Customization of Conductive Elastomer Based on PVA/PEI for Stretchable Sensors

Bioactive Deep Eutectic Solvent-Involved Sprayable Versatile Hydrogel for Monkeypox Virus Lesions Treatment

To address the issues of infectious virus, bacterial secondary infections, skin pigmentation, and scarring caused by monkeypox virus (MPXV), a sprayable hydrogel with versatile functions was developed with comprehensive properties. Based on current research, the bioactive deep eutectic solvent (DES) of rosmarinic acid-proanthocyanidin-glycol (RPG) was designed and synthesized as active agent, and molecular docking was applied to discover its binding to MPXV proteins through H-bonds and van der Waals interactions, and the docking results show the binding energies between RA, PC, Gly and MPXV proteins are -58.7188, -50.2311, and -18.4755 kcal/mol, respectively. Additionally, poly(vinyl alcohol) (PVA), borate, and xylitol (Xyl) were integrated with RPG to prepare the PB-RPG-Xyl hydrogel, which was characterized by popular ways. The pH-responsive properties of the hydrogel accelerated the release of RPG under acidic conditions, resulting in an increased cumulative release percentage of 84.83% at pH 5.5 at 210 min. Besides that, it was proved to have the expected sprayability, self-healing, adhesion, and shape-adaptability. The results of molecular dynamic simulation were meaningful to understanding its formation and self-healing mechanisms. Furthermore, the hydrogel shows ideal degradability, removability, and biocompatibility. Lastly, its multiple functions were systematically explored, including UV-blocking, blood clotting, cooling, antioxidant, antibacterial, and virus inhibition properties. The developed sprayable PB-RPG-Xyl hydrogel represents the first promising dressing based on natural bioactive DES for MPXV lesions management, which not only expands the application of green solvents in health care but also provides a convenient and effective treatment process for MPXV infection in the face of difficult skin lesions and complex treatment needs.

Bioinspired adaptive lipid-integrated bilayer coating for enhancing dynamic water retention in hydrogel-based flexible sensors

While hydrogel-based flexible sensors find extensive applications in fields such as medicine and robotics, their performance can be hindered by the rapid evaporation of water, leading to diminished sensitivity and mechanical durability. Despite the exploration of some effective solutions, such as introducing organic solvents, electrolytes, and elastomer composites, these approaches still suffer from problems including diminished conductivity, interface misalignment, and insufficient protection under dynamic conditions. Inspired by cell membrane structures, we developed an adaptive lipid-integrated bilayer coating (ALIBC) to enhance water retention in hydrogel-based sensors. Lipid layers and long-chain amphiphilic molecules are used as compact coating and anchoring agents on the hydrogel surface, mimicking the roles of lipids and membrane proteins in cell membranes, while spare lipids from aggregates within hydrogels can migrate to the surface to combat dehydration under deformation. This lipid-integrated bilayer coating prevents the water evaporation of hydrogels at both static and dynamic states without affecting the inherent flexibility, conductivity, and no cytotoxicity. Hydrogel-based sensors with ALIBC exhibited significantly enhanced performance in conditions of body temperature, extensive deformation, and long-term dynamic sensing. This work presents a general approach for precisely controlling the water-retaining capacity of hydrogels and hydrogel-based sensors without compromising their intrinsic physical properties.

Highly Conducting and Ultra-Stretchable Wearable Ionic Liquid-Free Transducer for Wireless Monitoring of Physical Motions

Wearable strain transducers are poised to transform the field of healthcare owing to the promise of personalized devices capable of real-time collection of human physiological health indicators. For instance, monitoring patients' progress following injury and/or surgery during physiotherapy is crucial but rarely performed outside clinics. Herein, multifunctional liquid-free ionic elastomers are designed through the volume effect and the formation of dynamic hydrogen bond networks between polyvinyl alcohol (PVA) and weak acids (phosphoric acid, phytic acid, formic acid, citric acid). An ultra-stretchable (4600% strain), highly conducting (10 mS cm-1), self-repairable (77% of initial strain), and adhesive ionic elastomer is obtained at high loadings of phytic acid (4:1 weight to PVA). Moreover, the elastomer displayed durable performances, with intact mechanical properties after a year of storage. The elastomer is used as a transducer to monitor human motions in a device comprising an ESP32-based development board. The device detected walking and/or running biomechanics and communicated motion-sensing data (i.e., amplitude, frequency) wirelessly. The reported technology can also be applied to other body parts to monitor recovery after injury and/or surgery and inform practitioners of motion biomechanics remotely and in real time to increase convalescence effectiveness, reduce clinic appointments, and prevent injuries.

Highly Elastic, Fatigue-Resistant, and Antifreezing MXene Functionalized Organohydrogels as Flexible Pressure Sensors for Human Motion Monitoring

Conductive organohydrogels-based flexible pressure sensors have gained considerable attention in health monitoring, artificial skin, and human-computer interaction due to their excellent biocompatibility, wearability, and versatility. However, hydrogels' unsatisfactory mechanical and unstable electrical properties hinder their comprehensive application. Herein, an elastic, fatigue-resistant, and antifreezing poly(vinyl alcohol) (PVA)/lipoic acid (LA) organohydrogel with a double-network structure and reversible cross-linking interactions has been designed, and MXene as a conductive filler is functionalized into organohydrogel to further enhance the diverse sensing performance of flexible pressure sensors. The as-fabricated MXene-based PVA/LA organohydrogels (PLBM) exhibit stable fatigue resistance for over 450 cycles under 40% compressive strain, excellent elasticity, antifreezing properties (<-20 °C), and degradability. Furthermore, the pressure sensors based on the PLBM organohydrogels show a fast response time (62 ms), high sensitivity (S = 0.0402 kPa-1), and excellent stability (over 1000 cycles). The exceptional performance enables the sensors to monitor human movements, such as joint flexion and throat swallowing. Moreover, the sensors integrating with the one-dimensional convolutional neural networks and the long-short-term memory networks deep learning algorithms have been developed to recognize letters with a 93.75% accuracy, representing enormous potential in monitoring human motion and human-computer interaction.

