Ultrastretchable, Self-Healable Hydrogels Based on Dynamic Covalent Bonding and Triblock Copolymer Micellization

Towards Sustainable Materials: A Review of Acylhydrazone Chemistry for Reversible Polymers

Transitioning towards a circular economy, extensive research has focused on dynamic covalent bonds (DCBs) to pave the way for more sustainable materials. These bonds enable debonding and rebonding on demand, as well as facilitating end-of-life recycling. Acylhydrazone/hydrazone chemistry offers a material with high stability under neutral and basic conditions making it a promising candidate for materials research, though the material is susceptible to acid degradation. However, this degradation under acidic conditions can be exploited, making it widely applicable in self-healing and biomedical fields, with potential for reprocessing and recycling. This review highlights studies exploring the reversibility of acylhydrazone/hydrazone bonds in various polymers, altering their properties, and utilizing them in applications such as self-healing, reprocessing, and recycling. The review also focuses on how the mechanical properties are affected by the presence of dynamic linkages, and methods to improve the mechanical performance.

Development of nanocellulose hydrogels for application in the food and biomedical industries: A review

As the most abundant and renewable natural resource, cellulose has attracted significant attention and research interest for the production of hydrogels (HGs). To address environmental issues and emerging demands, the benefits of naturally produced HGs include excellent mechanical properties and superior biocompatibility. HGs are three-dimensional networks created by chemical or physical cross-linking of linear or branched hydrophilic polymers and have high capacity for absorption of water and biological fluids. Although widely used in the food and biomedical fields, most HGs are not biodegradable. Nanocellulose hydrogels (NC-HGs) have been extensively applied in the food industry for detection of freshness, chemical additives, and substitutes, as well as the biomedical field for use as bioengineering scaffolds and drug delivery systems owing to structural interchangeability and stimuli-responsive properties. In this review article, the sources, structures, and preparation methods of NC-HGs are described, applications in the food and biomedical industries are summarized, and current limitations and future trends are discussed.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Accelerating the remodeling of collagen in cutaneous full-thickness wound using FIR soldering technology with bio-targeting nanocomposites hydrogel

A novel composite wound dressing hydrogel by incorporating single-walled carbon nanotubes and indocyanine green into a dual-crosslinked hydrogel through Schiff base reaction was developed. The objective was to prevent wound infection and enhance the thermal effect induced by laser energy. The hydrogel matrix was constructed using oxidized gelatin, pre-crosslinked with calcium ions, along with carboxymethyl chitosan, crosslinked via Schiff base reaction. Optimization of the blank hydrogel's gelation time, swelling index, degradation rate, and mechanical properties was achieved by adding 0.1% SWCNT and 0.1% ICG. Among them, the SWCNT-loaded hydrogel BCG-SWCNT exhibited superior performance overall: a gelation time of 102 s; a swelling index above 30 after equilibrium swelling; a degradation rate of 100.5% on the seventh day; and a compressive modulus of 8.8 KPa. It displayed significant inhibition against methicillin-resistant Staphylococcus aureus infection in wounds. When combined with laser energy usage, the composite hydrogel demonstrated excellent pro-healing activity in rats.

Dynamic Covalent Amphiphilic Polymer Conetworks Based on End-Linked Pluronic F108: Preparation, Characterization, and Evaluation as Matrices for Gel Polymer Electrolytes

We present the development of a platform of well-defined, dynamic covalent amphiphilic polymer conetworks (APCN) based on an α,ω-dibenzaldehyde end-functionalized linear amphiphilic poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (PEG-b-PPG-b-PEG, Pluronic) copolymer end-linked with a triacylhydrazide oligo(ethylene glycol) triarmed star cross-linker. The developed APCNs were characterized in terms of their rheological (increase in the storage modulus by a factor of 2 with increase in temperature from 10 to 50 °C), self-healing, self-assembling, and mechanical properties and evaluated as a matrix for gel polymer electrolytes (GPEs) in both the stretched and unstretched states. Our results show that water-loaded APCNs almost completely self-mend, self-organize at room temperature into a body-centered cubic structure with long-range order exhibiting an aggregation number of around 80, and display an exceptional room temperature stretchability of ∼2400%. Furthermore, ionic liquid-loaded APCNs could serve as gel polymer electrolytes (GPEs), displaying a substantial ion conductivity in the unstretched state, which was gradually reduced upon elongation up to a strain of 4, above which it gradually increased. Finally, it was found that recycled (dissolved and re-formed) ionic liquid-loaded APCNs could be reused as GPEs preserving 50-70% of their original ion conductivity.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Progress in Pluronic F127 Derivatives for Application in Wound Healing and Repair

