Adaptive frequency-domain filtering for neural signal preprocessing

Electrical interference from various sources is a common issue for experimental extracellular electrophysiology recordings collected using multi-electrode array neural recording systems. This interference deteriorates the signal-to-noise ratio (SNR) of the raw electrophysiology signals and hampers the accuracy of data post-processing using techniques such as spike-sorting. Traditional signal processing methods to digitally remove electrical interference during post-processing include bandpass filtering to limit the signal to the relevant spectral range of the biological data, e.g., the spikes band (300 Hz - 7 kHz), targeted notch filtering to remove power line interference from standard alternating current mains electricity and common reference removal to minimize noise common to all electrodes. These methods require a priori knowledge of the frequency of the interfering signal source to address the unique electromagnetic interference environment of each experimental setup. We discuss an adaptive method for automatically removing narrow-band electrical interference through a spectral peak detection and removal (SPDR) step that can be applied during post-processing of the recorded data, based on the intuition that tall, narrowband signals localized in the signal spectrum correspond to interference, rather than the activity of neurons. A spectral peak prominence (SPP) threshold is used to detect these peaks in the frequency domain, which will then be removed via notch filtering. We applied this method to simulated waveforms and also experimental electrophysiology data collected from cerebral organoids to demonstrate its effectiveness for removing unwanted interference without significantly distorting the neural signals. We discuss that proper selection of the SPP threshold is required to avoid over-filtering, which can result in distortion of the electrophysiology data. We also compare the firing-rate activity in the filtered electrophysiology with fluorescence calcium imaging, a secondary cellular activity marker, to quantify signal distortion and provide bounds on SNR-based optimization of the SPP threshold. The adaptive filtering technique demonstrated in this paper is a powerful method that can automatically detect and remove interband interference in recorded neural signals, potentially enabling data collection in more naturalistic settings where external interference signals are difficult to eliminate.

Electrocatalytic on-site oxygenation for transplanted cell-based-therapies

Implantable cell therapies and tissue transplants require sufficient oxygen supply to function and are limited by a delay or lack of vascularization from the transplant host. Previous exogenous oxygenation strategies have been bulky and had limited oxygen production or regulation. Here, we show an electrocatalytic approach that enables bioelectronic control of oxygen generation in complex cellular environments to sustain engineered cell viability and therapy under hypoxic stress and at high cell densities. We find that nanostructured sputtered iridium oxide serves as an ideal catalyst for oxygen evolution reaction at neutral pH. We demonstrate that this approach exhibits a lower oxygenation onset and selective oxygen production without evolution of toxic byproducts. We show that this electrocatalytic on site oxygenator can sustain high cell loadings (>60k cells/mm3) in hypoxic conditions in vitro and in vivo. Our results showcase that exogenous oxygen production devices can be readily integrated into bioelectronic platforms, enabling high cell loadings in smaller devices with broad applicability.

A snapshot review on materials enabled multimodal bioelectronics for neurological and cardiac research

Seamless integration of the body and electronics toward the understanding, quantification, and control of disease states remains one of the grand scientific challenges of this era. As such, research efforts have been dedicated to developing bioelectronic devices for chemical, mechanical, and electrical sensing, and cellular and tissue functionality modulation. The technologies developed to achieve these capabilities cross a wide range of materials and scale (and dimensionality), e.g., from micrometer to centimeters (from 2-dimensional (2D) to 3-dimensional (3D) assemblies). The integration into multimodal systems which allow greater insight and control into intrinsically multifaceted biological systems requires careful design and selection. This snapshot review will highlight the state-of-the-art in cellular recording and modulation as well as the material considerations for the design and manufacturing of devices integrating their capabilities.

Photothermal Excitation of Neurons Using MXene: Cellular Stress and Phototoxicity Evaluation

Understanding the communication of individual neurons necessitates precise control of neural activity. Photothermal modulation is a remote and non-genetic technique to control neural activity with high spatiotemporal resolution. The local heat release by photothermally active nanomaterial will change the membrane properties of the interfaced neurons during light illumination. Recently, it is demonstrated that the two-dimensional Ti3 C2 Tx MXene is an outstanding candidate to photothermally excite neurons with low incident energy. However, the safety of using Ti3 C2 Tx for neural modulation is unknown. Here, the biosafety of Ti3 C2 Tx -based photothermal modulation is thoroughly investigated, including assessments of plasma membrane integrity, mitochondrial stress, and oxidative stress. It is demonstrated that culturing neurons on 25 µg cm-2 Ti3 C2 Tx films and illuminating them with laser pulses (635 nm) with different incident energies (2-10 µJ per pulse) and different pulse frequencies (1 pulse, 1 Hz, and 10 Hz) neither damage the cell membrane, induce cellular stress, nor generate oxidative stress. The threshold energy to cause damage (i.e., 14 µJ per pulse) exceeded the incident energy for neural excitation (<10 µJ per pulse). This multi-assay safety evaluation provides crucial insights for guiding the establishment of light conditions and protocols in the clinical translation of photothermal modulation.

