Adaptive and multifunctional hydrogel hybrid probes for long-term sensing and modulation of neural activity

Biomaterials and tissue engineering in traumatic brain injury: novel perspectives on promoting neural regeneration

Traumatic brain injury is a serious medical condition that can be attributed to falls, motor vehicle accidents, sports injuries and acts of violence, causing a series of neural injuries and neuropsychiatric symptoms. However, limited accessibility to the injury sites, complicated histological and anatomical structure, intricate cellular and extracellular milieu, lack of regenerative capacity in the native cells, vast variety of damage routes, and the insufficient time available for treatment have restricted the widespread application of several therapeutic methods in cases of central nervous system injury. Tissue engineering and regenerative medicine have emerged as innovative approaches in the field of nerve regeneration. By combining biomaterials, stem cells, and growth factors, these approaches have provided a platform for developing effective treatments for neural injuries, which can offer the potential to restore neural function, improve patient outcomes, and reduce the need for drugs and invasive surgical procedures. Biomaterials have shown advantages in promoting neural development, inhibiting glial scar formation, and providing a suitable biomimetic neural microenvironment, which makes their application promising in the field of neural regeneration. For instance, bioactive scaffolds loaded with stem cells can provide a biocompatible and biodegradable milieu. Furthermore, stem cells-derived exosomes combine the advantages of stem cells, avoid the risk of immune rejection, cooperate with biomaterials to enhance their biological functions, and exert stable functions, thereby inducing angiogenesis and neural regeneration in patients with traumatic brain injury and promoting the recovery of brain function. Unfortunately, biomaterials have shown positive effects in the laboratory, but when similar materials are used in clinical studies of human central nervous system regeneration, their efficacy is unsatisfactory. Here, we review the characteristics and properties of various bioactive materials, followed by the introduction of applications based on biochemistry and cell molecules, and discuss the emerging role of biomaterials in promoting neural regeneration. Further, we summarize the adaptive biomaterials infused with exosomes produced from stem cells and stem cells themselves for the treatment of traumatic brain injury. Finally, we present the main limitations of biomaterials for the treatment of traumatic brain injury and offer insights into their future potential.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

Characterization of a conductive hydrogel@Carbon fibers electrode as a novel intraneural interface

Peripheral neural interfaces facilitate bidirectional communication between the nervous system and external devices, enabling precise control for prosthetic limbs, sensory feedback systems, and therapeutic interventions in the field of Bioelectronic Medicine. Intraneural interfaces hold great promise since they ensure high selectivity in communicating only with the desired nerve fascicles. Despite significant advancements, challenges such as chronic immune response, signal degradation over time, and lack of long-term biocompatibility remain critical considerations in the development of such devices. Here we report on the development and benchtop characterization of a novel design of an intraneural interface based on carbon fiber bundles. Carbon fibers possess low impedance, enabling enhanced signal detection and stimulation efficacy compared to traditional metal electrodes. We provided a 3D-stabilizing structure for the carbon fiber bundles made of PEDOT:PSS hydrogel, to enhance the biocompatibility between the carbon fibers and the nervous tissue. We further coated the overall bundles with a thin layer of elastomeric material to provide electrical insulation. Taken together, our results demonstrated that our electrode possesses adequate structural and electrochemical properties to ensure proper stimulation and recording of peripheral nerve fibers and a biocompatible interface with the nervous tissue.

Manipulating immune activity of macrophages: a materials and mechanics perspective

Macrophage immune cells exist on a plastic spectrum of phenotypes governed by their physical and biochemical environment. Controlling macrophage function to facilitate immunological regeneration or fighting pathology has emerged as a therapeutic possibility. The rate-limiting step in translating macrophage immunomodulation therapies has been the absence of fundamental knowledge of how physics and biochemistry in the macrophage microenvironment converge to inform phenotype. In this review we explore recent trends in bioengineered model systems that integrate physical and biochemical variables applied to macrophage mechanosensing and plasticity. We focus on how tuning of mechanical forces and biomaterial composition orchestrate macrophage function in physiological and pathological contexts. Ultimately, a broader understanding of stimuli-responsiveness in macrophages leads to informed design for future modulatory therapies.

Softening implantable bioelectronics: Material designs, applications, and future directions

Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.

Novel Biomaterials for Wound Healing and Tissue Regeneration

Skin is the first defense barrier of the human body, which can resist the invasion of external dust, microorganisms and other pollutants, and ensure that the human body maintains the homeostasis of the internal environment. Once the skin is damaged, the health threat to the human body will increase. Wound repair and the human internal environment are a dynamic process. How to effectively accelerate the healing of wounds without affecting the internal environment of the human body and guarantee that the repaired tissue retains its original function as much as possible has become a research hotspot. With the advancement of technology, researchers have combined new technologies to develop and prepare various types of materials for wound healing. This article will introduce the wound repair materials developed and prepared in recent years from three types: nanofibers, composite hydrogels, and other new materials. The paper aims to provide reference for researchers in related fields to develop and prepare multifunctional materials. This may be helpful to design more ideal materials for clinical application, and then achieve better wound healing and regeneration effects.

