Hydrogen bonds play a crucial role in self-healing systems. Such systems rely on intermolecular interactions to achieve their self-healing function. Hydrogen bonds are strong non-covalent bonds that are essential for the performance of the system. When the material is damaged by external forces, the weaker hydrogen bonds dissociate; if the broken parts are realigned and held together for a certain period, the material can repair itself. Since new hydrogen bonds naturally form on the fractured surfaces, the mobility of polymer chains also positively influences the self-healing process of the material.

Zhu et al.^[17] developed a series of photocurable 3D-printable resin monomers and prepared a 3D-printed resin composed of mono-functional polyurethane acrylate and acrylic acid as monomers, with zinc dimethacrylate as the crosslinker. Under light irradiation, the liquid monomers rapidly polymerize and crosslink due to the presence of intermolecular hydrogen bonds and ionic bonds. By adjusting the monomer composition, polymer networks ranging from soft to hard were obtained, and the dynamically crosslinked polymer networks endowed the printed parts with excellent self-healing and recyclability.

Wang et al.^[18] developed a polymer gel system (PEGgel) based on (hydroxyethyl methacrylate-co-acrylic acid) (P(HEMA-co-AAc)), in which polyethylene glycol (PEG) serves as the liquid component within the PEGgel. This gel system exhibits excellent physical properties, including high tensile strength and toughness, rapid self-healing capability, and sustained stability under various environmental conditions, mainly due to the abundant weak hydrogen bonds in polyethylene glycol that significantly enhance the mechanical properties of PEGgel. Additionally, PEGgel can rapidly self-repair. PEGgel has been successfully utilized to fabricate self-healable pneumatic actuators through 3D printing technology.

Wu et al.^[19] proposed a widely applicable method for photopolymerization-based 3D printing of thermoplastic polymers by designing hydrogen bonding interactions among thermoplastic resin molecules, wherein the hydrogen bonds promote molecular aggregation within the ink, thereby enhancing the polymerization rate of monomers. At the same time, the hydrogen bonding interactions slow down the dissolution and diffusion rates of the polymer in the unpolymerized parent ink, making it more effective to fabricate thermoplastic polymers using LCD-based photopolymerization printing technology.

Although Zhu, Wang, Wu et al. have achieved self-healing and recyclability in 3D printed materials, the performance of such self-healing materials can be affected by hydrogen bonds and ionic bonds within the material, thereby imposing certain limitations in practical applications. For example, under conditions of higher temperature or humidity, the stability and self-healing effectiveness of hydrogen bonds may weaken, leading to a decline in material performance.

The introduction of metal-ligand (M-L) complexes brings tunable thermodynamic and kinetic properties to self-healing systems, which can be used to prepare materials with adjustable mechanical properties. As many metal-ligand interactions are reversible, these complexes can break under physical or chemical stimuli, and once the stimuli are removed, the interactions can reform, thereby healing the material. Moreover, the electron exchange between the metal center and ligands endows metal-ligand complexes with unique electronic properties, including optical, magnetic, and electrochemical characteristics, which are beneficial for further expanding the variety of self-healing materials.

Burnworth et al.^[20] developed a light-induced self-healing material, which was the first reported case of such material. They prepared various supramolecular materials by introducing 2,6-bis(1'-methylbenzimidazolyl)pyridine (Mebip) ligands at both ends of the polymer chains and utilizing the interactions between Zn²⁺ and La³⁺ ions with the ligands. These supramolecular materials can achieve self-healing through illumination after being damaged. When exposed to light of a specific wavelength, the damaged regions reconnect, restoring their initial properties. Experiments demonstrated that supramolecular materials containing either Zn²⁺ or La³⁺ ions could undergo self-healing under ultraviolet light irradiation, and under the same conditions, the supramolecular materials with La³⁺ ions exhibited superior self-healing capability.