High Thermal Conductivity of Sandwich-Structured Flexible Thermal Interface Materials

Thermal interfaces are vital for effective thermal management in modern electronics, especially in the emerging fields of flexible electronics and soft robotics that impose requirements for interface materials to be soft and flexible in addition to having high thermal performance. Here, a novel sandwich-structured thermal interface material (TIM) is developed that simultaneously possesses record-low thermal resistance and high flexibility. Frequency-domain thermoreflectance (FDTR) is employed to investigate the overall thermal performance of the sandwich structure. As the core of this sandwich, a vertically aligned copper nanowire (CuNW) array preserves its high intrinsic thermal conductivity, which is further enhanced by 60% via a thick 3D graphene (3DG) coating. The thin copper layers on the top and bottom play the critical roles in protecting the nanowires during device assembly. Through the bottom-up fabrication process, excellent contacts between the graphene-coated CuNWs and the top/bottom layer are realized, leading to minimal interfacial resistance. In total, the thermal resistance of the sandwich is determined as low as ~0.23 mm²  K W^-1 . This work investigates a new generation of flexible thermal interface materials with an ultralow thermal resistance, which therefore renders the great promise for advanced thermal management in a wide variety of electronics.

First Organoid Intelligence (OI) workshop to form an OI community

The brain is arguably the most powerful computation system known. It is extremely efficient in processing large amounts of information and can discern signals from noise, adapt, and filter faulty information all while running on only 20 watts of power. The human brain's processing efficiency, progressive learning, and plasticity are unmatched by any computer system. Recent advances in stem cell technology have elevated the field of cell culture to higher levels of complexity, such as the development of three-dimensional (3D) brain organoids that recapitulate human brain functionality better than traditional monolayer cell systems. Organoid Intelligence (OI) aims to harness the innate biological capabilities of brain organoids for biocomputing and synthetic intelligence by interfacing them with computer technology. With the latest strides in stem cell technology, bioengineering, and machine learning, we can explore the ability of brain organoids to compute, and store given information (input), execute a task (output), and study how this affects the structural and functional connections in the organoids themselves. Furthermore, understanding how learning generates and changes patterns of connectivity in organoids can shed light on the early stages of cognition in the human brain. Investigating and understanding these concepts is an enormous, multidisciplinary endeavor that necessitates the engagement of both the scientific community and the public. Thus, on Feb 22-24 of 2022, the Johns Hopkins University held the first Organoid Intelligence Workshop to form an OI Community and to lay out the groundwork for the establishment of OI as a new scientific discipline. The potential of OI to revolutionize computing, neurological research, and drug development was discussed, along with a vision and roadmap for its development over the coming decade.

3D Graphene-Nanowire "Sandwich" Thermal Interface with Ultralow Resistance and Stiffness

Despite the recent advancements of passive and active cooling solutions for electronics, interfaces between materials have generally become crucial barriers for thermal transport because of intrinsic material dissimilarity and surface roughness at interfaces. We demonstrate a 3D graphene-nanowire "sandwich" thermal interface that enables an ultralow thermal resistance of ∼0.24 mm²·K/W that is about 1 order of magnitude smaller than those of solders and several orders of magnitude lower than those of thermal greases, gels, and epoxies, as well as a low elastic and shear moduli of ∼1 MPa like polymers and foams. The flexible 3D "sandwich" exhibits excellent long-term reliability with >1000 cycles over a broad temperature range from -55 °C to 125 °C. This nanostructured thermal interface material can greatly benefit a variety of electronic systems and devices by allowing them to operate at lower temperatures or at the same temperature but with higher performance and higher power density.

Hysteresis-free and high sensitivity strain sensing of ionically conductive hydrogels

Hydrogels are promising materials for soft and implantable strain sensors owing to their large compliance (E<100 kPa) and significant extensibility (εmax >500%) compared to other polymer networks. Further, hydrogels can be functionalized to seamlessly integrate with many types of tissues. However, most current methods attempt to imbue additional electronic functionality to structural hydrogel materials by incorporating fillers with orthogonal properties such as electronic or mixed ionic conduction. Although composite strategies may improve performance or facilitate heterogeneous integration with downstream hardware, composites complicate the path for regulatory approval and may compromise the otherwise compelling properties of the underlying structural material. Here we report hydrogel strain sensors composed of genipin-crosslinked gelatin and dopamine-functionalized poly(ethylene glycol) for in vivo monitoring of cardiac function. By measuring their impedance only in their resistive regime (>10 kHz), hysteresis is reduced and the resulting gauge factor is increased by ~50x to 1.02±0.05 and 1.46±0.05 from approximately 0.03-0.05 for PEG-Dopa and genipin-crosslinked gelatin respectively. Adhesion and in vivo biocompatibility are studied to support implementation of strain sensors for monitoring cardiac output in porcine models. Impedance-based strain sensing in the kilohertz regime simplifies the piezoresistive behavior of these materials and expands the range of hydrogel-based strain sensors.

Modulation of Early Stage Neuronal Outgrowth through Out-of-Plane Graphene

The correct wiring of a neural network requires neuron to integrate an incredible repertoire of cues found in their extracellular environment. The astonishing efficiency of this process plays a pivotal role in the correct wiring of the brain during development and axon regeneration. Biologically inspired micro- and nanostructured substrates have been shown to regulate axonal outgrowth. In parallel, several studies investigated graphene's potential as a conductive neural interface, able to enhance cell adhesion, neurite sprouting and outgrowth. Here, we engineered a 3D single- to few-layer fuzzy graphene morphology (3DFG), 3DFG on a collapsed Si nanowire (SiNW) mesh template (NT-3DFGc), and 3DFG on a noncollapsed SiNW mesh template (NT-3DFGnc) as neural-instructive materials. The micrometric protruding features of the NWs templates dictated neuronal growth cone establishment, as well as influencing axon elongation and branching. Furthermore, neurons-to-graphene coupling was investigated with comprehensive view of integrin-mediated contact adhesion points and plasma membrane curvature processes.