Cracking modes and force dynamics in the insertion of neural probes into hydrogel brain phantom

Objective. The insertion of penetrating neural probes into the brain is crucial for advancing neuroscience, yet it involves various inherent risks. Prototype probes are typically inserted into hydrogel-based brain phantoms and the mechanical responses are analyzed in order to inform the insertion mechanics duringin vivoimplantation. However, the underlying mechanism of the insertion dynamics of neural probes in hydrogel brain phantoms, particularly the phenomenon of cracking, remains insufficiently understood. This knowledge gap leads to misinterpretations and discrepancies when comparing results obtained from phantom studies to those observed under thein vivoconditions. This study aims to elucidate the impact of probe sharpness and dimensions on the cracking mechanisms and insertion dynamics characterized during the insertion of probes in hydrogel phantoms.Approach. The insertion of dummy probes with different shank shapes defined by the tip angle, width, and thickness is systematically studied. The insertion-induced cracks in the transparent hydrogel were accentuated by an immiscible dye, tracked byin situimaging, and the corresponding insertion force was recorded. Three-dimensional finite element analysis models were developed to obtain the contact stress between the probe tip and the phantom.Main results. The findings reveal a dual pattern: for sharp, slender probes, the insertion forces remain consistently low during the insertion process, owing to continuously propagating straight cracks that align with the insertion direction. In contrast, blunt, thick probes induce large forces that increase rapidly with escalating insertion depth, mainly due to the formation of branched crack with a conical cracking surface, and the subsequent internal compression. This interpretation challenges the traditional understanding that neglects the difference in the cracking modes and regards increased frictional force as the sole factor contributing to higher insertion forces. The critical probe sharpness factors separating straight and branched cracking is identified experimentally, and a preliminary explanation of the transition between the two cracking modes is derived from three-dimensional finite element analysis.Significance. This study presents, for the first time, the mechanism underlying two distinct cracking modes during the insertion of neural probes into hydrogel brain phantoms. The correlations between the cracking modes and the insertion force dynamics, as well as the effects of the probe sharpness were established, offering insights into the design of neural probes via phantom studies and informing future investigations into cracking phenomena in brain tissue during probe implantations.

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.

In Situ Structural Densification of Hydrogel Network and Its Interface with Electrodes for High-Performance Multimodal Artificial Skin

The multisensory responsiveness of hydrogels positions them as promising candidates for artificial skin, whereas the mismatch of modulus between soft hydrogels and hard electrodes as well as the poor adhesion and conductance at the interface greatly impairs the stability of electronics devices. Herein, we propose an in situ postprocessing approach utilizing electrochemical reactions between metals (Zn, etc.) and hydrogels to synergistically achieve strong adhesion of the hydrogel-electrode interface, low interfacial impedance, and local strain isolation due to the structural densification of the hydrogel network. The mechanism is that Zn electrochemically oxidizes to Zn2+ and injects into the hydrogel, gradually forming a mechanically interlocked structure, Zn2+-polymer dual-helix structural nodes, and a high-modulus ZnO from the surface to the interior. Compared to untreated samples, the treated sample displays 8.7 times increased interfacial adhesion energy between the hydrogel and electrode (87 J/m2), 95% decreased interfacial impedance (218.8 Ω), and a high-strain isolation efficiency (εtotal/εisolation > 400). Akin to human skin, the prepared sensor demonstrates multimodal sensing capabilities, encompassing highly sensitive strain perception and simultaneous perception of temperature, humidity, and oxygen content unaffected by strain interference. This easy on-chip preparation of hydrogel-based multimodal sensor array shows great potential for health and environment monitoring as artificial skin.

High-performing fiber electrodes based on a gold-shelled silver nanowire framework for bioelectronics

Flexible fiber electrodes offer new opportunities for bioelectronics and are reliable in vivo applications, high flexibility, high electrical conductivity, and satisfactory biocompatibility are typically required. Herein, we present an all-metal flexible and biocompatible fiber electrode based on a metal nanowire hybrid strategy, i.e., silver nanowires were assembled on a freestanding framework, and further to render them inert, they were plated with a gold nanoshell. Our fiber electrodes exhibited a low modulus of ∼75 MPa and electrical conductivity up to ∼4.8 × 106 S m-1. They can resist chemical erosion with negligible leakage of biotoxic silver ions in the physiological environment, thus ensuring satisfactory biocompatibility. Finally, we demonstrated the hybrid fiber as a neural electrode that stimulated the sciatic nerve of a mouse, proving its potential for applications in bioelectronics.

Needle-Like Multifunctional Biphasic Microfiber for Minimally Invasive Implantable Bioelectronics

Implantable bioelectronics has attracted significant attention in electroceuticals and clinical medicine for precise diagnosis and efficient treatment of target diseases. However, conventional rigid implantable devices face challenges such as poor tissue-device interface and unavoidable tissue damage during surgical implantation. Despite continuous efforts to utilize various soft materials to address such issues, their practical applications remain limited. Here, a needle-like stretchable microfiber composed of a phase-convertible liquid metal (LM) core and a multifunctional nanocomposite shell for minimally invasive soft bioelectronics is reported. The sharp tapered microfiber can be stiffened by freezing akin to a conventional needle to penetrate soft tissue with minimal incision. Once implanted in vivo where the LM melts, unlike conventional stiff needles, it regains soft mechanical properties, which facilitate a seamless tissue-device interface. The nanocomposite incorporating with functional nanomaterials exhibits both low impedance and the ability to detect physiological pH, providing biosensing and stimulation capabilities. The fluidic LM embedded in the nanocomposite shell enables high stretchability and strain-insensitive electrical properties. This multifunctional biphasic microfiber conforms to the surfaces of the stomach, muscle, and heart, offering a promising approach for electrophysiological recording, pH sensing, electrical stimulation, and radiofrequency ablation in vivo.

Multifunctional and Flexible Neural Probe with Thermally Drawn Fibers for Bidirectional Synaptic Probing in the Brain

Synapses in the brain utilize two distinct communication mechanisms: chemical and electrical. For a comprehensive investigation of neural circuitry, neural interfaces should be capable of both monitoring and stimulating these types of physiological interactions. However, previously developed interfaces for neurotransmitter monitoring have been limited in interaction modality due to constraints in device size, fabrication techniques, and the usage of flexible materials. To address this obstacle, we propose a multifunctional and flexible fiber probe fabricated through the microwire codrawing thermal drawing process, which enables the high-density integration of functional components with various materials such as polymers, metals, and carbon fibers. The fiber enables real-time monitoring of transient dopamine release in vivo, real-time stimulation of cell-specific neuronal populations via optogenetic stimulation, single-unit electrophysiology of individual neurons localized to the tip of the neural probe, and chemical stimulation via drug delivery. This fiber will improve the accessibility and functionality of bidirectional interrogation of neurochemical mechanisms in implantable neural probes.

