Biological Tissue-Inspired Ultrasoft, Ultrathin, and Mechanically Enhanced Microfiber Composite Hydrogel for Flexible Bioelectronics

Effects of fragmented polycaprolactone electrospun nanofiber in a hyaluronic acid hydrogel on fibroblasts

Hyaluronic acid (HA) hydrogels have shown promise as biomaterials for soft tissue engineering applications due to their biocompatibility and ability to mimic the extracellular matrix (ECM). However, their limited cell adhesion properties and the need for improved crosslinking methods have hindered their widespread use. In this study, we developed an ECM-mimicking HA hydrogel reinforced with alkaline hydrolyzed (1 M NaOH) fragmented (1.5 cm×1.5 cm) electrospun polycaprolactone (PCL) fibers to enhance cell adhesion and mechanical properties of HA hydrogel. Formation of HA hydrogel was achieved through a thiol-ene click reaction, which is initiated by exposure to visible blue light-activated biocompatible photoinitiator, riboflavin phosphate. The incorporation of alkaline hydrolyzed PCL fiber fragments (PFF) (0 %, 0.1 %, and 1 % w/v) into HA hydrogel precursor solution significantly increased the mechanical stiffness of the HA hydrogel, with the storage modulus ranging from 18.6 ± 0.7 Pa to 216.0 ± 38.2 Pa. The cytocompatibility of the PCL fiber-reinforced HA hydrogel was evaluated using NIH/3T3 fibroblasts. The results demonstrated improved cell adhesion, proliferation, and enhanced cellular functions, including increased production of glycosaminoglycans (GAGs) and collagen, in the PCL fiber-reinforced HA hydrogel compared to the control HA hydrogel. These findings suggest that the developed PCL fiber-reinforced HA hydrogel system, with tunable mechanical properties and excellent cytocompatibility, has potential applications in soft tissue engineering and regenerative medicine.

Scalable Layered Heterogeneous Hydrogel Fibers with Strain-Induced Crystallization for Tough, Resilient, and Highly Conductive Soft Bioelectronics

The advancement of soft bioelectronics hinges critically on the electromechanical properties of hydrogels. Despite ongoing research into diverse material and structural strategies to enhance these properties, producing hydrogels that are simultaneously tough, resilient, and highly conductive for long-term, dynamic physiological monitoring remains a formidable challenge. Here, a strategy utilizing scalable layered heterogeneous hydrogel fibers (LHHFs) is introduced that enables synergistic electromechanical modulation of hydrogels. High toughness (1.4 MJ m-3) and resilience (over 92% recovery from 200% strain) of LHHFs are achieved through a damage-free toughening mechanism that involves dense long-chain entanglements and reversible strain-induced crystallization of sodium polyacrylate. The unique symmetrical layered structure of LHHFs, featuring distinct electrical and mechanical functional layers, facilitates the mixing of multi-walled carbon nanotubes to significantly enhance electrical conductivity (192.7 S m-1) without compromising toughness and resilience. Furthermore, high-performance LHHF capacitive iontronic strain/pressure sensors and epidermal electrodes are developed, capable of accurately and stably capturing biomechanical and bioelectrical signals from the human body under long-term, dynamic conditions. The LHHF offers a promising route for developing hydrogels with uniquely integrated electromechanical attributes, advancing practical wearable healthcare applications.

Wood Cell Wall Nanoengineering toward Anisotropic, Strong, and Flexible Cellulosic Hydrogel Sensors

Achieving highly ionic conductive hydrogels from natural wood remains challenging owing to their insufficient surface area and low number of active sites on the cell wall. This study proposes a viable strategy to design a strong and anisotropic wood-based hydrogel through cell wall nanoengineering. By manipulating the microstructure of the wood cell wall, a flexible cellulosic hydrogel is achieved through Schiff base bonding via the polyacrylamide and cellulose molecular chains. This results in excellent flexibility and mechanical properties of the wood hydrogel with tensile strengths of 22.3 and 6.1 MPa in the longitudinal and transverse directions, respectively. Moreover, confining aqueous salt electrolytes within the porous structure gives anisotropic ionic conductivities (19.5 and 6.02 S/m in the longitudinal and transverse directions, respectively). The wood-based hydrogel sensor has a favorable sensitivity and a stable working performance at a low temperature of -25 °C in monitoring human motions, thereby demonstrating great potential applications in wearable sensor devices.