Triggering Ion Diffusion and Electron Transport Dual Pathways for High Efficiency Electrochemical Li+ Extraction

Efficient electrochemical Li+ adsorption holds significant promise for lithium extraction, while the mismatched rate between Li+ diffusion and electron transport within the electrode material impedes the electrochemical activity and restricts the adsorption efficiency. To address this challenge, herein, we rationally design and integrate the ion and electron dual-conducting poly(vinyl alcohol)-polyaniline (PVA-PANI) copolymer (CP) within the H1.6Mn1.6O4 (HMO) electrode matrix to facilitate Li+ diffusion and electron transport. The Li+ diffusion coefficient (DLi+) increased from 3.03 × 10-10 to 5.92 × 10-10 cm2/s, while the charge transfer resistance (Rct) decreased from 53.73 to 29.57 ohm. Consequently, the HMO@CP electrode exhibits superior adsorption kinetics and a state-of-the-art high adsorption capacity of up to 49.48 mg/g. Comprehensive mechanistic studies reveal that the negatively charged hydroxyl groups (-OH) in PVA accelerate Li+ diffusion and that the conjugated structure and redox-active quinoid sites in PANI offer denser electron distribution and promote electron transport. This synergistic effect in CP significantly enhanced Li+ diffusion and electron transport, leading to electrochemical activity and adsorption efficiency. Our work highlights the critical role of simultaneously regulating the ion diffusion and electron transport dual pathways for optimizing Li+ adsorption performance and inspires development of the next generation electrochemical adsorption electrodes.

High-strength fibrous sensors with an enhanced aggregate state for biomechanical monitoring of the Achilles tendon

Continuous monitoring of biomechanical signals generated from the injured Achilles tendon is essential for the deep understanding of the recovery or rehabilitation process, thus decreasing the risk of secondary injuries. With tissue-like components and adjustable properties, hydrogel-based biomechanical sensors are considered promising materials for human motion detection. However, existing hydrogels are characterized by inferior mechanical properties with strength and modulus typically lower than 1 MPa, as well as poor stability under physiological conditions, which hampers their applications in implantable devices. Moreover, acquiring the stress signal from collected electrical signals remains challenging. Herein, based on the regulation of polymer aggregation, a high-strength fibrous sensor composed of polyvinyl alcohol (PVA) and reduced graphene oxide (rGO) for in vivo monitoring is prepared through a two-step procedure, including freeze-thaw and freeze-soak. Benefiting from the synergy of crystallization, Hofmeister effect and nanocomposite, the hydrogel fibers feature high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa). Meanwhile, the removal of salt ions during fabrication improves the water content (69.18 ± 1.47%) and anti-swelling performance of such fibers and minimizes side effects after implantation. It is demonstrated that the fibrous sensor could record the relative resistance changes upon stretching with ideal sensitivity (GF = 1.57) and convert them into bearing stress through formula derivation and calculations. In vitro and in vivo assays further confirm its feasibility for real-time monitoring of joint motion, providing important references for medical diagnosis and treatment.

Green Strong Cornstalk Rind-Based Cellulose-PVA Aerogel for Oil Adsorption and Thermal Insulation

Cellulose-based aerogel has attracted considerable attention for its excellent adsorption capacity, biodegradability, and renewability. However, it is considered eco-unfriendly due to defibrillation of agriculture waste and requires harmful/expensive chemical agents. In this study, cornstalk rind-based aerogel was obtained via the following steps: green H2O2/HAc delignification of cornstalk rind to obtain cellulose fibers, binding with carboxymethyl cellulose (CMC)/polyvinyl alcohol (PVA) and freeze-drying treatment, and hydrophobic modification with stearic acid. The obtained aerogel showed high compressive strength (200 KPa), which is apparently higher (about 32 kPa) than NaClO-delignified cornstalk-based cellulose/PVA aerogel. Characterization of the obtained aerogel through SEM, water contact angle, etc., showed high porosity (95%), low density (0.0198 g/cm-3), and hydrophobicity (water contact angle, 159°), resulting in excellent n-hexane adsorption capacity (35 g/g), higher (about 29.5 g/g) than NaClO-delignified cornstalk-based cellulose/PVA aerogel. The adsorbed oil was recovered by the extrusion method, and the aerogel showed excellent recyclability in oil adsorption.

PEI-based functional materials: Fabrication techniques, properties, and biomedical applications

Cationic polymers have recently attracted considerable interest as research breakthroughs for various industrial and biomedical applications. They are particularly interesting due to their highly positive charges, acceptable physicochemical properties, and ability to undergo further modifications, making them attractive candidates for biomedical applications. Polyethyleneimines (PEIs), as the most extensively utilized polymers, are one of the valuable and prominent classes of polycations. Owing to their flexible polymeric chains, broad molecular weight (MW) distribution, and repetitive structural units, their customization for functional composites is more feasible. The specific beneficial attributes of PEIs could be introduced by purposeful functionalization or modification, long service life, biocompatibility, and distinct geometry. Therefore, PEIs have significant potential in biotechnology, medicine, and bioscience. In this review, we present the advances in PEI-based nanomaterials, their transfection efficiency, and their toxicity over the past few years. Furthermore, the potential and suitability of PEIs for various applications are highlighted and discussed in detail. This review aims to inspire readers to investigate innovative approaches for the design and development of next-generation PEI-based nanomaterials possessing cutting-edge functionalities and appealing characteristics.

Construction methods and biomedical applications of PVA-based hydrogels

Polyvinyl alcohol (PVA) hydrogel is favored by researchers due to its good biocompatibility, high mechanical strength, low friction coefficient, and suitable water content. The widely distributed hydroxyl side chains on the PVA molecule allow the hydrogels to be branched with various functional groups. By improving the synthesis method and changing the hydrogel structure, PVA-based hydrogels can obtain excellent cytocompatibility, flexibility, electrical conductivity, viscoelasticity, and antimicrobial properties, representing a good candidate for articular cartilage restoration, electronic skin, wound dressing, and other fields. This review introduces various preparation methods of PVA-based hydrogels and their wide applications in the biomedical field.

Recent Advances in Flexible CNC-Based Chiral Nematic Film Materials

Cellulose nanocrystal (CNC) is a renewable resource derived from lignocellulosic materials, known for its optical permeability, biocompatibility, and unique self-assembly properties. Recent years have seen great progresses in cellulose nanocrystal-based chiral photonic materials. However, due to its inherent brittleness, cellulose nanocrystal shows limitations in the fields of flexible materials, optical sensors and food freshness testing. In order to solve the above limitations, attempts have been made to improve the flexibility of cellulose nanocrystal materials without destroying their structural color. Despite these progresses, a systematic review on them is lacking. This review aims to fill this gap by providing an overview of the main strategies and the latest research findings on the flexibilization of cellulose nanocrystal-based chiral nematic film materials (FCNM). Specifically, typical substances and methods used for their preparation are summarized. Moreover, different kinds of cellulose nanocrystal-based composites are compared in terms of flexibility. Finally, potential applications and future challenges of flexible cellulose nanocrystal-based chiral nematic materials are discussed, inspiring further research in this field.

