Flexible Self-Repairing Materials for Wearable Sensing Applications: Elastomers and Hydrogels

A Review of Developments in Carbon-Based Nanocomposite Electrodes for Noninvasive Electroencephalography

Wearable biosensors have been of interest for their wide range of uses, varying from recording biological signals to measuring strain of bending joints. Carbon nanoparticles have been utilized in biocompatible polymers to create nanocomposites with highly tunable mechanical and electrical properties. These nanocomposites have been demonstrated to be highly effective as wearable sensors for recording physiological signals such as electroencephalography (EEG), offering advantages in mechanical and electrical properties and signal quality over commercially available sensors while maintaining feasibility and scalability in manufacturing. This review aims to provide a critical summary of the recent literature on the properties, design, fabrication, and performance of carbon-based nanocomposites for EEG electrodes. The goal of this review is to highlight the various design configurations and properties thereof, manufacturing methods, performance measurements, and related challenges associated with these promising noninvasive dry soft electrodes. While this technology offers many advantages over either other noninvasive or their invasive counterparts, there are still various challenges and opportunities for improvements and innovation. For example, the investigation of gradient composite structures, hybrid nanocomposite/composite materials, hierarchical contact surfaces, and the influence of loading and alignment of the dispersal phase in the performance of these electrodes could lead to novel and better designs. Finally, current practices for evaluating the performance of novel EEG electrodes are discussed and challenged, emphasizing the critical need for the development of standardized assessment protocols, which could provide reliability in the field, enable benchmarking, and hence promote innovation.

High-Sensitivity Flexible Self-Powered Pressure Sensor Based on Solid-Liquid Triboelectrification

Self-powered sensors based on triboelectric nanogenerators (TENG) possess advantages such as lightweight, small size, and low cost. However, the trade-off among response time, sensitivity, and wear resistance of the triboelectric layer in such sensors remains unresolved. In this paper, a hydrophobic triboelectric layer was prepared using polydimethylsiloxane (PDMS) doped with polytetrafluoroethylene (PTFE), generating triboelectric signals through the contact and separation of the triboelectric layer with water. Compared to the traditional solid-solid triboelectric charging method in self-powered sensors, this sensor primarily relies on solid-liquid triboelectric charging, effectively avoiding the wear issues caused by traditional solid-solid triboelectric charging. Simultaneously, thanks to the solid-liquid contact mode, the response time of the self-powered sensor is significantly improved, and it maintains high sensitivity under extremely small trigger forces. Finally, this paper demonstrates the application of this self-powered sensor in scenarios such as detecting human joint movements, mechanical finger states, and robotic hand grasping states, showing its promising application prospects in the field of intelligent monitoring.

Advanced flexible self-healing triboelectric nanogenerators for applications in complex environments

With the advent of the smart era, the demand for clean energy is rising, and flexible triboelectric nanogenerators (F-TENGs) based on elastomers have garnered significant attention. Based on the principles of electrostatic induction and coupling, F-TENGs can convert mechanical motion into electrical energy and are widely utilized in wearable devices and blue energy. F-TENGs offer a simple design, ease of manufacturing, and flexible usage scenarios. However, several weaknesses still limit their development. For example, F-TENG materials cannot recover from fatigue damage and are prone to output performance degradation under frequent friction or complex external conditions, leading to failure. To address these issues, researchers have explored the use of self-healable flexible polymer-based friction layers and electrodes. This review will provide a detailed summary of the key scientific and technological challenges faced by F-TENGs in complex and harsh environments, including ambient, high and low temperatures, high humidity, and strong acids and bases. Furthermore, the detailed research progress addressing these issues and the future development of F-TENGs will also be presented and explored. This paper aims to provide valuable insights and guidance for in-depth research and broad applications of flexible TENGs.

Wearable Arduino-Based Electronic Interactive Tattoo: A New Type of High-Tech Humanized Emotional Expression for Electronic Skin

Skin is the largest organ of the human body and holds the functions of sensing, protecting, and regulating. Since ancient times, people have decorated their skin by painting themselves, cutting, and using accessories to express their personality and aesthetic consciousness as a kind of artistic expression, one that shows the development and change of aesthetic consciousness. However, there are concerns regarding the inconvenience, high time cost, and negative body perception with traditional tattoos. In addition, the trend of skin decoration has gradually withdrawn due to a lack of intelligent interaction. In response to these problems, we proposed a wearable electronic skin tattoo that offers a novel means of communication and emotional expression for individuals with communication impairments, WABEIT. The tattoo uses skin-friendly PDMS as the base material, combines multi-mode sensing components such as silver wire circuit, a programmable Surface-Mounted Device (SMD), a thin-film-pressure sensor, and a heart rate sensor, and combines the embedded development board Arduino Nano for intelligent interaction, forming a wearable electronic interactive tattoo capable of sensing the environment, human-computer interaction, and the changeable performance of intelligent perception. The sensor is also equipped with a mobile power supply to support portability. The advantages of WABEIT are as follows: first, it avoids the pain, allergy, and long production process of traditional tattoos. Second, the patterns can adapt to different needs and generate feedback for users, which can effectively express personal emotions. Thirdly, the facility of removal reduces social discrimination and occupational constraints, which is especially suitable for East Asia. Experimental results indicate that the device exhibits a high sensitivity in signal response, a wide variety of pattern changes, and reliable interactive capabilities. The study demonstrates that the proposed design philosophy and implementation strategy can be generalized to the interactive design of other wearable devices, thereby providing novel insights and methodologies for human-computer interaction, electronic devices, and sensor applications.