Real-Time In Vivo Sensing of Nitric Oxide Using Photonic Microring Resonators

Real-time in vivo detection of biomarkers, particularly nitric oxide (NO), is of utmost importance for critical healthcare monitoring, therapeutic dosing, and fundamental understanding of NO's role in regulating many physiological processes. However, detection of NO in a biological medium is challenging due to its short lifetime and low concentration. Here, we demonstrate for the first time that photonic microring resonators (MRRs) can provide real-time, direct, and in vivo detection of NO in a mouse wound model. The MRR encodes the NO concentration information into its transfer function in the form of a resonance wavelength shift. We show that these functionalized MRRs, fabricated using complementary metal oxide semiconductor (CMOS) compatible processes, can achieve sensitive detection of NO (sub-μM) with excellent specificity and no apparent performance degradation for more than 24 h of operation in biological medium. With alternative functionalizations, this compact lab-on-chip optical sensing platform could support real-time in vivo detection of myriad of biochemical species.

Flexible Glassy Carbon Multielectrode Array for In Vivo Multisite Detection of Tonic and Phasic Dopamine Concentrations

Dopamine (DA) plays a central role in the modulation of various physiological brain functions, including learning, motivation, reward, and movement control. The DA dynamic occurs over multiple timescales, including fast phasic release, as a result of neuronal firing and slow tonic release, which regulates the phasic firing. Real-time measurements of tonic and phasic DA concentrations in the living brain can shed light on the mechanism of DA dynamics underlying behavioral and psychiatric disorders and on the action of pharmacological treatments targeting DA. Current state-of-the-art in vivo DA detection technologies are limited in either spatial or temporal resolution, channel count, longitudinal stability, and ability to measure both phasic and tonic dynamics. We present here an implantable glassy carbon (GC) multielectrode array on a SU-8 flexible substrate for integrated multichannel phasic and tonic measurements of DA concentrations. The GC MEA demonstrated in vivo multichannel fast-scan cyclic voltammetry (FSCV) detection of electrically stimulated phasic DA release simultaneously at different locations of the mouse dorsal striatum. Tonic DA measurement was enabled by coating GC electrodes with poly(3,4-ethylenedioxythiophene)/carbon nanotube (PEDOT/CNT) and using optimized square-wave voltammetry (SWV). Implanted PEDOT/CNT-coated MEAs achieved stable detection of tonic DA concentrations for up to 3 weeks in the mouse dorsal striatum. This is the first demonstration of implantable flexible MEA capable of multisite electrochemical sensing of both tonic and phasic DA dynamics in vivo with chronic stability.

Graphene nanostructures for input-output bioelectronics

The ability to manipulate the electrophysiology of electrically active cells and tissues has enabled a deeper understanding of healthy and diseased tissue states. This has primarily been achieved via input/output (I/O) bioelectronics that interface engineered materials with biological entities. Stable long-term application of conventional I/O bioelectronics advances as materials and processing techniques develop. Recent advancements have facilitated the development of graphene-based I/O bioelectronics with a wide variety of functional characteristics. Engineering the structural, physical, and chemical properties of graphene nanostructures and integration with modern microelectronics have enabled breakthrough high-density electrophysiological investigations. Here, we review recent advancements in 2D and 3D graphene-based I/O bioelectronics and highlight electrophysiological studies facilitated by these emerging platforms. Challenges and present potential breakthroughs that can be addressed via graphene bioelectronics are discussed. We emphasize the need for a multidisciplinary approach across materials science, micro-fabrication, and bioengineering to develop the next generation of I/O bioelectronics.

Thermal Transport in Multidimensional Silicon-Graphene Hybrid Nanostructures

In this work, we fabricate multidimensional silicon-graphene hybrid nanostructures composed of three-dimensional (3D) out-of-plane graphene flakes on a silicon nanowire core. By changing the synthesis temperature (700 and 1100 °C) and time (5, 10, and 20 min), we obtain two different types of 3D graphene flakes with tunable dimensions and structure parameters. We characterize the thermal transport behavior of this hybrid multidimensional material in a broad temperature range of 20-460 K. With different morphologies and structures, the effective thermal conductivity of the silicon-graphene hybrid nanostructures varies from 1 to 7 W/(m·K) at room temperature. We also apply molecular dynamics simulation and density functional theory to elucidate the thermal transport mechanisms in the silicon-graphene hybrid nanostructures.

Ti3C2Tx MXene Flakes for Optical Control of Neuronal Electrical Activity

Understanding cellular electrical communications in both health and disease necessitates precise subcellular electrophysiological modulation. Nanomaterial-assisted photothermal stimulation was demonstrated to modulate cellular activity with high spatiotemporal resolution. Ideal candidates for such an application are expected to have high absorbance at the near-infrared window, high photothermal conversion efficiency, and straightforward scale-up of production to allow future translation. Here, we demonstrate two-dimensional Ti₃C₂T_x (MXene) as an outstanding candidate for remote, nongenetic, optical modulation of neuronal electrical activity with high spatiotemporal resolution. Ti₃C₂T_x's photothermal response measured at the single-flake level resulted in local temperature rises of 2.31 ± 0.03 and 3.30 ± 0.02 K for 635 and 808 nm laser pulses (1 ms, 10 mW), respectively. Dorsal root ganglion (DRG) neurons incubated with Ti₃C₂T_x film (25 μg/cm²) or Ti₃C₂T_x flake dispersion (100 μg/mL) for 6 days did not show a detectable influence on cellular viability, indicating that Ti₃C₂T_x is noncytotoxic. DRG neurons were photothermally stimulated using Ti₃C₂T_x films and flakes with as low as tens of microjoules per pulse incident energy (635 nm, 2 μJ for film, 18 μJ for flake) with subcellular targeting resolution. Ti₃C₂T_x's straightforward and large-scale synthesis allows translation of the reported photothermal stimulation approach in multiple scales, thus presenting a powerful tool for modulating electrophysiology from single-cell to additive manufacturing of engineered tissues.

