Flexible fiber-based optoelectronics for neural interfaces

Low flow-resistance solid phase extraction of fluoroquinolones in water and food samples by high-pressure wet spinning porous polyimide microfibers

In this work, porous polyimide microfibers (PI-μF) were prepared by high-pressure wet spinning method, and successfully applied as adsorbents for solid phase extraction (SPE) of fluoroquinolones (FQs) in water and food samples. The PI-μFs of ∼10, 25, 50, 100 μm in diameter could be controlled by the inner diameter of quartz capillary nozzles. The flow resistance of SPE cartridges packed with 10 μm PI microfiber (10-PI-μF) and 25-PI-μF was comparable to or even lower than that of commercial SPE cartridges, while the flow resistance of 50-PI-μF and 100-PI-μF SPE cartridges was increased obviously due to tiny broken pieces. The 10-PI-μF and 25-PI-μF have a specific surface area of 102 m2 g-1 and 76 m2 g-1, mesopores of 22-32 nm, and large breakthrough volume of 110 mL/5 mg and 85 mL/5 mg for FQs, while the 50-PI-μF and 100-PI-μF had much lower specific surface area and hardly had retention for FQs. FQs from tap water, egg and milk samples were then extracted by PI-μF SPE, and analyzed by high performance liquid chromatography-fluorescence detector (HPLC-FLD). SPE parameters as type of elution solvent, elution solvent volume, pH value of sample solution, flow rate of sample solution, and breakthrough volume were first optimized in detail. Under the optimal conditions, the PI-μF SPE/HPLC-FLD method showed high recoveries (96.8%-107%), wide linearity (0.05-50 μg L-1, or 0.01-10 μg L-1), high determination coefficients (R2 ≥0.9992), and low limits of detection (LODs, 0.005-0.014 μg L-1). For the real tap water, egg and milk samples, the recoveries and RSDs were 81-119% and 0.8-9.8%, respectively. The results show that porous microfiber up to 25 μm in diameter is a promising solid-phase extraction adsorbent with the lowest flow resistance that can be used for trace organic pollutants in water and food samples.

Development of wafer-scale multifunctional nanophotonic neural probes for brain activity mapping

Optical techniques, such as optogenetic stimulation and functional fluorescence imaging, have been revolutionary for neuroscience by enabling neural circuit analysis with cell-type specificity. To probe deep brain regions, implantable light sources are crucial. Silicon photonics, commonly used for data communications, shows great promise in creating implantable devices with complex optical systems in a compact form factor compatible with high volume manufacturing practices. This article reviews recent developments of wafer-scale multifunctional nanophotonic neural probes. The probes can be realized on 200 or 300 mm wafers in commercial foundries and integrate light emitters for photostimulation, microelectrodes for electrophysiological recording, and microfluidic channels for chemical delivery and sampling. By integrating active optical devices to the probes, denser emitter arrays, enhanced on-chip biosensing, and increased ease of use may be realized. Silicon photonics technology makes possible highly versatile implantable neural probes that can transform neuroscience experiments.

Control of polymers' amorphous-crystalline transition enables miniaturization and multifunctional integration for hydrogel bioelectronics

Soft bioelectronic devices exhibit motion-adaptive properties for neural interfaces to investigate complex neural circuits. Here, we develop a fabrication approach through the control of metamorphic polymers' amorphous-crystalline transition to miniaturize and integrate multiple components into hydrogel bioelectronics. We attain an about 80% diameter reduction in chemically cross-linked polyvinyl alcohol hydrogel fibers in a fully hydrated state. This strategy allows regulation of hydrogel properties, including refractive index (1.37-1.40 at 480 nm), light transmission (>96%), stretchability (139-169%), bending stiffness (4.6 ± 1.4 N/m), and elastic modulus (2.8-9.3 MPa). To exploit the applications, we apply step-index hydrogel optical probes in the mouse ventral tegmental area, coupled with fiber photometry recordings and social behavioral assays. Additionally, we fabricate carbon nanotubes-PVA hydrogel microelectrodes by incorporating conductive nanomaterials in hydrogel for spontaneous neural activities recording. We enable simultaneous optogenetic stimulation and electrophysiological recordings of light-triggered neural activities in Channelrhodopsin-2 transgenic mice.

Homogeneous wet-spinning construction of skin-core structured PANI/cellulose conductive fibers for gas sensing and e-textile applications

Fiber-based wearable electronic textiles have broad applications, but non-degradable substrates may contribute to electronic waste. The application of cellulose-based composite fibers as e-textiles is hindered by the lack of fast and effective preparation methods. Here, we fabricated polyaniline (PANI)/cellulose fibers (PC) with a unique skin-core structure through a wet-spinning homogeneous blended system. The conductive network formation was enabled at a mere 1 wt% PANI. Notably, PC15 (15 wt% PANI) shows higher electrical conductivity of 21.50 mS cm-1. Further, PC15 exhibits excellent ammonia sensing performance with a sensitivity of 2.49 %/ppm and a low limit of detection (LOD) of 0.6 ppm. Cellulose-based composite fibers in this work demonstrate good gas sensing and anti-static properties as potential devices for smart e-textiles.

