Next-generation probes, particles, and proteins for neural interfacing

A Soft-Fiber Bioelectronic Device with Axon-Like Architecture Enables Reliable Neural Recording In Vivo under Vigorous Activities

Implantable neural devices that record neurons in various states, including static states, light activities such as walking, and vigorous activities such as running, offer opportunities for understanding brain functions and dysfunctions. However, recording neurons under vigorous activities remains a long-standing challenge because it leads to intense brain deformation. Thus, three key requirements are needed simultaneously for neural devices, that is, low modulus, low specific interfacial impedance, and high electrical conductivity, to realize stable device/brain interfaces and high-quality transmission of neural signals. However, they always contradict each other in current material strategies. Here, a soft fiber neural device capable of stably tracking individual neurons in the deep brain of medium-sized animals under vigorous activity is reported. Inspired by the axon architecture, this fiber neural device is constructed with a conductive gel fiber possessing a network-in-liquid structure using conjugated polymers and liquid matrices and then insulated with soft fluorine rubber. This strategy reconciles the contradictions and simultaneously confers the fiber neural device with low modulus (300 kPa), low specific impedance (579 kΩ µm2), and high electrical conductivity (32 700 S m-1) - ≈1-3 times higher than hydrogels. Stable single-unit spike tracking in running cats, which promises new opportunities for neuroscience is demonstrated.

Multifunctional Nanomaterials for Advancing Neural Interfaces: Recording, Stimulation, and Beyond

ConspectusNeurotechnology has seen dramatic improvements in the last three decades. The major focus in the field has been to design electrical communication platforms with high spatial resolution, stability, and translatability for understanding and affecting neural pathways. The deployment of nanomaterials in bioelectronics has enhanced the capabilities of conventional approaches employing microelectrode arrays (MEAs) for electrical interfaces, allowing the construction of miniaturized, high-performance neuroelectronics (Garg, R.; et al. ACS Appl. Nano Mater. 2023, 6, 8495). While these advancements in the electrical neuronal interface have revolutionized neurotechnology both in scale and breadth, an in-depth understanding of neurons' interactions is challenging due to the complexity of the environments where the cells and tissues are laid. The activity of large, three-dimensional neuronal systems has proven difficult to accurately monitor and modulate, and chemical cell-cell communication is often completely neglected. Recent breakthroughs in nanotechnology have provided opportunities to use new nonelectric modes of communication with neurons and to significantly enhance electrical signal interface capabilities. The enhanced electrochemical activity and optical activity of nanomaterials owing to their nonbulk electronic properties and surface nanostructuring have seen extensive utilization. Nanomaterials' enhanced optical activity enables remote neural state modulation, whereas the defect-rich surfaces provide an enormous number of available electrocatalytic sites for neurochemical detection and electrochemical modulation of cell microenvironments through Faradaic processes. Such unique properties can allow multimodal neural interrogation toward generating closed-loop interfaces with access to more complete neural state descriptors. In this Account, we will review recent advances and our efforts spearheaded toward utilizing nanostructured electrodes for enhanced bidirectional interfaces with neurons, the application of unique hybrid nanomaterials for remote nongenetic optical stimulation of neurons, tunable nanomaterials for highly sensitive and selective neurotransmitter detection, and the utilization of nanomaterials as electrocatalysts toward electrochemically modulating cellular activity. We highlight applications of these technologies across cell types through nanomaterial engineering with a focus on multifunctional graphene nanostructures applied though several modes of neural modulation but also an exploration of broad material classes for maximizing the potency of closed-loop bioelectronics.

Spatially Precise Genetic Engineering at the Electrode-Tissue Interface

The interface between electrodes and neural tissues plays a pivotal role in determining the efficacy and fidelity of neural activity recording and modulation. While considerable efforts have been made to improve the electrode-tissue interface, the majority of studies have primarily concentrated on the development of biocompatible neural electrodes through abiotic materials and structural engineering. In this study, an approach is presented that seamlessly integrates abiotic and biotic engineering principles into the electrode-tissue interface. Specifically, ultraflexible neural electrodes with short hairpin RNAs (shRNAs) designed to silence the expression of endogenous genes within neural tissues are combined. The system facilitates shRNA-mediated knockdown of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and polypyrimidine tract-binding protein 1 (PTBP1), two essential genes associated in neural survival/growth and neurogenesis, within specific cell populations located at the electrode-tissue interface. Additionally, it is demonstrated that the downregulation of PTEN in neurons can result in an enlargement of neuronal cell bodies at the electrode-tissue interface. Furthermore, the system enables long-term monitoring of neuronal activities following PTEN knockdown in a mouse model of Parkinson's disease and traumatic brain injury. The system provides a versatile approach for genetically engineering the electrode-tissue interface with unparalleled precision, paving the way for the development of regenerative electronics and next-generation brain-machine interfaces.

Soft Bioelectronics Using Nanomaterials and Nanostructures for Neuroengineering

The identification of neural networks for large areas and the regulation of neuronal activity at the single-neuron scale have garnered considerable attention in neuroscience. In addition, detecting biochemical molecules and electrically, optically, and chemically controlling neural functions are key research issues. However, conventional rigid and bulky bioelectronics face challenges for neural applications, including mechanical mismatch, unsatisfactory signal-to-noise ratio, and poor integration of multifunctional components, thereby degrading the sensing and modulation performance, long-term stability and biocompatibility, and diagnosis and therapy efficacy. Implantable bioelectronics have been developed to be mechanically compatible with the brain environment by adopting advanced geometric designs and utilizing intrinsically stretchable materials, but such advances have not been able to address all of the aforementioned challenges.Recently, the exploration of nanomaterial synthesis and nanoscale fabrication strategies has facilitated the design of unconventional soft bioelectronics with mechanical properties similar to those of neural tissues and submicrometer-scale resolution comparable to typical neuron sizes. The introduction of nanotechnology has provided bioelectronics with improved spatial resolution, selectivity, single neuron targeting, and even multifunctionality. As a result, this state-of-the-art nanotechnology has been integrated with bioelectronics in two main types, i.e., bioelectronics integrated with synthesized nanomaterials and bioelectronics with nanoscale structures. The functional nanomaterials can be synthesized and assembled to compose bioelectronics, allowing easy customization of their functionality to meet specific requirements. The unique nanoscale structures implemented with the bioelectronics could maximize the performance in terms of sensing and stimulation. Such soft nanobioelectronics have demonstrated their applicability for neuronal recording and modulation over a long period at the intracellular level and incorporation of multiple functions, such as electrical, optical, and chemical sensing and stimulation functions.In this Account, we will discuss the technical pathways in soft bioelectronics integrated with nanomaterials and implementing nanostructures for application to neuroengineering. We traced the historical development of bioelectronics from rigid and bulky structures to soft and deformable devices to conform to neuroengineering requirements. Recent approaches that introduced nanotechnology into neural devices enhanced the spatiotemporal resolution and endowed various device functions. These soft nanobioelectronic technologies are discussed in two categories: bioelectronics with synthesized nanomaterials and bioelectronics with nanoscale structures. We describe nanomaterial-integrated soft bioelectronics exhibiting various functionalities and modalities depending on the integrated nanomaterials. Meanwhile, soft bioelectronics with nanoscale structures are explained with their superior resolution and unique administration methods. We also exemplified the neural sensing and stimulation applications of soft nanobioelectronics across various modalities, showcasing their clinical applications in the treatment of neurological diseases, such as brain tumors, epilepsy, and Parkinson's disease. Finally, we discussed the challenges and direction of next-generation technologies.

