsive materials rely on localized changes in the tissue environment. Furthermore, these materials have the ability to deliver active pharmaceutical agents at a varied concentration levels and across a broad spectrum of action. Drug delivery, biomedical implant technology, biosensor technology, and tissue engineering can be listed as a few prominent applications that have sparked immense interest for conductive polymers-based research and advancements in academia as well as in industry. This review comprehensively covers a cutting-edge collection of electrically conductive polymers and composites, and provide detailed insights of recent trends and advancements allied to conductive polymers for their potential applicability in an array of diverse meadows primarily focusing on drug delivery, biosensing and therapeutics. Furthermore, progressions in their synthesis, structural and functional properties have been presented in conjunction with futuristic directions for the smooth clinical translations.

Pro-myelinating Clemastine administration improves recording performance of chronically implanted microelectrodes and nearby neuronal health

Intracortical microelectrodes have become a useful tool in neuroprosthetic applications in the clinic and to understand neurological disorders in basic neurosciences. Many of these brain-machine interface technology applications require successful long-term implantation with high stability and sensitivity. However, the intrinsic tissue reaction caused by implantation remains a major failure mechanism causing loss of recorded signal quality over time. Oligodendrocytes remain an underappreciated intervention target to improve chronic recording performance. These cells can accelerate action potential propagation and provides direct metabolic support for neuronal health and functionality. However, implantation injury causes oligodendrocyte degeneration and leads to progressive demyelination in surrounding brain tissue. Previous work highlighted that healthy oligodendrocytes are necessary for greater electrophysiological recording performance and the prevention of neuronal silencing around implanted microelectrodes over chronic implantation. Thus, we hypothesize that enhancing oligodendrocyte activity with a pharmaceutical drug, Clemastine, will prevent the chronic decline of microelectrode recording performance. Electrophysiological evaluation showed that the promyelination Clemastine treatment significantly elevated the signal detectability and quality, rescued the loss of multi-unit activity, and increased functional interlaminar connectivity over 16-weeks of implantation. Additionally, post-mortem immunohistochemistry showed that increased oligodendrocyte density and myelination coincided with increased survival of both excitatory and inhibitory neurons near the implant. Overall, we showed a positive relationship between enhanced oligodendrocyte activity and neuronal health and functionality near the chronically implanted microelectrode. This study shows that therapeutic strategy that enhance oligodendrocyte activity is effective for integrating the functional device interface with brain tissue over chronic implantation period.

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Zwitterionic Polymer Coated and Aptamer Functionalized Flexible Micro-Electrode Arrays for In Vivo Cocaine Sensing and Electrophysiology

The number of people aged 12 years and older using illicit drugs reached 59.3 million in 2020, among which 5.2 million are cocaine users based on the national data. In order to fully understand cocaine addiction and develop effective therapies, a tool is needed to reliably measure real-time cocaine concentration and neural activity in different regions of the brain with high spatial and temporal resolution. Integrated biochemical sensing devices based upon flexible microelectrode arrays (MEA) have emerged as a powerful tool for such purposes; however, MEAs suffer from undesired biofouling and inflammatory reactions, while those with immobilized biologic sensing elements experience additional failures due to biomolecule degradation. Aptasensors are powerful tools for building highly selective sensors for analytes that have been difficult to detect. In this work, DNA aptamer-based electrochemical cocaine sensors were integrated on flexible MEAs and protected with an antifouling zwitterionic poly (sulfobetaine methacrylate) (PSB) coating, in order to prevent sensors from biofouling and degradation by the host tissue. In vitro experiments showed that without the PSB coating, both adsorption of plasma protein albumin and exposure to DNase-1 enzyme have detrimental effects on sensor performance, decreasing signal amplitude and the sensitivity of the sensors. Albumin adsorption caused a 44.4% sensitivity loss, and DNase-1 exposure for 24 hr resulted in a 57.2% sensitivity reduction. The PSB coating successfully protected sensors from albumin fouling and DNase-1 enzyme digestion. In vivo tests showed that the PSB coated MEA aptasensors can detect repeated cocaine infusions in the brain for 3 hrs after implantation without sensitivity degradation. Additionally, the same MEAs can record electrophysiological signals at different tissue depths simultaneously. This novel flexible MEA with integrated cocaine sensors can serve as a valuable tool for understanding the mechanisms of cocaine addiction, while the PSB coating technology can be generalized to improve all implantable devices suffering from biofouling and inflammatory host responses.