Fatigue-resistant hydrogel optical fibers enable peripheral nerve optogenetics during locomotion

We develop soft and stretchable fatigue-resistant hydrogel optical fibers that enable optogenetic modulation of peripheral nerves in naturally behaving animals during persistent locomotion. The formation of polymeric nanocrystalline domains within the hydrogels yields fibers with low optical losses of 1.07 dB cm-1, Young's modulus of 1.6 MPa, stretchability of 200% and fatigue strength of 1.4 MPa against 30,000 stretch cycles. The hydrogel fibers permitted light delivery to the sciatic nerve, optogenetically activating hindlimb muscles in Thy1::ChR2 mice during 6-week voluntary wheel running assays while experiencing repeated deformation. The fibers additionally enabled optical inhibition of pain hypersensitivity in an inflammatory model in TRPV1::NpHR mice over an 8-week period. Our hydrogel fibers offer a motion-adaptable and robust solution to peripheral nerve optogenetics, facilitating the investigation of somatosensation.

Redesigning Natural Materials for Energy, Water, Environment, and Devices

The United Nations Framework Convention on Climate Change (UNFCCC) acknowledges that global cooperation is paramount to mitigate climate change and further warming. The global community is committed to renewable energy and natural materials to tackle this challenge for all humankind. The widespread use of natural materials is embraced as one such action to reach net-zero carbon emissions. Given the hierarchical framework and earth abundance, cellulose-based materials extend their negative carbon benefits to our daily products and accelerate our pace toward carbon neutrality. Here, we present an overview of recent developments of cellulose-based materials in upsurging applications in radiative cooling, thermal insulation, nanofluidics, and wearable devices. We also highlight various modifications and functionalized processes that transform massive amounts of cellulose into green products. The prosperous development of functionalized cellulose materials aligns with a circular economy. Expedited interdisciplinary fundamental investigations are expected to make fibrillated cellulose penetrate more into carbon downdraw at speed and scale.

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

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

In vivo brain temperature mapping using polymer optical fiber Bragg grating sensors

Variation of the brain temperature is strongly affected by blood flow, oxygen supply, and neural cell metabolism. Localized monitoring of the brain temperature is one of the most effective ways to correlate brain functions and diseases such as stroke, epilepsy, and mood disorders. While polymer optical fibers (POFs) are considered ideal candidates for temperature sensing in the brain, they have never been used so far in vivo. Here, we developed for the first, to the best of our knowledge, time an implantable probe based on a microstructured polymer optical fiber Bragg grating (FBG) sensor for intracranial brain temperature mapping. The temperature at different depths of the brain (starting from the cerebral cortex) and the correlation between the brain and body core temperature of a rat were recorded with a sensitivity of 33 pm/°C and accuracy <0.2°C. Our in vivo experimental results suggest that the proposed device can achieve real-time and high-resolution local temperature measurement in the brain, as well as being integrated with existing neural interfaces.

Conductance stable and mechanically durable bi-layer EGaIn composite-coated stretchable fiber for 1D bioelectronics

Deformable semi-solid liquid metal particles (LMP) have emerged as a promising substitute for rigid conductive fillers due to their excellent electrical properties and stable conductance under strain. However, achieving a compact and robust coating of LMP on fibers remains a persistent challenge, mainly due to the incompatibility of conventional coating techniques with LMP. Additionally, the limited durability and absence of initial electrical conductivity of LMP restrict their widespread application. In this study, we propose a solution process that robustly and compactly assembles mechanically durable and initially conductive LMP on fibers. Specifically, we present a shearing-based deposition of polymer-attached LMP followed by additional coating with CNT-attached LMP to create bi-layer LMP composite with exceptional durability, electrical conductivity, stretchability, and biocompatibility on various fibers. The versatility and reliability of this manufacturing strategy for 1D electronics are demonstrated through the development of sewn electrical circuits, smart clothes, stretchable biointerfaced fiber, and multifunctional fiber probes.

Optoelectronic and mechanical properties of microstructured polymer optical fiber neural probes

Multifunctional optical fiber-based neural interfaces have attracted significant attention for neural stimulation, recording, and photopharmacology towards understanding the central nervous system. In this work, we demonstrate the fabrication, optoelectrical characterization, and mechanical analysis of four types of microstructured polymer optical fiber neural probes using different soft thermoplastic polymers. The developed devices have integrated metallic elements for electrophysiology and microfluidic channels for localized drug delivery, and can be used for optogenetics in the visible spectrum at wavelengths spanning from 450 nm up to 800 nm. Their impedance, measured by electrochemical impedance spectroscopy, was found to be as low as 21 kΩ and 4.7 kΩ at 1kHz when indium and tungsten wires are used as the integrated electrodes, respectively. Uniform on-demand drug delivery can be achieved by the microfluidic channels with a measured delivery rate from 10 up to 1000 nL/min. In addition, we identified the buckling failure threshold (defined as the conditions for successful implantation) as well as the bending stiffness of the fabricated fibers. Using finite element analysis, we calculated the main critical mechanical properties of the developed probes to avoid buckling during implantation and maintain high flexibility of the probe within the tissue. Our results aim to demonstrate the impact of design, fabrication, and characteristics of the materials on the development of polymer fibers as next-generation implants and neural interfaces.

