Unbreakable Hydrogels with Self-Recoverable 10 200% Stretchability

Multifunctional natural starch-based hydrogels: Critical characteristics, formation mechanisms, various applications, future perspectives

With the growth of the global population and increasing concern for environmental issues, the development of sustainable and eco-friendly materials has become increasingly important. Starch, as a renewable resource, is one of the most abundant polysaccharides in nature, with the advantages of good biocompatibility, high biodegradability, and low cost. Starch-based hydrogels (SBHs) have attracted widespread attention due to their unique physical and chemical properties. This article provides a comprehensive review of the latest research progress in SBHs, discussing their main characteristics, formation mechanisms, diverse applications, and future development trends. First, it outlines the biocompatibility, degradability, water absorption and retention, environmental responsiveness, and mechanical strength of SBHs. Then, it elaborates in detail on the formation mechanisms of SBHs, including physical crosslinking (hydrogen bonding, electrostatic interactions, host-guest and coordination interactions), chemical crosslinking (such as initiators, heat, light, radiation, and click reactions), and synergistic effects. Subsequently, it analyzes the applications of SBHs in cutting-edge fields such as flexible sensors, medical dressings, drug delivery, tissue engineering, soil protection, wastewater treatment, and food packaging. Finally, it summarizes the challenges in current research and provides an outlook on future development trends, emphasizing the importance of further optimizing the performance of SBHs to meet broader industrial needs and environmental protection goals. This review not only provides a systematic theoretical framework for the study of SBHs but also charts a course for their innovative applications in the field of sustainable materials, playing a significant role in advancing the continuous development of this area.

Hierarchical Engineering of Amphiphilic Peptides Nanofibrous Crosslinkers toward Mechanically Robust, Functionally Customable, and Sustainable Supramolecular Hydrogels

Hierarchical architectures spanning multiple length scales are ubiquitous in biological tissues, conferring them with both mechanical robustness and dynamic functionalities via structural reorganization under loads. The design of hierarchical architectures within synthetic hydrogels to concurrently achieve mechanical reinforcement and functional integration remains challenging. Here, a biomimetic hierarchical engineering approach is reported to develop mechanically robust and function-customizable supramolecular hydrogels by utilizing strong yet dynamic fibrous nanoarchitectures of amphiphilic peptides as crosslinkers. This design, on one hand, resolves the strength-toughness trade-off in hydrogel design through energy-dissipative mechanisms involving dynamic detachment and reinsertion of peptides within their assembled nanostructures upon loading. On the other hand, the amphiphilicity and sequence programmability of peptides allow spatially orthogonal integration of multiple dynamic functionalities across distinct structural domains, including lipophilic fluorophore encapsulation, photopatterning capability, and anisotropic contraction. Capitalizing on its ultralow hysteresis and rapid recovery properties, the hydrogel's effectiveness is demonstrated as high-sensitivity strain sensors. Moreover, the fully noncovalent crosslinking strategy permits closed-loop recycling and reprocessing via reversible crosslinker disassembly-reassembly processes. Through systematic extension of this principle across diverse peptide systems, a generalized platform is demonstrated for creating advanced soft materials that synergistically integrate traditionally incompatible attributes of mechanical robustness, customable dynamic functionality, and environmental sustainability.

Boosting Electronic Charge Transport in Conductive Hydrogels via Rapid Ion-Electron Transduction

Traditional design of conductive hydrogels involves embedding conductive components within a hydrated polymeric network to establish interconnected electron pathways. While the hydration shell of the polymeric network is typically considered insulating, we demonstrate that it can enhance electron transport. Using a PEDOT:PSS hydrogel, we propose a hierarchical network with an inhomogeneous topological structure, consisting of entangled PSS chains, dense PSS assemblies, and PEDOT microcrystals. In the hydrated state, the dense PSS assemblies significantly lower the energy barrier for electron hopping between PEDOT microcrystals, thereby promoting electron transport. As a result, the charge transport mechanism in these hydrogels is predominantly electronic rather than ionic, effectively mimicking the behavior of electronic conductors. The charge transport rate reaches up to 2 × 106 m s-1, which is approximately five orders of magnitude higher than that of ion-based processes. This characteristic imparts the hydrogels with kinetically sensitive ion-electron transduction, enabling time-resolved electrochemical analysis of biochemical processes.