Plant-based Biomass/Polyvinyl Alcohol Gels for Flexible Sensors

Flexible sensors show great application potential in wearable electronics, human-computer interaction, medical health, bionic electronic skin and other fields. Compared with rigid sensors, hydrogel-based devices are more flexible and biocompatible and can easily fit the skin or be implanted into the body, making them more advantageous in the field of flexible electronics. In all designs, polyvinyl alcohol (PVA) series hydrogels exhibit high mechanical strength, excellent sensitivity and fatigue resistance, which make them promising candidates for flexible electronic sensing devices. This paper has reviewed the latest progress of PVA/plant-based biomass hydrogels in the construction of flexible sensor applications. We first briefly introduced representative plant biomass materials, including sodium alginate, phytic acid, starch, cellulose and lignin, and summarized their unique physical and chemical properties. After that, the design principles and performance indicators of hydrogel sensors are highlighted, and representative examples of PVA/plant-based biomass hydrogel applications in wearable electronics are illustrated. Finally, the future research is briefly prospected. We hope it can promote the research of novel green flexible sensors based on PVA/biomass hydrogel.

Artificial intelligence enhanced sensors - enabling technologies to next-generation healthcare and biomedical platform

The fourth industrial revolution has led to the development and application of health monitoring sensors that are characterized by digitalization and intelligence. These sensors have extensive applications in medical care, personal health management, elderly care, sports, and other fields, providing people with more convenient and real-time health services. However, these sensors face limitations such as noise and drift, difficulty in extracting useful information from large amounts of data, and lack of feedback or control signals. The development of artificial intelligence has provided powerful tools and algorithms for data processing and analysis, enabling intelligent health monitoring, and achieving high-precision predictions and decisions. By integrating the Internet of Things, artificial intelligence, and health monitoring sensors, it becomes possible to realize a closed-loop system with the functions of real-time monitoring, data collection, online analysis, diagnosis, and treatment recommendations. This review focuses on the development of healthcare artificial sensors enhanced by intelligent technologies from the aspects of materials, device structure, system integration, and application scenarios. Specifically, this review first introduces the great advances in wearable sensors for monitoring respiration rate, heart rate, pulse, sweat, and tears; implantable sensors for cardiovascular care, nerve signal acquisition, and neurotransmitter monitoring; soft wearable electronics for precise therapy. Then, the recent advances in volatile organic compound detection are highlighted. Next, the current developments of human-machine interfaces, AI-enhanced multimode sensors, and AI-enhanced self-sustainable systems are reviewed. Last, a perspective on future directions for further research development is also provided. In summary, the fusion of artificial intelligence and artificial sensors will provide more intelligent, convenient, and secure services for next-generation healthcare and biomedical applications.

A sweat-pH-enabled strongly adhesive hydrogel for self-powered e-skin applications

On-skin hydrogel electrodes are poorly conformable in sweaty scenarios due to low electrode-skin adhesion resulting from the sweat film formed on the skin surface, which seriously hinders practical applications. In this study, we fabricated a tough adhesive cellulose-nanofibril/poly(acrylic acid) (CNF/PAA) hydrogel with tight hydrogen-bond (H-bond) networks based on a common monomer and a biomass resource. Furthermore, inherent H-bonded network structures can be disrupted through judicious engineering using excess hydronium ions produced through sweating, which facilitate the transition to protonation and modulate the release of active groups (i.e., hydroxyl and carboxyl groups) accompanied by a pH drop. The lower pH enhances adhesive performance, especially on skin, with a 9.7-fold higher interfacial toughness (453.47 vs. 46.74 J m^-2), an 8.6-fold higher shear strength (600.14 vs. 69.71 kPa), and a 10.4-fold higher tensile strength (556.44 vs. 53.67 kPa) observed at pH 4.5 compared to the corresponding values at pH 7.5. Our prepared hydrogel electrode remains conformable on sweaty skin when assembled as a self-powered electronic skin (e-skin) and enables electrophysiological signals to be reliably collected with high signal-to-noise ratios when exercising. The strategy presented here promotes the design of high-performance adhesive hydrogels that may serve to record continuous electrophysiological signals under real-life conditions (beyond sweating) for various intelligent monitoring systems.

A Controlled Biodegradable Triboelectric Nanogenerator Based on PEGDA/Laponite Hydrogels

Implantable and wearable transient electronics based on nanogenerators have been applied in self-powered sensing, electrical-stimulation therapy, and other fields. However, the existing devices have a poor ability to match with the shapes of human tissues, and the degradation processes cannot meet individual needs. In this work, a PEGDA/Lap nanocomposite hydrogel was prepared that was based on biocompatible polyglycol diacrylate (PEGDA) and laponite, and a biodegradable single-electrode triboelectric nanogenerator (BS-TENG) was built. The PEGDA/Lap hydrogel has enhanced flexibility and mechanical and electrical performance. Its strain was 1001.8%, and the resistance was 10.8. The composite hydrogel had a good biocompatibility and could effectively promote the adhesion of cells. The BS-TENG could be used as a self-powered device to light an LED and serve as an active sensor for real-time monitoring of breath and various human movements. More importantly, the device could be degraded controllably without any harm. Therefore, BS-TENGs will be mainstream in diagnosis and treatment and play an important role in biomedical science.

Wet-Adhesive Multifunctional Hydrogel with Anti-swelling and a Skin-Seamless Interface for Underwater Electrophysiological Monitoring and Communication

A stable and seamless adhesion between the human skin and the hydrogel-based electronic skin is necessary for accurate sensing and human health monitoring in aquatic environments. Despite achieving significant progress in this field, it remains a great challenge to design skin-interfaced conductive hydrogels with high electrical conductivity, stablility, and seamless underwater adhesion to skin. Herein, a skin-inspired conductive multifunctional hydrogel is proposed, which has a wet-adhesive/hydrophilic and a non-adhesive/hydrophobic bilayer structure. The hydrogel shows high stretchability (∼2400%) and an ultra-low modulus (4.5 kPa), which facilitate the conformal and seamless attachment of the hydrogel to the skin with reduced motion artifacts. Owing to synergistic physical and chemical interactions, this hydrogel can achieve reliable underwater adhesion and display remarkable underwater adhesion strength (388.1 kPa) to porcine skin. In addition, MXene has been employed to obtain high electrical conductivity, create a route for stable electron transport, and reinforce mechanical properties. The hydrogel also possesses self-healing ability, a low swelling ratio (∼3.8%), biocompatibility, and specific adhesion to biological tissues in water. Facilitated with these advantages, the hydrogel-based electrodes achieve reliable electrophysiological signal detection in both air and wet conditions and demonstrate a higher signal-to-noise ratio (28.3 dB) than that of commercial Ag/AgCl gel electrodes (18.5 dB). Also, the hydrogel can be utilized as a strain sensor with high sensitivity for underwater communication. This multifunctional hydrogel improves the stability of the skin-hydrogel interface in aquatic environments and is expected to be promising for the next-generation bio-integrated electronics.

