Mesh Nanoelectronics: Seamless Integration of Electronics with Tissues

A perspective on translating genomic discoveries into targets for brain-machine interface and deep brain stimulation devices

Mental illnesses have a huge impact on individuals, families, and society, so there is a growing need for more efficient treatments. In this context, brain-computer interface (BCI) technology has the potential to revolutionize the options for neuropsychiatric therapies. However, the development of BCI-based therapies faces enormous challenges, such as power dissipation constraints, lack of credible feedback mechanisms, uncertainty of which brain areas and frequencies to target, and even which patients to treat. Some of these setbacks are due to the large gap in our understanding of brain function. In recent years, large-scale genomic analyses uncovered an unprecedented amount of information regarding the biology of the altered brain function observed across the psychopathology spectrum. We believe findings from genetic studies can be useful to refine BCI technology to develop novel treatment options for mental illnesses. Here, we assess the latest advancements in both fields, the possibilities that can be generated from their intersection, and the challenges that these research areas will need to address to ensure that translational efforts can lead to effective and reliable interventions. Specifically, starting from highlighting the overlap between mechanisms uncovered by large-scale genetic studies and the current targets of deep brain stimulation treatments, we describe the steps that could help to translate genomic discoveries into BCI targets. Because these two research areas have not been previously presented together, the present article can provide a novel perspective for scientists with different research backgrounds. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Biomedical Engineering.

An Integrated Optogenetic and Bioelectronic Platform for Regulating Cardiomyocyte Function

We report an integrated optogenetic and bioelectronic platform for stable and long-term modulation and monitoring of cardiomyocyte function in vitro. Optogenetic inputs were achieved through expression of a photoactivatable adenylyl cyclase (bPAC), that when activated by blue light caused a dose-dependent and time-limited increase in autonomous cardiomyocyte beat rate. Bioelectronic readouts were achieved through an integrated planar multi-electrode array (MEA) that provided real-time readouts of electrophysiological activity from 32 spatially-distinct locations. Irradiation at 27 μW/mm2 resulted in a ca. 14% increase in beat rate within 20-25 minutes, which remained stable for at least 2 hours. The beating rate could be cycled through repeated "on" and "off' states, and its magnitude was a monotonic function of irradiation intensity. Our integrated platform opens new avenues in bioelectronic medicine, including closed-loop feedback systems, with potential applications for cardiac regulation including arrhythmia diagnosis and intervention.

A flexible implant for acute intrapancreatic electrophysiology

Microelectrode arrays (MEAs) have proven to be a powerful tool to study electrophysiological processes over the last decades with most technology developed for investigation of the heart or brain. Other targets in the field of bioelectronic medicine are the peripheral nervous system and its innervation of various organs. Beyond the heart and nervous systems, the beta cells of the pancreatic islets of Langerhans generate action potentials during the production of insulin. In vitro experiments have demonstrated that their activity is a biomarker for blood glucose levels, suggesting that recording their activity in vivo could support patients suffering from diabetes mellitus with long-term automated read-out of blood glucose concentrations. Here, we present a flexible polymer-based implant having 64 low impedance microelectrodes designed to be implanted to a depth of 10 mm into the pancreas. As a first step, the implant will be used in acute experiments in pigs to explore the electrophysiological processes of the pancreas in vivo. Beyond use in the pancreas, our flexible implant and simple implantation method may also be used in other organs such as the brain.

Biomimetic Flexible Sensors and Their Applications in Human Health Detection

Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments.

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

Cleanroom strategies for micro- and nano-fabricating flexible implantable neural electronics

Implantable electronic neural interfaces typically take the form of probes and are made with rigid materials such as silicon and metals. These have advantages such as compatibility with integrated microchips, simple implantation and high-density nanofabrication but tend to be lacking in terms of biointegration, biocompatibility and durability due to their mechanical rigidity. This leads to damage to the device or, more importantly, the brain tissue surrounding the implant. Flexible polymer-based probes offer superior biocompatibility in terms of surface biochemistry and better matched mechanical properties. Research which aims to bring the fabrication of electronics on flexible polymer substrates to the nano-regime is remarkably sparse, despite the push for flexible consumer electronics in the last decade or so. Cleanroom-based nanofabrication techniques such as photolithography have been used as pattern transfer methods by the semiconductor industry to produce single nanometre scale devices and are now also used for making flexible circuit boards. There is still much scope for miniaturizing flexible electronics further using photolithography, bringing the possibility of nanoscale, non-invasive, high-density flexible neural interfacing. This work explores the fabrication challenges of using photolithography and complementary techniques in a cleanroom for producing flexible electronic neural probes with nanometre-scale features. This article is part of the theme issue 'Advanced neurotechnologies: translating innovation for health and well-being'.

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

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

Flexible Electrodes for In Vivo and In Vitro Electrophysiological Signal Recording

A variety of electrophysiological signals (electrocardiography, electromyography, electroencephalography, etc.) are generated during the physiological activities of human bodies, which can be collected by electrodes and thus provide critical insights into health status or facilitate fundamental scientific research. The long-term stable and high-quality recording of electrophysiological signals is the premise for their further applications, leading to demands for flexible electrodes with similar mechanical modulus and minimized irritation to human bodies. This review summarizes the latest advances in flexible electrodes for the acquisition of various electrophysiological signals. First, the concept of electrophysiological signals and the characteristics of different subcategory signals are introduced. Second, the invasive and noninvasive methods are reviewed for electrophysiological signal recording with a highlight on the design of flexible electrodes, followed by a discussion on their material selection. Subsequently, the applications of the electrophysiological signal acquisition in pathological diagnosis and restoration of body functions are discussed, showing the advantages of flexible electrodes. Finally, the main challenges and opportunities in this field are discussed. It is believed that the further exploration of materials for flexible electrodes and the combination of multidisciplinary technologies will boost the applications of flexible electrodes for medical diagnosis and human-machine interface.