Lai et al.^[21]designed a special polymethylsilsesquioxane (PMSSQ) monomer containing numerous carboxylic acid groups in its side chain, where the carboxyl group is a functional group capable of forming coordination bonds with metal ions. They utilized the weak coordination bond interaction between this monomer and zinc metal to synthesize a polymer material with high mechanical strength, exhibiting a Young's modulus close to 500 MPa, indicating excellent mechanical properties. Additionally, since the coordination equilibrium reaction between zinc metal and carboxylic acid is temperature-dependent, when the temperature increases, the equilibrium shifts toward dissociation of the complexes, causing the polymeric network to dissociate into free PMSSQ monomers and zinc ions, implying that the originally rigid polymeric network becomes softened and dispersed at elevated temperatures. Conversely, when the temperature decreases, the coordination equilibrium shifts back toward complex formation, allowing the monomers and metal ions to reconnect via coordination bonds and reconstruct the three-dimensional cross-linked polymeric network. As a result, the material exhibits hard and brittle characteristics at low temperatures but becomes soft and elastic at high temperatures, showing good thermoplasticity and thermal healing properties. In summary, this polymeric material utilizes the temperature-sensitive coordination equilibrium reaction between zinc metal and carboxylic acid groups to achieve unique temperature responsiveness, enabling the material to exhibit distinctly different physical properties under varying temperatures and to self-heal upon heating after damage.

Self-healing materials based on coordination bonds demonstrate significant advantages in terms of multiple healing cycles and mechanical properties. However, they are highly sensitive to environmental conditions and exhibit poor stability under varying pH levels, temperatures, and humidity, which limits their potential applications in extreme environments. At the same time, the effectiveness of material healing depends on precise control of specific conditions, increasing the complexity of practical applications. Moreover, metal ions used in these materials are relatively expensive and some possess toxicity, hindering their application in biological and environmentally friendly fields. During long-term use, such materials also face issues of fatigue and reduced stability; after repeated breaking and reformation, their healing efficiency and strength gradually decline, affecting performance in applications requiring high strength or durability. Large-scale production remains challenging as well, particularly regarding cost-effectiveness, synthesis consistency, and durability. Future development needs to focus on improving environmental adaptability, optimizing long-term performance, and developing low-toxicity, low-cost metal ions to unlock broader industrial application potential.

Host-guest interactions (H-G interactions) are a special type of intermolecular force that is often applied in the design and synthesis of self-healing materials. Most self-healing hydrogels based on H-G interactions can reorganize and recover their structures and functions after damage due to the dynamic interactions between host and guest molecules, thereby enabling automatic self-repair after physical injury without external intervention. Additionally, these types of hydrogels are generally sensitive to environmental conditions (such as temperature, pH, light, and electromagnetic fields), allowing them to change their physical or chemical properties under specific stimuli. Currently, such materials demonstrate significant application potential in the fields of medical materials and novel wearable sensors.

Liu et al.^[22] utilized low molecular weight polyvinyl alcohol (13,000) as the polymer backbone and introduced biocompatible host-guest natural molecule pairs, β-cyclodextrin and cholic acid, to prepare self-healing hydrogels at room temperature via host-guest interactions. This method for preparing self-healing hydrogels is not only simple to operate but also offers potential applications in the biomedical field due to its biocompatibility. The obtained hydrogel can achieve self-healing through host-guest interactions at room temperature after mechanical damage, recovering some of its mechanical properties.

Wang et al.^[23] developed a host-guest supramolecular (HGSM) system with three "arms" that can covalently crosslink with the natural polymer gelatin methacryloyl (GelMA), thereby constructing a novel supramolecular hydrogel (HGGelMA) integrating covalent crosslinking and non-covalent host-guest interactions, which is self-healable, can be 3D printed into various shapes, and exhibits significantly improved mechanical properties. The construction strategy of the host-guest supramolecular hydrogel is illustrated in Figure 1.

Self-healing materials based on host-guest interactions offer flexibility and multifunctionality in design, particularly excelling in the self-repair of material structures and functions. However, they also face challenges such as susceptibility of the repair effect to environmental influences, low repair efficiency, difficulty in fully restoring initial strength within a short period, complex design, high costs, and significant performance degradation after repeated repairs, making it difficult to meet the demands of applications requiring high strength and durability during long-term use.

Diels-Alder (DA) reaction is a common [ 4 + 2 ] cycloaddition reaction, in which a diene and a dienophile combine to form stable six-membered cycloadducts^[24]. Figure 2 shows typical DA reactions, where furan and maleimide derivatives are the most commonly used reaction systems^[25]. The DA reaction is thermally reversible, a characteristic that makes it suitable for designing self-healing materials. During heating, the DA adduct undergoes a retro-DA reaction, leading to the cleavage of previously formed covalent bonds within the material. When the temperature decreases, the diene and dienophile can reinitiate the cycloaddition reaction, thereby reforming the covalent bonds and enabling the material to repair itself.

Li et al.^[27] designed and synthesized a polymer/graphene-based self-healing material through a Diels-Alder (D-A) reaction. This material, which features a cross-linked network structure, can rapidly, efficiently, and via multiple channels repair itself under combined stimulation by heat, infrared light, and microwaves. It holds significant potential for applications in fields such as military equipment, protective coatings, and building materials.