Portable Iontophoresis Device for Efficient Drug Delivery

The timely delivery of drugs to specific locations in the body is imperative to ensure the efficacy of treatment. This study introduces a portable facial device that can deliver drugs efficiently using iontophoresis. Two types of power supplies-direct current and pulse ionization supplies-were manufactured by injection molding. Electrical stimulation elements, which contained Ag metal wires, were woven into facial mask packs. The diffusion phenomenon in the skin and iontophoresis were numerically modeled. Injection molding was simulated before the device was manufactured. Analysis using rhodamine B demonstrated a remarkable increase in the moisture content of the skin and effective absorption of the drug under an applied electric field upon the application of iontophoresis. The proposed concept and design constitute a new method of achieving effective drug absorption with wearable devices.

Implantable intracortical microelectrodes: reviewing the present with a focus on the future

Implantable intracortical microelectrodes can record a neuron's rapidly changing action potentials (spikes). In vivo neural activity recording methods often have either high temporal or spatial resolution, but not both. There is an increasing need to record more neurons over a longer duration in vivo. However, there remain many challenges to overcome before achieving long-term, stable, high-quality recordings and realizing comprehensive, accurate brain activity analysis. Based on the vision of an idealized implantable microelectrode device, the performance requirements for microelectrodes are divided into four aspects, including recording quality, recording stability, recording throughput, and multifunctionality, which are presented in order of importance. The challenges and current possible solutions for implantable microelectrodes are given from the perspective of each aspect. The current developments in microelectrode technology are analyzed and summarized.

Electrodeposited PEDOT/Nafion as Catalytic Counter Electrodes for Cobalt and Copper Bipyridyl Redox Mediators in Dye-Sensitized Solar Cells

PEDOT-based counter electrodes for dye-sensitized solar cells (DSSCs) are generally prepared by electrodeposition, which produces polymer films endowed with the best electrocatalytic properties. This translates in fast regeneration of the redox mediator, which allows the solar cell to sustain efficient photoconversion. The sustainable fabrication of DSSCs must consider the scaling up of the entire process, and when possible, it should avoid the use of large amounts of hazardous and/or inflammable chemicals, such as organic solvents for instance. This is why electrodeposition of PEDOT-based counter electrodes should preferably be carried out in aqueous media. In this study, PEDOT/Nafion was electrodeposited on FTO and comparatively evaluated as a catalytic material in DSSCs based on either cobalt or copper electrolytes. Our results show that the electrochemical response of PEDOT/Nafion toward Co(II/III-) or Cu(I/II)-based redox shuttles was comparable to that of PEDOT/ClO4 and significantly superior to that of PEDOT/PSS. In addition, when tested for adhesion, PEDOT/Nafion films were more stable for delamination if compared to PEDOT/ClO4, a feature that may prove beneficial in view of the long-term stability of solar devices.

Patternable Gelatin Methacrylate/PEDOT/Polystyrene Sulfonate Microelectrode Coatings for Neuronal Recording

This manuscript addresses the need for new soft biomaterials that can be fabricated on the surface of microelectrodes to reduce the mechanical mismatch between biological tissues and electrodes and improve the performance at the neural interface. By electrochemical polymerization of poly(3,4-dioxythiophene) (PEDOT)/polystyrene sulfonate (PSS) through a gelatin methacrylate (GelMA) hydrogel, we demonstrate the synthesis of a conducting polymer hydrogel (CPH) to meet the performance criteria of bioelectrodes. The hybrid material can be photolithographically patterned and covalently attached to gold microelectrodes, forming an interpenetrating network, as confirmed by infrared spectroscopy. The GelMA/PEDOT/PSS coatings were found to be reversibly electroactive by cyclic voltammetry and had low impedance compared to bare gold and GelMA-coated microelectrodes. The CPH coatings showed impedance at levels similar to conventional PEDOT/PSS coatings at a frequency of 1000 Hz. CPH exhibited electrochemical stability over 1000 CV cycles, and its performance was maintained over 14 days. Biocompatibility of the CPH coatings was confirmed by primary hippocampal neuronal cultures via a neuronal viability assay. The CPH-coated microelectrode arrays (MEAs) successfully recorded neuronal activity from primary hippocampal neuronal cells. The CPH GelMA/PEDOT/PSS is a highly promising coating material to enhance microelectrode performance at the neural interface.