Nano- and Microscale Optical and Electrical Biointerfaces and Their Relevance to Energy Research

Different research fields in energy sciences, such as photovoltaics for solar energy conversion, supercapacitors for energy storage, electrocatalysis for clean energy conversion technologies, and materials-bacterial hybrid for CO₂ fixation have been under intense investigations over the past decade. In recent years, new platforms for biointerface designs have emerged from the energy conversion and storage principles. This paper reviews recent advances in nano- and microscale materials/devices for optical and electrical biointerfaces. First, a connection is drawn between biointerfaces and energy science, and how these two distinct research fields can be connected is summarized. Then, a brief overview of current available tools for biointerface studies is presented. Third, three representative biointerfaces are reviewed, including neural, cardiac, and bacterial biointerfaces, to show how to apply these tools and principles to biointerface design and research. Finally, two possible future research directions for nano- and microscale biointerfaces are proposed.

Nanowire-Enabled Bioelectronics

Bioelectronics explores the use of electronic devices for applications in signal transduction at their interfaces with biological systems. The miniaturization of the bioelectronic systems has enabled seamless integration at these interfaces and is providing new scientific and technological opportunities. In particular, nanowire-based devices can yield smaller sized and unique geometry detectors that are difficult to access with standard techniques, and thereby can provide advantages in sensitivity with reduced invasiveness. In this review, we focus on nanowire-enabled bioelectronics. First, we provide an overview of synthetic studies for designed growth of semiconductor nanowires of which structure and composition are controlled to enable key elements for bioelectronic devices. Second, we review nanowire field-effect transistor sensors for highly sensitive detection of biomolecules, their applications in diagnosis and drug discovery, and methods for sensitivity enhancement. We then turn to recent progress in nanowire-enabled studies of electrogenic cells, including cardiomyocytes and neurons. Representative advances in electrical recording using nanowire electronic devices for single cell measurements, cell network mapping, and three-dimensional recordings of synthetic and natural tissues, and in vivo brain mapping are highlighted. Finally, we overview the key challenges and opportunities of nanowires for fundamental research and translational applications.

Compliant 3D frameworks instrumented with strain sensors for characterization of millimeter-scale engineered muscle tissues

Tissue-on-chip systems represent promising platforms for monitoring and controlling tissue functions in vitro for various purposes in biomedical research. The two-dimensional (2D) layouts of these constructs constrain the types of interactions that can be studied and limit their relevance to three-dimensional (3D) tissues. The development of 3D electronic scaffolds and microphysiological devices with geometries and functions tailored to realistic 3D tissues has the potential to create important possibilities in advanced sensing and control. This study presents classes of compliant 3D frameworks that incorporate microscale strain sensors for high-sensitivity measurements of contractile forces of engineered optogenetic muscle tissue rings, supported by quantitative simulations. Compared with traditional approaches based on optical microscopy, these 3D mechanical frameworks and sensing systems can measure not only motions but also contractile forces with high accuracy and high temporal resolution. Results of active tension force measurements of engineered muscle rings under different stimulation conditions in long-term monitoring settings for over 5 wk and in response to various chemical and drug doses demonstrate the utility of such platforms in sensing and modulation of muscle and other tissues. Possibilities for applications range from drug screening and disease modeling to biohybrid robotic engineering.

The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics

Neurological diseases are a prevalent cause of global mortality and are of growing concern when considering an ageing global population. Traditional treatments are accompanied by serious side effects including repeated treatment sessions, invasive surgeries, or infections. For example, in the case of deep brain stimulation, large, stiff, and battery powered neural probes recruit thousands of neurons with each pulse, and can invoke a vigorous immune response. This paper presents challenges in engineering and neuroscience in developing miniaturized and biointegrated alternatives, in the form of microelectrode probes. Progress in design and topology of neural implants has shifted the goal post toward highly specific recording and stimulation, targeting small groups of neurons and reducing the foreign body response with biomimetic design principles. Implantable device design recommendations, fabrication techniques, and clinical evaluation of the impact flexible, integrated probes will have on the treatment of neurological disorders are provided in this report. The choice of biocompatible material dictates fabrication techniques as novel methods reduce the complexity of manufacture. Wireless power, the final hurdle to truly implantable neural interfaces, is discussed. These aspects are the driving force behind continued research: significant breakthroughs in any one of these areas will revolutionize the treatment of neurological disorders.

Toward nanobioelectronic medicine: Unlocking new applications using nanotechnology

Bioelectronic medicine aims to interface electronic technology with biological components and design more effective therapeutic and diagnostic tools. Advances in nanotechnology have moved the field forward improving the seamless interaction between biological and electronic components. In the lab many of these nanobioelectronic devices have the potential to improve current treatment approaches, including those for cancer, cardiovascular disorders, and disease underpinned by malfunctions in neuronal electrical communication. While promising, many of these devices and technologies require further development before they can be successfully applied in a clinical setting. Here, we highlight recent work which is close to achieving this goal, including discussion of nanoparticles, carbon nanotubes, and nanowires for medical applications. We also look forward toward the next decade to determine how current developments in nanotechnology could shape the growing field of bioelectronic medicine. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Biosensing.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Cellular-scale silicon probes for high-density, precisely localized neurophysiology

Neural implants with large numbers of electrodes have become an important tool for examining brain functions. However, these devices typically displace a large intracranial volume compared with the neurons they record. This large size limits the density of implants, provokes tissue reactions that degrade chronic performance, and impedes the ability to accurately visualize recording sites within intact circuits. Here we report next-generation silicon-based neural probes at a cellular scale (5 × 10 µm cross section), with ultra-high-density packing (as little as 66 µm between shanks) and 64 or 256 closely spaced recording sites per probe. We show that these probes can be inserted into superficial or deep brain structures and record large spikes in freely behaving rats for many weeks. Finally, we demonstrate a slice-in-place approach for the precise registration of recording sites relative to nearby neurons and anatomical features, including striatal µ-opioid receptor patches. This scalable technology provides a valuable tool for examining information processing within neural circuits and potentially for human brain-machine interfaces.NEW & NOTEWORTHY Devices with many electrodes penetrating into the brain are an important tool for investigating neural information processing, but they are typically large compared with neurons. This results in substantial damage and makes it harder to reconstruct recording locations within brain circuits. This paper presents high-channel-count silicon probes with much smaller features and a method for slicing through probe, brain, and skull all together. This allows probe tips to be directly observed relative to immunohistochemical markers.

