Recent Progress on the Healing Mechanisms of Self-Healing Superhydrophilic Surfaces

Abstract

Superhydrophilic surfaces have important applications in fields such as energy, military, and medicine due to their unique wettability. However, the micro-/nano-structures of superhydrophilic surfaces are fragile and prone to damage, which can cause them to lose their superhydrophilicity and reduce their service life, severely limiting their applications. This paper discusses recent research progress and self-healing mechanisms of self-healing superhydrophilic surfaces from the perspectives of composition and structure self-healing. Additionally, it also introduces the research progress of superhydrophilic surfaces healed in air and underwater environments. Finally, the limitations of the self-healing superhydrophilic surfaces are summarized, and perspectives on future development are discussed.

1. Introduction

Superhydrophilic surfaces can improve the energy utilization efficiency of systems; reduce energy loss as a protective barrier and medium for interaction with the external environment; and have important applications in marine pollution prevention, transportation resistance reduction, membrane separation technology, biomedicine, photovoltaic power generation, catalysis, new energy, and other fields [1,2,3,4]. However, the micro-/nano-structures and hydrophilic composition of superhydrophilic surfaces suffer from insufficient mechanical and chemical stability, making it extremely important to improve surface durability. This can not only extend the surface service life but also reduce energy consumption caused by maintenance [5]. However, surface damage is inevitable, such as cracks, fractures, and even local detachment in extreme cases during use, which reduces surface durability and greatly limits its application.

At present, the improvement of superhydrophilic surface durability mainly focuses on two aspects: mechanical durability and chemical durability. Mechanical durability mainly focuses on improving the mechanical properties of surface structure. For example, Zhao et al. [6] prepared a composite micro-/nano-superhydrophilic surface using laser etching method, which has excellent durability characteristics and can withstand a wear distance of more than 6 m. Chemical durability mainly focuses on improving the stability of surface chemical composition [7]. For example, Zhuang et al. [8] deposited hydroxides of Fe, Co, and Ni on amino grafted carbon substrates by electrodeposition to prepare superhydrophilic surfaces with alkaline and salt resistance. However, the superhydrophilicity may decrease once the structure of the surface is damaged by external forces. Therefore, how to heal the damaged surface structure is crucial for extending the service life of superhydrophilic surfaces.

Inspired by the self-healing function of organisms, endowing the surface with a self-healing property can effectively enhance its durability. The preparation of self-healing superhydrophilic surfaces is usually to introduce dynamic reversible bonds or substances capable of restoring damaged areas, enabling the surface to autonomously restore its structure integrity after physical damage. Concurrently, by grafting hydrophilic chemical groups and constructing micro-/nano-scale rough structures, the high surface energy and extremely low water contact angles are ensured, thereby achieving the superhydrophilic effect. Common preparation methods include coating, surface functionalization, grafting modification, layer-by-layer assembly, and in situ synthesis. The preparation of self-healing superhydrophilic surfaces typically combines these methods synergistically to endow the surface with durable anti-fouling, anti-fogging, and self-cleaning properties, significantly extending its service life.

Some researchers have reported studies of self-healing superhydrophilic surfaces, but few papers have reported the progress of superhydrophilic surfaces in integrating fabrication methods, damage categories, ways of healing, and respective advantages and disadvantages. In this review, we summarize the latest research progress in self-healing superhydrophilic surfaces based on the basic principle of wettability, focusing on the composition self-healing and structure self-healing (Figure 1). The advantages and disadvantages of self-healing strategies are also summarized, followed by a look forward to their future development.

2. Thermodynamic Principle of Superhydrophilic Surfaces

Under the idealized assumption of a perfectly smooth and flat surface, Young’s equation [9] could quantitatively describe the contact angle θ at the three-phase interface of a droplet based on the balance of the surface free energy:

$$\cos \theta ={\displaystyle \frac{{\mathsf{\gamma }}^{\mathrm{s}}-{\mathsf{\gamma }}^{\mathrm{sl}}}{{\mathsf{\gamma }}^{\mathrm{l}}}}$$

where γ^s is the surface tension of the solid, γ^sl is the solid–liquid interfacial tension, and γ^l is the surface tension of liquid (Figure 2).

Thus, the wettability of the solid surface can be classified into the following four categories based on the contact angle θ:

(1)

When θ = 0°, the liquid completely spreads across the solid surface, achieving a full wetting state.