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.

Bioelectrical interfaces with cortical spheroids in three-dimensions

Objective.Three-dimensional (3D) neuronal spheroid culture serves as a powerful model system for the investigation of neurological disorders and drug discovery. The success of such a model system requires techniques that enable high-resolution functional readout across the entire spheroid. Conventional microelectrode arrays and implantable neural probes cannot monitor the electrophysiology (ephys) activity across the entire native 3D geometry of the cellular construct.Approach.Here, we demonstrate a 3D self-rolled biosensor array (3D-SR-BA) integrated with a 3D cortical spheroid culture for simultaneousin vitroephys recording, functional Ca²⁺imaging, while monitoring the effect of drugs. We have also developed a signal processing pipeline to detect neural firings with high spatiotemporal resolution from the ephys recordings based on established spike sorting methods.Main results.The 3D-SR-BAs cortical spheroid interface provides a stable, high sensitivity recording of neural action potentials (<50µV peak-to-peak amplitude). The 3D-SR-BA is demonstrated as a potential drug screening platform through the investigation of the neural response to the excitatory neurotransmitter glutamate. Upon addition of glutamate, the neural firing rates increased notably corresponding well with the functional Ca²⁺imaging.Significance.Our entire system, including the 3D-SR-BA integrated with neuronal spheroid culture, enables simultaneous ephys recording and functional Ca²⁺imaging with high spatiotemporal resolution in conjunction with chemical stimulation. We demonstrate a powerful toolset for future studies of tissue development, disease progression, and drug testing and screening, especially when combined with native spheroid cultures directly extracted from humans.

Intracellular action potential recordings from cardiomyocytes by ultrafast pulsed laser irradiation of fuzzy graphene microelectrodes

Graphene with its unique electrical properties is a promising candidate for carbon-based biosensors such as microelectrodes and field effect transistors. Recently, graphene biosensors were successfully used for extracellular recording of action potentials in electrogenic cells; however, intracellular recordings remain beyond their current capabilities because of the lack of an efficient cell poration method. Here, we present a microelectrode platform consisting of out-of-plane grown three-dimensional fuzzy graphene (3DFG) that enables recording of intracellular cardiac action potentials with high signal-to-noise ratio. We exploit the generation of hot carriers by ultrafast pulsed laser for porating the cell membrane and creating an intimate contact between the 3DFG electrodes and the intracellular domain. This approach enables us to detect the effects of drugs on the action potential shape of human-derived cardiomyocytes. The 3DFG electrodes combined with laser poration may be used for all-carbon intracellular microelectrode arrays to allow monitoring of the cellular electrophysiological state.

Toward sustainable desalination using food waste: capacitive desalination with bread-derived electrodes

Each year approximately 1.3 billion tons of food is either wasted or lost. One of the most wasted foods in the world is bread. The ability to reuse wasted food in another area of need, such as water scarcity, would provide a tremendous sustainable outcome. To address water scarcity, many areas of the world are now implementing desalination. One desalination technology that could benefit from food waste reuse is capacitive deionization (CDI). CDI has emerged as a powerful desalination technology that essentially only requires a pair of electrodes and a low-voltage power supply. Developing freestanding carbon electrodes from food waste could lower the overall cost of CDI systems and the environmental and economic impact from food waste. We created freestanding CDI electrodes from bread. The electrodes possessed a hierarchical pore structure that enabled both high salt adsorption capacity and one of the highest reported values for hydraulic permeability to date in a flow-through CDI system. We also developed a sustainable technique for electrode fabrication that does not require the use of common laboratory equipment and could be deployed in decentralized locations and developing countries with low-financial resources.

Multidimensional graphene nanostructures – synthesis and applications

Conventional graphene electronics fail to leverage the exceptional surface-area-to-volume ratio of graphene due to the challenges imposed by arranging two-dimensional (2D) nanomaterials in three-dimensional (3D) spaces. Recently, a new topology of graphene, nanowire template 3D fuzzy graphene (NT-3DFG), has been developed to overcome this limitation. We provide an overview of the unique physical and chemical properties of NT-3DFG that are a direct consequence of the material structure and describe NT-3DFG’s promising applications in bioelectronics and energy-related research.

Electron Transport in Multidimensional Fuzzy Graphene Nanostructures

Atomically thin two-dimensional (2D) materials offer a range of superlative electronic and electrochemical properties that facilitate applications in sensing, energy conversion, and storage. Graphene, a 2D allotrope of carbon, has exceptional surface area per unit mass and highly catalytic edges. To leverage these properties, efforts have been made to synthesize complex three-dimensional (3D) geometries of graphene, with an eye toward integration into functional electronic devices. However, the electronic transport properties of such complex 3D structures are not well understood at a microscopic level. Here, we report electron transport in a 3D arrangement of free-standing 2D graphene flakes along an isolated one-dimensional Si nanowire. We show that transport through the free-standing graphene network is dominated by variable-range hopping and leads to negative magnetoresistance, from cryogenic conditions up to room temperature. Our findings lay the foundation for studying transport mechanisms in 2D material-based multidimensional nanostructures.