Toward Intelligent Materials with the Promise of Self-Healing Hydrogels in Flexible Devices

Flexible sensors are revolutionizing wearable and implantable devices, with conductive hydrogels emerging as key materials due to their biomimetic structure, biocompatibility, tunable transparency, and stimuli-responsive electrical properties. However, their fragility and limited durability pose significant challenges for broader applications. Drawing inspiration from the self-healing capabilities of natural organisms like mussels, researchers are embedding self-repair mechanisms into hydrogels to improve their reliability and lifespan. This review highlights recent advances in self-healing (SH) conductive hydrogels, focusing on synthesis methods, healing mechanisms, and strategies to enhance multifunctionality. It also explores their wide-ranging applications, including in vivo signal monitoring, wearable biochemical sensors, supercapacitors, flexible displays, triboelectric nanogenerators, and implantable bioelectronics. While progress has been made, challenges remain in balancing self-healing efficiency, mechanical strength, and sensing performance. This review offers insights into overcoming these obstacles and discusses future research directions for advancing SH hydrogel-based bioelectronics, aiming to pave the way for durable, high-performance devices in next-generation wearable and implantable technologies.

Self-healing actuatable electroluminescent fibres

Alternating-current electroluminescent fibres are promising candidates as light sources for smart textiles and soft machines. However, physical damage from daily use causes device deterioration or failure, making self-healable electroluminescent fibres attractive. In addition, soft robots could benefit from light-emitting combined with magnetically actuated functions. Here, we present a self-healing and actuatable Scalable Hydrogel-clad Ionotronic Nickel-core Electroluminescent (SHINE) fibre which achieves a record luminance of 1068 cd × m-2 at 5.7 V × μm-1. The SHINE fibre can self-heal across all constituent layers after being severed, recovering 98.6% of pristine luminance and maintaining for over 10 months. SHINE fibre is also magnetically actuatable due to the ferromagnetic nickel electrode core, enabling a soft robotic fibre with omnidirectional actuation and electro-luminescence. Our approach to this multifunctional fibre broadens the design of fibre electronics and fibre robots, with applications in interactive displays and damage-resilient navigation.

Smart hydrogel-based trends in future tendon injury repair: A review

Despite advances in tissue engineering for tendon repair, rapid functional repair is still challenging due to its specificity and is prone to complications such as postoperative infections and tendon adhesions. Smart responsive hydrogels provide new ideas for tendon therapy with their flexibly designed three-dimensional cross-linked polymer networks that respond to specific stimuli. In recent years, a variety of smart-responsive hydrogels have been developed for the treatment of tendon disorders, showing great research promise and ability to address complex challenges. This article provides a comprehensive review of recent advances in the field of smart-responsive hydrogels for the treatment of tendon disorders, with a special focus on their response properties to different physical, chemical and biological stimuli. The multiple functional properties of these innovative materials are discussed in depth, including excellent biocompatibility and biodegradability, excellent mechanical properties, biomimetic structural design, convenient injectability, and unique self-healing capabilities. These properties enable the smart-responsive hydrogels to demonstrate significant advantages in solving difficult problems in the treatment of tendon disorders, such as precise drug delivery, tendon adhesion prevention and postoperative infection control. In addition, the article looks at the future prospects of smart-responsive hydrogels and analyses the challenges they may face in achieving widespread application.

Highly Stretchable, Transparent, Self-Healing Ion-Conducting Elastomers for Long-Term Reliable Human Motion Detection

The flexible electronic sensor is a critical component of wearable devices, generally requiring high stretchability, excellent transmittance, conductivity, self-healing capability, and strong adhesion. However, designing ion-conducting elastomers meeting all these requirements simultaneously remains a challenge. In this study, a novel approach is presented to fabricate highly stretchable, transparent, and self-healing ion-conducting elastomers, which are synthesized via photo-polymerization of two polymerizable deep eutectic solvents (PDESs) monomers, i.e., methacrylic acid (MAA)/choline chloride (ChCl) and itaconic acid (IA)/ChCl. The as-prepared ion-conducting elastomers possess outstanding properties, including high transparency, conductivity, and the capability to adhere to various substrates. The elastomers also demonstrate ultra-stretchability (up to 3900%) owing to a combination of covalent cross-linking and noncovalent cross-linking. In addition, the elastomers can recover up to 3250% strain and over 94.5% of their original conductivity after self-healing at room temperature for 5 min, indicating remarkable mechanical and conductive self-healing abilities. When utilized as strain sensors to monitor real-time motion of human fingers, wrist, elbow, and knee joints, the elastomers exhibit stable and strong repetitive electrical signals, demonstrating excellent sensing performance for large-scale movements of the human body. It is anticipated that these ion-conducting elastomers will find promising applications in flexible and wearable electronics.

