Ultra-low Impedance Graphene Microelectrodes with High Optical Transparency for Simultaneous Deep 2-photon Imaging in Transgenic Mice

Transparent MXene Microelectrode Arrays for Multimodal Mapping of Neural Dynamics

Transparent microelectrode arrays have proven useful in neural sensing, offering a clear interface for monitoring brain activity without compromising high spatial and temporal resolution. The current landscape of transparent electrode technology faces challenges in developing durable, highly transparent electrodes while maintaining low interface impedance and prioritizing scalable processing and fabrication methods. To address these limitations, we introduce artifact-resistant transparent MXene microelectrode arrays optimized for high spatiotemporal resolution recording of neural activity. With 60% transmittance at 550 nm, these arrays enable simultaneous imaging and electrophysiology for multimodal neural mapping. Electrochemical characterization shows low impedance of 563 ± 99 kΩ at 1 kHz and a charge storage capacity of 58 mC cm⁻² without chemical doping. In vivo experiments in rodent models demonstrate the transparent arrays' functionality and performance. In a rodent model of chemically-induced epileptiform activity, we tracked ictal wavefronts via calcium imaging while simultaneously recording seizure onset. In the rat barrel cortex, we recorded multi-unit activity across cortical depths, showing the feasibility of recording high-frequency electrophysiological activity. The transparency and optical absorption properties of Ti₃C₂Tx MXene microelectrodes enable high-quality recordings and simultaneous light-based stimulation and imaging without contamination from light-induced artifacts.

Linking brain activity across scales with simultaneous opto- and electrophysiology

The brain enables adaptive behavior via the dynamic coordination of diverse neuronal signals across spatial and temporal scales: from fast action potential patterns in microcircuits to slower patterns of distributed activity in brain-wide networks. Understanding principles of multiscale dynamics requires simultaneous monitoring of signals in multiple, distributed network nodes. Combining optical and electrical recordings of brain activity is promising for collecting data across multiple scales and can reveal aspects of coordinated dynamics invisible to standard, single-modality approaches. We review recent progress in combining opto- and electrophysiology, focusing on mouse studies that shed new light on the function of single neurons by embedding their activity in the context of brain-wide activity patterns. Optical and electrical readouts can be tailored to desired scales to tackle specific questions. For example, fast dynamics in single cells or local populations recorded with multi-electrode arrays can be related to simultaneously acquired optical signals that report activity in specified subpopulations of neurons, in non-neuronal cells, or in neuromodulatory pathways. Conversely, two-photon imaging can be used to densely monitor activity in local circuits while sampling electrical activity in distant brain areas at the same time. The refinement of combined approaches will continue to reveal previously inaccessible and under-appreciated aspects of coordinated brain activity.

Integrated Microprism and Microelectrode Array for Simultaneous Electrophysiology and Two-Photon Imaging across All Cortical Layers

Cerebral neural electronics play a crucial role in neuroscience research with increasing translational applications such as brain-computer interfaces for sensory input and motor output restoration. While widely utilized for decades, the understanding of the cellular mechanisms underlying this technology remains limited. Although two-photon microscopy (TPM) has shown great promise in imaging superficial neural electrodes, its application to deep-penetrating electrodes is technically difficult. Here, a novel device integrating transparent microelectrode arrays with glass microprisms, enabling electrophysiology recording and stimulation alongside TPM imaging across all cortical layers in a vertical plane, is introduced. Tested in Thy1-GCaMP6 mice for over 4 months, the integrated device demonstrates the capability for multisite electrophysiological recording/stimulation and simultaneous TPM calcium imaging. As a proof of concept, the impact of microstimulation amplitude, frequency, and depth on neural activation patterns is investigated using the setup. With future improvements in material stability and single unit yield, this multimodal tool greatly expands integrated electrophysiology and optical imaging from the superficial brain to the entire cortical column, opening new avenues for neuroscience research and neurotechnology development.