A biodegradable, flexible photonic patch for in vivo phototherapy

Diagnostic and therapeutic illumination on internal organs and tissues with high controllability and adaptability in terms of spectrum, area, depth, and intensity remains a major challenge. Here, we present a flexible, biodegradable photonic device called iCarP with a micrometer scale air gap between a refractive polyester patch and the embedded removable tapered optical fiber. ICarP combines the advantages of light diffraction by the tapered optical fiber, dual refractions in the air gap, and reflection inside the patch to obtain a bulb-like illumination, guiding light towards target tissue. We show that iCarP achieves large area, high intensity, wide spectrum, continuous or pulsatile, deeply penetrating illumination without puncturing the target tissues and demonstrate that it supports phototherapies with different photosensitizers. We find that the photonic device is compatible with thoracoscopy-based minimally invasive implantation onto beating hearts. These initial results show that iCarP could be a safe, precise and widely applicable device suitable for internal organs and tissue illumination and associated diagnosis and therapy.

Soft monolithic infrared neural interface for simultaneous neurostimulation and electrophysiology

Controlling neuronal activity using implantable neural interfaces constitutes an important tool to understand and develop novel strategies against brain diseases. Infrared neurostimulation is a promising alternative to optogenetics for controlling the neuronal circuitry with high spatial resolution. However, bi-directional interfaces capable of simultaneously delivering infrared light and recording electrical signals from the brain with minimal inflammation have not yet been reported. Here, we have developed a soft fibre-based device using high-performance polymers which are >100-fold softer than conventional silica glass used in standard optical fibres. The developed implant is capable of stimulating the brain activity in localized cortical domains by delivering laser pulses in the 2 μm spectral region while recording electrophysiological signals. Action and local field potentials were recorded in vivo from the motor cortex and hippocampus in acute and chronic settings, respectively. Immunohistochemical analysis of the brain tissue indicated insignificant inflammatory response to the infrared pulses while the signal-to-noise ratio of recordings still remained high. Our neural interface constitutes a step forward in expanding infrared neurostimulation as a versatile approach for fundamental research and clinically translatable therapies.

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.

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.

Ferromagnetic Flexible Electronics for Brain-Wide Selective Neural Recording

Flexible microelectronics capable of straightforward implantation, remotely controlled navigation, and stable long-term recording hold great promise in diverse medical applications, particularly in deciphering complex functions of neural circuits in the brain. Existing flexible electronics, however, are often limited in bending and buckling during implantation, and unable to access a large brain region. Here, an injectable class of electronics with stable recording, omnidirectional steering, and precise navigating capabilities based on magnetic actuation is presented. After simple transcriptional injection, the rigid coatings are biodegraded quickly and the bundles of magnetic-nanoparticles-coated microelectrodes become separated, ultra-flexible, and magnetic actuated for further minimally invasive three-dimensional interpenetration in the brain. As proof of concept, this paradigm-shifting approach is demonstrated for selective and multiplexed neural activities recording across distant regions in the deep rodent brains. Coupling with optogenetic neural stimulation, the unique capabilities of this platform in electrophysiological readouts of projection dynamics in vivo are also demonstrated. The ability of these miniaturized, remotely controllable, and biocompatible ferromagnetic flexible electronics to afford minimally invasive manipulations in the soft tissues of the mammalian brain foreshadows applications in other organ systems, with great potential for broad utility in biomedical science and engineering.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Engineering Electrodes with Robust Conducting Hydrogel Coating for Neural Recording and Modulation

Coating conventional metallic electrodes with conducting polymers has enabled the essential characteristics required for bioelectronics, such as biocompatibility, electrical conductivity, mechanical compliance, and the capacity for structural and chemical functionalization of the bioelectrodes. However, the fragile interface between the conducting polymer and the electrode in wet physiological environment greatly limits their utility and reliability. Here, a general yet reliable strategy to seamlessly interface conventional electrodes with conducting hydrogel coatings is established, featuring tissue-like modulus, highly-desirable electrochemical properties, robust interface, and long-term reliability. Numerical modeling reveals the role of toughening mechanism, synergy of covalent anchorage of long-chain polymers, and chemical cross-linking, in improving the long-term robustness of the interface. Through in vivo implantation in freely-moving mouse models, it is shown that stable electrophysiological recording can be achieved, while the conducting hydrogel-electrode interface remains robust during the long-term low-voltage electrical stimulation. This simple yet versatile design strategy addresses the long-standing technical challenges in functional bioelectrode engineering, and opens up new avenues for the next-generation diagnostic brain-machine interfaces.

Semiconductor Multimaterial Optical Fibers for Biomedical Applications

Integrated sensors and transmitters of a wide variety of human physiological indicators have recently emerged in the form of multimaterial optical fibers. The methods utilized in the manufacture of optical fibers facilitate the use of a wide range of functional elements in microscale optical fibers with an extensive variety of structures. This article presents an overview and review of semiconductor multimaterial optical fibers, their fabrication and postprocessing techniques, different geometries, and integration in devices that can be further utilized in biomedical applications. Semiconductor optical fiber sensors and fiber lasers for body temperature regulation, in vivo detection, volatile organic compound detection, and medical surgery will be discussed.

