A Stretchable and Transparent Electrode Based on PEGylated Silk Fibroin for In Vivo Dual-Modal Neural-Vascular Activity Probing

Soft bioelectronics for diagnostic and therapeutic applications in neurological diseases

Physical and chemical signals in the central nervous system yield crucial information that is clinically relevant under both physiological and pathological conditions. The emerging field of bioelectronics focuses on the monitoring and manipulation of neurophysiological signals with high spatiotemporal resolution and minimal invasiveness. Significant advances have been realized through innovations in materials and structural design, which have markedly enhanced mechanical and electrical properties, biocompatibility, and overall device performance. The diagnostic and therapeutic potential of soft bioelectronics has been corroborated across a diverse array of pre-clinical settings. This review summarizes recent studies that underscore the developments and applications of soft bioelectronics in neurological disorders, including neuromonitoring, neuromodulation, tumor treatment, and biosensing. Limitations and outlooks of soft devices are also discussed in terms of power supply, wireless control, biocompatibility, and the integration of artificial intelligence. This review highlights the potential of soft bioelectronics as a future platform to promote deciphering brain functions and clinical outcomes of neurological diseases.

Polysaccharide-protein based scaffolds for cartilage repair and regeneration

Cartilage repair and regeneration have become a global issue that millions of patients from all over the world need surgical intervention to repair the articular cartilage annually due to the limited self-healing capability of the cartilage tissues. Cartilage tissue engineering has gained significant attention in cartilage repair and regeneration by integration of the chondrocytes (or stem cells) and the artificial scaffolds. Recently, polysaccharide-protein based scaffolds have demonstrated unique and promising mechanical and biological properties as the artificial extracellular matrix of natural cartilage. In this review, we summarize the modification methods for polysaccharides and proteins. The preparation strategies for the polysaccharide-protein based hydrogel scaffolds are presented. We discuss the mechanical, physical and biological properties of the polysaccharide-protein based scaffolds. Potential clinical translation and challenges on the artificial scaffolds are also discussed.

Interfacing Neuron-Motor Pathways with Stretchable and Biocompatible Electrode Arrays

ConspectusIn the field of neuroscience, understanding the complex interactions within the intricate neuron-motor system depends crucially on the use of high-density, physiological multiple electrode arrays (MEAs). In the neuron-motor system, the transmission of biological signals primarily occurs through electrical and chemical signaling. Taking neurons for instance, when a neuron receives external stimuli, it generates an electrical signal known as the action potential. This action potential propagates along the neuron's axon and is transmitted to other neurons via synapses. At the synapse, chemical signals (neurotransmitters) are released, allowing the electrical signal to traverse the synaptic gap and influence the next neuron. MEAs can provide unparalleled insights into neural signal patterns when interfacing with the nerve systems through their excellent spatiotemporal resolution. However, the inherent differences in mechanical and chemical properties between these artificial devices and biological tissues can lead to serious complications after chronic implantation, such as body rejection, infection, tissue damage, or device malfunction. A promising strategy to enhance MEAs' biocompatibility involves minimizing their thickness, which aligns their bending stiffness with that of surrounding tissues, thereby minimizing damage over time. However, this solution has its limits; the resulting ultrathin devices, typically based on plastic films, lack the necessary stretchability, restricting their use to organs that neither stretch nor grow.For practical deployments, devices must exhibit certain levels of stretchability (ranging from 20 to 70%), tailored to the specific requirements of the target organs. In this Account, we outline recent advancements in developing stretchable MEAs that balance stretchability with sufficient electrical conductivity for effective use in physiological research, acting as sensors and stimulators. By concentrating on the neuron-motor pathways, we summarize how the stretchable MEAs meet various application needs and examine their effectiveness. We distinguish between on-skin and implantable uses, given their vastly different requirements. Within each application scope, we highlight cutting-edge technologies, evaluating their strengths and shortcomings. Recognizing that most current devices rely on elastic films with a Young's modulus value between ∼0.5 and 5 MPa, we delve into the potential for softer MEAs, particularly those using multifunctional hydrogels for an optimizing tissue-device interface and address the challenges in adapting such hydrogel-based MEAs for chronic implants. Additionally, transitioning soft MEAs from lab to fab involves connecting them to a rigid adapter and external machinery, highlighting a critical challenge at the soft-rigid interface due to strain concentration, especially in chronic studies subject to unforeseen mechanical strains. We discuss innovative solutions to this integration challenge, being optimistic that the development of durable, biocompatible, stretchable MEAs will significantly advance neuroscience and related fields.