Super-strong hydrogel reinforced by an interconnected hollow microfiber network via regulating the water-cellulose-copolymer interplay

The discontinuous fiber reinforced hydrogels are easy to fail due to the fracture of the fiber matrix during load-bearing. Here, we propose a novel strategy based on the synergistic reinforcement of interconnected natural fiber networks at multiple scales to fabricate hydrogels with extraordinary mechanical properties. Specifically, the P(AA-AM)/Cel (P(AA-AM), poly(acrylic acid-acrylamide); Cel, cellulose) hydrogel is synthesized by copolymerizing AA and AM on a substrate of paper with an interconnected hollow cellulose microfiber network. This innovative design achieves a collaborative improvement of mechanical properties, including a 253-times increase in strength (27.8 vs. 0.11 MPa), 137-times increase in work of fracture (3.59 vs. 0.026 MJ m-3), and 235-times increase in fracture energy (16.48 vs. 0.07 kJ m-2). These outstanding mechanical properties benefit from the P(AA-AM) network formed by the copolymerization, which fills both the inside and outside of the hollow cellulose fibers, thus establishing abundant strong hydrogen bonds with the fibers and welding the fiber junctions. Consequently, the hydrogel exhibits enhanced resistance to the slippage and fracture of fibers. This strategy demonstrates the mechanical strengthening effectiveness of a variety of hydrogels by regulating the water-cellulose-copolymer interplay, representing a practical and universal route for designing super-strong hydrogels.

Puncture-resistant hydrogels with high mechanical performance achieved supersaturated salt

Sufficient mechanical performance is the basic requirement for load-bearing and damage-resistant materials. However, the simultaneous optimization of mechanical properties is usually difficult in a single hydrogel. Herein, a supersaturated salt was employed to enhance the mechanical performance and damage resistance of hydrogels. By immersing the pre-formed hydrogel based on hydrophobic associations into supersaturated Na2SO4 solution (3.3 M), high-density and strong hydrophobic associations were constructed simultaneously in the network due to the contraction of hydrophilic chains and improvement of hydrophobic associations. Compared to the pristine hydrogel, this salt-treated hydrogel was transparent and showed a simultaneous enhancement in stiffness (E of 253 ± 7 MPa), strength (σ of 12.65 ± 0.07 MPa), and toughness (Γ of 19.6 ± 3.2 MJ m-3). It also displayed remarkable puncture and tear resistance with a puncture force of 66 N, a puncture energy of 370 mJ, and a tearing energy of 34 kJ m-2. This work provides a simple method to simultaneously optimize the contradictory mechanical properties and puncture resistance in a single hydrogel.

An Oriented Interpenetrating Network Structure Multi-Stimuli Responsive Hydrogel

As a recent focal point of research, soft electronics encompass various factors that synergistically enhance their mechanical properties and ensure stable electrical performance. However, challenges such as immiscible conductive fillers, poor phase interfaces, and unstable conductive networks hinder the overall efficacy of these materials. To address these issues, a hydrogel featuring an oriented interpenetrating network structure (OIPN) is developed. The pyrrole monomer is in situ polymerized within the confined space of PVA macromolecular chains at low temperatures, resulting in a double network structure. Subsequently, the conductive hydrogel with an OIPN configuration is synthesized through directional freezing combined with salting out techniques. After doping phytic acid (IP6), non-covalent bonds dynamically reinforce the dual network architecture and the pathways for conductivity transfer. Due to its distinctive OIPN structure, the hydrogel containing 50% PPy and 2.3% IP6 exhibits remarkable conductivity (75 µs mm-1), excellent stretchability (400%), optimal multi-stimuli sensing responses (mechanical and gaseous stimuli), and outstanding device stability (over 2600 cycles at 40% strain). This multifunctional hydrogel presents a promising strategy for advancing applications in soft electronic materials.