Pluronic F127 hydrogel biomaterial has garnered considerable attention in wound healing and repair due to its remarkable properties including temperature sensitivity, injectability, biodegradability, and maintain a moist wound environment. This comprehensive review provides an in-depth exploration of the recent advancements in Pluronic F127-derived hydrogels, such as F127-CHO, F127-NH2, and F127-DA, focusing on their applications in the treatment of various types of wounds, ranging from burns and acute wounds to infected wounds, diabetic wounds, cutaneous tumor wounds, and uterine scars. Furthermore, the review meticulously examines the intricate interaction mechanisms employed by these hydrogels within the wound microenvironment. By elucidating the underlying mechanisms, discussing the strengths and weaknesses of Pluronic F127, analyzing the current state of wound healing development, and expanding on the trend of targeting mitochondria and cells with F127 as a nanomaterial. The review enhances our understanding of the therapeutic effects of these hydrogels aims to foster the development of effective and safe wound-healing modalities. The valuable insights provided this review have the potential to inspire novel ideas for clinical treatment and facilitate the advancement of innovative wound management approaches.

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.

Dynamic Pluronic F127 Crosslinking Enhancement of Biopolymeric Nanocomposites for Piezo-Triboelectric Single-Hybrid Nanogenerators and Self-Powered Sensors

Biomechanical and nanomechanical energy harvesting systems have gained a wealth of interest, resulting in a plethora of research into the development of biopolymeric-based devices as sustainable alternatives. Piezoelectric, triboelectric, and hybrid nanogenerator devices for electrical applications are engineered and fabricated using innovative, sustainable, facile-approach flexible composite films with high performance based on bacterial cellulose and BaTiO₃ , intrinsically and structurally enhanced by Pluronic F127, a micellar cross-linker. The voltage and current outputs of the modified versions with multiwalled carbon nanotube as a conductivity enhancer and post-poling effect are 38 V and 2.8 µA cm^-2 , respectively. The multiconnective devices' power density can approach 10 µW cm^-2 . The rectified output power is capable of charging capacitors, driving light-emitting diode lights, powering a digital watch and interfacing with a commercial microcontroller board to operate as a piezoresistive force sensor switch as a proof of concept. Magnetoelectric studies show that the composites have the potential to be incorporated into magnetoelectric systems. The biopolymeric composites prove to be desirable candidates for multifunctional energy harvesters and electronic devices.

Magnetic-responsive Covalent Adaptable Networks

Covalent adaptable networks (CANs) are reprocessable polymers whose structural arrangement is based on the recombination of dynamic covalent bonds. Composite materials prepared by incorporating magnetic particles into CANs attract much attention due to their remote and precise control, fast response speed, high biological safety and strong penetration of magnetic stimuli. These properties often involve magnetothermal effect and direct magnetic-field guidance. Besides, some of them can also respond to light, electricity or pH values. Thus, they are favorable for soft actuators since various functions are achieved such as magnetic-assisted self-healing (heating or at ambient temperature), welding (on land or under water), shape-morphing, and so on. Although magnetic CANs just start to be studied in recent two years, their advances are promised to expand the practical applications in both cutting-edge academic and engineering fields. This review aims to summarize recent progress in magnetic-responsive CANs, including their design, synthesis and application.

Unbreakable Hydrogels with Self-Recoverable 10 200% Stretchability

Design of tough hydrogels maintaining structural integrity under multivariable mechanical loads remains hugely challenging because the anticipated characteristics such as stretchability, strength, toughness, and fracture resistance can hardly be compatible. Herein, a simple but robust hydrogel network formed by copolymerization of divinyl benzene with acrylamide in micellar solutions for ultra-high fracture resistance and self-recoverable stretchability is proposed. The network provides dynamic association of hydrophobic domains and homogeneous crosslinking of hydrophilic chains, which shows step-by-step deformation process. The dynamic associations allow recoverable small deformations, then the homogeneous crosslinking ensures reversible unfolding and alignment of polymer chains to self-strengthen for ultra-large deformations without crack propagations. The resultant hydrogels exhibit comprehensive unbreakable feature with self-recoverable ultra-high stretchability (100% recovery from 10 200% strain), superior fracture resistance (toughness > 26 kJ m^-2 ), and anticrack propagation and fatigue (fatigue threshold: ≈2.5 kJ m^-2 ). Even the prenotched hydrogels can undergo tens cyclic loads at 10 200% strain and thousands cyclic loads at 200% strain without noticeable changes in mechanical performance. The robust network prepared from homogeneous hydrophobic crosslinking provides a facile approach and a new mechanism to explore tough hydrogels with superior antifracture and extreme self-recoverable deformability for diverse applications.

