Polymer Composite with Carbon Nanofibers Aligned during Thermal Drawing as a Microelectrode for Chronic Neural Interfaces

Self-Poled Piezoelectric Nanocomposite Fiber Sensors for Wireless Monitoring of Physiological Signals

Self-powered sensors have the potential to enable real-time health monitoring without contributing to the ever-growing demand for energy. Piezoelectric nanogenerators (PENGs) respond to mechanical deformations to produce electrical signals, imparting a sensing capability without external power sources. Textiles conform to the human body and serve as an interactive biomechanical energy harvesting and sensing medium without compromising comfort. However, the textile-based PENG fabrication process is complex and lacks scalability, making these devices impractical for mass production. Here, we demonstrate the fabrication of a long-length PENG fiber compatible with industrial-scale manufacturing. The thermal drawing process enables the one-step fabrication of self-poled MoS2-poly(vinylidene fluoride) (PVDF) nanocomposite fiber devices integrated with electrodes. Heat and stress during thermal drawing and MoS2 nanoparticle addition facilitate interfacial polarization and dielectric modulation to enhance the output performance. The fibers show a 57 and 70% increase in the output voltage and current compared to the pristine PVDF fiber, respectively, at a considerably low MoS2 loading of 3 wt %. The low Young's modulus of the outer cladding ensures an effective stress transfer to the piezocomposite domain and allows minute motion detection. The flexible fibers demonstrate wireless, self-powered physiological sensing and biomotion analysis capability. The study aims to guide the large-scale production of highly sensitive integrated fibers to enable textile-based and plug-and-play wearable sensors.

Structurally Aligned Multifunctional Neural Probe (SAMP) Using Forest-Drawn CNT Sheet onto Thermally Drawn Polymer Fiber for Long-Term In Vivo Operation

Neural probe engineering is a dynamic field, driving innovation in neuroscience and addressing scientific and medical demands. Recent advancements involve integrating nanomaterials to improve performance, aiming for sustained in vivo functionality. However, challenges persist due to size, stiffness, complexity, and manufacturing intricacies. To address these issues, a neural interface utilizing freestanding CNT-sheets drawn from CNT-forests integrated onto thermally drawn functional polymer fibers is proposed. This approach yields a device with structural alignment, resulting in exceptional electrical, mechanical, and electrochemical properties while retaining biocompatibility for prolonged periods of implantation. This Structurally Aligned Multifunctional neural Probe (SAMP) employing forest-drawn CNT sheets demonstrates in vivo capabilities in neural recording, neurotransmitter detection, and brain/spinal cord circuit manipulation via optogenetics, maintaining functionality for over a year post-implantation. The straightforward fabrication method's versatility, coupled with the device's functional reliability, underscores the significance of this technique in the next-generation carbon-based implants. Moreover, the device's longevity and multifunctionality position it as a promising platform for long-term neuroscience research.

Wireless closed-loop deep brain stimulation using microelectrode array probes

Deep brain stimulation (DBS), including optical stimulation and electrical stimulation, has been demonstrated considerable value in exploring pathological brain activity and developing treatments for neural disorders. Advances in DBS microsystems based on implantable microelectrode array (MEA) probes have opened up new opportunities for closed-loop DBS (CL-DBS) in situ. This technology can be used to detect damaged brain circuits and test the therapeutic potential for modulating the output of these circuits in a variety of diseases simultaneously. Despite the success and rapid utilization of MEA probe-based CL-DBS microsystems, key challenges, including excessive wired communication, need to be urgently resolved. In this review, we considered recent advances in MEA probe-based wireless CL-DBS microsystems and outlined the major issues and promising prospects in this field. This technology has the potential to offer novel therapeutic options for psychiatric disorders in the future.

Biohybrid tongue based on hypothalamic neuronal network-on-a-chip for real-time blood glucose sensing and assessment

The expression of sweet receptors in the hypothalamus has been implicated in energy homeostasis control and the pathogenesis of obesity and diabetes. However, the exact mechanism by which hypothalamic glucose-sensing neurons function remains unclear. Conventional detection methods, such as fiber photometry, optogenetics, brain-machine interfaces, patch clamp and calcium imaging, pose limitations for real-time glucose perception due to their complexity, cytotoxicity and so on. Therefore, this study proposes a biohybrid tongue based on hypothalamic neuronal network (HNN)-on-a-chip coupling with microelectrode array (MEA) for real-time glucose perception. Hypothalamic neuronal cultures were cultivated on a two-dimensional "brain-on-chip" device, enabling the formation of neuronal networks and electrophysiological signal detection. Additionally, we investigated the endogenous expression of sweet taste receptors (T1R2/T1R3) in hypothalamic neuronal cells, providing the basis for the biohybrid tongue based on HNN-on-a-chip's sweetness detection capabilities. The spike signal response to sucrose and glucose stimulation was detected, and concentration-dependent responses were explored with glucose concentrations ranging from 0.01 mM to 8 mM. MEAs allow for real-time recordings, enabling the observation of dynamic changes in neuronal responses to glucose fluctuations over time. The biohybrid tongue based on HNN-on-a-chip can measure various parameters, including spike frequency and amplitude, providing insights into neuronal firing patterns and excitability. Moreover, hypothalamic glucoregulatory neurons that sense and respond to changes in blood glucose was identified, including glucose-excited neurons (GE-Neurons) and glucose-inhibited neurons (GI-Neurons). The detection range for GE-Neurons spans from 0.4 to 6 mM, while GI-Neurons demonstrate sensitivity within the range of 1-8 mM. And the glucose detection limit was firmly established at 0.01 mM. Through non-linear regression analysis, the IC50 for GI-Neurons' spike firing was determined to be 4.18 mM. In conclusion, the biohybrid tongue based on HNN-on-a-chip offers a valuable in vitro tool for studying hypothalamic neurons, elucidating glucose sensing mechanisms, and understanding hypothalamic neuronal function.

