Transparent Electrophysiology Microelectrodes and Interconnects from Metal Nanomesh

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

An inorganic-blended p-type semiconductor with robust electrical and mechanical properties

Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm2/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 μs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.

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.

Metallic Micro-Nano Network-Based Soft Transparent Electrodes: Materials, Processes, and Applications

Soft transparent electrodes (TEs) have received tremendous interest from academia and industry due to the rapid development of lightweight, transparent soft electronics. Metallic micro-nano networks (MMNNs) are a class of promising soft TEs that exhibit excellent optical and electrical properties, including low sheet resistance and high optical transmittance, as well as superior mechanical properties such as softness, robustness, and desirable stability. They are genuinely interesting alternatives to conventional conductive metal oxides, which are expensive to fabricate and have limited flexibility on soft surfaces. This review summarizes state-of-the-art research developments in MMNN-based soft TEs in terms of performance specifications, fabrication methods, and application areas. The review describes the implementation of MMNN-based soft TEs in optoelectronics, bioelectronics, tactile sensors, energy storage devices, and other applications. Finally, it presents a perspective on the technical difficulties and potential future possibilities for MMNN-based TE development.

Emerging trends in the development of flexible optrode arrays for electrophysiology

Optical-electrode (optrode) arrays use light to modulate excitable biological tissues and/or transduce bioelectrical signals into the optical domain. Light offers several advantages over electrical wiring, including the ability to encode multiple data channels within a single beam. This approach is at the forefront of innovation aimed at increasing spatial resolution and channel count in multichannel electrophysiology systems. This review presents an overview of devices and material systems that utilize light for electrophysiology recording and stimulation. The work focuses on the current and emerging methods and their applications, and provides a detailed discussion of the design and fabrication of flexible arrayed devices. Optrode arrays feature components non-existent in conventional multi-electrode arrays, such as waveguides, optical circuitry, light-emitting diodes, and optoelectronic and light-sensitive functional materials, packaged in planar, penetrating, or endoscopic forms. Often these are combined with dielectric and conductive structures and, less frequently, with multi-functional sensors. While creating flexible optrode arrays is feasible and necessary to minimize tissue-device mechanical mismatch, key factors must be considered for regulatory approval and clinical use. These include the biocompatibility of optical and photonic components. Additionally, material selection should match the operating wavelength of the specific electrophysiology application, minimizing light scattering and optical losses under physiologically induced stresses and strains. Flexible and soft variants of traditionally rigid photonic circuitry for passive optical multiplexing should be developed to advance the field. We evaluate fabrication techniques against these requirements. We foresee a future whereby established telecommunications techniques are engineered into flexible optrode arrays to enable unprecedented large-scale high-resolution electrophysiology systems.

Transparent and Stretchable Au─Ag Nanowire Recording Microelectrode Arrays

Transparent microelectrodes have received much attention from the biomedical community due to their unique advantages in concurrent crosstalk-free electrical and optical interrogation of cell/tissue activity. Despite recent progress in constructing transparent microelectrodes, a major challenge is to simultaneously achieve desirable mechanical stretchability, optical transparency, electrochemical performance, and chemical stability for high-fidelity, conformal, and stable interfacing with soft tissue/organ systems. To address this challenge, we have designed microelectrode arrays (MEAs) with gold-coated silver nanowires (Au─Ag NWs) by combining technical advances in materials, fabrication, and mechanics. The Au coating improves both the chemical stability and electrochemical impedance of the Au─Ag NW microelectrodes with only slight changes in optical properties. The MEAs exhibit a high optical transparency >80% at 550 nm, a low normalized 1 kHz electrochemical impedance of 1.2-7.5 Ω cm2, stable chemical and electromechanical performance after exposure to oxygen plasma for 5 min, and cyclic stretching for 600 cycles at 20% strain, superior to other transparent microelectrode alternatives. The MEAs easily conform to curvilinear heart surfaces for colocalized electrophysiological and optical mapping of cardiac function. This work demonstrates that stretchable transparent metal nanowire MEAs are promising candidates for diverse biomedical science and engineering applications, particularly under mechanically dynamic conditions.