Swellable microneedle-coupled light-addressable photoelectrochemical sensor for in-situ tracking of multiple pesticides pollution in vivo

In vivo monitoring of multiple pesticide contamination is of great significance for evaluating the health risks of different pesticides, agricultural production safety, and ecological and environmental assessment. Here, we report a hydrogel microneedle array coupled light-addressable photoelectrochemical sensor for tracking multiple pesticide uptake and elimination in living animals and plants, holding three prominent merits: i) enables in-situ detection of in vivo pesticides, avoiding cumbersome and complex sample transportation and handling processes; ii) allows repeated in vivo sampling of the same organism, improving tracking test controllability and accuracy; iii) avoids lethal sampling, providing a better understanding of the pesticides fate in living organisms. The coupled sensor is mechanically robust for withstanding more than 0.35 N per needle and highly swellable (800 %) for timely extraction of sufficient in vivo solution for analysis. For proof-of-concept, it achieves in-situ detection of atrazine, acetamiprid, and carbendazim efficiently and quantitatively in artificial agarose skin models, mouse skin interstitial fluids, and plant leaves with little inflammatory reaction. This simple, highly integrated, minimally invasive, and high-throughput in vivo monitoring method is ideal for future field environmental monitoring and plant and animal disease diagnosis.

Postoperative Long-Term Monitoring of Mechanical Characteristics in Reconstructed Soft Tissues Using Biocompatible, Immune-Tolerant, and Wireless Electronic Sutures

Accurate postoperative assessment of varying mechanical properties is crucial for customizing patient-specific treatments and optimizing rehabilitation strategies following Achilles tendon (AT) rupture and reconstruction surgery. This study introduces a wireless, chip-less, and immune-tolerant in vivo strain-sensing suture designed to continuously monitor mechanical stiffness variations in the reconstructed AT throughout the healing process. This innovative sensing suture integrates a standard medical suturing thread with a wireless fiber strain-sensing system, which incorporates a fiber strain sensor and a double-layered inductive coil for wireless readout. The winding design of Au nanoparticle-based fiber electrodes and a hollow core contribute to the fiber strain sensor's high sensitivity (factor of 6.2 and 15.1 pF for revised sensitivity), negligible hysteresis, and durability over 10,000 stretching cycles. To ensure biocompatibility and immune tolerance during extended in vivo periods, an antibiofouling lubricant layer was applied to the sensing suture. Using this sensing system, we successfully monitored the strain responses of the reconstructed AT in an in vivo porcine model. This facilitated the postoperative assessment of mechanical stiffness variations through a well-established analytical model during the healing period.

Implantable Neural Microelectrodes: How to Reduce Immune Response

Implantable neural microelectrodes exhibit the great ability to accurately capture the electrophysiological signals from individual neurons with exceptional submillisecond precision, holding tremendous potential for advancing brain science research, as well as offering promising avenues for neurological disease therapy. Although significant advancements have been made in the channel and density of implantable neural microelectrodes, challenges persist in extending the stable recording duration of these microelectrodes. The enduring stability of implanted electrode signals is primarily influenced by the chronic immune response triggered by the slight movement of the electrode within the neural tissue. The intensity of this immune response increases with a higher bending stiffness of the electrode. This Review thoroughly analyzes the sequential reactions evoked by implanted electrodes in the brain and highlights strategies aimed at mitigating chronic immune responses. Minimizing immune response mainly includes designing the microelectrode structure, selecting flexible materials, surface modification, and controlling drug release. The purpose of this paper is to provide valuable references and ideas for reducing the immune response of implantable neural microelectrodes and stimulate their further exploration in the field of brain science.

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.

Flexible high-density microelectrode arrays for closed-loop brain-machine interfaces: a review

Flexible high-density microelectrode arrays (HDMEAs) are emerging as a key component in closed-loop brain-machine interfaces (BMIs), providing high-resolution functionality for recording, stimulation, or both. The flexibility of these arrays provides advantages over rigid ones, such as reduced mismatch between interface and tissue, resilience to micromotion, and sustained long-term performance. This review summarizes the recent developments and applications of flexible HDMEAs in closed-loop BMI systems. It delves into the various challenges encountered in the development of ideal flexible HDMEAs for closed-loop BMI systems and highlights the latest methodologies and breakthroughs to address these challenges. These insights could be instrumental in guiding the creation of future generations of flexible HDMEAs, specifically tailored for use in closed-loop BMIs. The review thoroughly explores both the current state and prospects of these advanced arrays, emphasizing their potential in enhancing BMI technology.

Macrophage microRNA-146a is a central regulator of the foreign body response to biomaterial implants

Host recognition and immune-mediated foreign body response (FBR) to biomaterials can adversely affect the functionality of implanted materials. To identify key targets underlying the generation of FBR, here we perform analysis of microRNAs (miR) and mRNAs responses to implanted biomaterials. We found that (a) miR-146a levels inversely affect macrophage accumulation, foreign body giant cell (FBGC) formation, and fibrosis in a murine implant model; (b) macrophage-derived miR-146a is a crucial regulator of the FBR and FBGC formation, as confirmed by global and cell-specific knockout of miR-146a; (c) miR-146a modulates genes related to inflammation, fibrosis, and mechanosensing; (d) miR-146a modulates tissue stiffness near the implant during FBR; and (e) miR-146a is linked to F-actin production and cellular traction force induction, which are vital for FBGC formation. These novel findings suggest that targeting macrophage miR-146a could be a selective strategy to inhibit FBR, potentially improving the biocompatibility of biomaterials.