Strain-Temperature Dual Sensor Based on Deep Learning Strategy for Human-Computer Interaction Systems

Thermoelectric (TE) hydrogels, mimicking human skin, possessing temperature and strain sensing capabilities, are well-suited for human-machine interaction interfaces and wearable devices. In this study, a TE hydrogel with high toughness and temperature responsiveness was created using the Hofmeister effect and TE current effect, achieved through the cross-linking of PVA/PAA/carboxymethyl cellulose triple networks. The Hofmeister effect, facilitated by Na+ and SO42- ions coordination, notably increased the hydrogel's tensile strength (800 kPa). Introduction of Fe2+/Fe3+ as redox pairs conferred a high Seebeck coefficient (2.3 mV K-1), thereby enhancing temperature responsiveness. Using this dual-responsive sensor, successful demonstration of a feedback mechanism combining deep learning with a robotic hand was accomplished (with a recognition accuracy of 95.30%), alongside temperature warnings at various levels. It is expected to replace manual work through the control of the manipulator in some high-temperature and high-risk scenarios, thereby improving the safety factor, underscoring the vast potential of TE hydrogel sensors in motion monitoring and human-machine interaction applications.

A stretchable conductive elastomer sensor with self-healing and highly linear strain for human movement detection and pressure response

Expanding the detection information of wearable smart devices in applications has practical implications for their use in daily life and healthcare. Damage and breakage caused by mechanical injuries and continuous use are unavoidable for polymer matrices so self-healing properties are expected to be conferred on flexible sensors to extend their life and durability. In addition, a good linearity of relative resistance change vs. strain (gauge factor, GF) facilitates the streamlined conversion of electrical signals to 3D information of human motion, whereas existing works on sensors neglect the quantitative analysis of signals. This letter reports a self-healable flexible electronic sensor based on hydrogen bonding and electrostatic interaction between maleic acid-grafted natural rubber (MNR), polyaniline (PANI), and phytic acid (PA). MNR is the flexible matrix and the template for aniline (ANI) polymerization, and PA acts as the dopant and crosslinking agent. The MNR-PANI-PA sensor shows easy self-healing at room temperature, enhanced mechanical behaviour (∼2.5 MPa, 1000% strain), and excellent linearity (GF of 13.8 over 250% strain and GF of 32.0 over 250-100% strain). Due to the highly linear relationship between ΔR/R and bending angle, the electrical signals of human limb movement can output relevant information on bending angle and frequency. By constructing a sensing array, changes in the position and magnitude of applied pressure could also be detected in real-time. Based on these advantages, the MNR-PANI-PA composite sensor is expected to have potential applications in health monitoring, body motion detection, and electronic skins.

Smart Physical-Based Transdermal Drug Delivery System:Towards Intelligence and Controlled Release

Transdermal drug delivery systems based on physical principles have provided a stable, efficient, and safe strategy for disease therapy. However, the intelligent device with real-time control and precise drug release is required to enhance treatment efficacy and improve patient compliance. This review summarizes the recent developments, application scenarios, and drug release characteristics of smart transdermal drug delivery systems fabricated with physical principle. Special attention is paid to the progress of intelligent design and concepts in of physical-based transdermal drug delivery technologies for real-time monitoring and precise drug release. In addition, facing with the needs of clinical treatment and personalized medicine, the recent progress and trend of physical enhancement are further highlighted for transdermal drug delivery systems in combination with pharmaceutical dosage forms to achieve better transdermal effects and facilitate the development of smart medical devices. Finally, the next generation and future application scenarios of smart physical-based transdermal drug delivery systems are discussed, a particular focus in vaccine delivery and tumor treatment.

Synergistic Self-Healing Enhancement in Multifunctional Silicone Elastomers and Their Application in Smart Materials

Organosilicon polymers (silicones) are of enduring interest both as an established branch of polymer chemistry and as a segment of commercial products. Their unique properties were exploited in a wide range of everyday applications. However, current silicone trends in chemistry and materials engineering are focused on new smart applications, including stretchable electronics, wearable stress sensors, protective coatings, and soft robotics. Such applications require a fresh approach to methods for increasing the durability and mechanical strength of polysiloxanes, including crosslinked systems. The introduction of self-healing options to silicones has been recognized as a promising alternative in this field, but only carefully designed multifunctional systems operating with several different self-healing mechanisms can truly address the demands placed on such valuable materials. In this review, we summarized the progress of research efforts dedicated to the synthesis and applications of self-healing hybrid materials through multi-component systems that enable the design of functional silicon-based polymers for smart applications.

Flexible and ultrasensitive piezoresistive electronic skin based on chitin/sulfonated carbon nanotube films

Wearable electronic skin has gained increasing popularity due to its remarkable properties of high flexibility, sensitivity, and lightweight, making it an ideal choice for detecting human physiological activity. In this study, we successfully prepared e-skin using regenerated chitin (RCH) and sulfonated carbon nanotubes (SCNTs). The e-skin demonstrated brilliant mechanical and sensing properties, exhibiting a sensitivity of 1.75 kPa-1 within the 0-5 kPa range and a fast response-recovery time of <10 ms. Furthermore, it displayed an ultra-low detection limit of 1.39 Pa (5 mg), exceptional stability (up to 11,000 cycles), and a remarkable mechanical strength, reaching up to 50 MPa. Moreover, the e-skin was fabricated through a simple and economical approach. With the popularity of micro sensing devices, the e-skin holds tremendous potential for various applications, including wearable electronic devices, health and sports monitoring, artificial intelligence and other fields.