Anti-Fouling, Adhesive Polyzwitterionic Hydrogel Electrodes Toughened Using a Tannic Acid Nanoflower

Conductive polyzwitterionic hydrogels with good adhesion properties show potential prospect in implantable electrodes and electronic devices. Adhesive property of polyzwitterionic hydrogels in humid environments can be improved by the introduction of catechol groups. However, common catechol modifiers can usually quench free radicals, resulting in a contradiction between long-term tissue adhesion and hydrogel toughness. By adding tannic acid (TA) to the dispersion of clay nanosheets and nanofibers, we designed TA-coated nanoflowers and nanofibers as the reinforcing phase to prepare polyzwitterionic hydrogels with adhesion properties. The hydrogel combines the mussel-like and zwitterionic co-adhesive mechanism to maintain long-term adhesion in underwater environments. In particular, the noncovalent cross-linking provided by the nanoflower structure effectively compensates for the defects caused by free-radical quenching so that the hydrogel obtained a high stretchability of over 2900% and a toughness of 1.16 J/m³. The hydrogel also has excellent anti-biofouling property and shows resistance to bacteria and cells. In addition, the hydrogel possesses a low modulus (<10 kPa) and ionic conductivity (0.25 S/m), making it an ideal material for the preparation of implantable electrodes.

Towards Multiplexed and Multimodal Biosensor Platforms in Real-Time Monitoring of Metabolic Disorders

Metabolic syndrome (MS) is a cluster of conditions that increases the probability of heart disease, stroke, and diabetes, and is very common worldwide. While the exact cause of MS has yet to be understood, there is evidence indicating the relationship between MS and the dysregulation of the immune system. The resultant biomarkers that are expressed in the process are gaining relevance in the early detection of related MS. However, sensing only a single analyte has its limitations because one analyte can be involved with various conditions. Thus, for MS, which generally results from the co-existence of multiple complications, a multi-analyte sensing platform is necessary for precise diagnosis. In this review, we summarize various types of biomarkers related to MS and the non-invasively accessible biofluids that are available for sensing. Then two types of widely used sensing platform, the electrochemical and optical, are discussed in terms of multimodal biosensing, figure-of-merit (FOM), sensitivity, and specificity for early diagnosis of MS. This provides a thorough insight into the current status of the available platforms and how the electrochemical and optical modalities can complement each other for a more reliable sensing platform for MS.

Electro-optical mechanically flexible coaxial microprobes for minimally invasive interfacing with intrinsic neural circuits

Central to advancing our understanding of neural circuits is developing minimally invasive, multi-modal interfaces capable of simultaneously recording and modulating neural activity. Recent devices have focused on matching the mechanical compliance of tissue to reduce inflammatory responses. However, reductions in the size of multi-modal interfaces are needed to further improve biocompatibility and long-term recording capabilities. Here a multi-modal coaxial microprobe design with a minimally invasive footprint (8-14 µm diameter over millimeter lengths) that enables efficient electrical and optical interrogation of neural networks is presented. In the brain, the probes allowed robust electrical measurement and optogenetic stimulation. Scalable fabrication strategies can be used with various electrical and optical materials, making the probes highly customizable to experimental requirements, including length, diameter, and mechanical properties. Given their negligible inflammatory response, these probes promise to enable a new generation of readily tunable multi-modal devices for long-term, minimally invasive interfacing with neural circuits.

Neurotechnological Approaches to the Diagnosis and Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common cause of dementia in the elderly, clinically defined by progressive cognitive decline and pathologically, by brain atrophy, neuroinflammation, and accumulation of extracellular amyloid plaques and intracellular neurofibrillary tangles. Neurotechnological approaches, including optogenetics and deep brain stimulation, have exploded as new tools for not only the study of the brain but also for application in the treatment of neurological diseases. Here, we review the current state of AD therapeutics and recent advancements in both invasive and non-invasive neurotechnologies that can be used to ameliorate AD pathology, including neurostimulation via optogenetics, photobiomodulation, electrical stimulation, ultrasound stimulation, and magnetic neurostimulation, as well as nanotechnologies employing nanovectors, magnetic nanoparticles, and quantum dots. We also discuss the current challenges in developing these neurotechnological tools and the prospects for implementing them in the treatment of AD and other neurodegenerative diseases.

Multichannel optogenetics combined with laminar recordings for ultra-controlled neuronal interrogation

Simultaneous large-scale recordings and optogenetic interventions may hold the key to deciphering the fast-paced and multifaceted dialogue between neurons that sustains brain function. Here we have taken advantage of thin, cell-sized, optical fibers for minimally invasive optogenetics and flexible implantations. We describe a simple procedure for making those fibers side-emitting with a Lambertian emission distribution. Here we combined those fibers with silicon probes to achieve high-quality recordings and ultrafast multichannel optogenetic inhibition. Furthermore, we developed a multi-channel optical commutator and general-purpose patch-cord for flexible experiments. We demonstrate that our framework allows to conduct simultaneous laminar recordings and multifiber stimulations, 3D optogenetic stimulation, connectivity inference, and behavioral quantification in freely moving animals. Our framework paves the way for large-scale photo tagging and controlled interrogation of rapid neuronal communication in any combination of brain areas.

Researchers from Freiburg University developed an ultraflexible fiber-based 3D light delivery system for electrophysiology and optogenetic manipulation in freely moving animals. The system allows multiside modulation of neuronal activity combined with neuronal measurements.