Recent Advances in Transparent Electrodes and Their Multimodal Sensing Applications

This review examines the recent advancements in transparent electrodes and their crucial role in multimodal sensing technologies. Transparent electrodes, notable for their optical transparency and electrical conductivity, are revolutionizing sensors by enabling the simultaneous detection of diverse physical, chemical, and biological signals. Materials like graphene, carbon nanotubes, and conductive polymers, which offer a balance between optical transparency, electrical conductivity, and mechanical flexibility, are at the forefront of this development. These electrodes are integral in various applications, from healthcare to solar cell technologies, enhancing sensor performance in complex environments. The paper addresses challenges in applying these electrodes, such as the need for mechanical flexibility, high optoelectronic performance, and biocompatibility. It explores new materials and innovative techniques to overcome these hurdles, aiming to broaden the capabilities of multimodal sensing devices. The review provides a comparative analysis of different transparent electrode materials, discussing their applications and the ongoing development of novel electrode systems for multimodal sensing. This exploration offers insights into future advancements in transparent electrodes, highlighting their transformative potential in bioelectronics and multimodal sensing technologies.

High-density transparent graphene arrays for predicting cellular calcium activity at depth from surface potential recordings

Optically transparent neural microelectrodes have facilitated simultaneous electrophysiological recordings from the brain surface with the optical imaging and stimulation of neural activity. A remaining challenge is to scale down the electrode dimensions to the single-cell size and increase the density to record neural activity with high spatial resolution across large areas to capture nonlinear neural dynamics. Here we developed transparent graphene microelectrodes with ultrasmall openings and a large, transparent recording area without any gold extensions in the field of view with high-density microelectrode arrays up to 256 channels. We used platinum nanoparticles to overcome the quantum capacitance limit of graphene and to scale down the microelectrode diameter to 20 µm. An interlayer-doped double-layer graphene was introduced to prevent open-circuit failures. We conducted multimodal experiments, combining the recordings of cortical potentials of microelectrode arrays with two-photon calcium imaging of the mouse visual cortex. Our results revealed that visually evoked responses are spatially localized for high-frequency bands, particularly for the multiunit activity band. The multiunit activity power was found to be correlated with cellular calcium activity. Leveraging this, we employed dimensionality reduction techniques and neural networks to demonstrate that single-cell and average calcium activities can be decoded from surface potentials recorded by high-density transparent graphene arrays.

The potential of graphene coatings as neural interfaces

Recent advances in nanotechnology design and fabrication have shaped the landscape for the development of ideal cell interfaces based on biomaterials. A holistic evaluation of the requirements for a cell interface is a highly complex task. Biocompatibility is a crucial requirement which is affected by the interface's properties, including elemental composition, morphology, and surface chemistry. This review explores the current state-of-the-art on graphene coatings produced by chemical vapor deposition (CVD) and applied as neural interfaces, detailing the key properties required to design an interface capable of physiologically interacting with neural cells. The interfaces are classified into substrates and scaffolds to differentiate the planar and three-dimensional environments where the cells can adhere and proliferate. The role of specific features such as mechanical properties, porosity and wettability are investigated. We further report on the specific brain-interface applications where CVD graphene paved the way to revolutionary advances in biomedicine. Future studies on the long-term effects of graphene-based materials in vivo will unlock even more potentially disruptive neuro-applications.

Surface modification of multilayer graphene electrodes by local printing of platinum nanoparticles using spark ablation for neural interfacing

In this paper, we present the surface modification of multilayer graphene electrodes with platinum (Pt) nanoparticles (NPs) using spark ablation. This method yields an individually selective local printing of NPs on an electrode surface at room temperature in a dry process. NP printing is performed as a post-process step to enhance the electrochemical characteristics of graphene electrodes. The NP-printed electrode shows significant improvements in impedance, charge storage capacity (CSC), and charge injection capacity (CIC), versus the equivalent electrodes without NPs. Specifically, electrodes with 40% NP surface density demonstrate 4.5 times lower impedance, 15 times higher CSC, and 4 times better CIC. Electrochemical stability, assessed via continuous cyclic voltammetry (CV) and voltage transient (VT) tests, indicated minimal deviations from the initial performance, while mechanical stability, assessed via ultrasonic vibration, is also improved after the NP printing. Importantly, NP surface densities up to 40% maintain the electrode optical transparency required for compatibility with optical imaging and optogenetics. These results demonstrate selective NP deposition and local modification of electrochemical properties in graphene electrodes for the first time, enabling the cohabitation of graphene electrodes with different electrochemical and optical characteristics on the same substrate for neural interfacing.

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.