Room-Temperature Self-Healing Conductive Elastomers for Modular Assembly as a Microfluidic Electrochemical Biosensing Platform for the Detection of Colorectal Cancer Exosomes

Modular components for rapid assembly of microfluidics must put extra effort into solving leakage and alignment problems between individual modules. Here, we demonstrate a conductive elastomer with self-healing properties and propose a modular microfluidic component configuration system that utilizes self-healing without needing external interfaces as an alternative to the traditional chip form. Specifically, dual dynamic covalent bond crosslinks (imine and borate ester bonds) established between Polyurethane (PU) and 2-Formylbenzeneboronic acid (2-FPBA) are the key to a hard room-temperature self-healing elastomeric substrate PP (PU/2-FPBA). An MG (MXene/GO) conductive network with stable layer spacing (Al-O bonds) obtained from MXene and graphene oxide (GO) by in situ reduction of metals confers photothermal conductivity to PP. One-step liquid molding obtained a standardized modular component library of puzzle shapes from PP and MGPP (MG/PP). The exosomes were used to validate the performance of the constructed microfluidic electrochemical biosensing platform. The device has a wide detection range (50-10⁵ particles/μL) and a low limit of detection (LOD) (42 particles/μL) (S/N = 3), providing a disposable, reusable, cost-effective, and rapid analysis platform for quantitative detection of colorectal cancer exosomes. In addition, to our knowledge, this is the first exploration of self-healing conductive elastomers for a modular microfluidic electrochemical biosensing platform.

Polymeric Amines Induce Nitric Oxide Release from S-Nitrosothiols

Catalytic generation of nitric oxide (NO) from NO donors by nanomaterials has enabled prolonged NO delivery for various biomedical applications, but this approach requires laborious synthesis routes. In this study, a new class of materials, that is, polymeric amines including polyethyleneimine (PEI), poly-L-lysine, and poly(allylamine hydrochloride), is discovered to induce NO generation from S-nitrosothiols (RSNOs) at physiological conditions. Controlled NO generation can be readily achieved by tuning the concentration of the NO donors (RSNOs) and polymers, and the type and molecular weight of the polymers. Importantly, the mechanism of NO generation by these polymers is deciphered to be attributed to the nucleophilic reaction between primary amines on polymers and the SNO groups of RSNOs. The NO-releasing feature of the polymers can be integrated into a suite of materials, for example, simply by embedding PEI into poly(vinyl alcohol) (PVA) hydrogels. The functionality of the PVA/PEI hydrogels is demonstrated for Pseudomonas aeruginosa biofilm prevention with a ≈4 log reduction within 6 h. As NO has potential therapeutic implications in various diseases, the identification of polymeric amines to induce NO release will open new opportunities in NO-generating biomaterials for antibacterial, antiviral, anticancer, antithrombotic, and wound healing applications.

Development of an Innovative Soft Piezoresistive Biomaterial Based on the Interconnection of Elastomeric PDMS Networks and Electrically-Conductive PEDOT:PSS Sponges

A deeply interconnected flexible transducer of polydimethylsiloxane (PDMS) and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was obtained as a material for the application of soft robotics. Firstly, transducers were developed by crosslinking PEDOT:PSS with 3-glycidyloxypropryl-trimethoxysilane (GPTMS) (1, 2 and 3% v/v) and using freeze-drying to obtain porous sponges. The PEDOT:PSS sponges were morphologically characterized, showing porosities mainly between 200 and 600 µm2; such surface area dimensions tend to decrease with increasing degrees of crosslinking. A stability test confirmed a good endurance for up to 28 days for the higher concentrations of the crosslinker tested. Consecutively, the sponges were electromechanically characterized, showing a repeatable and linear resistance variation by the pressure triggers within the limits of their working range (∆RR0 max = 80% for 1-2% v/v of GPTMS). The sponges containing 1% v/v of GPTMS were intertwined with a silicon elastomer to increase their elasticity and water stability. The flexible transducer obtained with this method exhibited moderately lower sensibility and repeatability than the PEDOT:PSS sponges, but the piezoresistive response remained stable under mechanical compression. Furthermore, the transducer displayed a linear behavior when stressed within the limits of its working range. Therefore, it is still valid for pressure sensing and contact detection applications. Lastly, the flexible transducer was submitted to preliminary biological tests that indicate a potential for safe, in vivo sensing applications.

An Ultra-Stretchable Polyvinyl Alcohol Hydrogel Based on Tannic Acid Modified Aramid Nanofibers for Use as a Strain Sensor

The mechanical performance is critical for hydrogels that are used as strain sensors. p-Aramid nanofiber (ANF) is preferable as an additive to the reinforce the mechanical performance of a poly(vinyl alcohol) (PVA). However, due to the limited hydrogen bond sites, the preparation of ultra-stretchable, ANF-based hydrogel strain sensor is still a challenge. Herein, we reported an ultra-stretchable PVA hydrogel sensor based on tea stain-inspired ANFs. Due to the presence of numerous phenol groups in the tannic acid (TA) layer, the interaction between PVA and the ANFs was significantly enhanced even though the mass ratio of TA@ANF in the hydrogel was 2.8 wt‰. The tensile breaking modulus of the PVA/TA@ANF/Ag hydrogel sensor was increased from 86 kPa to 326 kPa, and the tensile breaking elongation was increased from 356% to 602%. Meanwhile, the hydrogel became much softer, and no obvious deterioration of the flexibility was observed after repeated use. Moreover, Ag NPs were formed in situ on the surfaces of the ANFs, which imparted the sensor with electrical conductivity. The hydrogel-based strain sensor could be used to detect the joint movements of a finger, an elbow, a wrist, and a knee, respectively. This ultra-stretchable hydrogel described herein was a promising candidate for detecting large-scale motions.

Dynamic grafting of carboxylates onto poly(vinyl alcohol) polymers for supramolecularly-crosslinked hydrogel formation

Supramolecular hydrogels have attracted considerable interest due to their unique stimuli-responsive and self-healing properties. However, these hydrogel systems are usually achieved by covalent grafting of supramolecular units onto the polymer backbone, which in turn limits their reprocessability. Herein, we prepared a supramolecular hydrogel system by forming dynamic covalent crosslinks between 4-carboxyphenylboronic acid (CPBA) and polyvinyl alcohol (PVA). The system was then further crosslinked with either calcium ions or branched polyethylenimine (PEI) to generate hydrogels with distinctly different properties. Incorporation of calcium ions resulted in the formation of hydrogels with higher storage modulus of 7290 Pa but without self-healing properties. On the other hand, PEI-crosslinked hydrogel (PVA-CPBA-PEI) exhibited >2000% critical strain value, demonstrated high stability over 52 days and showed sustained antibacterial effect. A combination of supramolecular interactions and dynamic covalent crosslinks can be an alternate strategy to fabricate next generation hydrogel materials.