High-Fidelity sEMG Signals Recorded by an on-Skin Electrode Based on AgNWs for Hand Gesture Classification Using Machine Learning

The human forearm is one of the most densely distributed parts of the human body, with the most irregular spatial distribution of muscles. A number of specific forearm muscles control hand motions. Acquiring high-fidelity sEMG signals from human forearm muscles is vital for human-machine interface (HMI) applications based on gesture recognition. Currently, the most commonly used commercial electrodes for detecting sEMG or other electrophysiological signals have a rigid nature without stretchability and cannot maintain conformal contact with the human skin during deformation, and the adhesive hydrogel used in them to reduce skin-electrode impedance may shrink and cause skin inflammation after long-term use. Therefore, developing elastic electrodes with stretchability and biocompatibility for sEMG signal recording is essential for developing HMI. Here, we fabricated a nanocomposite hybrid on-skin electrode by infiltrating silver nanowires (AgNWs), a one-dimensional (1D) nano metal material with conductivity, into polydimethylsiloxane (PDMS), a silicone elastomer with a similar Young's modulus to that of the human skin. The AgNW on-skin electrode has a thickness of 300 μm and low sheet resistance of 0.481 ± 0.014 Ω/sq and can withstand the mechanical strain of up to 54% and maintain a sheet resistance lower than 1 Ω/sq after 1000 dynamic strain cycles. The AgNW on-skin electrode can record high signal-to-noise ratio (SNR) sEMG signals from forearm muscles and can reflect various force levels of muscles by sEMG signals. Besides, four typical hand gestures were recognized by the multichannel AgNW on-skin electrodes with a recognition accuracy of 92.3% using machine learning method. The AgNW on-skin electrode proposed in this study has great potential and promise in various HMI applications that employ sEMG signals as control signals.

Highly Stretchable Graphene Scrolls Transistors for Self-Powered Tribotronic Non-Mechanosensation Application

Owing to highly desired requirements in advanced disease diagnosis, therapy, and health monitoring, noncontact mechanosensation active matrix has drawn considerable attention. To satisfy the practical demands of high energy efficiency, in this report, combining the advantage of multiparameter monitoring, high sensitivity, and high resolution of active matrix field-effect transistor (FET) with triboelectric nanogenerators (TENG), we successfully developed the tribotronic mechanosensation active matrix based on tribotronic ion gel graphene scrolls field-effect transistors (GSFET). The tribopotential produced by TENG served as a gate voltage to modulate carrier transport along the semiconductor channel and realized self-powered ability with considerable decreased energy consumption. To achieve high spatial utilization and more pronounced responsivity of the dielectric of this transistor, ion gel was used to act as a triboelectric layer to conduct friction and contact electrification with external materials directly to produce triboelectric charges to power GFET. This tribopotential-driving device has excellent tactile sensing properties with high sensitivity (1.125 mm^-1), rapid response time (~16 ms), and a durability operation of thousands of cycles. Furthermore, the device was transparent and flexible with the capability of spatially mapping touch stimuli and monitoring real-time temperature. Due to all these unique characteristics, this novel noncontact mechanosensation GSFET active matrix provided a new method for self-powered E-skin with promising potential for self-powered wearable devices and intelligent robots.

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.

Bilayer-Nanomesh Transparent Neuroelectrodes on 10μm-Thick PDMS

Transparent electrode arrays have emerged as promising platforms for neural interfacing by enabling simultaneous electrophysiological recording and optical measurements. Soft and thin devices also have compelling advantages due to their less mechanical mismatch with the brain tissue. Here we demonstrate a bilayer-nanomesh-based transparent microelectrode array (MEA) on ultrathin Polydimethylsiloxane (PDMS) substrate. We have successfully fabricated 32-channel, bilayer-nanomesh microelectrodes on PDMS with total device thickness down to only 10μm. In addition to excellent electrode performance, device reliability, and optical transparency, we have also demonstrated successful hydrophilic surface modification and great sterilization compatibility.