Injectable Hybrid-Crosslinked Hydrogels as Fatigue-Resistant and Shape-Stable Skin Depots

Injectable hydrogels have gained considerable attention, but they are typically mechanically weak and subject to repeated physiological stresses in the body. Herein, we prepared polyurethane diacrylate (EPC-DA) hydrogels, which are injectable and can be photocrosslinked into fatigue-resistant implants. The mechanical properties can be tuned by changing photocrosslinking conditions, and the hybrid-crosslinked EPC-DA hydrogels exhibited high stability and sustained release properties. In contrast to common injectable hydrogels, EPC-DA hydrogels exhibited excellent antifatigue properties with >90% recovery during cyclic compression tests and showed shape stability after application of force and immersion in an aqueous buffer for 35 days. The EPC-DA hydrogel formed a shape-stable hydrogel depot in an ex vivo porcine skin model, with establishment of a temporary soft gel before in situ fixing by UV crosslinking. Hybrid crosslinking using injectable polymeric micelles or nanoparticles may be a general strategy for producing hydrogel implants resistant to physiological stresses.

Superstretchable, yet stiff, fatigue-resistant ligament-like elastomers

Ligaments are flexible and stiff tissues around joints to support body movements, showing superior toughness and fatigue-resistance. Such a combination of mechanical properties is rarely seen in synthetic elastomers because stretchability, stiffness, toughness, and fatigue resistance are seemingly incompatible in materials design. Here we resolve this long-standing mismatch through a hierarchical crosslinking design. The obtained elastomer can endure 30,000% stretch and exhibit a Young’s modulus of 18 MPa and toughness of 228 MJ m⁻³, outperforming all the reported synthetic elastomers. Furthermore, the fatigue threshold is as high as 2,682 J m⁻², the same order of magnitude as the ligaments (~1,000 J m⁻²). We reveal that the dynamic double-crosslinking network composed of Li⁺-O interactions and PMMA nanoaggregates allows for a hierarchical energy dissipation, enabling the elastomers as artificial ligaments in soft robotics.

Stiffness, toughness, and fatigue resistance are seemingly incompatible in materials design. Here the authors demonstrate a hierarchical crosslinking strategy using lithium ion oxygen interactions and PMMA nanoaggregates to enable energy dissipation in the network, leading to stiff yet tough polymer materials.

Injectable Self-Healing First-Aid Tissue Adhesives with Outstanding Hemostatic and Antibacterial Performances for Trauma Emergency Care

Soft-tissue trauma emergency caused by natural disasters and traffic accidents is highly prevalent, which can result in massive bleeding, pathogen infection, and even death. Although numerous tissue adhesives can bind to tissue surfaces and cover wounds, most of them still have several deficiencies, including long gelation time, poor adhesive strength, and anti-infection, making them inappropriate for use as first-aid bandages. Herein, injectable and self-healing four-arm-PEG-CHO/polyethyleneimine (PEI) tissue adhesives as liquid first-aid supplies are developed via the dynamic Schiff base reaction for trauma emergency. It is found that the prepared hydrogel adhesives exhibit short and controlled gelation time (9∼88 s), strong adhesive strength, and excellent antibacterial ability. Their hemostatic and antimicrobial performances can be tailored by the mass ratio of four-arm-PEG-CHO/PEI. Moreover, in vitro biological assays display that the developed tissue adhesives possess satisfactory cyto/hemocompatibility. Importantly, in vivo the designed adhesives show fast hemostatic capacity and excellent anti-infection as compared to commercial Prontosan gel. Thus, this work indicates that the four-arm-PEG-CHO/PEI first-aid tissue adhesives display great potential for wound emergency management.

