Thermodynamically stable whilst kinetically labile coordination bonds lead to strong and tough self-healing polymers

Recent Advances in Environmentally Friendly Dual-crosslinking Polymer Networks

Environmentally friendly crosslinked polymer networks feature degradable covalent or non-covalent bonds, with many of them manifesting dynamic characteristics. These attributes enable convenient degradation, facile reprocessibility, and self-healing capabilities. However, the inherent instability of these crosslinking bonds often compromises the mechanical properties of polymer networks, limiting their practical applications. In this context, environmentally friendly dual-crosslinking polymer networks (denoted EF-DCPNs) have emerged as promising alternatives to address this challenge. These materials effectively balance the need for high mechanical properties with the ability to degrade, recycle, and/or self-heal. Despite their promising potential, investigations into EF-DCPNs remain in their nascent stages, and several gaps and limitations persist. This Review provides a comprehensive overview of the synthesis, properties, and applications of recent progress in EF-DCPNs. Firstly, synthetic routes to a rich variety of EF-DCPNs possessing two distinct types of dynamic bonds (i.e., imine, disulfide, ester, hydrogen bond, coordination bond, and other bonds) are introduced. Subsequently, complex structure- and dynamic nature-dependent mechanical, thermal, and electrical properties of EF-DCPNs are discussed, followed by their exemplary applications in electronics and biotechnology. Finally, future research directions in this rapidly evolving field are outlined.

Regulation of Hard Segment Cluster Structures for High-performance Poly(urethane-urea) Elastomers

Elastomers are widely used in daily life; however, the preparation of degradable and recyclable elastomers with high strength, high toughness, and excellent crack resistance remains a challenging task. In this report, a polycaprolactone-based poly(urethane-urea) elastomer is presented with excellent mechanical properties by optimizing the arrangement of hard segment clusters. It is found that long alkyl chains of the chain extenders lead to small and evenly distributed hard segment clusters, which is beneficial for improving mechanical properties. Together with the multiple hydrogen bond structure and stress-induced crystallization, the obtained elastomer exhibits a high strength of 63.3 MPa, an excellent toughness of 431 MJ m-3 and an outstanding fracture energy of 489 kJ m-2, while maintaining good recyclability and degradability. It is believed that the obtained elastomer holds great promise in various application fields and it contributes to the development of a sustainable society.

Synthesis of Photoresponsive Fast Self-healing Polyolefin Composites by Nickel-Catalyzed Copolymerization of Ethylene and Lignin Cluster Monomers

Polymers may suffer from sudden mechanical damages during long-term use under various harsh operating environments. Rapid and real-time self-healing will extend their service life, which is particularly attractive in the context of circular economy. In this work, a lignin cluster polymerization strategy (LCPS) was designed to prepare a series of lignin functionalized polyolefin composites with excellent mechanical properties through nickel catalyzed copolymerization of ethylene and lignin cluster monomers. These composites can achieve rapid self-healing within 30 seconds under a variety of extreme usage environments (underwater, seawater, extremely low temperatures as low as -60 °C, organic solvents, acid/alkali solvents, etc.), which is of great significance for real-time self-healing of sudden mechanical damage. More importantly, the dynamic cross-linking network within these composites enable great re-processability and amazing sealing performances.

Retrospective Analysis of Structure-Property Relationship of Emergent Metallo-Supramolecular Polymer Networks

Metallo-supramolecular polymer networks (MSPNs) are fabricated from the crosslinking of polymers by discrete supramolecular coordination complexes. Due to the availability of various coordination complexes, e. g., 2D macrocycles and 3D nanocages, the MSPNs have been recently developed with broadly tunable visco-elasticity and enriched functions inherited from the coordination complexes. The coordination complexes possess enriched topologies and unique structural relaxation dynamics, rendering them the capability to break the traditional tradeoffs of polymer systems for the design of materials with enhanced mechanical performance. The structure-property relationship studies are critical for the material-by-design of MSPNs, while the spatiotemporal investigations are desired for the exploration of dynamics information. The work summarizes recent studies on the unique ligand-exchange kinetics and the multi-level structural relaxation dynamics of MSPNs. The MSPNs' mechanical properties can be quantitatively correlated with the dynamics for understanding the structure-property relationship. This concept will not only serve to attract more researchers to engage in the study of the structure-activity relationship of MSPNs but also inspire innovative research findings pertaining to the application of MSPNs.

Biomass Waste Utilization as Nanocomposite Anodes through Conductive Polymers Strengthened SiO2/C from Streblus asper Leaves for Sustainable Energy Storages

Sustainable anode materials, including natural silica and biomass-derived carbon materials, are gaining increasing attention in emerging energy storage applications. In this research, we highlighted a silica/carbon (SiO2/C) derived from Streblus asper leaf wastes using a simple method. Dried Streblus asper leaves, which have plenty of biomass in Thailand, have a unique leaf texture due to their high SiO2 content. We can convert these worthless leaves into SiO2/C nanocomposites in one step, producing eco-materials with distinctive microstructures that influence electrochemical energy storage performance. Through nanostructured design, SiO2/C is thoroughly covered by a well-connected framework of conductive hybrid polymers based on the sodium alginate-polypyrrole (SA-PPy) network, exhibiting impressive morphology and performance. In addition, an excellent electrically conductive SA-PPy network binds to the SiO2/C particle surface through crosslinker bonding, creating a flexible porous space that effectively facilitates the SiO2 large volume expansion. At a current density of 0.3 C, this synthesized SA-PPy@Nano-SiO2/C anode provides a high specific capacity of 756 mAh g-1 over 350 cycles, accounting for 99.7% of the theoretical specific capacity. At the high current of 1 C (758 mA g-1), a superior sustained cycle life of over 500 cycles was evidenced, with over 93% capacity retention. The research also highlighted the potential for this approach to be scaled up for commercial production, which could have a significant impact on the sustainability of the lithium-ion battery industry. Overall, the development of green nanocomposites along with polymers having a distinctive structure is an exciting area of research that has the potential to address some of the key challenges associated with lithium-ion batteries, such as capacity degradation and safety concerns, while also promoting sustainability and reducing environmental impact.