Nanosphere Lithography: A Versatile Approach to Develop Transparent Conductive Films for Optoelectronic Applications

Transparent conductive films (TCFs) are irreplaceable components in most optoelectronic applications such as solar cells, organic light-emitting diodes, sensors, smart windows, and bioelectronics. The shortcomings of existing traditional transparent conductors demand the development of new material systems that are both transparent and electrically conductive, with variable functionality to meet the requirements of new generation optoelectronic devices. In this respect, TCFs with periodic or irregular nanomesh structures have recently emerged as promising candidates, which possess superior mechanical properties in comparison with conventional metal oxide TCFs. Among the methods for nanomesh TCFs fabrication, nanosphere lithography (NSL) has proven to be a versatile platform, with which a wide range of morphologically distinct nanomesh TCFs have been demonstrated. These materials are not only functionally diverse, but also have advantages in terms of device compatibility. This review provides a comprehensive description of the NSL process and its most relevant derivatives to fabricate nanomesh TCFs. The structure-property relationships of these materials are elaborated and an overview of their application in different technologies across disciplines related to optoelectronics is given. It is concluded with a perspective on current shortcomings and future directions to further advance the field.

Advances in Implantable Optogenetic Technology for Cardiovascular Research and Medicine

Optogenetic technology provides researchers with spatiotemporally precise tools for stimulation, sensing, and analysis of function in cells, tissues, and organs. These tools can offer low-energy and localized approaches due to the use of the transgenically expressed light gated cation channel Channelrhodopsin-2 (ChR2). While the field began with many neurobiological accomplishments it has also evolved exceptionally well in animal cardiac research, both in vitro and in vivo. Implantable optical devices are being extensively developed to study particular electrophysiological phenomena with the precise control that optogenetics provides. In this review, we highlight recent advances in novel implantable optogenetic devices and their feasibility in cardiac research. Furthermore, we also emphasize the difficulties in translating this technology toward clinical applications and discuss potential solutions for successful clinical translation.

Ga Based Particles, Alloys and Composites: Fabrication and Applications

Liquid metal (LM) materials, including pure gallium (Ga) LM, eutectic alloys and their composites with organic polymers and inorganic nanoparticles, are cutting-edge functional materials owing to their outstanding electrical conductivity, thermal conductivity, extraordinary mechanical compliance, deformability and excellent biocompatibility. The unique properties of LM-based materials at room temperatures can overcome the drawbacks of the conventional electronic devices, particularly high thermal, electrical conductivities and their fluidic property, which would open tremendous opportunities for the fundamental research and practical applications of stretchable and wearable electronic devices. Therefore, research interest has been increasingly devoted to the fabrication methodologies of LM nanoparticles and their functional composites. In this review, we intend to present an overview of the state-of-art protocols for the synthesis of Ga-based materials, to introduce their potential applications in the fields ranging from wearable electronics, energy storage batteries and energy harvesting devices to bio-applications, and to discuss challenges and opportunities in future studies.

Hybrid Electrical and Optical Neural Interfaces

Neural interfaces bridge the nervous system and the outside world by recording and stimulating neurons. Combining electrical and optical modalities in a single, hybrid neural interface system could lead to complementary and powerful new ways to explore the brain. It has gained robust and exciting momentum recently in neuroscience and neural engineering research. Here, we review developments in the past several years aiming to achieve such hybrid electrical and optical microsystem platforms. Specifically, we cover three major categories of technological advances: transparent neuroelectrodes, optical neural fibers with electrodes, and neural probes/grids integrating electrodes and microscale light-emitting diodes. We discuss examples of these probes tailored to combine electrophysiological recording with optical imaging or optical neural stimulation of the brain and possible directions of future innovation.