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.

Electrospun Fibrous Silica for Bone Tissue Engineering Applications

The production of highly porous and three-dimensional (3D) scaffolds with biomimicking abilities has gained extensive attention in recent years for tissue engineering (TE) applications. Considering the attractive and versatile biomedical functionality of silica (SiO₂) nanomaterials, we propose herein the development and validation of SiO₂-based 3D scaffolds for TE. This is the first report on the development of fibrous silica architectures, using tetraethyl orthosilicate (TEOS) and polyvinyl alcohol (PVA) during the self-assembly electrospinning (ES) processing (a layer of flat fibers must first be created in self-assembly electrospinning before fiber stacks can develop on the fiber mat). The compositional and microstructural characteristics of obtained fibrous materials were evaluated by complementary techniques, in both the pre-ES aging period and post-ES calcination. Then, in vivo evaluation confirmed their possible use as bioactive scaffolds in bone TE.

A Short Review on Nanostructured Carbon Containing Biopolymer Derived Composites for Tissue Engineering Applications

The development of new scaffolds and materials for tissue engineering is a wide and open realm of material science. Among solutions, the use of biopolymers represents a particularly interesting area of study due to their great chemical complexity that enables creation of specific molecular architectures. However, biopolymers do not exhibit the properties required for direct application in tissue repair-such as mechanical and electrical properties-but they do show very attractive chemical functionalities which are difficult to produce through in vitro synthesis. The combination of biopolymers with nanostructured carbon fillers could represent a robust solution to enhance composite properties, producing composites with new and unique features, particularly relating to electronic conduction. In this paper, we provide a review of the field of carbonaceous nanostructure-containing biopolymer composites, limiting our investigation to tissue-engineering applications, and providing a complete overview of the recent and most outstanding achievements.

Scalable Fabrication of MXene-PVDF Nanocomposite Triboelectric Fibers via Thermal Drawing

In the data-driven world, textile is a valuable resource for improving the quality of life through continuous monitoring of daily activities and physiological signals of humans. Triboelectric nanogenerators (TENG) are an attractive option for self-powered sensor development by coupling energy harvesting and sensing ability. In this study, to the best of the knowledge, scalable fabrication of Ti₃ C₂ T_x MXene-embedded polyvinylidene fluoride (PVDF) nanocomposite fiber using a thermal drawing process is presented for the first time. The output open circuit voltage and short circuit current show 53% and 58% improvement, respectively, compared to pristine PVDF fiber. The synergistic interaction between the surface termination groups of MXene and polar PVDF polymer enhances the performance of the fiber. The flexibility of the fiber enables the weaving of fabric TENG devices for large-area applications. The fabric TENG (3 × 2 cm² ) demonstrates a power density of 40.8 mW m^-2 at the matching load of 8 MΩ by maintaining a stable performance over 12 000 cycles. Moreover, the fabric TENG has shown the capability of energy harvesting by operating a digital clock and a calculator. A distributed self-powered sensor for human activities and walking pattern monitoring are demonstrated with the fabric.

Microelectronic fibers for multiplexed sweat sensing

Wearable bioelectronics are gaining extraordinary attention due to their capabilities to achieve continuous monitoring of human health status. However, mainstream manufacturing technologies, including photolithography and printing technology, limit current wearable bioelectronics on 2D planar structures with little surface area in contact with the body. It thus limits the amount of physiological information that current wearable bioelectronics could obtain. Furthermore, they need to be firmly attached to the body, affecting the wearing comfort. In this study, we leveraged the versatile thermal drawing process and developed a flexible microelectronic fiber with bioanalytical functions that could be woven into textiles as a new form of wearable bioelectronics. Within a single strand of fiber, we successfully integrated all-in-one multiplexed electrochemical sensing capabilities, with the sweat as the primary object. Adopting the laser micromachining technique, we developed biosensing functions on the longitudinal surface of the fiber with two sensing electrodes for Na⁺ and uric acid (UA), respectively, together with a pseudo reference electrode (p-RE). We carefully characterized the all-in-one multiplexed sensing performance of the fiber and demonstrated its successful application in sweat sensing based on its textile forms. The results show significant potential for application in wearable textiles for monitoring key health signals of humans.

Highly Integrated Multi-Material Fibers for Soft Robotics

Soft robots are envisioned as the next generation of safe biomedical devices in minimally invasive procedures. Yet, the difficulty of processing soft materials currently limits the size, aspect-ratio, manufacturing throughput, as well as, the design complexity and hence capabilities of soft robots. Multi-material thermal drawing is introduced as a material and processing platform to create soft robotic fibers imparted with multiple actuations and sensing modalities. Several thermoplastic and elastomeric material options for the fibers are presented, which all exhibit the rheological processing attributes for thermal drawing but varying mechanical properties, resulting in adaptable actuation performance. Moreover, numerous different fiber designs with intricate internal architectures, outer diameters of 700 µm, aspect ratios of 10³ , and a fabrication at a scale of 10s of meters of length are demonstrated. A modular tendon-driven mechanism enables 3-dimensional (3D) motion, and embedded optical guides, electrical wires, and microfluidic channels give rise to multifunctionality. The fibers can perceive and autonomously adapt to their environments, as well as, probe electrical properties, and deliver fluids and mechanical tools to spatially distributed targets.