Gels, jets, mosquitoes, and magnets: a review of implantation strategies for soft neural probes

Implantable neuroelectronic interfaces have enabled breakthrough advances in the clinical diagnosis and treatment of neurological disorders, as well as in fundamental studies of brain function, behavior, and disease. Intracranial electroencephalography (EEG) mapping with stereo-EEG (sEEG) depth electrodes is routinely adopted for precise epilepsy diagnostics and surgical treatment, while deep brain stimulation has become the standard of care for managing movement disorders. Intracortical microelectrode arrays for high-fidelity recordings of neural spiking activity have led to impressive demonstrations of the power of brain-machine interfaces for motor and sensory functional recovery. Yet, despite the rapid pace of technology development, the issue of establishing a safe, long-term, stable, and functional interface between neuroelectronic devices and the host brain tissue still remains largely unresolved. A body of work spanning at least the last 15 years suggests that safe, chronic integration between invasive electrodes and the brain requires a close match between the mechanical properties of man-made components and the neural tissue. In other words, the next generation of invasive electrodes should be soft and compliant, without sacrificing biological and chemical stability. Soft neuroelectronic interfaces, however, pose a new and significant surgical challenge: bending and buckling during implantation that can preclude accurate and safe device placement. In this topical review, we describe the next generation of soft electrodes and the surgical implantation methods for safe and precise insertion into brain structures. We provide an overview of the most recent innovations in the field of insertion strategies for flexible neural electrodes such as dissolvable or biodegradable carriers, microactuators, biologically-inspired support structures, and electromagnetic drives. In our analysis, we also highlight approaches developed in different fields, such as robotic surgery, which could be potentially adapted and translated to the insertion of flexible neural probes.

Bioinspired flexible electronics for seamless neural interfacing and chronic recording

Implantable neural probes are among the most widely applied tools for the understanding of neural circuit functions and the treatment of neurological disorders. Despite remarkable progress in recent years, it is still challenging for conventional rigid probes to achieve stable neural recording over long periods of time. Recently, flexible electronics with biomimetic structures and mechanical properties have been demonstrated for the formation of seamless probe-neural interfaces, enabling long-term recording stability. In this review, we provide an overview of bioinspired flexible electronics, from their structural design to probe-brain interfaces and chronic neural recording applications. Opportunities of bioinspired flexible electronics in fundamental neuroscience and clinical studies are also discussed.

Photo-cross-linkable, insulating silk fibroin for bioelectronics with enhanced cell affinity

Bioelectronic scaffolds that support devices while promoting tissue integration could enable tissue hybrids with augmented electronic capabilities. Here, we demonstrate a photo-cross-linkable silk fibroin (PSF) derivative and investigate its structural, electrical, and chemical properties. Lithographically defined PSF films offered tunable thickness and <1-µm spatial resolution and could be released from a relief layer yielding freestanding scaffolds with centimeter-scale uniformity. These constructs were electrically insulating; multielectrode arrays with PSF-passivated interconnects provided stable electrophysiological readouts from HL-1 cardiac model cells, brain slices, and hearts. Compared to SU8, a ubiquitous biomaterial, PSF exhibited superior affinity toward neurons which we attribute to its favorable surface charge and enhanced attachment of poly-d-lysine adhesion factors. This finding is of significant importance in bioelectronics, where tight junctions between devices and cell membranes are necessary for electronic communication. Collectively, our findings are generalizable to a variety of geometries, devices, and tissues, establishing PSF as a promising bioelectronic platform.

Soft-Hard Composites for Bioelectric Interfaces

Bioelectric devices can probe fundamental biological dynamics and improve the lives of human beings. However, direct application of traditional rigid electronics onto soft tissues can cause signal transduction and biocompatibility issues. One common mitigation strategy is the use of soft-hard composites to form more biocompatible interfaces with target cells or tissues. Here, we identify several soft-hard composite designs in naturally occurring systems. We use these designs to categorize the existing bioelectric interfaces and to suggest future opportunities. We discuss the utility of soft-hard composites for a variety of interfaces, such as in vitro and in vivo electronic or optoelectronic sensing and genetic and non-genetic modulation. We end the review by proposing new soft-hard composites for future bioelectric studies.

Nano-enabled cellular engineering for bioelectric studies

Engineered cells have opened up a new avenue for scientists and engineers to achieve specialized biological functions. Nanomaterials, such as silicon nanowires and quantum dots, can establish tight interfaces with cells either extra- or intracellularly, and they have already been widely used to control cellular functions. The future exploration of nanomaterials in cellular engineering may reveal numerous opportunities in both fundamental bioelectric studies and clinic applications. In this review, we highlight several nanomaterials-enabled non-genetic approaches to fabricating engineered cells. First, we briefly review the latest progress in engineered or synthetic cells, such as protocells that create cell-like behaviors from nonliving building blocks, and cells made by genetic or chemical modifications. Next, we illustrate the need for non-genetic cellular engineering with semiconductors and present some examples where chemical synthesis yields complex morphology or functions needed for biointerfaces. We then provide discussions in detail about the semiconductor nanostructure-enabled neural, cardiac, and microbial modulations. We also suggest the need to integrate tissue engineering with semiconductor devices to carry out more complex functions. We end this review by providing our perspectives for future development in non-genetic cellular engineering.