Ultraflexible electrodes for recording neural activity in the mouse spinal cord during motor behavior

Implantable electrode arrays are powerful tools for directly interrogating neural circuitry in the brain, but implementing this technology in the spinal cord in behaving animals has been challenging due to the spinal cord's significant motion with respect to the vertebral column during behavior. Consequently, the individual and ensemble activity of spinal neurons processing motor commands remains poorly understood. Here, we demonstrate that custom ultraflexible 1-μm-thick polyimide nanoelectronic threads can conduct laminar recordings of many neuronal units within the lumbar spinal cord of unrestrained, freely moving mice. The extracellular action potentials have high signal-to-noise ratio, exhibit well-isolated feature clusters, and reveal diverse patterns of activity during locomotion. Furthermore, chronic recordings demonstrate the stable tracking of single units and their functional tuning over multiple days. This technology provides a path for elucidating how spinal circuits compute motor actions.

Neurophotonics: a comprehensive review, current challenges and future trends

The human brain, with its vast network of billions of neurons and trillions of synapses (connections) between diverse cell types, remains one of the greatest mysteries in science and medicine. Despite extensive research, an understanding of the underlying mechanisms that drive normal behaviors and response to disease states is still limited. Advancement in the Neuroscience field and development of therapeutics for related pathologies requires innovative technologies that can provide a dynamic and systematic understanding of the interactions between neurons and neural circuits. In this work, we provide an up-to-date overview of the evolution of neurophotonic approaches in the last 10 years through a multi-source, literature analysis. From an initial corpus of 243 papers retrieved from Scopus, PubMed and WoS databases, we have followed the PRISMA approach to select 56 papers in the area. Following a full-text evaluation of these 56 scientific articles, six main areas of applied research were identified and discussed: (1) Advanced optogenetics, (2) Multimodal neural interfaces, (3) Innovative therapeutics, (4) Imaging devices and probes, (5) Remote operations, and (6) Microfluidic platforms. For each area, the main technologies selected are discussed according to the photonic principles applied, the neuroscience application evaluated and the more indicative results of efficiency and scientific potential. This detailed analysis is followed by an outlook of the main challenges tackled over the last 10 years in the Neurophotonics field, as well as the main technological advances regarding specificity, light delivery, multimodality, imaging, materials and system designs. We conclude with a discussion of considerable challenges for future innovation and translation in Neurophotonics, from light delivery within the brain to physical constraints and data management strategies.

Flexible, Biodegradable, and Wireless Magnetoelectric Paper for Simple In Situ Personalization of Bioelectric Implants

Bioelectronic implants delivering electrical stimulation offer an attractive alternative to traditional pharmaceuticals in electrotherapy. However, achieving simple, rapid, and cost-effective personalization of these implants for customized treatment in unique clinical and physical scenarios presents a substantial challenge. This challenge is further compounded by the need to ensure safety and minimal invasiveness, requiring essential attributes such as flexibility, biocompatibility, lightness, biodegradability, and wireless stimulation capability. Here, a flexible, biodegradable bioelectronic paper with homogeneously distributed wireless stimulation functionality for simple personalization of bioelectronic implants is introduced. The bioelectronic paper synergistically combines i) lead-free magnetoelectric nanoparticles (MENs) that facilitate electrical stimulation in response to external magnetic field and ii) flexible and biodegradable nanofibers (NFs) that enable localization of MENs for high-selectivity stimulation, oxygen/nutrient permeation, cell orientation modulation, and biodegradation rate control. The effectiveness of wireless electrical stimulation in vitro through enhanced neuronal differentiation of neuron-like PC12 cells and the controllability of their microstructural orientation are shown. Also, scalability, design flexibility, and rapid customizability of the bioelectronic paper are shown by creating various 3D macrostructures using simple paper crafting techniques such as cutting and folding. This platform holds promise for simple and rapid personalization of temporary bioelectronic implants for minimally invasive wireless stimulation therapies.

Multimodal Technologies for Closed-Loop Neural Modulation and Sensing

Existing methods for studying neural circuits and treating neurological disorders are typically based on physical and chemical cues to manipulate and record neural activities. These approaches often involve predefined, rigid, and unchangeable signal patterns, which cannot be adjusted in real time according to the patient's condition or neural activities. With the continuous development of neural interfaces, conducting in vivo research on adaptive and modifiable treatments for neurological diseases and neural circuits is now possible. In this review, current and potential integration of various modalities to achieve precise, closed-loop modulation, and sensing in neural systems are summarized. Advanced materials, devices, or systems that generate or detect electrical, magnetic, optical, acoustic, or chemical signals are highlighted and utilized to interact with neural cells, tissues, and networks for closed-loop interrogation. Further, the significance of developing closed-loop techniques for diagnostics and treatment of neurological disorders such as epilepsy, depression, rehabilitation of spinal cord injury patients, and exploration of brain neural circuit functionality is elaborated.

Resorbable conductive materials for optimally interfacing medical devices with the living

Implantable and wearable bioelectronic systems are arising growing interest in the medical field. Linking the microelectronic (electronic conductivity) and biological (ionic conductivity) worlds, the biocompatible conductive materials at the electrode/tissue interface are key components in these systems. We herein focus more particularly on resorbable bioelectronic systems, which can safely degrade in the biological environment once they have completed their purpose, namely, stimulating or sensing biological activity in the tissues. Resorbable conductive materials are also explored in the fields of tissue engineering and 3D cell culture. After a short description of polymer-based substrates and scaffolds, and resorbable electrical conductors, we review how they can be combined to design resorbable conductive materials. Although these materials are still emerging, various medical and biomedical applications are already taking shape that can profoundly modify post-operative and wound healing follow-up. Future challenges and perspectives in the field are proposed.

Effects of Micromachining on Anti-oxidant Elution from a Mechanically-Adaptive Polymer

Intracortical microelectrodes (IMEs) can be used to restore motor and sensory function as a part of brain-computer interfaces in individuals with neuromusculoskeletal disorders. However, the neuroinflammatory response to IMEs can result in their premature failure, leading to reduced therapeutic efficacy. Mechanically-adaptive, resveratrol-eluting (MARE) neural probes target two mechanisms believed to contribute to the neuroinflammatory response by reducing the mechanical mismatch between the brain tissue and device, as well as locally delivering an antioxidant therapeutic. To create the mechanically-adaptive substrate, a dispersion, casting, and evaporation method is used, followed by a microfabrication process to integrate functional recording electrodes on the material. Resveratrol release experiments were completed to generate a resveratrol release profile and demonstrated that the MARE probes are capable of long-term controlled release. Additionally, our results showed that resveratrol can be degraded by laser-micromachining, an important consideration for future device fabrication. Finally, the electrodes were shown to have a suitable impedance for single-unit neural recording and could record single units in vivo.

