A self-healing conductive and stretchable aligned carbon nanotube/hydrogel composite with a sandwich structure

Stretch-Induced Conductivity Enhancement in Highly Conductive and Tough Hydrogels

The resistance of gels and elastomers increases significantly with tensile strain, which reduces conductive stability and restricts their use in stable and reliable electronics. Here, highly conductive tough hydrogels composed of silver nanowires (AgNWs), liquid metal (LM), and poly(vinyl alcohol) (PVA) are fabricated. The stretch-induced orientations of AgNWs, deformable LM, and PVA nanocrystalline create conductive pathways, enhancing the mechanical properties of the hydrogels, including increased ultimate fracture stress (13-33 MPa), strain (3000-5300%), and toughness (390.9-765.1 MJ m-3). Notably, the electrical conductivity of the hydrogels is significantly improved from 4.05 × 10-3 to 24 S m-1 when stretched to 4200% strain, representing a 6000-fold enhancement. The incorporation of PVA nanocrystalline, deformable LM, and AgNWs effectively mitigates stress concentration at the crack tip, thereby conferring crack propagation insensitivity and fatigue resistance to the hydrogels. Moreover, the hydrogels are designed with a reversible crosslinking network, allowing for water-induced recycling.

Self-healing amino acid-bearing acrylamides/n-butyl acrylate copolymers via multiple noncovalent bonds

Four amino acid-bearing acrylamides, N-acryloyl-l-threonine (AThrOH), N-acryloyl-l-glutamic acid (AGluOH), N-acryloyl-l-phenylalanine (APheOH), and N-acryloyl-l, l-diphenylalanine (APhePheOH), were selected for copolymerization with n-butyl acrylate (nBA) to develop amino acid-based self-healable copolymers. A series of copolymers comprising amino acid-bearing acrylamides and nBA with tunable comonomer compositions and molecular weights were synthesized by free radical and reversible addition-fragmentation chain-transfer copolymerization. Self-healing and mechanical properties originated from the noncovalent bonds between the carboxyl, hydroxyl, and amide groups, and π-π stacking interactions among the amino acid residues in the side chains were evaluated. Among these copolymers, P(nBA-co-AGluOH) with suitable comonomer compositions and molecular weights (nBA : AGluOH = 82 : 18, Mn = 18 300, Mw/Mn = 2.58) exhibited good mechanical properties (modulus of toughness = 17.3 MJ m-3) and self-healing under ambient conditions. The multiple noncovalent bonds of P(nBA-co-AGluOH)s were also efficient in improving the optical properties with an enhanced refractive index and good transparency.

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.

Stretchable, freezing-tolerant conductive hydrogel for wearable electronics reinforced by cellulose nanocrystals toward multiple hydrogen bonding

Conductive hydrogels have the mechanical properties and the electronic transport properties of conductive polymers, which have been widely used in the fields of energy storage and bioelectronics. However, the rigidity and brittleness of conductive polymers hinder the long-term stability of hydrogels and limit its application in new flexible electronic devices. It is a high challenging task to prepare ion-conductive hydrogels with excellent mechanical properties, anti-freeze and electrical conductivity through a simple preparation process under the action of hydrogen bonds. We present a facile strategy to prepare mechanically tough, swelling ability hydrogels reinforced by cellulose nanocrystals (CNCs). Herein, CNCs were produced by high pressure homogeneous and pretreated with deep eutectic solvent (DES). The conductivity of the hydrogel is 0.021 S/cm at room temperature. Due to the function of DMSO/H₂O in organic solvent system, the ion-conducting hydrogel remains flexible and conductive (0.014 S/cm) at -70 °C. Hydrogel has excellent mechanical properties (stress about 1.4 MPa, strain about 1018%), excellent transparency, freezing resistance (-70 °C) and other comprehensive characteristics. The hydrogel can be assembled into a sensor for use in monitoring large and small movements of the human body, showing good responsiveness and stability.