Flexible silk-fibroin-based microelectrode arrays for high-resolution neural recording

High-precision neural recording plays a pivotal role in unraveling the intricate mechanisms that underlie information transmission of the nervous system, raising increasing interest in the development of implantable microelectrode arrays (MEAs). The challenge lies in providing a truly soft, highly conductive and low-impedance neural interface for precise recording of the electrophysiological signals of individual neurons or neural networks. Herein, by implementing a novel topological regulation strategy of silk fibroin (SF) crosslinking, we prepared a flexible, hydrophilic, and biocompatible MEA substrate, facilitating a biocompatible neural interface that minimizes mechanical mismatch with biological tissues. Additionally, we established a strategy involving screen-printing combined with post-coating to prepare MEAs with high conductivity, low impedance and high capacitance, by coating PEDOT:PSS on titanium carbide (Ti3C2) microarrays. The Ti3C2 nanosheets, as the conductive track of the MEAs, avoided the charge drifting associated with metals and facilitated the processing of the MEAs. Further coating PEDOT:PSS on the electrode points reduced the impedance 100-fold, from 105 to 103 Ω. Experimental validation confirmed the superior electrophysiological signal recording capabilities of the SF-based MEA (SMEA) in peripheral and cerebral nerves with a much higher signal-to-noise ratio (SNR) of 20. In particular, we achieved high-precision recording of the action potential (AP) induced by flash visual stimulation, demonstrating high performance in weak signal recording. In summary, the development of SMEA provides a robust foundation for future investigations into the mechanisms and principles of neural circuit information transmission in complex nervous systems.

Mild preparation of hyaluronic acid/silk fibroin sponges by modified crosslinking method

The composites composed of hyaluronic acid (HA) and silk fibroin (SF) exhibit great potential in diverse biomedical applications. However, the utilization of commercial crosslinkers such as 1,4-butanediol diglycidyl ether (BDDE) for crosslinking HA typically necessitates harsh conditions involving strong alkaline, which greatly limits its potential applications. In this study, a mild modified approach was developed to fabricate HA/SF blend sponges crosslinked by BDDE without alkaline conditions. The blend solutions were cryo-concentrated to induce crosslinking reactions. The mechanism of freezing crosslinking was elucidated by investigating the effects of ice crystal growth and HA molecular weight on the degree of crosslinking. The results revealed that HA achieved efficient crosslinking when its molecular weight exceeds 1000 kDa and freezing temperatures ranged from -40 °C to -20 °C. After introducing SF, multiple crosslinks were formed between SF and HA chains, producing water-stable porous sponges. The SEM results demonstrated that the introduction of SF effectively enhanced the interconnectivity between macropores through creating subordinate holes onto the pores wall. Raising the SF content significantly enhanced compression strength, resistance to enzymatic degradation and cell viability of blend sponges. This study provides a novel strategy for designing bioactive HA/SF blend sponges as substitutes for tissue repair and wound dressing.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Biomaterial strategies for regulating the neuroinflammatory response

Injury and disease in the central nervous system (CNS) can result in a dysregulated inflammatory environment that inhibits the repair of functional tissue. Biomaterials present a promising approach to tackle this complex inhibitory environment and modulate the mechanisms involved in neuroinflammation to halt the progression of secondary injury and promote the repair of functional tissue. In this review, we will cover recent advances in biomaterial strategies, including nanoparticles, hydrogels, implantable scaffolds, and neural probe coatings, that have been used to modulate the innate immune response to injury and disease within the CNS. The stages of inflammation following CNS injury and the main inflammatory contributors involved in common neurodegenerative diseases will be discussed, as understanding the inflammatory response to injury and disease is critical for identifying therapeutic targets and designing effective biomaterial-based treatment strategies. Biomaterials and novel composites will then be discussed with an emphasis on strategies that deliver immunomodulatory agents or utilize cell-material interactions to modulate inflammation and promote functional tissue repair. We will explore the application of these biomaterial-based strategies in the context of nanoparticle- and hydrogel-mediated delivery of small molecule drugs and therapeutic proteins to inflamed nervous tissue, implantation of hydrogels and scaffolds to modulate immune cell behavior and guide axon elongation, and neural probe coatings to mitigate glial scarring and enhance signaling at the tissue-device interface. Finally, we will present a future outlook on the growing role of biomaterial-based strategies for immunomodulation in regenerative medicine and neuroengineering applications in the CNS.