Flexible neural probes: a review of the current advantages, drawbacks, and future demands

Brain diseases affect millions of people and have a huge social and economic impact. The use of neural probes for studies in animals has been the main approach to increasing knowledge about neural network functioning. Ultimately, neuroscientists are trying to develop new and more effective therapeutic approaches to treating neurological disorders. The implementation of neural probes with multifunctionalities (electrical, optical, and fluidic interactions) has been increasing in the last few years, leading to the creation of devices with high temporal and spatial resolution. Increasing the applicability of, and elements integrated into, neural probes has also led to the necessity to create flexible interfaces, reducing neural tissue damage during probe implantation and increasing the quality of neural acquisition data. In this paper, we review the fabrication, characterization, and validation of several types of flexible neural probes, exploring the main advantages and drawbacks of these devices. Finally, future developments and applications are covered. Overall, this review aims to present the currently available flexible devices and future appropriate avenues for development as possible guidance for future engineered devices.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

An Emerging Era: Conformable Ultrasound Electronics

Conformable electronics are regarded as the next generation of personal healthcare monitoring and remote diagnosis devices. In recent years, piezoelectric-based conformable ultrasound electronics (cUSE) have been intensively studied due to their unique capabilities, including nonradiative monitoring, soft tissue imaging, deep signal decoding, wireless power transfer, portability, and compatibility. This review provides a comprehensive understanding of cUSE for use in biomedical and healthcare monitoring systems and a summary of their recent advancements. Following an introduction to the fundamentals of piezoelectrics and ultrasound transducers, the critical parameters for transducer design are discussed. Next, five types of cUSE with their advantages and limitations are highlighted, and the fabrication of cUSE using advanced technologies is discussed. In addition, the working function, acoustic performance, and accomplishments in various applications are thoroughly summarized. It is noted that application considerations must be given to the tradeoffs between material selection, manufacturing processes, acoustic performance, mechanical integrity, and the entire integrated system. Finally, current challenges and directions for the development of cUSE are highlighted, and research flow is provided as the roadmap for future research. In conclusion, these advances in the fields of piezoelectric materials, ultrasound transducers, and conformable electronics spark an emerging era of biomedicine and personal healthcare.

A Nanozyme-Based Electrode for High-Performance Neural Recording

Implanted neural electrodes have been widely used to treat brain diseases that require high sensitivity and biocompatibility at the tissue-electrode interface. However, currently used clinical electrodes cannot meet both these requirements simultaneously, which hinders the effective recording of electronic signals. Herein, nanozyme-based neural electrodes incorporating bioinspired atomically precise clusters are developed as a general strategy with a heterogeneous design for multiscale and ultrasensitive neural recording via quantum transport and biocatalytic processes. Owing to the dual high-speed electronic and ionic currents at the electrode-tissue interface, the impedance of nanozyme electrodes is 26 times lower than that of state-of-the-art metal electrodes, and the acquisition sensitivity for the local field potential is ≈10 times higher than that of clinical PtIr electrodes, enabling a signal-to-noise ratio (SNR) of up to 14.7 dB for single-neuron recordings in rats. The electrodes provide more than 100-fold higher antioxidant and multi-enzyme-like activities, which effectively decrease 67% of the neuronal injury area by inhibiting glial proliferation and allowing sensitive and stable neural recording. Moreover, nanozyme electrodes can considerably improve the SNR of seizures in acute epileptic rats and are expected to achieve precise localization of seizure foci in clinical settings.

Tuning Microelectrodes' Impedance to Improve Fast Ripples Recording

Epilepsy is a chronic neurological disorder characterized by recurrent seizures resulting from abnormal neuronal hyperexcitability. In the case of pharmacoresistant epilepsy requiring resection surgery, the identification of the Epileptogenic Zone (EZ) is critical. Fast Ripples (FRs; 200-600 Hz) are one of the promising biomarkers that can aid in EZ delineation. However, recording FRs requires physically small electrodes. These microelectrodes suffer from high impedance, which significantly impacts FRs' observability and detection. In this study, we investigated the potential of a conductive polymer coating to enhance FR observability. We employed biophysical modeling to compare two types of microelectrodes: Gold (Au) and Au coated with the conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (Au/PEDOT:PSS). These electrodes were then implanted into the CA1 hippocampal neural network of epileptic mice to record FRs during epileptogenesis. The results showed that the polymer-coated electrodes had a two-order lower impedance as well as a higher transfer function amplitude and cut-off frequency. Consequently, FRs recorded with the PEDOT:PSS-coated microelectrode yielded significantly higher signal energy compared to the uncoated one. The PEDOT:PSS coating improved the observability of the recorded FRs and thus their detection. This work paves the way for the development of signal-specific microelectrode designs that allow for better targeting of pathological biomarkers.

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

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

Manipulation of cross-linking in PEDOT:PSS hydrogels for biointerfacing

Conducting hydrogels can be used to fabricate bioelectronic devices that are soft for improved cell- and tissue-interfacing. Those based on conjugated polymers, such as poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS), can be made simply with solution-based processing techniques, yet the influence of fabrication variables on final gel properties is not fully understood. In this study, we investigated if PEDOT:PSS cross-linking could be manipulated by changing the concentration of a gelling agent, ionic liquid, in the hydrogel precursor mixture. Rheology and gelation kinetics of precursor mixtures were investigated, and aqueous stability, swelling, conductivity, stiffness, and cytocompatibility of formed hydrogels were characterized. Increasing ionic liquid concentration was found to increase cross-linking as measured by decreased swelling, decreased non-network fraction, increased stiffness, and increased conductivity. Such manipulation of IL concentration thus afforded control of final gel properties and was utilized in further investigations of biointerfacing. When cross-linked sufficiently, PEDOT:PSS hydrogels were stable in sterile cell culture conditions for at least 28 days. Additionally, hydrogels supported a viable and proliferating population of human dermal fibroblasts for at least two weeks. Collectively, these characterizations of stability and cytocompatibility illustrate that these PEDOT:PSS hydrogels have significant promise for biointerfacing applications that require soft materials for direct interaction with cells.