Multifunctional Double-Layer Composite Hydrogel Conduit Based on Chitosan for Peripheral Nerve Repairing

Peripheral nerve regeneration and functional recovery is a major challenge in clinical practice. Nerve conduit is an effective treatment for peripheral nerve repair, but the traditional hollow nerve conduit is not satisfactory in peripheral nerve repair due to the limitation of cell migration and nutrient transport. Herein, the double cross-linked hydrogels with injectable, self-healing, and conductive properties are synthesized by the Schiff base reaction between polyaniline-modified carboxymethyl chitosan and aldehyde-modified Pluronic F-127 (F127-CHO), and the hydrophobic interaction of F127-CHO. The conductive hydrogel is injected into the cavity of chitosan conduit prepared by electrodeposition. The inner conductive hydrogel and the outer chitosan conduit are formed into a whole through the Schiff base reaction to obtain a double-layer composite hydrogel nerve conduit. The double-layer composite hydrogel neural conduit loaded with 7,8-dihydroxyflavone (DHF) has excellent degradability, biocompatibility, antioxidant activity, and Schwann cell proliferation activity. In the rat sciatic nerve defect model, the double-layer composite hydrogel nerve conduit significantly promotes sciatic nerve regeneration compared with the chitosan hollow conduit. Surprisingly, the repair ability of double-layered hydrogel nerve conduit loaded with DHF is comparable to that of autologous transplantation. Therefore, this multifunctional double-layer composite hydrogel conduit has great potential for peripheral nerve repairing.

Chemical syntheses of bioinspired and biomimetic polymers toward biobased materials

The rich structures and hierarchical organizations in nature provide many sources of inspiration for advanced material designs. We wish to recapitulate properties such as high mechanical strength, colour-changing ability, autonomous healing and antimicrobial efficacy in next-generation synthetic materials. Common in nature are non-covalent interactions such as hydrogen bonding, ionic interactions and hydrophobic effects, which are all useful motifs in tailor-made materials. Among these are biobased components, which are ubiquitously conceptualized in the space of recently developed bioinspired and biomimetic materials. In this regard, sustainable organic polymer chemistry enables us to tune the properties and functions of such materials that are essential for daily life. In this Review, we discuss recent progress in bioinspired and biomimetic polymers and provide insights into biobased materials through the evolution of chemical approaches, including networking/crosslinking, dynamic interactions and self-assembly. We focus on advances in biobased materials; namely polymeric mimics of resilin and spider silk, mechanically and optically adaptive materials, self-healing elastomers and hydrogels, and antimicrobial polymers.

Tough and Resilient Hydrogels Enabled by a Multifunctional Initiating and Cross-Linking Agent

Many high-strength hydrogels have been developed in recent years; however, few of them are both tough and resilient, and their intrinsic paradoxical nature makes designing a gel with both high toughness and high resilience a great challenge. To address this problem, we introduced both N,N,N,N-pentamethyldiethylenetriamine (PA) and N,N-methylenebisacrylamide (MBA) into polyacrylamide hydrogel networks to construct an entangled network that contains chemically cross-linked chains and branched chains simultaneously. The entanglements of branched chains can act as a physical cross-linking point to uniformly disperse stress on molecular chains, and chemical cross-linking ensures the stability of the hydrogel network. The increase in the number and length of branched chains is able to achieve an enhancement in strength while the slip of the entangled polymer chains can effectively achieve energy dissipation and can improve the toughness of the gel. Moreover, the resultant hydrogels exhibit an excellent resilience (>98%). Therefore, high toughness and resilience are achieved simultaneously. In addition, we also investigated the initiation mechanism of PA. This strategy creates a new way for the preparation of next-generation high toughness and high resilience hydrogel-based materials, which have promising applications in wearable, flexible strain/pressure sensors.

The Manufacture of Unbreakable Bionics via Multifunctional and Self-Healing Silk-Graphene Hydrogels

Biomaterials capable of transmitting signals over longer distances than those in rigid electronics can open new opportunities for humanity by mimicking the way tissues propagate information. For seamless mirroring of the human body, they also have to display conformability to its curvilinear architecture, as well as, reproducing native-like mechanical and electrical properties combined with the ability to self-heal on demand like native organs and tissues. Along these lines, a multifunctional composite is developed by mixing silk fibroin and reduced graphene oxide. The material is coined "CareGum" and capitalizes on a phenolic glue to facilitate sacrificial and hierarchical hydrogen bonds. The hierarchal bonding scheme gives rise to high mechanical toughness, record-breaking elongation capacity of ≈25 000%, excellent conformability to arbitrary and complex surfaces, 3D printability, a tenfold increase in electrical conductivity, and a fourfold increase in Young's modulus compared to its pristine counterpart. By taking advantage of these unique properties, a durable and self-healing bionic glove is developed for hand gesture sensing and sign translation. Indeed, CareGum is a new advanced material with promising applications in fields like cyborganics, bionics, soft robotics, human-machine interfaces, 3D-printed electronics, and flexible bioelectronics.