Hybrid Endometrial-Derived Hydrogels: Human Organoid Culture Models and In Vivo Perspectives

The endometrium plays a vital role in fertility, providing a receptive environment for embryo implantation and development. Understanding the endometrial physiology is essential for developing new strategies to improve reproductive healthcare. Human endometrial organoids (hEOs) are emerging as powerful models for translational research and personalized medicine. However, most hEOs are cultured in a 3D microenvironment that significantly differs from the human endometrium, limiting their applicability in bioengineering. This study presents a hybrid endometrial-derived hydrogel that combines the rigidity of PuraMatrix (PM) with the natural scaffold components and interactions of a porcine decellularized endometrial extracellular matrix (EndoECM) hydrogel. This hydrogel provides outstanding support for hEO culture, enhances hEO differentiation efficiency due to its biochemical similarity with the native tissue, exhibits superior in vivo stability, and demonstrates xenogeneic biocompatibility in mice over a 2-week period. Taken together, these attributes position this hybrid endometrial-derived hydrogel as a promising biomaterial for regenerative treatments in reproductive medicine.

Fully bioresorbable hybrid opto-electronic neural implant system for simultaneous electrophysiological recording and optogenetic stimulation

Bioresorbable neural implants based on emerging classes of biodegradable materials offer a promising solution to the challenges of secondary surgeries for removal of implanted devices required for existing neural implants. In this study, we introduce a fully bioresorbable flexible hybrid opto-electronic system for simultaneous electrophysiological recording and optogenetic stimulation. The flexible and soft device, composed of biodegradable materials, has a direct optical and electrical interface with the curved cerebral cortex surface while exhibiting excellent biocompatibility. Optimized to minimize light transmission losses and photoelectric artifact interference, the device was chronically implanted in the brain of transgenic mice and performed to photo-stimulate the somatosensory area while recording local field potentials. Thus, the presented hybrid neural implant system, comprising biodegradable materials, promises to provide monitoring and therapy modalities for versatile applications in biomedicine.

In-vivo integration of soft neural probes through high-resolution printing of liquid electronics on the cranium

Current soft neural probes are still operated by bulky, rigid electronics mounted to a body, which deteriorate the integrity of the device to biological systems and restrict the free behavior of a subject. We report a soft, conformable neural interface system that can monitor the single-unit activities of neurons with long-term stability. The system implements soft neural probes in the brain, and their subsidiary electronics which are directly printed on the cranial surface. The high-resolution printing of liquid metals forms soft neural probes with a cellular-scale diameter and adaptable lengths. Also, the printing of liquid metal-based circuits and interconnections along the curvature of the cranium enables the conformal integration of electronics to the body, and the cranial circuit delivers neural signals to a smartphone wirelessly. In the in-vivo studies using mice, the system demonstrates long-term recording (33 weeks) of neural activities in arbitrary brain regions. In T-maze behavioral tests, the system shows the behavior-induced activation of neurons in multiple brain regions.

An Antidehydration Hydrogel Based on Zwitterionic Oligomers for Bioelectronic Interfacing

Hydrogels are ideal interfacing materials for on-skin healthcare devices, yet their susceptibility to dehydration hinders their practical use. While incorporating hygroscopic metal salts can prevent dehydration and maintain ionic conductivity, concerns arise regarding metal toxicity due to the passage of small ions through the skin barrier. Herein, an antidehydration hydrogel enabled by the incorporation of zwitterionic oligomers into its network is reported. This hydrogel exhibits exceptional water retention properties, maintaining ≈88% of its weight at 40% relative humidity, 25 °C for 50 days and about 84% after being heated at 50 °C for 3 h. Crucially, the molecular weight design of the embedded oligomers prevents their penetration into the epidermis, as evidenced by experimental and molecular simulation results. The hydrogel allows stable signal acquisition in electrophysiological monitoring of humans and plants under low-humidity conditions. This research provides a promising strategy for the development of epidermis-safe and biocompatible antidehydration hydrogel interfaces for on-skin devices.

Nanoscale surface coatings and topographies for neural interfaces

With the lack of minimally invasive tools for probing neuronal systems across spatiotemporal scales, understanding the working mechanism of the nervous system and limited assessments available are imperative to prevent or treat neurological disorders. In particular, nanoengineered neural interfaces can provide a solution to this technological barrier. This review covers recent surface engineering approaches, including nanoscale surface coatings, and a range of topographies from the microscale to the nanoscale, primarily focusing on neural-interfaced biosystems. Specifically, the immobilization of bioactive molecules to fertilize the neural cell lineage, topographical engineering to induce mechanotransduction in neural cells, and enhanced cell-chip coupling using three-dimensional structured surfaces are highlighted. Advances in neural interface design will help us understand the nervous system, thereby achieving the effective treatments for neurological disorders. STATEMENT OF SIGNIFICANCE: • This review focuses on designing bioactive neural interface with a nanoscale chemical modification and topographical engineering at multiscale perspective. • Versatile nanoscale surface coatings and topographies for neural interface are summarized. • Recent advances in bioactive materials applicable for neural cell culture, electrophysiological sensing, and neural implants are reviewed.

Region-specific targeting of microglia in vivo using direct delivery of tamoxifen metabolites via microfluidic polymer fibers

Region-specific genetic manipulation of glial cells remains challenging due to the lack of anatomically selective transgenic models. Although local transduction is achievable with viral vectors, uniform recombination can be challenging in larger brain regions. We investigated the efficacy of intraparenchymal delivery of the tamoxifen metabolite endoxifen using inducible cre reporter mice. After observing localized reporter induction following stereotaxic injections of endoxifen in CX3CR1creERT2 mice, we carried out chronic delivery via osmotic pumps attached to bilateral cannulas made of stainless steel or microfluidic polymer fibers. Analysis of reporter expression in sections or iDISCO-cleared brains from TMEM119creERT2 mice revealed widespread induction following chronic infusion. Neuronal damage and gliosis were more prevalent around steel cannulas than polymer fibers, and glial reactivity was further attenuated when devices were implanted two months before drug delivery. In summary, region-specific recombination is achievable in glia with minimal tissue damage after endoxifen delivery via microfluidic polymer implants.