Illuminating the Brain: Advances and Perspectives in Optoelectronics for Neural Activity Monitoring and Modulation

Optogenetic modulation of brain neural activity that combines optical and electrical modes in a unitary neural system has recently gained robust momentum. Controlling illumination spatial coverage, designing light-activated modulators, and developing wireless light delivery and data transmission are crucial for maximizing the use of optical neuromodulation. To this end, biocompatible electrodes with enhanced optoelectrical performance, device integration for multiplexed addressing, wireless transmission, and multimodal operation in soft systems have been developed. This review provides an outlook for uniformly illuminating large brain areas while spatiotemporally imaging the neural responses upon optoelectrical stimulation with little artifacts. Representative concepts and important breakthroughs, such as head-mounted illumination, multiple implanted optical fibers, and micro-light-delivery devices, are discussed. Examples of techniques that incorporate electrophysiological monitoring and optoelectrical stimulation are presented. Challenges and perspectives are posed for further research efforts toward high-density optoelectrical neural interface modulation, with the potential for nonpharmacological neurological disease treatments and wireless optoelectrical stimulation.

Graphene-based cardiac sensors and actuators

Graphene, a 2D carbon allotrope, is revolutionizing many biomedical applications due to its unique mechanical, electrical, thermal, and optical properties. When bioengineers realized that these properties could dramatically enhance the performance of cardiac sensors and actuators and may offer fundamentally novel technological capabilities, the field exploded with numerous studies developing new graphene-based systems and testing their limits. Here we will review the link between specific properties of graphene and mechanisms of action of cardiac sensors and actuators, analyze the performance of these systems from inaugural studies to the present, and offer future perspectives.

Recent developments in multifunctional neural probes for simultaneous neural recording and modulation

Neural probes are among the most widely applied tools for studying neural circuit functions and treating neurological disorders. Given the complexity of the nervous system, it is highly desirable to monitor and modulate neural activities simultaneously at the cellular scale. In this review, we provide an overview of recent developments in multifunctional neural probes that allow simultaneous neural activity recording and modulation through different modalities, including chemical, electrical, and optical stimulation. We will focus on the material and structural design of multifunctional neural probes and their interfaces with neural tissues. Finally, future challenges and prospects of multifunctional neural probes will be discussed.

Multifunctional 3D-printed bioceramic scaffolds: Recent strategies for osteosarcoma treatment

Osteosarcoma is the most prevalent bone malignant tumor in children and teenagers. The bone defect, recurrence, and metastasis after surgery severely affect the life quality of patients. Clinically, bone grafts are implanted. Primary bioceramic scaffolds show a monomodal osteogenesis function. With the advances in three-dimensional printing technology and materials science, while maintaining the osteogenesis ability, scaffolds become more patient-specific and obtain additional anti-tumor ability with functional agents being loaded. Anti-tumor therapies include photothermal, magnetothermal, old and novel chemo-, gas, and photodynamic therapy. These strategies kill tumors through novel mechanisms to treat refractory osteosarcoma due to drug resistance, and some have shown the potential to reverse drug resistance and inhibit metastasis. Therefore, multifunctional three-dimensional printed bioceramic scaffolds hold excellent promise for osteosarcoma treatments. To better understand, we review the background of osteosarcoma, primary 3D-printed bioceramic scaffolds, and different therapies and have a prospect for the future.

Transparent neural interfaces: challenges and solutions of microengineered multimodal implants designed to measure intact neuronal populations using high-resolution electrophysiology and microscopy simultaneously

The aim of this review is to present a comprehensive overview of the feasibility of using transparent neural interfaces in multimodal in vivo experiments on the central nervous system. Multimodal electrophysiological and neuroimaging approaches hold great potential for revealing the anatomical and functional connectivity of neuronal ensembles in the intact brain. Multimodal approaches are less time-consuming and require fewer experimental animals as researchers obtain denser, complex data during the combined experiments. Creating devices that provide high-resolution, artifact-free neural recordings while facilitating the interrogation or stimulation of underlying anatomical features is currently one of the greatest challenges in the field of neuroengineering. There are numerous articles highlighting the trade-offs between the design and development of transparent neural interfaces; however, a comprehensive overview of the efforts in material science and technology has not been reported. Our present work fills this gap in knowledge by introducing the latest micro- and nanoengineered solutions for fabricating substrate and conductive components. Here, the limitations and improvements in electrical, optical, and mechanical properties, the stability and longevity of the integrated features, and biocompatibility during in vivo use are discussed.