A fully transparent, flexible PEDOT:PSS-ITO-Ag-ITO based microelectrode array for ECoG recording

Integrative neural interfaces combining neurophysiology and optogenetics with neural imaging provide numerous opportunities for neuroscientists to study the structure and function of neural circuits in the brain. Such a comprehensive interface demands miniature electrode arrays with high transparency, mechanical flexibility, electrical conductivity, and biocompatibility. Conventional transparent microelectrodes made of a single material, such as indium tin oxide (ITO), ultrathin metals, graphene and poly-(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), hardly possess the desired combination of those properties. Herein, ultra-flexible, highly conductive and fully transparent microscale electrocorticogram (μECoG) electrode arrays made of a PEDOT:PSS-ITO-Ag-ITO assembly are constructed on thin parylene C films. The PEDOT:PSS-ITO-Ag-ITO assembly achieves a maximum ∼14% enhancement in light transmission over a broad spectrum (350-650 nm), a significant reduction in electrochemical impedance by 91.25%, and an increase in charge storage capacitance by 1229.78 μC cm^-2. Peeling, bending, and Young's modulus tests verify the enhanced mechanical flexibility and robustness of the multilayer assembly. The μECoG electrodes enable electrical recordings with high signal-to-noise ratios (SNRs) (∼35-36 dB) under different color photostimulations, suggesting that the electrodes are resilient to photon-induced artifacts. In vivo animal experiments confirm that our array can successfully record light-evoked ECoG oscillations from the primary visual cortex (V1) of an anesthetized rat.

Non-Iridescent Metal Nanomesh with Disordered Nanoapertures Fabricated by Phase Separation Lithography of Polymer Blend as Transparent Conductive Film

Metallic nanomesh, one of the emerging transparent conductive film (TCF) materials with both high electrical conductivity and optical transmittance, shows great potential to replace indium tin oxide (ITO) in optoelectronic devices. However, lithography-fabricated metallic nanomeshes suffer from an iridescence problem caused by the optical diffraction of periodic nanostructures, which has negative effects on display performance. In this work, we propose a novel approach to fabricate large-scale metallic nanomesh as TCFs on flexible polyethylene terephthalate (PET) sheets by maskless phase separation lithography of polymer blends in a low-cost and facile process. Polystyrene (PS)/polyphenylsilsequioxane (PPSQ) polymer blend was chosen as resist material for phase separation lithography due to their different etching selectivity under O₂ reactive ion etching (RIE). The PS constituent was selectively removed by O₂ RIE and the remained PPSQ nanopillars with varying sizes in random distribution were used as masks for further pattern transfer and metal deposition process. Gold (Au) nanomeshes with adjustable nanostructures were achieved after the lift-off step. Au nanomesh exhibited good optoelectronic properties (R_S = 41 Ω/sq, T = 71.9%) and non-iridescence, without angle dependence owing to the aperiodic structures of disordered apertures. The results indicate that this Au nanomesh has high potential application in high-performance and broad-viewing-angle optoelectronic devices.

Materials, Devices, and Systems of On-Skin Electrodes for Electrophysiological Monitoring and Human-Machine Interfaces

On-skin electrodes function as an ideal platform for collecting high-quality electrophysiological (EP) signals due to their unique characteristics, such as stretchability, conformal interfaces with skin, biocompatibility, and wearable comfort. The past decade has witnessed great advancements in performance optimization and function extension of on-skin electrodes. With continuous development and great promise for practical applications, on-skin electrodes are playing an increasingly important role in EP monitoring and human-machine interfaces (HMI). In this review, the latest progress in the development of on-skin electrodes and their integrated system is summarized. Desirable features of on-skin electrodes are briefly discussed from the perspective of performances. Then, recent advances in the development of electrode materials, followed by the analysis of strategies and methods to enhance adhesion and breathability of on-skin electrodes are examined. In addition, representative integrated electrode systems and practical applications of on-skin electrodes in healthcare monitoring and HMI are introduced in detail. It is concluded with the discussion of key challenges and opportunities for on-skin electrodes and their integrated systems.