Organ-on-e-chip: Three-dimensional self-rolled biosensor array for electrical interrogations of human electrogenic spheroids

Cell-cell communication plays a pivotal role in coordination and function of biological systems. Three-dimensional (3D) spheroids provide venues to explore cellular communication for tissue development and drug discovery, as their 3D architecture mimics native in vivo microenvironments. Cellular electrophysiology is a prevalent signaling paradigm for studying electroactive cells. Currently, electrophysiological studies do not provide direct, multisite, simultaneous investigation of tissues in 3D. In this study, 3D self-rolled biosensor arrays (3D-SR-BAs) of either active field-effect transistors or passive microelectrodes were implemented to interface human cardiac spheroids in 3D. The arrays provided continuous and stable multiplexed recordings of field potentials with high sensitivity and spatiotemporal resolution, supported with simultaneous calcium imaging. Our approach enables electrophysiological investigation and monitoring of the complex signal transduction in 3D cellular assemblies toward an organ-on-an-electronic-chip (organ-on-e-chip) platform for tissue maturation investigations and development of drugs for disease treatment, such as arrhythmias.

Intact mangrove root electrodes for desalination

Through the benefit of billions of years of evolution, biology has developed tremendous strategies on how to co-exist in high salinity and water scarce environments. Biologically-inspired abiotic systems are becoming a central pillar in how we respond to critical grand challenges that accompany exponential population growth, uncontrolled climate change and the harsh reality that 96.5% of the water on the planet is saltwater. One fascinating biologic adaptation to saltwater is the growth of mangrove trees in brackish swamps and along the coasts. Through a process of salt exclusion, the mangrove maintains a near freshwater flow from roots to leaves to survive. One abiotic approach to water desalination is capacitive deionization, which aims to desalinate low-salinity water sources at energy costs below current technologies, such as reverse osmosis and thermal distillation. In this work, we use one-step carbonization of a plant with developed aerenchyma tissue to enable highly-permeable, freestanding flow-through capacitive deionization electrodes. We show that carbonized aerenchyma from red mangrove roots reduces the resistance to water flow through electrodes by 65-fold relative to carbonized common woody biomass. We then demonstrate the practical use of the intact carbonized red mangrove roots as electrodes in a flow-through capacitive deionization system. These findings have implications in a range of fields including water desalination, bioinspired materials, and plant functionality.

Biological adaptation in mangrove root enables freestanding carbonized architecture to be used as a highly permeable flow-through capacitive deionization electrode.

Bioelectronics with nanocarbons

Characterizing the electrical activity of cardiomyocytes and neurons is crucial in understanding the complex processes in the heart and brain tissues, both in healthy and diseased states. Micro- and nanotechnologies have significantly improved the electrophysiological investigation of cellular networks. Carbon-based nanomaterials or nanocarbons, such as carbon nanotubes (CNTs), nanodiamonds (NDs) and graphene are promising building blocks for bioelectronics platforms owing to their outstanding chemical and physical properties. In this review, we discuss the various bioelectronics applications of nanocarbons and their derivatives. Furthermore, we touch upon the challenges that remain in the field and describe the emergence of carbon-based hybrid-nanomaterials that will potentially address those limitations, thus improving the capabilities to investigate the electrophysiology of excitable cells, both as a network and at the single cell level.

Nanowire-Mesh-Templated Growth of Out-of-Plane Three-Dimensional Fuzzy Graphene

Graphene, a honeycomb sp² hybridized carbon lattice, is a promising building block for hybrid-nanomaterials due to its electrical, mechanical, and optical properties. Graphene can be readily obtained through mechanical exfoliation, solution-based deposition of reduced graphene oxide (rGO), and chemical vapor deposition (CVD). The resulting graphene films' topology is two-dimensional (2D) surface. Recently, synthesis of three-dimensional (3D) graphitic networks supported or templated by nanoparticles, foams, and hydrogels was reported. However, the resulting graphene films lay flat on the surface, exposing 2D surface topology. Out-of-plane grown carbon nanostructures, such as vertically aligned graphene sheets (VAGS) and vertical carbon nanowalls (CNWs), are still tethered to 2D surface. 3D morphology of out-of-plane growth of graphene hybrid-nanomaterials which leverages graphene's outstanding surface-to-volume ratio has not been achieved to date. Here we demonstrate highly controlled synthesis of 3D out-of-plane single- to few-layer fuzzy graphene (3DFG) on a Si nanowire (SiNW) mesh template. By varying graphene growth conditions (CH₄ partial pressure and process time), we control the size, density, and electrical properties of the NW templated 3DFG (NT-3DFG). 3DFG growth can be described by a diffusion-limited-aggregation (DLA) model. The porous NT-3DFG meshes exhibited high electrical conductivity of ca. 2350 S m^-1. NT-3DFG demonstrated exceptional electrochemical functionality, with calculated specific electrochemical surface area as high as ca. 1017 m² g^-1 for a ca. 7 μm thick mesh. This flexible synthesis will inspire formation of complex hybrid-nanomaterials with tailored optical and electrical properties to be used in future applications such as sensing, and energy conversion and storage.