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.

Engineering Proteins for PEDOT Dispersions: A New Horizon for Highly Mixed Ionic-Electronic Biocompatible Conducting Materials

Poly (3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrene sulfonate (PSS) is the most used conducting polymer from energy to biomedical applications. Despite its exceptional properties, there is a need for developing new materials that can improve some of its inherent limitations, e.g., biocompatibility. In this context, doping PEDOT is propose with a robust recombinant protein with tunable properties, the consensus tetratricopeptide repeated protein (CTPR). The doping consists of an oxidative polymerization, where the PEDOT chains are stabilized by the negative charges of the CTPR protein. CTPR proteins are evaluated with three different lengths (3, 10, and 20 identical CTPR units) and optimized varied synthetic conditions. These findings revealed higher doping rate and oxidized state of the PEDOT chains when doped with the smallest scaffold (CTPR3). These PEDOT:CTPR hybrids possess ionic and electronic conductivity. Notably, PEDOT:CTPR3 displayed an electronic conductivity of 0.016 S cm-1, higher than any other reported protein-doped PEDOT. This result places PEDOT:CTPR3 at the level of PEDOT-biopolymer hybrids, and brings it closer in performance to PEDOT:PSS gold standard. Furthermore, PEDOT:CTPR3 dispersion is successfully optimized for inkjet printing, preserving its electroactivity properties after printing. This approach opens the door to the use of these novel hybrids for bioelectronics.

Sensing, Imaging, and Therapeutic Strategies Endowing by Conjugate Polymers for Precision Medicine

Conjugated polymers (CPs) have promising applications in biomedical fields, such as disease monitoring, real-time imaging diagnosis, and disease treatment. As a promising luminescent material with tunable emission, high brightness and excellent stability, CPs are widely used as fluorescent probes in biological detection and imaging. Rational molecular design and structural optimization have broadened absorption/emission range of CPs, which are more conductive for disease diagnosis and precision therapy. This review provides a comprehensive overview of recent advances in the application of CPs, aiming to elucidate their structural and functional relationships. The fluorescence properties of CPs and the mechanism of detection signal amplification are first discussed, followed by an elucidation of their emerging applications in biological detection. Subsequently, CPs-based imaging systems and therapeutic strategies are illustrated systematically. Finally, recent advancements in utilizing CPs as electroactive materials for bioelectronic devices are also investigated. Moreover, the challenges and outlooks of CPs for precision medicine are discussed. Through this systematic review, it is hoped to highlight the frontier progress of CPs and promote new breakthroughs in fundamental research and clinical transformation.

Advancing Intelligent Organ-on-a-Chip Systems with Comprehensive In Situ Bioanalysis

In vitro models are essential to a broad range of biomedical research, such as pathological studies, drug development, and personalized medicine. As a potentially transformative paradigm for 3D in vitro models, organ-on-a-chip (OOC) technology has been extensively developed to recapitulate sophisticated architectures and dynamic microenvironments of human organs by applying the principles of life sciences and leveraging micro- and nanoscale engineering capabilities. A pivotal function of OOC devices is to support multifaceted and timely characterization of cultured cells and their microenvironments. However, in-depth analysis of OOC models typically requires biomedical assay procedures that are labor-intensive and interruptive. Herein, the latest advances toward intelligent OOC (iOOC) systems, where sensors integrated with OOC devices continuously report cellular and microenvironmental information for comprehensive in situ bioanalysis, are examined. It is proposed that the multimodal data in iOOC systems can support closed-loop control of the in vitro models and offer holistic biomedical insights for diverse applications. Essential techniques for establishing iOOC systems are surveyed, encompassing in situ sensing, data processing, and dynamic modulation. Eventually, the future development of iOOC systems featuring cross-disciplinary strategies is discussed.