Design of Stretchable Holey Gold Biosensing Electrode for Real-Time Cell Monitoring

In bioelectronics, gold thin films have been widely used as sensing electrodes for probing biological events due to their high conductivity, chemical inertness, biocompatibility, wide electrochemical window, and facile surface modification. However, they are intrinsically not stretchable, which limits their applications in detecting biological reactions when a soft biological system is mechanically deformed. Here, we report on a nanosphere lithography-based strategy to generate ordered microhole gold thin-film electrodes supported by elastomeric substrates. Both experimental and theoretical studies show that the presence of microholes substantially suppresses the catastrophic crack propagation-the main reason for electrical failure for a continuous gold film. As a result, the holey gold film achieves a ∼94% stretchable limit, after which the conductivity is lost, in contrast to ∼4% for the nonstructured counterpart. Furthermore, the pinhole gold electrode is successfully used to monitor the H₂O₂ released from living cells under dynamic stretching conditions.

Flexible and Transparent Metal Oxide/Metal Grid Hybrid Interfaces for Electrophysiology and Optogenetics

Flexible and transparent microelectrodes and interconnects provide the unique capability for a wide range of emerging biological applications, including simultaneous optical and electrical interrogation of biological systems. For practical biointerfacing, it is important to further improve the optical, electrical, electrochemical, and mechanical properties of the transparent conductive materials. Here, high-performance microelectrodes and interconnects with high optical transmittance (59-81%), superior electrochemical impedance (5.4-18.4 Ω cm2), and excellent sheet resistance (5.6-14.1 Ω sq-1), using indium tin oxide (ITO) and metal grid (MG) hybrid structures are demonstrated. Notably, the hybrid structures retain the superior mechanical properties of flexible MG other than brittle ITO with no changes in sheet resistance even after 5000 bending cycles against a small radius at 5 mm. The capabilities of the ITO/MG microelectrodes and interconnects are highlighted by high-fidelity electrical recordings of transgenic mouse hearts during co-localized programmed optogenetic stimulation. In vivo histological analysis reveals that the ITO/MG structures are fully biocompatible. Those results demonstrate the great potential of ITO/MG interfaces for broad fundamental and translational physiological studies.

Improvement of the Optoelectrical Properties of a Transparent Conductive Polymer via a Simple Mechanical Pressure Treatment

In this study, a simple and novel mechanical pressure treatment (MPT) was used to effectively improve the electrical and optical properties of ethylene glycol (EG)-doped PEDOT:PSS (EG-PEDOT:PSS) thin films, one of the most successful organic conductor materials ever which is are widely used in organic electronics because of their admirable film-forming property, high light transmittance, and excellent thermal stability. It is found that the conductivity of the EG-PEDOT:PSS films increased by 32% due to dramatically enhanced carrier mobility because an MPT improves the phase separation between PEDOT and PSS and then yields an interpenetrating conductive network. Meanwhile, the transmittance of the EG-PEDOT:PSS films in the near-infrared band was enhanced, and the surface roughness was reduced. These thin films retain their incredible flexibility as well; after 5000 times of 180° bending, the sheet resistance is basically unchanged. Considering that this MPT approach is already well developed in industrial applications, it is very hopeful to extend this technique in the field of organic electronics.