Heterogeneous and Cooperative Rupture of Histidine-Ni2+ Metal-Coordination Bonds on Rationally Designed Protein Templates

Metal-coordination bonds, a highly tunable class of dynamic noncovalent interactions, are pivotal to the function of a variety of protein-based natural materials and have emerged as binding motifs to produce strong, tough, and self-healing bioinspired materials. While natural proteins use clusters of metal-coordination bonds, synthetic materials frequently employ individual bonds, resulting in mechanically weak materials. To overcome this current limitation, we rationally designed a series of elastin-like polypeptide templates with the capability of forming an increasing number of intermolecular histidine-Ni2+ metal-coordination bonds. Using single-molecule force spectroscopy and steered molecular dynamics simulations, we show that templates with three histidine residues exhibit heterogeneous rupture pathways, including the simultaneous rupture of at least two bonds with more-than-additive rupture forces. The methodology and insights developed improve our understanding of the molecular interactions that stabilize metal-coordinated proteins and provide a general route for the design of new strong, metal-coordinated materials with a broad spectrum of dissipative time scales.

Highly Durable Silicone-Based Elastomers Achieved Through the Synergy of Bi-Incompatible Soft Segments and Multi-Scale Hydrogen Bonds

Developing a silicone elastomer with high strength, exceptional toughness, good crack tolerance, healability, and recyclability, poses significant challenges due to the inherent trade-offs between these properties. Herein, the design of silicone-based elastomers with a nanoscopic microphase separation structure and comprehensive mechanical properties is achieved by combining bi-incompatible soft segments and multi-scale hydrogen bonds. The formation of multi-scale hydrogen bonds involving urethane, urea, and 2-ureido-4[1H]-pyrimidinone (UPy) facilitates efficient reversible crosslinking of the synthesized polymer containing thermodynamically incompatible poly(dimethylsiloxane) (PDMS) and poly(propylene glycol) (PPG). The dynamic dissociation and recombination of hydrogen bonds, coupled with the forced compatibility and spontaneous separation of bi-incompatible soft segments, can effectively dissipate energy, particularly in the crack region during the stretching process. The obtained silicone-based elastomer exhibits a high break strength of 8.0 MPa, good elongation at break of 1910%, ultrahigh toughness of 67.8 MJ m-3, and unprecedented fracture energy of 31.8 kJ m-2 while maintaining their thermal stability, hydrophobicity, healability, and recyclability. This resilient and long-lasting silicone-based elastomer exhibits significant potential for use in flexible electronic devices.

Robust, Self-Healing, and Multi-Use Poly(Urethane-Urea-Imide) Elastomer as a Durable Adhesive for Thermal Interface Materials

Currently, research on thermal interface materials (TIMs) is primarily focused on enhancing thermal conductivity. However, strong adhesion and multifunctionality are also important characteristics for TIMs when pursing more stable interface heat conduction. Herein, a novel poly(urethane-urea-imide) (PUUI) elastomer containing abundant dynamic hydrogen bonds network and reversible disulfide linkages is successfully synthesized for application as a TIM matrix. The PUUI can self-adapt to the metal substrate surface at moderate temperatures (80 °C) and demonstrates a high adhesion strength of up to 7.39 MPa on aluminum substrates attributed its noncovalent interactions and strong intrinsic cohesion. Additionally, the PUUI displays efficient self-healing capability, which can restore 94% of its original mechanical properties after self-healing for 6 h at room temperature. Furthermore, PUUI composited with aluminum nitride and liquid metal hybrid fillers demonstrates a high thermal conductivity of 3.87 W m-1 K-1 while maintaining remarkable self-healing capability and adhesion. When used as an adhesive-type TIM, it achieves a low thermal contact resistance of 22.1 mm2 K W-1 at zero pressure, only 16.7% of that of commercial thermal pads. This study is expected to break the current research paradigm of TIMs and offers new insights for the development of advanced, reliable, and sustainable TIMs.

Dynamic Chemistry Toolbox for Advanced Sustainable Materials

Developing dynamic chemistry for polymeric materials offers chemical solutions to solve key problems associated with current plastics. Mechanical performance and dynamic function are equally important in material design because the former determines the application scope and the latter enables chemical recycling and hence sustainability. However, it is a long-term challenge to balance the subtle trade-off between mechanical robustness and dynamic properties in a single material. The rise of dynamic chemistry, including supramolecular and dynamic covalent chemistry, provides many opportunities and versatile molecular tools for designing constitutionally dynamic materials that can adapt, repair, and recycle. Facing the growing social need for developing advanced sustainable materials without compromising properties, recent progress showing how the toolbox of dynamic chemistry can be explored to enable high-performance sustainable materials by molecular engineering strategies is discussed here. The state of the art and recent milestones are summarized and discussed, followed by an outlook toward future opportunities and challenges present in this field.