Strong and anti-swelling nanofibrous hydrogel composites inspired by biological tissue for amphibious motion sensors

Conductive hydrogel-based sensors are increasingly favored for flexible electronics due to their skin-like characteristics. However, conventional hydrogels suffer from significant swelling in humid environments and poor mechanical properties which largely restrict their applications in wearable electronic devices, especially for underwater sensing. Herein, drawing inspiration from the extracellular matrix (ECM) structure, a TPU-PVAc@AgNPs/MXene nanofibrous hydrogel composite (TPAMH) with excellent mechanical robustness and anti-swelling properties is developed. The TPAMH nanofibrous hydrogel composite is created by integrating the silver nanoparticles (AgNPs) and MXene nanosheets into an interwoven network comprising of stiff TPU nanofibers as the fibril scaffold and formic acid-crosslinked PVA hydrogel fibers as the elastic matrix (PVAc). Benefiting from the unique ECM structure, the obtained nanofibrous hydrogel composites exhibit exceptional tensile strength (4.47 MPa), remarkable elongation at break (621%), excellent anti-swelling properties, and high detection sensitivity (maximum gauge factor = 105.02), which are sufficient to monitor body motions in both air and water environments effectively. They can detect large strain movements of fingers, elbows, wrists, and knees, as well as small strain physiological signals such as frown, smile, and pulse beats, with high accuracy. Particularly noteworthy is their ability to accurately identify underwater multidirectional motions and facilitate underwater smart alarms using Morse code.

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.

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.

Ultrasoft Long-Lasting Reusable Hydrogel-Based Sensor Patch for Biosignal Recording

Here, we report an ultrasoft extra long-lasting, reusable hydrogel-based sensor that enables high-quality electrophysiological recording with low-motion artifacts. The developed sensor can be used and stored in an ambient environment for months before being reused. The developed sensor is made of a self-adhesive electrical-conductivity-enhanced ultrasoft hydrogel mounted in an Ecoflex-based frame. The hydrogel's conductivity was enhanced by incorporating polypyrrole (PPy), resulting in a conductivity of 0.25 S m-1. Young's modulus of the sensor is only 12.9 kPa, and it is stretchable up to 190%. The sensor was successfully used for electrocardiography (ECG) and electromyography (EMG). Our results indicate that using the developed hydrogel-based sensor, the signal-to-noise ratio of recorded electrophysiological signals was improved in comparison to that when medical-grade silver/silver chloride (Ag/AgCl) wet gel electrodes were used (33.55 dB in comparison to 22.16 dB). Due to the ultra-softness, high stretchability, and self-adhesion of the developed sensor, it can conform to the skin and, therefore, shows low susceptibility to motion. In addition, the sensor shows no sign of irritation or allergic reaction, which usually occurs after long-term wearing of medical-grade Ag/AgCl wet gel electrodes on the skin. Further, the sensor is fabricated using a low-cost and scalable fabrication process.