Effect of Graphene on Nonneuronal and Neuronal Cell Viability and Stress

In recent years graphene has drawn considerable research interest for biomedical applications. However, applications of graphene in biological systems also raise concerns about its possible toxicity. Here, by using live cell imaging techniques, we investigate the effect of pristine graphene on the viability as well as stress of both nonneuronal and neuronal cells under physiological conditions. We find that graphene promotes cell adhesion and proliferation. Furthermore, we find that graphene has no detectable adverse effect on mitochondrial membrane potential and morphology, or autophagy levels in the cell, indicating that graphene does not induce cell stress. Our results highlight the potential of graphene to be used in biomedical applications by providing long-term and stable nonneural and neural interfaces.

Synthesis of Group IV Nanowires on Graphene: The Case of Ge Nanocrawlers

In recent years, there has been a growing interest in using graphene as a synthesis platform for polymers, zero-dimensional (0D) materials, one-dimensional materials (1D), and two-dimensional (2D) materials. Here, we report the investigation of the growth of germanium nanowires (GeNWs) and germanium nanocrawlers (GeNCs) on single-layer graphene surfaces. GeNWs and GeNCs are synthesized on graphene films by gold nanoparticles catalyzed vapor-liquid-solid growth mechanism. The addition of hydrogen chloride gas (HCl) at the nucleation step increased the propensity toward GeNCs growth on the surface. As the time lag before HCl introduction during the nucleation step increased, a significant change in the number of out-of-plane GeNWs versus in-plane GeNCs was observed. The nucleation temperature and time played a key role in the formation of GeNCs as well. The fraction of GeNCs (χNCs) decreased from 0.95 ± 0.01 to 0.66 ± 0.07 when the temperature was kept at 305 °C for 15 s versus maintained at 305 °C throughout the process, respectively. GeNCs exhibit ⟨112⟩ as the preferred growth direction whereas GeNWs exhibit both ⟨112⟩ and ⟨111⟩ as the preferred growth directions. Finally, our growth model suggests a possible mechanism for the preference of an in-plane GeNC growth on graphene versus GeNW on SiO2. These findings open up unique opportunities for fundamental studies of crystal growth on graphene, as well as enable exploration of new electronic interfaces between group IV materials and graphene, potentially toward designing new geometries for hybrid materials sensors.

Photoactive electrospun fibers for inducing cell death

A photoactive electrospun material producing reactive oxygen species (ROS) upon light irradiation is reported. The phototoxicity of the generated ROS is spatially restricted to the fiber-tissue interface by conjugation of the photosensitizer to a macromolecule. Photo-triggered ROS is produced on demand and repeatedly. It induces death of mammalian cells growing on the material surface with high spatial resolution.

Slow DNA transport through nanopores in hafnium oxide membranes

We present a study of double- and single-stranded DNA transport through nanopores fabricated in ultrathin (2-7 nm thick) freestanding hafnium oxide (HfO2) membranes. The high chemical stability of ultrathin HfO2 enables long-lived experiments with <2 nm diameter pores that last several hours, in which we observe >50 000 DNA translocations with no detectable pore expansion. Mean DNA velocities are slower than velocities through comparable silicon nitride pores, providing evidence that HfO2 nanopores have favorable physicochemical interactions with nucleic acids that can be leveraged to slow down DNA in a nanopore.

Nanocomposite gold-silk nanofibers

Cell-biomaterial interactions can be controlled by modifying the surface chemistry or nanotopography of the material, to induce cell proliferation and differentiation if desired. Here we combine both approaches in forming silk nanofibers (SNFs) containing gold nanoparticles (AuNPs) and subsequently chemically modifying the fibers. Silk fibroin mixed with gold seed nanoparticles was electrospun to form SNFs doped with gold seed nanoparticles (SNF(seed)). Following gold reduction, there was a 2-fold increase in particle diameter confirmed by the appearance of a strong absorption peak at 525 nm. AuNPs were dispersed throughout the AuNP-doped silk nanofibers (SNFs(Au)). The Young's modulus of the SNFs(Au) was almost 70% higher than that of SNFs. SNFs(Au) were modified with the arginine-glycine-aspartic acid (RGD) peptide. Human mesenchymal stem cells that were cultured on RGD-modified SNF(Au) had a more than 2-fold larger cell area compared to the cells cultured on bare SNFs; SNF(Au) also increased cell size. This approach may be used to alter the cell-material interface in tissue engineering and other applications.

The smartest materials: the future of nanoelectronics in medicine

Electronics have become central to many aspects of biomedicine, ranging from fundamental biophysical studies of excitable tissues to medical monitoring and electronic implants to restore limb movement. The development of new materials and approaches is needed to enable enhanced tissue integration, interrogation, and stimulation and other functionalities. Nanoscale materials offer many avenues for progress in this respect. New classes of molecular-scale bioelectronic interfaces can be constructed using either one-dimensional nanostructures, such as nanowires and nanotubes, or two-dimensional nanostructures, such as graphene. Nanodevices can create ultrasensitive sensors and can be designed with spatial resolution as fine as the subcellular regime. Structures on the nanoscale can enable the development of engineered tissues within which sensing elements are integrated as closely as the nervous system within native tissues. In addition, the close integration of nanomaterials with cells and tissues will also allow the development of in vitro platforms for basic research or diagnostics. Such lab-on-a-chip systems could, for example, enable testing of the effects of candidate therapeutic molecules on intercellular, single-cell, and even intracellular physiology. Finally, advances in nanoelectronics can lead to extremely sophisticated smart materials with multifunctional capabilities, enabling the spectrum of biomedical possibilities from diagnostic studies to the creation of cyborgs.