3D stretchable and self-encapsulated multimaterial triboelectric fibers

A robust power device for wearable technologies and soft electronics must feature good encapsulation, high deformability, and reliable electrical outputs. Despite substantial progress in materials and architectures for two-dimensional (2D) planar power configurations, fiber-based systems remain limited to relatively simple configurations and low performance due to challenges in processing methods. Here, we extend complex 2D triboelectric nanogenerator configurations to 3D fiber formats based on scalable thermal processing of water-resistant thermoplastic elastomers and composites. We perform mechanical analysis using finite element modeling to understand the fiber's deformation and the level of control and engineering on its mechanical behavior and thus to guide its dimensional designs for enhanced electrical performance. With microtexture patterned functional surfaces, the resulting fibers can reliably produce state-of-the-art electrical outputs from various mechanical deformations, even under harsh conditions. These mechanical and electrical attributes allow their integration with large and stretchable surfaces for electricity generation of hundreds of microamperes.

Recent Advances in Stretchable and Wearable Capacitive Electrophysiological Sensors for Long-Term Health Monitoring

Over the past several years, wearable electrophysiological sensors with stretchability have received significant research attention because of their capability to continuously monitor electrophysiological signals from the human body with minimal body motion artifacts, long-term tracking, and comfort for real-time health monitoring. Among the four different sensors, i.e., piezoresistive, piezoelectric, iontronic, and capacitive, capacitive sensors are the most advantageous owing to their reusability, high durability, device sterilization ability, and minimum leakage currents between the electrode and the body to reduce the health risk arising from any short circuit. This review focuses on the development of wearable, flexible capacitive sensors for monitoring electrophysiological conditions, including the electrode materials and configuration, the sensing mechanisms, and the fabrication strategies. In addition, several design strategies of flexible/stretchable electrodes, body-to-electrode signal transduction, and measurements have been critically evaluated. We have also highlighted the gaps and opportunities needed for enhancing the suitability and practical applicability of wearable capacitive sensors. Finally, the potential applications, research challenges, and future research directions on stretchable and wearable capacitive sensors are outlined in this review.

Thermally Drawn CNT-Based Hybrid Nanocomposite Fiber for Electrochemical Sensing

Nowadays, bioelectronic devices are evolving from rigid to flexible materials and substrates, among which thermally-drawn-fiber-based bioelectronics represent promising technologies thanks to their inherent flexibility and seamless integration of multi-functionalities. However, electrochemical sensing within fibers remains a poorly explored area, as it imposes new demands for material properties-both the electrochemical sensitivity and the thermomechanical compatibility with the fiber drawing process. Here, we designed and fabricated microelectrode fibers made of carbon nanotube (CNT)-based hybrid nanocomposites and further evaluated their detailed electrochemical sensing performances. Carbon-black-impregnated polyethylene (CB-CPE) was chosen as the base material, into which CNT was loaded homogeneously in a concentration range of 3.8 to 10 wt%. First, electrical impedance characterization of CNT nanocomposites showed a remarkable decrease of the resistance with the increase in CNT loading ratio, suggesting that CNTs notably increased the effective electrical current pathways inside the composites. In addition, the proof-of-principle performance of fiber-based microelectrodes was characterized for the detection of ferrocenemethanol (FcMeOH) and dopamine (DA), exhibiting an ultra-high sensitivity. Additionally, we further examined the long-term stability of such composite-based electrode in exposure to the aqueous environment, mimicking the in vivo or in vitro settings. Later, we functionalized the surface of the microelectrode fiber with ion-sensitive membranes (ISM) for the selective sensing of Na+ ions. The miniature fiber-based electrochemical sensor developed here holds great potential for standalone point-of-care sensing applications. In the future, taking full advantage of the thermal drawing process, the electrical, optical, chemical, and electrochemical modalities can be all integrated together within a thin strand of fiber. This single fiber can be useful for fundamental multi-mechanistic studies for biological applications and the weaved fibers can be further applied for daily health monitoring as functional textiles.

A High-Performance Electrode Based on van der Waals Heterostructure for Neural Recording

Neural electrodes have been widely used to monitor neurological disorders and have a major impact on neuroscience, whereas traditional electrodes are limited to their inherent high impedance, which makes them insensitive to weak signals during recording neural signals. Herein, we developed a neural electrode based on the graphene/Ag van der Waals heterostructure for improving the detection sensitivity and signal-to-noise ratio (SNR). The impedance of the graphene/Ag electrode is reduced to 161.4 ± 13.4 MΩ μm², while the cathode charge-storage capacity (CSCc) reaches 24.2 ± 1.9 mC cm^-2, which is 6.3 and 48.4 times higher than those of the commercial Ag electrodes, respectively. Density functional theory (DFT) results find that the Ag-graphene interface has more doped electronic states, providing faster electron transfer and enhanced interfacial transport. In vivo detection sensitivity and SNR of graphene/Ag electrodes are significantly improved. The current work provides a feasible solution for designing brain electrodes to monitor neural signals more sensitively and accurately.