Recent Advances in Poly(vinyl alcohol)-Based Hydrogels

Poly(vinyl alcohol) (PVA) is a versatile synthetic polymer, used for the design of hydrogels, porous membranes and films. Its solubility in water, film- and hydrogel-forming capabilities, non-toxicity, crystallinity and excellent mechanical properties, chemical inertness and stability towards biological fluids, superior oxygen and gas barrier properties, good printability and availability (relatively low production cost) are the main aspects that make PVA suitable for a variety of applications, from biomedical and pharmaceutical uses to sensing devices, packaging materials or wastewater treatment. However, pure PVA materials present low stability in water, limited flexibility and poor biocompatibility and biodegradability, which restrict its use alone in various applications. PVA mixed with other synthetic polymers or biomolecules (polysaccharides, proteins, peptides, amino acids etc.), as well as with inorganic/organic compounds, generates a wide variety of materials in which PVA's shortcomings are considerably improved, and new functionalities are obtained. Also, PVA's chemical transformation brings new features and opens the door for new and unexpected uses. The present review is focused on recent advances in PVA-based hydrogels.

Core-Shell Gel Nanofiber Scaffolds Constructed by Microfluidic Spinning toward Wound Repair and Tissue Regeneration

Growing demand for wound care resulting from the increasing chronic diseases and trauma brings intense pressure to global medical health service system. Artificial skin provides mechanical and microenvironmental support for wound, which is crucial in wound healing and tissue regeneration. However, challenges still remain in the clinical application of artificial skin since the lack of the synergy effect of necessary performance. In this study, a multi-functional artificial skin is fabricated through microfluidic spinning technology by using core-shell gel nanofiber scaffolds (NFSs). This strategy can precisely manipulate the microstructure of artificial skin under microscale. The as-prepared artificial skin demonstrates superior characteristics including surface wettability, breathability, high mechanical strength, strain sensitivity, biocompatibility and biodegradability. Notably, this artificial skin has the capability to deliver medications in a controlled and sustained manner, thereby accelerating the wound healing process. This innovative approach paves the way for the development of a new generation of artificial skin and introduces a novel concept for the structural design of the unique core-shell gel NFSs.

Degradable and Recyclable Hydrogels for Sustainable Bioelectronics

Hydrogel bioelectronics has been widely used in wearable sensors, electronic skin, human-machine interfaces, and implantable tissue-electrode interfaces, providing great convenience for human health, safety, and education. The generation of electronic waste from bioelectronic devices jeopardizes human health and the natural environment. The development of degradable and recyclable hydrogels is recognized as a paradigm for realizing the next generation of environmentally friendly and sustainable bioelectronics. This review first summarizes the wide range of applications for bioelectronics, including wearable and implantable devices. Then, the employment of natural and synthetic polymers in hydrogel bioelectronics is discussed in terms of degradability and recyclability. Finally, this work provides constructive thoughts and perspectives on the current challenges toward hydrogel bioelectronics, providing valuable insights and guidance for the future evolution of sustainable hydrogel bioelectronics.

Ferried Albumin-Inspired Bioadhesive With Dynamic Interfacial Bonds for Emergency Rescue

Emergency prehospital wound closure and hemorrhage control are the first priorities for life-saving. Majority of bioadhesives form bonds with tissues through irreversible cross-linking, and the remobilization of misalignment may cause severe secondary damage to tissues. Therefore, developing an adhesive that can quickly and tolerably adhere to traumatized dynamic tissue or organ surfaces in emergency situations is a major challenge. Inspired by the structure of human serum albumin (HSA), a branched polymer with multitentacled sulfhydryl is synthesized, then, an instant and fault-tolerant tough wet-tissue adhesion (IFA) hydrogel is prepared. Adhesive application time is just 5 s (interfacial toughness of ≈580 J m-2), and favorable tissue-adhesion is maintained after ten cycles. IFA hydrogel shows unchangeable adhesive performance after 1 month of storage based on the internal oxidation-reduction mechanism. It not only can efficiently seal various organs but also achieves effective hemostasis in models of the rat femoral artery and rabbit-ear artery. This work also proposes an effective strategy for controllable adhesion, enabling the production of asymmetric adhesives with on-demand detachment. Importantly, IFA hydrogel has sound antioxidation, antibacterial property, hemocompatibility, and cytocompatibility. Hence, the HSA-inspired bioadhesive emerges as a promising first-aid supply for human-machine interface-based health management and non-invasive wound closure.