A Subdural Bioelectronic Implant to Record Electrical Activity from the Spinal Cord in Freely Moving Rats

Bioelectronic devices have found use at the interface with neural tissue to investigate and treat nervous system disorders. Here, the development and characterization of a very thin flexible bioelectronic implant inserted along the thoracic spinal cord in rats directly in contact with and conformable to the dorsal surface of the spinal cord are presented. There is no negative impact on hind-limb functionality nor any change in the volume or shape of the spinal cord. The bioelectronic implant is maintained in rats for a period of 12 weeks. The first subdural recordings of spinal cord activity in freely moving animals are presented; rats are plugged in via a recording cable and allowed to freely behave and move around on a raised platform. Recordings contained multiple distinct voltage waveforms spatially localize to individual electrodes. This device has great potential to monitor electrical signaling in the spinal cord after an injury and in the future, this implant will facilitate the identification of biomarkers in spinal cord injury and recovery, while enabling the delivery of localized electroceutical and chemical treatments.

Recent Advances in Intelligent Wearable Medical Devices Integrating Biosensing and Drug Delivery

The primary roles of precision medicine are to perform real-time examination, administer on-demand medication, and apply instruments continuously. However, most current therapeutic systems implement these processes separately, leading to treatment interruption and limited recovery in patients. Personalized healthcare and smart medical treatment have greatly promoted research on and development of biosensing and drug-delivery integrated systems, with intelligent wearable medical devices (IWMDs) as typical systems, which have received increasing attention because of their non-invasive and customizable nature. Here, the latest progress in research on IWMDs is reviewed, including their mechanisms of integrating biosensing and on-demand drug delivery. The current challenges and future development directions of IWMDs are also discussed.

3D-Printed Polypyrrole Microneedle Arrays for Electronically Controlled Transdural Drug Release

After the spinal cord injury, inflammation and cytotoxicity cause further damage to neural cells. The progression of this secondary injury might be reduced by the administration of anti-inflammatory drugs. To allow the local delivery of such drugs while minimizing dural opening, we have created a polypyrrole (PPy)-coated microneedle array using a microscale three-dimensional (3D) printing technology that facilitates electronically controlled encapsulation and the transdural release of drugs. PPy microneedles demonstrated an electronically controlled release of steroid dexamethasone (Dexa) in a novel in vitro transdural model and in vivo. The biological activity of the device was then tested by the electronic release of Dexa into an in vitro model of neuroinflammation, using activated microglia. Following electrically activated Dexa release, inflammation was reduced, as demonstrated by a decrease in nitric oxide and proinflammatory cytokines Il-6 and MCP-1. These results demonstrate the feasibility of PPy-coated microneedles for the transdural delivery of anti-inflammatory drugs to the central nervous system.

Poly(3,4-ethylenedioxythiophene)-Based Neural Interfaces for Recording and Stimulation: Fundamental Aspects and In Vivo Applications

Next-generation neural interfaces for bidirectional communication with the central nervous system aim to achieve the intimate integration with the neural tissue with minimal neuroinflammatory response, high spatio-temporal resolution, very high sensitivity, and readout stability. The design and manufacturing of devices for low power/low noise neural recording and safe and energy-efficient stimulation that are, at the same time, conformable to the brain, with matched mechanical properties and biocompatibility, is a convergence area of research where neuroscientists, materials scientists, and nanotechnologists operate synergically. The biotic-abiotic neural interface, however, remains a formidable challenge that prompts for new materials platforms and innovation in device layouts. Conductive polymers (CP) are attractive materials to be interfaced with the neural tissue and to be used as sensing/stimulating electrodes because of their mixed ionic-electronic conductivity, their low contact impedance, high charge storage capacitance, chemical versatility, and biocompatibility. This manuscript reviews the state-of-the-art of poly(3,4-ethylenedioxythiophene)-based neural interfaces for extracellular recording and stimulation, focusing on those technological approaches that are successfully demonstrated in vivo. The aim is to highlight the most reliable and ready-for-clinical-use solutions, in terms of materials technology and recording performance, other than spot major limitations and identify future trends in this field.