Printing Flexible and Hybrid Electronics for Human Skin and Eye-Interfaced Health Monitoring Systems

Advances in printing materials and techniques for flexible and hybrid electronics in the domain of connected healthcare have enabled rapid development of innovative body-interfaced health monitoring systems at a tremendous pace. Thin, flexible, and stretchable biosensors that are printed on a biocompatible soft substrate provide the ability to noninvasively and unobtrusively integrate with the human body for continuous monitoring and early detection of diseases and other conditions affecting health and well being. Hybrid integration of such biosensors with extremely well-established silicon-based microcircuit chips offers a viable route for in-sensor data processing and wireless transmission in many medical and clinical settings. Here, a set of advanced and hybrid printing techniques is summarized, covering diverse aspects ranging from active electronic materials to process capability, for their use in human skin and eye-interfaced health monitoring systems with different levels of complexity. Essential components of the devices, including constituent biomaterials, structural layouts, assembly methods, and power and data processing configurations, are outlined and discussed in a categorized manner tailored to specific clinical needs. Perspectives on the benefits and challenges of these systems in basic and applied biomedical research are presented and discussed.

Injectable Biomedical Devices for Sensing and Stimulating Internal Body Organs

The rapid pace of progress in implantable electronics driven by novel technology has created devices with unconventional designs and features to reduce invasiveness and establish new sensing and stimulating techniques. Among the designs, injectable forms of biomedical electronics are explored for accurate and safe targeting of deep-seated body organs. Here, the classes of biomedical electronics and tools that have high aspect ratio structures designed to be injected or inserted into internal organs for minimally invasive monitoring and therapy are reviewed. Compared with devices in bulky or planar formats, the long shaft-like forms of implantable devices are easily placed in the organs with minimized outward protrusions via injection or insertion processes. Adding flexibility to the devices also enables effortless insertions through complex biological cavities, such as the cochlea, and enhances chronic reliability by complying with natural body movements, such as the heartbeat. Diverse types of such injectable implants developed for different organs are reviewed and the electronic, optoelectronic, piezoelectric, and microfluidic devices that enable stimulations and measurements of site-specific regions in the body are discussed. Noninvasive penetration strategies to deliver the miniscule devices are also considered. Finally, the challenges and future directions associated with deep body biomedical electronics are explained.

Conducting Polymer-Based Composite Materials for Therapeutic Implantations: From Advanced Drug Delivery System to Minimally Invasive Electronics

Conducting polymer-based composites have recently becoming popular in both academic research and industrial practices due to their high conductivity, ease of process, and tunable electrical properties. The multifunctional conducting polymer-based composites demonstrated great application potential for in vivo therapeutics and implantable electronics, including drug delivery, neural interfacing, and minimally invasive electronics. In this review article, the state-of-the-art conducting polymer-based composites in the mentioned biological fields are discussed and summarized. The recent progress on the synthesis, structure, properties, and application of the conducting polymer-based composites is presented, aimed at revealing the structure-property relationship and the corresponding functional applications of the conducting polymer-based composites. Furthermore, key issues and challenges regarding the implantation performance of these composites are highlighted in this paper.

Single-Molecule Electrical Detection: A Promising Route toward the Fundamental Limits of Chemistry and Life Science

The ultimate limit of analytical chemistry is single-molecule detection, which allows one to visualize the dynamic processes of chemical/biological interactions with single-molecule or single-event sensitivity and hence enables the study of stochastic fluctuations under equilibrium conditions and the observation of time trajectories and reaction pathways of individual species in nonequilibrated systems. In addition, such studies may also allow the direct observation of novel microscopic quantum effects and fundamental discoveries of underlying molecular mechanisms in organic reactions and biological processes that are not accessible in ensemble experiments, thus providing unique opportunities to solve the key problems of physical, chemical, and life sciences. Consequently, the field of single-molecule detection has received considerable attention and has witnessed tremendous advances in different directions in combination with other disciplines. This Account describes our ongoing work on the development of groundbreaking methods (termed "single-molecule electrical approaches") of translating the detailed processes of chemical reactions or biological functions into detectable electrical signals at the single-event level on the platform of single-molecule electronic devices, with a particular focus on graphene-molecule-graphene single-molecule junctions (GMG-SMJs) and silicon-nanowire-based single-molecule electrical nanocircuits. These nanocircuit-based architectures are complementary to conventional optical or mechanical techniques but exhibit obvious advantages such as the absence of problems associated with bleaching and fluorescent labeling. Dash-line lithography (DLL) is an efficient lithographic method of cutting graphene and forming carboxylic-acid-functionalized nanogapped graphene point contact arrays developed to address the formidable challenges of molecular device fabrication difficulty and poor stability. Molecules of interest terminated by amines on both ends can be covalently sandwiched between graphene point contacts to create high-throughput robust GMG-SMJs containing only one molecule as the conductive element. In conjunction with the ease of device fabrication and device stability, this feature distinguishes GMG-SMJs as a new testbed platform for single-molecule analysis characterized by high temporal resolution and superior signal-to-noise ratios. By exploiting the DLL method, we have fabricated molecular devices that are sensitive to external stimuli and are capable of transducing chemical/biochemical events into electrical signals at the single-molecule level, with notable examples including host-guest interaction, hydrogen bond dynamics, DNA intercalation, photoinduced conformational transition, carbocation formation, nucleophilic addition, and stereoelectronic effect. In addition to GMG-SMJs and considering compatibility with the silicon-based industry, we have also developed a reliable method of point-functionalizing silicon-nanowire-based nanotransistors to afford single-molecule electrical nanocircuits. This approach proved to be a robust platform for single-molecule electrical analysis capable of probing fast dynamic processes such as single-protein detection, DNA hybridization/polymorphism, and motor rotation dynamics. The above systematic investigations emphasize the importance and unique advantages of universal single-molecule electrical approaches for realizing direct, label-free, real-time electrical measurements of reaction dynamics with single-event sensitivity. These approaches promise a fascinating mainstream platform to explore the dynamics of stochastic processes in chemical/biological systems as well as gain information in fields ranging from reaction chemistry for elucidating the intrinsic mechanisms to genomics or proteomics for accurate molecular and even point-of-care clinical diagnoses.