In Vivo Penetrating Microelectrodes for Brain Electrophysiology

In recent decades, microelectrodes have been widely used in neuroscience to understand the mechanisms behind brain functions, as well as the relationship between neural activity and behavior, perception and cognition. However, the recording of neuronal activity over a long period of time is limited for various reasons. In this review, we briefly consider the types of penetrating chronic microelectrodes, as well as the conductive and insulating materials for microelectrode manufacturing. Additionally, we consider the effects of penetrating microelectrode implantation on brain tissue. In conclusion, we review recent advances in the field of in vivo microelectrodes.

E-Cannula reveals anatomical diversity in sharp-wave ripples as a driver for the recruitment of distinct hippocampal assemblies

The hippocampus plays a critical role in spatial navigation and episodic memory. However, research on in vivo hippocampal activity dynamics mostly relies on single modalities, such as electrical recordings or optical imaging, with respectively limited spatial and temporal resolution. Here, we develop the E-Cannula, integrating fully transparent graphene microelectrodes with imaging cannula, which enables simultaneous electrical recording and two-photon calcium imaging from the exact same neural populations across an anatomically extended region of the mouse hippocampal CA1 stably across several days. The large-scale multimodal recordings show that sharp wave ripples (SWRs) exhibit spatiotemporal wave patterns along multiple axes in two-dimensional (2D) space with different spatial extents and temporal propagation modes. Notably, distinct SWR wave patterns are associated with the selective recruitment of orthogonal CA1 cell assemblies. These results demonstrate the utility of the E-Cannula as a versatile neurotechnology with the potential for future integration with other optical components.

Polymer Skulls With Integrated Transparent Electrode Arrays for Cortex-Wide Opto-Electrophysiological Recordings

Electrophysiology and optical imaging provide complementary neural sensing capabilities - electrophysiological recordings have high temporal resolution, while optical imaging allows recording of genetically-defined populations at high spatial resolution. Combining these two modalities for simultaneous large-scale, multimodal sensing of neural activity across multiple brain regions can be very powerful. Here, transparent, inkjet-printed electrode arrays with outstanding optical and electrical properties are seamlessly integrated with morphologically conformant transparent polymer skulls. Implanted on transgenic mice expressing the Calcium (Ca2+ ) indicator GCaMP6f in excitatory neurons, these "eSee-Shells" provide a robust opto-electrophysiological interface for over 100 days. eSee-Shells enable simultaneous mesoscale Ca2+ imaging and electrocorticography (ECoG) acquisition from multiple brain regions covering 45 mm2 of cortex under anesthesia and in awake animals. The clarity and transparency of eSee-Shells allow recording single-cell Ca2+ signals directly below the electrodes and interconnects. Simultaneous multimodal measurement of cortical dynamics reveals changes in both ECoG and Ca2+ signals that depend on the behavioral state.

Strategies for interface issues and challenges of neural electrodes

Neural electrodes, as a bridge for bidirectional communication between the body and external devices, are crucial means for detecting and controlling nerve activity. The electrodes play a vital role in monitoring the state of neural systems or influencing it to treat disease or restore functions. To achieve high-resolution, safe and long-term stable nerve recording and stimulation, a neural electrode with excellent electrochemical performance (e.g., impedance, charge storage capacity, charge injection limit), and good biocompatibility and stability is required. Here, the charge transfer process in the tissues, the electrode-tissue interfaces and the electrode materials are discussed respectively. Subsequently, the latest research methods and strategies for improving the electrochemical performance and biocompatibility of neural electrodes are reviewed. Finally, the challenges in the development of neural electrodes are proposed. It is expected that the development of neural electrodes will offer new opportunities for the evolution of neural prosthesis, bioelectronic medicine, brain science, and so on.

Neurophotonic tools for microscopic measurements and manipulation: status report

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics' agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

Omnidirectional / Unidirectional Antireflection-Switchable Structures Inspired by Dragonfly Wings

Randomly arranged irregular inclined conical structure-covered dragonfly wings, distinguished from periodic conical structure-covered cicada wings, are with high optical transparency for wide viewing angles. Bioinspired by the antireflective structures, we develop a colloidal lithography approach for engineering randomly arranged irregular conical structures with shape memory polymer-based tips. The structures establish a gradual refractive index transition to suppresses optical reflection in the visible spectrum. By manipulating the configuration of structure tips through applying common solvent stimulations or contact pressures under ambient conditions, the resulting unidirectional antireflection and omnidirectional antireflection performances are able to be instantaneously and reversibly switched. The dependences of structure shape, structure inclination, structure arrangement, and structure composition on the switchable antireflection capability are also systematically investigated in this study.