Transparent Flexible Nanoline Field-Effect Transistor Array with High Integration in a Large Area

Transparent flexible transistor array requests large-area fabrication, high integration, high manufacturing throughput, inexpensive process, uniformity in transistor performance, and reproducibility. This study suggests a facile and reliable approach to meet the requirements. We use the Al-coated polymer nanofiber patterns obtained by electrohydrodynamic (EHD) printing as a photomask. We use the lithography and deposition to produce highly aligned nanolines (NLs) of metals, insulators, and semiconductors on large substrates. With these NLs, we demonstrate a highly integrated NL field-effect transistor (NL-FET) array (10⁵/(4 × 4 in²), 254 pixel-per-inch) made of pentacene and indium zinc oxide semiconductor NLs. In addition, we demonstrate a NL complementary inverter (NL-CI) circuit consisting of pentacene and fullerene NLs. The NL-FET array shows high transparency (∼90%), flexibility (stable at 2.5 mm bending radius), uniformity (∼90%), and high performances (mobility = 0.52 cm²/(V s), on-off ratio = 7.0 × 10⁶). The NL-CI circuit also shows high transparency, flexibility, and typical switching characteristic with a gain of 21. The reliable large-scale fabrication of the various NLs proposed in this study is expected to be applied for manufacturing transparent flexible nanoelectronic devices.

A Review: Electrode and Packaging Materials for Neurophysiology Recording Implants

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

MEA Recordings and Cell-Substrate Investigations with Plasmonic and Transparent, Tunable Holey Gold

Microelectrode arrays are widely used in different fields such as neurobiology or biomedicine to read out electrical signals from cells or biomolecules. One way to improve microelectrode applications is the development of novel electrode materials with enhanced or additional functionality. In this study, we fabricated macroelectrodes and microelectrode arrays containing gold penetrated by nanohole arrays as a conductive layer. We used this holey gold to optically excite surface plasmon polaritons, which lead to a strong increase in transparency, an effect that is further enhanced by the plasmon's interaction with cell culture medium. By varying the nanohole diameter in finite-difference time domain simulations, we demonstrate that the transmission can be increased to above 70% with its peak at a wavelength depending on the holey gold's lattice constant. Further, we demonstrate that the novel transparent microelectrode arrays are as suitable for recording cellular electrical activity as standard devices. Moreover, we prove using spectral measurements and finite-difference time domain simulations that plasmonically induced transmission peaks of holey gold red-shift upon sensing medium or cells in close vicinity (<30 nm) to the substrate. Thus, we establish plasmonic and transparent holey gold as a tunable material suitable for cellular electrical recordings and biosensing applications.

Long-Lived, Transferred Crystalline Silicon Carbide Nanomembranes for Implantable Flexible Electronics

Implantable electronics are of great interest owing to their capability for real-time and continuous recording of cellular-electrical activity. Nevertheless, as such systems involve direct interfaces with surrounding biofluidic environments, maintaining their long-term sustainable operation, without leakage currents or corrosion, is a daunting challenge. Herein, we present a thin, flexible semiconducting material system that offers attractive attributes in this context. The material consists of crystalline cubic silicon carbide nanomembranes grown on silicon wafers, released and then physically transferred to a final device substrate (e.g., polyimide). The experimental results demonstrate that SiC nanomembranes with thicknesses of 230 nm do not experience the hydrolysis process (i.e., the etching rate is 0 nm/day at 96 °C in phosphate-buffered saline (PBS)). There is no observable water permeability for at least 60 days in PBS at 96 °C and non-Na⁺ ion diffusion detected at a thickness of 50 nm after being soaked in 1× PBS for 12 days. These properties enable Faradaic interfaces between active electronics and biological tissues, as well as multimodal sensing of temperature, strain, and other properties without the need for additional encapsulating layers. These findings create important opportunities for use of flexible, wide band gap materials as essential components of long-lived neurological and cardiac electrophysiological device interfaces.