Research Progress on the Flexibility of an Implantable Neural Microelectrode

Neural microelectrode is the important bridge of information exchange between the human body and machines. By recording and transmitting nerve signals with electrodes, people can control the external machines. At the same time, using electrodes to electrically stimulate nerve tissue, people with long-term brain diseases will be safely and reliably treated. Young's modulus of the traditional rigid electrode probe is not matched well with that of biological tissue, and tissue immune rejection is easy to generate, resulting in the electrode not being able to achieve long-term safety and reliable working. In recent years, the choice of flexible materials and design of electrode structures can achieve modulus matching between electrode and biological tissue, and tissue damage is decreased. This review discusses nerve microelectrodes based on flexible electrode materials and substrate materials. Simultaneously, different structural designs of neural microelectrodes are reviewed. However, flexible electrode probes are difficult to implant into the brain. Only with the aid of certain auxiliary devices, can the implant be safe and reliable. The implantation method of the nerve microelectrode is also reviewed.

Customizing MRI-Compatible Multifunctional Neural Interfaces through Fiber Drawing

Fiber drawing enables scalable fabrication of multifunctional flexible fibers that integrate electrical, optical and microfluidic modalities to record and modulate neural activity. Constraints on thermomechanical properties of materials, however, have prevented integrated drawing of metal electrodes with low-loss polymer waveguides for concurrent electrical recording and optical neuromodulation. Here we introduce two fabrication approaches: (1) an iterative thermal drawing with a soft, low melting temperature (Tm) metal indium, and (2) a metal convergence drawing with traditionally non-drawable high Tm metal tungsten. Both approaches deliver multifunctional flexible neural interfaces with low-impedance metallic electrodes and low-loss waveguides, capable of recording optically-evoked and spontaneous neural activity in mice over several weeks. We couple these fibers with a light-weight mechanical microdrive (1g) that enables depth-specific interrogation of neural circuits in mice following chronic implantation. Finally, we demonstrate the compatibility of these fibers with magnetic resonance imaging (MRI) and apply them to visualize the delivery of chemical payloads through the integrated channels in real time. Together, these advances expand the domains of application of the fiber-based neural probes in neuroscience and neuroengineering.

All-in-Fiber Electrochemical Sensing

Electrochemical sensors have found a wide range of applications in analytical chemistry thanks to the advent of high-throughput printing technologies. However, these techniques are usually limited to two-dimensional (2D) geometry with relatively large minimal feature sizes. Here, we report on the scalable fabrication of monolithically integrated electrochemical devices with novel and customizable fiber-based architectures. The multimaterial thermal drawing technique is employed to co-process polymer composites and metallic glass into uniform electroactive and pseudoreference electrodes embedded in an insulating polymer cladding fiber. To demonstrate the versatility of the process, we tailor the fiber microstructure to two configurations: a small-footprint fiber tip sensor and a high-surface-area capillary cell. We demonstrate the performance of our devices using cyclic voltammetry and chronoamperometry for the direct detection and quantification of paracetamol, a common anesthetic drug. Finally, we showcase a fully portable pipet-based analyzer using low-power electronics and an "electrochemical pipet tip" for direct sampling and analysis of microliter-range volumes. Our approach paves the way toward novel materials and architectures for efficient electrochemical sensing to be deployed in existing and novel personal care and surgical configurations.

Intracortical Microelectrode Array Unit Yield under Chronic Conditions: A Comparative Evaluation

While microelectrode arrays (MEAs) offer the promise of elucidating functional neural circuitry and serve as the basis for a cortical neuroprosthesis, the challenge of designing and demonstrating chronically reliable technology remains. Numerous studies report “chronic” data but the actual time spans and performance measures corresponding to the experimental work vary. In this study, we reviewed the experimental durations that constitute chronic studies across a range of MEA types and animal species to gain an understanding of the widespread variability in reported study duration. For rodents, which are the most commonly used animal model in chronic studies, we examined active electrode yield (AEY) for different array types as a means to contextualize the study duration variance, as well as investigate and interpret the performance of custom devices in comparison to conventional MEAs. We observed wide-spread variance within species for the chronic implantation period and an AEY that decayed linearly in rodent models that implanted commercially-available devices. These observations provide a benchmark for comparing the performance of new technologies and highlight the need for consistency in chronic MEA studies. Additionally, to fully derive performance under chronic conditions, the duration of abiotic failure modes, biological processes induced by indwelling probes, and intended application of the device are key determinants.

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.

A comprehensive review summarizing the recent biomedical applications of functionalized carbon nanofibers

Since the discovery and fabrication of carbon nanofibers (CNFs) over a decade ago, scientists foster to discover novel myriad potential applications for this material in both biomedicine and industry. The unique economic viability, mechanical, electrical, optical, thermal, and structural properties of CNFs led to their rapid emergence. CNFs become an artificial intelligence platform for different uses, including a wide range of biomedical applications. Furthermore, CNFs have exceptionally large surface areas that make them flexible for tailoring and functionalization on demand. This review highlights the recent progress and achievements of CNFs in a wide range of biomedical fields, including cancer therapy, biosensing, tissue engineering, and wound dressing. Besides the synthetic techniques of CNFs, their potential toxicity and limitations, as biomaterials in real clinical settings, will be presented. This review discusses CNF's future investigations in other biomedical fields, including gene delivery and bioimaging and CNFs risk assessment.