Graphene and graphene-related materials as brain electrodes

Neural electrodes are used for acquiring neuron signals in brain-machine interfaces, and they are crucial for next-generation neuron engineering and related medical applications. Thus, developing flexible, stable and high-resolution neural electrodes will play an important role in stimulation, acquisition, recording and analysis of signals. Compared with traditional metallic electrodes, electrodes based on graphene and other two-dimensional materials have attracted wide attention in electrophysiological recording and stimulation due to their excellent physical properties such as unique flexibility, low resistance, and high optical transparency. In this review, we have reviewed the recent progress of electrodes based on graphene, graphene/polymer compounds and graphene-related materials for neuron signal recording, stimulation, and related optical signal coupling technology, which provides an outlook on the role of electrodes in the nanotechnology-neuron interface as well as medical diagnosis.

Emerging approaches for sensing and modulating neural activity enabled by nanocarbons and carbides

Devices that can record or modulate neural activity are essential tools in clinical diagnostics and monitoring, basic research, and consumer electronics. Realizing stable functional interfaces between manmade electronics and biological tissues is a longstanding challenge that requires device and material innovations to meet stringent safety and longevity requirements and to improve functionality. Compared to conventional materials, nanocarbons and carbides offer a number of specific advantages for neuroelectronics that can enable advances in functionality and performance. Here, we review the latest emerging trends in neuroelectronic interfaces based on nanocarbons and carbides, with a specific emphasis on technologies developed for use in vivo. We highlight specific applications where the ability to tune fundamental material properties at the nanoscale enables interfaces that can safely and precisely interact with neural circuits at unprecedented spatial and temporal scales, ranging from single synapses to the whole human body.

Decoding of cortex-wide brain activity from local recordings of neural potentials

Objective. Electrical recordings of neural activity from brain surface have been widely employed in basic neuroscience research and clinical practice for investigations of neural circuit functions, brain-computer interfaces, and treatments for neurological disorders. Traditionally, these surface potentials have been believed to mainly reflect local neural activity. It is not known how informative the locally recorded surface potentials are for the neural activities across multiple cortical regions.Approach. To investigate that, we perform simultaneous local electrical recording and wide-field calcium imaging in awake head-fixed mice. Using a recurrent neural network model, we try to decode the calcium fluorescence activity of multiple cortical regions from local electrical recordings.Main results. The mean activity of different cortical regions could be decoded from locally recorded surface potentials. Also, each frequency band of surface potentials differentially encodes activities from multiple cortical regions so that including all the frequency bands in the decoding model gives the highest decoding performance. Despite the close spacing between recording channels, surface potentials from different channels provide complementary information about the large-scale cortical activity and the decoding performance continues to improve as more channels are included. Finally, we demonstrate the successful decoding of whole dorsal cortex activity at pixel-level using locally recorded surface potentials.Significance. These results show that the locally recorded surface potentials indeed contain rich information of the large-scale neural activities, which could be further demixed to recover the neural activity across individual cortical regions. In the future, our cross-modality inference approach could be adapted to virtually reconstruct cortex-wide brain activity, greatly expanding the spatial reach of surface electrical recordings without increasing invasiveness. Furthermore, it could be used to facilitate imaging neural activity across the whole cortex in freely moving animals, without requirement of head-fixed microscopy configurations.

2D Materials for Skin-Mountable Electronic Devices

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Graphene-Based Electrode Materials for Neural Activity Detection

The neural electrode technique is a powerful tool for monitoring and regulating neural activity, which has a wide range of applications in basic neuroscience and the treatment of neurological diseases. Constructing a high-performance electrode-nerve interface is required for the long-term stable detection of neural signals by electrodes. However, conventional neural electrodes are mainly fabricated from rigid materials that do not match the mechanical properties of soft neural tissues, thus limiting the high-quality recording of neuroelectric signals. Meanwhile, graphene-based nanomaterials can form stable electrode-nerve interfaces due to their high conductivity, excellent flexibility, and biocompatibility. In this literature review, we describe various graphene-based electrodes and their potential application in neural activity detection. We also discuss the biological safety of graphene neural electrodes, related challenges, and their prospects.