High-quality-factor flexible and transparent capacitors with Cr-Au nanomeshes as bottom electrodes

Flexible and transparent capacitors have been fabricated on polyethylene naphthalate (PEN) substrates. The Cr-Au nanomeshes, ZrO₂, and aluminum-doped zinc oxide (AZO) films were prepared in sequence as bottom electrodes, dielectric layers and top electrodes. A high quality factor of 82 was obtained, nearly 4.9 times larger than that ever reported, which directly indicates much lower power consumption. The flexible capacitors can normally work at a bending radius of 7 mm without any loss of electrical performance, and even maintain electrical characteristics after cyclical bending tests. Moreover, a high capacitance density of 12.3 fF μm^-2 and an ultralow leakage current of 1.5 × 10^-8 A cm^-2 at 1 V have been achieved. Transmittance measurements indicate that the capacitors represent an average optical transmittance of over 50% at a visible region. This is, therefore, a dramatic step toward real applications in flexible and transparent integrated circuits.

Softening gold for elastronics

Gold, one of the noble metals, has played a significant role in human society throughout history. Gold's excellent electrical, optical and chemical properties make the element indispensable in maintaining a prosperous modern electronics industry. In the emerging field of stretchable electronics (elastronics), the main challenge is how to utilize these excellent material properties under various mechanical deformations. This review covers the recent progress in developing "softening" gold chemistry for various applications in elastronics. We systematically present material synthesis and design principles, applications, and challenges and opportunities ahead.

Matrix-Independent Highly Conductive Composites for Electrodes and Interconnects in Stretchable Electronics

Electrically conductive composites (ECCs) hold great promise in stretchable electronics because of their printability, facile preparation, elasticity, and possibility for large-area fabrication. A high conductivity at steady state and during mechanical deformation is a critical property for ECCs, and extensive efforts have been made to improve the conductivity. However, most of those approaches are exclusively functional to a specific polymer matrix, restricting their capability to meet other requirements, such as mechanical, adhesive, and thermomechanical properties. Here, we report a generic approach to prepare ECCs with conductivity close to that of bulk metals and maintain their conductivity during stretching. This approach iodizes the surfactants on the commercial silver flakes, and subsequent photo exposure converts these silver iodide nanoparticles to silver nanoparticles. The ECCs based on silver nanoparticle-covered silver flakes exhibit high conductivity because of the removal of insulating surfactants as well as the enhanced contact between flakes. The treatment of silver flakes is independent of the polymer matrix and provides the flexibility in matrix selection. In the development of stretchable interconnects, ECCs can be prepared with the same polymer as the substrate to ensure strong adhesion between interconnects and the substrate. For the fabrication of on-skin electrodes, a polymer matrix of low modulus can be selected to enhance conformal contact with the skin for reduced impedance.

Transparent, Flexible, Penetrating Microelectrode Arrays with Capabilities of Single-Unit Electrophysiology

Accurately mapping neuronal activity across brain networks is critical to understand behaviors, yet it is very challenging due to the need of tools with both high spatial and temporal resolutions. Here, penetrating arrays of flexible microelectrodes made of low-impedance nanomeshes are presented, which are capable of recording single-unit electrophysiological neuronal activity and at the same time, transparent, allowing to bridge electrical and optical brain mapping modalities. These 32 transparent penetrating electrodes with site area, 225 µm² , have a low impedance of ≈149 kΩ at 1 kHz, an adequate charge injection limit of ≈0.76 mC cm^-2 , and up to 100% yield. Mechanical bending tests reveal that the array is robust up to 1000 bending cycles, and its high transmittance of 67% at 550 nm makes it suitable for combining with various optical methods. A temporary stiffening using polyethylene glycol allows the penetrating nanomesh arrays to be inserted into the brain minimally invasively, with in vivo validation of recordings of spontaneous and evoked single-unit activity of neurons across layers of the mouse visual cortex. Together, these results establish a novel neurotechnology-transparent, flexible, penetrating microelectrode arrays-which possesses great potential for brain research.