Hemoadhican-Based Bioabsorbable Hydrogel for Preventing Postoperative Adhesions

Postoperative peritoneal adhesions are a prevalent clinical issue following abdominal and pelvic surgery, frequently resulting in heightened personal and societal health burdens. Traditional biomedical barriers offer limited benefits because of practical challenges for doctors and their incompatibility with laparoscopic surgery. Hydrogel materials, represented by hyaluronic acid gels, are receiving increasing attention. However, existing antiadhesive gels still have limited effectiveness or carry the risk of complications in clinical applications. Herein, we developed a novel hydrogel using polysaccharide hemoadhican (HD) as the base material and polyethylene glycol diglycidyl ether (PEGDE) as the cross-linking agent. The HD hydrogels exhibit appropriate mechanical properties, injectability, and excellent cytocompatibility. We demonstrate resistance to protein adsorption and L929 fibroblast cell adhesion to the HD hydrogel. The biodegradability and efficacy against peritoneal adhesion are further evaluated in C57BL/6 mice. Our results suggest a potential strategy for anti-postoperative tissue adhesion barrier biomaterials.

Low-impedance tissue-device interface using homogeneously conductive hydrogels chemically bonded to stretchable bioelectronics

Stretchable bioelectronics has notably contributed to the advancement of continuous health monitoring and point-of-care type health care. However, microscale nonconformal contact and locally dehydrated interface limit performance, especially in dynamic environments. Therefore, hydrogels can be a promising interfacial material for the stretchable bioelectronics due to their unique advantages including tissue-like softness, water-rich property, and biocompatibility. However, there are still practical challenges in terms of their electrical performance, material homogeneity, and monolithic integration with stretchable devices. Here, we report the synthesis of a homogeneously conductive polyacrylamide hydrogel with an exceptionally low impedance (~21 ohms) and a reasonably high conductivity (~24 S/cm) by incorporating polyaniline-decorated poly(3,4-ethylenedioxythiophene:polystyrene). We also establish robust adhesion (interfacial toughness: ~296.7 J/m2) and reliable integration between the conductive hydrogel and the stretchable device through on-device polymerization as well as covalent and hydrogen bonding. These strategies enable the fabrication of a stretchable multichannel sensor array for the high-quality on-skin impedance and pH measurements under in vitro and in vivo circumstances.

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

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

Stretchable, Self-Rolled, Microfluidic Electronics Enable Conformable Neural Interfaces of Brain and Vagus Neuromodulation

Implantable neuroelectronic interfaces have gained significant importance in long-term brain-computer interfacing and neuroscience therapy. However, due to the mechanical and geometrical mismatches between the electrode-nerve interfaces, personalized and compatible neural interfaces remain serious issues for peripheral neuromodulation. This study introduces the stretchable and flexible electronics class as a self-rolled neural interface for neurological diagnosis and modulation. These stretchable electronics are made from liquid metal-polymer conductors with a high resolution of 30 μm using microfluidic printing technology. They exhibit high conformability and stretchability (over 600% strain) during body movements and have good biocompatibility during long-term implantation (over 8 weeks). These stretchable electronics offer real-time monitoring of epileptiform activities with excellent conformability to soft brain tissue. The study also develops self-rolled microfluidic electrodes that tightly wind the deforming nerves with minimal constraint (160 μm in diameter). The in vivo signal recording of the vagus and sciatic nerve demonstrates the potential of self-rolled cuff electrodes for sciatic and vagus neural modulation by recording action potential and reducing heart rate. The findings of this study suggest that the robust, easy-to-use self-rolled microfluidic electrodes may provide useful tools for compatible neuroelectronics and neural modulation.

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.

Portable Electrochemical Sensor for Micromotor Speed Monitoring

Autonomous movement promotes practical applications of micromotors. Understanding the moving speeds is a crucial step in micromotor studies. The current analysis method relies on an expensive optical microscope, which is limited to laboratory settings. Herein, we have developed a lightweight (0.15 g), portable (2.0 × 3.5 cm2), and low-cost (approximately $0.26) micromotor sensor (μ-Motor sensor), composed of water-sensitive materials for micromotor speed monitoring. Moving micromotors induce fluid flow, enhancing the evaporation rate of the liquid medium. Consequently, a high correlation between motor speed and water molecule concentration above the moving medium has been established. The μ-Motor sensor enables a real-time readout of the moving speed in various settings, with high accuracy (≥95% in the lab and ≥90% in field studies at a local beach). The μ-Motor sensor opens up a new way for detecting micro/nanomachine movements, illuminating future applications of micro/nanorobotics for diverse scenarios.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Silk fibroin carriers with sustained release capacity for treating neurological diseases