Graphene on glassy carbon microelectrodes demonstrate long-term structural and functional stability in neurophysiological recording and stimulation

Objective.There is a growing interest in the use of carbon and its allotropes for microelectrodes in neural probes because of their inertness, long-term electrical and electrochemical stability, and versatility. Building on this interest, we introduce a new electrode material system consisting of an ultra-thin monoatomic layer of graphene (Gr) mechanically supported by a relatively thicker layer of glassy carbon (GC).Approach.Due to its high electrical conductivity and high double-layer capacitance, Gr has impressive electrical and electrochemical properties, two key properties that are useful for neural recording and stimulation applications. However, because of its two-dimensional nature, Gr exhibits a lack of stiffness in the transverse direction and hence almost non-existent flexural and out-of-plane rigidity that will severely limit its wider use. On the other hand, GC is one of carbon's important allotropes and consists of three-dimensional microstructures of Gr fragments with a natural molecular similarity to Gr. Further, GC has exceptional chemical inertness, good electrical properties, high electrochemical stability, purely capacitive charge injection, and fast surface electrokinetics coupled with lithography patternability. This makes GC an ideal candidate for addressing Gr's lack of out-of-plane rigidity through providing a matching sturdier and robust mechanical backing. Combining the strengths of these two allotropes of carbon, we introduce a new neural probe that consists of ∼1 nm thick layer of patterned Gr microelectrodes supported by another layer of 3-5μm thick patterned GC.Main results. We present the fabrication technology for the newGr on GC(graphene on glassy carbon) microelectrodes and the accompanying pattern transfer technology on flexible substrate and report on the bond between these two allotropes of carbon through FTIR, surface morphology through SEM, topography through atomic force microscopy, and microstructure imaging through scanning transmission electron microscopy. A long-term (18 weeks)in vivostudy of the use of theseGr on GCmicroelectrodes assessed the quality of the electrocorticography-based neural signal recording and stimulation through electrophysiological measurements. The probes were demonstrated to be functionally and structurally stable over the 18 week period with minimal glial response-the longest reported so far for Gr-based microelectrodes.Significance.TheGr on GCmicroelectrodes presented here offers a compelling case for expanding the potentials of Gr-based technology in the broad areas of neural probes.

3D fuzzy graphene microelectrode array for dopamine sensing at sub-cellular spatial resolution

The development of a high sensitivity real-time sensor for multi-site detection of dopamine (DA) with high spatial and temporal resolution is of fundamental importance to study the complex spatial and temporal pattern of DA dynamics in the brain, thus improving the understanding and treatments of neurological and neuropsychiatric disorders. In response to this need, here we present high surface area out-of-plane grown three-dimensional (3D) fuzzy graphene (3DFG) microelectrode arrays (MEAs) for highly selective, sensitive, and stable DA electrochemical sensing. 3DFG microelectrodes present a remarkable sensitivity to DA (2.12 ± 0.05 nA/nM, with LOD of 364.44 ± 8.65 pM), the highest reported for nanocarbon MEAs using Fast Scan Cyclic Voltammetry (FSCV). The high surface area of 3DFG allows for miniaturization of electrode down to 2 × 2 μm², without compromising the electrochemical performance. Moreover, 3DFG MEAs are electrochemically stable under 7.2 million scans of continuous FSCV cycling, present exceptional selectivity over the most common interferents in vitro with minimum fouling by electrochemical byproducts and can discriminate DA and serotonin (5-HT) in response to the injection of their 50:50 mixture. These results highlight the potential of 3DFG MEAs as a promising platform for FSCV based multi-site detection of DA with high sensitivity, selectivity, and spatial resolution.

Multimodal neural recordings with Neuro-FITM uncover diverse patterns of cortical-hippocampal interactions

Many cognitive processes require communication between the neocortex and the hippocampus. However, coordination between large-scale cortical dynamics and hippocampal activity is not well understood, partially due to the difficulty in simultaneously recording from those regions. In the present study, we developed a flexible, insertable and transparent microelectrode array (Neuro-FITM) that enables investigation of cortical-hippocampal coordinations during hippocampal sharp-wave ripples (SWRs). Flexibility and transparency of Neuro-FITM allow simultaneous recordings of local field potentials and neural spiking from the hippocampus during wide-field calcium imaging. These experiments revealed that diverse cortical activity patterns accompanied SWRs and, in most cases, cortical activation preceded hippocampal SWRs. We demonstrated that, during SWRs, different hippocampal neural population activity was associated with distinct cortical activity patterns. These results suggest that hippocampus and large-scale cortical activity interact in a selective and diverse manner during SWRs underlying various cognitive functions. Our technology can be broadly applied to comprehensive investigations of interactions between the cortex and other subcortical structures.