Connective tissue inspired elastomer-based hydrogel for artificial skin via radiation-indued penetrating polymerization

Robust hydrogels offer a candidate for artificial skin of bionic robots, yet few hydrogels have a comprehensive performance comparable to real human skin. Here, we present a general method to convert traditional elastomers into tough hydrogels via a unique radiation-induced penetrating polymerization method. The hydrogel is composed of the original hydrophobic crosslinking network from elastomers and grafted hydrophilic chains, which act as elastic collagen fibers and water-rich substances. Therefore, it successfully combines the advantages of both elastomers and hydrogels and provides similar Young's modulus and friction coefficients to human skin, as well as better compression and puncture load capacities than double network and polyampholyte hydrogels. Additionally, responsive abilities can be introduced during the preparation process, granting the hybrid hydrogels shape adaptability. With these unique properties, the hybrid hydrogel can be a candidate for artificial skin, fluid flow controller, wound dressing layer and many other bionic application scenarios.

Polysaccharide-based hydrogels for medical devices, implants and tissue engineering: A review

Hydrogels are highly biocompatible biomaterials composed of crosslinked three-dimensional networks of hydrophilic polymers. Owing to their natural origin, polysaccharide-based hydrogels (PBHs) possess low toxicity, high biocompatibility and demonstrate in vivo biodegradability, making them great candidates for use in various biomedical devices, implants, and tissue engineering. In addition, many polysaccharides also show additional biological activities such as antimicrobial, anticoagulant, antioxidant, immunomodulatory, hemostatic, and anti-inflammatory, which can provide additional therapeutic benefits. The porous nature of PBHs allows for the immobilization of antibodies, aptamers, enzymes and other molecules on their surface, or within their matrix, potentiating their use in biosensor devices. Specific polysaccharides can be used to produce transparent hydrogels, which have been used widely to fabricate ocular implants. The ability of PBHs to encapsulate drugs and other actives has been utilized for making neural implants and coatings for cardiovascular devices (stents, pacemakers and venous catheters) and urinary catheters. Their high water-absorption capacity has been exploited to make superabsorbent diapers and sanitary napkins. The barrier property and mechanical strength of PBHs has been used to develop gels and films as anti-adhesive formulations for the prevention of post-operative adhesion. Finally, by virtue of their ability to mimic various body tissues, they have been explored as scaffolds and bio-inks for tissue engineering of a wide variety of organs. These applications have been described in detail, in this review.

A review on self-healing featured soft robotics

Soft robots are becoming more popular because they can solve issues stiff robots cannot. Soft component and system design have seen several innovations recently. Next-generation robot-human interactions will depend on soft robotics. Soft material technologies integrate safety at the material level, speeding its integration with biological systems. Soft robotic systems must be as resilient as biological systems in unexpected, uncontrolled situations. Self-healing materials, especially polymeric and elastomeric ones, are widely studied. Since most currently under-development soft robotic systems are composed of polymeric or elastomeric materials, this finding may provide immediate assistance to the community developing soft robots. Self-healing and damage-resilient systems are making their way into actuators, structures, and sensors, even if soft robotics remains in its infancy. In the future, self-repairing soft robotic systems composed of polymers might save both money and the environment. Over the last decade, academics and businesses have grown interested in soft robotics. Despite several literature evaluations of the soft robotics subject, there seems to be a lack of systematic research on its intellectual structure and development despite the rising number of articles. This article gives an in-depth overview of the existing knowledge base on damage resistance and self-healing materials' fundamental structure and classifications. Current uses, problems with future implementation, and solutions to those problems are all included in this overview. Also discussed are potential applications and future directions for self-repairing soft robots.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Water-Resistant Conductive Gels toward Underwater Wearable Sensing

Conductive gels are developing vigorously as superior wearable sensing materials due to their intrinsic conductivity, softness, stretchability, and biocompatibility, showing a great potential in many aspects of lives. However, compared to their wide application on land, it is significant yet rather challenging for traditional conductive gels to realize sensing application under water. The swelling of gels and the loss of conductive components in the aqueous environment, resulted from the diffusion across the interface, lead to structural instability and sensing performance decline. Fortunately, great efforts are devoted to improving the water resistance of conductive gels and employing them in the field of underwater wearable sensing in recent years, and some exciting achievements are obtained, which are of great significance for promoting the safety and efficiency of underwater activities. However, there is no review to thoroughly summarize the underwater sensing application of conductive gels. This review presents a brief overview of the representative design strategies for developing water-resistant conductive gels and their diversified applications in the underwater sensing field as wearable sensors. Finally, the ongoing challenges for further developing water-resistant conductive gels for underwater wearable sensing are also discussed along with recommendations for the future.

