Progress towards biocompatible intracortical microelectrodes for neural interfacing applications

Systemically administered platelet-inspired nanoparticles to reduce inflammation surrounding intracortical microelectrodes

Intracortical microelectrodes (IMEs) are essential for neural signal acquisition in neuroscience and brain-machine interface (BMI) systems, aiding patients with neurological disorders, paralysis, and amputations. However, IMEs often fail to maintain robust signal quality over time, partly due to neuroinflammation caused by vascular damage during insertion. Platelet-inspired nanoparticles (PIN), which possess injury-targeting functions, mimic the adhesion and aggregation of active platelets through conjugated collagen-binding peptides (CBP), von Willebrand Factor-binding peptides (VBP), and fibrinogen-mimetic peptides (FMP). Systemically administered PINs can potentially enhance hemostasis and promote the resealing of IME insertion-induced leaky blood-brain barrier (BBB), thereby attenuating the influx of blood-derived proteins into the brain parenchyma that trigger neuroinflammation. This study explores the potential of PINs to mitigate neuroinflammation at implant sites. Male Sprague Dawley rats underwent craniotomies and IME implantations, followed by a single dose of Cy5 labeled PINs (2 mg/kg). Rats were sacrificed at intervals from 0 to 4 days post-implantation (DPI) for biodistribution analysis using an in vivo live imaging system (IVIS) and immunohistochemistry (IHC) to assess neuroinflammation, BBB permeability, and active platelet distribution. Another cohort of rats received weekly PINs, trehalose buffer (TH, diluent control), or control nanoparticles (CP, PEG-coated liposomes) for 4 weeks, with similar endpoint analyses. Results indicated that PIN concentrations were significantly elevated near IME interfaces acutely (0-4 DPI) and after 4 weeks of repeated dosing. At 3 DPI, peak intensities of active platelets (CD62P), activated microglia/macrophages (CD68), and PINs were observed. Immunoglobulin G (IgG) was upregulated during the first 24 h near implant sites but declined thereafter. At 4 weeks, the PINs group exhibited higher intensities of active platelets and PINs, and reduced CD68 and IgG levels compared to controls. PINs effectively targeted the IME-tissue interface, alongside endogenous activated platelets, resulting in reduced neuroinflammatory and BBB-leakage markers compared to the diluent-only-infused control group. Repeated dosing of PINs presents a promising approach for enhancing the quality of neural recordings in future studies.

Overcoming failure: improving acceptance and success of implanted neural interfaces

Implanted neural interfaces are electronic devices that stimulate or record from neurons with the purpose of improving the quality of life of people who suffer from neural injury or disease. Devices have been designed to interact with neurons throughout the body to treat a growing variety of conditions. The development and use of implanted neural interfaces is increasing steadily and has shown great success, with implants lasting for years to decades and improving the health and quality of life of many patient populations. Despite these successes, implanted neural interfaces face a multitude of challenges to remain effective for the lifetime of their users. The devices are comprised of several electronic and mechanical components that each may be susceptible to failure. Furthermore, implanted neural interfaces, like any foreign body, will evoke an immune response. The immune response will differ for implants in the central nervous system and peripheral nervous system, as well as over time, ultimately resulting in encapsulation of the device. This review describes the challenges faced by developers of neural interface systems, particularly devices already in use in humans. The mechanical and technological failure modes of each component of an implant system is described. The acute and chronic reactions to devices in the peripheral and central nervous system and how they affect system performance are depicted. Further, physical challenges such as micro and macro movements are reviewed. The clinical implications of device failures are summarized and a guide for determining the severity of complication was developed and provided. Common methods to diagnose and examine mechanical, technological, and biological failure modes at various stages of development and testing are outlined, with an emphasis on chronic in vivo characterization of implant systems. Finally, this review concludes with an overview of some of the innovative solutions developed to reduce or resolve the challenges faced by implanted neural interface systems.

Bacteria invade the brain following intracortical microelectrode implantation, inducing gut-brain axis disruption and contributing to reduced microelectrode performance

Brain-machine interface performance can be affected by neuroinflammatory responses due to blood-brain barrier (BBB) damage following intracortical microelectrode implantation. Recent findings suggest that certain gut bacterial constituents might enter the brain through damaged BBB. Therefore, we hypothesized that damage to the BBB caused by microelectrode implantation could facilitate microbiome entry into the brain. In our study, we found bacterial sequences, including gut-related ones, in the brains of mice with implanted microelectrodes. These sequences changed over time. Mice treated with antibiotics showed a reduced presence of these bacteria and had a different inflammatory response, which temporarily improved microelectrode recording performance. However, long-term antibiotic use worsened performance and disrupted neurodegenerative pathways. Many bacterial sequences found were not present in the gut or in unimplanted brains. Together, the current study established a paradigm-shifting mechanism that may contribute to chronic intracortical microelectrode recording performance and affect overall brain health following intracortical microelectrode implantation.

Dexamethasone-loaded platelet-inspired nanoparticles improve intracortical microelectrode recording performance

Long-term robust intracortical microelectrode (IME) neural recording quality is negatively affected by the neuroinflammatory response following microelectrode insertion. This adversely impacts brain-machine interface (BMI) performance for patients with neurological disorders or amputations. Recent studies suggest that the leakage of blood-brain barrier (BBB) and microhemorrhage caused by the IME insertions lead to the increased neuroinflammation and reduced neural recording performance. Additionally, a sustained presence of activated platelets and coagulation factors is found near the insertion site. Thus, we hypothesized that the systemic administration of dexamethasone sodium phosphate-loaded platelet-inspired nanoparticle (SPPINDEX) can improve the neural recording performance of intracortical microelectrodes (IMEs) by promoting hemostasis, facilitating blood-brain barrier (BBB) healing, and achieving implant-targeted drug delivery. Leveraging the hemostatic and coagulation factor-binding properties of the platelet-inspired nanoparticle (PIN) drug delivery platform, SPPINDEX treatment can initially attenuate the invasion of neuroinflammatory triggers into the brain parenchyma caused by insertion-induced microhemorrhages or a compromised BBB. Furthermore, targeted delivery of the anti-inflammatory drug dexamethasone sodium phosphate (DEXSP) to the implant site via these nanoparticles can attenuate ongoing neuroinflammation, enhancing overall therapeutic efficacy. Weekly treatment with SPPINDEX for 8 weeks significantly improved the recording capabilities of IMEs compared to platelet-inspired nanoparticles alone (PIN), free dexamethasone sodium phosphate (Free DEXSP), and a diluent control trehalose buffer (TH), as assessed through extracellular single-unit recordings. Immunohistochemical analyses of neuron density, activated microglia/macrophage density, astrocyte density, and BBB permeability suggest that the improved neural recording performance may be attributed to reduced neuron degeneration, activated microglia and astrocytes at the implant interface caused by the decreased infiltration of blood-derived proteins that trigger neuroinflammation and the therapeutic effects from DEXSP. Overall, SPPINDEX treatment promotes an anti-inflammatory environment that improves neuronal density and enhances recording performance.