Hydrogels for active photonics

Conventional photonic devices exhibit static optical properties that are design-dependent, including the material's refractive index and geometrical parameters. However, they still possess attractive optical responses for applications and are already exploited in devices across various fields. Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli. Over the past few years, various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties. Herein, we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels. In particular, fabrication techniques for hydrogel photonic devices are categorized into film growth, photolithography (PL), electron-beam lithography (EBL), and nanoimprint lithography (NIL). Furthermore, we provide insights into future directions and prospects for deformable hydrogel photonics, along with their potential practical applications.

Multifunctional wet-adhesive chitosan/acrylic conduit for sutureless repair of peripheral nerve injuries

The incidence of peripheral nerve injury (PNI) is high worldwide, and a poor prognosis is common. Surgical closure and repair of the affected area are crucial to ensure the effective treatment of peripheral nerve injuries. Despite being the standard treatment approach, reliance on sutures to seal the severed nerve ends introduces several limitations and restrictions. This technique is intricate and time-consuming, and the application of threading and punctate sutures may lead to tissue damage and heightened tension concentrations, thus increasing the risk of fixation failure and local inflammation. This study aimed to develop easily implantable chitosan-based peripheral nerve repair conduits that combine acrylic acid and cleavable N-hydroxysuccinimide to reduce nerve damage during repair. In ex vivo tissue adhesion tests, the conduit achieved maximal interfacial toughness of 705 J m-2 ± 30 J m-2, allowing continuous bridging of the severed nerve ends. Adhesive repair significantly reduces local inflammation caused by conventional sutures, and the positive charge of chitosan disrupts the bacterial cell wall and reduces implant-related infections. This promises to open new avenues for sutureless nerve repair and reliable medical implants.

Soft Bioelectronics for Neuroengineering: New Horizons in the Treatment of Brain Tumor and Epilepsy

Soft bioelectronic technologies for neuroengineering have shown remarkable progress, which include novel soft material technologies and device design strategies. Such technological advances that are initiated from fundamental brain science are applied to clinical neuroscience and provided meaningful promises for significant improvement in the diagnosis efficiency and therapeutic efficacy of various brain diseases recently. System-level integration strategies in consideration of specific disease circumstances can enhance treatment effects further. Here, recent advances in soft implantable bioelectronics for neuroengineering, focusing on materials and device designs optimized for the treatment of intracranial disease environments, are reviewed. Various types of soft bioelectronics for neuroengineering are categorized and exemplified first, and then details for the sensing and stimulating device components are explained. Next, application examples of soft implantable bioelectronics to clinical neuroscience, particularly focusing on the treatment of brain tumor and epilepsy are reviewed. Finally, an ideal system of soft intracranial bioelectronics such as closed-loop-type fully-integrated systems is presented, and the remaining challenges for their clinical translation are discussed.

Advances in Wireless, Batteryless, Implantable Electronics for Real-Time, Continuous Physiological Monitoring

This review summarizes recent progress in developing wireless, batteryless, fully implantable biomedical devices for real-time continuous physiological signal monitoring, focusing on advancing human health care. Design considerations, such as biological constraints, energy sourcing, and wireless communication, are discussed in achieving the desired performance of the devices and enhanced interface with human tissues. In addition, we review the recent achievements in materials used for developing implantable systems, emphasizing their importance in achieving multi-functionalities, biocompatibility, and hemocompatibility. The wireless, batteryless devices offer minimally invasive device insertion to the body, enabling portable health monitoring and advanced disease diagnosis. Lastly, we summarize the most recent practical applications of advanced implantable devices for human health care, highlighting their potential for immediate commercialization and clinical uses.

Scalable, flexible carbon fiber electrode thread arrays for three-dimensional probing of neurochemical activity in deep brain structures of rodents

We developed a flexible "electrode-thread" array for recording dopamine neurochemicals from a lateral distribution of subcortical targets (up to 16) transverse to the axis of insertion. Ultrathin (∼10 μm diameter) carbon fiber (CF) electrode-threads (CFETs) are clustered into a tight bundle to introduce them into the brain from a single-entry point. The individual CFETs splay laterally in deep brain tissue during insertion due to their innate flexibility. This spatial redistribution allows navigation of the CFETs towards deep brain targets spreading horizontally from the axis of insertion. Commercial "linear" arrays provide single-entry insertion but only allow measurements along the axis of insertion. Horizontally configured arrays inflict separate penetrations for each individual channel. We tested functional performance of our CFET arrays in vivo for recording dopamine and for providing lateral spread to multiple distributed sites in the rat striatum. Spatial spread was further characterized in agar brain phantoms as a function of insertion depth. We also developed protocols to slice the embedded CFETs within fixed brain tissue using standard histology. This method allowed extraction of the precise spatial coordinates of the implanted CFETs and their recording sites as integrated with immunohistochemical staining for surrounding anatomical, cytological, and protein expression labels. Our CFET array has the potential to unlock a wide range of applications, from uncovering the role of neuromodulators in synaptic plasticity, to addressing critical safety barriers in clinical translation towards diagnostic and adaptive treatment in Parkinson's disease and major mood disorders.

The ultra-thin, minimally invasive surface electrode array NeuroWeb for probing neural activity

Electrophysiological recording technologies can provide valuable insights into the functioning of the central and peripheral nervous systems. Surface electrode arrays made of soft materials or implantable multi-electrode arrays with high electrode density have been widely utilized as neural probes. However, neither of these probe types can simultaneously achieve minimal invasiveness and robust neural signal detection. Here, we present an ultra-thin, minimally invasive neural probe (the "NeuroWeb") consisting of hexagonal boron nitride and graphene, which leverages the strengths of both surface electrode array and implantable multi-electrode array. The NeuroWeb open lattice structure with a total thickness of 100 nm demonstrates high flexibility and strong adhesion, establishing a conformal and tight interface with the uneven mouse brain surface. In vivo electrophysiological recordings show that NeuroWeb detects stable single-unit activity of neurons with high signal-to-noise ratios. Furthermore, we investigate neural interactions between the somatosensory cortex and the cerebellum using transparent dual NeuroWebs and optical stimulation, and measure the times of neural signal transmission between the brain regions depending on the pathway. Therefore, NeuroWeb can be expected to pave the way for understanding complex brain networks with optical and electrophysiological mapping of the brain.