Self-Healable PEDOT:PSS-PVA Nanocomposite Hydrogel Strain Sensor for Human Motion Monitoring

Strain sensors based on conducting polymer hydrogels are considered highly promising candidates for wearable electronic devices. However, existing conducting polymer hydrogels are susceptible to aging, damage, and failure, which can greatly deteriorate the sensing performance of strain sensors based on these substances and the accuracy of data collection under large deformation. Developing conductive polymer hydrogels with concurrent high sensing performance and self-healing capability is a critical yet challenging task to improve the stability and lifetime of strain sensors. Herein, we design a self-healable conducting polymer hydrogel by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers and poly(vinyl alcohol) (PVA) via both physical and chemical crosslinking. This PEDOT:PSS-PVA nanocomposite hydrogel strain sensor displays an excellent strain monitoring range (>200%), low hysteresis (<1.6%), a high gauge factor (GF = 3.18), and outstanding self-healing efficiency (>83.5%). Electronic skins based on such hydrogel strain sensors can perform the accurate monitoring of various physiological signals, including swallowing, finger bending, and knee bending. This work presents a novel conducting polymer hydrogel strain sensor demonstrating both high sensing performance and self-healability, which can satisfy broad application scenarios, such as wearable electronics, health monitoring, etc.

Designable Electrical/Thermal Coordinated Dual-Regulation Based on Liquid Metal Shape Memory Polymer Foam for Smart Switch

Electronic components with tunable resistance, especially with synergistic regulation of thermal conductivity, play important roles in the fields of electronics, smart switch, soft robots, and so on. However, it is still a challenge to get the material with various resistance and thermal conductivity stably without lasting external force. Herein, a liquid metal shape memory polymer foam (LM-SMF) is developed by loading electrically and thermally conductive liquid metal (LM) on deformable foam skeleton. Based on thermal response shape memory effect, the foam skeleton can be reversibly pressed, the process of which enables LM to transfer between connected and disconnected states. As a result, obtained LM-SMF shows that the resistance stably changes from 0.8 Ω (conductor) to 200 MΩ (insulator), and the thermal conductivity difference is up to 4.71 times (0.108 to 0.509 W m-1 K-1 ), which indicates that LM-SMF can achieve the electrical and thermal dual-regulation. Moreover, LM-SMF can be used as a designable self-feedback/-warning integrated smart switch or tunable infrared stealth switch. This work proposes a novel strategy to get the material with electrical-thermal coordinated dual-regulation, which is possibly applied in intelligent heating system with real-time monitoring function, electrothermal sensor in the future.

Anti-freezing, recoverable and transparent conductive hydrogels co-reinforced by ethylene glycol as flexible sensors for human motion monitoring

Wearable flexible sensors based on conductive hydrogels have received extensive attention in the fields of electronic skin and smart monitoring. However, conductive hydrogels contain a large amount of water, which greatly affects their performances in harsh environments. It is therefore necessary to prepare hydrogel sensors that are stable at low temperatures. Herein, metal ions (MgCl₂) and ethylene glycol (EG) were combined with polyvinyl alcohol (PVA) to obtain a conductive PVA/EG hydrogel with tensile strength and elongation at break of 1.1 MPa and 442.3 %, respectively, which could withstand >6000-fold its own weight. The binary solvent system composed of water and EG contributed to the excellent anti-freezing properties and long-term storage (>1 week), flexibility, and stability of the hydrogel even at -20 °C. The wearable PVA/EG hydrogel as a flexible sensor possessed desirable sensing performances with a competitive GF value of 0.725 and fatigue resistance (50 cycles) when used to monitor various human motions and physiological signals. Overall, this hydrogel sensor shows strong potential for application in the fields of human motion monitoring, written information sensing, and information encryption and transmission.

Role of Hydrophobic Associations in Self-Healing Hydrogels Based on Amphiphilic Polysaccharides

Self-healing hydrogels have the ability to recover their original properties after the action of an external stress, due to presence in their structure of reversible chemical or physical cross-links. The physical cross-links lead to supramolecular hydrogels stabilized by hydrogen bonds, hydrophobic associations, electrostatic interactions, or host-guest interactions. Hydrophobic associations of amphiphilic polymers can provide self-healing hydrogels with good mechanical properties, and can also add more functionalities to these hydrogels by creating hydrophobic microdomains inside the hydrogels. This review highlights the main general advantages brought by hydrophobic associations in the design of self-healing hydrogels, with a focus on hydrogels based on biocompatible and biodegradable amphiphilic polysaccharides.

Hierarchically Structured Carbon Nanofiber-Enabled Skin-Like Strain Sensors with Full-Range Human Motion Monitoring and Autonomous Self-Healing Capability

Flexible strain sensors that mimic the properties of human skin have recently attracted tremendous attention. However, integrating multiple functions of skin into one strain sensor, e.g., stretchability, full-range motion response, and self-healing capability, is still an enormous challenge. Herein, a skin-like strain sensor was presented by the construction of hierarchically structured carbon nanofibers (CNFs), followed by encapsulation of elastic self-healing polyurethane (PU). The hierarchical sensing structure was composed of diversified CNFs with orientations from highly aligned to randomly oriented, and their different fracture mechanisms enabled the resultant strain sensor to successfully integrate key sensing properties including high sensitivity (gauge factor of 90), wide sensing range (∼80% strain), and fast response (52 ms). These properties, combined with high stretchability (870%) and excellent stability (>2000 cycles), allowed the sensor to precisely detect full-range human motions from large joint motions to subtle physiological signals. Moreover, the strain sensor had spontaneous self-healing capability at room temperature with high healing efficiencies of 97.7%, while the healing process could substantially be accelerated by the natural sunlight (24 h → 0.5 h). The healed sensor possessed comparable stretchability, sensing performance, and accurate monitoring ability of subtle body signals with the original sensor. The biomimetic self-healing functionality along with skin-like sensing properties makes it attractive for next-generation wearable electronics.