Starch/ionic liquid/hydrophobic association hydrogel with high stretchability, fatigue resistance, self-recovery and conductivity for sensitive wireless wearable sensors

Conductive hydrogels have been widely used in wearable electronics due to their flexible, conductive and adjustable properties. With ever-growing demand for sustainable and biocompatible sensing materials, biopolymer-based hydrogels have drawn significant attention. Among them, starch-based hydrogels have a great potential for wearable electronics. However, it remains challenging to develop multifunctional starch-based hydrogels with high stretchability, good conductivity, excellent durability and high sensitivity. Herein, amylopectin and ionic liquid were introduced into a hydrophobic association hydrogel to endow it with versatility. Benefiting from the synergistic effect of amylopectin and ionic liquid, the hydrogel exhibited excellent mechanical properties (the elongation of 2540 % with a Young's modulus of 12.0 kPa and a toughness of 1.3 MJ·m-3), self-recovery, good electrical properties (a conductivity of 1.8 S·m-1 and electrical self-healing), high sensitivity (gauge factor up to 26.85) and excellent durability (5850 cycles). The above properties of the hydrogel were closely correlated to its internal structure from hydrophobic association, H-bonding and electrostatic interaction, and can be regulated by changing the component contents. A wireless wearable sensor based on the hydrogel realized accurate and stable monitoring of joint motions and expression changes. This work demonstrates a kind of promising biopolymer-based materials as candidates for high-performance flexible wearable sensors.

Enhancement of hybrid organohydrogels by interpenetrating crosslinking strategies for multi-source signal recognition over a wide temperature range

With substantial temperature differentials between summer and winter in polar regions, there exists a pressing necessity for flexible sensors capable of functioning across a broad temperature spectrum to facilitate the construction of a more intelligent human-machine interface. Nevertheless, developing flexible sensors resilient to extremely low temperatures remains a significant challenge. In this study, we present an organohydrogel capable of functioning ranging from ambient to -78 °C, enabling real-time monitoring of multi-source signals, including motion, physiology, speech, and pressure. We synthesize organohydrogel employing a singular methodology: interpenetrating network structures as matrix frameworks, dynamic hydrophobic linkages as the physical cross-linking points, and incorporating a bionic binder. H-Bonding and chain entanglement synergistic supramolecular interactions build the organohydrogel matrix with microphase-separated domains, which, together with the combination of binary solvents and inorganic salts, allows it to exhibit excellent properties, including large stretchability (≈1700%), high ionic conductivity (1.57 S m-1), admirable sensing sensitivity performance (gauge factor: GF = 6.47, S = 0.32 kPa-1), an exceptionally low-pressure detection threshold (≈1 Pa), enables wireless transmission of distress signals through human-machine interaction even at -78 °C, which makes it possible to use it in polar exploration and to give robots a "sense of touch" for a variety of deep-diving tasks.

Multi-Modal Sensing Ionogels with Tunable Mechanical Properties and Environmental Stability for Aquatic and Atmospheric Environments

Ionogels have garnered significant interest due to their great potential in flexible iontronic devices. However, their limited mechanical tunability and environmental intolerance have posed significant challenges for their integration into next-generation flexible electronics in different scenarios. Herein, the synergistic effect of cation-oxygen coordination interaction and hydrogen bonding is leveraged to construct a 3D supramolecular network, resulting in ionogels with tunable modulus, stretchability, and strength, achieving an unprecedented elongation at break of 10 800%. Moreover, the supramolecular network endows the ionogels with extremely high fracture energy, crack insensitivity, and high elasticity. Meanwhile, the high environmental stability and hydrophobic network of the ionogels further shield them from the unfavorable effects of temperature variations and water molecules, enabling them to operate within a broad temperature range and exhibit robust underwater adhesion. Then, the ionogel is assembled into a wearable sensor, demonstrating its great potential in flexible sensing (temperature, pressure, and strain) and underwater signal transmission. This work can inspire the applications of ionogels in multifunctional sensing and wearable fields.