Ouyang et al.^[28] designed and synthesized novel dynamic cross-linked self-healing polyurethanes containing Diels-Alder bonds, as well as their carbon nanotube (CNTs) composites, achieving SLS (selective laser sintering) 3D/4D printing. They initially synthesized a series of dynamic cross-linked polyurethane CANs containing DA bonds (Figure 3). By designing DA-bonded diol molecules and optimizing the molecular weight of polycaprolactone diols, types of isocyanates, and degree of crosslinking, they obtained a polyurethane with excellent self-healing properties and mechanical performance suitable for SLS 3D printing, exhibiting a tensile strength of approximately 23 MPa and an elongation at break of approximately 307%. The material achieved a healing efficiency of 98.2% after being repaired for 1 h at 120 °C.

Guo et al.^[29] developed a novel reprocessable and degradable dynamic cross-linked elastomeric polymer based on the Diels-Alder synergistic reaction. By incorporating nanocomposites, they constructed a conductive elastomer with excellent toughness and stretchability, enabling convenient customization of wearable electronic devices through 3D printing. The core of their research involved the development of a polyester elastomer PFB with favorable dynamic properties. PFB achieves dynamic cross-linking through the Diels-Alder cycloaddition reaction between furan and maleimide structures. This cycloaddition reaction is a highly specific concerted process, allowing PFB to maintain its durable dynamic characteristics in various environments. This feature endows PFB with excellent thermoplasticity, facilitating processing and recyclability via 3D printing technology.

Wang Dingwen^[30] prepared self-healing cross-linked non-isocyanate polyurethane (NIPU) materials using a gradual heating method. Unsaturated polyesters and NIPUs with low molecular weight and high C \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}C content were synthesized through the DA reaction. The high C \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}C content in the unsaturated polyester enables SD-NIPU to exhibit excellent self-healing properties, achieving complete surface crack repair within only 5 min at 120 °C.

Feng et al.^[31] developed a novel thermoplastic polyurethane (PU-DA) with excellent self-healing properties, utilizing both the thermally reversible DA reaction and molecular chain thermal motion to repair cracks formed during use. In this system, the thermally reversible DA reaction primarily restores the material's mechanical properties, while the thermal motion of molecular chains accelerates the entire self-healing process. Thanks to this dual mechanism, the material exhibits rapid repair capability and high self-healing efficiency. For instance, under conditions of heating at 120 °C for 15 minutes, its self-healing efficiency can reach 71%. The healing mechanism and self-healing performance of this material are illustrated in Fig. 4.

Although the photo-curable polymer materials prepared via Diels-Alder addition reactions exhibit higher polymer strength and self-healing properties similar to those from ester exchange reactions, these reactions typically require higher temperatures, and their self-healing processes demand greater energy input^[33].

The acylhydrazone bond is a chemical bond formed through a condensation reaction between aldehyde compounds and acylhydrazine. Under acidic catalysis, the reaction exhibits reversibility under suitable mild conditions^[34]. The acylhydrazone structure demonstrates its dynamic reversible characteristics through the imine bond. Meanwhile, the amide groups in acylhydrazones can provide additional stability via hydrogen bonding interactions; however, these hydrogen bonds do not significantly influence the self-healing process. Moreover, introducing aldehyde or acylhydrazine compounds into the acylhydrazone can trigger dynamic reversible exchange reactions, thereby affecting the physical properties of the polymer.

Kim et al.^[35] synthesized transparent hydrogels by adding a mixture of glycol chitosan (GC) and adipic acid dihydrazide (ADH) into an oxidized hyaluronic acid (OHA) solution. The mechanism involves the formation of imine bonds (C \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}N) between the aldehyde groups of OHA and the amino groups of GC, as well as acylhydrazone bonds (C \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}N—N) formed between the aldehyde groups of OHA and the acylhydrazide groups of ADH. Through extrusion-based bioprinting of the OHA/GC/ADH hydrogel, model objects of various shapes and sizes were 3D printed. The resulting 3D constructs exhibited structural stability and self-healing properties without requiring additional support materials or secondary crosslinking for further stabilization.

Most dynamic reversible exchange capabilities of acylhydrazone bonds are mainly exhibited in environments with higher mobility and the presence of acidic catalysts. Therefore, materials capable of self-healing at low temperatures typically require repair in solution or gel states. In contrast, solid-state materials generally need higher temperatures to achieve self-healing.