Poly(curcumin-co-poly(ethylene glycol)) films provide neuroprotection following reactive oxygen species insultin vitro

Objective.Curcumin is an antioxidant and anti-inflammatory molecule that may provide neuroprotection following central nervous system injury. However, curcumin is hydrophobic, limiting its ability to be loaded and then released from biomaterials for neural applications. We previously developed polymers containing curcumin, and these polymers may be applied to neuronal devices or to neural injury to promote neuroprotection. Thus, our objective was to evaluate two curcumin polymers as potential neuroprotective materials for neural applications.Approach.For each curcumin polymer, we created three polymer solutions by varying the weight percentage of curcumin polymer in solvent. These solutions were subsequently coated onto glass coverslips, and the thickness of the polymer was assessed using profilometry. Polymer degradation and dissolution was assessed using brightfield microscopy, scanning electron microscopy, and gel permeation chromatography. The ability of the polymers to protect cortical neurons from free radical insult was assessed using anin vitrocortical culture model.Main results.The P50 curcumin polymer (containing greater poly(ethylene glycol) content than the P75 polymer), eroded readily in solution, with erosion dependent on the weight percentage of polymer in solvent. Unlike the P50 polymer, the P75 polymer did not undergo erosion. Since the P50 polymer underwent erosion, we expected that the P50 polymer would more readily protect cortical neurons from free radical insult. Unexpectedly, even though P75 films did not erode, P75 polymers protected neurons from free radical insult, suggesting that erosion is not necessary for these polymers to enable neuroprotection.Significance.This study is significant as it provides a framework to evaluate polymers for future neural applications. Additionally, we observed that some curcumin polymers do not require dissolution to enable neuroprotection. Future work will assess the ability of these materials to enable neuroprotection withinin vivomodels of neural injury.

Recent advances in polymer-based thin-film electrodes for ECoG applications

Electrocorticography (ECoG) has garnered widespread attention owing to its superior signal resolution compared to conventional electroencephalogram (EEG). While ECoG signal acquisition entails invasiveness, the invasive rigid electrode used inevitably inflicts damage on brain tissue. Polymer electrodes that combine conductivity and transparency have garnered great interest because they not only facilitate high-quality signal acquisition but also provide additional insights while preserving the health of the brain, positioning them as the future frontier in the brain-computer interface (BCI). This review summarizes the multifaceted functions of polymers in ECoG thin-film electrodes for the BCI. We present the abilities of sensitive and structural polymers focusing on impedance reduction, signal quality improvement, good flexibility, and transparency. Typically, two sensitive polymers and four structural polymers are analyzed in detail in terms of ECoG electrode properties. Moreover, the underlying mechanism of polymer-based electrodes in signal quality enhancement is revealed. Finally, the remaining challenges and perspectives are discussed.

Cell-specific spatial profiling of targeted protein expression to characterize the impact of intracortical microelectrode implantation on neuronal health

Intracortical microelectrode arrays (MEAs) can record neuronal activity and advance brain-computer interface (BCI) devices. Implantation of the invasive MEA kills local neurons, which has been documented using immunohistochemistry (IHC). Neuronal nuclear protein (NeuN), a protein that lines the nuclei of exclusively neuronal cells, has been used as a marker for neuronal health and survival for decades in neuroscience and neural engineering. NeuN staining is often used to describe the neuronal response to intracortical microelectrode array (MEA) implantation. However, IHC is semiquantitative, relying on intensity readings rather than directly counting expressed proteins. To supplement previous IHC studies, we evaluated the expression of proteins representing different aspects of neuronal structure or function: microtubule-associated protein 2 (MAP2), neurofilament light (NfL), synaptophysin (SYP), myelin basic protein (MBP), and oligodendrocyte transcription factor 2 (OLIG2) following a neural injury caused by intracortical MEA implantation. Together, these five proteins evaluate the cytoskeletal structure, neurotransmitter release, and myelination of neurons. To fully evaluate neuronal health in NeuN-positive (NeuN+) regions, we only quantified protein expression in NeuN+ regions, making this the first-ever cell-specific spatial profiling evaluation of targeted proteins by multiplex immunochemistry following MEA implantation. We performed our protein quantification along with NeuN IHC to compare the results of the two techniques directly. We found that NeuN immunohistochemical analysis does not show the same trends as MAP2, NfL, SYP, MBP, and OLIG2 expression. Further, we found that all five quantified proteins show a decreased expression pattern that aligns more with historic intracortical MEA recording performance.

Implantable Self-Powered Systems for Electrical Stimulation Medical Devices

With the integration of bioelectronics and materials science, implantable self-powered systems for electrical stimulation medical devices have emerged as an innovative therapeutic approach, garnering significant attention in medical research. These devices achieve self-powering through integrated energy conversion modules, such as triboelectric nanogenerators (TENGs) and piezoelectric nanogenerators (PENGs), significantly enhancing the portability and long-term efficacy of therapeutic equipment. This review delves into the design strategies and clinical applications of implantable self-powered systems, encompassing the design and optimization of energy harvesting modules, the selection and fabrication of adaptable electrode materials, innovations in systematic design strategies, and the extensive utilization of implantable self-powered systems in biological therapies, including the treatment of neurological disorders, tissue regeneration engineering, drug delivery, and tumor therapy. Through a comprehensive analysis of the latest research progress, technical challenges, and future directions in these areas, this paper aims to provide valuable insights and inspiration for further research and clinical applications of implantable self-powered systems.

A review on the performance of brain-computer interface systems used for patients with locked-in and completely locked-in syndrome

Patients with locked-in syndrome (LIS) and complete locked-in syndrome (CLIS) own a fully functional brain restricted within a non-functional body. In order to help LIS patients stay connected with their surroundings, brain-computer interfaces (BCIs) and related technologies have emerged. BCIs translate brain activity into actions that can be performed by external devices enabling LIS patients to communicate, leading to an increase in their quality of life. The past decade has seen the rapid development of BCIs that have the potential to be used for patients with locked-in syndrome, from which a great deal is tested only on healthy subjects and not on actual patients. This study aims to (1) provide the readers with a comprehensive study that contributes to this growing area of research by exploring the performance of BCIs tested specifically on LIS and CLIS patients, (2) give an overview of different modalities and paradigms used in different stages of the locked-in syndrome, and (3) discuss the contributions and limitations of BCIs introduced for the LIS and CLIS patients in the state-of-the-art and lay a groundwork for researchers interested in this field.