Smart textile lighting/display system with multifunctional fibre devices for large scale smart home and IoT applications

Smart textiles consist of discrete devices fabricated from—or incorporated onto—fibres. Despite the tremendous progress in smart textiles for lighting/display applications, a large scale approach for a smart display system with integrated multifunctional devices in traditional textile platforms has yet to be demonstrated. Here we report the realisation of a fully operational 46-inch smart textile lighting/display system consisting of RGB fibrous LEDs coupled with multifunctional fibre devices that are capable of wireless power transmission, touch sensing, photodetection, environmental/biosignal monitoring, and energy storage. The smart textile display system exhibits full freedom of form factors, including flexibility, bendability, and rollability as a vivid RGB lighting/grey-level-controlled full colour display apparatus with embedded fibre devices that are configured to provide external stimuli detection. Our systematic design and integration strategies are transformational and provide the foundation for realising highly functional smart lighting/display textiles over large area for revolutionary applications on smart homes and internet of things (IoT).

A large scale approach for multifunctional smart display systems in traditional textiles has yet to be demonstrated. Here, authors present a foldable, rollable 46-inch smart textile lighting/display system for smart homes and internet of things applications via the systematic design and integration of versatile fibre devices into textile form factors.

Adaptive polymer fiber neural device for drug delivery and enlarged illumination angle for neuromodulation

OBJECTIVE

Optical fiber devices constitute significant tools for the modulation and interrogation of neuronal circuitry in the mid and deep brain regions. The illuminated brain area during neuromodulation has a direct impact on the spatio-temporal properties of the brain activity and depends solely on the material and geometrical characteristics of the optical fibers. In the present work, we developed two different flexible polymer optical fibers (POFs) with integrated microfluidic channels (MFCs) and an ultra-high numerical aperture (UHNA) for enlarging the illumination angle to achieve efficient neuromodulation.

APPROACH

Three distinct thermoplastic polymers: polysulfone (PSU), polycarbonate (PC), and fluorinated ethylene propylene (FEP) were used to fabricate two step-index UHNA POF neural devices using a scalable thermal drawing process. The POFs were characterized in terms of their illumination map as well as their fluid delivery capability in phantom and adult rat brain slices.

MAIN RESULTS

A 100-fold reduced bending stiffness of the proposed fiber devices compared to their commercially available counterparts has been found. The integrated MFCs can controllably deliver dye (trypan blue) on-demand over a wide range of injection rates spanning from 10 nL/min to 1000 nL/min. Compared with commercial silica fibers, the proposed UHNA POFs exhibited an increased illumination area by 17% and 21% under 470 and 650 nm wavelength, respectively. In addition, a fluorescent light recording experiment has been conducted to demonstrate the ability of our UHNA POFs to be used as optical waveguides in fiber photometry.

SIGNIFICANCE

Our results overcome the current technological limitations of fiber implants that have limited illumination area and we suggest that soft neural fiber devices can be developed using different custom designs for illumination, collection, and photometry applications. We anticipate our work to pave the way towards the development of next-generation functional optical fibers for neuroscience.

Multimodal neural probes for combined optogenetics and electrophysiology

To understand how brain functions arise from interconnected neural networks, it is necessary to develop tools that can allow simultaneous manipulation and recording of neural activities. Multimodal neural probes, especially those that combine optogenetics with electrophysiology, provide a powerful tool for the dissection of neural circuit functions and understanding of brain diseases. In this review, we provide an overview of recent developments in multimodal neural probes. We will focus on materials and integration strategies of multimodal neural probes to achieve combined optogenetic stimulation and electrical recordings with high spatiotemporal precision and low invasiveness. In addition, we will also discuss future opportunities of multimodal neural interfaces in basic and translational neuroscience.

Microfluidic platforms for single neuron analysis

Single-neuron actions are the basis of brain function, as clinical sequelae, neuronal dysfunction or failure for most of the central nervous system (CNS) diseases and injuries can be identified via tracing single-neurons. The bulk analysis methods tend to miscue critical information by assessing the population-averaged outcomes. However, its primary requisite in neuroscience to analyze single-neurons and to understand dynamic interplay of neurons and their environment. Microfluidic systems enable precise control over nano-to femto-liter volumes via adjusting device geometry, surface characteristics, and flow-dynamics, thus facilitating a well-defined micro-environment with spatio-temporal control for single-neuron analysis. The microfluidic platform not only offers a comprehensive landscape to study brain cell diversity at the level of transcriptome, genome, and/or epigenome of individual cells but also has a substantial role in deciphering complex dynamics of brain development and brain-related disorders. In this review, we highlight recent advances of microfluidic devices for single-neuron analysis, i.e., single-neuron trapping, single-neuron dynamics, single-neuron proteomics, single-neuron transcriptomics, drug delivery at the single-neuron level, single axon guidance, and single-neuron differentiation. Moreover, we also emphasize limitations and future challenges of single-neuron analysis by focusing on key performances of throughput and multiparametric activity analysis on microfluidic platforms.

Recent advances in recording and modulation technologies for next-generation neural interfaces

Along with the advancement in neural engineering techniques, unprecedented progress in the development of neural interfaces has been made over the past few decades. However, despite these achievements, there is still room for further improvements especially toward the possibility of monitoring and modulating neural activities with high resolution and specificity in our daily lives. In an effort of taking a step toward the next-generation neural interfaces, we want to highlight the recent progress in neural technologies. We will cover a wide scope of such developments ranging from novel platforms for highly specific recording and modulation to system integration for practical applications of novel interfaces.