Strong and Tough Supramolecular Covalent Adaptable Networks with Room-Temperature Closed-Loop Recyclability

Development of closed-loop chemically recyclable plastics (CCRPs) that can be widely used in daily life can be a fundamental solution to the global plastic waste crisis. Hence, it is of great significance to develop easy-to-recycle CCRPs that possess superior or comparable material properties to the commodity plastics. Here, a novel dual crosslinked CCRP, namely, supramolecular covalent adaptable networks (supra-CANs), is reported, which not only displays mechanical properties higher than the strong and tough commodity polycarbonate, but also exhibits excellent solvent resistance as thermosets. The supra-CANs are constructed by introducing reversible noncovalent crosslinks into the dynamic covalent polymer networks, resulting in highly stiff and strong thermosets that also exhibit thermoplastic-like ductile and tough behaviors as well as reprocessability and rehealability. In great contrast, the analogs that do not have noncovalent crosslinks (CANs) show elastomeric properties with significantly decreased mechanical strength. Importantly, the developed supra-CANs and CANs can be converted back into the initial monomers in high yields and purity at room temperature, even with additives, which enables the sustainable polymer-monomer-polymer circulation. This work provides new design principles for high-performance chemically recyclable polymers as sustainable substitutes for the conventional plastics.

Stretchable Semi-Interpenetrating Carboxymethyl Guar Gum-Based Composite Hydrogel for Moisture-Proof Wearable Strain Sensor

Wearable strain sensors of conductive hydrogels have very broad application prospects in electronic skins and human-machine interfaces. However, conductive hydrogels suffer from unstable signal transmission due to environmental humidity and inherent shortcomings of their materials. Herein, we introduce a novel moisture-proof conductive hydrogel with high toughness (2.89 MJ m-3), mechanical strength (1.00 MPa), and high moisture-proof sensing performance by using dopamine-functionalized gold nanoparticles as conductive fillers into carboxymethyl guar gum and acrylamide. Moreover, the hydrogel can realize real-time monitoring of major and subtle human movements with good sensitivity and repeatability. In addition, the hydrogel-assembled strain sensor exhibits stable sensing signals after being left for 1 h, and the relative resistance change rate under different strains (25-300%) shows no obvious noise signal up to 99% relative humidity. Notably, the wearable strain sensing is suitable for wearable sensor devices with high relative humidity.

High-Performance Zwitterionic Organohydrogel Fiber in Bioelectronics for Monitoring Bioinformation

Bioinformation plays an imperative role in day-to-day life. Wearable bioelectronics are important for sensing bioinformation in real-time and conductive hydrogel fibers are a key component in next generation wearable bioelectronics. However, current conductive hydrogel fibers have remarkable disadvantages such as insufficient conductivity, stability, and bioinformation sensing ability. Here, we report the synthesis of a zwitterionic organohydrogel (ZOH) fiber by the combination of the mold method and solvent replacement strategy. The ZOH fiber shows transparency (92.1%), stretchability (905.8%), long-term stability, anti-freezing ability (-35-60 °C), and low light transmission loss (0.17 dB/cm). Then, we integrate the ZOH fiber into fabric for use as a bioinformation sensor, the results prove its capability as a bioinformation monitor, monitoring information such as motion and bioelectric signals. In addition, the potential of the ZOH fiber in optogenetic applications is also confirmed.

Highly stretchable, self-healing, and degradable ionic conductive cellulose hydrogel for human motion monitoring

Self-healing biomass-based conductive hydrogels are applied as flexible strain sensors for wearable devices and human movement monitoring. Cellulose is the most abundant biomass-based materials and exhibits excellent toughness, dispersion and degradability. In this paper, nanocellulose crystals (NCCs) prepared from sisal, used as reinforcing fillers were coated with tannic acid (TA) to prepare inexpensive bio-nanocomposite hydrogels that also included polyvinyl alcohol, okra polysaccharide (OP), and borax. These hydrogels exhibit excellent self-healing and mechanical properties with the maximum elongation, toughness, and self-healing efficiency (9 min) of 1426.2 %, 264.4 kJ/m³, and 62.1 %, respectively. A fabricated hydrogel strain sensor was successfully used to detect and monitor various human movements such as wrist bending, elbow bending, and slight changes in facial expression. In addition, this sensor possessed excellent durability and good working stability after repeated circulation. The nanocomposite hydrogel synthesized in this work utilized natural polysaccharide to manufacture flexible functional materials with good application prospects in the field of flexible sensors.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Mechanically Strong, Wet Adhesive, and Self-Healing Polyurethane Ionogel Enhanced with a Semi-interpenetrating Network for Underwater Motion Detection