High-toughness, extensile and self-healing PDMS elastomers constructed by decuple hydrogen bonding

Elastomers are widely used in traditional industries and new intelligent fields. However, they are inevitably damaged by electricity, heat, force, etc. during the working process. With the continuous improvement of reliability and environmental protection requirements in human production and living, it is vital to develop elastomer materials with good mechanical properties that are not easily damaged and can self-heal after being damaged. Nevertheless, there are often contradictions between mechanical properties and self-healing as well as toughness, strength, and ductility. Herein, a strong and dynamic decuple hydrogen bonding based on carbon hydrazide (CHZ) is reported, accompanied with soft polydimethylsiloxane (PDMS) chains to prepare self-healing (efficiency 98.7%), recyclable, and robust elastomers (CHZ-PDMS). The strategy of decuple hydrogen bonding will significantly impact the study of the mechanical properties of elastomers. High stretchability (1731%) and a high toughness of 23.31 MJ m-3 are achieved due to the phase-separated structure and energy dissipation. The recyclability of CHZ-PDMS further supports the concept of environmental protection. The application of CHZ-PDMS as a flexible strain sensor exhibited high sensitivity.

Dynamic carboxymethyl chitosan prodrug hydrogel precisely mediates robust therapy on wound infection

Dynamic antibacterial polysaccharide prodrug hydrogels are in great demand for treatment of wound infection owing to their unique advantages such as excellent biocompatibility, superior antimicrobial property as well as favorable wound healing capacity. Herein, this work highlights the successful development of a dynamic carboxymethyl chitosan (CMC) prodrug hydrogel, which is facilely constructed through Schiffer base reaction between antibacterial components (amikacin and CMC) and crosslinker (dialdehyde PEG). Moderate dynamic imine linkages endow the hydrogel with excellent injectable and self-healing capability as well as targeted on-demand drug release in slightly alkaline condition at infected wound. All ingredients and their strong intermolecular interactions endow the hydrogel with favorable swelling and moisture retention capability. Moreover, the covalent and non-covalent interactions also endow the hydrogel with superior adhesion and mechanical property. These attractive characteristics enable hydrogel to effectively kill pathogens, promote wound healing and reduce side effects of amikacin. Thereby, such a dynamic CMC prodrug hydrogel may open a new avenue for a robust therapy on wound infection, greatly advancing their use in clinics.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Rapid Self-Healing Hydrogel with Ultralow Electrical Hysteresis for Wearable Sensing

Self-healing hydrogels are in high demand for wearable sensing applications due to their remarkable deformability, high ionic and electrical conductivity, self-adhesiveness to human skin, as well as resilience to both mechanical and electrical damage. However, these hydrogels face challenges such as delayed healing times and unavoidable electrical hysteresis, which limit their practical effectiveness. Here, we introduce a self-healing hydrogel that exhibits exceptionally rapid healing with a recovery time of less than 0.12 s and an ultralow electrical hysteresis of less than 0.64% under cyclic strains of up to 500%. This hydrogel strikes an ideal balance, without notable trade-offs, between properties such as softness, deformability, ionic and electrical conductivity, self-adhesiveness, response and recovery times, durability, overshoot behavior, and resistance to nonaxial deformations such as twisting, bending, and pressing. Owing to this unique combination of features, the hydrogel is highly suitable for long-term, durable use in wearable sensing applications, including monitoring body movements and electrophysiological activities on the skin.

Dynamic metal-linker bonds in metal-organic frameworks

Metal-linker bonds serve as the "glue" that binds metal ions to multitopic organic ligands in the porous materials known as metal-organic frameworks (MOFs). Despite ample evidence of bond lability in molecular and polymeric coordination compounds, the metal-linker bonds of MOFs were long assumed to be rigid and static. Given the importance of ligand fields in determining the behaviour of metal species, labile bonding in MOFs would help explain outstanding questions about MOF behaviour, while providing a design tool for controlling dynamic and stimuli-responsive optoelectronic, magnetic, catalytic, and mechanical phenomena. Here, we present emerging evidence that MOF metal-linker bonds exist in dynamic equilibria between weakly and tightly bond conformations, and that these equilibria respond to guest-host chemistry, drive phase change behavior, and exhibit size-dependence in MOF nanoparticles.

Low-temperature, ultra-fast, and recyclable self-healing nanocomposites reinforced with non-solvent silylated modified cellulose nanocrystals

Developing polymeric materials with remarkable mechanical properties and fast self-healing performance even at low temperatures is challenging. Herein, the polymeric nanocomposites containing silane-treated cellulose nanocrystals (SCNC) with ultrafast self-healing and exceptional mechanical characteristics were developed even at low temperatures. First, CNC is modified with a cyclic silane coupling agent using an eco-friendly chemical vapor deposition method. The nanocomposite was then fabricated by blending SCNC with matrix prepolymer, prepared from monomers that possess lower critical solution temperature, followed by the inclusion of dibutyltin dilaurate and hexamethylene diisocyanate. The self-healing capability of the novel SCNC/polymer nanocomposites was enhanced remarkably by increasing the content of SCNC (0-3 wt%) and reaching (≥99 %) at temperatures (5 & 25 °C) within <20 min. Moreover, SCNC-3 showed a toughness of (2498 MJ/m3) and SCNC-5 displayed a robust tensile strength of (22.94 ± 0.4 MPa) whereas SCNC-0 exhibited a lower tensile strength (7.4 ± 03 MPa) and toughness of (958 MJ/m3). Additionally, the nanocomposites retain their original mechanical properties after healing at temperatures (5 & 25 °C) owing to the formation of hydrogen bonds via incorporation of the SCNC. These novel SCNC-based self-healable nanocomposites with tunable mechanical properties offer novel insight into preparing damage and temperature-responsive flexible and wearable devices.