Comprehensive proteomic analysis of the differential expression of 62 proteins following intracortical microelectrode implantation

Intracortical microelectrodes (IMEs) are devices designed to be implanted into the cerebral cortex for various neuroscience and neuro-engineering applications. A critical feature of IMEs is their ability to detect neural activity from individual neurons. Currently, IMEs are limited by chronic failure, largely considered to be caused by the prolonged neuroinflammatory response to the implanted devices. Over the past few years, the characterization of the neuroinflammatory response has grown in sophistication, with the most recent advances focusing on mRNA expression following IME implantation. While gene expression studies increase our broad understanding of the relationship between IMEs and cortical tissue, advanced proteomic techniques have not been reported. Proteomic evaluation is necessary to describe the diverse changes in protein expression specific to neuroinflammation, neurodegeneration, or tissue and cellular viability, which could lead to the further development of targeted intervention strategies designed to improve IME functionality. In this study, we have characterized the expression of 62 proteins within 180 μm of the IME implant site at 4-, 8-, and 16-weeks post-implantation. We identified potential targets for immunotherapies, as well as key pathways that contribute to neuronal dieback around the IME implant.

Bioelectronic Neural Interfaces: Improving Neuromodulation Through Organic Conductive Coatings

Integration of bioelectronic devices in clinical practice is expanding rapidly, focusing on conditions ranging from sensory to neurological and mental health disorders. While platinum (Pt) electrodes in neuromodulation devices such as cochlear implants and deep brain stimulators have shown promising results, challenges still affect their long-term performance. Key among these are electrode and device longevity in vivo, and formation of encapsulating fibrous tissue. To overcome these challenges, organic conductors with unique chemical and physical properties are being explored. They hold great promise as coatings for neural interfaces, offering more rapid regulatory pathways and clinical implementation than standalone bioelectronics. This study provides a comprehensive review of the potential benefits of organic coatings in neuromodulation electrodes and the challenges that limit their effective integration into existing devices. It discusses issues related to metallic electrode use and introduces physical, electrical, and biological properties of organic coatings applied in neuromodulation. Furthermore, previously reported challenges related to organic coating stability, durability, manufacturing, and biocompatibility are thoroughly reviewed and proposed coating adhesion mechanisms are summarized. Understanding organic coating properties, modifications, and current challenges of organic coatings in clinical and industrial settings is expected to provide valuable insights for their future development and integration into organic bioelectronics.

Antioxidant Coated Microelectrode Arrays: Effects on Putative Inhibitory and Excitatory Neurons

Intracortical microelectrode arrays (MEAs) are used to record neural activity in vivo at single-cell resolution for both neuroscience studies and for engineering restorative devices such as brain-computer interfaces (BCIs). The recording performance of these devices are known to degrade over weeks to months after implantation due, in part, to neuroinflammation and oxidative stress. Characterizing and mitigating the degradation of recording performance is of particular interest for chronic applications. Literature suggests that inhibitory neurons may be more susceptible to oxidative stress than excitatory neurons. In this study, we classify recorded neural signals as either putative inhibitory or excitatory based on their waveform characteristics and aim to identify if one preferentially benefits from the use of a Mn(III)tetrakis94-benzoic acid)porphyrin (MnTBAP) coating to reduce reactive oxygen species, which we have previously demonstrated improves chronic neural recordings. In this study, we found that the MnTBAP coating affects these two classes of neurons differently, depending on the cortical depth. The MnTBAP coating improves the number of putative inhibitory signals recorded on the middle electrode sites (L5) and putative excitatory units on the superficial (L2/3 & L4) electrode sites. Our results suggest that decreases in recording performance may be influenced by both cortical depth and neuronal cell type. Furthermore, we show that the benefits of a MnTBAP coating to chronic neural recordings differ between putative inhibitory and excitatory neurons with a depth dependence.

Depletion of complement factor 3 delays the neuroinflammatory response to intracortical microelectrodes

The neuroinflammatory response to intracortical microelectrodes (IMEs) used with brain-machine interfacing (BMI) applications is regarded as the primary contributor to poor chronic performance. Recent developments in high-plex gene expression technologies have allowed for an evolution in the investigation of individual proteins or genes to be able to identify specific pathways of upregulated genes that may contribute to the neuroinflammatory response. Several key pathways that are upregulated following IME implantation are involved with the complement system. The complement system is part of the innate immune system involved in recognizing and eliminating pathogens - a significant contributor to the foreign body response against biomaterials. Specifically, we have identified Complement 3 (C3) as a gene of interest because it is the intersection of several key complement pathways. In this study, we investigated the role of C3 in the IME inflammatory response by comparing the neuroinflammatory gene expression at the microelectrode implant site between C3 knockout (C3-/-) and wild-type (WT) mice. We have found that, like in WT mice, implantation of intracortical microelectrodes in C3-/- mice yields a dramatic increase in the neuroinflammatory gene expression at all post-surgery time points investigated. However, compared to WT mice, C3 depletion showed reduced expression of many neuroinflammatory genes pre-surgery and 4 weeks post-surgery. Conversely, depletion of C3 increased the expression of many neuroinflammatory genes at 8 weeks and 16 weeks post-surgery, compared to WT mice. Our results suggest that C3 depletion may be a promising therapeutic target for acute, but not chronic, relief of the neuroinflammatory response to IME implantation. Additional compensatory targets may also be required for comprehensive long-term reduction of the neuroinflammatory response for improved intracortical microelectrode performance.

Impact of Impedance Levels on Recording Quality in Flexible Neural Probes

Flexible neural probes are attractive emerging technologies for brain recording because they can effectively record signals with minimal risk of brain damage. Reducing the electrode impedance of the probe before recording is a common practice of many researchers. However, studies investigating the impact of low impedance levels on high-quality recordings using flexible neural probes are lacking. In this study, we electrodeposited Pt onto a commercial flexible polyimide neural probe and investigated the relationship between the impedance level and the recording quality. The probe was inserted into the brains of anesthetized mice. The electrical signals of neurons in the brain, specifically the ventral posteromedial nucleus of the thalamus, were recorded at impedance levels of 50, 250, 500 and 1000 kΩ at 1 kHz. The study results demonstrated that as the impedance decreased, the quality of the signal recordings did not consistently improve. This suggests that extreme lowering of the impedance may not always be advantageous in the context of flexible neural probes.