Outside looking in: nanotube transistor intracellular sensors

Nanowire-based field-effect transistors, including devices with planar and three-dimensional configurations, are being actively explored as detectors for extra- and intracellular recording due to their small size and high sensitivities. Here we report the synthesis, fabrication, and characterization of a new needle-shaped nanoprobe based on an active silicon nanotube transistor, ANTT, that enables high-resolution intracellular recording. In the ANTT probe, the source/drain contacts to the silicon nanotube are fabricated on one end, passivated from external solution, and then time-dependent changes in potential can be recorded from the opposite nanotube end via the solution filling the tube. Measurements of conductance versus water-gate potential in aqueous solution show that the ANTT probe is selectively gated by potential changes within the nanotube, thus demonstrating the basic operating principle of the ANTT device. Studies interfacing the ANTT probe with spontaneously beating cardiomyocytes yielded stable intracellular action potentials similar to those reported by other electrophysiological techniques. In addition, the straightforward fabrication of ANTT devices was exploited to prepare multiple ANTT structures at the end of single probes, which enabled multiplexed recording of intracellular action potentials from single cells and multiplexed arrays of single ANTT device probes. These studies open up unique opportunities for multisite recordings from individual cells through cellular networks.

Synthetically encoded ultrashort-channel nanowire transistors for fast, pointlike cellular signal detection

Nanostructures, which have sizes comparable to biological functional units involved in cellular communication, offer the potential for enhanced sensitivity and spatial resolution compared to planar metal and semiconductor structures. Silicon nanowire (SiNW) field-effect transistors (FETs) have been used as a platform for biomolecular sensors, which maintain excellent signal-to-noise ratios while operating on lengths scales that enable efficient extra- and intracellular integration with living cells. Although the NWs are tens of nanometers in diameter, the active region of the NW FET devices typically spans micrometers, limiting both the length and time scales of detection achievable with these nanodevices. Here, we report a new synthetic method that combines gold-nanocluster-catalyzed vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) NW growth modes to produce synthetically encoded NW devices with ultrasharp (<5 nm) n-type highly doped (n(++)) to lightly doped (n) transitions along the NW growth direction, where n(++) regions serve as source/drain (S/D) electrodes and the n-region functions as an active FET channel. Using this method, we synthesized short-channel n(++)/n/n(++) SiNW FET devices with independently controllable diameters and channel lengths. SiNW devices with channel lengths of 50, 80, and 150 nm interfaced with spontaneously beating cardiomyocytes exhibited well-defined extracellular field potential signals with signal-to-noise values of ca. 4 independent of device size. Significantly, these "pointlike" devices yield peak widths of ∼500 μs, which is comparable to the reported time constant for individual sodium ion channels. Multiple FET devices with device separations smaller than 2 μm were also encoded on single SiNWs, thus enabling multiplexed recording from single cells and cell networks with device-to-device time resolution on the order of a few microseconds. These short-channel SiNW FET devices provide a new opportunity to create nanoscale biomolecular sensors that operate on the length and time scales previously inaccessible by other techniques but necessary to investigate fundamental, subcellular biological processes.

Intracellular recordings of action potentials by an extracellular nanoscale field-effect transistor

The ability to make electrical measurements inside cells has led to many important advances in electrophysiology. The patch clamp technique, in which a glass micropipette filled with electrolyte is inserted into a cell, offers both high signal-to-noise ratio and temporal resolution. Ideally, the micropipette should be as small as possible to increase the spatial resolution and reduce the invasiveness of the measurement, but the overall performance of the technique depends on the impedance of the interface between the micropipette and the cell interior, which limits how small the micropipette can be. Techniques that involve inserting metal or carbon microelectrodes into cells are subject to similar constraints. Field-effect transistors (FETs) can also record electric potentials inside cells, and because their performance does not depend on impedance, they can be made much smaller than micropipettes and microelectrodes. Moreover, FET arrays are better suited for multiplexed measurements. Previously, we have demonstrated FET-based intracellular recording with kinked nanowire structures, but the kink configuration and device design places limits on the probe size and the potential for multiplexing. Here, we report a new approach in which a SiO2 nanotube is synthetically integrated on top of a nanoscale FET. This nanotube penetrates the cell membrane, bringing the cell cytosol into contact with the FET, which is then able to record the intracellular transmembrane potential. Simulations show that the bandwidth of this branched intracellular nanotube FET (BIT-FET) is high enough for it to record fast action potentials even when the nanotube diameter is decreased to 3 nm, a length scale well below that accessible with other methods. Studies of cardiomyocyte cells demonstrate that when phospholipid-modified BIT-FETs are brought close to cells, the nanotubes can spontaneously penetrate the cell membrane to allow the full-amplitude intracellular action potential to be recorded, thus showing that a stable and tight seal forms between the nanotube and cell membrane. We also show that multiple BIT-FETs can record multiplexed intracellular signals from both single cells and networks of cells.

Three-dimensional, flexible nanoscale field-effect transistors as localized bioprobes

Nanoelectronic devices offer substantial potential for interrogating biological systems, although nearly all work has focused on planar device designs. We have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis versus trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aqueous solution, independent of large mechanical deflections, and demonstrated high pH sensitivity. Additionally, 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.