A Suite of Neurophotonic Tools to Underpin the Contribution of Internal Brain States in fMRI

Recent developments in optical microscopy, applicable for large-scale and longitudinal imaging of cortical activity in behaving animals, open unprecedented opportunities to gain a deeper understanding of neurovascular and neurometabolic coupling during different brain states. Future studies will leverage these tools to deliver foundational knowledge about brain state-dependent regulation of cerebral blood flow and metabolism as well as regulation as a function of brain maturation and aging. This knowledge is of critical importance to interpret hemodynamic signals observed with functional magnetic resonance imaging (fMRI).

Nano-optoelectrodes Integrated with Flexible Multifunctional Fiber Probes by High-Throughput Scalable Fabrication

Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical-electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

Multimodal in vivo recording using transparent graphene microelectrodes illuminates spatiotemporal seizure dynamics at the microscale

Neurological disorders such as epilepsy arise from disrupted brain networks. Our capacity to treat these disorders is limited by our inability to map these networks at sufficient temporal and spatial scales to target interventions. Current best techniques either sample broad areas at low temporal resolution (e.g. calcium imaging) or record from discrete regions at high temporal resolution (e.g. electrophysiology). This limitation hampers our ability to understand and intervene in aberrations of network dynamics. Here we present a technique to map the onset and spatiotemporal spread of acute epileptic seizures in vivo by simultaneously recording high bandwidth microelectrocorticography and calcium fluorescence using transparent graphene microelectrode arrays. We integrate dynamic data features from both modalities using non-negative matrix factorization to identify sequential spatiotemporal patterns of seizure onset and evolution, revealing how the temporal progression of ictal electrophysiology is linked to the spatial evolution of the recruited seizure core. This integrated analysis of multimodal data reveals otherwise hidden state transitions in the spatial and temporal progression of acute seizures. The techniques demonstrated here may enable future targeted therapeutic interventions and novel spatially embedded models of local circuit dynamics during seizure onset and evolution.

Evaluation of Durability of Transparent Graphene Electrodes Fabricated on Different Flexible Substrates for Chronic In Vivo Experiments

OBJECTIVE

To investigate chronic durability of transparent graphene electrodes fabricated on polyethylene terephthalate (PET) and SU-8 substrates for chronic in vivo studies.

METHODS

We perform systematic accelerated aging tests to understand the chronic reliability and failure modes of transparent graphene microelectrode arrays built on PET and SU-8 substrates. We employ graphene microelectrodes fabricated on PET substrate in chronic in vivo experiments with transgenic mice.

RESULTS

Our results show that graphene microelectrodes fabricated on PET substrate work reliably after 30 days accelerated aging test performed at 87 °C, equivalent to 960 days in vivo lifetime. We demonstrate stable chronic recordings of cortical potentials in multimodal imaging/recording experiments using transparent graphene microelectrodes fabricated on PET substrate. On the other hand, graphene microelectrode arrays built on SU-8 substrate exhibit extensive crack formation across microelectrode sites and wires after one to two weeks, resulting in total failure of recording capability for chronic studies.

CONCLUSION

PET shows superior reliability as a substrate for graphene microelectrode arrays for chronic in vivo experiments.

SIGNIFICANCE

Graphene is a unique neural interface material enabling cross-talk free integration of electrical and optical recording and stimulation techniques in the same experiment. To date, graphene-based microelectrode arrays have been demonstrated in various multi-modal acute experiments involving electrophysiological sensing or stimulation, optical imaging and optogenetics stimulation. Understanding chronic reliability of graphene-based transparent interfaces is very important to expand the use of this technology for long-term behavioral studies with animal models.