Molecular design of an ultra-strong tissue adhesive hydrogel with tunable multifunctionality

Designing adhesive hydrogels with optimal properties for the treatment of injured tissues is challenging due to the tradeoff between material stiffness and toughness while maintaining adherence to wet tissue surfaces. In most cases, bioadhesives with improved mechanical strength often lack an appropriate elastic compliance, hindering their application for sealing soft, elastic, and dynamic tissues. Here, we present a novel strategy for engineering tissue adhesives in which molecular building blocks are manipulated to allow for precise control and optimization of the various aforementioned properties without any tradeoffs. To introduce tunable mechanical properties and robust tissue adhesion, the hydrogel network presents different modes of covalent and noncovalent interactions using N-hydroxysuccinimide ester (NHS) conjugated alginate (Alg-NHS), poly (ethylene glycol) diacrylate (PEGDA), tannic acid (TA), and Fe³⁺ ions. Through combining and tuning different molecular interactions and a variety of crosslinking mechanisms, we were able to design an extremely elastic (924%) and tough (4697 kJ/m³) multifunctional hydrogel that could quickly adhere to wet tissue surfaces within 5 s of gentle pressing and deform to support physiological tissue function over time under wet conditions. While Alg-NHS provides covalent bonding with the tissue surfaces, the catechol moieties of TA molecules synergistically adopt a mussel-inspired adhesive mechanism to establish robust adherence to the wet tissue. The strong adhesion of the engineered bioadhesive patch is showcased by its application to rabbit conjunctiva and porcine cornea. Meanwhile, the engineered bioadhesive demonstrated painless detachable characteristics and in vitro biocompatibility. Additionally, due to the molecular interactions between TA and Fe³⁺, antioxidant and antibacterial properties required to support the wound healing pathways were also highlighted. Overall, by tuning various molecular interactions, we were able to develop a single-hydrogel platform with an "all-in-one" multifunctionality that can address current challenges of engineering hydrogel-based bioadhesives for tissue repair and sealing.

Submillimeter Multifunctional Ferromagnetic Fiber Robots for Navigation, Sensing, and Modulation

Small-scale robots capable of remote active steering and navigation offer great potential for biomedical applications. However, the current design and manufacturing procedure impede their miniaturization and integration of various diagnostic and therapeutic functionalities. Herein, submillimeter fiber robots that can integrate navigation, sensing, and modulation functions are presented. These fiber robots are fabricated through a scalable thermal drawing process at a speed of 4 meters per minute, which enables the integration of ferromagnetic, electrical, optical, and microfluidic composite with an overall diameter of as small as 250 µm and a length of as long as 150 m. The fiber tip deflection angle can reach up to 54o under a uniform magnetic field of 45 mT. These fiber robots can navigate through complex and constrained environments, such as artificial vessels and brain phantoms. Moreover, Langendorff mouse hearts model, glioblastoma micro platforms, and in vivo mouse models are utilized to demonstrate the capabilities of sensing electrophysiology signals and performing a localized treatment. Additionally, it is demonstrated that the fiber robots can serve as endoscopes with embedded waveguides. These fiber robots provide a versatile platform for targeted multimodal detection and treatment at hard-to-reach locations in a minimally invasive and remotely controllable manner.

Flexible Electrodes for Brain-Computer Interface System

Brain-computer interface (BCI) has been the subject of extensive research recently. Governments and companies have substantially invested in relevant research and applications. The restoration of communication and motor function, the treatment of psychological disorders, gaming, and other daily and therapeutic applications all benefit from BCI. The electrodes hold the key to the essential, fundamental BCI precondition of electrical brain activity detection and delivery. However, the traditional rigid electrodes are limited due to their mismatch in Young's modulus, potential damages to the human body, and a decline in signal quality with time. These factors make the development of flexible electrodes vital and urgent. Flexible electrodes made of soft materials have grown in popularity in recent years as an alternative to conventional rigid electrodes because they offer greater conformance, the potential for higher signal-to-noise ratio (SNR) signals, and a wider range of applications. Therefore, the latest classifications and future developmental directions of fabricating these flexible electrodes are explored in this paper to further encourage the speedy advent of flexible electrodes for BCI. In summary, the perspectives and future outlook for this developing discipline are provided.

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.

Functional material-mediated wireless physical stimulation for neuro-modulation and regeneration

Nerve injuries and neurological diseases remain intractable clinical challenges. Despite the advantages of stem cell therapy in treating neurological disorders, uncontrollable cell fates and loss of cell function in vivo are still challenging. Recently, increasing attention has been given to the roles of external physical signals, such as electricity and ultrasound, in regulating stem cell fate as well as activating or inhibiting neuronal activity, which provides new insights for the treatment of neurological disorders. However, direct physical stimulations in vivo are short in accuracy and safety. Functional materials that can absorb energy from a specific physical field exerted in a wireless way and then release another localized physical signal hold great advantages in mediating noninvasive or minimally invasive accurate indirect physical stimulations to promote the therapeutic effect on neurological disorders. In this review, the mechanism by which various physical signals regulate stem cell fate and neuronal activity is summarized. Based on these concepts, the approaches of using functional materials to mediate indirect wireless physical stimulation for neuro-modulation and regeneration are systematically reviewed. We expect that this review will contribute to developing wireless platforms for neural stimulation as an assistance for the treatment of neurological diseases and injuries.