High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines

Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semiseparated network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alcohol) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mechanical twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiological signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and technical platforms toward stretchable electronic skins and intelligent robotic systems.

Red carbon dot directed biocrystalline alignment for piezoelectric energy harvesting

Herein, using chitin-derived chitosan, we first demonstrate the luminous carbon dot-directed large-scale biocrystalline piezo-phase alignment. This further significantly facilitates the piezo-energy harvesting of Earth-abundant natural biopolymers. A very small, yet moderate, number of red-emission carbon quantum dots (R-CQDs) allow a highly preferential macroscopic alignment of chitosan based, electrospun hybrid nanofibers and a highly preferential microscopic alignment of internal chitosan piezo-phase crystalline lamellae. Meanwhile, R-CQD hybridized bionanofibers maintain the long-wavelength photoluminescence excitation/emission of encapsulated, monodisperse R-CQDs. The piezoelectric voltage output and piezoelectric current output of hybrid bionanofibers reach up to 125 V cm^-3 and 1.5 μA cm^-3, respectively. They are more than 5 and 6 times higher than those of the state-of-the-art pristine ones, respectively. Moreover, the proof-of-concept red-emission bionanofibrous piezoelectric nanogenerator shows a highly durable, highly stable, and highly reproducible piezoresponse in over 10 000 continuous load cycles. As a reliable renewable energy source, it demonstrates the fast charging of external capacitors and the direct operation of commercial electronics. In particular, as a self-powered wearable tactile healthcare sensor, it attains ultrahigh mechanosensitivity in sensing a broad range of human biophysiological pressures and strains.

Degradable and stretchable bio-based strain sensor for human motion detection

In recent years, flexible strain sensors have attracted considerable attention for the great application potential in the emerging fields of wearable devices, electronic skin and health monitoring. However, most of flexible strain sensors are nondegradable, and the produced numerous electronic wastes after uselessness will seriously threaten environment and ecology. Herein, we propose a new strategy to fabricate degradable and stretchable bio-based strain sensor using candle soot (CS) particles to construct conductive pathways and chitosan, potato starch (PS), and polyvinyl alcohol (PVA) to form stretchable matrix in the presence of Fe³⁺ ions. Owing to the formation of multiple hydrogen bonding constructed by chitosan, PS and PVA as well as coordination bonding by Fe³⁺ ions, the obtained strain sensor showed high elongation at break up to 200% and good fatigue resistance. Furthermore, the firm embedding of the CS particles into the surface of the stretchable matrix endowed the strain sensor with steady sensitivity (gauge factors of 1.49 at 0-60% strain and 2.71 at 60-100% strain), fast response (0.22 s) and good repeatability even after 1000 stretching-releasing cycles. In addition, the strain sensor was successfully applied to detect various human motions including finger and wrist bending, swallowing and pronunciation. Interestingly, after connecting to an Arduino microcontroller circuit with a Bluetooth module, the strain sensor was able to wirelessly detect real-time movements of index finger joints. Different from most previously reported sensors, the prepared strain sensor in this work was completely degraded in 2 wt% CH₃COOH solution at 90 °C only within 10 min, thus effectively avoiding the production of electrical waste after the updating and upgrading of the sensors. The findings conceivably stand out as a new methodology to prepare environmental-friendly sensors in the field of flexible electronics, which is very beneficial for the sustainable development of environment and society.

Body Temperature Enhanced Adhesive, Antibacterial and Recyclable Ionic Hydrogel for Epidermal Electrophysiological Monitoring

Hydrogel-based epidermal electrodes have attracted widespread attention in health monitoring and human-machine interfaces for their good biocompatibility, skin-matched Young's modulus, and stable in situ electrophysiological recording performance. However, it is difficult to make the exact conformal attachment between skin and electrodes because of the hair, wrinkles as well as complex, curved contours of the skin. This also results in signal distortion and large noise. Here, a body temperature enhanced skin-adhesive epidermal electrode is proposed based on non-covalent cross-linked network ionic hydrogel. The ionic hydrogel is fabricated by the polyvinyl alcohol (PVA), branched polyethyleneimine (b-PEI) and calcium chloride (CaCl₂ ), which demonstrates impressive performances including ultra-stretchability of 1291%, great adhesion to skin and other non-biological materials, stable conductivity of 3.09 S/m, recyclability and outstanding antibacterial ability, simultaneously. Specifically, the adhesion of the ionic hydrogel behaves as temperature-sensitive and could be enhanced by body temperature. Furthermore, the ionic hydrogel is utilized as epidermal electrodes, which displays seductive capability to record multifarious electrophysiological signals with high signal-to-noise ratio and ultra-low detection limit, including electrocardiogram (ECG), electromyogram (EMG) and electroencephalogram (EEG). The as-proposed body temperature enhanced skin-adhesive ionic hydrogel brings intelligent functions and broadens the way for epidermal electronics, promoting the development of healthcare electronics. This article is protected by copyright. All rights reserved.

PEIGel: A biocompatible and injectable scaffold with innate immune adjuvanticity for synergized local immunotherapy

Intratumoral immunotherapeutic hydrogel administration is emerging as an effective method for inducing a durable and robust antitumor immune response. However, scaffold hydrogels that can synergize with the loaded drugs, thus potentiating therapeutic efficacy, are limited. Here, we report a ternary hydrogel composed of polyvinyl alcohol (PVA), polyethylenimine (PEI)‒a cationic polymer with potential immunoactivation effects, and magnesium ions‒a stimulator of the adaptive immune response, which exhibits an intrinsic immunomodulation function of reversing the immunologically "cold" phenotype of a murine breast tumor to a "hot" phenotype by upregulating PD-L1 expression and promoting M1-like macrophage polarization. PEI hydrogel (PEIGel) encapsulating an immune checkpoint blockade (ICB) inhibitor‒anti-PD-L1 antibody (α-PDL1) exhibits synergistic effects resulting in elimination of primary tumors and remote metastases and prevention of tumor relapse after surgical resection. A preliminary mechanistic study revealed a probably hidden role of PEI in modulating the polyamine metabolism/catabolism of tumors to potentiate the immune adjuvant effect. These results deepen our understanding of the innate immune activation function of PEI and pave the way for harnessing PEI as an immune adjuvant for ICB therapy.