Flexible optoelectric neural interfaces

Understanding the neural basis of brain function and dysfunction and designing effective therapeutics require high resolution targeted stimulation and recording of neural activity. Optical methods have been recently developed for neural stimulation as well as functional and structural imaging. These methods call for implantable devices to deliver light into the neural tissue at depth with high spatiotemporal resolution. To address this need, rigid and flexible neurophotonic implants have been recently designed. This article reviews the state-of-the-art flexible passive and active penetrating optical neural probes developed for light delivery with minimal damage to the tissue. Passive and active flexible neurophotonic implants are compared and insights about future directions are provided.

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.

A non-printed integrated-circuit textile for wireless theranostics

While the printed circuit board (PCB) has been widely considered as the building block of integrated electronics, the world is switching to pursue new ways of merging integrated electronic circuits with textiles to create flexible and wearable devices. Herein, as an alternative for PCB, we described a non-printed integrated-circuit textile (NIT) for biomedical and theranostic application via a weaving method. All the devices are built as fibers or interlaced nodes and woven into a deformable textile integrated circuit. Built on an electrochemical gating principle, the fiber-woven-type transistors exhibit superior bending or stretching robustness, and were woven as a textile logical computing module to distinguish different emergencies. A fiber-type sweat sensor was woven with strain and light sensors fibers for simultaneously monitoring body health and the environment. With a photo-rechargeable energy textile based on a detailed power consumption analysis, the woven circuit textile is completely self-powered and capable of both wireless biomedical monitoring and early warning. The NIT could be used as a 24/7 private AI "nurse" for routine healthcare, diabetes monitoring, or emergencies such as hypoglycemia, metabolic alkalosis, and even COVID-19 patient care, a potential future on-body AI hardware and possibly a forerunner to fabric-like computers.

Eutectics: formation, properties, and applications

Various eutectic systems have been proposed and studied over the past few decades. Most of the studies have focused on three typical types of eutectics: eutectic metals, eutectic salts, and deep eutectic solvents. On the one hand, they are all eutectic systems, and their eutectic principle is the same. On the other hand, they are representative of metals, inorganic salts, and organic substances, respectively. They have applications in almost all fields related to chemistry. Their different but overlapping applications stem from their very different properties. In addition, the proposal of new eutectic systems has greatly boosted the development of cross-field research involving chemistry, materials, engineering, and energy. The goal of this review is to provide a comprehensive overview of these typical eutectics and describe task-specific strategies to address growing demands.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Digital electronics in fibres enable fabric-based machine-learning inference

Digital devices are the essential building blocks of any modern electronic system. Fibres containing digital devices could enable fabrics with digital system capabilities for applications in physiological monitoring, human-computer interfaces, and on-body machine-learning. Here, a scalable preform-to-fibre approach is used to produce tens of metres of flexible fibre containing hundreds of interspersed, digital temperature sensors and memory devices with a memory density of ~7.6 × 10⁵ bits per metre. The entire ensemble of devices are individually addressable and independently operated through a single connection at the fibre edge, overcoming the perennial single-fibre single-device limitation and increasing system reliability. The digital fibre, when incorporated within a shirt, collects and stores body temperature data over multiple days, and enables real-time inference of wearer activity with an accuracy of 96% through a trained neural network with 1650 neuronal connections stored within the fibre. The ability to realise digital devices within a fibre strand which can not only measure and store physiological parameters, but also harbour the neural networks required to infer sensory data, presents intriguing opportunities for worn fabrics that sense, memorise, learn, and infer situational context.

Implementation of digital electronics into fibres can enable real time monitoring of human physiological functions. Loke et al. show how digital functionalities can be incorporated into thin flexible polymeric fibre strands and applied for on-body machine-learning and intelligent textiles.

Diving beetle-like miniaturized plungers with reversible, rapid biofluid capturing for machine learning-based care of skin disease

Recent advances in bioinspired nano/microstructures have received attention as promising approaches with which to implement smart skin-interfacial devices for personalized health care. In situ skin diagnosis requires adaptable skin adherence and rapid capture of clinical biofluids. Here, we report a simple, all-in-one device consisting of microplungers and hydrogels that can rapidly capture biofluids and conformally attach to skin for stable, real-time monitoring of health. Inspired by the male diving beetle, the microplungers achieve repeatable, enhanced, and multidirectional adhesion to human skin in dry/wet environments, revealing the role of the cavities in these architectures. The hydrogels within the microplungers instantaneously absorb liquids from the epidermis for enhanced adhesiveness and reversibly change color for visual indication of skin pH levels. To realize advanced biomedical technologies for the diagnosis and treatment of skin, our suction-mediated device is integrated with a machine learning framework for accurate and automated colorimetric analysis of pH levels.