Innovative solutions for disease management

The increasing prevalence of chronic diseases is a driver for emerging big data technologies for healthcare including digital platforms for data collection, systems for active patient engagement and education, therapy specific predictive models, optimized patient pathway models. Powerful bioelectronic medicine tools for data collection, analysis and visualization allow for joint processing of large volumes of heterogeneous data, which in turn can produce new insights about patient outcomes and alternative interpretations of clinical patterns that can lead to implementation of optimized clinical decisions and clinical patient pathway by healthcare professionals.With this perspective, we identify innovative solutions for disease management and evaluate their impact on patients, payers and society, by analyzing their impact in terms of clinical outcomes (effectiveness, safety, and quality of life) and economic outcomes (cost-effectiveness, savings, and productivity).As a result, we propose a new approach based on the main pillars of innovation in the disease management area, i.e. progressive patient care models, patient-centric approaches, bioelectronics for precise medicine, and lean management that, combined with an increase in appropriate private-public-citizen-partnership, leads towards Patient-Centric Healthcare.

On-demand electrically controlled melatonin release from PEDOT/SNP composite improves quality of chronic neural recording

Long-time and high-quality signal acquisition performance from implantable electrodes is the key to establish stable and efficient brain-computer interface (BCI) connections. The chronic performance of implantable electrodes is hindered by the inflammatory response of brain tissue. In order to solve the material limitation of biological interface electrodes, we designed sulfonated silica nanoparticles (SNPs) as the dopant of Poly (3,4-ethylenedioxythiophene) (PEDOT) to modify the implantable electrodes. In this work, melatonin (MT) loaded SNPs were incorporated in PEDOT via electrochemical deposition on nickel-chromium (Ni-Cr) alloy electrode and carbon nanotube (CNT) fiber electrodes, without affecting the acute neural signal recording capacity. After coating with PEDOT/SNP-MT, the charge storage capacity of both electrodes was significantly increased, and the electrochemical impedance at 1 kHz of the Ni-Cr alloy electrodes was significantly reduced, while that of the CNT electrodes was significantly increased. In addition, this study inspected the effect of electrically triggered MT release every other day on the quality and longevity of neural recording from implanted neural electrodes in rat hippocampus for 1 month. Both MT modified Ni-Cr alloy electrodes and CNT electrodes showed significantly higher spike amplitude after 26-day recording. Significantly, the histological studies showed that the number of astrocytes around the implanted Ni-Cr alloy electrodes was significantly reduced after MT release. These results demonstrate the potent outcome of PEDOT/SNP-MT treatment in improving the chronic neural recording quality possibly through its anti-inflammatory property.

Neurotoxin-Derived Optical Probes for Biological and Medical Imaging

The superb specificity and potency of biological toxins targeting various ion channels and receptors are of major interest for the delivery of therapeutics to distinct cell types and subcellular compartments. Fused with reporter proteins or labelled with fluorophores and nanocomposites, animal toxins and their detoxified variants also offer expanding opportunities for visualisation of a range of molecular processes and functions in preclinical models, as well as clinical studies. This article presents state-of-the-art optical probes derived from neurotoxins targeting ion channels, with discussions of their applications in basic and translational biomedical research. It describes the design and production of probes and reviews their applications with advantages and limitations, with prospects for future improvements. Given the advances in imaging tools and expanding research areas benefiting from the use of optical probes, described here resources should assist the discovery process and facilitate high-precision interrogation and therapeutic interventions.

Soft Bioelectronics for Therapeutics

Soft bioelectronics play an increasingly crucial role in high-precision therapeutics due to their softness, biocompatibility, clinical accuracy, long-term stability, and patient-friendliness. In this review, we provide a comprehensive overview of the latest representative therapeutic applications of advanced soft bioelectronics, ranging from wearable therapeutics for skin wounds, diabetes, ophthalmic diseases, muscle disorders, and other diseases to implantable therapeutics against complex diseases, such as cardiac arrhythmias, cancer, neurological diseases, and others. We also highlight key challenges and opportunities for future clinical translation and commercialization of soft therapeutic bioelectronics toward personalized medicine.

A nanoscale inorganic coating strategy for stabilizing hydrogel neural probes in vivo

Hydrogels with adaptable optical and mechanical characteristics show considerable promise for light delivery in vivo with neuroengineering applications. However, the unlinked amorphous polymer chains within hydrogels can cause volumetric swelling after water absorption under physiological conditions over time. Chemically cross-linked poly(vinyl alcohol) (PVA) hydrogels showcase fatigue-resistant attributes and promising biocompatibility for the manufacture of soft neural probes. However, possible swelling of the PVA hydrogel matrix could impact the structural stability of hydrogel-based bioelectronics and their long-term in vivo functionality. In this study, we utilized an atomic layer deposition (ALD) technique to generate an inorganic, silicon dioxide (SiO2) coating layer on chemically cross-linked PVA hydrogel fibers. To evaluate the stability of SiO2-coated PVA hydrogel fibers mimicking the in vivo environment, we conducted accelerated stability tests. SiO2-coated PVA hydrogel fibers showed improved stability over a one-week incubation period under a harsh environment, preventing swelling and preserving their mechanical and optical properties compared to uncoated fibers. These SiO2-coated PVA hydrogel fibers demonstrated nanoscale polymeric crystalline domains (6.5 ± 0.1 nm), an elastic modulus of 73.7 ± 31.7 MPa, a maximum elongation of 113.6 ± 24.2%, and minimal light transmission loss (1.9 ± 0.2 dB cm-1). Lastly, we applied these SiO2-coated PVA hydrogel fibers in vivo to optically activate the motor cortex of transgenic Thy1::ChR2 mice during locomotor behavioral tests. This mouse cohort was genetically modified to express the light-sensitive ion channel, channelrhodopsin-2 (ChR2), and implanted with hydrogel fibers to deliver light to the motor cortex area (M2). Light stimulation via hydrogel fibers resulted in optogenetically modulated mouse locomotor behaviors, including increased contralateral rotation, mobility speeds, and travel distances.

Hydrogel Bioelectronics for Health Monitoring

Hydrogels are considered an ideal platform for personalized healthcare due to their unique characteristics, such as their outstanding softness, appealing biocompatibility, excellent mechanical properties, etc. Owing to the high similarity between hydrogels and biological tissues, hydrogels have emerged as a promising material candidate for next generation bioelectronic interfaces. In this review, we discuss (i) the introduction of hydrogel and its traditional applications, (ii) the work principles of hydrogel in bioelectronics, (iii) the recent advances in hydrogel bioelectronics for health monitoring, and (iv) the outlook for future hydrogel bioelectronics' development.

Bioadhesive polymer semiconductors and transistors for intimate biointerfaces

The use of bioelectronic devices relies on direct contact with soft biotissues. For transistor-type bioelectronic devices, the semiconductors that need to have direct interfacing with biotissues for effective signal transduction do not adhere well with wet tissues, thereby limiting the stability and conformability at the interface. We report a bioadhesive polymer semiconductor through a double-network structure formed by a bioadhesive brush polymer and a redox-active semiconducting polymer. The resulting semiconducting film can form rapid and strong adhesion with wet tissue surfaces together with high charge-carrier mobility of ~1 square centimeter per volt per second, high stretchability, and good biocompatibility. Further fabrication of a fully bioadhesive transistor sensor enabled us to produce high-quality and stable electrophysiological recordings on an isolated rat heart and in vivo rat muscles.