Reporter Scaffolds for Clinically Relevant Cell Transplantation Studies

There are a number of cell therapies that are either in clinical trials or moving toward clinical trials, particularly for diseases of the retina. One of the challenges with cell therapies is tracking the status of cells over time. Genetic manipulation can facilitate this, but it can limit the clinical application of the cells. There are a host of fluorophores that have been developed to assess the status of cells, but these molecules tend to be cleared rapidly from cells. There are preclinical strategies that use degradable scaffolds, and we hypothesized that these scaffolds could be used to track the state of cells during preclinical studies. In this work, we explored whether fluorophores could be delivered from simple scaffolds fabricated under extremely harsh conditions, be active upon release, and report on the cells growing on the scaffolds over time. We encapsulated CellROX® Green Reagent, and pHrodo™ Red AM in poly(lactic-co-glycolic acid) (PLGA) scaffolds, showed that they could be delivered over weeks and were still active upon release and taken up by cells. These experiments provide the foundation for using scaffolds to deliver molecules to report on cells.

Modularized Field-Effect Transistor Biosensors

Field-effect transistors (FETs), when functionalized with proper biorecognition elements (such as antibodies or enzymes), represent a unique platform for real-time, specific, label-free transduction of biochemical signals. However, direct immobilization of biorecognition molecules on FETs imposes limitations on reprogrammability, sensor regeneration, and robust device handling. Here we demonstrate a modularized design of FET biosensors with separate biorecognition and transducer modules, which are capable of reversible assembly and disassembly. In particular, hydrogel "stamps" immobilizing bioreceptors have been chosen to build biorecognition modules to reliably interface with FET transducers structurally and functionally. Successful detection of penicillin down to 0.25 mM has been achieved with a penicillinase-encoded hydrogel module, demonstrating effective signal transduction across the hybrid interface. Moreover, sequential integration of urease- and penicillinase-encoded modules on the same FET device allows us to reprogram the sensing modality without cross-contamination. In addition to independent bioreceptor encoding, the modular design also fosters sophisticated control of sensing kinetics by modulating the physiochemical microenvironment in the biorecognition modules. Specifically, the distinction in hydrogel porosity between polyethylene glycol and gelatin enables controlled access and detection of larger molecules, such as poly-l-lysine (MW 150-300 kDa), only through the gelatin module. Biorecognition modules with standardized interface designs have also been exploited to comply with additive mass fabrication by 3D printing, demonstrating potential for low cost, ease of storage, multiplexing, and great customizability for personalized biosensor production. This generic concept presents a unique integration strategy for modularized bioelectronics and could broadly impact hybrid device development.

Nanowired Bioelectric Interfaces

Biological systems have evolved biochemical, electrical, mechanical, and genetic networks to perform essential functions across various length and time scales. High-aspect-ratio biological nanowires, such as bacterial pili and neurites, mediate many of the interactions and homeostasis in and between these networks. Synthetic materials designed to mimic the structure of biological nanowires could also incorporate similar functional properties, and exploiting this structure-function relationship has already proved fruitful in designing biointerfaces. Semiconductor nanowires are a particularly promising class of synthetic nanowires for biointerfaces, given (1) their unique optical and electronic properties and (2) their high degree of synthetic control and versatility. These characteristics enable fabrication of a variety of electronic and photonic nanowire devices, allowing for the formation of well-defined, functional bioelectric interfaces at the biomolecular level to the whole-organ level. In this Focus Review, we first discuss the history of bioelectric interfaces with semiconductor nanowires. We next highlight several important, endogenous biological nanowires and use these as a framework to categorize semiconductor nanowire-based biointerfaces. Within this framework we then review the fundamentals of bioelectric interfaces with semiconductor nanowires and comment on both material choice and device design to form biointerfaces spanning multiple length scales. We conclude with a discussion of areas with the potential for greatest impact using semiconductor nanowire-enabled biointerfaces in the future.

Nanoenabled Direct Contact Interfacing of Syringe-Injectable Mesh Electronics

Polymer-based electronics with low bending stiffnesses and high flexibility, including recently reported macroporous syringe-injectable mesh electronics, have shown substantial promise for chronic studies of neural circuitry in the brains of live animals. A central challenge for exploiting these highly flexible materials for in vivo studies has centered on the development of efficient input/output (I/O) connections to an external interface with high yield, low bonding resistance, and long-term stability. Here we report a new paradigm applied to the challenging case of injectable mesh electronics that exploits the high flexibility of nanoscale thickness two-sided metal I/O pads that can deform and contact standard interface cables in high yield with long-term electrical stability. First, we describe the design and facile fabrication of two-sided metal I/O pads that allow for contact without regard to probe orientation. Second, systematic studies of the contact resistance as a function of I/O pad design and mechanical properties demonstrate the key role of the I/O pad bending stiffness in achieving low-resistance stable contacts. Additionally, computational studies provide design rules for achieving high-yield multiplexed contact interfacing in the case of angular misalignment such that adjacent channels are not shorted. Third, the in vitro measurement of 32-channel mesh electronics probes bonded to interface cables using the direct contact method shows a reproducibly high yield of electrical connectivity. Finally, in vivo experiments with 32-channel mesh electronics probes implanted in live mice demonstrate the chronic stability of the direct contact interface, enabling consistent tracking of single-unit neural activity over at least 2 months without a loss of channel recording. The direct contact interfacing methodology paves the way for scalable long-term connections of multiplexed mesh electronics neural probes for neural recording and modulation and moreover could be used to facilitate a scalable interconnection of other flexible electronics in biological studies and therapeutic applications.