An optically transparent multi-electrode array for combined electrophysiology and optophysiology at the mesoscopic scale

OBJECTIVE

A number of tissue penetrating opto-electrodes to simultaneously record and optogenetically influence brain activity have been developed. For experiments at the surface of the brain, such as electrocorticogram (ECoG) recordings and surface optogenetics, fewer devices have been described and no device has found widespread adoption for neuroscientific experiments. One issue slowing adoption is the complexity and fragility of existing devices, typically based on transparent electrode materials like graphene and indium-tin oxide (ITO). We focused here on improving existing processes based on metal traces and polyimide (PI), which produce more robust and cost-effective devices, to develop a multi-electrode array for optophysiology.

APPROACH

The most widely used substrate material for surface electrodes, PI, has seen little use for optophysiologicalμECoG/ECoG arrays. This is due to its lack of transparency at optogenetically relevant short wavelengths. Here we use very thin layers of PI in combination with chrome-gold-platinum electrodes to achieve the necessary substrate transparency and high mechanical flexibility in a device that still rejects light artifacts well.

MAIN RESULTS

The manufactured surface arrays have a thickness of only 6.5 µm, resulting in 80% transparency for blue light. We demonstrate immunity against opto-electric artifacts, long term stability and biocompatibility as well as suitability for optical voltage imaging. The biocompatible arrays are capable of recording stable ECoGs over months without any measurable degradation and can be used to map the tonotopic organization of the curved rodent auditory cortex.

SIGNIFICANCE

Our novel probes combine proven materials and processing steps to create optically near-transparent electrode arrays with superior longevity. In contrast to previous opto-electrodes, our probes are simple to manufacture, robust, offer long-term stability, and are a practical engineering solution for optophysiological experiments not requiring transparency of the electrode sites themselves.

Graphene-based wearable sensors

The human body is a "delicate machine" full of sensors such as the fingers, nose, and mouth. In addition, numerous physiological signals are being created every moment, which can reflect the condition of the body. The quality and the quantity of the physiological signals are important for diagnoses and the execution of therapies. Due to the incompact interface between the sensors and the skin, the signals obtained by commercial rigid sensors do not bond well with the body; this decreases the quality of the signal. To increase the quantity of the data, it is important to detect physiological signals in real time during daily life. In recent years, there has been an obvious trend of applying graphene devices with excellent performance (flexibility, biocompatibility, and electronic characters) in wearable systems. In this review, we will first provide an introduction about the different methods of synthesis of graphene, and then techniques for graphene patterning will be outlined. Moreover, wearable graphene sensors to detect mechanical, electrophysiological, fluid, and gas signals will be introduced. Finally, the challenges and prospects of wearable graphene devices will be discussed. Wearable graphene sensors can improve the quality and quantity of the physiological signals and have great potential for health-care and telemedicine in the future.

Graphene-Based Nanomaterials: From Production to Integration With Modern Tools in Neuroscience

Graphene, a two-dimensional carbon crystal, has emerged as a promising material for sensing and modulating neuronal activity in vitro and in vivo. In this review, we provide a primer for how manufacturing processes to produce graphene and graphene oxide result in materials properties that may be tailored for a variety of applications. We further discuss how graphene may be composited with other bio-compatible materials of interest to make novel hybrid complexes with desired characteristics for bio-interfacing. We then highlight graphene's ever-widen utility and unique properties that may in the future be multiplexed for cross-modal modulation or interrogation of neuronal network. As the biological effects of graphene are still an area of active investigation, we discuss recent development, with special focus on how surface coatings and surface properties of graphene are relevant to its biological effects. We discuss studies conducted in both non-murine and murine systems, and emphasize the preclinical aspect of graphene's potential without undermining its tangible clinical implementation.

Graphene-Based Sensors for Human Health Monitoring

Since the desire for real-time human health monitoring as well as seamless human-machine interaction is increasing rapidly, plenty of research efforts have been made to investigate wearable sensors and implantable devices in recent years. As a novel 2D material, graphene has aroused a boom in the field of sensor research around the world due to its advantages in mechanical, thermal, and electrical properties. Numerous graphene-based sensors used for human health monitoring have been reported, including wearable sensors, as well as implantable devices, which can realize the real-time measurement of body temperature, heart rate, pulse oxygenation, respiration rate, blood pressure, blood glucose, electrocardiogram signal, electromyogram signal, and electroencephalograph signal, etc. Herein, as a review of the latest graphene-based sensors for health monitoring, their novel structures, sensing mechanisms, technological innovations, components for sensor systems and potential challenges will be discussed and outlined.