Genetically targeted chemical assembly of polymers specifically localized extracellularly to surface membranes of living neurons

Multicellular biological systems, particularly living neural networks, exhibit highly complex organization properties that pose difficulties for building cell-specific biocompatible interfaces. We previously developed an approach to genetically program cells to assemble structures that modify electrical properties of neurons in situ, opening up the possibility of building minimally invasive cell-specific structures and interfaces. However, the efficiency and biocompatibility of this approach were challenged by limited membrane targeting of the constructed materials. Here, we design a method for highly localized expression of enzymes targeted to the plasma membrane of primary neurons, with minimal intracellular retention. Next, we show that polymers synthesized in situ by this approach form dense extracellular clusters selectively on the targeted cell membrane and that neurons remain viable after polymerization. Last, we show generalizability of this method across a range of design strategies. This platform can be readily extended to incorporate a broad diversity of materials onto specific cell membranes within tissues and may further enable next-generation biological interfaces.

Multifunctional Filler-Free PEDOT:PSS Hydrogels with Ultrahigh Electrical Conductivity Induced by Lewis-Acid-Promoted Ion Exchange

Highly conductive hydrogels with biotissue-like mechanical properties are of great interest in the emerging field of hydrogel bioelectronics due to their good biocompatibility, deformability, and stability. Fully polymeric hydrogels may exhibit comparable Young's modulus to biotissues. However, most of these filler-free hydrogels have a low electrical conductivity of <10 S cm-1 , which limits their wide applications of them in digital circuits or bioelectronic devices. In this work, a series of metal-halides-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels with an ultrahigh electrical conductivity up to 547 S cm-1 is reported, which is 1.5 times to 104 times higher than previously reported filler-free polymeric hydrogels. Theoretical calculation demonstrated that the ion exchange between PEDOT:PSS and the metal halides played an important role to promote phase separation in the hydrogels, which thus leads to ultrahigh electrical conductivity. The high electrical conductivity resulted in multifunctional hydrogels with high performance in thermoelectrics, electromagnetic shielding, Joule heating, and sensing. Such flexible and stretchable hydrogels with ultrahigh electrical conductivity and stability upon various deformations are promising for soft bioelectronics devices and wearable electronics.

Chemically revised conducting polymers with inflammation resistance for intimate bioelectronic electrocoupling

Conducting polymers offer attractive mixed ionic-electronic conductivity, tunable interfacial barrier with metal, tissue matchable softness, and versatile chemical functionalization, making them robust to bridge the gap between brain tissue and electronic circuits. This review focuses on chemically revised conducting polymers, combined with their superior and controllable electrochemical performance, to fabricate long-term bioelectronic implants, addressing chronic immune responses, weak neuron attraction, and long-term electrocommunication instability challenges. Moreover, the promising progress of zwitterionic conducting polymers in bioelectronic implants (≥4 weeks stable implantation) is highlighted, followed by a comment on their current evolution toward selective neural coupling and reimplantable function. Finally, a critical forward look at the future of zwitterionic conducting polymers for in vivo bioelectronic devices is provided.

Achieving tissue-level softness on stretchable electronics through a generalizable soft interlayer design

Soft and stretchable electronics have emerged as highly promising tools for biomedical diagnosis and biological studies, as they interface intimately with the human body and other biological systems. Most stretchable electronic materials and devices, however, still have Young's moduli orders of magnitude higher than soft bio-tissues, which limit their conformability and long-term biocompatibility. Here, we present a design strategy of soft interlayer for allowing the use of existing stretchable materials of relatively high moduli to versatilely realize stretchable devices with ultralow tissue-level moduli. We have demonstrated stretchable transistor arrays and active-matrix circuits with moduli below 10 kPa-over two orders of magnitude lower than the current state of the art. Benefiting from the increased conformability to irregular and dynamic surfaces, the ultrasoft device created with the soft interlayer design realizes electrophysiological recording on an isolated heart with high adaptability, spatial stability, and minimal influence on ventricle pressure. In vivo biocompatibility tests also demonstrate the benefit of suppressing foreign-body responses for long-term implantation. With its general applicability to diverse materials and devices, this soft-interlayer design overcomes the material-level limitation for imparting tissue-level softness to a variety of bioelectronic devices.

In situ assembly of bioresorbable organic bioelectronics in the brain

Bioelectronics can potentially complement classical therapies in nonchronic treatments, such as immunotherapy and cancer. In addition to functionality, minimally invasive implantation methods and bioresorbable materials are central to nonchronic treatments. The latter avoids the need for surgical removal after disease relief. Self-organizing substrate-free organic electrodes meet these criteria and integrate seamlessly into dynamic biological systems in ways difficult for classical rigid solid-state electronics. Here we place bioresorbable electrodes with a brain-matched shear modulus-made from water-dispersed nanoparticles in the brain-in the targeted area using a capillary thinner than a human hair. Thereafter, we show that an optional auxiliary module grows dendrites from the installed conductive structure to seamlessly embed neurons and modify the electrode's volume properties. We demonstrate that these soft electrodes set off a controlled cellular response in the brain when relaying external stimuli and that the biocompatible materials show no tissue damage after bioresorption. These findings encourage further investigation of temporary organic bioelectronics for nonchronic treatments assembled in vivo.

3D printable high-performance conducting polymer hydrogel for all-hydrogel bioelectronic interfaces

Owing to the unique combination of electrical conductivity and tissue-like mechanical properties, conducting polymer hydrogels have emerged as a promising candidate for bioelectronic interfacing with biological systems. However, despite the recent advances, the development of hydrogels with both excellent electrical and mechanical properties in physiological environments is still challenging. Here we report a bi-continuous conducting polymer hydrogel that simultaneously achieves high electrical conductivity (over 11 S cm^-1), stretchability (over 400%) and fracture toughness (over 3,300 J m^-2) in physiological environments and is readily applicable to advanced fabrication methods including 3D printing. Enabled by these properties, we further demonstrate multi-material 3D printing of monolithic all-hydrogel bioelectronic interfaces for long-term electrophysiological recording and stimulation of various organs in rat models.

Direct-print three-dimensional electrodes for large- scale, high-density, and customizable neural inter- faces

Silicon-based planar microelectronics is a powerful tool for scalably recording and modulating neural activity at high spatiotemporal resolution, but it remains challenging to target neural structures in three dimensions (3D). We present a method for directly fabricating 3D arrays of tissue-penetrating microelectrodes onto silicon microelectronics. Leveraging a high-resolution 3D printing technology based on 2-photon polymerization and scalable microfabrication processes, we fabricated arrays of 6,600 microelectrodes 10-130 μm tall and at 35-μm pitch onto a planar silicon-based microelectrode array. The process enables customizable electrode shape, height and positioning for precise targeting of neuron populations distributed in 3D. As a proof of concept, we addressed the challenge of specifically targeting retinal ganglion cell (RGC) somas when interfacing with the retina. The array was customized for insertion into the retina and recording from somas while avoiding the axon layer. We verified locations of the microelectrodes with confocal microscopy and recorded high-resolution spontaneous RGC activity at cellular resolution. This revealed strong somatic and dendritic components with little axon contribution, unlike recordings with planar microelectrode arrays. The technology could be a versatile solution for interfacing silicon microelectronics with neural structures and modulating neural activity at large scale with single-cell resolution.