Deterministic Assembly of Three-Dimensional Suspended Nanowire Structures

Controlled assembly of nanowire three-dimensional (3D) geometry in an addressable way can lead to advanced 3D device integration and application. By combining a deterministic planar nanowire assembly and a transfer process, we show here a versatile method to construct vertically protruding and suspending nanowire structures. The method harnesses the merits from both processes to yield positional and geometric control in individual nanowires. Multiple transfers can further lead to hierarchical multiwire 3D structures. Assembled 3D nanowire structures have well-defined on-substrate terminals that allow scalable addressing and integration. Proof-of-concept nanosenors based on assembled 3D nanowire structures can achieve high sensitivity in force detection.

Hybrid Silicon Nanowire Devices and Their Functional Diversity

In the pool of nanostructured materials, silicon nanostructures are known as conventionally used building blocks of commercially available electronic devices. Their application areas span from miniaturized elements of devices and circuits to ultrasensitive biosensors for diagnostics. In this Review, the current trends in the developments of silicon nanowire-based devices are summarized, and their functionalities, novel architectures, and applications are discussed from the point of view of analog electronics, arisen from the ability of (bio)chemical gating of the carrier channel. Hybrid nanowire-based devices are introduced and described as systems decorated by, e.g., organic complexes (biomolecules, polymers, and organic films), aimed to substantially extend their functionality, compared to traditional systems. Their functional diversity is explored considering their architecture as well as areas of their applications, outlining several groups of devices that benefit from the coatings. The first group is the biosensors that are able to represent label-free assays thanks to the attached biological receptors. The second group is represented by devices for optoelectronics that acquire higher optical sensitivity or efficiency due to the specific photosensitive decoration of the nanowires. Finally, the so-called new bioinspired neuromorphic devices are shown, which are aimed to mimic the functions of the biological cells, e.g., neurons and synapses.

An atlas of nano-enabled neural interfaces

Advances in microscopy and molecular strategies have allowed researchers to gain insight into the intricate organization of the mammalian brain and the roles that neurons play in processing information. Despite vast progress, therapeutic strategies for neurological disorders remain limited, owing to a lack of biomaterials for sensing and modulating neuronal signalling in vivo. Therefore, there is a pressing need for developing material-based tools that can form seamless biointerfaces and interrogate the brain with unprecedented resolution. In this Review, we discuss important considerations in material design and implementation, highlight recent breakthroughs in neural sensing and modulation, and propose future directions in neurotechnology research. Our goal is to create an atlas for nano-enabled neural interfaces and to demonstrate how emerging nanotechnologies can interrogate neural systems spanning multiple biological length scales.

Advanced One- and Two-Dimensional Mesh Designs for Injectable Electronics

The unique structure and mechanical properties of syringe-injectable mesh electronics have enabled seamless tissue integration and stable chronic recording of the activities of the same neurons on a year scale. Here, we report studies of a series of structural and mechanical mesh electronics design variations that allow injection using needles at least 4-fold smaller than those previously reported to minimize the footprint during injection of the electronics in soft matter and tissue. Characterization of new ultraflexible two-dimensional (2D) and one-dimensional (1D) probes has demonstrated reproducible injection of the newly developed mesh electronics designs via needles as small as 100 μm in inner diameter (ID) with reduced injection volumes. In vitro hydrogel and in vivo mouse brain studies have shown that ultraflexible 2D and 1D probes maintain their structural integrity and conformation post-injection after being transferred through the reduced diameter needles. In addition, analysis of the variation of the post-injection mesh cross sections suggests a smaller degree of tissue deformation and relaxation with decreasing needle diameters. The capability to implement rational design for mesh electronic probes that can be delivered via much smaller diameter needles should open up new opportunities for integration of electronics with tissue and soft matter in fundamental and translational studies.

Novel electrode technologies for neural recordings

Neural recording electrode technologies have contributed considerably to neuroscience by enabling the extracellular detection of low-frequency local field potential oscillations and high-frequency action potentials of single units. Nevertheless, several long-standing limitations exist, including low multiplexity, deleterious chronic immune responses and long-term recording instability. Driven by initiatives encouraging the generation of novel neurotechnologies and the maturation of technologies to fabricate high-density electronics, novel electrode technologies are emerging. Here, we provide an overview of recently developed neural recording electrode technologies with high spatial integration, long-term stability and multiple functionalities. We describe how these emergent neurotechnologies can approach the ultimate goal of illuminating chronic brain activity with minimal disruption of the neural environment, thereby providing unprecedented opportunities for neuroscience research in the future.

Multifunctional Freestanding Microprobes for Potential Biological Applications

Deep-level sensors for detecting the local temperatures of inner organs and tissues of an animal are rarely reported. In this paper, we present a method to fabricate multifunctional micro-probes with standard cleanroom procedures, using a piece of stainless-steel foil as the substrate. On each of the as-fabricated micro-probes, arrays of thermocouples made of Pd-Cr thin-film stripes with reliable thermal sensing functions were built, together with Pd electrode openings for detecting electrical signals. The as-fabricated sword-shaped freestanding microprobes with length up to 30 mm showed excellent mechanical strength and elastic properties when they were inserted into the brain and muscle tissues of live rats, as well as suitable electrochemical properties and, therefore, are promising for potential biological applications.