Recent Progress in Advanced Polyester Elastomers for Tissue Engineering and Bioelectronics

Polyester elastomers are highly flexible and elastic materials that have demonstrated considerable potential in various biomedical applications including cardiac, vascular, neural, and bone tissue engineering and bioelectronics. Polyesters are desirable candidates for future commercial implants due to their biocompatibility, biodegradability, tunable mechanical properties, and facile synthesis and fabrication methods. The incorporation of bioactive components further improves the therapeutic effects of polyester elastomers in biomedical applications. In this review, novel structural modification methods that contribute to outstanding mechanical behaviors of polyester elastomers are discussed. Recent advances in the application of polyester elastomers in tissue engineering and bioelectronics are outlined and analyzed. A prospective of the future research and development on polyester elastomers is also provided.

Functional PDMS Elastomers: Bulk Composites, Surface Engineering, and Precision Fabrication

Polydimethylsiloxane (PDMS)-the simplest and most common silicone compound-exemplifies the central characteristics of its class and has attracted tremendous research attention. The development of PDMS-based materials is a vivid reflection of the modern industry. In recent years, PDMS has stood out as the material of choice for various emerging technologies. The rapid improvement in bulk modification strategies and multifunctional surfaces has enabled a whole new generation of PDMS-based materials and devices, facilitating, and even transforming enormous applications, including flexible electronics, superwetting surfaces, soft actuators, wearable and implantable sensors, biomedicals, and autonomous robotics. This paper reviews the latest advances in the field of PDMS-based functional materials, with a focus on the added functionality and their use as programmable materials for smart devices. Recent breakthroughs regarding instant crosslinking and additive manufacturing are featured, and exciting opportunities for future research are highlighted. This review provides a quick entrance to this rapidly evolving field and will help guide the rational design of next-generation soft materials and devices.

Toughening self-healing elastomer crosslinked by metal-ligand coordination through mixed counter anion dynamics

Mechanically tough and self-healable polymeric materials have found widespread applications in a sustainable future. However, coherent strategies for mechanically tough self-healing polymers are still lacking due to a trade-off relationship between mechanical robustness and viscoelasticity. Here, we disclose a toughening strategy for self-healing elastomers crosslinked by metal-ligand coordination. Emphasis was placed on the effects of counter anions on the dynamic mechanical behaviors of polymer networks. As the coordinating ability of the counter anion increases, the binding of the anion leads to slower dynamics, thus limiting the stretchability and increasing the stiffness. Additionally, multimodal anions that can have diverse coordination modes provide unexpected dynamicity. By simply mixing multimodal and non-coordinating anions, we found a significant synergistic effect on mechanical toughness ( > 3 fold) and self-healing efficiency, which provides new insights into the design of coordination-based tough self-healing polymers.

Dual Reversible Network Nanoarchitectonics for Ultrafast Light-Controlled Healable and Tough Polydimethylsiloxane-Based Composite Elastomers

It is highly desirable to develop polydimethylsiloxane (PDMS) elastomers with high self-healing efficiency and excellent mechanical properties. However, most self-healable materials reported to date still take several hours to self-heal and improving the self-healing property often comes at the expense of mechanical properties. Herein, a simple design strategy of dual reversible network nanoarchitectonics is reported for constructing ultrafast light-controlled healable (40 s) and tough (≈7.2 MJ m-3) PDMS-based composite elastomers. The rupture reconstruction of dynamic bonds and the reinforcement effect of carbon nanotubes (10 wt %) endowed our composite elastomer with excellent fracture toughness that originated from a good yield strength (≈1.1 MPa) and stretchability (≈882%). Moreover, carbon nanotubes can quickly and directly heat the damaged area of the composite to achieve its ultrafast repair with the assistance of dynamic polymer/filler interfacial interaction, greatly shortening the self-healing time (12 h). The self-healing performance is superior to that of reported self-healable PDMS-based materials. This novel strategy and the as-prepared supramolecular elastomer can inspire further various practical applications, such as remote anti-icing/deicing materials.

Self-Healable and Reprocessable Silicon Elastomers Based on Imine-Boroxine Bonds for Flexible Strain Sensor

Silicon elastomers with excellent self-healing and reprocessing abilities are highly desirable for the advancement of next-generation energy, electronic, and robotic applications. In this study, a dual cross-linked self-healing polysiloxane elastomer was facilely fabricated by introducing an exchangeable imine bond and boroxine into polydimethylsiloxane (PDMS) networks. The PDMS elastomers exhibited excellent self-healing properties due to the synergistic effect of dynamic reversible imine bonds and boroxine. After healing for 2 h, the mechanical strength of the damaged elastomers completely and rapidly recovered at room temperature. Furthermore, the prepared PDMS elastomers could be repeatedly reprocessed multiple times under milder conditions without significant degradation in mechanical performance. In addition, a stretchable and self-healable electrical sensor was developed by integrating carbon nanotubes (CNTs) with the PDMS elastomer, which can be employed to monitor multifarious human motions in real time. Therefore, this work provides a new inspiration for preparing self-healable and reprocessable silicone elastomers for future flexible electronics.