Self-powered multifunctional sensing based on super-elastic fibers by soluble-core thermal drawing

The well-developed preform-to-fiber thermal drawing technique owns the benefit to maintain the cross-section architecture and obtain an individual micro-scale strand of fiber with the extended length up to thousand meters. In this work, we propose and demonstrate a two-step soluble-core fabrication method by combining such an inherently scalable manufacturing method with simple post-draw processing to explore the low viscosity polymer fibers and the potential of soft fiber electronics. As a result, an ultra-stretchable conductive fiber is achieved, which maintains excellent conductivity even under 1900% strain or 1.5 kg load/impact freefalling from 0.8-m height. Moreover, by combining with triboelectric nanogenerator technique, this fiber acts as a self-powered self-adapting multi-dimensional sensor attached on sports gears to monitor sports performance while bearing sudden impacts. Next, owing to its remarkable waterproof and easy packaging properties, this fiber detector can sense different ion movements in various solutions, revealing the promising applications for large-area undersea detection.

Nano-optoelectrodes Integrated with Flexible Multifunctional Fiber Probes by High-Throughput Scalable Fabrication

Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical-electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

Miniature multiplexed label-free pH probe in vivo

Correlating in-brain pH fluctuations with the pathophysiology has been impeded by the lack of in vivo techniques to precisely determine local pH changes. Here, we developed an all-in-one pH probe for spatially-resolved and label-free pH sensing in vivo, based on a field-effect pH sensor, i.e., a light-addressable potentiometric sensor (LAPS), coupled to a flexible multimodal fiber. A readout photocurrent from the LAPS, elicited from a modulated light source, registers the localized surface potential change, proportional to the pH change. Upon simultaneous illuminations at multi-spot by a plurality of light sources with different modulation frequencies, pH changes at multiple designated spots are obtained via demultiplexing this photocurrent. To enable its in vivo applications, we combined the LAPS with a multimodal fiber fabricated by the convergence thermal drawing. Such fiber seamlessly integrates a multicore optical waveguide in the center for the light delivery, surrounded by electrodes for leading out photocurrent and serving as a pseudo-reference electrode, respectively. Such hybrid all-in-one pH probes can measure pH changes at 14 pixels simultaneously with a spatial resolution of 250 μm and a temporal resolution of 30 Hz. The pH sensitivity was characterized as 57.5 ± 2.2 mV/pH homogeneously across all measurable pixels. Such probes have been implanted into the hippocampal formation of rats and their capabilities to capture pH changes at multiple pixels were evaluated at both physiological and pathological conditions. Technologies developed here represents a new class of in vivo chemical sensing technologies enabling the spatially-resolved investigation of intrinsic chemical signals in deep brain structures with high spatial and temporal resolutions.

Micelle-enabled self-assembly of porous and monolithic carbon membranes for bioelectronic interfaces

Real-world bioelectronics applications, including drug delivery systems, biosensing and electrical modulation of tissues and organs, largely require biointerfaces at the macroscopic level. However, traditional macroscale bioelectronic electrodes usually exhibit invasive or power-inefficient architectures, inability to form uniform and subcellular interfaces, or faradaic reactions at electrode surfaces. Here, we develop a micelle-enabled self-assembly approach for a binder-free and carbon-based monolithic device, aimed at large-scale bioelectronic interfaces. The device incorporates a multi-scale porous material architecture, an interdigitated microelectrode layout and a supercapacitor-like performance. In cell training processes, we use the device to modulate the contraction rate of primary cardiomyocytes at the subcellular level to target frequency in vitro. We also achieve capacitive control of the electrophysiology in isolated hearts, retinal tissues and sciatic nerves, as well as bioelectronic cardiac sensing. Our results support the exploration of device platforms already used in energy research to identify new opportunities in bioelectronics.

Selectively Micro-Patternable Fibers via In-Fiber Photolithography

Multimaterial fibers engineered to integrate glasses, metals, semiconductors, and composites found applications in ubiquitous sensing, biomedicine, and robotics. The longitudinal symmetry typical of fibers, however, limits the density of functional interfaces with fiber-based devices. Here, thermal drawing and photolithography are combined to produce a scalable method for deterministically breaking axial symmetry within multimaterial fibers. Our approach harnesses a two-step polymerization in thiol-epoxy and thiol-ene photopolymer networks to create a photoresist compatible with high-throughput thermal drawing in atmospheric conditions. This, in turn, delivers meters of fiber that can be patterned along the length increasing the density of functional points. This approach may advance applications of fiber-based devices in distributed sensors, large area optoelectronic devices, and smart textiles.