Flexible MXene/Bacterial Cellulose Film Sound Detector Based on Piezoresistive Sensing Mechanism

Flexible pressure sensors have aroused extensive attention in health monitoring, human-computer interaction, soft robotics, and more, as a staple member of wearable electronics. However, a majority of traditional research focuses solely on foundational mechanical sensing tests and ordinary human-motion monitoring, ignoring its other applications in daily life. In this work, a paper-based pressure sensor is prepared by using MXene/bacterial cellulose film with three-dimensional isolation layer structure, and its sensing capability as a wearable sound detector has also been studied. The as-prepared device exhibits great comprehensive mechanical sensing performance as well as accurate detection of human physiological signals. As a sound detector, not only can it recognize different voice signals and sound attributes by monitoring movement of throat muscles, but also it will distinguish a variety of natural sounds through air pressure waves caused by sound transmission (also called sound waves), like the eardrum. Besides, it plays an important role in sound visualization technology because of the ability for capturing and presenting music signals. Moreover, millimeter-scale thickness, lightweight, and degradable raw materials make the sensor convenient and easy to carry, meeting requirements of environmental protection as well.

Environment tolerant, adaptable and stretchable organohydrogels: preparation, optimization, and applications

Multiple stretchable materials have been successively developed and applied to wearable devices, soft robotics, and tissue engineering. Organohydrogels are currently being widely studied and formed by dispersing immiscible hydrophilic/hydrophobic polymer networks or only hydrophilic polymer networks in an organic/water solvent system. In particular, they can not only inherit and carry forward the merits of hydrogels, but also have some unique advantageous features, such as anti-freezing and water retention abilities, solvent resistance, adjustable surface wettability, and shape memory effect, which are conducive to the wide environmental adaptability and intelligent applications. This review first summarizes the structure, preparation strategy, and unique advantages of the reported organohydrogels. Furthermore, organohydrogels can be optimized for electro-mechanical properties or endowed with various functionalities by adding or modifying various functional components owing to their modifiability. Correspondingly, different optimization strategies, mechanisms, and advanced developments are described in detail, mainly involving the mechanical properties, conductivity, adhesion, self-healing properties, and antibacterial properties of organohydrogels. Moreover, the applications of organohydrogels in flexible sensors, energy storage devices, nanogenerators, and biomedicine have been summarized, confirming their unlimited potential in future development. Finally, the existing challenges and future prospects of organohydrogels are provided.

Advanced Implantable Biomedical Devices Enabled by Triboelectric Nanogenerators

Implantable biomedical devices (IMDs) play essential roles in healthcare. Subject to the limited battery life, IMDs cannot achieve long-term in situ monitoring, diagnosis, and treatment. The proposal and rapid development of triboelectric nanogenerators free IMDs from the shackles of batteries and spawn a self-powered healthcare system. This review aims to overview the development of IMDs based on triboelectric nanogenerators, divided into self-powered biosensors, in vivo energy harvesting devices, and direct electrical stimulation therapy devices. Meanwhile, future challenges and opportunities are discussed according to the development requirements of current-level self-powered IMDs to enhance output performance, develop advanced triboelectric nanogenerators with multifunctional materials, and self-driven close-looped diagnosis and treatment systems.

Ambient Humidity-Induced Phase Separation for Fiber Morphology Engineering toward Piezoelectric Self-Powered Sensing

Electrospun polymeric piezoelectric fibers have a considerable potential for shape-adaptive mechanical energy harvesting and self-powered sensing in biomedical, wearable, and industrial applications. However, their unsatisfactory piezoelectric performance remains an issue to be overcome. While strategies for increasing the crystallinity of electroactive β phases have thus far been the major focus in realizing enhanced piezoelectric performance, tailoring the fiber morphology can also be a promising alternative. Herein, a design strategy that combines the nonsolvent-induced phase separation of a polymer/solvent/water ternary system and electrospinning for fabricating piezoelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE) fibers with surface porosity under ambient humidity is presented. Notably, electrospun P(VDF-TrFE) fibers with higher surface porosity outperform their smooth-surfaced counterparts with a higher β phase content in terms of output voltage and power generation. Theoretical and numerical studies also underpin the contribution of the structural porosity to the harvesting performance, which is attributable to local stress concentration and reduced dielectric constant due to the air in the pores. This porous fiber design can broaden the application prospects of shape-adaptive energy harvesting and self-powered sensing based on piezoelectric polymer fibers with enhanced voltage and power performance, as successfully demonstrated in this work by developing a communication system based on self-powered motion sensing.

Ultra-Stretchable and Fast Self-Healing Ionic Hydrogel in Cryogenic Environments for Artificial Nerve Fiber

Self-healing materials behave with irreplaceable advantages in biomimetic intelligent robots (BIR) for avoiding or reducing safety hazards and economic losses from accidental damage during service. However, the self-healing ability is unreservedly lost and even becomes rigid and fragile in the cryogenic environment where BIR are precisely needed. Here, the authors report a versatile ionic hydrogel with fast self-healing ability, ultra-stretchability, and stable conductivity, even at -80 °C. The hydrogel is systematically optimized to improve a hydrogen-bonded network nanostructure, coordinated achieving a quick self-healing ability within 10 min, large deformation tolerance of over 7000%, superior conductivity of 11.76 S cm^-1 and anti-freezing ability, which is difficult to obtain simultaneously. Such a hydrogel provides new opportunities for artificial electronic devices in harsh environments. As a prospective application, they fabricate an artificial nerve fiber by mimicking the structure and functions of the myelinated axon, exhibiting the property of fast and potential-gated signal transmission. This artificial nerve fiber is integrated into a robot for demonstrating a real-time high fidelity and high throughput information interaction under big deformation and cryogenic temperature. The hydrogel and bionic device will bring pioneering functions for robots and open a broad application scenario in extreme conditions.

Highly Stretchable Conductive Covalent Coacervate Gels for Electronic Skin

Highly stretchable electrically conductive hydrogels have been extensively researched in recent years, especially for applications in strain and pressure sensing, electronic skin, and implantable bioelectronic devices. Herein, we present a new cross-linked complex coacervate approach to prepare conductive hydrogels that are both highly stretchable and compressive. The gels involve a complex coacervate between carboxylated nanogels and branched poly(ethylene imine), whereby the latter is covalently cross-linked by poly(ethylene glycol) diglycidyl ether (PEGDGE). Inclusion of graphene nanoplatelets (Gnp) provides electrical conductivity as well as tensile and compressive strain-sensing capability to the hydrogels. We demonstrate that judicious selection of the molecular weight of the PEGDGE cross-linker enables the mechanical properties of these hydrogels to be tuned. Indeed, the gels prepared with a PEGDGE molecular weight of 6000 g/mol defy the general rule that toughness decreases as strength increases. The conductive hydrogels achieve a compressive strength of 25 MPa and a stretchability of up to 1500%. These new gels are both adhesive and conformal. They provide a self-healable electronic circuit, respond rapidly to human motion, and can act as strain-dependent sensors while exhibiting low cytotoxicity. Our new approach to conductive gel preparation is efficient, involves only preformed components, and is scalable.