High-tear resistant gels crosslinked by DA@CNC for 3D printing flexible wearable devices

Photocurable gels have broad application prospects in biomedicine, bionics, flexible wearable devices and other fields. However, there are still some problems in the current photocurable gels, such as notch sensitivity, that is, poor tear resistance. In this study, we provided a photocurable gel with excellent tear resistance. The gel prepolymer is mainly composed of hydroxymethylacrylamide (NAM) and cellulose nanocrystals (CNC) modified with dopamine hydrochloride (DA), referred to as DA@CNC. After photocuring, the prepared gels show excellent mechanical properties such as tear resistance, elasticity and toughness. The introduction of DA@CNC not only endows gels with a large amount of energy dissipation through hydrogen bond crosslinking, but also effectively resists crack expansion as a nano-sized reinforcing phase, which greatly improves the tear resistance of the gels. Even at a 40 % gap, the elongation at break of the gel can still reach 1445 %. In addition, the DA can endow the gel with good electrical conductivity and excellent sensitivity (GF = 23.8). Some flexible wearable devices like finger sleeve and wristband can be customized by photocurable 3D printing using the gel with high toughness. This high-performance gel has great application potential in flexible wearable devices.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Advances and applications of biomimetic biomaterials for endogenous skin regeneration

Endogenous regeneration is becoming an increasingly important strategy for wound healing as it facilitates skin's own regenerative potential for self-healing, thereby avoiding the risks of immune rejection and exogenous infection. However, currently applied biomaterials for inducing endogenous skin regeneration are simplistic in their structure and function, lacking the ability to accurately mimic the intricate tissue structure and regulate the disordered microenvironment. Novel biomimetic biomaterials with precise structure, chemical composition, and biophysical properties offer a promising avenue for achieving perfect endogenous skin regeneration. Here, we outline the recent advances in biomimetic materials induced endogenous skin regeneration from the aspects of structural and functional mimicry, physiological process regulation, and biophysical property design. Furthermore, novel techniques including in situ reprograming, flexible electronic skin, artificial intelligence, single-cell sequencing, and spatial transcriptomics, which have potential to contribute to the development of biomimetic biomaterials are highlighted. Finally, the prospects and challenges of further research and application of biomimetic biomaterials are discussed. This review provides reference to address the clinical problems of rapid and high-quality skin regeneration.

Functionalized hydrogels as smart gene delivery systems to treat musculoskeletal disorders

Despite critical advances in regenerative medicine, the generation of definitive, reliable treatments for musculoskeletal diseases remains challenging. Gene therapy based on the delivery of therapeutic genetic sequences has strong value to offer effective, durable options to decisively manage such disorders. Furthermore, scaffold-mediated gene therapy provides powerful alternatives to overcome hurdles associated with classical gene therapy, allowing for the spatiotemporal delivery of candidate genes to sites of injury. Among the many scaffolds for musculoskeletal research, hydrogels raised increasing attention in addition to other potent systems (solid, hybrid scaffolds) due to their versatility and competence as drug and cell carriers in tissue engineering and wound dressing. Attractive functionalities of hydrogels for musculoskeletal therapy include their injectability, stimuli-responsiveness, self-healing, and nanocomposition that may further allow to upgrade of them as "intelligently" efficient and mechanically strong platforms, rather than as just inert vehicles. Such functionalized hydrogels may also be tuned to successfully transfer therapeutic genes in a minimally invasive manner in order to protect their cargos and allow for their long-term effects. In light of such features, this review focuses on functionalized hydrogels and demonstrates their competence for the treatment of musculoskeletal disorders using gene therapy procedures, from gene therapy principles to hydrogel functionalization methods and applications of hydrogel-mediated gene therapy for musculoskeletal disorders, while remaining challenges are being discussed in the perspective of translation in patients. STATEMENT OF SIGNIFICANCE: Despite advances in regenerative medicine, the generation of definitive, reliable treatments for musculoskeletal diseases remains challenging. Gene therapy has strong value in offering effective, durable options to decisively manage such disorders. Scaffold-mediated gene therapy provides powerful alternatives to overcome hurdles associated with classical gene therapy. Among many scaffolds for musculoskeletal research, hydrogels raised increasing attention. Functionalities including injectability, stimuli-responsiveness, and self-healing, tune them as "intelligently" efficient and mechanically strong platforms, rather than as just inert vehicles. This review introduces functionalized hydrogels for musculoskeletal disorder treatment using gene therapy procedures, from gene therapy principles to functionalized hydrogels and applications of hydrogel-mediated gene therapy for musculoskeletal disorders, while remaining challenges are discussed from the perspective of translation in patients.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