A review on the features, performance and potential applications of hydrogel-based wearable strain/pressure sensors

Over the past few years, development of wearable devices has gained increasing momentum. Notably, the demand for stretchable strain sensors has significantly increased due to many potential and emerging applications such as human motion monitoring, prosthetics, robotic systems, and touch panels. Recently, hydrogels have been developed to overcome the drawbacks of the elastomer-based wearable strain sensors, caused by insufficient biocompatibility, brittle mechanical properties, complicated fabrication process, as the hydrogels can provide a combination of various exciting properties such as intrinsic electrical conductivity, suitable mechanical properties, and biocompatibility. There are numerous research works reported in the literature which consider various aspects as preparation approaches, design strategies, properties control, and applications of hydrogel-based strain sensors. This article aims to present a review on this exciting topic with a new insight on the hydrogel-based wearable strain sensors in terms of their features, strain sensory performance, and prospective applications. In this respect, we first briefly review recent advances related to designing the materials and the methods for promoting hydrogels' intrinsic features. Then, strain (both tensile and pressure) sensing performance of prepared hydrogels is critically studied, and alternative approaches for their high-performance sensing are proposed. Subsequently, this review provides several promising applications of hydrogel-based strain sensors, including bioapplications and human-machine interface devices. Finally, challenges and future outlooks of conductive and stretchable hydrogels employed in the wearable strain sensors are discussed.

Self-Healing Silicone Elastomer with Stable and High Adhesion in Harsh Environments

Adhesive and self-healing elastomers are urgently needed for their convenience and intelligence in biological medicine, flexible electronics, intelligent residential systems, etc. However, their inevitable use in harsh environments results in further enhancement requirements of the structure and performance of adhesive and self-healing elastomers. Herein, a novel self-healing and high-adhesion silicone elastomer was designed by the synergistic effect of multiple dynamic bonds. It revealed excellent stretchability (368%) and self-healing properties at room temperature (98.1%, 5 h) and in a water environment (96.4% for 5 h). Meanwhile, the resultant silicone elastomer exhibited high adhesion to metal and nonmetal and showed stable adhesion in harsh environments, such as under acidic (pH 1) and alkaline (pH 12) environments, salt water, petroleum ether, water, etc. Furthermore, it was applied as a shatter-proof protective layer and a rust-proof coating, proving its significant potential in intelligent residential system applications.

Self-Healing Mechanism and Conductivity of the Hydrogel Flexible Sensors: A Review

Sensors are devices that can capture changes in environmental parameters and convert them into electrical signals to output, which are widely used in all aspects of life. Flexible sensors, sensors made of flexible materials, not only overcome the limitations of the environment on detection devices but also expand the application of sensors in human health and biomedicine. Conductivity and flexibility are the most important parameters for flexible sensors, and hydrogels are currently considered to be an ideal matrix material due to their excellent flexibility and biocompatibility. In particular, compared with flexible sensors based on elastomers with a high modulus, the hydrogel sensor has better stretchability and can be tightly attached to the surface of objects. However, for hydrogel sensors, a poor mechanical lifetime is always an issue. To address this challenge, a self-healing hydrogel has been proposed. Currently, a large number of studies on the self-healing property have been performed, and numerous exciting results have been obtained, but there are few detailed reviews focusing on the self-healing mechanism and conductivity of hydrogel flexible sensors. This paper presents an overview of self-healing hydrogel flexible sensors, focusing on their self-healing mechanism and conductivity. Moreover, the advantages and disadvantages of different types of sensors have been summarized and discussed. Finally, the key issues and challenges for self-healing flexible sensors are also identified and discussed along with recommendations for the future.

Induction heating induced self-healing of nanocomposites based on surface-functionalized cationic iron oxide particles and polyelectrolytes

Supramolecular interactions represent versatile, reversible, and intrinsic mechanisms for bond formation after the failure of materials. Ionic interactions excel through high flexibility and binding strength. In this study, ionic interactions between polymer matrices and inorganic nanoparticles were used to induce self-healing properties. Random, anionic polyelectrolyte copolymers consisting of di(ethylene glycol) methyl ether methacrylate and sodium-4-(methacryloyloxy)butan-1-sulfonate were synthesized by atom transfer radical polymerization. Differential scanning calorimetry measurements confirmed the adjustability of the glass transition temperature via the polymer composition. Within the glass transition temperature window of the homopolymers from -23 °C to 126 °C, the range between -18 °C to 50 °C was examined, generating suitable matrices for self-healing. Superparamagnetic iron oxide nanoparticles with a size of 8 nm were synthesized by thermal decomposition of iron(iii) acetylacetonate and used as the inorganic filler. Positive surface charges were introduced by functionalization with N,N,N-trimethyl-6-phosphonhexan-1-aminium bromide. Functionalization was confirmed with FTIR, TGA, and zeta potential measurements. Ionic interactions between filler and polymer promote a uniform particle dispersion within the material. Self-healing experiments were performed at 80 °C and without the addition of further healing agents. Utilizing the magnetic properties induced by the iron oxide nanoparticles, spatially resolved healing within an alternating magnetic field was achieved on a μm scale.