Design and Implementation of Functional Nanoelectronic Interfaces With Biomolecules, Cells, and Tissue Using Nanowire Device Arrays

Nanowire FETs (NWFETs) are promising building blocks for nanoscale bioelectronic interfaces with cells and tissue since they are known to exhibit exquisite sensitivity in the context of chemical and biological detection, and have the potential to form strongly coupled interfaces with cell membranes. We present a general scheme that can be used to assemble NWs with rationally designed composition and geometry on either planar inorganic or biocompatible flexible plastic surfaces. We demonstrate that these devices can be used to measure signals from neurons, cardiomyocytes, and heart tissue. Reported signals are in millivolts range, which are equal to or substantially greater than those recorded with either planar FETs or multielectrode arrays, and demonstrate one unique advantage of NW-based devices. Basic studies showing the effect of device sensitivity and cell/substrate junction quality on signal magnitude are presented. Finally, our demonstrated ability to design high-density arrays of NWFETs enables us to map signal at the subcellular level, a functionality not enabled by conventional microfabricated devices. These advances could have broad applications in high-throughput drug assays, fundamental biophysical studies of cellular function, and development of powerful prosthetics.

Graphene and nanowire transistors for cellular interfaces and electrical recording

Nanowire field-effect transistors (NW-FETs) have been shown to be powerful building blocks for nanoscale bioelectronic interfaces with cells and tissue due to their excellent sensitivity and their capability to form strongly coupled interfaces with cell membranes. Graphene has also been shown to be an attractive building block for nanoscale electronic devices, although little is known about its interfaces with cells and tissue. Here we report the first studies of graphene field effect transistors (Gra-FETs) as well as combined Gra- and NW-FETs interfaced to electrogenic cells. Gra-FET conductance signals recorded from spontaneously beating embryonic chicken cardiomyocytes yield well-defined extracellular signals with signal-to-noise ratio routinely >4. The conductance signal amplitude was tuned by varying the Gra-FET working region through changes in water gate potential, V(wg). Signals recorded from cardiomyocytes for different V(wg) result in constant calibrated extracellular voltage, indicating a robust graphene/cell interface. Significantly, variations in V(wg) across the Dirac point demonstrate the expected signal polarity flip, thus allowing, for the first time, both n- and p-type recording to be achieved from the same Gra-FET simply by offsetting V(wg). In addition, comparisons of peak-to-peak recorded signal widths made as a function of Gra-FET device sizes and versus NW-FETs allowed an assessment of relative resolution in extracellular recording. Specifically, peak-to-peak widths increased with the area of Gra-FET devices, indicating an averaged signal from different points across the outer membrane of the beating cells. One-dimensional silicon NW- FETs incorporated side by side with the two-dimensional Gra-FET devices further highlighted limits in both temporal resolution and multiplexed measurements from the same cell for the different types of devices. The distinct and complementary capabilities of Gra- and NW-FETs could open up unique opportunities in the field of bioelectronics in the future.

Flexible electrical recording from cells using nanowire transistor arrays

Semiconductor nanowires (NWs) have unique electronic properties and sizes comparable with biological structures involved in cellular communication, thus making them promising nanostructures for establishing active interfaces with biological systems. We report a flexible approach to interface NW field-effect transistors (NWFETs) with cells and demonstrate this for silicon NWFET arrays coupled to embryonic chicken cardiomyocytes. Cardiomyocyte cells were cultured on thin, optically transparent polydimethylsiloxane (PDMS) sheets and then brought into contact with Si-NWFET arrays fabricated on standard substrates. NWFET conductance signals recorded from cardiomyocytes exhibited excellent signal-to-noise ratios with values routinely >5 and signal amplitudes that were tuned by varying device sensitivity through changes in water gate-voltage potential, V(g). Signals recorded from cardiomyocytes for V(g) from -0.5 to +0.1 V exhibited amplitude variations from 31 to 7 nS whereas the calibrated voltage remained constant, indicating a robust NWFET/cell interface. In addition, signals recorded as a function of increasing/decreasing displacement of the PDMS/cell support to the device chip showed a reversible >2x increase in signal amplitude (calibrated voltage) from 31 nS (1.0 mV) to 72 nS (2.3 mV). Studies with the displacement close to but below the point of cell disruption yielded calibrated signal amplitudes as large as 10.5 +/- 0.2 mV. Last, multiplexed recording of signals from NWFET arrays interfaced to cardiomyocyte monolayers enabled temporal shifts and signal propagation to be determined with good spatial and temporal resolution. Our modular approach simplifies the process of interfacing cardiomyocytes and other cells to high-performance Si-NWFETs, thus increasing the experimental versatility of NWFET arrays and enabling device registration at the subcellular level.

Electrical recording from hearts with flexible nanowire device arrays

We show that nanowire field-effect transistor (NWFET) arrays fabricated on both planar and flexible polymeric substrates can be reproducibly interfaced with spontaneously beating embryonic chicken hearts in both planar and bent conformations. Simultaneous recordings from glass microelectrode and NWFET devices show that NWFET conductance variations are synchronized with the beating heart. The conductance change associated with beating can be tuned substantially by device sensitivity, although the voltage-calibrated signals, 4-6 mV, are relatively constant and typically larger than signals recorded by microelectrode arrays. Multiplexed recording from NWFET arrays yielded signal propagation times across the myocardium with high spatial resolution. The transparent and flexible NWFET chips also enable simultaneous electrical recording and optical registration of devices to heart surfaces in three-dimensional conformations not possible with planar microdevices. The capability of simultaneous optical imaging and electrical recording also could be used to register devices to a specific region of the myocardium at the cellular level, and more generally, NWFET arrays fabricated on increasingly flexible plastic and/or biopolymer substrates have the potential to become unique tools for electrical recording from other tissue/organ samples or as powerful implants.