High Aspect Ratio and Light-Sensitive Micropillars Based on a Semiconducting Polymer Optically Regulate Neuronal Growth

Many nano- and microstructured devices capable of promoting neuronal growth and network formation have been previously investigated. In certain cases, topographical cues have been successfully complemented with external bias, by employing electrically conducting scaffolds. However, the use of optical stimulation with topographical cues was rarely addressed in this context, and the development of light-addressable platforms for modulating and guiding cellular growth and proliferation remains almost completely unexplored. Here, we develop high aspect ratio micropillars based on a prototype semiconducting polymer, regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), as an optically active, three-dimensional platform for embryonic cortical neurons. P3HT micropillars provide a mechanically compliant environment and allow a close contact with neuronal cells. The combined action of nano/microtopography and visible light excitation leads to effective optical modulation of neuronal growth and orientation. Embryonic neurons cultured on polymer pillars show a clear polarization effect and, upon exposure to optical excitation, a significant increase in both neurite and axon length. The biocompatible, microstructured, and light-sensitive platform developed here opens up the opportunity to optically regulate neuronal growth in a wireless, repeatable, and spatio-temporally controlled manner without genetic modification. This approach may be extended to other cell models, thus uncovering interesting applications of photonic devices in regenerative medicine.

Advances in Carbon-Based Microfiber Electrodes for Neural Interfacing

Neural interfacing devices using penetrating microelectrode arrays have emerged as an important tool in both neuroscience research and medical applications. These implantable microelectrode arrays enable communication between man-made devices and the nervous system by detecting and/or evoking neuronal activities. Recent years have seen rapid development of electrodes fabricated using flexible, ultrathin carbon-based microfibers. Compared to electrodes fabricated using rigid materials and larger cross-sections, these microfiber electrodes have been shown to reduce foreign body responses after implantation, with improved signal-to-noise ratio for neural recording and enhanced resolution for neural stimulation. Here, we review recent progress of carbon-based microfiber electrodes in terms of material composition and fabrication technology. The remaining challenges and future directions for development of these arrays will also be discussed. Overall, these microfiber electrodes are expected to improve the longevity and reliability of neural interfacing devices.

Computational and Histological Analyses for Investigating Mechanical Interaction of Thermally Drawn Fiber Implants with Brain Tissue

The development of a compliant neural probe is necessary to achieve chronic implantation with minimal signal loss. Although fiber-based neural probes fabricated by the thermal drawing process have been proposed as a solution, their long-term effect on the brain has not been thoroughly investigated. Here, we examined the mechanical interaction of thermally drawn fiber implants with neural tissue through computational and histological analyses. Specifically, finite element analysis and immunohistochemistry were conducted to evaluate the biocompatibility of various fiber implants made with different base materials (steel, silica, polycarbonate, and hydrogel). Moreover, the effects of the coefficient of friction and geometric factors including aspect ratio and the shape of the cross-section on the strain were investigated with the finite element model. As a result, we observed that the fiber implants fabricated with extremely softer material such as hydrogel exhibited significantly lower strain distribution and elicited a reduced immune response. In addition, the implants with higher coefficient of friction (COF) and/or circular cross-sections showed a lower strain distribution and smaller critical volume. This work suggests the materials and design factors that need to be carefully considered to develop future fiber-based neural probes to minimize mechanical invasiveness.

Fiber-Based Electrochemical Biosensors for Monitoring pH and Transient Neurometabolic Lactate

Developing tools that are able to monitor transient neurochemical dynamics is important to decipher brain chemistry and function. Multifunctional polymer-based fibers have been recently applied to monitor and modulate neural activity. Here, we explore the potential of polymer fibers comprising six graphite-doped electrodes and two microfluidic channels within a flexible polycarbonate body as a platform for sensing pH and neurometabolic lactate. Electrodes were made into potentiometric sensors (responsive to pH) or amperometric sensors (lactate biosensors). The growth of an iridium oxide layer made the fiber electrodes responsive to pH in a physiologically relevant range. Lactate biosensors were fabricated via platinum black growth on the fiber electrode, followed by an enzyme layer, making them responsive to lactate concentration. Lactate fiber biosensors detected transient neurometabolic lactate changes in an in vivo mouse model. Lactate concentration changes were associated with spreading depolarizations, known to be detrimental to the injured brain. Induced waves were identified by a signature lactate concentration change profile and measured as having a speed of ∼2.7 mm/min (n = 4 waves). Our work highlights the potential applications of fiber-based biosensors for direct monitoring of brain metabolites in the context of injury.

Hybrid Electrical and Optical Neural Interfaces

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

Soft Electronics Based on Stretchable and Conductive Nanocomposites for Biomedical Applications

Research on the field of implantable electronic devices that can be directly applied in the body with various functionalities is increasingly intensifying due to its great potential for various therapeutic applications. While conventional implantable electronics generally include rigid and hard conductive materials, their surrounding biological objects are soft and dynamic. The mechanical mismatch between implanted devices and biological environments induces damages in the body especially for long-term applications. Stretchable electronics with outstanding mechanical compliance with biological objects effectively improve such limitations of existing rigid implantable electronics. In this article, the recent progress of implantable soft electronics based on various conductive nanocomposites is systematically described. In particular, representative fabrication approaches of conductive and stretchable nanocomposites for implantable soft electronics and various in vivo applications of implantable soft electronics are focused on. To conclude, challenges and perspectives of current implantable soft electronics that should be considered for further advances are discussed.