A disulfide bond is a covalent bond formed between two sulfur atoms. In recent years, self-healing mechanisms based on disulfide bonds have been applied to develop novel self-healing materials. These materials can repair themselves and restore their original physical and chemical properties after sustaining damage. The mechanism involves the dynamic breaking and reformation of disulfide bonds through chemical reactions. When the material sustains damage, the disulfide bonds break, releasing reactive sulfur atoms. Subsequently, under appropriate conditions, such as heating or the addition of a catalyst, the disulfide bonds can reform, thereby "repairing" the damaged areas of the material.

Miao et al.^[36] obtained a self-healable 3D smart structure by casting an EPSS/CNTs composite material with dynamic disulfide bonds and sacrificing a 3D printed mold. The self-healing mechanism of the EPSS/CNTs is illustrated in Fig. 5.

Rahman et al.^[37] synthesized a novel elastomer using photo-initiated thiol-ene click chemistry through inkjet 3D printing, which was post-processed by compression molding. The self-healing and reprocessing characteristics were achieved by introducing dynamic disulfide bonds into the cross-linked network structure of the elastomer, where the disulfide metathesis reaction could be accelerated by a phosphine catalyst. The repaired and reprocessed elastomers exhibited mechanical and thermal properties similar to those of the original. The disulfide bond metathesis reaction during the self-healing process of the elastomer is illustrated in Fig. 6.

Li et al.^[38] prepared a polyurethane elastomer containing disulfide bonds using DLP printing technology, which exhibited excellent self-healing capability. They mixed polyurethane acrylate containing disulfide bonds with reactive diluents and photoinitiators to produce a photosensitive resin with good fluidity and rapid curing characteristics. Using this resin, they printed various three-dimensional components with high precision, high tensile strength (3.39 MPa), high elongation at break (400.38%), and superior elasticity and toughness, along with self-healing properties. After heat treatment at 80 °C for 12 h, the self-healing efficiency of the printed components reached up to 95%. The obtained printed components, due to their excellent self-healing ability and mechanical properties, hold great potential for applications in fields such as sensors and smart materials.

Due to the fact that the self-healing capability of disulfide bonds usually requires specific temperature conditions, their healing efficiency may significantly decrease under excessively low or high temperatures. In addition, compared with certain other types of self-healing mechanisms, the self-healing process based on disulfide bonds is relatively slow, making it unsuitable for applications requiring rapid repair. At the same time, disulfide bonds are also highly sensitive to oxygen and are prone to oxidation in oxygen-rich environments, leading to a reduction in the material's self-healing ability. These factors greatly restrict the performance of disulfide bond-based self-healing materials in practical applications.

However, these intrinsic self-healing polymer materials exhibit varying mechanical properties and healing efficiencies, as shown in Table 1.

White^[39] first proposed a new concept in 2001: polymer self-healing composites through microcapsule-external healing agents, designed to repair resin matrix composites. When the material cracks, the microcapsules release a healing agent, which infiltrates the crack and reacts with a pre-embedded catalyst, thereby bonding the crack and achieving a repair effect. The mechanism is illustrated in Figure 7. In White's experiment, dicyclopentadiene (DCPD) was encapsulated in polyurea-formaldehyde (PUF) microcapsules and embedded into an epoxy resin matrix. Upon damage, the microcapsules rupture, releasing DCPD monomers. These DCPD monomers subsequently come into contact with Grubbs' first-generation catalyst distributed throughout the matrix, triggering a ring-opening metathesis polymerization (ROMP) reaction that forms a cross-linked network structure to repair the crack. Using tapered double cantilever beam (TDCB) specimens to evaluate the recovery of fracture toughness, studies showed that the self-healing efficiency reached as high as 75%. The content of Grubbs' catalyst is a key factor affecting the self-healing efficiency; its concentration is positively correlated with the ROMP reaction rate^[40]. Although this self-healing system offers advantages such as low viscosity and rapid reaction at room temperature, it suffers from limitations including the high cost of Grubbs' catalyst and its susceptibility to deactivation.

Zhang Qingqing et al.^[41] designed a dual self-healing anticorrosive coating based on polyaniline microcapsules. By combining the emulsion templating method, photopolymerization, and interfacial polymerization of aniline, linseed oil-loaded polyaniline microcapsules were prepared and integrated with a waterborne epoxy resin coating to construct a dual self-healing anticorrosive coating with excellent photothermal conversion capability. The linseed oil loaded in the microcapsules acts as a self-healing agent, while the polyaniline in the shell provides both corrosion inhibition and photothermal repair functionalities.