Strong Acid Enabled Comprehensive Training of Poly (Sodium Acrylate) Hydrogel Networks

The design of admirable hydrogel networks is of both practical and fundamental importance for diverse applications of hydrogels. Herein a general strategy of acid-assisted training is designed to enable multiple improvements of conventional poly (sodium acrylate) networks for hydrogels. Hydrophobic homogeneous crosslinked poly (sodium acrylate) hydrogels are prepared to verify the strategy. The multiple improvements of poly (sodium acrylate) networks are simply achieved by immersing the hydrogel networks into 4 M H2SO4 solutions. The introduced acids would induce transformation of poly (sodium acrylate) into poly (acrylic acid) at hydrogel surface, which constructs dynamic hydrogen bonding interactions to tighten the network. The acid-containing poly (sodium acrylate) hydrogels newly generate anti-swelling and self-healing performance, and show mechanical improvement. The internal poly (sodium acrylate) of the pristine acid-containing hydrogels is further fully transformed via acid-infiltration after following cyclic stretch/release training to significantly improve the mechanical performance. The Young's modulus, stress, and toughness of the fully-trained hydrogels are 187.6 times, 35.6 times, and 5.4 times enhanced, respectively. The polymeric networks retain isotropic in fully-trained hydrogels to ensure superior stretchability of 8.6. The acid-assisted training performance of the hydrogels can be reversibly recovered by NaOH neutralization. The acid-assisted training strategy here is general for poly (sodium acrylate) hydrogels.

Superabsorbent quaternary ammonium guar gum hydrogel with controlled release of humic acid for soil improvement and plant growth

Growing plants in karst areas tends to be difficult due to the easy loss of water and soil. To enhance soil agglomeration, water retention, and soil fertility, this study developed a physically and chemically crosslinked hydrogel prepared from quaternary ammonium guar gum and humic acid. The results showed that non-covalent dynamic bonds between the two components delayed humic acid release into the soil, with a release rate of only 35 % after 240 h. The presence of four hydrophilic groups (quaternary ammonium, hydroxyl, carboxyl, and carbonyl) in the hydrogel more than doubled the soil's water retention capacity. The interaction between hydrogel and soil minerals (especially carbonate and silica) promoted hydrogel-soil and soil‑carbonate adhesion, and the adhesion strength between soil particles was enhanced by 650 %. Moreover, compared with direct fertilization, this degradable hydrogel not only increased the germination rate (100 %) and growth status of mung beans but also reduced the negative effects of excessive fertilization on plant roots. The study provides an eco-friendly, low-cost, and intelligent system for soil improvement in karst areas. It further proves the considerable application potential of hydrogels in agriculture.

Toughening Hydrogels with Fibrillar Connected Double Networks

Biological tissues, such as tendons or cartilage, possess high strength and toughness with very low plastic deformations. In contrast, current strategies to prepare tough hydrogels commonly utilize energy dissipation mechanisms based on physical bonds that lead to irreversible large plastic deformations, thus limiting their load-bearing applications. This article reports a strategy to toughen hydrogels using fibrillar connected double networks (fc-DN), which consist of two distinct but chemically interconnected polymer networks, that is, a polyacrylamide network and an acrylated agarose fibril network. The fc-DN design allows efficient stress transfer between the two networks and high fibril alignment during deformation, both contributing to high strength and toughness, while the chemical crosslinking ensures low plastic deformations after undergoing high strains. The mechanical properties of the fc-DN network can be readily tuned to reach an ultimate tensile strength of 8 MPa and a toughness of above 55 MJ m-3, which is 3 and 3.5 times more than that of fibrillar double network hydrogels without chemical connections, respectively. The application potential of the fc-DN hydrogel is demonstrated as load-bearing damping material for a jointed robotic lander. The fc-DN design provides a new toughening mechanism for hydrogels that can be used for soft robotics or bioelectronic applications.