Spatially expandable fiber-based probes as a multifunctional deep brain interface

Understanding the cytoarchitecture and wiring of the brain requires improved methods to record and stimulate large groups of neurons with cellular specificity. This requires miniaturized neural interfaces that integrate into brain tissue without altering its properties. Existing neural interface technologies have been shown to provide high-resolution electrophysiological recording with high signal-to-noise ratio. However, with single implantation, the physical properties of these devices limit their access to one, small brain region. To overcome this limitation, we developed a platform that provides three-dimensional coverage of brain tissue through multisite multifunctional fiber-based neural probes guided in a helical scaffold. Chronic recordings from the spatially expandable fiber probes demonstrate the ability of these fiber probes capturing brain activities with a single-unit resolution for long observation times. Furthermore, using Thy1-ChR2-YFP mice we demonstrate the application of our probes in simultaneous recording and optical/chemical modulation of brain activities across distant regions. Similarly, varying electrographic brain activities from different brain regions were detected by our customizable probes in a mouse model of epilepsy, suggesting the potential of using these probes for the investigation of brain disorders such as epilepsy. Ultimately, this technique enables three-dimensional manipulation and mapping of brain activities across distant regions in the deep brain with minimal tissue damage, which can bring new insights for deciphering complex brain functions and dynamics in the near future.

Existing neural interfaces are limited in accessing one, small brain region. Here, the authors introduce a scaffold with helix hollow channels, which direct multisite multifunctional fibre probes into the brain at different angles, allowing for simultaneous recording and stimulation across distant regions.

Structured nanoscale metallic glass fibres with extreme aspect ratios

Micro- and nanoscale metallic glasses offer exciting opportunities for both fundamental research and applications in healthcare, micro-engineering, optics and electronics. The scientific and technological challenges associated with the fabrication and utilization of nanoscale metallic glasses, however, remain unresolved. Here, we present a simple and scalable approach for the fabrication of metallic glass fibres with nanoscale architectures based on their thermal co-drawing within a polymer matrix with matched rheological properties. Our method yields well-ordered and uniform metallic glasses with controllable feature sizes down to a few tens of nanometres, and aspect ratios greater than 10¹⁰. We combine fluid dynamics and advanced in situ transmission electron microscopy analysis to elucidate the interplay between fluid instability and crystallization kinetics that determines the achievable feature sizes. Our approach yields complex fibre architectures that, combined with other functional materials, enable new advanced all-in-fibre devices. We demonstrate in particular an implantable metallic glass-based fibre probe tested in vivo for a stable brain-machine interface that paves the way towards innovative high-performance and multifunctional neuro-probes.

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.

Enhanced nerve cell proliferation and differentiation on electrically conductive scaffolds embedded with graphene and carbon nanotubes

Conduits that promote nerve regeneration are currently of great medical concern, particularly when gaps exist between nerve endings. To address this issue, our laboratory previously developed a nerve conduit from biodegradable poly(caprolactone fumarate) (PCLF) that supports peripheral nerve regeneration. The present study improves upon this work by further developing an electrically conductive, positively charged PCLF scaffold through the incorporation of graphene, carbon nanotubes (CNTs), and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MTAC) (PCLF-Graphene-CNT-MTAC) using ultraviolet (UV) induced photocrosslinking. Scanning electron microscopy, transmission electron microscopy, and atomic force microscopy were used to assess the incorporation of CNTs and graphene into PCLF-Graphene-CNT-MTAC scaffolds, which displayed enhanced surface roughness and reduced electrochemical impedance when compared to neat PCLF. Scaffolds with these surface modifications also showed improved growth and differentiation of rat pheochromocytoma 12 cells in vitro, with enhanced cell growth, neurite extension, and cellular migration. Furthermore, an increased number of neurite protrusions were observed when the conduit was electrically stimulated. These results show that the electrically conductive PCLF-Graphene-CNT-MTAC nerve scaffolds presented here support the cellular behaviors that are critical for nerve regeneration, ultimately making this material an attractive candidate for regenerative medicine applications.

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.

A Route Toward Smart System Integration: From Fiber Design to Device Construction

Fiber is a symbol of human civilization, being ubiquitous but obscure in society over most of history. Fiber has been revived upon the advent of fiber-based electronic devices in the past two decades. This is due to its desirable lightweight, flexible, and conformable characteristics, which enable it to play a fundamental role in the electronic and information era. Numerous fiber-based electronic devices have sprung up in energy conversion, energy storage, sensing, actuation, etc. A possibility is thereby conceived that they can be integrated into smart systems compatible with the human body, consisting of biotic fiber-based organs and tissues, which possess similar but more advanced functions. However, the design of mono-/multifibers, the construction of fiber-based devices, and the integration of these smart systems represent great challenges in fundamental understanding and practical implementation. A systematic review of the current state of the art with respect to the design and fabrication of electronic fiber materials, construction of fiber-based devices, and integration of smart systems is presented. In addition, limitations of current fiber-based devices and perspectives are explored toward potential and promising smart integration.

A Review: Electrode and Packaging Materials for Neurophysiology Recording Implants

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

Multimaterial and multifunctional neural interfaces: from surface-type and implantable electrodes to fiber-based devices

Development of neural interfaces from surface electrodes to fibers with various type, functionality, and materials.

Scalable Fabrication of Highly Flexible Porous Polymer-Based Capacitive Humidity Sensor Using Convergence Fiber Drawing

In this study, we fabricated a highly flexible fiber-based capacitive humidity sensor using a scalable convergence fiber drawing approach. The sensor’s sensing layer is made of porous polyetherimide (PEI) with its porosity produced in situ during fiber drawing, whereas its electrodes are made of copper wires. The porosity induces capillary condensation starting at a low relative humidity (RH) level (here, 70%), resulting in a significant increase in the response of the sensor at RH levels ranging from 70% to 80%. The proposed humidity sensor shows a good sensitivity of 0.39 pF/% RH in the range of 70%–80% RH, a maximum hysteresis of 9.08% RH at 70% RH, a small temperature dependence, and a good stability over a 48 h period. This work demonstrates the first fiber-based humidity sensor fabricated using convergence fiber drawing.