Kong et al.^[42] developed a microcapsule-based self-healing anticorrosive coating using epoxy resin. The shell material of the microcapsules consists of isophorone diisocyanate and trimethylolpropane, while the healing agent comprises isophorone diisocyanate and dipentaerythritol. When the material sustains damage and is exposed to the atmosphere, the exposed diisocyanate groups react with water molecules in the air, achieving efficient self-healing with a repair efficiency exceeding 80%.

Li et al.^[43] adopted a conductive aqueous solution as a healing agent for electronic devices, successfully fabricating microcapsules with melamine-formaldehyde resin as the shell and the conductive aqueous solution as the core. The obtained microcapsules exhibited uniform core-shell spherical structures and demonstrated excellent thermodynamic properties and phase change latent heat, making them suitable not only for self-healing of electronic devices but also for reducing heat generated during long-term operation of circuits.

Microcapsule-based self-healing technology enables efficient repair, but its capability for multiple repairs at the same location is limited, and there are several technical challenges, such as determining the capsule diameter and shell thickness. Additionally, an inappropriate core-shell ratio may affect the mechanical properties and surface finish of the material. Due to these limitations, microcapsule self-healing technology may be more suitable for applications in 3D printing where repeated damage is uncommon, precision requirements are not stringent, and frequent surface treatments are unnecessary.

Both microcapsule-type and hollow fiber-type self-healing materials have a common drawback: they are mostly unable to repeatedly repair cracks at the same location multiple times, as cracks tend to reappear at the original fracture surfaces.^[44] Microvascular self-healing materials utilize a method in which a flowable healing agent fluid is contained within a three-dimensional network of microchannels. This approach can overcome the limitations of microcapsule-type and hollow fiber-type self-healing materials, enabling multiple self-healing events for microcracks. When the microvasculature ruptures, the healing agent is released into the composite material, allowing for self-repair. Microvascular self-healing materials not only enable repeated repairs at the same location but also enhance the material's toughness, with the added advantage that the healing agent can be replenished multiple times, ensuring continuous supply. Therefore, microvascular self-healing materials represent a superior self-healing system compared to hollow fiber and microcapsule-based repair technologies.

Microvascular self-healing materials are a type of smart material first introduced by Toohey et al.^[11] in 2007. These materials can utilize Grubbs catalyst and dicyclopentadiene (DCPD) healing agent to repeatedly repair damage at the same location within the material. Studies have shown that the healing efficiency of DCPD can reach 70%, and the catalyst content does not affect the healing efficiency. However, the mass fraction of the catalyst limits the number of healing cycles; when the catalyst's mass fraction is 10%, up to seven healing cycles can be achieved.

Dean et al.^[45] employed a segmented gas-liquid flow (SGLF) approach to enhance reagent mixing within the microvascular self-healing system. Their study revealed that SGLF can achieve more uniform mixing of healing agents in large-scale damaged regions, thereby improving the healing effectiveness and mechanical properties of the material. At the same time, SGLF is also capable of expanding the repairable range of the material and increasing its degree of polymerization. Based on these findings, SGLF shows promise for further optimizing the performance of microvascular self-healing systems by varying material types, damage patterns, and healing chemical reactions.

Gergely et al.^[46] utilized 3D printing technology to fabricate sacrificial templates from polylactic acid (PLA) containing a catalyst. Subsequently, the catalyst accelerated the pyrolysis of PLA, and the template structure was optimized. They embedded the template within a thermosetting polymer and then removed it through a heat treatment process, creating multidimensional, multiscale, interconnected vascular and porous networks within the thermosetting polymer. This method enables the fabrication of vascular and porous structures with various sizes and shapes, offering an approach for engineering materials that references biologically optimized vascular design.

Microvascular self-healing is a method that utilizes the three-dimensional network structure of microchannels to repeatedly supply healing agent fluids, thereby achieving multiple self-repair in damaged areas of materials^[47], which has significant application value but also faces several challenges, mainly in the fabrication and optimization of the three-dimensional network structure. The manufacturing of such three-dimensional network structures is relatively complex, requiring considerations of factors such as catalyst activity, healing agent fluidity, and mechanical properties of the material. Therefore, there remains considerable room for advancement in the research of microvascular self-healing materials, necessitating the exploration of more effective methods for constructing three-dimensional network structures in future studies.