Neurological diseases such as traumatic brain injury, cerebral ischemia, Parkinson's, and Alzheimer's disease usually occur in the central and peripheral nervous system and result in nervous dysfunction, such as cognitive impairment and motor dysfunction. Long-term clinical intervention is necessary for neurological diseases where neural stem cell transplantation has made substantial progress. However, many risks remain for cell therapy, such as puncture bleeding, postoperative infection, low transplantation success rate, and tumor formation. Sustained drug delivery, which aims to maintain the desired steady-state drug concentrations in plasma or local injection sites, is considered as a feasible option to help overcome side effects and improve the therapeutic efficiency of drugs on neurological diseases. Natural polymers such as silk fibroin have excellent biocompatibility, which can be prepared for various end-use material formats, such as microsphere, gel, coating/film, scaffold/conduit, microneedle, and enables the dynamic release of loaded drugs to achieve a desired therapeutic response. Sustained-release drug delivery systems are based on the mechanism of diffusion and degradation by altering the structures of silk fibroin and drugs, factors, and cells, which can induce nerve recovery and restore the function of the nervous system in a slow and persistent manner. Based on these desirable properties of silk fibroin as a carrier with sustained-release capacity, this paper discusses the role of various forms of silk fibroin-based drug delivery materials in treating neurological diseases in recent years.

Sprayable Berberine-Silk Fibroin Microspheres with Extracellular Matrix Anchoring Function Accelerate Infected Wound Healing through Antibacterial and Anti-inflammatory Effects

The conventional method of applying local medications for treating wound infections is often ineffective because of the dilution of drugs by the excess wound exudate. In addition, there have been insufficient studies investigating the adhesion between drug-loaded nanomaterials and cells or tissue. To address this intractable problem, berberine-silk fibroin microspheres (Ber@MPs) with an extracellular matrix-anchoring function were developed in this study. The microspheres were prepared from silk fibroin using the polyethylene glycol emulsion precipitation method. Subsequently, berberine was loaded onto the microspheres. Our results revealed that Ber@MPs firmly anchored to cells, continuously releasing berberine in the microenvironment. Moreover, both Ber@MPs and Ber@MPs-cell complexes exerted a strong and long-lasting antibacterial effect against Staphylococcus aureus and Staphylococcus epidermidis in the microenvironment, despite the large amount of wound exudate. In addition, Ber@MPs effectively resisted the inflammatory response induced by lipopolysaccharides and accelerated the migration of fibroblasts and neovascularization of endothelial cells cultured in inflammation-induced media. Finally, the in vivo experiments confirmed that the Ber@MP spray accelerated the healing of infected wounds via its antibacterial and anti-inflammatory effects. Therefore, this study provides a novel strategy for treating infected wounds in the presence of excess exudate.

A Nonswelling Hydrogel with Regenerable High Wet Tissue Adhesion for Bioelectronics

Reducing the swelling of tissue-adhesive hydrogels is crucial for maintaining stable tissue adhesion and inhibiting tissue inflammation. However, reported strategies for reducing swelling always result in a simultaneous decrease in the tissue adhesive strength of the hydrogel. Furthermore, once the covalent bonds break in the currently reported hydrogels, they cannot be rebuilt, and the hydrogel loses its tissue adhesive ability. In this work, a nonswelling hydrogel (named as "PAACP") possessing regenerable high tissue adhesion is synthesized by copolymerizing and crosslinking poly(vinyl butyral) with acrylic acid, gelatin, and chitosan-grafted N-acetyl-l-cysteine. The tissue adhesive strength of the obtained PAACP reaches 211.4 kPa, which is approximately ten times higher than that of the reported nonswelling hydrogels, and the hydrogel can be reused for multiple cycles. The as-prepared hydrogel shows great potential in soft bioelectronics, as muscle fatigue is successfully monitored via the electrode array and strain sensor integrated on PAACP substrates. The success of these bioelectronics offers potential applicability in the long-term diagnosis of muscle-related health conditions and prosthetic manipulations.