Comprehensive Proteomic Analysis of the Differential Expression of 83 Proteins Following Intracortical Microelectrode Implantation

Intracortical microelectrodes (IMEs) are devices designed to be implanted into the cerebral cortex for various neuroscience and neuro-engineering applications. A critical feature of these devices is their ability to detect neural activity from individual neurons. Currently, IMEs are limited by chronic failure, largely considered to be caused by the prolonged neuroinflammatory response to the implanted devices. Over the decades, characterization of the neuroinflammatory response has grown in sophistication, with the most recent advances including advanced genomics and spatially resolved transcriptomics. While gene expression studies increase our broad understanding of the relationship between IMEs and cortical tissue, advanced proteomic techniques have not been reported. Proteomic evaluation is necessary to describe the diverse changes in protein expression specific to neuroinflammation, neurodegeneration, or tissue and cellular viability, which could lead to the development of more targeted intervention strategies designed to improve IME function. In this study, we have characterized the expression of 83 proteins within 180 μm of the IME implant site at 4-, 8-, and 16-weeks post-implantation. We identified potential targets for immunotherapies, as well as key pathways and functions that contribute to neuronal dieback around the IME implant.

Bacteria Invade the Brain Following Sterile Intracortical Microelectrode Implantation

Brain-machine interface performance is largely affected by the neuroinflammatory responses resulting in large part from blood-brain barrier (BBB) damage following intracortical microelectrode implantation. Recent findings strongly suggest that certain gut bacterial constituents penetrate the BBB and are resident in various brain regions of rodents and humans, both in health and disease. Therefore, we hypothesized that damage to the BBB caused by microelectrode implantation could amplify dysregulation of the microbiome-gut-brain axis. Here, we report that bacteria, including those commonly found in the gut, enter the brain following intracortical microelectrode implantation in mice implanted with single-shank silicon microelectrodes. Systemic antibiotic treatment of mice implanted with microelectrodes to suppress bacteria resulted in differential expression of bacteria in the brain tissue and a reduced acute inflammatory response compared to untreated controls, correlating with temporary improvements in microelectrode recording performance. Long-term antibiotic treatment resulted in worsening microelectrode recording performance and dysregulation of neurodegenerative pathways. Fecal microbiome composition was similar between implanted mice and an implanted human, suggesting translational findings. However, a significant portion of invading bacteria was not resident in the brain or gut. Together, the current study established a paradigm-shifting mechanism that may contribute to chronic intracortical microelectrode recording performance and affect overall brain health following intracortical microelectrode implantation.

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.

In Vivo Characterization of Intracortical Probes with Focused Ion Beam-Etched Nanopatterned Topographies

(1) Background: Intracortical microelectrodes (IMEs) are an important part of interfacing with the central nervous system (CNS) and recording neural signals. However, recording electrodes have shown a characteristic steady decline in recording performance owing to chronic neuroinflammation. The topography of implanted devices has been explored to mimic the nanoscale three-dimensional architecture of the extracellular matrix. Our previous work used histology to study the implant sites of non-recording probes and showed that a nanoscale topography at the probe surface mitigated the neuroinflammatory response compared to probes with smooth surfaces. Here, we hypothesized that the improvement in the neuroinflammatory response for probes with nanoscale surface topography would extend to improved recording performance. (2) Methods: A novel design modification was implemented on planar silicon-based neural probes by etching nanopatterned grooves (with a 500 nm pitch) into the probe shank. To assess the hypothesis, two groups of rats were implanted with either nanopatterned (n = 6) or smooth control (n = 6) probes, and their recording performance was evaluated over 4 weeks. Postmortem gene expression analysis was performed to compare the neuroinflammatory response from the two groups. (3) Results: Nanopatterned probes demonstrated an increased impedance and noise floor compared to controls. However, the recording performances of the nanopatterned and smooth probes were similar, with active electrode yields for control probes and nanopatterned probes being approximately 50% and 45%, respectively, by 4 weeks post-implantation. Gene expression analysis showed one gene, Sirt1, differentially expressed out of 152 in the panel. (4) Conclusions: this study provides a foundation for investigating novel nanoscale topographies on neural probes.

Comparative analysis of energy transfer mechanisms for neural implants

As neural implant technologies advance rapidly, a nuanced understanding of their powering mechanisms becomes indispensable, especially given the long-term biocompatibility risks like oxidative stress and inflammation, which can be aggravated by recurrent surgeries, including battery replacements. This review delves into a comprehensive analysis, starting with biocompatibility considerations for both energy storage units and transfer methods. The review focuses on four main mechanisms for powering neural implants: Electromagnetic, Acoustic, Optical, and Direct Connection to the Body. Among these, Electromagnetic Methods include techniques such as Near-Field Communication (RF). Acoustic methods using high-frequency ultrasound offer advantages in power transmission efficiency and multi-node interrogation capabilities. Optical methods, although still in early development, show promising energy transmission efficiencies using Near-Infrared (NIR) light while avoiding electromagnetic interference. Direct connections, while efficient, pose substantial safety risks, including infection and micromotion disturbances within neural tissue. The review employs key metrics such as specific absorption rate (SAR) and energy transfer efficiency for a nuanced evaluation of these methods. It also discusses recent innovations like the Sectored-Multi Ring Ultrasonic Transducer (S-MRUT), Stentrode, and Neural Dust. Ultimately, this review aims to help researchers, clinicians, and engineers better understand the challenges of and potentially create new solutions for powering neural implants.

Mutable Collagenous Tissue: A Concept Generator for Biomimetic Materials and Devices

Echinoderms (starfish, sea-urchins and their close relations) possess a unique type of collagenous tissue that is innervated by the motor nervous system and whose mechanical properties, such as tensile strength and elastic stiffness, can be altered in a time frame of seconds. Intensive research on echinoderm 'mutable collagenous tissue' (MCT) began over 50 years ago, and over 20 years ago, MCT first inspired a biomimetic design. MCT, and sea-cucumber dermis in particular, is now a major source of ideas for the development of new mechanically adaptable materials and devices with applications in diverse areas including biomedical science, chemical engineering and robotics. In this review, after an up-to-date account of present knowledge of the structural, physiological and molecular adaptations of MCT and the mechanisms responsible for its variable tensile properties, we focus on MCT as a concept generator surveying biomimetic systems inspired by MCT biology, showing that these include both bio-derived developments (same function, analogous operating principles) and technology-derived developments (same function, different operating principles), and suggest a strategy for the further exploitation of this promising biological resource.