Water-Soluble, Self-Healing, and Debonding Primer for the Interface between Silicone Leather and Polydimethylsiloxane Composites

The exceptional hydrophobicity and antifouling properties of polydimethylsiloxane (PDMS) composites have attracted extensive interest as a result of low surface energy. However, PDMS composites are hardly bound by common primers, greatly limiting their practical applications. To address this issue, an EPMS/VTMS (EV) primer synthesized by hydrolytic polycondensation of 3-[(2,3)-epoxypropoxypropyl]methyldiethoxysilane (EPMS) and vinyltrimethoxysilane (VTMS) in acidic conditions is proposed. Interestingly, the EV primer exhibits exceptional reactivity, self-healing capabilities, and strong adhesion to various substrates, including metals, plastics, and glass. The bonding aluminum plates are easily debonded by immersion in water without any residue left. Subsequently, the EV primer has been applied to the interface between silicone leather and the PDMS composite. Results show that the bonding strength for the silicone leather coated with the EV/PDMS composite is 16 times higher than that of the silicone leather modified with the PDMS composite. Meanwhile, the modified silicone leather exhibits impressive antifouling performance, including aqueous and oily stains, appreciable breathability, and excellent wear resistance, even if suffering from 20 000 cycles of abrasion. These excellent advantages for the modified silicone leather should be attributable to the synergistic effect of the EV primer and the PDMS composite. These findings pave the way to develop an ecofriendly debonding primer for PDMS composites, greatly facilitating applications of silicone leather.

Tough Hydrogels for Load-Bearing Applications

Tough hydrogels have emerged as a promising class of materials to target load-bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high-performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state-of-the-art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high-performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high-performance hydrogels are compared with existing materials, and promising future opportunities are discussed.

Hydrophobic association: A facile approach to prepare physical cross-linked gelatin hydrogel with desirable thermal stability, flexibility and self-healing ability

Methacrylic anhydride was added to 20 % gelatin solution to prepare gelatin methacryloyl (GelMA) but an unexpected gelation process was observed within several minutes. The experimental data revealed that the methacryloyl substitution can increase the hydrophobicity of gelatin and the micellar diameter in solution. Therefore, we speculated that the methacryloyl substitution caused the formation of micellar cross-links based on the hydrophobic residues of gelatin and the methacryloyl groups, thus obtaining the hydrophobic association hydrogels. The thixotropic and tensile experiments confirmed that GelMA hydrogel possessed the features of hydrophobic association hydrogels like self-healing and stretchable abilities. The rheological experiments revealed that the gelation rate and the mechanical strength of the GelMA hydrogels were in direct proportion to the concentration of GelMA and the degree of methacryloyl substitution. GelMA hydrogels possessed desirable thermal stability that it didn't melt after being heated to 90 °C. Furthermore, the MTT assays and calcein AM/PI staining revealed that GelMA hydrogel was biocompatible. These results collectively confirm that the hydrophobic association is a prospective and facile approach to prepare gelatin hydrogel with desirable properties for further application.

Evolutionary Reinforcement of Polymer Networks: A Stepwise-Enhanced Strategy for Ultrarobust Eutectogels

Gel materials are appealing due to their diverse applications in biomedicine, soft electronics, sensors, and actuators. Nevertheless, the existing synthetic gels are often plagued by feeble network structures and inherent defects associated with solvents, which compromise their mechanical load-bearing capacity and cast persistent doubts about their reliability. Herein, combined with attractive deep eutectic solvent (DES), a stepwise-enhanced strategy is presented to fabricate ultrarobust eutectogels. It focuses on the continuous modulation and optimization of polymer networks through complementary annealing and solvent exchange processes, which drives a progressive increase in both quantity and mass of the interconnected polymer chains at microscopic scale, hence contributing to the evolutionary enhancement of network structure. The resultant eutectogel exhibits superb mechanical properties, including record-breaking strength (31.8 MPa), toughness (76.0 MJ m-3 ), and Young's modulus (25.6 MPa), together with exceptional resistance ability to tear and crack propagation. Moreover, this eutectogel is able to be further programmed through photolithography to in situ create patterned eutectogel for imparting specific functionalities. Enhanced by its broad applicability to various DES combinations, this stepwise-enhanced strategy is poised to serve as a crucial template and methodology for the future development of robust gels.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Ultra-thin amphiphilic hydrogel electrolyte for flexible zinc-ion paper batteries

The paper-like ZIBs can be folded and unfolded using the Miura folding technique, enhancing the areal energy density by a factor of 18.