Silk chemistry and biomedical material designs

Silk fibroin has applications in different medical fields such as tissue engineering, regenerative medicine, drug delivery and medical devices. Advances in silk chemistry and biomaterial designs have yielded exciting tools for generating new silk-based materials and technologies. Selective chemistries can enhance or tune the features of silk, such as mechanics, biodegradability, processability and biological interactions, to address challenges in medically relevant materials (hydrogels, films, sponges and fibres). This Review details the design and utility of silk biomaterials for different applications, with particular focus on chemistry. This Review consists of three segments: silk protein fundamentals, silk chemistries and functionalization mechanisms. This is followed by a description of different crosslinking chemistries facilitating network formation, including the formation of composite biomaterials. Utility in the fields of tissue engineering, drug delivery, 3D printing, cell coatings, microfluidics and biosensors are highlighted. Looking to the future, we discuss silk biomaterial design strategies to continue to improve medical outcomes.

Recent Progress in Biomedical Sensors Based on Conducting Polymer Hydrogels

Biosensors are increasingly taking a more active role in health science. The current needs for the constant monitoring of biomedical signals, as well as the growing spending on public health, make it necessary to search for materials with a combination of properties such as biocompatibility, electroactivity, resorption, and high selectivity to certain bioanalytes. Conducting polymer hydrogels seem to be a very promising materials, since they present many of the necessary properties to be used as biosensors. Furthermore, their properties can be shaped and enhanced by designing conductive polymer hydrogel-based composites with more specific functionalities depending on the end application. This work will review the recent state of the art of different biological hydrogels for biosensor applications, discuss the properties of the different components alone and in combination, and reveal their high potential as candidate materials in the fabrication of all-organic diagnostic, wearable, and implantable sensor devices.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Bioactive polymer-enabled conformal neural interface and its application strategies

Neural interface is a powerful tool to control the varying neuron activities in the brain, where the performance can directly affect the quality of recording neural signals and the reliability of in vivo connection between the brain and external equipment. Recent advances in bioelectronic innovation have provided promising pathways to fabricate flexible electrodes by integrating electrodes on bioactive polymer substrates. These bioactive polymer-based electrodes can enable the conformal contact with irregular tissue and result in low inflammation when compared to conventional rigid inorganic electrodes. In this review, we focus on the use of silk fibroin and cellulose biopolymers as well as certain synthetic polymers to offer the desired flexibility for constructing electrode substrates for a conformal neural interface. First, the development of a neural interface is reviewed, and the signal recording methods and tissue response features of the implanted electrodes are discussed in terms of biocompatibility and flexibility of corresponding neural interfaces. Following this, the material selection, structure design and integration of conformal neural interfaces accompanied by their effective applications are described. Finally, we offer our perspectives on the evolution of desired bioactive polymer-enabled neural interfaces, regarding the biocompatibility, electrical properties and mechanical softness.

Electron Conductive and Transparent Hydrogels for Recording Brain Neural Signals and Neuromodulation

Recording brain neural signals and optogenetic neuromodulations open frontiers in decoding brain neural information and neurodegenerative disease therapeutics. Conventional implantable probes suffer from modulus mismatch with biological tissues and an irreconcilable tradeoff between transparency and electron conductivity. Herein, a strategy is proposed to address these tradeoffs, which generates conductive and transparent hydrogels with polypyrrole-decorated microgels as cross-linkers. The optical transparency of the electrodes can be attributed to the special structures that allow light waves to bypass the microgel particles and minimize their interaction. Demonstrated by probing the hippocampus of rat brains, the biomimetic electrode shows a prolonged capacity for simultaneous optogenetic neuromodulation and recording of brain neural signals. More importantly, an intriguing brain-machine interaction is realized, which involves signal input to the brain, brain neural signal generation, and controlling limb behaviors. This breakthrough work represents a significant scientific advancement toward decoding brain neural information and developing neurodegenerative disease therapies.

Transparent neural interfaces: challenges and solutions of microengineered multimodal implants designed to measure intact neuronal populations using high-resolution electrophysiology and microscopy simultaneously

The aim of this review is to present a comprehensive overview of the feasibility of using transparent neural interfaces in multimodal in vivo experiments on the central nervous system. Multimodal electrophysiological and neuroimaging approaches hold great potential for revealing the anatomical and functional connectivity of neuronal ensembles in the intact brain. Multimodal approaches are less time-consuming and require fewer experimental animals as researchers obtain denser, complex data during the combined experiments. Creating devices that provide high-resolution, artifact-free neural recordings while facilitating the interrogation or stimulation of underlying anatomical features is currently one of the greatest challenges in the field of neuroengineering. There are numerous articles highlighting the trade-offs between the design and development of transparent neural interfaces; however, a comprehensive overview of the efforts in material science and technology has not been reported. Our present work fills this gap in knowledge by introducing the latest micro- and nanoengineered solutions for fabricating substrate and conductive components. Here, the limitations and improvements in electrical, optical, and mechanical properties, the stability and longevity of the integrated features, and biocompatibility during in vivo use are discussed.