Recent Advancements in Graphene-Based Implantable Electrodes for Neural Recording/Stimulation

Implantable electrodes represent a groundbreaking advancement in nervous system research, providing a pivotal tool for recording and stimulating human neural activity. This capability is integral for unraveling the intricacies of the nervous system's functionality and for devising innovative treatments for various neurological disorders. Implantable electrodes offer distinct advantages compared to conventional recording and stimulating neural activity methods. They deliver heightened precision, fewer associated side effects, and the ability to gather data from diverse neural sources. Crucially, the development of implantable electrodes necessitates key attributes: flexibility, stability, and high resolution. Graphene emerges as a highly promising material for fabricating such electrodes due to its exceptional properties. It boasts remarkable flexibility, ensuring seamless integration with the complex and contoured surfaces of neural tissues. Additionally, graphene exhibits low electrical resistance, enabling efficient transmission of neural signals. Its transparency further extends its utility, facilitating compatibility with various imaging techniques and optogenetics. This paper showcases noteworthy endeavors in utilizing graphene in its pure form and as composites to create and deploy implantable devices tailored for neural recordings and stimulations. It underscores the potential for significant advancements in this field. Furthermore, this paper delves into prospective avenues for refining existing graphene-based electrodes, enhancing their suitability for neural recording applications in in vitro and in vivo settings. These future steps promise to revolutionize further our capacity to understand and interact with the neural research landscape.

Antioxidant Dimethyl Fumarate Temporarily but Not Chronically Improves Intracortical Microelectrode Performance

Intracortical microelectrode arrays (MEAs) can be used in a range of applications, from basic neuroscience research to providing an intimate interface with the brain as part of a brain-computer interface (BCI) system aimed at restoring function for people living with neurological disorders or injuries. Unfortunately, MEAs tend to fail prematurely, leading to a loss in functionality for many applications. An important contributing factor in MEA failure is oxidative stress resulting from chronically inflammatory-activated microglia and macrophages releasing reactive oxygen species (ROS) around the implant site. Antioxidants offer a means for mitigating oxidative stress and improving tissue health and MEA performance. Here, we investigate using the clinically available antioxidant dimethyl fumarate (DMF) to reduce the neuroinflammatory response and improve MEA performance in a rat MEA model. Daily treatment of DMF for 16 weeks resulted in a significant improvement in the recording capabilities of MEA devices during the sub-chronic (Weeks 5-11) phase (42% active electrode yield vs. 35% for control). However, these sub-chronic improvements were lost in the chronic implantation phase, as a more exacerbated neuroinflammatory response occurs in DMF-treated animals by 16 weeks post-implantation. Yet, neuroinflammation was indiscriminate between treatment and control groups during the sub-chronic phase. Although worse for chronic use, a temporary improvement (<12 weeks) in MEA performance is meaningful. Providing short-term improvement to MEA devices using DMF can allow for improved use for limited-duration studies. Further efforts should be taken to explore the mechanism behind a worsened neuroinflammatory response at the 16-week time point for DMF-treated animals and assess its usefulness for specific applications.

Platelets and hemostatic proteins are co-localized with chronic neuroinflammation surrounding implanted intracortical microelectrodes

Intracortical microelectrodes induce vascular injury upon insertion into the cortex. As blood vessels rupture, blood proteins and blood-derived cells (including platelets) are introduced into the 'immune privileged' brain tissues at higher-than-normal levels, passing through the damaged blood-brain barrier. Blood proteins adhere to implant surfaces, increasing the likelihood of cellular recognition leading to activation of immune and inflammatory cells. Persistent neuroinflammation is a major contributing factor to declining microelectrode recording performance. We investigated the spatial and temporal relationship of blood proteins fibrinogen and von Willebrand Factor (vWF), platelets, and type IV collagen, in relation to glial scarring markers for microglia and astrocytes following implantation of non-functional multi-shank silicon microelectrode probes into rats. Together with type IV collagen, fibrinogen and vWF augment platelet recruitment, activation, and aggregation. Our main results indicate blood proteins participating in hemostasis (fibrinogen and vWF) persisted at the microelectrode interface for up to 8-weeks after implantation. Further, type IV collagen and platelets surrounded the probe interface with similar spatial and temporal trends as vWF and fibrinogen. In addition to prolonged blood-brain barrier instability, specific blood and extracellular matrix proteins may play a role in promoting the inflammatory activation of platelets and recruitment to the microelectrode interface. STATEMENT OF SIGNIFICANCE: Implanted microelectrodes have substantial potential for restoring function to people with paralysis and amputation by providing signals that feed into natural control algorithms that drive prosthetic devices. Unfortunately, these microelectrodes do not display robust performance over time. Persistent neuroinflammation is widely thought to be a primary contributor to the devices' progressive decline in performance. Our manuscript reports on the highly local and persistent accumulation of platelets and hemostatic blood proteins around the microelectrode interface of brain implants. To our knowledge neuroinflammation driven by cellular and non-cellular responses associated with hemostasis and coagulation has not been rigorously quantified elsewhere. Our findings identify potential targets for therapeutic intervention and a better understanding of the driving mechanisms to neuroinflammation in the brain.

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.

Toward a New Generation of Bio-Scaffolds for Neural Tissue Engineering: Challenges and Perspectives

Neural tissue engineering presents a compelling technological breakthrough in restoring brain function, holding immense promise. However, the quest to develop implantable scaffolds for neural culture that fulfill all necessary criteria poses a remarkable challenge for material science. These materials must possess a host of desirable characteristics, including support for cellular survival, proliferation, and neuronal migration and the minimization of inflammatory responses. Moreover, they should facilitate electrochemical cell communication, display mechanical properties akin to the brain, emulate the intricate architecture of the extracellular matrix, and ideally allow the controlled release of substances. This comprehensive review delves into the primary requisites, limitations, and prospective avenues for scaffold design in brain tissue engineering. By offering a panoramic overview, our work aims to serve as an essential resource, guiding the creation of materials endowed with bio-mimetic properties, ultimately revolutionizing the treatment of neurological disorders by developing brain-implantable scaffolds.

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.

Fabrication Methods and Chronic In Vivo Validation of Mechanically Adaptive Microfluidic Intracortical Devices

Intracortical neural probes are both a powerful tool in basic neuroscience studies of brain function and a critical component of brain computer interfaces (BCIs) designed to restore function to paralyzed patients. Intracortical neural probes can be used both to detect neural activity at single unit resolution and to stimulate small populations of neurons with high resolution. Unfortunately, intracortical neural probes tend to fail at chronic timepoints in large part due to the neuroinflammatory response that follows implantation and persistent dwelling in the cortex. Many promising approaches are under development to circumvent the inflammatory response, including the development of less inflammatory materials/device designs and the delivery of antioxidant or anti-inflammatory therapies. Here, we report on our recent efforts to integrate the neuroprotective effects of both a dynamically softening polymer substrate designed to minimize tissue strain and localized drug delivery at the intracortical neural probe/tissue interface through the incorporation of microfluidic channels within the probe. The fabrication process and device design were both optimized with respect to the resulting device mechanical properties, stability, and microfluidic functionality. The optimized devices were successfully able to deliver an antioxidant solution throughout a six-week in vivo rat study. Histological data indicated that a multi-outlet design was most effective at reducing markers of inflammation. The ability to reduce inflammation through a combined approach of drug delivery and soft materials as a platform technology allows future studies to explore additional therapeutics to further enhance intracortical neural probes performance and longevity for clinical applications.