Self-Healing Silicone Materials: Looking Back and Moving Forward

This review is dedicated to self-healing silicone materials, which can partially or entirely restore their original characteristics after mechanical or electrical damage is caused to them, such as formed (micro)cracks, scratches, and cuts. The concept of self-healing materials originated from biomaterials (living tissues) capable of self-healing and regeneration of their functions (plants, human skin and bones, etc.). Silicones are ones of the most promising polymer matrixes to create self-healing materials. Self-healing silicones allow an increase of the service life and durability of materials and devices based on them. In this review, we provide a critical analysis of the current existing types of self-healing silicone materials and their functional properties, which can be used in biomedicine, optoelectronics, nanotechnology, additive manufacturing, soft robotics, skin-inspired electronics, protection of surfaces, etc.

Complexation of Sulfonate-Containing Polyurethane and Polyacrylic Acid Enables Fabrication of Self-Healing Hydrogel Membranes with High Mechanical Strength and Excellent Elasticity

Artificial hydrogel membranes with good biocompatibility are strongly needed in biological fields. The preparation of biocompatible hydrogel membranes simultaneously possessing high mechanical strength, excellent elasticity, and satisfactory self-healing properties remains a challenge. Herein, we demonstrate the preparation of such hydrogel membranes by complexation of sulfonate-containing polyurethane (SPU) and poly(acrylic acid) (PAA) in the presence of Zn²⁺ ions followed by swelling in water (denoted as SPU-PAA/Zn). Originating from the synergy of the coordination and hydrogen-bonding interactions and the reinforcement effect of the in situ formed hydrophobic domains, the SPU-PAA/Zn hydrogel membrane exhibits a high tensile strength of ∼7.1 MPa and a toughness of ∼30.4 MJ m^-3. Moreover, the hydrogel membrane is highly elastic, which can restore to its initial state from an ∼500% strain within 40 min rest at room temperature without any external assistance. The dynamic noncovalent interactions and hydrophobic domains allow the fractured hydrogel membrane to heal and completely regain its original integrity and mechanical properties at room temperature. Both in vitro and in vivo tests confirm that the hydrogel membrane exhibits satisfactory biocompatibility and could be potentially used as a biological barrier membrane in surgical operations or artificial organs.

Bond clusters control rupture force limit in shear loaded histidine-Ni2+ metal-coordinated proteins

Dynamic noncovalent interactions are pivotal to the structure and function of biological proteins and have been used in bioinspired materials for similar roles. Metal-coordination bonds, in particular, are especially tunable and enable control over static and dynamic properties when incorporated into synthetic materials. Despite growing efforts to engineer metal-coordination bonds to produce strong, tough, and self-healing materials, the systematic characterization of the exact contribution of these bonds towards mechanical strength and the effect of geometric arrangements is missing, limiting the full design potential of these bonds. In this work, we engineer the cooperative rupture of metal-coordination bonds to increase the rupture strength of metal-coordinated peptide dimers. Utilizing all-atom steered molecular dynamics simulations on idealized bidentate histidine-Ni²⁺ coordinated peptides, we show that histidine-Ni²⁺ bonds can rupture cooperatively in groups of two to three bonds. We find that there is a strength limit, where adding additional coordination bonds does not contribute to the additional increase in the protein rupture strength, likely due to the highly heterogeneous rupture behavior exhibited by the coordination bonds. Further, we show that this coordination bond limit is also found natural metal-coordinated biological proteins. Using these insights, we quantitatively suggest how other proteins can be rationally designed with dynamic noncovalent interactions to exhibit cooperative bond breaking behavior. Altogether, this work provides a quantitative analysis of the cooperativity and intrinsic strength limit for metal-coordination bonds with the aim of advancing clear guiding molecular principles for the mechanical design of metal-coordinated materials.

Self-Healing Binder for High-Voltage Batteries

Lithium-ion batteries are core components of flexible electronic devices. However, deformation types, such as impinging, bending, stretching, folding, and twisting, can cause internal cracks and, eventually, damage these batteries. The cracks separate the active particles from the conductive particles and the binder, as well as the electrode from the collector. Self-healing binders can alleviate this mechanical damage and improve the stress response of active material particles during high rates of charging and discharging of these batteries and the operation at a high voltage, thereby enhancing their cycle performance. In the present study, a thermoplastic intrinsic self-healing polymer (TISP) binder is proposed. The TISP is obtained by polymerization of butanediol (2,3-BDO), propylene glycol (1,3-PDO), succinic acid (SuA), sebacic acid (SeA), and iconic acid (IA). The hydroxyl and ester groups in its structure can form diverse bonds including the hydrogen and ion-dipole with active particles and the current collector, thereby producing elevated adhesion. Its properties, including a low glass transition temperature (-60 °C), amorphous structure, and low cross-link density, improve the mobility of polymer chains at 40 °C, and this facilitates structural recovery and the maintenance of strong adhesions. Owing to its higher occupied molecular orbital (HOMO) level than the electrolyte solvent, the TISP is likely oxidized before the main component of the electrolyte during charging. This decomposition produces a chemical passivation interphase on the cathode which reduces side reactions of LiCoO₂ and the electrolyte under high-voltage conditions. Tests reveal that a LiCoO₂ electrode battery using the TISP as a binder retains 162.4 mAh g^-1 after 349 cycles at 4.5 V, and this represents an 86.5% capacity retention. In addition, heating (40 °C, 1 h) of a scratch-damaged electrode can recover a specific capacity of 156.6 mAh g^-1 after 349 cycles at 4.5 V. Relative to a battery without any mechanical scratch, this capacity recovery represents approximately 96%, and this demonstrates the importance of the TISP to the high-voltage damaged electrode.