One-pot freezing-thawing preparation of cellulose nanofibrils reinforced polyvinyl alcohol based ionic hydrogel strain sensor for human motion monitoring

Ionic conductive hydrogels have been widely applied in sensors, energy storage and soft electronics recently. However, most of the polyvinyl alcohol (PVA) based ionic hydrogels are mainly fabricated by soaking the hydrogels in high concentration electrolyte solution which can induce the waste of electrolyte and solvent. Herein, we have designed cellulose nanofibrils (CNF) and ZnSO₄ reinforced PVA based hydrogels through a one-pot simple freezing-thawing method at low ZnSO₄ concentration without any soaking process. Furthermore, the hydrogel with 0.4% CNF exhibited stress up to 0.79 MPa (242% strain) and high ionic conductivity of 0.32 S m^-1 (0.07 M ZnSO₄). Moreover, hydrogel sensor displayed high linear gauge factor 1.70 (0-200% strain), excellent stability, durability and reliability. The integrated hydrogel sensor also showed excellent sensor performance for human motion monitoring. This work provides a new prospect for the design of cellulose reinforced conductive hydrogels via a facile method.

Lignin reinforced hydrogels with multi-functional sensing and moist-electric generating applications

Hydrogels, including PVA hydrogels, have numerous applications in many fields; however, their poor mechanical strength limits their utilization potential. Lignin, the most abundant aromatic biopolymer in nature from lignocellulosic biomass, is presently under-utilized. Herein, we used lignin to improve strength and impart pH-responsive properties of PVA hydrogel. The lignin reinforced PVA (LRP) hydrogel has a maximum storage modulus of 83.1 kPa, which is much higher than the PVA hydrogel. The LRP hydrogel exhibits great ionic conductivity, mechanical properties, and strain-sensitivity even at -30 °C. The LRP hydrogel is subsequently applied for a moisture-induced electric generator, which delivers a voltage output of 226.6 mV from moisture flow. The eco-friendly, pH responsive, high antifreezing, ionic conductive, strain sensitive, and moist-electric generating hydrogels have potential applications in many fields, including biomedicine, flexible electrodes, pH-responsive switch, strain sensor, and next-generation self-powered device systems.

Healable, Adhesive, and Conductive Nanocomposite Hydrogels with Ultrastretchability for Flexible Sensors

In recent years, conductive hydrogels have generated tremendous attention in biomedicals and bioelectronics fields due to their excellent physiochemical properties. In this study, a physically cross-linked conducting hydrogel has been designed in combination with cellulose nanocrystalline (CNC), polyacrylic acid (PAA) chains, laurel methacrylate, and sodium dodecyl sulfate. The obtained result shows that the hydrogel prepared is ultrastretchable, mechanically robust, transparent, biocompatible, conductive, and self-healing. The mechanical property of the prepared hydrogel is optimized through variation of the CNC content. The optimal hydrogel (CNC-1/PAA) exhibits an impressive mechanics, including high stretchability (∼1800%) and compressibility, good elasticity, and fatigue resistance. Furthermore, the conductivity of the hydrogel enables tensile strain- and pressure-sensing capabilities. The CNC/PAA-based flexible sensors are successfully designed, which shows high sensitivity, fast response (290 ms), and excellent cycle stability as well as the pressure sensing capability. As a result, the designed hydrogel has the ability to sense and detect diverse human motion, including elbow/finger/wrist bending and speaking, which demonstrates that the designed self-healing conductive hydrogels have significant potential for applications in flexible electronics.

Design of ultra-stretchable, highly adhesive and self-healable hydrogels via tannic acid-enabled dynamic interactions

Hydrogels have emerged as a landmark soft material for a wide range of applications such as in biomedical devices, soft robotics, artificial electronic skins, and the Internet of Things (IoT). To date, engineering hydrogels that simultaneously possess high stretchability (>3000%) and strong on-skin adhesion (>30 kPa) has not been an easy task. Generally, good stretchability is mainly dominated by the bulk interactions of hydrogels, whereas robust adhesion relies on the interfacial interactions of hydrogels with their surroundings. Here, we report a facile strategy to engineer an ultra-stretchable, highly adhesive and self-healable hydrogel, by virtue of tannic-acid-enabled dynamic interactions (TEDI) to fully substitute conventional covalent crosslinking. The TEDI strategy allows us to synchronously regulate both bulk and interfacial interactions to obtain exciting properties that outperform conventional hydrogels, including an extraordinary stretchability of over 7300%, remarkable self-healing abilities, and a robust on-skin adhesion of 50 kPa. With these intriguing merits, TEDI hydrogels are demonstrated to be a wearable strain sensor that accurately detect the motion of the human body. Moreover, our TEDI strategy unlocks new opportunities to design next-generation ionic hydrogels that may be valuable for applications in wearable electronic devices and healthcare monitoring.

Self-Healing Functional Electronic Devices

Electronic devices with various functions bring great convenience and revolutionize the way we live. They are inevitable to degrade over time because of physical or chemical fatigue and damage during practical operation. To make these devices have the ability to autonomously heal from cracks and restore their mechanical and electrical properties, self-healing materials emerged as the time requires for constructing robust and self-healing electronic devices. Here the development of self-healing electronic devices with different functions, for example, energy harvesting, energy storage, sensing, and transmission, is reviewed. The new application scenarios and existing challenges are explored, and possible strategies and perspectives for future practical applications are discussed.