Multimodal Electrocorticogram Active Electrode Array Based on Zinc Oxide-Thin Film Transistors

Active electrocorticogram (ECoG) electrodes can amplify weak electrophysiological signals and improve anti-interference ability; however, traditional active electrodes are opaque and cannot realize photoelectric collaborative observation. In this study, an active and fully transparent ECoG array based on zinc oxide thin-film transistors (ZnO TFTs) is developed as a local neural signal amplifier for electrophysiological monitoring. The transparency of the proposed ECoG array is up to 85%, which is superior to that of the previously reported active electrode arrays. Various electrical characterizations have demonstrated its ability to record electrophysiological signals with a higher signal-to-noise ratio of 19.9 dB compared to the Au grid (13.2 dB). The high transparency of the ZnO-TFT electrode array allows the concurrent collection of high-quality electrophysiological signals (32.2 dB) under direct optical stimulation of the optogenetic mice brain. The ECoG array can also work under 7-Tesla magnetic resonance imaging to record local brain signals without affecting brain tissue imaging. As the most transparent active ECoG array to date, it provides a powerful multimodal tool for brain observation, including recording brain activity under synchronized optical modulation and 7-Tesla magnetic resonance imaging.

Integration of hydrogels in microfabrication processes for bioelectronic medicine: Progress and outlook

Recent research aiming at the development of electroceuticals for the treatment of medical conditions such as degenerative diseases, cardiac arrhythmia and chronic pain, has given rise to microfabricated implanted bioelectronic devices capable of interacting with host biological tissues in synergistic modalities. Owing to their multimodal affinity to biological tissues, hydrogels have emerged as promising interface materials for bioelectronic devices. Here, we review the state-of-the-art and forefront in the techniques used by research groups for the integration of hydrogels into the microfabrication processes of bioelectronic devices, and present the manufacturability challenges to unlock their further clinical deployment.

Regulation of the Inevitable Water-Responsivity of Silk Fibroin Biopolymer by Polar Amino Acid Activation

In nature, water is vital for maintaining homeostasis. Particularly, organisms (e.g., plant leaf, bird feather) exploit water fluidics for motions. Hydration-adaptive crystallization is the representative water-responsive actuation of biopolymers. This crystallization has inspired the development of intelligent human-robot interfaces. At the same time, it hinders the consistent adhesion of tissue adhesive. As hydration-adaptive crystallization is inevitable, the on-demand control of crystallization is desirable in the innovative biopolymeric biomedical systems. To this end, this study developed an amino acid-based technology to artificially up- or down-regulate the inevitable crystallization of silk fibroin. A case II diffusion model was constructed, and it revealed that the activity of polar amino acid is related to crystallization kinetics. Furthermore, the water dynamics study suggested that active amino acid stabilizes crystallization-triggering water molecules. As a proof-of-concept, we verified that a 30% increase in the activity of serine resulted in a 50% decrease in the crystallization rate. Furthermore, the active amino acid-based suppression of hydration-adaptive crystallization enabled the silk fibroin to keep its robust adhesion (approximately 160 kJ m^-3) by reducing the water-induced loss of adhesive force. The proposed silk fibroin was demonstrated as a stable tissue adhesive applied on ex vivo porcine mandible tissue. This amino acid-based regulation of hydration-adaptive crystallization will pioneer next-generation biopolymer-based healthcare.