The gel-based sensors have developed rapidly in recent years toward multifunctionality. However, there are still some challenges that need to be solved, such as poor mechanical properties and inaccessibility to wet or water environments. To address these issues, we have developed an ionogel with a semi-interpenetrating network structure by adopting poly(vinylidene fluoride-co-hexafluoropropylene) as the linear non-cross-linked network, a double-bonded ionic liquid and double-bonded capped polyurethane as the cross-linked network, and an ionic liquid as the conductive media. The obtained ionogel exhibits tunable mechanical properties (3.67-8.76 MPa) and excellent sensing properties (IG-20, GF = 8.2). The superb environmental stability and self-healing properties of the ionogel were also demonstrated. Meanwhile, adhesion, self-healing, and sensing performance were guaranteed for underwater due to the presence of a large number of C-F bonds. We strongly believe that this ionogel with excellent mechanical properties and underwater communication is expected for monitoring the health of the human body and information transmission in the future.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

A 3D printable, highly stretchable, self-healing hydrogel-based sensor based on polyvinyl alcohol/sodium tetraborate/sodium alginate for human motion monitoring

Self-healing hydrogels have great application potential in the field of bio-sensors due to their self-healing, flexibility and excellent tensile properties. However, most hydrogel-based sensors are processed by template method, which is unable to fabricate complex three-dimensional (3D) structures, and limits the development of hydrogel-based sensor devices. A simple yet efficient one-pot method was proposed to fabricate polyvinyl alcohol/sodium tetraborate/sodium alginate hydrogel inks (SPB), also a fabricating process of self-healing hydrogel based on 3D printing technology has been proposed. The SPB hydrogel rapidly healed (<30 s) at room temperature, while its mechanical properties and conductivity also recovered quickly after healing. Besides, it could be used as wearable strain sensors, whose high stretchability (>2800 % strain) and sensitivity (gauge factor: 18.56 at 2000 % strain) could not only detect very large stretch deformations, but also detect the tiny pressure changes in the human body, such as finger flexion, knee flexion, and respiration. This study provides a method for the rapid fabrication of complex-structured hydrogel-based sensors, which is helpful for the hydrogel-based sensor applications in human motion detection and wearable devices.

Nature-inspired preparation of self-adhesive, frost-resistant, and ion-conductive hydrogels for flexible strain sensors

A nature-inspired strategy has been developed to prepare polyvinyl alcohol (PVA)/catechol-modified quaternized chitosan (QCS-C)/MXene hydrogels with good self-adhesion, frost-resistance, and high ion-conductivity. The PVA/QCS-C/MXene hydrogel shows an ionic conductivity of 8.82 S m-1 and a gauge factor of 33.53 at low strain (0-10%), and remains flexible and conductive at -47 °C. The PVA/QCS-C15/MXene hydrogel displays promising potential as an ionically conductive hydrogel sensor for applications in flexible electronic devices.

Sponge-Hosting Polyaniline Array Microstructures for Piezoresistive Sensors with a Wide Detection Range and High Sensitivity

The development of highly elastic spongy conductors is attractive for use in piezoresistive sensors due to their low cost, easy fabrication, and wide sensing range. However, the combination of high sensitivity and broad sensing range in a single piezoresistive sensor, though highly demanded, is challenging because they are contradictory in principle. Herein, a highly elastic spongy conductor with sponge-hosting polyaniline (PANI) fluff-like array microstructures is fabricated through cryopolymerization. The as-obtained sponge-hosting conductors exhibited an excellent elasticity under high compression strain and stress of up to 80% and 101 kPa, respectively, covering typical deformation ranges for detecting complex human activities. Benefiting from the presence of the sponge-hosting fluff-like PANI arrays, the as-prepared sponge-hosting conductors for piezoresistive sensors possessed a high sensitivity of 0.54 kPa^-1 in a broad pressure range of 0.1-101 kPa, ascribing to the formation of the sponge-hosting fluff arrays with graded conductive network structures of array contacts and sponge-skeleton contacts at low and high compressions, respectively. As a result, the as-assembled piezoresistive sensors were demonstrated for tactile sensing and human-motion monitoring. This work reveals new approaches for tailored fabrication of sponge-hosting conducting polymers with tunable fluff-like microstructures for highly sensitive and wide-range wearable piezoresistive sensors.

Recent Advances in Intelligent Wearable Medical Devices Integrating Biosensing and Drug Delivery

The primary roles of precision medicine are to perform real-time examination, administer on-demand medication, and apply instruments continuously. However, most current therapeutic systems implement these processes separately, leading to treatment interruption and limited recovery in patients. Personalized healthcare and smart medical treatment have greatly promoted research on and development of biosensing and drug-delivery integrated systems, with intelligent wearable medical devices (IWMDs) as typical systems, which have received increasing attention because of their non-invasive and customizable nature. Here, the latest progress in research on IWMDs is reviewed, including their mechanisms of integrating biosensing and on-demand drug delivery. The current challenges and future development directions of IWMDs are also discussed.