Soft Electronic Materials with Combinatorial Properties Generated via Mussel-Inspired Chemistry and Halloysite Nanotube Reinforcement

Soft and electrically active materials are currently being utilized for intelligent systems, including electronic skin, cybernetics, soft robotics, and wearable devices. However, fabricating materials that fulfill the complex requirements of such advanced applications remains a challenge. These attributes include electronic, adhesive, self-healing, flexible, moldable, printable, and strong mechanical properties. Inspired by the recent interest in transforming monofunctional materials into multifunctional ones through nanoreinforcement and mussel-inspired chemistry, we have designed a simple two-step methodology based on halloysite nanotube (HNT) and polydopamine (PDA) to address the grand challenges in the field. In brief, HNTs were coated with PDA and embedded within a poly(vinyl alcohol) (PVA)-based polymeric matrix in combination with ferric ions (Fe³⁺). The final composite displayed a 3-fold increase in electrical conductivity, a 20-fold increase in mechanical stiffness, and a 7-fold increase in energy dissipation in comparison to their nonfunctional counterparts, which arose from a combination of nanotube alignment and mussel-inspired chemistry. Moreover, the developed composite could elongate up to 30000% of its original length, maintain its electrical properties after 600% strain, self-heal within seconds (both electrically and mechanically), and display strain-sensitivity. Finally, it was 3D-printable and thus amenable for engineering of customized wearable electronics.

Self-healing and easy-to-shape mineralized hydrogels for iontronics

Hydrogel-based multifunctional materials have attracted much attention. In this work, novel mineralized hydrogels were fabricated through physically cross-linked polyvinylpyrrolidone (PVP) and CaCO3. The mineralized hydrogels were prepared by simply mixing CaCl2, Na2CO3, and PVP in aqueous solutions. The CO32- induced the aggregation of the PVP chains and the CaCO3 particles in situ generated in the aqueous solution worked as fillers to strengthen the hydrogels. Based on this method, other kinds of mineralized hydrogels were prepared by replacing the Ca2+ with different metal ions. The mineralized hydrogels displayed shapeable, self-healing and thixotropic properties. Moreover, the mineralized hydrogel-based sensor showed good and stable sensitivity to compressive pressure, and could be used to monitor human actions. This work presents a facile method for preparing mineralized hydrogels, which are promising for various applications due to their outstanding properties.

A porous self-healing hydrogel with an island-bridge structure for strain and pressure sensors

Conductive hydrogels have attracted widespread attention in wearable electronic devices and human motion detection. However, designing self-healing hydrogels with high conductivity and excellent mechanical properties remains a challenge. In this work, polyvinyl alcohol/carbon nanotubes/graphene (PVA/CNTs/graphene) with an island-bridge hydrogel structure and self-healing properties was designed by merging PVA/CNTs hydrogel and PVA/graphene hydrogel, in which the PVA/graphene hydrogel acts as an "island" and PVA/CNTs hydrogel acts as a "bridge" to bridge the entire conductive network. Hydrogen-bonding between the borate ion and the -OH group of PVA allows the conductive hydrogel to heal without any external stimulation. The PVA/CNTs/graphene hydrogel can be used for both stretchable strain and pressure sensors. The obtained PVA/CNTs/graphene composite hydrogel exhibits excellent electrical conductivity, extreme high elastic strain (up to 900%) and strong mechanical pressure (up to 10 kPa). The strain sensor based on the PVA/CNTs/graphene hydrogel exhibits excellent tensile strain sensitivity (a gauge factor of 152.6 in the strain region of 316-600%) and wide detection working range (1-600%) with high durability and repeatability. The sensor also remains highly sensitive when being used as a pressure sensor (-0.127 kPa-1 at 0-5 kPa). Additionally, the PVA/CNTs/graphene hydrogel-based sensor can detect human motions after multiple cuts and self-healing with excellent stability and repeatability. The PVA/CNTs/graphene hydrogel provides a new idea in the development of wearable electronics, demonstrating the potential of the next generation of wearable electronics.

Self-healing hydrogel sensors with multiple shape memory properties for human motion monitoring

Shape memory hydrogels offer new opportunities for the development of smart wearables due to their intelligent responsiveness.