Bioinspired silk fibroin materials: From silk building blocks extraction and reconstruction to advanced biomedical applications

Silk fibroin has become a promising biomaterial owing to its remarkable mechanical property, biocompatibility, biodegradability, and sufficient supply. However, it is difficult to directly construct materials with other formats except for yarn, fabric and nonwoven based on natural silk. A promising bioinspired strategy is firstly extracting desired building blocks of silk, then reconstructing them into functional regenerated silk fibroin (RSF) materials with controllable formats and structures. This strategy could give it excellent processability and modifiability, thus well meet the diversified needs in biomedical applications. Recently, to engineer RSF materials with properties similar to or beyond the hierarchical structured natural silk, novel extraction and reconstruction strategies have been developed. In this review, we seek to describe varied building blocks of silk at different levels used in biomedical field and their effective extraction and reconstruction strategies. This review also present recent discoveries and research progresses on how these functional RSF biomaterials used in advanced biomedical applications, especially in the fields of cell-material interactions, soft tissue regeneration, and flexible bioelectronic devices. Finally, potential study and application for future opportunities, and current challenges for these bioinspired strategies and corresponding usage were also comprehensively discussed. In this way, it aims to provide valuable references for the design and modification of novel silk biomaterials, and further promote the high-quality-utilization of silk or other biopolymers.

Fast Self-Assembly of Photonic Crystal Hydrogel for Wearable Strain and Temperature Sensor

Structural colors from photonic crystals (PCs) have attracted emerging attention in the research area of wearable sensors. Conventional self-assembly of PC takes days to weeks. Here, a fast self-assembly method of PC with horizontal precipitation of silica nanoparticles (NPs) in a polydimethylsiloxane fence, which can be completed within 1-4 h depending on the fence parameters, is introduced. The resultant PC exhibits tunable structural colors in the entire visible spectrum. With infiltration of composite hydrogels containing acrylic acid, acrylamide, chitosan, and carbon nanotubes (CNTs) into the gaps of NPs to form an inverse opal PC, a structural color hydrogel that can quickly respond to different stimuli, including strain and temperature, is obtained. Moreover, with the addition of CNTs, the composite PC hydrogel can also output an electronic signal together with optical color changes. Based on these extraordinary responsive behaviors, the PC hydrogel sensor for quantitative feedback to external stimuli of stretching, bending, pressing, and thermal stimuli, with brilliant color change and electronic signal outputs simultaneously, is demonstrated. This fast-assembled PC hydrogel with excellent responsive properties has great potential for applications in wearable devices, mechanical sensors, temperature sensors, and colorimetric displays.

Preparation and Characterization of Natural Silk Fibroin Hydrogel for Protein Drug Delivery

In recent years, hydrogels have been widely used as drug carriers, especially in the area of protein delivery. The natural silk fibroin produced from cocoons of the Bombyx mori silkworm possesses excellent biocompatibility, significant bioactivity, and biodegradability. Therefore, silk fibroin-based hydrogels are arousing widespread interest in biomedical research. In this study, a process for extracting natural silk fibroin from raw silk textile yarns was established, and three aqueous solutions of silk fibroin with different molecular weight distributions were successfully prepared by controlling the degumming time. Silk fibroin was dispersed in the aqueous solution as “spherical” aggregate particles, and the smaller particles continuously accumulated into large particles. Finally, a silk fibroin hydrogel network was formed. A rheological analysis showed that as the concentration of the silk fibroin hydrogel increased its storage modulus increased significantly. The degradation behavior of silk fibroin hydrogel in different media verified its excellent stability, and the prepared silk fibroin hydrogel had good biocompatibility and an excellent drug-loading capacity. After the protein model drug BSA was loaded, the cumulative drug release within 12 h reached 80%. We hope that these investigations will promote the potential utilities of silk fibroin hydrogels in clinical medicine.

Engineering Natural and Recombinant Silks for Sustainable Biodevices

Silk fibroin (SF) is a structural protein derived from natural silkworm silks. Materials fabricated based on SF usually inherit extraordinary physical and biological properties, including high mechanical strength, toughness, optical transparency, tailorable biodegradability, and biocompatibility. Therefore, SF has attracted interest in the development of sustainable biodevices, especially for emergent bio-electronic technologies. To expand the function of current silk devices, the SF characteristic sequence has been used to synthesize recombinant silk proteins that benefit from SF and other functional peptides, such as stimuli-responsive elastin peptides. In addition to genetic engineering methods, innovated chemistry modification approaches and improved material processing techniques have also been developed for fabricating advanced silk materials with tailored chemical features and nanostructures. Herein, this review summarizes various methods to synthesize functional silk-based materials from different perspectives. This review also highlights the recent advances in the applications of natural and recombinant silks in tissue regeneration, soft robotics, and biosensors, using B. mori SF and silk-elastin-like proteins (SELPs) as examples.