A Hybrid Injectable and Self-Healable Hydrogel System as 3D Cell Culture Scaffold

Cell therapies have great potential for the treatment of many different diseases, while the direct application of cells to the targeted location leads to limited therapeutic outcomes due to the low cell engraftment and cell survival rate. Injectable hydrogels have been developed to facilitate cell delivery; however, those currently developed hydrogel systems still face the limited cell survival rate. Here, an injectable and self-healable hydrogel is reported through the combination of hyperbranched PEG-based multi-hydrazide macro-crosslinker (HB-PEG-HDZ) and aldehyde-functionalized hyaluronic acid (HA-CHO), with gelatin added to increase the crosslinking density and cell activity. The hydrogels can be formed only in 7 s due to the relatively high content of the functional end groups. The reversible crosslinking mechanism between the hydrazide and aldehyde groups endows the hydrogel with shear-thinning and self-healing properties. The hydrogels with gelatin exhibit relatively better mechanical properties and cell activity. The hydrogels can improve the survival, attachment, and engraftment of injected cells due to the rapid sol-gel transition, which can promote an enhanced regenerative response.

Hemostatic Self-Healing Hydrogel with Excellent Biocompatibility Composed of Polyphosphate-Conjugated Functional PNIPAM-Bearing Acylhydrazide

Biocompatible self-healing hydrogels present an effective application as drug-releasing vehicles for tissue engineering and wound repairing. At the same time, the effective hemostatic property of the hydrogels also improves the application property as wound dressing materials. In this research, the PNIPAM-bearing acylhydrazide P(NIPAM-co-AH) was synthesized and then hemostatic polyphosphate (PolyP) was imported to prepare polyphosphate-conjugated P(NIPAM-co-AH) (PNAP). Through the acylhydrazone connection of PNAP and aldehyde functional PEO (PEO DA), the self-healing hydrogel with a hemostatic property was fabricated with good flexibility and sealing effect. The resultant hydrogels kept excellent biocompatibility and showed controlled drug release behavior. More importantly, the hydrogel accelerated the coagulation rate in vitro and presented a strong hemostatic effect as the binder in the hemorrhage model in vivo, which endow the hemostatic hydrogel with a very useful drug delivery carrier for wound healing applications or first aid treatment of the wounded in critical situations.

An Extremely Stretchable and Self-Healable Supramolecular Polymer Network

The construction of a single polymer network with extreme stretchability, relatively high mechanical strength, and fast and facile autonomous room-temperature self-healing capability still remains a challenge. Herein, supramolecular polymer networks are fabricated by synergistically incorporating metal-ligand and hydrogen bonds in poly(propylene glycol) (PPG). The representative specimen, PPG-Im-MDA-1.5-0.25-Cu, shows a combination of notable mechanical properties involving an extreme stretching ratio of 346 ± 14× and a Young's modulus of 2.10 ± 0.14 MPa, which are superior to the previously reported extremely stretchable polymeric materials. Notably, the destroyed specimen can fully recover mechanical performances within 1 h. The tunability of mechanical properties and self-healing capability has been actualized by merely tailoring the content of a chain extender. The application of the as-prepared supramolecular PPG network in constructing a flexible and self-healable conductor has been demonstrated. This strategy provides some insights for preparing extremely stretchable and self-healable polymeric materials.

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

Dual-faced borax mediated synthesis of self-healable hydrogels merging dynamic covalent bonding and micellization

A facile route towards multifunctional hydrogels was proposed via borax catalyzed thiol-acrylate and borax–diol chemistry. The hydrogels exhibited self-healable ability and anti-fatigue property by merging dynamic boronate ester bonds and micellization in one system.