Multistrand Twisted Triboelectric Kevlar Yarns for Harvesting High Impact Energy, Body Injury Location and Levels Evaluation

Developing ultrahigh-strength fabric-based triboelectric nanogenerators for harvesting high-impact energy and sensing biomechanical signals is still a great challenge. Here, the constraints are addressed by design of a multistrand twisted triboelectric Kevlar (MTTK) yarn using conductive and non-conductive Kevlar fibers. Manufactured using a multistrand twisting process, the MTTK yarn offers superior tensile strength (372 MPa), compared to current triboelectric yarns. In addition, a self-powered impact sensing fabric patch (SP-ISFP) comprising signal acquisition, processing, communication circuit, and MTTK yarns is integrated. The SP-ISFP features withstanding impact (4 GPa) and a sensitivity and response time under the high impact condition (59.68 V GPa-1; 0.4 s). Furthermore, a multi-channel smart bulletproof vest is developed by the array of 36 SP-ISFPs, enabling the reconstruction of impact mapping and assessment of body injury location and levels by real-time data acquisition. Their potential to reduce body injuries, professional security, and construct a multi-point personal vital signs dynamic monitoring platform holds great promise.

Diamond-Like Carbon Depositing on the Surface of Polylactide Membrane for Prevention of Adhesion Formation During Tendon Repair

Post-traumatic peritendinous adhesion presents a significant challenge in clinical medicine. This study proposes the use of diamond-like carbon (DLC) deposited on polylactic acid (PLA) membranes as a biophysical mechanism for anti-adhesion barrier to encase ruptured tendons in tendon-injured rats. The results indicate that PLA/DLC composite membrane exhibits more efficient anti-adhesion effect than PLA membrane, with histological score decreasing from 3.12 ± 0.27 to 2.20 ± 0.22 and anti-adhesion effectiveness increasing from 21.61% to 44.72%. Mechanistically, the abundant C=O bond functional groups on the surface of DLC can reduce reactive oxygen species level effectively; thus, the phosphorylation of NF-κB and M1 polarization of macrophages are inhibited. Consequently, excessive inflammatory response augmented by M1 macrophage-originated cytokines including interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) is largely reduced. For biocompatibility evaluation, PLA/DLC membrane is slowly absorbed within tissue and displays prolonged barrier effects compared to traditional PLA membranes. Further studies show the DLC depositing decelerates the release of degradation product lactic acid and its induction of macrophage M2 polarization by interfering esterase and PLA ester bonds, which further delays the fibrosis process. It was found that the PLA/DLC membrane possess an efficient biophysical mechanism for treatment of peritendinous adhesion.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring

Hydrogel-enabled skin bioelectronics that can continuously monitor health for extended periods is crucial for early disease detection and treatment. However, it is challenging to engineer ultrathin gas-permeable hydrogel sensors that can self-adhere to the human skin for long-term daily use (>1 week). Here, we present a ~10-micrometer-thick polyurethane nanomesh-reinforced gas-permeable hydrogel sensor that can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 8 days under daily life conditions. This research involves two key steps: (i) material design by gelatin-based thermal-dependent phase change hydrogels and (ii) robust thinness geometry achieved through nanomesh reinforcement. The resulting ultrathin hydrogels exhibit a thickness of ~10 micrometers with superior mechanical robustness, high skin adhesion, gas permeability, and anti-drying performance. To highlight the potential applications in early disease detection and treatment that leverage the collective features, we demonstrate the use of ultrathin gas-permeable hydrogels for long-term, continuous high-precision electrophysiological monitoring under daily life conditions up to 8 days.