In-fibre particle manipulation and device assembly via laser induced thermocapillary convection

The ability to manipulate in-fibre particles is of technological and scientific significance, yet particle manipulation inside solid environment remains fundamentally challenging. Here we show an accurately controlled, non-contact, size- and material-independent method for manipulating in-fibre particles based on laser-induced thermocapillary convection. The laser liquefaction process transforms the fibre from a solid media into an ideal fluid environment and triggers the in-fibre thermocapillary convection. In-fibre particles, with diameter from submicron to hundreds of microns, can be migrated toward the designated position. The number of particles being migrated, the particle migration velocity and direction can be precisely controlled. As a proof-of-concept, the laser-induced flow currents lead to the migration-to-contact of dislocated in-fibre p- and n-type semiconductor particles and the forming of dual-particle p-n homo- and heterojunction directly in a fibre. This approach not only enables in-fibre device assembly to achieve multi-component fibre devices, but also provide fundamental insight for in-solid particle manipulation.

Particle manipulation inside a fibre is challenging yet meaningful to achieve for fibre devices. Here, the authors demonstrate a method to manipulate in-fibre particles via laser induced thermocapillary convection, enabling the formation of semiconductor junctions directly in a fibre.

The Rise of Fiber Electronics

As a new direction in applied chemistry, fiber electronics allow device configuration to evolve from three to two dimensions and then to one dimension. The reduction in dimension brings unique properties, such as ultraflexibility, tissue adaptability, and weavability, enabling their use in a variety of applications, particularly in various emerging fields related to implantable devices and wearable systems. The different types of fiber electrode materials are summarized based on the one-dimensional configuration and their distinctive interfaces, various devices, and promising applications. The remaining challenges and future directions are finally highlighted.

Self-Powered Direct Muscle Stimulation Using a Triboelectric Nanogenerator (TENG) Integrated with a Flexible Multiple-Channel Intramuscular Electrode

Muscle function loss can result from multiple nervous system diseases including spinal cord injury (SCI), stroke, and multiple sclerosis (MS). Electrical muscle stimulation is clinically employed for rehabilitative and therapeutic purpose and typically requires mA-level stimulation current. Here, we report electrical muscle stimulation, which is directly powered by a stacked-layer triboelectric nanogenerator (TENG) through a flexible multiple-channel intramuscular electrode. This multiple-channel intramuscular electrode allows mapping of motoneurons that is sparsely distributed in the muscle tissue and thus enables high efficiency TENG muscle stimulation, although the short-circuit current of the TENG is only 35 μA. With a stimulation efficiency matrix, we find the electrical muscle stimulation efficiency is affected by two factors, namely, the electrode-motoneuron position, and the stimulation waveform polarity. To test whether it is a universal phenomenon for electrical stimulation, we then further investigate with the conventional square wave current stimulation and confirm that the stimulation efficiency is also affected by these two factors. Thus, we develop a self-powered direct muscle stimulation system with a TENG as power source and waveform generator, and a multiple-channel intramuscular electrode to allow motoneuron mapping for stimulation efficiency optimization. We believe such self-powered system could be potentially used for rehabilitative and therapeutic purpose to treat muscle function loss.

Flexible fiber-based optoelectronics for neural interfaces

Neurological and psychiatric conditions pose an increasing socioeconomic burden on our aging society. Our ability to understand and treat these conditions relies on the development of reliable tools to study the dynamics of the underlying neural circuits. Despite significant progress in approaches and devices to sense and modulate neural activity, further refinement is required on the spatiotemporal resolution, cell-type selectivity, and long-term stability of neural interfaces. Guided by the principles of neural transduction and by the materials properties of the neural tissue, recent advances in neural interrogation approaches rely on flexible and multifunctional devices. Among these approaches, multimaterial fibers have emerged as integrated tools for sensing and delivering of multiple signals to and from the neural tissue. Fiber-based neural probes are produced by thermal drawing process, which is the manufacturing approach used in optical fiber fabrication. This technology allows straightforward incorporation of multiple functional components into microstructured fibers at the level of their macroscale models, preforms, with a wide range of geometries. Here we will introduce the multimaterial fiber technology, its applications in engineering fields, and its adoption for the design of multifunctional and flexible neural interfaces. We will discuss examples of fiber-based neural probes tailored to the electrophysiological recording, optical neuromodulation, and delivery of drugs and genes into the rodent brain and spinal cord, as well as their emerging use for studies of nerve growth and repair.

Advances in Nano Neuroscience: From Nanomaterials to Nanotools

During the last decades, neuroscientists have increasingly exploited a variety of artificial, de-novo synthesized materials with controlled nano-sized features. For instance, a renewed interest in the development of prostheses or neural interfaces was driven by the availability of novel nanomaterials that enabled the fabrication of implantable bioelectronics interfaces with reduced side effects and increased integration with the target biological tissue. The peculiar physical-chemical properties of nanomaterials have also contributed to the engineering of novel imaging devices toward sophisticated experimental settings, to smart fabricated scaffolds and microelectrodes, or other tools ultimately aimed at a better understanding of neural tissue functions. In this review, we focus on nanomaterials and specifically on carbon-based nanomaterials, such as carbon nanotubes (CNTs) and graphene. While these materials raise potential safety concerns, they represent a tremendous technological opportunity for the restoration of neuronal functions. We then describe nanotools such as nanowires and nano-modified MEA for high-performance electrophysiological recording and stimulation of neuronal electrical activity. We finally focus on the fabrication of three-dimensional synthetic nanostructures, used as substrates to interface biological cells and tissues in vitro and in vivo.