Translational opportunities and challenges of invasive electrodes for neural interfaces

Invasive brain-machine interfaces can restore motor, sensory and cognitive functions. However, their clinical adoption has been hindered by the surgical risk of implantation and by suboptimal long-term reliability. In this Review, we highlight the opportunities and challenges of invasive technology for clinically relevant electrophysiology. Specifically, we discuss the characteristics of neural probes that are most likely to facilitate the clinical translation of invasive neural interfaces, describe the neural signals that can be acquired or produced by intracranial electrodes, the abiotic and biotic factors that contribute to their failure, and emerging neural-interface architectures.

Significantly reduced inflammatory foreign-body-response to neuroimplants and improved recording performance in young compared to adult rats

The multicellular inflammatory encapsulation of implanted intracortical multielectrode arrays (MEA) is associated with severe deterioration of their field potentials' (FP) recording performance, which thus limits the use of brain implants in basic research and clinical applications. Therefore, extensive efforts have been made to identify the conditions in which the inflammatory foreign body response (FBR) is alleviated, or to develop methods to mitigate the formation of the inflammatory barrier. Here, for the first time, we show that (1) in young rats (74±8 gr, 4 weeks old at the onset of the experiments), cortical tissue recovery following MEA implantation proceeds with ameliorated inflammatory scar as compared to adult rats (242 ± 18 gr, 9 weeks old at the experimental onset); (2) in contrast to adult rats in which the Colony Stimulating factor 1 Receptor (CSF1R) antagonist chow eliminated ∼95% of the cortical microglia but not microglia adhering to the implant surfaces, in young rats the microglia adhering to the implant were eliminated along with the parenchymal microglia population. The removal of microglia adhering to the implant surfaces was correlated with improved recording performance by in-house fabricated Perforated Polyimide MEA Platforms (PPMP). These results support the hypothesis that microglia adhering to the surface of the electrodes, rather than the multicellular inflammatory scar, is the major underlying mechanism that deteriorates implant recording performance, and that young rats provide an advantageous model to study months-long, multisite electrophysiology in freely behaving rats. STATEMENT OF SIGNIFICANCE: Multisite electrophysiological recordings and stimulation devices play central roles in basic brain research and medical applications. The insertion of multielectrode-array platforms into the brain's parenchyma unavoidably injures the tissue, and initiates a multicellular inflammatory cascade culminating in the formation of an encapsulating scar tissue (the foreign body response-FBR). The dominant view, which directs most current research efforts to mitigate the FBR, holds that the FBR is the major hurdle to effective electrophysiological use of neuroprobes. By contrast, this report demonstrates that microglia adhering to the surface of a neuroimplants, rather than the multicellular FBR, underlie the performance deterioration of neuroimplants. These findings pave the way to the development of novel and focused strategies to overcome the functional deterioration of neuroimplants.

Turning tissues into conducting matter

An electrically conducting soft polymer is synthesized within living tissue.

Development of a Slow-Degrading Polymerized Curcumin Coating for Intracortical Microelectrodes

Intracortical microelectrodes are used with brain-computer interfaces to restore lost limb function following nervous system injury. While promising, recording ability of intracortical microelectrodes diminishes over time due, in part, to neuroinflammation. As curcumin has demonstrated neuroprotection through anti-inflammatory activity, we fabricated a 300 nm-thick intracortical microelectrode coating consisting of a polyurethane copolymer of curcumin and polyethylene glycol (PEG), denoted as poly(curcumin-PEG1000 carbamate) (PCPC). The uniform PCPC coating reduced silicon wafer hardness by two orders of magnitude and readily absorbed water within minutes, demonstrating that the coating is soft and hydrophilic in nature. Using an in vitro release model, curcumin eluted from the PCPC coating into the supernatant over 1 week; the majority of the coating was intact after an 8-week incubation in buffer, demonstrating potential for longer term curcumin release and softness. Assessing the efficacy of PCPC within a rat intracortical microelectrode model in vivo, there were no significant differences in tissue inflammation, scarring, neuron viability, and myelin damage between the uncoated and PCPC-coated probes. As the first study to implant nonfunctional probes with a polymerized curcumin coating, we have demonstrated the biocompatibility of a PCPC coating and presented a starting point in the design of poly(pro-curcumin) polymers as coating materials for intracortical electrodes.

Conformable neural interface based on off-stoichiometry thiol-ene-epoxy thermosets

Off-stoichiometry thiol-ene-epoxy (OSTE+) thermosets show low permeability to gases and little absorption of dissolved molecules, allow direct low-temperature dry bonding without surface treatments, have a low Young's modulus, and can be manufactured via UV polymerisation. For these reasons, OSTE+ thermosets have recently gained attention for the rapid prototyping of microfluidic chips. Moreover, their compatibility with standard clean-room processes and outstanding mechanical properties make OSTE+ an excellent candidate as a novel material for neural implants. Here we exploit OSTE+ to manufacture a conformable multilayer micro-electrocorticography array with 16 platinum electrodes coated with platinum black. The mechanical properties allow conformability to curved surfaces such as the brain. The low permeability and strong adhesion between layers improve the stability of the device. Acute experiments in mice show the multimodal capacity of the array to record and stimulate the neural tissue by smoothly conforming to the mouse cortex. Devices are not cytotoxic, and immunohistochemistry stainings reveal only modest foreign body reaction after two and six weeks of chronic implantation. This work introduces OSTE+ as a promising material for implantable neural interfaces.

Manufacturing Processes of Implantable Microelectrode Array for In Vivo Neural Electrophysiological Recordings and Stimulation: A State-Of-the-Art Review

Electrophysiological recording and stimulation of neuron activities are important for us to understand the function and dysfunction of the nervous system. To record/stimulate neuron activities as voltage fluctuation extracellularly, microelectrode array (MEA) implants are a promising tool to provide high temporal and spatial resolution for neuroscience studies and medical treatments. The design configuration and recording capabilities of the MEAs have evolved dramatically since their invention and manufacturing process development has been a key driving force for such advancement. Over the past decade, since the White House Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative launched in 2013, advanced manufacturing processes have enabled advanced MEAs with increased channel count and density, access to more brain areas, more reliable chronic performance, as well as minimal invasiveness and tissue reaction. In this state-of-the-art review paper, three major types of electrophysiological recording MEAs widely used nowadays, namely, microwire-based, silicon-based, and flexible MEAs are introduced and discussed. Conventional design and manufacturing processes and materials used for each type are elaborated, followed by a review of further development and recent advances in manufacturing technologies and the enabling new designs and capabilities. The review concludes with a discussion on potential future directions of manufacturing process development to enable the long-term goal of large-scale high-density brain-wide chronic recordings in freely moving animals.