Self-healing and mechanical performance of dynamic glycol chitosan hydrogel nanocomposites

Evaluation of Schiff base nanocomposite hydrogels properties using a benzaldehyde multifunctional amphiphilic polyacrylamide crosslinker in conjunction with glycol chitosan.

Materials engineering, processing, and device application of hydrogel nanocomposites

Hydrogels are widely implemented as key materials in various biomedical applications owing to their soft, flexible, hydrophilic, and quasi-solid nature. Recently, however, new material properties over those of bare hydrogels have been sought for novel applications. Accordingly, hydrogel nanocomposites, i.e., hydrogels converged with nanomaterials, have been proposed for the functional transformation of conventional hydrogels. The incorporation of suitable nanomaterials into the hydrogel matrix allows the hydrogel nanocomposite to exhibit multi-functionality in addition to the biocompatible feature of the original hydrogel. Therefore, various hydrogel composites with nanomaterials, including nanoparticles, nanowires, and nanosheets, have been developed for diverse purposes, such as catalysis, environmental purification, bio-imaging, sensing, and controlled drug delivery. Furthermore, novel technologies for the patterning of such hydrogel nanocomposites into desired shapes have been developed. The combination of such material engineering and processing technologies has enabled the hydrogel nanocomposite to become a key soft component of electronic, electrochemical, and biomedical devices. We herein review the recent research trend in the field of hydrogel nanocomposites, particularly focusing on materials engineering, processing, and device applications. Furthermore, the conclusions are presented with the scope of future research outlook, which also includes the current technical limitations.

Polyvinyl Alcohol-Stabilized Liquid Metal Hydrogel for Wearable Transient Epidermal Sensors

Wearable epidermal sensors are attracting growing interests in human activity monitoring and flexible touch display, but they are still limited by the poor self-healing property and the difficult dissolvable feature. Herein, we report polyvinyl alcohol (PVA)-stabilized liquid metal particles (LMPs) (PVA-LMPs) hydrogels with excellent self-healing performance and the dissolvable feature for wearable epidermal sensors, constructed by dispersing LMPs of eutectic gallium and indium into the borate-modified PVA polymer networks. Interestingly, the PVA-LMPs hydrogels exhibited excellent electrically and mechanically self-healing ability. Moreover, the PVA-LMPs hydrogel can be fabricated as epidermal sensors, which can accurately monitor the human activities. Additionally, the epidermal sensors are dissolvable, showing an attractive feature for on demand transient electronics. It is demonstrated that the hydroxyl groups of PVA can stabilize LMPs via hydrogen-bonding interactions. Furthermore, the dynamic cross-linking bonds between hydrogels and LMPs can rupture and coalesce reversibly in the hydrogel network, which endow the hydrogels with both electrically and mechanically self-healing ability. This work shows the potential of constructing next-generation multifunctional hydrogel-based epidermal sensors for human activity monitoring, wearable healthcare diagnosis, portable electronics, and robot tactile systems.

Highly compressive and stretchable poly(ethylene glycol) based hydrogels synthesised using pH-responsive nanogels without free-radical chemistry

Poly(ethylene glycol) (PEG) based hydrogels are amongst the most studied synthetic hydrogels. However, reports on PEG-based hydrogels with high mechanical strength are limited. Herein, a class of novel, well-defined PEG-based nanocomposite hydrogels with tunable mechanical strength are synthesised via ring-opening reactions of diglycidyl ethers with carboxylate ions. The pH responsive crosslinked polyacid nanogels (NG) in the dispersed phase act as high functionality crosslinkers which covalently bond to the poly(ethylene glycol) diglycidyl ethers (PEGDGE) as the continuous matrix. A series of NG-x-PEG-y-z gels are prepared where x, y and z are concentrations of NGs, PEGDGE and the PEGDGE molecular weight, respectively. The hydrogel compositions and nano-structural homogeneity of the NGs have strong impact on the enhancement of mechanical properties which enables property tuning. Based on this design, a highly compressive PEG-based nanocomposite hydrogel (NG-13-PEG-20-6000) exhibits a compressive stress of 24.2 MPa, compressive fracture strain greater than 98% and a fracture energy density as high as 1.88 MJ m-3. The tensile fracture strain is 230%. This is amongst one of the most compressive PEG-based hydrogels reported to-date. Our chemically crosslinked gels are resilient and show highly recoverable dissipative energy. The cytotoxicity test shows that human nucleus pulposus (NP) cells remained viable after 8 days of culture time. The overall results highlight their potential for applications as replacements for intervertebral discs or articular cartilages.