Spinal cord bioelectronic interfaces: opportunities in neural recording and clinical challenges

Bioelectronic stimulation of the spinal cord has demonstrated significant progress in restoration of motor function in spinal cord injury (SCI). The proximal, uninjured spinal cord presents a viable target for the recording and generation of control signals to drive targeted stimulation. Signals have been directly recorded from the spinal cord in behaving animals and correlated with limb kinematics. Advances in flexible materials, electrode impedance and signal analysis will allow SCR to be used in next-generation neuroprosthetics. In this review, we summarize the technological advances enabling progress in SCR and describe systematically the clinical challenges facing spinal cord bioelectronic interfaces and potential solutions, from device manufacture, surgical implantation to chronic effects of foreign body reaction and stress-strain mismatches between electrodes and neural tissue. Finally, we establish our vision of bi-directional closed-loop spinal cord bioelectronic bypass interfaces that enable the communication of disrupted sensory signals and restoration of motor function in SCI.

In Vivo Organic Bioelectronics for Neuromodulation

The nervous system poses a grand challenge for integration with modern electronics and the subsequent advances in neurobiology, neuroprosthetics, and therapy which would become possible upon such integration. Due to its extreme complexity, multifaceted signaling pathways, and ∼1 kHz operating frequency, modern complementary metal oxide semiconductor (CMOS) based electronics appear to be the only technology platform at hand for such integration. However, conventional CMOS-based electronics rely exclusively on electronic signaling and therefore require an additional technology platform to translate electronic signals into the language of neurobiology. Organic electronics are just such a technology platform, capable of converting electronic addressing into a variety of signals matching the endogenous signaling of the nervous system while simultaneously possessing favorable material similarities with nervous tissue. In this review, we introduce a variety of organic material platforms and signaling modalities specifically designed for this role as "translator", focusing especially on recent implementation in in vivo neuromodulation. We hope that this review serves both as an informational resource and as an encouragement and challenge to the field.

Semiconducting Polymers for Neural Applications

Electronically interfacing with the nervous system for the purposes of health diagnostics and therapy, sports performance monitoring, or device control has been a subject of intense academic and industrial research for decades. This trend has only increased in recent years, with numerous high-profile research initiatives and commercial endeavors. An important research theme has emerged as a result, which is the incorporation of semiconducting polymers in various devices that communicate with the nervous system—from wearable brain-monitoring caps to penetrating implantable microelectrodes. This has been driven by the potential of this broad class of materials to improve the electrical and mechanical properties of the tissue–device interface, along with possibilities for increased biocompatibility. In this review we first begin with a tutorial on neural interfacing, by reviewing the basics of nervous system function, device physics, and neuroelectrophysiological techniques and their demands, and finally we give a brief perspective on how material improvements can address current deficiencies in this system. The second part is a detailed review of past work on semiconducting polymers, covering electrical properties, structure, synthesis, and processing.

Assessing the Feasibility of Developing in vivo Neuroprobes for Parallel Intracellular Recording and Stimulation: A Perspective

Developing novel neuroprobes that enable parallel multisite, long-term intracellular recording and stimulation of neurons in freely behaving animals is a neuroscientist's dream. When fulfilled, it is expected to significantly enhance brain research at fundamental mechanistic levels including that of subthreshold signaling and computations. Here we assess the feasibility of merging the advantages of in vitro vertical nanopillar technologies that support intracellular recordings with contemporary concepts of in vivo extracellular field potential recordings to generate the dream neuroprobes that read the entire electrophysiological signaling repertoire.

Engineering strategies towards overcoming bleeding and glial scar formation around neural probes

Neural probes are sophisticated electrophysiological tools used for intra-cortical recording and stimulation. These microelectrode arrays, designed to penetrate and interface the brain from within, contribute at the forefront of basic and clinical neuroscience. However, one of the challenges and currently most significant limitations is their ‘seamless’ long-term integration into the surrounding brain tissue. Following implantation, which is typically accompanied by bleeding, the tissue responds with a scarring process, resulting in a gliotic region closest to the probe. This glial scarring is often associated with neuroinflammation, neurodegeneration, and a leaky blood–brain interface (BBI). The engineering progress on minimizing this reaction in the form of improved materials, microfabrication, and surgical techniques is summarized in this review. As research over the past decade has progressed towards a more detailed understanding of the nature of this biological response, it is time to pose the question: Are penetrating probes completely free from glial scarring at all possible?