Tough, Photoluminescent, Self-Healing Waterborne Polyurethane Elastomers Resulting from Synergistic Action of Multiple Dynamic Bonds

Polymers that integrate multiple functions into one system broaden the application range of materials, but it remains a great challenge to obtain polymer materials with simultaneously high strength, high toughness, and high self-healing rate. In this work, we prepared waterborne polyurethane (WPU) elastomers using Schiff bases containing disulfide and acylhydrazone bonds (PD) as chain extenders. Acylhydrazone forming a hydrogen bond not only acts as a physical cross-linking point, which promotes the microphase separation of polyurethane to increase the thermal stability, tensile strength, and toughness of the elastomer, but also serves as a "clip" to integrate various dynamic bonds together to synergistically reduce the activation energy of the polymer chain movement and endow the molecular chain with faster fluidity. Therefore, WPU-PD exhibits excellent mechanical properties at room temperature, such as a tensile strength and a fracture energy of 25.91 MPa and 121.66 kJ m^-2, respectively, and a high self-healing efficiency of 93.7% in a short time under moderate heating conditions. In addition, the photoluminescence property of WPU-PD enables us to track its self-healing process by monitoring change of the fluorescence intensity at the cracks, which helps to avoid the accumulation of cracks and improve the reliability of the elastomer. This self-healing polyurethane has a great potential application value in optical anticounterfeiting, flexible electronics devices, functional automobile protective films, and so on.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

A supramolecular gel-elastomer system for soft iontronic adhesives

Electroadhesion provides a promising route to augment robotic functionalities with continuous, astrictive, and reversible adhesion force. However, the lack of suitable conductive/dielectric materials and processing capabilities have impeded the integration of electroadhesive modules into soft robots requiring both mechanical compliance and robustness. We present herein an iontronic adhesive based on a dynamically crosslinked gel-elastomer system, including an ionic organohydrogel as adhesive electrodes and a resilient polyurethane with high electrostatic energy density as dielectric layers. Through supramolecular design and synthesis, the dual-material system exhibits cohesive heterolayer bonding and autonomous self-healing from damages. Iontronic soft grippers that seamlessly integrate actuation, adhesive prehension, and exteroceptive sensation are devised via additive manufacturing. The grippers can capture soft and deformable items, bear high payload under reduced voltage input, and rapidly release foreign objects in contrast to electroadhesives. Our materials and iontronic mechanisms pave the way for future advancement in adhesive-enhanced multifunctional soft devices.

Enzymatically mediated Gleditsia sinensis galactomannan based hydrogel inspired by wound healing process

The self-healing property based on metal-ligand physical coordination is particularly interesting in bio-hydrogel science due to its allowance for multiple local healing events to process. As the most abundant renewable green resource in nature, Gleditsia sinensis galactomannan has great potential as a starting material for functional materials. In this study, the biocompatible Gleditsia sinensis galactomannan and cellulose were firstly chemically modified and then taken as the main constituent for constructing the metal-ligand coordination through an enzyme-regulated strategy. The hydrogel could quickly gelatinize in the surrounding environment, corresponding to the violent exothermic phenomenon, and exhibit extraordinary self-healing behavior. The molecular dynamics simulation of the hydrogel confirmed the more stable coordinated configuration from Fe(III)-chelates than Fe(II)-chelates. The morphology, mechanical property, antibacterial, and cytotoxicity of the prepared hydrogel were also studied. Our results indicated that galactomannan hydrogel based on the metal-ligand networks could balance the kinetic stability and intrinsic healability through the enzyme-induced route, which provide a new perspective in the field of biomaterial applications.

Multivalent Design of Low-Entropy-Penalty Ion-Dipole Interactions for Dynamic Yet Thermostable Supramolecular Networks

Dynamic supramolecular networks are constantly accompanied by thermal instability. The fundamental reason is most reversible noncovalent bonds quickly decay at elevated temperatures and dissociate below 100 °C. Here, in this paper, we realize a reversible ion-dipole interaction with high-temperature stability exceeding 150 °C. The resultant supramolecular network can simultaneously possess mechanical strength of 1.32 MPa (14.8 times that of pristine material), dynamic self-healing capability, and a stable working temperature of up to 200 °C. From the prolonged characteristic relaxation time of 600 s even at 100 °C, our material represents one of the most thermally stable dynamic supramolecular polymers. These remarkable performances are achieved by using a new multivalent yet low-entropy-penalty molecular design. In this way, the noncovalent bond can reach a high enthalpy while minimizing the entropy-dominated thermal dissociations.

Intrinsically Self-Healing Polymers: From Mechanistic Insight to Current Challenges

Self-healing materials open new prospects for more sustainable technologies with improved material performance and devices' longevity. We present an overview of the recent developments in the field of intrinsically self-healing polymers, the broad class of materials based mostly on polymers with dynamic covalent and noncovalent bonds. We describe the current models of self-healing mechanisms and discuss several examples of systems with different types of dynamic bonds, from various hydrogen bonds to dynamic covalent bonds. The recent advances indicate that the most intriguing results are obtained on the systems that have combined different types of dynamic bonds. These materials demonstrate high toughness along with a relatively fast self-healing rate. There is a clear trade-off relationship between the rate of self-healing and mechanical modulus of the materials, and we propose design principles of polymers toward surpassing this trade-off. We also discuss various applications of intrinsically self-healing polymers in different technologies and summarize the current challenges in the field. This review intends to provide guidance for the design of intrinsic self-healing polymers with required properties.