Advanced Multimaterial Electronic and Optoelectronic Fibers and Textiles

The ability to integrate complex electronic and optoelectronic functionalities within soft and thin fibers is one of today's key advanced manufacturing challenges. Multifunctional and connected fiber devices will be at the heart of the development of smart textiles and wearable devices. These devices also offer novel opportunities for surgical probes and tools, robotics and prostheses, communication systems, and portable energy harvesters. Among the various fiber-processing methods, the preform-to-fiber thermal drawing technique is a very promising process that is used to fabricate multimaterial fibers with complex architectures at micro- and nanoscale feature sizes. Recently, a series of scientific and technological breakthroughs have significantly advanced the field of multimaterial fibers, allowing a wider range of functionalities, better performance, and novel applications. Here, these breakthroughs, in the fundamental understanding of the fluid dynamics, rheology, and tailoring of materials microstructures at play in the thermal drawing process, are presented and critically discussed. The impact of these advances on the research landscape in this field and how they offer significant new opportunities for this rapidly growing scientific and technological platform are also discussed.

Clinical Applications of Carbon Nanomaterials in Diagnostics and Therapy

Nanomaterials have the potential to improve how patients are clinically treated and diagnosed. While there are a number of nanomaterials that can be used toward improved drug delivery and imaging, how these nanomaterials confer an advantage over other nanomaterials, as well as current clinical approaches is often application or disease specific. How the unique properties of carbon nanomaterials, such as nanodiamonds, carbon nanotubes, carbon nanofibers, graphene, and graphene oxides, make them promising nanomaterials for a wide range of clinical applications are discussed herein, including treating chemoresistant cancer, enhancing magnetic resonance imaging, and improving tissue regeneration and stem cell banking, among others. Additionally, the strategies for further improving drug delivery and imaging by carbon nanomaterials are reviewed, such as inducing endothelial leakiness as well as applying artificial intelligence toward designing optimal nanoparticle-based drug combination delivery. While the clinical application of carbon nanomaterials is still an emerging field of research, there is substantial preclinical evidence of the translational potential of carbon nanomaterials. Early clinically trial studies are highlighted, further supporting the use of carbon nanomaterials in clinical applications for both drug delivery and imaging.

Multifunctional Fibers as Tools for Neuroscience and Neuroengineering

Multifunctional devices for modulation and probing of neuronal activity during free behavior facilitate studies of functions and pathologies of the nervous system. Probes composed of stiff materials, such as metals and semiconductors, exhibit elastic and chemical mismatch with the neural tissue, which is hypothesized to contribute to sustained tissue damage and gliosis. Dense glial scars have been found to encapsulate implanted devices, corrode their surfaces, and often yield poor recording quality in long-term experiments. Motivated by the hypothesis that reducing the mechanical stiffness of implanted probes may improve their long-term reliability, a variety of probes based on soft materials have been developed. In addition to enabling electrical neural recording, these probes have been engineered to take advantage of genetic tools for optical neuromodulation. With the emergence of optogenetics, it became possible to optically excite or inhibit genetically identifiable cell types via expression of light-sensitive opsins. Optogenetics experiments often demand implantable multifunctional devices to optically stimulate, deliver viral vectors and drugs, and simultaneously record electrophysiological signals from the specified cells within the nervous system. Recent advances in microcontact printing and microfabrication techniques have equipped flexible probes with microscale light-emitting diodes (μLEDs), waveguides, and microfluidic channels. Complementary to these approaches, fiber drawing has emerged as a scalable route to integration of multiple functional features within miniature and flexible neural probes. The thermal drawing process relies on the fabrication of macroscale models containing the materials of interest, which are then drawn into microstructured fibers with predefined cross-sectional geometries. We have recently applied this approach to produce fibers integrating conductive electrodes for extracellular recording of single- and multineuron potentials, low-loss optical waveguides for optogenetic neuromodulation, and microfluidic channels for drug and viral vector delivery. These devices allowed dynamic investigation of the time course of opsin expression across multiple brain regions and enabled pairing of optical stimulation with local pharmacological intervention in behaving animals. Neural probes designed to interface with the spinal cord, a viscoelastic tissue undergoing repeated strain during normal movement, rely on the integration of soft and flexible materials to avoid injury and device failure. Employing soft substrates, such as parylene C and poly-(dimethylsiloxane), for electrode and μLED arrays permitted stimulation and recording of neural activity on the surface of the spinal cord. Similarly, thermally drawn flexible and stretchable optoelectronic fibers that resemble the fibrous structure of the spinal cord were implanted without any significant inflammatory reaction in the vicinity of the probes. These fibers enabled simultaneous recording and optogenetic stimulation of neural activity in the spinal cord. In this Account, we review the applications of multifunctional fibers and other integrated devices for optoelectronic probing of neural circuits and discuss engineering directions that may facilitate future studies of nerve repair and accelerate the development of bioelectronic medical devices.