Perspectives on tissue-like bioelectronics for neural modulation

Advances in bioelectronic implants have been offering valuable chances to interface and modulate neural systems. Potential mismatches between bioelectronics and targeted neural tissues require devices to exhibit "tissue-like" properties for better implant-bio integration. In particular, mechanical mismatches pose a significant challenge. In the past years, efforts were made in both materials synthesis and device design to achieve bioelectronics mechanically and biochemically mimicking biological tissues. In this perspective, we mainly summarized recent progress of developing "tissue-like" bioelectronics and categorized them into different strategies. We also discussed how these "tissue-like" bioelectronics were utilized for modulating in vivo nervous systems and neural organoids. We concluded the perspective by proposing further directions including personalized bioelectronics, novel materials design and the involvement of artificial intelligence and robotic techniques.

A tissue-engineered neural interface with photothermal functionality

Neural interfaces are well-established as a tool to understand the behaviour of the nervous system via recording and stimulation of living neurons, as well as serving as neural prostheses. Conventional neural interfaces based on metals and carbon-based materials are generally optimised for high conductivity; however, a mechanical mismatch between the interface and the neural environment can significantly reduce long-term neuromodulation efficacy by causing an inflammatory response. This paper presents a soft composite material made of gelatin methacryloyl (GelMA) containing graphene oxide (GO) conjugated with gold nanorods (AuNRs). The soft hydrogel presents stiffness within the neural environment range of modulus below 5 kPa, while the AuNRs, when exposed to light in the near infrared range, provide a photothermal response that can be used to improve the spatial and temporal precision of neuromodulation. These favourable properties can be maintained at safer optical power levels when combined with electrical stimulation. In this paper we provide mechanical and biological characterization of the optical activity of the GO-AuNR composite hydrogel. The optical functionality of the material has been evaluated via photothermal stimulation of explanted rat retinal tissue. The outcomes achieved with this study encourage further investigation into optical and electrical costimulation parameters for a range of biomedical applications.

Liquid Metal-Based Electrode Array for Neural Signal Recording

Neural electrodes are core devices for research in neuroscience, neurological diseases, and neural-machine interfacing. They build a bridge between the cerebral nervous system and electronic devices. Most of the neural electrodes in use are based on rigid materials that differ significantly from biological neural tissue in flexibility and tensile properties. In this study, a liquid-metal (LM) -based 20-channel neural electrode array with a platinum metal (Pt) encapsulation material was developed by microfabrication technology. The in vitro experiments demonstrated that the electrode has stable electrical properties and excellent mechanical properties such as flexibility and bending, which allows the electrode to form conformal contact with the skull. The in vivo experiments also recorded electroencephalographic signals using the LM-based electrode from a rat under low-flow or deep anesthesia, including the auditory-evoked potentials triggered by sound stimulation. The auditory-activated cortical area was analyzed using source localization technique. These results indicate that this 20-channel LM-based neural electrode array satisfies the demands of brain signal acquisition and provides high-quality-electroencephalogram (EEG) signals that support source localization analysis.

Blood-Glucose-Powered Metabolic Fuel Cell for Self-Sufficient Bioelectronics

Currently available bioelectronic devices consume too much power to be continuously operated on rechargeable batteries, and are often powered wirelessly, with attendant issues regarding reliability, convenience, and mobility. Thus, the availability of a robust, self-sufficient, implantable electrical power generator that works under physiological conditions would be transformative for many applications, from driving bioelectronic implants and prostheses to programing cellular behavior and patients' metabolism. Here, capitalizing on a new copper-containing, conductively tuned 3D carbon nanotube composite, an implantable blood-glucose-powered metabolic fuel cell is designed that continuously monitors blood-glucose levels, converts excess glucose into electrical power during hyperglycemia, and produces sufficient energy (0.7 mW cm^-2 , 0.9 V, 50 mm glucose) to drive opto- and electro-genetic regulation of vesicular insulin release from engineered beta cells. It is shown that this integration of blood-glucose monitoring with elimination of excessive blood glucose by combined electro-metabolic conversion and insulin-release-mediated cellular consumption enables the metabolic fuel cell to restore blood-glucose homeostasis in an automatic, self-sufficient, and closed-loop manner in an experimental model of type-1 diabetes.

Polymer nanofiber network reinforced gold electrode array for neural activity recording

Flexible and stretchable neural electrodes are promising tools for high-fidelity interfacing with soft and curvilinear brain surface. Here, we describe a flexible and stretchable neural electrode array that consists of polyacrylonitrile (PAN) nanofiber network reinforced gold (Au) film electrodes. Under stretching, the interweaving PAN nanofibers effectively terminate the formation of propagating cracks in the Au films and thus enable the formation of a dynamically stable electrode-tissue interface. Moreover, the PAN nanofibers increase the surface roughness and active surface areas of the Au electrodes, leading to reduced electrochemical impedance and improved signal-to-noise ratio. As a result, PAN nanofiber network reinforced Au electrode arrays can allow for reliable in vivo multichannel recording of epileptiform activities in rats.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13534-022-00257-5.

Stitching Flexible Electronics into the Brain

Understanding complex neuronal networks requires monitoring long-term neuronal activity in various regions of the brain. Significant progress has been made in multisite implantations of well-designed probes, such as multisite implantation of Si-based and polymer-based probes. However, these multiprobe strategies are limited by the sizes and weights of interfaces to the multiple probes and the inability to track the activity of the same neurons and changes in neuronal activity over longer time periods. Here, a long single flexible probe that can be implanted by stitching into multiple regions of the mouse brain and subsequently transmit chronically stable neuronal signals from the multiple sites via a single low-mass interface is reported. The probe at four different sites is implemented using a glass capillary needle or two sites using an ultrathin metal needle. In vitro tests in brain-mimicking hydrogel show that multisite probe implantations achieve a high connection yield of >86%. In vivo histological images at each site of probes, implanted by stitching using either glass capillary or ultrathin metal insertion needles exhibit seamless tissue-probe interfaces with negligible chronic immune response. In addition, electrophysiology studies demonstrate the ability to track single neuron activities at every injection site with chronic stability over at least one month. Notably, the measured spike amplitudes and signal-to-noise ratios at different implantation sites show no statistically significant differences. Multisite stitching implantation of flexible electronics in the brain opens up new opportunities for both fundamental neuroscience research and electrotherapeutic applications.