Insights into the Correlation of Microscopic Motions of [c2]Daisy Chains with Macroscopic Mechanical Performance for Mechanically Interlocked Networks

Mimicking filament sliding in sarcomeres using artificial molecular muscles such as [c2]daisy chains has aroused increasing interest in developing advanced polymeric materials. Although few bistable [c2]daisy chain-based mechanically interlocked polymers (MIPs) with stimuli-responsive behaviors have been constructed, it remains a significant challenge to establish the relationship between microscopic responsiveness of [c2]daisy chains and macroscopic mechanical properties of the corresponding MIPs. Herein, we report two mechanically interlocked networks (MINs) consisting of dense [c2]daisy chains with individual extension (MIN-1) or contraction (MIN-2) conformations decoupled from a bistable precursor, which serve as model systems to address the challenge. Upon external force, the extended [c2]daisy chains in MIN-1 mainly undergo elastic deformation, which is able to assure the strength, elasticity, and creep resistance of the corresponding material. For the contracted [c2]daisy chains, long-range sliding motion occurs along with the release of latent alkyl chains between the two DB24C8 wheels, and accumulating lots of such microscopic motions endows MIN-2 with enhanced ductility and ability of energy dissipation. Therefore, by decoupling a bistable [c2]daisy chain into individual extended and contracted ones, we directly correlate the microscopic motion of [c2]daisy chains with macroscopic mechanical properties of MINs.

Digital light 3D printing of a polymer composite featuring robustness, self-healing, recyclability and tailorable mechanical properties

Producing lightweight structures with high weight-specific strength and stiffness, self-healing abilities, and recyclability, is highly attractive for engineering applications such as aerospace, biomedical devices, and smart robots. Most self-healing polymer systems used to date for mechanical components lack 3D printability and satisfactory load-bearing capacity. Here, we report a new self-healable polymer composite for Digital Light Processing 3D Printing, by combining two monomers with distinct mechanical characteristics. It shows a desirable and superior combination of properties among 3D printable self-healing polymers, with tensile strength and elastic modulus up to 49 MPa and 810 MPa, respectively. Benefiting from dual dynamic bonds between the linear chains, a healing efficiency of above 80% is achieved after heating at a mild temperature of 60 °C without additional solvents. Printed objects are also endowed with multi-materials assembly and recycling capabilities, allowing robotic components to be easily reassembled or recycled after failure. Mechanical properties and deformation behaviour of printed composites and lattices can be tuned significantly to suit various practical applications by altering formulation. Lattice structures with three different architectures were printed and tested in compression: honeycomb, re-entrant, and chiral. They can regain their structural integrity and stiffness after damage, which is of great value for robotic applications. This study extends the performance space of composites, providing a pathway to design printable architected materials with simultaneous mechanical robustness/healability, efficient recoverability, and recyclability.

Current Self-Healing Binders for Energetic Composite Material Applications

Energetic composite materials (ECMs) are the basic materials of polymer binder explosives and composite solid propellants, which are mainly composed of explosive crystals and binders. During the manufacturing, storage and use of ECMs, the bonding surface is prone to micro/fine cracks or defects caused by external stimuli such as temperature, humidity and impact, affecting the safety and service of ECMs. Therefore, substantial efforts have been devoted to designing suitable self-healing binders aimed at repairing cracks/defects. This review describes the research progress on self-healing binders for ECMs. The structural designs of these strategies to manipulate macro-molecular and/or supramolecular polymers are discussed in detail, and then the implementation of these strategies on ECMs is discussed. However, the reasonable configuration of robust microstructures and effective dynamic exchange are still challenges. Therefore, the prospects for the development of self-healing binders for ECMs are proposed. These critical insights are emphasized to guide the research on developing novel self-healing binders for ECMs in the future.

Elastomeric Liquid-Free Conductor for Iontronic Devices

For a long time, the potential application of gel-based ionic devices was limited by the problem of liquid leakage or evaporation. Here, we utilized amorphous, irreversible and reversible cross-linked polyTA (PTA) as a matrix and lithium bis(trifluoromethane sulfonamide) (LiTFSI) as an electrolyte to prepare a stretchable (495%) and self-healing (94%) solvent-free elastomeric ionic conductor. The liquid-free ionic elastomer can be used as a stable strain sensor to monitor human activities sensitively under extreme temperatures. Moreover, the prepared elastic conductor (TEOA_0.10-PTA@LiTFSI) was also considered an electrode to assemble with self-designed repairable dielectric organosilicon layers (RD-PDMS) to develop a sustainable triboelectric nanogenerator (SU-TENG) with outstanding performance. SU-TENG maintained good working ability under extreme conditions (-20 °C, 60 °C, and 200% strain). This work provided a low-cost and simple idea for the development of reliable iontronic equipment for human-computer interaction, motion sensing, and sustainable energy.

Repulsive segregation of fluoroalkyl side chains turns a cohesive polymer into a mechanically tough, ultrafast self-healable, nonsticky elastomer

Dynamic crosslinking of flexible polymer chains via attractive and reversible interactions is widely employed to obtain autonomously self-healable elastomers. However, this design leads to a trade-off relationship between the strength and self-healing speed of the material, i.e., strong crosslinks provide a mechanically strong elastomer with slow self-healing property. To address this issue, we report an "inversion" concept, in which attractive poly(ethyl acrylate-random-methyl acrylate) chains are dynamically crosslinked via repulsively segregated fluoroalkyl side chains attached along the main chain. The resulting elastomer self-heals rapidly (> 90% within 15 min) via weak but abundant van der Waals interactions among matrix polymers, while the dynamic crosslinking provides high fracture stress (≈2 MPa) and good toughness (≈17 MJ m^-3). The elastomer has a nonsticky surface and selectively self-heals only at the damaged faces due to the surface segregation of the fluoroalkyl chains. Moreover, our elastomer strongly adheres to polytetrafluoroethylene plates (≈60 N cm^-2) via hot pressing.