Hydrogel Gate Graphene Field-Effect Transistors as Multiplexed Biosensors

Nanoscale field-effect transistors (FETs) represent a unique platform for real time, label-free transduction of biochemical signals with unprecedented sensitivity and spatiotemporal resolution, yet their translation toward practical biomedical applications remains challenging. Herein, we demonstrate the potential to overcome several key limitations of traditional FET sensors by exploiting bioactive hydrogels as the gate material. Spatially defined photopolymerization is utilized to achieve selective patterning of polyethylene glycol on top of individual graphene FET devices, through which multiple biospecific receptors can be independently encapsulated into the hydrogel gate. The hydrogel-mediated integration of penicillinase was demonstrated to effectively catalyze enzymatic reaction in the confined microenvironment, enabling real time, label-free detection of penicillin down to 0.2 mM. Multiplexed functionalization with penicillinase and acetylcholinesterase has been demonstrated to achieve highly specific sensing. In addition, the microenvironment created by the hydrogel gate has been shown to significantly reduce the nonspecific binding of nontarget molecules to graphene channels as well as preserve the encapsulated enzyme activity for at least one week, in comparison to free enzymes showing significant signal loss within one day. This general approach presents a new biointegration strategy and facilitates multiplex detection of bioanalytes on the same platform, which could underwrite new advances in healthcare research.

Human Brain/Cloud Interface

The Internet comprises a decentralized global system that serves humanity's collective effort to generate, process, and store data, most of which is handled by the rapidly expanding cloud. A stable, secure, real-time system may allow for interfacing the cloud with the human brain. One promising strategy for enabling such a system, denoted here as a "human brain/cloud interface" ("B/CI"), would be based on technologies referred to here as "neuralnanorobotics." Future neuralnanorobotics technologies are anticipated to facilitate accurate diagnoses and eventual cures for the ∼400 conditions that affect the human brain. Neuralnanorobotics may also enable a B/CI with controlled connectivity between neural activity and external data storage and processing, via the direct monitoring of the brain's ∼86 × 109 neurons and ∼2 × 1014 synapses. Subsequent to navigating the human vasculature, three species of neuralnanorobots (endoneurobots, gliabots, and synaptobots) could traverse the blood-brain barrier (BBB), enter the brain parenchyma, ingress into individual human brain cells, and autoposition themselves at the axon initial segments of neurons (endoneurobots), within glial cells (gliabots), and in intimate proximity to synapses (synaptobots). They would then wirelessly transmit up to ∼6 × 1016 bits per second of synaptically processed and encoded human-brain electrical information via auxiliary nanorobotic fiber optics (30 cm3) with the capacity to handle up to 1018 bits/sec and provide rapid data transfer to a cloud based supercomputer for real-time brain-state monitoring and data extraction. A neuralnanorobotically enabled human B/CI might serve as a personalized conduit, allowing persons to obtain direct, instantaneous access to virtually any facet of cumulative human knowledge. Other anticipated applications include myriad opportunities to improve education, intelligence, entertainment, traveling, and other interactive experiences. A specialized application might be the capacity to engage in fully immersive experiential/sensory experiences, including what is referred to here as "transparent shadowing" (TS). Through TS, individuals might experience episodic segments of the lives of other willing participants (locally or remote) to, hopefully, encourage and inspire improved understanding and tolerance among all members of the human family.

Wearable and Implantable Soft Bioelectronics Using Two-Dimensional Materials

Soft bioelectronics intended for application to wearable and implantable biomedical devices have attracted great attention from material scientists, device engineers, and clinicians because of their extremely soft mechanical properties that match with a variety of human organs and tissues, including the brain, heart, skin, eye, muscles, and neurons, as well as their wide diversity in device designs and biomedical functions that can be finely tuned for each specific case of applications. These unique features of the soft bioelectronics have allowed minimal mechanical and biological damage to organs and tissues integrated with bioelectronic devices and reduced side effects including inflammation, skin irritation, and immune responses even after long-term biointegration. These favorable properties for biointegration have enabled long-term monitoring of key biomedical indicators with high signal-to-noise ratio, reliable diagnosis of the patient's health status, and in situ feedback therapy with high treatment efficacy optimized for the requirements of each specific disease model. These advantageous device functions and performances could be maximized by adopting novel high-quality soft nanomaterials, particularly ultrathin two-dimensional (2D) materials, for soft bioelectronics. Two-dimensional materials are emerging material candidates for the channels and electrodes in electronic devices (semiconductors and conductors, respectively). They can also be applied to various biosensors and therapeutic actuators in soft bioelectronics. The ultrathin vertically layered nanostructure, whose layer number can be controlled in the synthesis step, and the horizontally continuous planar molecular structure, which can be found over a large area, have conferred unique mechanical, electrical, and optical properties upon the 2D materials. The atomically thin nanostructure allows mechanical softness and flexibility and high optical transparency of the device, while the large-area continuous thin film structure allows efficient carrier transport within the 2D plane. In addition, the quantum confinement effect in the atomically thin 2D layers introduces interesting optoelectronic properties and superb photodetecting capabilities. When fabricated as soft bioelectronic devices, these interesting and useful material features of the 2D materials enable unconventional device functions in biological and optical sensing, as well as superb performance in electrical and biochemical therapeutic actuations. In this Account, we first summarize the distinctive characteristics of the 2D materials in terms of the mechanical, optical, chemical, electrical, and biomedical aspects and then present application examples of the 2D materials to soft bioelectronic devices based on each aforementioned unique material properties. Among various kinds of 2D materials, we particularly focus on graphene and MoS₂. The advantageous material features of graphene and MoS₂ include ultrathin thickness, facile functionalization, large surface-to-volume ratio, biocompatibility, superior photoabsorption, and high transparency, which allow the development of high-performance multifunctional soft bioelectronics, such as a wearable glucose patch, a highly sensitive humidity sensor, an ultrathin tactile sensor, a soft neural probe, a soft retinal prosthesis, a smart endoscope, and a cell culture platform. A brief comparison of their characteristics and performances is also provided. Finally, this Account concludes with a future outlook on next-generation soft bioelectronics based on 2D materials.