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

Design of admirable conductive hydrogels combining robust toughness, soft flexibility, desirable conductivity, and freezing resistance remains daunting challenges for meeting the customized and critical demands of flexible and wearable electronics. Herein, a promising and facile strategy to prepare hydrogels tailored to these anticipated demands is proposed, which is prepared in one step by homogeneous cross-linking of acrylamide using hydrophobic divinylbenzene stabilized by micelles under saturated high-saline solutions. The influence of high-saline environments on the hydrogel topology and mechanical performance is investigated. The high-saline environments suppress the size of hydrophobic cross-linkers in micelles during hydrogel polymerization, which weaken the dynamic hydrophobic associations to soften the hydrogels. Nevertheless, the homogeneous cross-linked networks ensure antifracture during ultralarge deformations. The obtained hydrogels show special mechanical performance combining extremely soft deformability and antifracture features (Young's modulus, 5 kPa; stretchability, 10200%; toughness, 134 kJ m-2; and excellent anticrack propagation). The saturated-saline environments also endow the hydrogels with desirable ion conductivity (106 mS cm-1) and freezing resistance (<20 °C). These comprehensive properties of the obtained hydrogels are quite suitable for flexible electronic applications, which is demonstrated by the high sensitivity and durability of the derived strain sensors.

Cartilage-Inspired Inhomogeneous Salt-Hydrogel for Hydrated Drag-Reducing and Strain Sensing

Articular cartilages exhibit load-bearing capacity and durability due to their inhomogeneous structure. Inspired by this unique structure, a tough and inhomogeneous salt-hydrogel was developed by trapping sodium acetate (NaAc) crystals in polyacrylamide (PAM) polymer networks and then partially redissolving the NaAc crystals. The compressive and tensile stresses of the salt-hydrogel increase significantly by more than 20 times when oversaturated Ac- and Na+ are introduced into the gel network. Such an enhancement in mechanical strength is primarily attributed to the formation of NaAc crystals within the gel network. Further investigations reveal that the mechanical strength of the salt-hydrogel is temperature-dependent as the NaAc crystals gradually redissolve in the gel network with increasing temperature. Furthermore, redissolving NaAc crystals in an aqueous solution can yield an inhomogeneous salt-hydrogel. The topmost soft surface of the salt-hydrogel offers hydration lubrication, while the inhomogeneous network confers load-bearing capacity and durability. Compared to regular hydrogels, the inhomogeneous salt-hydrogel surface can realize drag reduction and remain smooth without damage after the friction tests. Moreover, a salt-hydrogel coating is also fabricated to visually demonstrate its drag-reducing property. In addition, this salt-hydrogel possesses conductivity and can be utilized in the development of inhomogeneous salt-hydrogel fibers (diameter = 438 ± 7 μm) for strain detection. The produced salt-hydrogel fiber exhibits excellent durability and reproducibility as a strain sensor, capable of detecting both small strains (e.g., 1%) and large strains (e.g., 40%). This work provides fundamental insights into developing hydrogels with an inhomogeneous network and explores their potential applications (e.g., hydrated drag-reducing, strain sensing).

An Anti-Fracture and Super Deformable Soft Hydrogel Network Insensitive to Extremely Harsh Environments

Design of hydrogels with superior flexible deformability, anti-fracture toughness, and reliable environment adaption is fundamentally and practically important for diverse hydrogel-based flexible devices. However, these features can hardly be compatible even in elaborately designed hydrogels. Herein soft hydrogel networks with superior anti-fracture and deformability are proposed, which show good adaption to extremely harsh saline or alkaline environments. The hydrogel network is one-step constructed via hydrophobic homogenous cross-linking of poly (sodium acrylate), which is expected to provide hydrophobic associations and homogeneous cross-linking for energy dissipation. The obtained hydrogels are quite soft and deformable (tensile modulus: ≈20 kPa, stretchability: 3700%), but show excellent anti-fracture toughness (10.6 kJ m-2 ). The energy dissipation mechanism can be further intensified under saline or alkaline environments. The mechanical performance of the hydrophobic cross-linking topology is inspired rather than weakened by extremely saline or alkaline environments (stretchability: 3900% and 5100%, toughness: 16.1 and 17.1 kJ m-2 under saturated NaCl and 6 mol L-1 NaOH environments, respectively). The hydrogel network also shows good performance in reversible deformations, ion conductivity, sensing strain, monitoring human motions, and freezing resistance under high-saline environments. The hydrogel network show unique mechanical performance and robust environment adaption, which is quite promising for diverse applications.