Full-field, conformal epiretinal electrode array using hydrogel and polymer hybrid technology

Shape-morphable electrode arrays can form 3D surfaces to conform to complex neural anatomy and provide consistent positioning needed for next-generation neural interfaces. Retinal prostheses need a curved interface to match the spherical eye and a coverage of several cm to restore peripheral vision. We fabricated a full-field array that can (1) cover a visual field of 57° based on electrode position and of 113° based on the substrate size; (2) fold to form a compact shape for implantation; (3) self-deploy into a curvature fitting the eye after implantation. The full-field array consists of multiple polymer layers, specifically, a sandwich structure of elastomer/polyimide-based-electrode/elastomer, coated on one side with hydrogel. Electrodeposition of high-surface-area platinum/iridium alloy significantly improved the electrical properties of the electrodes. Hydrogel over-coating reduced electrode performance, but the electrodes retained better properties than those without platinum/iridium. The full-field array was rolled into a compact shape and, once implanted into ex vivo pig eyes, restored to a 3D curved surface. The full-field retinal array provides significant coverage of the retina while allowing surgical implantation through an incision 33% of the final device diameter. The shape-changing material platform can be used with other neural interfaces that require conformability to complex neuroanatomy.

Spatiotemporal Expression of RNA-Seq Identified Proteins at the Electrode Interface

Implantation of electrodes in the brain can be used to record from or stimulate neural tissues to treat neurological disease and injury. However, the tissue response to implanted devices can limit their functional longevity. Recent RNA-seq datasets identify hundreds of genes associated with gliosis, neuronal function, myelination, and cellular metabolism that are spatiotemporally expressed in neural tissues following the insertion of microelectrodes. To validate mRNA as a predictor of protein expression, this study evaluates a sub-set of RNA-seq identified proteins (RSIP) at 24-hours, 1-week, and 6-weeks post-implantation using quantitative immunofluorescence methods. This study found that expression of RSIPs associated with glial activation (Glial fibrillary acidic protein (GFAP), Polypyrmidine tract binding protein-1 (Ptbp1)), neuronal structure (Neurofilament heavy chain (Nefh), Proteolipid protein-1 (Plp1), Myelin Basic Protein (MBP)), and iron metabolism (Transferrin (TF), Ferritin heavy chain-1 (Fth1)) reinforce transcriptional data. This study also provides additional context to the cellular distribution of RSIPs using a MATLAB-based approach to quantify immunofluorescence intensity within specific cell types. Ptbp1, TF, and Fth1 were found to be spatiotemporally distributed within neurons, astrocytes, microglia, and oligodendrocytes at the device interface relative to distal and contralateral tissues. The altered distribution of RSIPs relative to distal tissue is largely localized within 100µm of the device injury, which approaches the functional recording range of implanted electrodes. This study provides evidence that RNA-sequencing can be used to predict protein-level changes in cortical tissues and that RSIPs can be further investigated to identify new biomarkers of the tissue response that influence signal quality. STATEMENT OF SIGNIFICANCE: : Microelectrode arrays implanted into the brain are useful tools that can be used to study neuroscience and to treat pathological conditions in a clinical setting. The tissue response to these devices, however, can severely limit their functional longevity. Transcriptomics has deepened the understandings of the tissue response by revealing numerous genes which are differentially expressed following device insertion. This manuscript provides validation for the use of transcriptomics to characterize the tissue response by evaluating a subset of known differentially expressed genes at the protein level around implanted electrodes over time. In additional to validating mRNA-to-protein relationships at the device interface, this study has identified emerging trends in the spatiotemporal distribution of proteins involved with glial activation, neuronal remodeling, and essential iron binding proteins around implanted silicon devices. This study additionally provides a new MATLAB based methodology to quantify protein distribution within discrete cell types at the device interface which may be used as biomarkers for further study or therapeutic intervention in the future.

Wireless and battery-free technologies for neuroengineering

Tethered and battery-powered devices that interface with neural tissues can restrict natural motions and prevent social interactions in animal models, thereby limiting the utility of these devices in behavioural neuroscience research. In this Review Article, we discuss recent progress in the development of miniaturized and ultralightweight devices as neuroengineering platforms that are wireless, battery-free and fully implantable, with capabilities that match or exceed those of wired or battery-powered alternatives. Such classes of advanced neural interfaces with optical, electrical or fluidic functionality can also combine recording and stimulation modalities for closed-loop applications in basic studies or in the practical treatment of abnormal physiological processes.

Rapid improvement of heart repair in rats after myocardial infarction by precise magnetic stimulation on the vagus nerve with an injectable magnetic hydrogel

The imbalance between the sympathetic and the parasympathetic nervous system is one of the main pathogeneses of myocardial infarction (MI). Vagus nerve stimulation (VNS), which restores autonomic nervous balance by enhancing the parasympathetic drive, is shown to have benefits for patients with MI. As a clinically safe and effective remote neuromodulation method, magnetic stimulation is expected to overcome the problems of infection and nerve injury caused by electrode implantation. However, it is difficult to achieve precise stimulation on a single vagus nerve due to the poor focus of the magnetic field. Here, we described a novel magnetic vagus nerve stimulation (mVNS) system, which consisted of an injectable chitosan/β-glycerophosphate (CS/GP) hydrogel loaded with superparamagnetic iron oxide (SPIO) nanoparticles and a mild magnetic pulse sequence. The injectable hydrogel prepared from clinically safe materials ensured minimally invasive implantation, and the SPIO nanoparticles in the hydrogel mediated the precise magnetic stimulation of a single vagus nerve. Under a mild magnetic field (∼100 mT), a decrease in heart rate and a change in vagus nerve potential were found in rats under in situ injection of a magnetic CS/GP hydrogel. Magnetic stimulation on the vagus nerve for 4 weeks (20 Hz, three times daily, 5 minutes each time) significantly improved the cardiac function and reduced the infarct size of the rats subjected to myocardial infarction, accompanied by suppression of inflammatory cell infiltration and inflammation factor expression. Taken together, these results demonstrated that the mVNS exhibited promising potential for treating myocardial infarction in the clinic.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

Commonly Overlooked Factors in Biocompatibility Studies of Neural Implants

Biocompatibility of cutting-edge neural implants, surgical tools and techniques, and therapeutic technologies is a challenging concept that can be easily misjudged. For example, neural interfaces are routinely gauged on how effectively they determine active neurons near their recording sites. Tissue integration and toxicity of neural interfaces are frequently assessed histologically in animal models to determine tissue morphological and cellular changes in response to surgical implantation and chronic presence. A disconnect between histological and efficacious biocompatibility exists, however, as neuronal numbers frequently observed near electrodes do not match recorded neuronal spiking activity. The downstream effects of the myriad surgical and experimental factors involved in such studies are rarely examined when deciding whether a technology or surgical process is biocompatible. Such surgical factors as anesthesia, temperature excursions, bleed incidence, mechanical forces generated, and metabolic conditions are known to have strong systemic and thus local cellular and extracellular consequences. Many tissue markers are extremely sensitive to the physiological state of cells and tissues, thus significantly impacting histological accuracy. This review aims to shed light on commonly overlooked factors that can have a strong impact on the assessment of neural biocompatibility and to address the mismatch between results stemming from functional and histological methods.