Photoreversible Bond-Based Shape Memory Polyurethanes with Light-Induced Self-Healing, Recyclability, and 3D Fluorescence Encryption

Developing a shape memory polyurethane with high mechanical properties, excellent self-healing has become a huge challenge for the development of smart materials. Herein, we report the design and fabrication of a shape memory polyurethane network terminated with coumarin units (HEOMC-PU) to address this conundrum. The synthesized HEOMC-PU exhibits exceptional mechanical performance with a breaking elongation of 746% and toughness of 55.5 MJ·m-3. By utilizing the dynamically reversible behavior of coumarin units to repair the damaged network, the efficient self-healing performance (99.2%) of HEOMC-PU is obtained. In addition, the prepared network and light-induced dynamic reversibility endow the HEOMC-PU with both liquid-state remoldability and solid-state plasticity, respectively, enabling polyurethane to be recycled and processed multiple times. Furthermore, based on the fluorescence responsive characteristic of coumarin, HEOMC-PU with a fluorescent pattern can be deformed into specific three-dimensional configurations by combining photolithography, self-healing, and the shape memory effect. Such a multilevel and multidimensional anti-counterfeiting platform with rewritable fluorescent patterns and reconfigurable shapes can open up a new encryption approach for future intelligent anti-counterfeiting.

Controllable Physical Synergized Triboelectricity, Shape Memory, Self-Healing, and Optical Sensing with Rollable Form Factor by Zn cluster

To build devices offering users comfortable experience, it is important to focus on form factor and multifunctionality. In this study, for the first time, multifunctional Zn clusters with shape memory, self-healing, triboelectricity, and optical sensing synergized with rollable form factor are designed and fabricated by coordinating COO^- and Zn²⁺ . As pore forming agent, Zn clusters produce hierarchical porous structure depending on Zn amount. Zn clusters are applied as message transmitters and charge containers in optical sensing and corona charge injection, respectively. Moreover, Zn clusters in PVB-COO-Zn serve as positive tribomaterial due to Zn ion doping effect, increasing the output performance as the Zn amount reaches 20 wt%. In addition, injecting positive charge into PVB-COO-Zn 20 lead to more than 24 times increase in output performance compared to those of non-porous structures. The reversibility of Zn clusters endows shape memory and self-healing, synergized with the rollable form factor. The rollability is implemented using the long alkyl chain and the energy absorption of porous structure, providing damage resistance. The advancements in this work provide opportunities for multifunctional and unique applications (shape memory rotating-triboelectric nanogenerator, rollable self-healing touchpad, hidden tag) synergized with rollability that accomplishes working in broadened condition in near future.

Recent Progress on Self-Healable Conducting Polymers

Materials able to regenerate after damage have been the object of investigation since the ancient times. For instance, self-healing concretes, able to resist earthquakes, aging, weather, and seawater have been known since the times of ancient Rome and are still the object of research. During the last decade, there has been an increasing interest in self-healing electronic materials, for applications in electronic skin (E-skin) for health monitoring, wearable and stretchable sensors, actuators, transistors, energy harvesting, and storage devices. Self-healing materials based on conducting polymers are particularly attractive due to their tunable high conductivity, good stability, intrinsic flexibility, excellent processability and biocompatibility. Here recent developments are reviewed in the field of self-healing electronic materials based on conducting polymers, such as poly 3,4-ethylenedioxythiophene (PEDOT), polypyrrole (PPy), and polyaniline (PANI). The different types of healing, the strategies adopted to optimize electrical and mechanical properties, and the various possible healing mechanisms are introduced. Finally, the main challenges and perspectives in the field are discussed.

Tailored modular assembly derived self-healing polythioureas with largely tunable properties covering plastics, elastomers and fibers

To impart self-healing polymers largely adjustable dynamicity and mechanical performance, here we develop libraries of catalyst-free reversible polythioureas directly from commodity 1,4-phenylene diisothiocyanate and amines via facile click chemistry based modular assembly. By using the amine modules with various steric hindrances and flexibilities, the reversible thiourea units acquire triggering temperatures from room temperature to 120 °C. Accordingly, the derived self-healable, recyclable and controlled degradable dynamically crosslinked polythioureas can take effect within wide temperature range. Moreover, mechanical properties of the materials can be tuned covering plastics, elastomers and fibers using (i) different assemble modules or (ii) solid-state stretching. Particularly, unidirectional stretching leads to the record-high tensile strength of 266 MPa, while bidirectional stretching provides the materials with biaxial strengths up to over 120 MPa. The molecular mechanism and technological innovations discussed in this work may benefit promotion and application of self-healing polymers towards greatly diverse demands and scenarios.

Intrinsic self-healing polymers attract increasing attention but often suffer from a narrow self-healing temperature range and unsatisfactory mechanical performance. Here, the authors use click chemistry to develop a library of catalyst-free reversible polythioureas and demonstrate that the self-healing temperature and mechanical properties can be adjusted by controlling the flexibility and the steric environment.