Spiral NeuroString: High-Density Soft Bioelectronic Fibers for Multimodal Sensing and Stimulation

Bioelectronic fibers hold promise for both research and clinical applications due to their compactness, ease of implantation, and ability to incorporate various functionalities such as sensing and stimulation. However, existing devices suffer from bulkiness, rigidity, limited functionality, and low density of active components. These limitations stem from the difficulty to incorporate many components on one-dimensional (1D) fiber devices due to the incompatibility of conventional microfabrication methods (e.g., photolithography) with curved, thin and long fiber structures. Herein, we introduce a fabrication approach, ‶spiral transformation″, to convert two-dimensional (2D) films containing microfabricated devices into 1D soft fibers. This approach allows for the creation of high density multimodal soft bioelectronic fibers, termed Spiral NeuroString (S-NeuroString), while enabling precise control over the longitudinal, angular, and radial positioning and distribution of the functional components. We show the utility of S-NeuroString for motility mapping, serotonin sensing, and tissue stimulation within the dynamic and soft gastrointestinal (GI) system, as well as for single-unit recordings in the brain. The described bioelectronic fibers hold great promises for next-generation multifunctional implantable electronics.

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.

Multifunctional Fiber-Based Optoacoustic Emitter as a Bidirectional Brain Interface

A bidirectional brain interface with both "write" and "read" functions can be an important tool for fundamental studies and potential clinical treatments for neurological diseases. Herein, a miniaturized multifunctional fiber-based optoacoustic emitter (mFOE) is reported thatintegrates simultaneous optoacoustic stimulation for "write" and electrophysiology recording of neural circuits for "read". Because of the intrinsic ability of neurons to respond to acoustic wave, there is no requirement of the viral transfection. The orthogonality between optoacoustic waves and electrical field provides a solution to avoid the interference between electrical stimulation and recording. The stimulation function of the mFOE is first validated in cultured ratcortical neurons using calcium imaging. In vivo application of mFOE for successful simultaneous optoacoustic stimulation and electrical recording of brain activities is confirmed in mouse hippocampus in both acute and chronical applications up to 1 month. Minor brain tissue damage is confirmed after these applications. The capability of simultaneous neural stimulation and recording enabled by mFOE opens up new possibilities for the investigation of neural circuits and brings new insights into the study of ultrasound neurostimulation.

Hemostatic patch with ultra-strengthened mechanical properties for efficient adhesion to wet surfaces

Controlling traumatic bleeding from damaged internal organs while effectively sealing the wound is critical for saving the lives of patients. Existing bioadhesives suffer from blood incompatibility, insufficient adhesion to wet surfaces, weak mechanical properties, and complex application procedures. Here, we engineered a ready-to-use hemostatic bioadhesive with ultra-strengthened mechanical properties and fatigue resistance, robust adhesion to wet tissues within a few seconds of gentle pressing, deformability to accommodate physiological function and action, and the ability to stop bleeding efficiently. The engineered hydrogel, which demonstrated high elasticity (>900%) and toughness (>4600 kJ/m3), was formed by fine-tuning a series of molecular interactions and crosslinking mechanisms involving N-hydroxysuccinimide (NHS) conjugated alginate (Alg-NHS), poly (ethylene glycol) diacrylate (PEGDA), tannic acid (TA), and Fe3+ ions. Dual adhesive moieties including mussel-inspired pyrogallol/catechol and NHS synergistically enhanced wet tissue adhesion (>400 kPa in a wound closure test). In conjunction with physical sealing, the high affinity of TA/Fe3+ for blood could further augment hemostasis. The engineered bioadhesive demonstrated excellent in vitro and in vivo biocompatibility as well as improved hemostatic efficacy as compared to commercial Surgicel®. Overall, the hydrogel design strategy described herein holds great promise for overcoming existing obstacles impeding clinical translation of engineered hemostatic bioadhesives.

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

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

Construction of a Flexible Optogenetic Device for Multisite and Multiregional Optical Stimulation Through Flexible µ-LED Displays on the Cerebral Cortex

Precisely delivering light to multiple locations in biological tissue is crucial for advancing multiregional optogenetics in neuroscience research. However, conventional implantable devices typically have rigid geometries and limited light sources, allowing only single or dual probe placement with fixed spacing. Here, a fully flexible optogenetic device with multiple thin-film microscale light-emitting diode (µ-LED) displays scattering from a central controller is presented. Each display is heterogeneously integrated with thin-film 5 × 10 µ-LEDs and five optical fibers 125 µm in diameter to achieve cellular-scale spatial resolution. Meanwhile, the device boasts a compact, flexible circuit capable of multichannel configuration and wireless transmission, with an overall weight of 1.31 g, enabling wireless, real-time neuromodulation of freely moving rats. Characterization results and finite element analysis have demonstrated excellent optical properties and mechanical stability, while cytotoxicity tests further ensure the biocompatibility of the device for implantable applications. Behavior studies under optogenetic modulation indicate great promise for wirelessly modulating neural functions in freely moving animals. The device with multisite and multiregional optogenetic modulation capability offers a comprehensive platform to advance both fundamental neuroscience studies and potential applications in brain-computer interfaces.

Fibrous wearable and implantable bioelectronics

Fibrous wearable and implantable devices have emerged as a promising technology, offering a range of new solutions for minimally invasive monitoring of human health. Compared to traditional biomedical devices, fibers offer a possibility for a modular design compatible with large-scale manufacturing and a plethora of advantages including mechanical compliance, breathability, and biocompatibility. The new generation of fibrous biomedical devices can revolutionize easy-to-use and accessible health monitoring systems by serving as building blocks for most common wearables such as fabrics and clothes. Despite significant progress in the fabrication, materials, and application of fibrous biomedical devices, there is still a notable absence of a comprehensive and systematic review on the subject. This review paper provides an overview of recent advancements in the development of fibrous wearable and implantable electronics. We categorized these advancements into three main areas: manufacturing processes, platforms, and applications, outlining their respective merits and limitations. The paper concludes by discussing the outlook and challenges that lie ahead for fiber bioelectronics, providing a holistic view of its current stage of development.