Deep brain stimulation by blood-brain-barrier-crossing piezoelectric nanoparticles generating current and nitric oxide under focused ultrasound

Deep brain stimulation via implanted electrodes can alleviate neuronal disorders. However, its applicability is constrained by side effects resulting from the insertion of electrodes into the brain. Here, we show that systemically administered piezoelectric nanoparticles producing nitric oxide and generating direct current under high-intensity focused ultrasound can be used to stimulate deep tissue in the brain. The release of nitric oxide temporarily disrupted tight junctions in the blood-brain barrier, allowing for the accumulation of the nanoparticles into brain parenchyma, and the piezoelectrically induced output current stimulated the release of dopamine by dopaminergic neuron-like cells. In a mouse model of Parkinson's disease, the ultrasound-responsive nanoparticles alleviated the symptoms of the disease without causing overt toxicity. The strategy may inspire the development of other minimally invasive therapies for neurodegenerative diseases.

Photoactive Nanomaterials for Wireless Neural Biomimetics, Stimulation, and Regeneration

Nanomaterials at the neural interface can provide the bridge between bioelectronic devices and native neural tissues and achieve bidirectional transmission of signals with our brain. Photoactive nanomaterials, such as inorganic and polymeric nanoparticles, nanotubes, nanowires, nanorods, nanosheets or related, are being explored to mimic, modulate, control, or even substitute the functions of neural cells or tissues. They show great promise in next generation technologies for the neural interface with excellent spatial and temporal accuracy. In this review, we highlight the discovery and understanding of these nanomaterials in precise control of an individual neuron, biomimetic retinal prosthetics for vision restoration, repair or regeneration of central or peripheral neural tissues, and wireless deep brain stimulation for treatment of movement or mental disorders. The most intriguing feature is that the photoactive materials fit within a minimally invasive and wireless strategy to trigger the flux of neurologically active molecules and thus influences the cell membrane potential or key signaling molecule related to gene expression. In particular, we focus on worthy pathways of photosignal transduction at the nanomaterial-neural interface and the behavior of the biological system. Finally, we describe the challenges on how to design photoactive nanomaterials specific to neurological disorders. There are also some open issues such as long-term interface stability and signal transduction efficiency to further explore for clinical practice.

Facile Fabrication of Injectable Alginate and Poly(3,4-ethylenedioxythiophene)-Based Soft Electrodes toward the Goal of Neuro-Regenerative Applications

Resident brain neural precursor cells (NPCs) are electrosensitive cells that respond to electric field application by proliferating, differentiating, and undergoing rapid and directed cathodal migration. Harnessing NPC potential is a promising strategy to facilitate neural repair following injury or disease. The use of electric fields to activate NPCs is limited by current electrode designs which are typically made of conductive metals that are stiff and can lead to neuroinflammation following implantation, in part due to the mechanical mismatch between physiological conditions and material. Herein, the design of a novel, injectable biobased soft electrode with properties suitable for electrical stimulation in vivo is explored. The recent interest in using biologically derived polymers which are relatively abundant and afford economic feasibility have been built upon. Sodium alginate is utilized to form soft hydrogels, thereby addressing the issue of mechanical mismatch, and the conductive polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), to generate an innovative new material. It is demonstrated that the optimized alginate PEDOT blend matches the modulus of the brain and is suitable for injection and is not cytotoxic to neural cells. Furthermore, in vivo studies demonstrate minimal activation of inflammatory cells upon implantation in the brain compared to classically used platinum-based electrodes.

Filtration-processed biomass nanofiber electrodes for flexible bioelectronics

An increasing demand for bioelectronics that interface with living systems has driven the development of materials to resolve mismatches between electronic devices and biological tissues. So far, a variety of different polymers have been used as substrates for bioelectronics. Especially, biopolymers have been investigated as next-generation materials for bioelectronics because they possess interesting characteristics such as high biocompatibility, biodegradability, and sustainability. However, their range of applications has been restricted due to the limited compatibility of classical fabrication methods with such biopolymers. Here, we introduce a fabrication process for thin and large-area films of chitosan nanofibers (CSNFs) integrated with conductive materials. To this end, we pattern carbon nanotubes (CNTs), silver nanowires, and poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) by a facile filtration process that uses polyimide masks fabricated via laser ablation. This method yields feedlines of conductive material on nanofiber paper and demonstrates compatibility with conjugated and high-aspect-ratio materials. Furthermore, we fabricate a CNT neural interface electrode by taking advantage of this fabrication process and demonstrate peripheral nerve stimulation to the rapid extensor nerve of a live locust. The presented method might pave the way for future bioelectronic devices based on biopolymer nanofibers.

Modulating cell signalling in vivo with magnetic nanotransducers

Weak magnetic fields offer nearly lossless transmission of signals within biological tissue. Magnetic nanomaterials are capable of transducing magnetic fields into a range of biologically relevant signals in vitro and in vivo. These nanotransducers have recently enabled magnetic control of cellular processes, from neuronal firing and gene expression to programmed apoptosis. Effective implementation of magnetically controlled cellular signalling relies on careful tailoring of magnetic nanotransducers and magnetic fields to the responses of the intended molecular targets. This primer discusses the versatility of magnetic modulation modalities and offers practical guidelines for selection of appropriate materials and field parameters, with a particular focus on applications in neuroscience. With recent developments in magnetic instrumentation and nanoparticle chemistries, including those that are commercially available, magnetic approaches promise to empower research aimed at connecting molecular and cellular signalling to physiology and behaviour in untethered moving subjects.

Distributed interfacing by nanoscale photodiodes enables single-neuron light activation and sensory enhancement in 3D spinal explants

Among emerging technologies developed to interface neuronal signaling, engineering electrodes at the nanoscale would yield more precise biodevices opening to progress in neural circuit investigations and to new therapeutic potential. Despite remarkable progress in miniature electronics for less invasive neurostimulation, most nano-enabled, optically triggered interfaces are demonstrated in cultured cells, which precludes the studies of natural neural circuits. We exploit here free-standing silicon-based nanoscale photodiodes to optically modulate single, identified neurons in mammalian spinal cord explants. With near-infrared light stimulation, we show that activating single excitatory or inhibitory neurons differently affects sensory circuits processing in the dorsal horn. We successfully functionalize nano-photodiodes to target single molecules, such as glutamate AMPA receptor subunits, thus enabling light activation of specific synaptic pathways. We conclude that nano-enabled neural interfaces can modulate selected sensory networks with low invasiveness. The use of nanoscale photodiodes can thus provide original perspective in linking neural activity to specific behavioral outcome.