Neuron matters: neuromodulation with electromagnetic stimulation must consider neurons as dynamic identities

Neuromodulation with electromagnetic stimulation is widely used for the control of abnormal neural activity, and has been proven to be a valuable alternative to pharmacological tools for the treatment of many neurological diseases. Tremendous efforts have been focused on the design of the stimulation apparatus (i.e., electrodes and magnetic coils) that delivers the electric current to the neural tissue, and the optimization of the stimulation parameters. Less attention has been given to the complicated, dynamic properties of the neurons, and their context-dependent impact on the stimulation effects. This review focuses on the neuronal factors that influence the outcomes of electromagnetic stimulation in neuromodulation. Evidence from multiple levels (tissue, cellular, and single ion channel) are reviewed. Properties of the neural elements and their dynamic changes play a significant role in the outcome of electromagnetic stimulation. This angle of understanding yields a comprehensive perspective of neural activity during electrical neuromodulation, and provides insights in the design and development of novel stimulation technology.

In vivo spatiotemporal dynamics of astrocyte reactivity following neural electrode implantation

Brain computer interfaces (BCIs), including penetrating microelectrode arrays, enable both recording and stimulation of neural cells. However, device implantation inevitably causes injury to brain tissue and induces a foreign body response, leading to reduced recording performance and stimulation efficacy. Astrocytes in the healthy brain play multiple roles including regulating energy metabolism, homeostatic balance, transmission of neural signals, and neurovascular coupling. Following an insult to the brain, they are activated and gather around the site of injury. These reactive astrocytes have been regarded as one of the main contributors to the formation of a glial scar which affects the performance of microelectrode arrays. This study investigates the dynamics of astrocytes within the first 2 weeks after implantation of an intracortical microelectrode into the mouse brain using two-photon microscopy. From our observation astrocytes are highly dynamic during this period, exhibiting patterns of process extension, soma migration, morphological activation, and device encapsulation that are spatiotemporally distinct from other glial cells, such as microglia or oligodendrocyte precursor cells. This detailed characterization of astrocyte reactivity will help to better understand the tissue response to intracortical devices and lead to the development of more effective intervention strategies to improve the functional performance of neural interfacing technology.

The effect of A1 and A2 reactive astrocyte expression on hydrocephalus shunt failure

Background

The composition of tissue obstructing neuroprosthetic devices is largely composed of inflammatory cells with a significant astrocyte component. In a first-of-its-kind study, we profile the astrocyte phenotypes present on hydrocephalus shunts.

Methods

qPCR and RNA in-situ hybridization were used to quantify pro-inflammatory (A1) and anti-inflammatory (A2) reactive astrocyte phenotypes by analyzing C3 and EMP1 genes, respectively. Additionally, CSF cytokine levels were quantified using ELISA. In an in vitro model of astrocyte growth on shunts, different cytokines were used to prevent the activation of resting astrocytes into the A1 and A2 phenotypes. Obstructed and non-obstructed shunts were characterized based on the degree of actual tissue blockage on the shunt surface instead of clinical diagnosis.

Results

The results showed a heterogeneous population of A1 and A2 reactive astrocytes on the shunts with obstructed shunts having a significantly higher proportion of A2 astrocytes compared to non-obstructed shunts. In addition, the pro-A2 cytokine IL-6 inducing proliferation of astrocytes was found at higher concentrations among CSF from obstructed samples. Consequently, in the in vitro model of astrocyte growth on shunts, cytokine neutralizing antibodies were used to prevent activation of resting astrocytes into the A1 and A2 phenotypes which resulted in a significant reduction in both A1 and A2 growth.

Conclusions

Therefore, targeting cytokines involved with astrocyte A1 and A2 activation is a promising intervention aimed to prevent shunt obstruction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00367-3.

Stable, long-term single-neuronal recording from the rat spinal cord with flexible carbon nanotube fiber electrodes

Objective. Flexible implantable electrodes enable months-long stable recording of single-unit signals from rat brains. Despite extensive efforts in the development of flexible probes for brain recording, thus far there are no conclusions on their application in long-term single neuronal recording from the spinal cord which is more mechanically active. To this end, we realized the chronic recording of single-unit signals from the spinal cord of freely-moving rats using flexible carbon nanotube fiber (CNTF) electrodes. Approach. We developed flexible CNTF electrodes for intraspinal recording. Continuous in vivo impedance monitoring and histology studies were conducted to explore the critical factors determining the longevity of the recording, as well as to illustrate the evolution of the electrode-tissue interface. Gait analysis were performed to evaluate the biosafety of the chronic intraspinal implantation of the CNTF electrodes. Main results. By increasing the insulation thickness of the CNTF electrodes, single-unit signals were continuously recorded from the spinal cord of freely-moving rats without electrode repositioning for 3-4 months. Single neuronal and local field potential activities in response to somatic mechanical stimulation were successfully recorded from the spinal dorsal horns. Histological data demonstrated the ability of the CNTF microelectrodes to form an improved intraspinal interfaces with greatly reduced gliosis compared to their stiff metal counterparts. Continuous impedance monitoring suggested that the longevity of the intraspinal recording with CNTF electrodes was determined by the insulation durability. Gait analysis showed that the chronic presence of the CNTF electrodes caused no noticeable locomotor deficits in rats. Significance. It was found that the chronic recording from the spinal cord faces more stringent requirements on the electrode structural durability than recording from the brain. The stable, long-term intraspinal recording provides unique capabilities for studying the physiological functions of the spinal cord relating to motor, sensation, and autonomic control in both health and disease.

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.

Neuroinflammatory Gene Expression Analysis Reveals Pathways of Interest as Potential Targets to Improve the Recording Performance of Intracortical Microelectrodes

Intracortical microelectrodes are a critical component of brain-machine interface (BMI) systems. The recording performance of intracortical microelectrodes used for both basic neuroscience research and clinical applications of BMIs decreases over time, limiting the utility of the devices. The neuroinflammatory response to the microelectrode has been identified as a significant contributing factor to its performance. Traditionally, pathological assessment has been limited to a dozen or so known neuroinflammatory proteins, and only a few groups have begun to explore changes in gene expression following microelectrode implantation. Our initial characterization of gene expression profiles of the neuroinflammatory response to mice implanted with non-functional intracortical probes revealed many upregulated genes that could inform future therapeutic targets. Emphasis was placed on the most significant gene expression changes and genes involved in multiple innate immune sets, including Cd14, C3, Itgam, and Irak4. In previous studies, inhibition of Cluster of Differentiation 14 (Cd14) improved microelectrode performance for up to two weeks after electrode implantation, suggesting CD14 can be explored as a potential therapeutic target. However, all measures of improvements in signal quality and electrode performance lost statistical significance after two weeks. Therefore, the current study investigated the expression of genes in the neuroinflammatory pathway at the tissue-microelectrode interface in Cd14-/- mice to understand better how Cd14 inhibition was connected to temporary improvements in recording quality over the initial 2-weeks post-surgery, allowing for the identification of potential co-therapeutic targets that may work synergistically with or after CD14 inhibition to improve microelectrode performance.