Flexible and Stretchable Electrochemical Sensors for Biological Monitoring

The rise of flexible and stretchable electronics has revolutionized biosensor techniques for probing biological systems. Particularly, flexible and stretchable electrochemical sensors (FSECSs) enable the in situ quantification of numerous biochemical molecules in different biological entities owing to their exceptional sensitivity, fast response, and easy miniaturization. Over the past decade, the fabrication and application of FSECSs have significantly progressed. This review highlights key developments in electrode fabrication and FSECSs functionalization. It delves into the electrochemical sensing of various biomarkers, including metabolites, electrolytes, signaling molecules, and neurotransmitters from biological systems, encompassing the outer epidermis, tissues/organs in vitro and in vivo, and living cells. Finally, considering electrode preparation and biological applications, current challenges and future opportunities for FSECSs are discussed.

Strongly adhesive zwitterionic composite hydrogel paints for surgical sutures and blood-contacting devices

Hydrogels show eminent advantages in biomedical and pharmaceutical fields. However, their application as coating materials for biomedical devices is limited by several key challenges, such as lack of universality, weak mechanical strength, and low adhesion to the substrate. Here we report versatile and tough adhesion composite hydrogel paints (CHPs), which consist of zwitterionic copolymers and microgels, both with reactive groups. The CHPs exhibit tunable rheology and thickness, hydrophilicity, biofouling resistance, durability, and convenient fabrication on metal, polymer, and inorganic surfaces with arbitrary shapes. As a proof-of-concept, the CHP-surgical sutures demonstrate exceptional lubrication, drug delivery, anti-infection, and anti-fibrous capsule properties. Moreover, the CHP-PVC tubing effectively prevents thrombus formation in vitro and ex vivo rabbit blood circulation without anticoagulants. This work provides valuable insights for enhancing and developing integrated hydrogel technologies for biomedical devices. STATEMENT OF SIGNIFICANCE: The combination of hydrogel and biomedical devices can enable numerous existing applications in medicine. In this study, inspired by the principle of microgel reinforcement in industrial paints, we propose a simple and versatile zwitterionic composite hydrogel paints (CHPs) strategy, which can be easily applied to diverse substrates with arbitrary shapes by covalent grafting between complementary groups by brush, dip, or spray. The CHPs integrated universality, tough adhesion, mechanical durability, and anti-biofouling properties because of their unique chemical composition and coating structure design. This strategy provides a simple and versatile route for surface modification of biomedical devices.

Ultraflexible endovascular probes for brain recording through micrometer-scale vasculature

Implantable neuroelectronic interfaces have enabled advances in both fundamental research and treatment of neurological diseases but traditional intracranial depth electrodes require invasive surgery to place and can disrupt neural networks during implantation. We developed an ultrasmall and flexible endovascular neural probe that can be implanted into sub-100-micrometer-scale blood vessels in the brains of rodents without damaging the brain or vasculature. In vivo electrophysiology recording of local field potentials and single-unit spikes have been selectively achieved in the cortex and olfactory bulb. Histology analysis of the tissue interface showed minimal immune response and long-term stability. This platform technology can be readily extended as both research tools and medical devices for the detection and intervention of neurological diseases.

A mosquito mouthpart-like bionic neural probe

Advancements in microscale electrode technology have revolutionized the field of neuroscience and clinical applications by offering high temporal and spatial resolution of recording and stimulation. Flexible neural probes, with their mechanical compliance to brain tissue, have been shown to be superior to rigid devices in terms of stability and longevity in chronic recordings. Shuttle devices are commonly used to assist flexible probe implantation; however, the protective membrane of the brain still makes penetration difficult. Hidden damage to brain vessels during implantation is a significant risk. Inspired by the anatomy of the mosquito mouthparts, we present a biomimetic neuroprobe system that integrates high-sensitivity sensors with a high-fidelity multichannel flexible electrode array. This customizable system achieves distributed and minimally invasive implantation across brain regions. Most importantly, the system's nonvisual monitoring capability provides an early warning detection for intracranial soft tissues, such as vessels, reducing the potential for injury during implantation. The neural probe system demonstrates exceptional sensitivity and adaptability to environmental stimuli, as well as outstanding performance in postoperative and chronic recordings. These findings suggest that our biomimetic neural-probe device offers promising potential for future applications in neuroscience and brain-machine interfaces. A mosquito mouthpart-like bionic neural probe consisting of a highly sensitive tactile sensor module, a flexible microelectrode array, and implanted modules that mimic the structure of mosquito mouthparts. The system enables distributed implantation of electrode arrays across multiple brain regions while making the implantation minimally invasive and avoiding additional dural removal. The tactile sensor array can monitor the implantation process to achieve early warning of vascular damage. The excellent postoperative short-term recording performance and long-term neural activity tracking ability demonstrate that the system is a promising tool in the field of brain-computer interfaces.

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.

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

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

Emerging Penetrating Neural Electrodes: In Pursuit of Large Scale and Longevity

Penetrating neural electrodes provide a powerful approach to decipher brain circuitry by allowing for time-resolved electrical detections of individual action potentials. This unique capability has contributed tremendously to basic and translational neuroscience, enabling both fundamental understandings of brain functions and applications of human prosthetic devices that restore crucial sensations and movements. However, conventional approaches are limited by the scarce number of available sensing channels and compromised efficacy over long-term implantations. Recording longevity and scalability have become the most sought-after improvements in emerging technologies. In this review, we discuss the technological advances in the past 5-10 years that have enabled larger-scale, more detailed, and longer-lasting recordings of neural circuits at work than ever before. We present snapshots of the latest advances in penetration electrode technology, showcase their applications in animal models and humans, and outline the underlying design principles and considerations to fuel future technological development.