Molecular Dynamics Simulations of Polymer Nanocomposites Welding: Interfacial Structure, Dynamics and Strength

Polymer welding has received numerous scientific attention, however, the welding of polymer nanocomposites (PNCs) has not been studied yet. In this work, via coarse-grained molecular dynamics simulation, we focus our attention on investigating the welding interfacial structure, dynamics and strength by constructing the upper and lower layers of PNCs, by varying the polymer-nanoparticle (NP) interaction strength ε_NP-p . Remarkably, at low ε_NP-p , the NPs gradually migrate into the top and bottom surface layer perpendicular to the z direction during the adhesion process, while they are distributed in the middle region at high ε_NP-p . Meanwhile, the dimension of polymer chains is found to exhibit a remarkable anisotropy evidenced by the root-mean-square radius of gyration in the xy- (R_g,xy ) and z- (R_g,z ) component. The welding interdiffusion depth increases the fastest at low ε_NP-p, attributed to the high mobility of polymer chains and NPs. Lastly, although the mechanical properties of PNCs at high ε_NP-p is the strongest because of the presence of the NPs in the bulk region, the welding efficiency is the greatest at low ε_NP-p . Generally, our work could provide a fundamental understanding of the interfacial welding of PNCs, in hopes of guiding to design and fabricate excellent self-healable PNCs. This article is protected by copyright. All rights reserved.

Humins Blending in Thermoreversible Diels-Alder Networks for Stiffness Tuning and Enhanced Healing Performance for Soft Robotics

Humins waste valorization is considered to be an essential pathway to improve the economic viability of many biorefinery processes and further promote their circularity by avoiding waste formation. In this research, the incorporation of humins in a Diels-Alder (DA) polymer network based on furan-maleimide thermoreversible crosslinks was studied. A considerable enhancement of the healing efficiency was observed by just healing for 1 h at 60 °C at the expense of a reduction of the material mechanical properties, while the unfilled material showed no healing under the same conditions. Nevertheless, the thermal healing step favored the irreversible humins polycondensation, thus strengthening the material while keeping the enhanced healing performance. Our hypothesis states a synergistic healing mechanism based on humins flowing throughout the damage, followed by thermal humins crosslinking during the healing trigger, together with DA thermoreversible bonds recombination. A multi-material soft robotic gripper was manufactured out of the proposed material, showing not only improved recovery of the functional performance upon healing but also stiffness-tunable features by means of humins thermal crosslinking. For the first time, both damage healing and zone reinforcement for further damage prevention are achieved in a single intrinsic self-healing system.

β-Cyclodextrin-Based Ultrahigh Stretchable, Flexible, Electro- and Pressure-Responsive, Adhesive, Transparent Hydrogel as Motion Sensor

In the present work, a multiple-stimuli-responsive hydrogel has been synthesized via polymerization of acrylamide (AAm) and N-hydroxy methyl acrylamide (HMAm) on β-cyclodextrin (β-CD). The synthesized hydrogel β-CD-g-(pAAm/pHMAm) exhibited various striking features like ultrahigh stretchability (>6000%), flexibility, stab resistivity, self-recoverability, electroresponsiveness, pressure-responsiveness, adhesiveness, and high transparency (>90%). Besides, the hydrogel has demonstrated enhanced biocompatibility, UV resistance, and thermoresponsive shape memory behaviors. On the basis of these attractive characteristics of the hydrogel, a flexible pressure sensor for the real-time monitoring of human motion with superior biocompatibility and transparency was fabricated. Moreover, due to the nanofibrillar surface morphology of the β-CD-g-(pAAm/pHMAm) hydrogel, the sensor based on the gel exhibited high sensitivity (0.053 kPa^-1 for 0-3.3 kPa). The flexible sensor demonstrates very fast response time (130 ms-210 ms) with adequate stability (5000 cycles). Interestingly, the sensor can rapidly sense both robust (index finger and wrist) motions as well as tiny (swallowing and phonation) physiological actions. In addition, this adhesive hydrogel patch also acts as a potential carrier for the sustained topical release of (∼80.8% in 48 h) the antibiotic drug gentamicin sulfate.

Polysaccharide-based electroconductive hydrogels: Structure, properties and biomedical applications

Architecting an appropriate platform for biomedical applications requires setting a balance between simplicity and complexity. Polysaccharides (PSAs) play essential roles in our life in food resources, structural materials, and energy storage capacitors. Moreover, the diversity and abundance of PSAs have made them an indispensable part of food ingredients and cosmetics. PSA-based hydrogels have been extensively reviewed in biomedical applications. These hydrogels can be designed in different forms to show optimum performance. For instance, electroactive PSA-based hydrogels respond under an electric stimulus. Such performance can be served in stimulus drug release and determining cell fate. This review classifies and discusses the structure, properties, and applications of the most important polysaccharide-based electroactive hydrogels (agarose, alginate, chitosan, cellulose, and dextran) in medicine, focusing on their usage in tissue engineering, flexible electronics, and drug delivery applications.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Processing of Self-Healing Polymers for Soft Robotics

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self-healing polymers address this vulnerability. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. Herein, the wide variety of processing techniques of self-healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross-links present in the self-healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application.