Dual-Stimuli-Responsive and Anti-Freezing Conductive Ionic Hydrogels for Smart Wearable Devices and Optical Display Devices

Stimuli-responsive hydrogels are a class of important materials for the preparation of flexible sensors, but the development of UV/stress dual-responsive ion-conductive hydrogels with excellent tunability for wearable devices remains a major challenge. In this study, a dual-responsive multifunctional ion-conductive hydrogel (PVA-GEL-GL-Mo₇) with high tensile strength, good stretchability, outstanding flexibility, and stability is successfully fabricated. The prepared hydrogel has an excellent tensile strength of 2.2 MPa, high tenacity of 5.26 MJ/m³, favorable extensibility (522%), and high transparency of 90%. Importantly, the hydrogels have dual responsiveness to UV light and stress, allowing it to be used as a wearable device while responding differently to the UV intensity of different outdoor environments (hydrogels can show different levels of color when exposed to different light intensities of UV light) and can remain flexible at -50 and 85 °C (sensing at both -25 and 85 °C). Therefore, the hydrogels developed in this study have good prospects in different applications, such as flexible wearable devices, duplicate paper, and dual-responsive interactive devices.

Tough and stretchable all-κ-carrageenan hydrogel based on the cooperative effects between chain conformation transition and stepwise mechanical training

The traditional κ-carrageenan (κCG)-based hydrogel obtained from hot water can rupture easily under mechanical loading. To address this vulnerability, here we presented a robust all-κCG hydrogel without employing the second synthetic network. By simply regulating the polymer chains from random coil to stiff chain conformation in NaOH/urea solvent system via the freeze-thawing process, the as-prepared hydrogel with homogeneous structure can display an enhanced stretchability from 42.1 to 156 %, while maintaining the similar fracture stress. Moreover, upon the stepwise mechanical training and subsequent incubation in KCl aqueous solution, more helical segments of κCG were aligned and involved into the association domains, thus leading to the increment in both the crystallinity and anisotropy. Consequently, a fast self-strengthening behavior occurred, and a more stretchable (fracture strain up to 396 %), strong (stress ∼ 0.55 MPa) and tough (∼1.52 MJ m^-3) κCG hydrogel was obtained. In comparison to the traditional one, the fracture strain and toughness are increased by 8.5 and 11.5 times, respectively. In addition, this κCG hydrogel can demonstrate good recovery and shape-memory behaviors under medium deformation. Hence, this tough all-κCG hydrogel is expected to be tailored into the biomaterials as the wearable device, artificial tendon, and cartilage in the future.

Carbon Quantum Dot/Chitosan-Derived Hydrogels with Photo-stress-pH Multiresponsiveness for Wearable Sensors

In recent years, hydrogels have attracted extensive attention in smart sensing owing to their biocompatibility and high elasticity. However, it is still a challenge to develop hydrogels with excellent multiple responsiveness for smart wearable sensors. In this paper, a facile synthesis of carbon quantum dots (CQDs)-doped cross-linked chitosan quaternary/carboxymethylcellulose hydrogels (CCCDs) is presented. Designing of dual network hydrogels decorated with CQDs provides abundant crosslinking and improves the mechanical properties of the hydrogels. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor: 9.88), linearity (R² : 0.97), stretchable ability (stress: 0.67 MPa; strain: 404%), good cyclicity, and durability. The luminescent properties are endowed by the CQDs further broaden the application of hydrogels for realizing flexible electronics. More interestingly, the strain sensor based on CCCDs hydrogel demonstrates photo responsiveness (ΔR/R₀ ≈20%) and pH responsiveness (pH range ≈4-7) performance. CCCDs hydrogels can be used for gesture recognition and light sensing switch. As a proof-of-concept, a smart wearable sensor is designed for monitoring human activities and detecting pH variation in human sweat during exercise. This study reveals new possibilities for further applications in wearable health monitoring.