Inorganic semiconductor biointerfaces

Biological systems respond to and communicate through biophysical cues, such as electrical, thermal, mechanical and topographical signals. However, precise tools for introducing localized physical stimuli and/or for sensing biological responses to biophysical signals with high spatiotemporal resolution are limited. Inorganic semiconductors display many relevant electrical and optical properties, and they can be fabricated into a broad spectrum of electronic and photonic devices. Inorganic semiconductor devices enable the formation of functional interfaces with biological material, ranging from proteins to whole organs. In this Review, we discuss fundamental semiconductor physics and operation principles, with a focus on their behaviour in physiological conditions, and highlight the advantages of inorganic semiconductors for the establishment of biointerfaces. We examine semiconductor device design and synthesis and discuss typical signal transduction mechanisms at bioelectronic and biophotonic interfaces for electronic and optoelectronic sensing, optoelectronic and photothermal stimulation and photoluminescent in vivo imaging of cells and tissues. Finally, we evaluate cytotoxicity and highlight possible new material components and biological targets of inorganic semiconductor devices.

Methods for powering bioelectronic microdevices

Bioelectronic microdevices are an emerging class of biomedical devices miniaturized at the scale of a millimeter or less, which promise new capabilities for monitoring and treating human disease. Although rapid progress has been made in the sensing and actuation capabilities of microdevices, a major technological challenge remains in the way that these devices are powered within the body. In this review, we revisit the power requirements of microdevices, describe current methods for storing, transferring or harvesting energy in microdevices, provide an overview of emerging powering approaches and discuss the promise of microdevices in biomedicine.

All carbon materials pn diode

Semiconductor pn junctions are elementary building blocks of many electronic devices such as transistors, solar cells, photodetectors, and integrated circuits. Due to the absence of an energy bandgap and massless Dirac-like behaviour of charge carriers, graphene pn junction with electrical current rectification characteristics is hardly achieved. Here we show a graphene pn junction diode can be made exclusively from carbon materials by laminating two layers of positively and negatively charged graphene oxides. As the interdiffusion of oppositely charged mobile counterions, a built-in potential is created to rectify the current by changing the tunnelling probability of electrons across the junction. This graphene diode is semi-transparent, can perform simple logic operations, and since it has carbon nanotubes electrodes, we demonstrate an all carbon materials pn diode. We expect this graphene diode will expand material choices and provide functionalities (e.g. grafting recognition units on graphene oxides) beyond that of traditional semiconductor pn junctions.

Chemically functionalized graphene oxide-based pn junction diodes have potential for future electronic device applications. Here, the authors report an all carbon pn diode with graphene oxide and carbon nanotubes electrodes showing excellent current rectification and efficient logic gates.

Atomic Fe-Nx Coupled Open-Mesoporous Carbon Nanofibers for Efficient and Bioadaptable Oxygen Electrode in Mg-Air Batteries

The recently emerging metal-air batteries equipped with advanced oxygen electrodes have provided enormous opportunities to develop the next generation of wearable and bio-adaptable power sources. Theoretically, neutral electrolyte-based Mg-air batteries possess potential advantages in electronics and biomedical applications over the other metal-air counterparts, especially the alkaline-based Zn-air batteries. However, the rational design of advanced oxygen electrode for Mg-air batteries with high discharge voltage and capacity under neutral conditions still remains a major challenge. Inspired by fibrous string structures of bufo-spawn, it is reported here that the scalable synthesis of atomic Fe-Nx coupled open-mesoporous N-doped-carbon nanofibers (OM-NCNF-FeNx ) as advanced oxygen electrode for Mg-air batteries. The fabricated OM-NCNF-FeNx electrodes present manifold advantages, including open-mesoporous and interconnected structures, 3D hierarchically porous networks, good bio-adaptability, homogeneously coupled atomic Fe-Nx sites, and high oxygen electrocatalytic performances. Most importantly, the assembled Mg-air batteries with neutral electrolytes reveal high open-circuit voltage, stable discharge voltage plateaus, high capacity, long operating life, and good flexibility. Overall, the discovery on fabricating atomic OM-NCNF-FeNx electrode will not only create new pathways for achieving flexible, wearable, and bio-adaptable power sources, but also take a step towards the scale-up production of advanced nanofibrous carbon electrodes for a broad range of applications.

Syringe-injectable Mesh Electronics for Stable Chronic Rodent Electrophysiology

Implantable brain electrophysiology probes are valuable tools in neuroscience due to their ability to record neural activity with high spatiotemporal resolution from shallow and deep brain regions. Their use has been hindered, however, by mechanical and structural mismatches between the probes and brain tissue that commonly lead to micromotion and gliosis with resulting signal instability in chronic recording experiments. In contrast, following the implantation of ultraflexible mesh electronics via syringe injection, the mesh probes form a seamless, gliosis-free interface with the surrounding brain tissue that enables stable tracking of individual neurons on at least a year timescale. This protocol details the key steps in a typical mouse neural recording experiment using syringe-injectable mesh electronics, including the fabrication of mesh electronics in a standard photolithography-based process possible at many universities, loading mesh electronics into standard capillary needles, stereotaxic injection in vivo, connection of the mesh input/output to standard instrumentation interfaces, restrained or freely moving recording sessions, and histological sectioning of brain tissue containing mesh electronics. Representative neural recordings and histology data are presented. Investigators familiar with this protocol will have the knowledge necessary to incorporate mesh electronics into their own experiments and take advantage of the unique opportunities afforded by long-term stable neural interfacing, such as studies of aging processes, brain development, and the pathogenesis of brain disease.