Hydrophilic Micro- and Macroelectrodes with Antibiofouling Properties for Biomedical Applications

Implantable neural electrodes are generally used to record the electrical activity of neurons and to stimulate neurons in the nervous system. Biofouling triggered by inflammatory responses can dramatically affect the performance of neural electrodes, resulting in decreased signal sensitivity and consistency over time. Thus, long-term clinical applications require electrically conducting electrode materials with reduced dimensions, high flexibility, and antibiofouling properties that can reduce the degree of inflammatory reactions and increase the lifetime of neural electrodes. Carbon nanotubes (CNTs) are well known to form flexible assemblies such as CNT fibers. Herein, we report the covalent functionalization of predefined CNT fiber and film surfaces with hydrophilic, antibiofouling phosphorylcholine (PC) molecules. The electrochemical and spectroscopic characteristics, impedance properties, hydrophilicity, and in vitro antifouling nature of the functionalized CNT surfaces were evaluated. The hydrophilicity of the functionalized CNT films was demonstrated by a decrease in the static contact angle from 134.4° ± 3.9° before to 15.7° ± 1.5° after one and fully wetting after three functionalization cycles, respectively. In addition, the extent of protein absorption on the functionalized CNT films was significantly lower than that on the nonfunctionalized CNT film. Surprisingly, the faradic charge-transfer properties and impedance of the CNT assemblies were preserved after functionalization with PC molecules. These functionalized CNT assemblies are promising for the development of low-impedance neural electrodes with higher hydrophilicity and protein-fouling resistance to inhibit inflammatory responses.

Mechanically and electrically biocompatible hydrogel ionotronic fibers for fabricating structurally stable implants and enabling noncontact physioelectrical modulation

Narrowing the mechanical and electrical mismatch between tissue and implantable microelectronics is essential for reducing immune responses and modulating physioelectrical signals. Nevertheless, the design of such implantable microelectronics remains a challenge due to the limited availability of suitable materials. Here, the fabrication of an electrically and mechanically biocompatible alginate hydrogel ionotronic fiber (AHIF) is reported, which is constructed by combing ionic chelation-assisted wet-spinning and mechanical training. The synergistic effects of these two processes allow the alginate to form a highly-oriented nanofibril and molecular network, with a hierarchical structure highly similar to that of natural fibers. These favourable structural features endow AHIF with tissue-mimicking mechanical characteristics, such as self-stiffening and soft tissue-like mechanical properties. In addition, tissue-like chemical components, i.e., biomacromolecules, Ca²⁺ ions, and water, endow AHIF with properties including biocompatibility and tissue-matching conductivity. These advantages bring light to the application of AHIFs in electrically-conductive implantable devices. As a prototype, an AHIF is designed to perform physioelectrical modulation through noncontact electromagnetic induction. Through experimental and machine learning optimizations, physioelectrical-like signals generated by the AHIF are used to identify the geometry and tension state of the implanted device in the body. Such an intelligent AHIF system has promising application prospects in bioelectronics, IntelliSense, and human-machine interactions.

Medical Applications of Porous Biomaterials: Features of Porosity and Tissue-Specific Implications for Biocompatibility

Porosity is an important material feature commonly employed in implants and tissue scaffolds. The presence of material voids permits the infiltration of cells, mechanical compliance, and outward diffusion of pharmaceutical agents. Various studies have confirmed that porosity indeed promotes favorable tissue responses, including minimal fibrous encapsulation during the foreign body reaction (FBR). However, increased biofilm formation and calcification is also described to arise due to biomaterial porosity. Additionally, the relevance of host responses like the FBR, infection, calcification, and thrombosis are dependent on tissue location and specific tissue microenvironment. In this review, the features of porous materials and the implications of porosity in the context of medical devices is discussed. Common methods to create porous materials are also discussed, as well as the parameters that are used to tune pore features. Responses toward porous biomaterials are also reviewed, including the various stages of the FBR, hemocompatibility, biofilm formation, and calcification. Finally, these host responses are considered in tissue specific locations including the subcutis, bone, cardiovascular system, brain, eye, and female reproductive tract. The effects of porosity across the various tissues of the body is highlighted and the need to consider the tissue context when engineering biomaterials is emphasized.

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.

Neurotechnological Approaches to the Diagnosis and Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common cause of dementia in the elderly, clinically defined by progressive cognitive decline and pathologically, by brain atrophy, neuroinflammation, and accumulation of extracellular amyloid plaques and intracellular neurofibrillary tangles. Neurotechnological approaches, including optogenetics and deep brain stimulation, have exploded as new tools for not only the study of the brain but also for application in the treatment of neurological diseases. Here, we review the current state of AD therapeutics and recent advancements in both invasive and non-invasive neurotechnologies that can be used to ameliorate AD pathology, including neurostimulation via optogenetics, photobiomodulation, electrical stimulation, ultrasound stimulation, and magnetic neurostimulation, as well as nanotechnologies employing nanovectors, magnetic nanoparticles, and quantum dots. We also discuss the current challenges in developing these neurotechnological tools and the prospects for implementing them in the treatment of AD and other neurodegenerative diseases.

Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications

Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells.

Characterization of Active Electrode Yield for Intracortical Arrays: Awake versus Anesthesia

Intracortical microelectrode arrays are used for recording neural signals at single-unit resolution and are promising tools for studying brain function and developing neuroprosthetics. Research is being done to increase the chronic performance and reliability of these probes, which tend to decrease or fail within several months of implantation. Although recording paradigms vary, studies focused on assessing the reliability and performance of these devices often perform recordings under anesthesia. However, anesthetics—such as isoflurane—are known to alter neural activity and electrophysiologic function. Therefore, we compared the neural recording performance under anesthesia (2% isoflurane) followed by awake conditions for probes implanted in the motor cortex of both male and female Sprague-Dawley rats. While the single-unit spike rate was significantly higher by almost 600% under awake compared to anesthetized conditions, we found no difference in the active electrode yield between the two conditions two weeks after surgery. Additionally, the signal-to-noise ratio was greater under anesthesia due to the noise levels being nearly 50% greater in awake recordings, even though there was a 14% increase in the peak-to-peak voltage of distinguished single units when awake. We observe that these findings are similar for chronic time points as well. Our observations indicate that either anesthetized or awake recordings are acceptable for studies assessing the chronic reliability and performance of intracortical microelectrode arrays.