Imaging Sodium Flux during Action Potentials in Neurons with Fluorescent Nanosensors and Transparent Microelectrodes

Sodium flux plays a pivotal role in neurobiological processes including initiation of action potentials and regulation of neuronal cell excitability. However, unlike the wide range of fluorescent calcium indicators used extensively for cellular studies, the choice of sodium probes remains limited. We have previously demonstrated optode-based nanosensors (OBNs) for detecting sodium ions with advantageous modular properties such as tunable physiological sensing range, full reversibility, and superb selectivity against key physiological interfering ion potassium. (1) Motivated by bridging the gap between the great interest in sodium imaging of neuronal cell activity as an alternative to patch clamp and limited choices of optical sodium indicators, in this Letter we report the application of nanosensors capable of detecting intracellular sodium flux in isolated rat dorsal root ganglion neurons during electrical stimulation using transparent microelectrodes. Taking advantage of the ratiometric detection scheme offered by this fluorescent modular sensing platform, we performed dual color imaging of the sensor to monitor the intracellular sodium currents underlying trains of action potentials in real time. The combination of nanosensors and microelectrodes for monitoring neuronal sodium dynamics is a novel tool for investigating the regulatory role of sodium ions involved during neural activities.

Transparent arrays of bilayer-nanomesh microelectrodes for simultaneous electrophysiology and two-photon imaging in the brain

Transparent microelectrode arrays have emerged as increasingly important tools for neuroscience by allowing simultaneous coupling of big and time-resolved electrophysiology data with optically measured, spatially and type resolved single neuron activity. Scaling down transparent electrodes to the length scale of a single neuron is challenging since conventional transparent conductors are limited by their capacitive electrode/electrolyte interface. In this study, we establish transparent microelectrode arrays with high performance, great biocompatibility, and comprehensive in vivo validations from a recently developed, bilayer-nanomesh material composite, where a metal layer and a low-impedance faradaic interfacial layer are stacked reliably together in a same transparent nanomesh pattern. Specifically, flexible arrays from 32 bilayer-nanomesh microelectrodes demonstrated near-unity yield with high uniformity, excellent biocompatibility, and great compatibility with state-of-the-art wireless recording and real-time artifact rejection system. The electrodes are highly scalable, with 130 kilohms at 1 kHz at 20 μm in diameter, comparable to the performance of microelectrodes in nontransparent Michigan arrays. The highly transparent, bilayer-nanomesh microelectrode arrays allowed in vivo two-photon imaging of single neurons in layer 2/3 of the visual cortex of awake mice, along with high-fidelity, simultaneous electrical recordings of visual-evoked activity, both in the multi-unit activity band and at lower frequencies by measuring the visual-evoked potential in the time domain. Together, these advances reveal the great potential of transparent arrays from bilayer-nanomesh microelectrodes for a broad range of utility in neuroscience and medical practices.

Transparent poly(3,4-ethylenedioxythiophene)-based microelectrodes for extracellular recording

It is well known that at the interface between neuronal tissue and recording electrode low electrical impedance is required. However, if simultaneous optical detection or stimulation is an issue, good optical transmittance of the electrode material is desirable as well. State-of-the-art titanium nitride electrodes provide superior low impedance compared to gold or iridium, but are nontransparent. Transparent electrode materials like the transparent conducting oxide, indium tin oxide (ITO), or graphene offer high light transmittance (>80%) but reveal relatively high impedance. In this paper, the authors propose the conducting polymer poly(3,4-ethylenedioxythiophene) with the counter ion NO₃^- as the electrode material for low impedance and good optical transmittance properties. The polymer is electrochemically deposited onto ITO improving the relatively high impedance of ITO. This multilayer electrode allows not only for electrophysiological recordings of cardiomyocytes but also for monitoring of cell contraction under the microscope. Electrochemical impedance spectroscopy and action potential recordings reveal that the new transparent electrodes are a good compromise in terms of low impedance and transparency if deposition parameters are optimized.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.