In Vivo Photopharmacology Enabled by Multifunctional Fibers

Photoswitchable ligands can add an optical switch to a target receptor or signaling cascade and enable reversible control of neural circuits. The application of this approach, termed photopharmacology, to behavioral experiments has been impeded by a lack of integrated hardware capable of delivering both light and compounds to deep brain regions in moving subjects. Here, we devise a hybrid photochemical genetic approach to target neurons using a photoswitchable agonist of the capsaicin receptor TRPV1, red-AzCA-4. Using multifunctional fibers with optical and microfluidic capabilities, we delivered a transgene coding for TRPV1 into the ventral tegmental area (VTA). This sensitized excitatory VTA neurons to red-AzCA-4, allowing us to optically control conditioned place preference in mice, thus extending applications of photopharmacology to behavioral experiments. Applied to endogenous receptors, our approach may accelerate future studies of molecular mechanisms underlying animal behavior.

Designer patterned functional fibers via direct imprinting in thermal drawing

Creating micro/nanostructures on fibers is beneficial for extending the application range of fiber-based devices. To achieve this using thermal fiber drawing is particularly important for the mass production of longitudinally uniform fibers up to tens of kilometers. However, the current thermal fiber drawing technique can only fabricate one-directional micro/nano-grooves longitudinally due to structure elongation and polymer reflow. Here, we develop a direct imprinting thermal drawing (DITD) technique to achieve arbitrarily designed surface patterns on entire fiber surfaces with high resolution in all directions. Such a thermal imprinting process is simulated and confirmed experimentally. Key process parameters are further examined, showing a process feature size as small as tens of nanometers. Furthermore, nanopatterns are fabricated on fibers as plasmonic metasurfaces, and double-sided patterned fibers are produced to construct self-powered wearable touch sensing fabric, revealing the bright future of the DITD technology in multifunctional fiber-based devices, wearable electronics, and smart textiles.

Creating micro/nanostructures on fibers is beneficial to many fiber-based devices, which remains a challenge in large-scale fabrication due to elongation and reflow. Here, the authors demonstrate a method for generating high-resolution, arbitrarily designed surface patterns on fiber during the thermal drawing process.

Induced neural stem cell differentiation on a drawn fiber scaffold-toward peripheral nerve regeneration

To achieve regeneration of long sections of damaged nerves, restoration methods such as direct suturing or autologous grafting can be inefficient. Solutions involving biohybrid implants, where neural stem cells are grown in vitro on an active support before implantation, have attracted attention. Using such an approach, combined with recent advancements in microfabrication technology, the chemical and physical environment of cells can be tailored in order to control their behaviors. Herein, a neural stem cell polycarbonate fiber scaffold, fabricated by 3D printing and thermal drawing, is presented. The combined effect of surface microstructure and chemical functionalization using poly-L-ornithine (PLO) and double-walled carbon nanotubes (DWCNTs) on the biocompatibility of the scaffold, induced differentiation of the neural stem cells (NSCs) and channeling of the neural cells was investigated. Upon treatment of the fiber scaffold with a suspension of DWCNTs in PLO (0.039 g l^-1) and without recombinants a high degree of differentiation of NSCs into neuronal cells was confirmed by using nestin, galactocerebroside and doublecortin immunoassays. These findings illuminate the potential use of this biohybrid approach for the realization of future nerve regenerative implants.

Optical Waveguides and Integrated Optical Devices for Medical Diagnosis, Health Monitoring and Light Therapies

Optical waveguides and integrated optical devices are promising solutions for many applications, such as medical diagnosis, health monitoring and light therapies. Despite the many existing reviews focusing on the materials that these devices are made from, a systematic review that relates these devices to the various materials, fabrication processes, sensing methods and medical applications is still seldom seen. This work is intended to link these multidisciplinary fields, and to provide a comprehensive review of the recent advances of these devices. Firstly, the optical and mechanical properties of optical waveguides based on glass, polymers and heterogeneous materials and fabricated via various processes are thoroughly discussed, together with their applications for medical purposes. Then, the fabrication processes and medical implementations of integrated passive and active optical devices with sensing modules are introduced, which can be used in many medical fields such as drug delivery and cardiovascular healthcare. Thirdly, wearable optical sensing devices based on light sensing methods such as colorimetry, fluorescence and luminescence are discussed. Additionally, the wearable optical devices for light therapies are introduced. The review concludes with a comprehensive summary of these optical devices, in terms of their forms, materials, light sources and applications.

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.

Recent Progress and Perspectives of Thermally Drawn Multimaterial Fiber Electronics

Fibers are the building blocks of a broad spectrum of products from textiles to composites, and waveguides to wound dressings. While ubiquitous, the capabilities of fibers have not rapidly increased compared to semiconductor chip technology, for example. Recognizing that fibers lack the composition, geometry, and feature sizes for more functions, exploration of the boundaries of fiber functionality began some years ago. The approach focuses on a particular form of fiber production, thermal-drawing from a preform. This process has been used for producing single material fibers, but by combining metals, insulators, and semiconductors all within a single strand of fiber, an entire world of functionality in fibers has emerged. Fibers with optical, electrical, acoustic, or optoelectronic functionalities can be produced at scale from relatively easy-to-assemble macroscopic preforms. Two significant opportunities now present themselves. First, can one expect that fiber functions escalate in a predictable manner, creating the context for a "Moore's Law" analog in fibers? Second, as fabrics occupy an enormous surface around the body, could fabrics offer a valuable service to augment the human body? Toward answering these questions, the materials, performance, and limitations of thermally drawn fibers in different electronic applications are detailed and their potential in new fields is envisioned.