Integration of Macro-Cross-Linker and Metal Coordination: A Super Stretchable Hydrogel with High Toughness

The development of multifunctional hydrogels with high strength and stretchability is one of the most important topics in soft-matter research owing to their potential applications in various fields. In this work, a dual physically cross-linked network was designed for the fabrication of ultrastretchable tough hydrogels. The hydrogels were prepared through in situ polymerization of acrylic acid and acrylamide in the presence of positively charged quaternary poly(ethylene imine) (Q-PEI) and micelle-forming Pluronic F127 diacrylate, thus introducing electrostatic interactions between the positively charged Q-PEI and negatively charged poly(acrylic acid-co-acrylamide). For further mechanical reinforcement, Ca²⁺ and Cu²⁺ ions were introduced into the hydrogel network to construct coordination bonds, significantly enhancing tensile strength as well as stretchability. The hydrogel prepared with Ca²⁺ ion coordination bonds was found to be stretchable to 108 times its original length and exhibited a maximum toughness of 177 MJ·m^-3, representing one of the most robust systems with both extraordinary toughness and superstretchability prepared to date. The hydrogels also exhibited excellent recovery of dimensions and reproducibility in terms of mechanical properties, providing a promising ultrastretchable soft-matter system with outstanding mechanical strength.

Instant Gelation System as Self-Healable and Printable 3D Cell Culture Bioink Based on Dynamic Covalent Chemistry

The rapid development of additive manufacturing techniques in the field of tissue regeneration offers unprecedented success for artificial tissue and organ fabrication. However, some limitations still remain for current bioinks, such as the compromised cell viability after printing, the low cross-linking efficiency leading to poor printing resolution and speed due to the relatively slow gelation rate, and the requirement of external stimuli for gelation. To address these problems, herein, a biocompatible and printable instant gelation hydrogel system has been developed based on a designed hyperbranched poly(ethylene glycol) (PEG)-based multihydrazide macro-cross-linker (HB-PEG-HDZ) and an aldehyde-functionalized hyaluronic acid (HA-CHO). HB-PEG-HDZ is prepared by the postfunctionalization of hyperbranched PEG-based multivinyl macromer via thiol-ene chemistry. Owing to the high functional group density of HB-PEG-HDZ, the hydrogel can be formed instantly upon mixing the solutions of two components. The reversible cross-linking mechanism between the hydrazide and aldehyde groups endows the hydrogel with shear-thinning and self-healing properties. The minimally toxic components and cross-linking chemistry allow the resulting hydrogel to be a biocompatible niche. Moreover, the fast sol-to-gel transition of the hydrogel, combining all of the advanced characteristics of this platform, protects the cells during the printing procedure, avoids their damage during extrusion, and improves the transplanted cell survival.

Extremely Tough, Puncture-Resistant, Transparent, and Photoluminescent Polyurethane Elastomers for Crack Self-Diagnose and Healing Tracking

Ensuring material performance reliability and lifetime is crucial for practical operations. Small cracks on the material surface are often detrimental to its safe operation. This study describes the development of a hydrogen bond-rich puncture-resistant polyurethane elastomer with supertoughness. The as-prepared polyurethane transparent films feature high tensile break strength (57.4 MPa) and great toughness (228 MJ m^-3). Additionally, a facile, low-cost, crack self-diagnostic approach through photoluminescence using a small luminous pen is reported. The materials efficiently achieved self-healing at 90 °C after the crack formation. The change of fluorescence intensity on the crack can be used to track the self-healing process. Therefore, this work provides a guideline for the material design of supertough, puncture-resistant, transparent, and healable elastomers and a crack self-diagnosis and healing approach.

Design of Slidable Polymer Networks: A Rational Strategy to Stretchable, Rapid Self-Healing Hydrogel Electrolytes for Flexible Supercapacitors

Hydrogel electrolytes are of particular interest in the fabrication of flexible supercapacitors that are able to withstand deformation and physical damage. Nevertheless, there still exists a huge space in the design of hydrogel electrolytes with comprehensive performances including high processability, conductivity, mechanical strength, and self-healability. Herein, a slidable polymer network is constructed through the cross-linking reaction among commercially available polyethyleneimine (PEI), polyvinyl alcohol (PVA), and 4-formylphenylboronic acid (Bn) to generate PEI-PVA-Bn hydrogels, which have high adaptability to various electrolytes such as LiCl, NaCl, KCl, and ionic liquids. The as formed hydrogel electrolytes not only show excellent mechanical property (elongation at break up to 1223%, strength of 34.6 kPa) and self-healability (highest strain self-healing efficiency reaches 94.3% within 2 min) but also exhibit high conductivity (up to 21.49 mS cm^-1). Flexible supercapacitors constructed by sandwiching the PEI-PVA-Bn-LiCl hydrogel electrolyte between two multiwalled carbon nanotube electrodes demonstrate a broadened operating potential window of 1.4 V, specific capacitance of 16.7 mF cm^-2, high cycling stability up to 10 000 charge/discharge cycles, and excellent mechanical stability.