A ionic liquid enhanced conductive hydrogel for strain sensing applications

Strain-sensitive and conductive hydrogels have attracted extensive research interest due to their potential applications in various fields, such as healthcare monitoring, human-machine interfaces and soft robots. However, low electrical signal transmission and poor tensile properties still limit the application of flexible sensing hydrogels in large amplitude and high frequency motion. In this study, a novel ionic liquid segmental polyelectrolyte hydrogel consisting of acrylic acid (AAc), 1-vinyl-3-butylimidazolium bromide (VBIMBr) and aluminum ion (Al³⁺) was prepared by molecular design and polymer synthesis. The cationic groups and amphiphilicity of ionic liquid chain segments effectively improve the tensile behavior of the polyelectrolyte hydrogel, with a maximum tensile strength of 0.16 MPa and a maximum breaking strain of 604%. The introduction of ionic liquid segments increased the current carrying concentration of polyelectrolyte hydrogel, and the conductivity reached the initial 4.8 times (12.5 S/m), which is a necessary condition for detecting various amplitude and high frequency limb movements. The flexible electronic sensor prepared by this polyelectrolyte hydrogel efficiently detects the movement of different parts of the human body stably and sensitively, even in extreme environment (-20 °C). These outstanding advantages demonstrate the great potential of this hydrogel in healthcare monitoring and wearable flexible strain sensors.

Integration of Internet of Things with Quantum Dots: A State-of-the-art of Medicine

Internet of Things (IoT) emerges as disruptive innovation and development in the fields of drug delivery and biomedical sciences using on-target active transportation, sensors, wearable devices, real-time diagnostics, etc. Semiconducting fluorescence emitting material, quantum dots on integration with IoT displayed interesting results in the healthcare sector, especially in hospitals and pathological laboratories. Presently, the integrated system is used to improve productivity without the interference of human and offer a cost-effective system. This integrated system can be used for the detection of various diseases like epilepsy, cancer, diabetes, etc., and various biomedical applications like energy storage, lights, sensor technology, light filters, etc. The integrated technology is implemented into the field of medicine for simplifying the approaches in therapeutics and diagnostic applications. The collected and analyzed data are further useful for healthcare professionals to find patient-centric solutions. Artificial Intelligence-aided IoT emerges as a novel technology for transmitting and securing health data. Despite some of the limitations like e-waste and the risk of hacking, an IoT-based QD system will be considered as a modern healthcare provider with life-saving products for enriching the medical quality and real-time accessibility.

Anchoring-mediated topology signature of self-assembled elastomers undergoing mechanochromic coupling/decoupling

Soft elastomers with their ability to integrate strain-adaptive stiffening and coloration have recently received significant research attention for application in artificial muscle and active camouflage. However, there is a lack of theoretical understanding of their complex molecular dynamics and mechanochromic coupling/decoupling. In this study, a topological dynamics model is proposed to understand the anchoring-mediated topology signature of self-assembled elastomers. Based on the constrained molecular junction model, a free-energy function is firstly formulated to describe the working principles of strain-adaptive stiffening and coloration in the self-assembled elastomer. A coupled ternary "rock-paper-scissors" model is proposed to describe the topological dynamics of self-assembly, mechanochromic coupling and mechanoresponsive stiffening of the self-assembled elastomers, in which there are three fractal geometry components in the topology network. Finally, the proposed models are verified using the experimental results reported in the literature. This study provides a fundamental approach to understand the working mechanism and topological dynamics in the self-assembled elastomers, with molecularly encoded stiffening and coloration.

Flexible Supercapacitors Based on Graphene/Boron Nitride Nanosheets Electrodes and PVA/PEI Gel Electrolytes

All-solid-state supercapacitors have gained increasing attention as wearable energy storage devices, partially due to their flexible, safe, and lightweight natures. However, their electrochemical performances are largely hampered by the low flexibility and durability of current polyvinyl alcohol (PVA) based electrolytes. Herein, a novel polyvinyl alcohol-polyethyleneimine (PVA-PEI) based, conductive and elastic hydrogel was devised as an all-in-one electrolyte platform for wearable supercapacitor (WSC). For proof-of-concept, we assembled all-solid-state supercapacitors based on boron nitride nanosheets (BNNS) intercalated graphene electrodes and PVA-PEI based gel electrolyte. Furthermore, by varying the electrolyte ions, we observed synergistic effects between the hydrogel and the electrode materials when KOH was used as electrolyte ions, as the Graphene/BNNS@PVA-PEI-KOH WSCs exhibited a significantly improved areal capacitance of 0.35 F/cm2 and a smaller ESR of 6.02 ohm/cm2. Moreover, due to the high flexibility and durability of the PVA-PEI hydrogel electrolyte, the developed WSCs behave excellent flexibility and cycling stability under different bending states and after 5000 cycles. Therefore, the conductive, yet elastic, PVA-PEI hydrogel represents an attractive electrolyte platform for WSC, and the Graphene/BNNS@PVA-PEI-KOH WSCs shows broad potentials in powering wearable electronic devices.

A Stretchable, Self-Healable Triboelectric Nanogenerator as Electronic Skin for Energy Harvesting and Tactile Sensing

Electronic skin that is deformable, self-healable, and self-powered has high competitiveness for next-generation energy/sense/robotic applications. Herein, we fabricated a stretchable, self-healable triboelectric nanogenerator (SH-TENG) as electronic skin for energy harvesting and tactile sensing. The elongation of SH-TENG can achieve 800% (uniaxial strain) and the SH-TENG can self-heal within 2.5 min. The SH-TENG is based on the single-electrode mode, which is constructed from ion hydrogels with an area of 2 cm × 3 cm, the output of short-circuit transferred charge (Qsc), open-circuit voltage (Voc), and short-circuit current (Isc) reaches ~6 nC, ~22 V, and ~400 nA, and the corresponding output power density is ~2.9 μW × cm⁻² when the matching resistance was ~140 MΩ. As a biomechanical energy harvesting device, the SH-TENG also can drive red light-emitting diodes (LEDs) bulbs. Meanwhile, SH-TENG has shown good sensitivity to low-frequency human touch and can be used as an artificial electronic skin for touch/pressure sensing. This work provides a suitable candidate for the material selection of the hydrogel-based self-powered electronic skin.

Adhesive and tough hydrogels: from structural design to applications

In recent years, multifunctional hydrogels have garnered great interest. Usually, there is a contradiction between the toughness and interface adhesion of traditional hydrogels. In engineering and medical applications, hydrogels need to have good adhesive properties and toughness. The design of functional hydrogels with strong adhesion and high toughness is key to their application. In this review, the research progress of adhesive and tough hydrogels in recent years is outlined. Specifically, the structural design (such as integrated, layered, and gradient structures) and applications (such as cartilage repair, drug delivery, strain sensors, tissue adhesives, soft actuators, and supercapacitors) of adhesive and tough hydrogels are classified and discussed, providing new insights on their design and development.

Effect of methacrylic acid and pendant vinyl groups on the mechanical properties of highly stretchable core–shell nanostructured films deposited from water

The mechanical properties of these highly stretchable, water deposited elastomers can be tuned by varying MAA content and vinyl functionalisation.

Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid-silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid-silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human-machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm^-1) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.