Fabrication of Tough Double-Network Hydrogels from Highly Cross-Linked Brittle Neutral Networks Using Alkaline Hydrolysis

This paper describes a simple method to synthesize tough hydrogels from a highly cross-linked neutral network. It was found that applying alkaline hydrolysis to a highly cross-linked hydrogel synthesized from acrylamide (AAm) can increase its swelling ratio dramatically. Double-network (DN) hydrogels synthesized from polymerization of loosely cross-linked AAm networks inside a highly cross-linked AAm gel were not tough. However, repeating the same recipes with a second polymerization step to synthesize a DN hydrogel from a hydrolyzed highly cross-linked AAm gel resulted in tough hydrogels. Those gels exhibited finite tensile behavior similar to that of conventional DN hydrogels. Moreover, craze-like patterns were observed during tensile loading of a DN hydrogel synthesized from a hydrolyzed highly cross-linked first network and a loosely cross-linked second network. The patterns remained in the gel even after strain hardening at high stretch ratios. The craze-like pattern formation was suppressed by increasing the concentration of cross-linking monomer in the second polymerization step. Crack propagation in DN hydrogels synthesized using hydrolysis was also studied by applying a tensile load on notched specimens.

Artificial Intelligence Meets Flexible Sensors: Emerging Smart Flexible Sensing Systems Driven by Machine Learning and Artificial Synapses

The recent wave of the artificial intelligence (AI) revolution has aroused unprecedented interest in the intelligentialize of human society. As an essential component that bridges the physical world and digital signals, flexible sensors are evolving from a single sensing element to a smarter system, which is capable of highly efficient acquisition, analysis, and even perception of vast, multifaceted data. While challenging from a manual perspective, the development of intelligent flexible sensing has been remarkably facilitated owing to the rapid advances of brain-inspired AI innovations from both the algorithm (machine learning) and the framework (artificial synapses) level. This review presents the recent progress of the emerging AI-driven, intelligent flexible sensing systems. The basic concept of machine learning and artificial synapses are introduced. The new enabling features induced by the fusion of AI and flexible sensing are comprehensively reviewed, which significantly advances the applications such as flexible sensory systems, soft/humanoid robotics, and human activity monitoring. As two of the most profound innovations in the twenty-first century, the deep incorporation of flexible sensing and AI technology holds tremendous potential for creating a smarter world for human beings.

Sensor-Based Wearable Systems for Monitoring Human Motion and Posture: A Review

In recent years, marked progress has been made in wearable technology for human motion and posture recognition in the areas of assisted training, medical health, VR/AR, etc. This paper systematically reviews the status quo of wearable sensing systems for human motion capture and posture recognition from three aspects, which are monitoring indicators, sensors, and system design. In particular, it summarizes the monitoring indicators closely related to human posture changes, such as trunk, joints, and limbs, and analyzes in detail the types, numbers, locations, installation methods, and advantages and disadvantages of sensors in different monitoring systems. Finally, it is concluded that future research in this area will emphasize monitoring accuracy, data security, wearing comfort, and durability. This review provides a reference for the future development of wearable sensing systems for human motion capture.

Skin-Inspired Patterned Hydrogel with Strain-Stiffening Capability for Strain Sensors

Flexible materials with ionic conductivity and stretchability are indispensable in emerging fields of flexible electronic devices as sensing and protecting layers. However, designing robust sensing materials with skin-like compliance remains challenging because of the contradiction between softness and strength. Herein, inspired by the modulus-contrast hierarchical structure of biological skin, we fabricated a biomimetic hydrogel with strain-stiffening capability by embedding the stiff array of poly(acrylic acid) (PAAc) in the soft polyacrylamide (PAAm) hydrogel. The stress distribution in both stiff and soft domains can be regulated by changing the arrangement of patterns, thus improving the mechanical properties of the patterned hydrogel. As expected, the resulting patterned hydrogel showed its nonlinear mechanical properties, which afforded a high strength of 1.20 MPa while maintaining a low initial Young's modulus of 31.0 kPa. Moreover, the array of PAAc enables the patterned hydrogel to possess protonic conductivity in the absence of additional ionic salts, thus endowing the patterned hydrogel with the ability to serve as a strain sensor for monitoring human motion.