(2)

When 0° < θ < 90°, the liquid spreads across a finite contact area on the solid surface, presenting partial wetting state.

(3)

When 90° < θ < 180°, the liquid contracts into a bead-like shape on the surface the solid surface, exhibiting a non-wetting state.

(4)

When θ = 180°, the liquid forms a perfect sphere on the solid surface, demonstrating a perfect non-wetting state.

However, the classical Young’s model is only applicable to ideally smooth surfaces. To expand its practical application, in 1936, Wenzel [10] established a quantitative relationship between the apparent contact angle θr and the intrinsic contact angle θ of a rough surface based on the thermodynamic equilibrium principle

$$\cos {\theta }_r=\mathrm{rcos}\theta$$

where r is the surface roughness factor, which is defined as the ratio of the real solid–liquid contact area to the apparent projection area. This model introduced the surface morphology parameter into the wetting theory system for the first time. In 1944, Cassie and Baxter [11] proposed different modifications to Young’s equation by investigating the non-homogeneous surface wettability of two different rough chemical materials in terms of the ratio of the solid surface to the gas surface at the three-phase junction:

$$\cos {\theta }_c=f_1\cos \theta +f_2\cos {\theta }_g$$

where f₁ and f₂ denote the solid–liquid and solid–gas contact area fractions, respectively, which satisfy f₁ + f₂ = 1. θ is the solid–liquid contact angle, and θ_g is the solid–gas contact angle. Compared with Wenzel’s formula, Cassie’s formula more accurately describes the real situation of the system interface.

Systematic comparison of the two modified models shows that surface roughening can significantly enhance the intrinsic wetting characteristics of the material [12].

For the design of superhydrophilic and underwater superoleophobic materials, the contact angle of oil in air and the contact angle of oil underwater have been formulated according to Young’s equation:

$$\begin{gathered} \cos {\theta }_o={\displaystyle \frac{{\gamma }_{sv}-{\gamma }_{so}}{{\gamma }_{ov}}} \\ \cos {\theta }_{\mathrm{ow}}={\displaystyle \frac{{\gamma }_{sw}-{\gamma }_{so}}{{\gamma }_{ov}}} \end{gathered}$$

γ_sv, γ_so, and γ_ov denote the interfacial tensions at the solid–gas, solid–oil, and oil–gas interfaces, respectively, and γ_sw and γ_ow correspond to those at the solid–water and oil–water interfaces. Combining the above equations and Young’s equation, the underwater contact angle can also be written as

$$\cos {\theta }_{\mathrm{ow}}={\displaystyle \frac{{\gamma }_{ov}\cos {\theta }_o-{\gamma }_{wv}cos{\theta }_w}{{\gamma }_{ov}}}$$

As the superhydrophilic surface is immersed in water, water spreads rapidly over it, forming a stable film of water, thus isolating it from the oil phase. Hence, hydrophilic surfaces are usually oleophobic [13].

During the dynamic spreading of the liquid film, the apparent contact angle at the three-phase contact line changes: the contact angle θ gradually evolves from an initial obtuse angle to an acute angle as the free energy of the solid–liquid interface continues to decrease. When θ < 10° [14], the solid–liquid interaction energy significantly exceeds the free energy of the liquid–gas interface, and the surface is superhydrophilic.

The hydrophilicity of the surface is primarily influenced by hydrophilic components and surface micro-/nano-structures. The composition determines whether the surface is hydrophilic, while the micro-/nano-structures enhance surface wettability, such as porous structures and hierarchical structures. Typically, hydrophilic surfaces are influenced by hydrophilic composition and exhibit high surface energy. These hydrophilic components primarily consist of substances containing polar functional groups such as hydroxyl, carboxyl, amino groups, and other partially oxygen-containing functional groups. For example, Wang et al. [15] used ultraviolet-visible light to irradiate ZnO@stearic acid nanoarrays prepared on a Zn substrate to excite electron–hole pairs. It will continuously oxidize and reduce the adsorbed oxygen and water on the surface, form hydroxyl radicals adsorbed at oxygen vacancies on the ZnO surface, create stable hydrophilic sites, and induce the surface to transition from superhydrophobic to superhydrophilic. Additionally, Yu et al. [16] combined NH₂–MIL-125(Ti) with reduced graphene oxide (RGO) and modified it with polydopamine (PDA) to prepare a superhydrophilic membrane containing polar functional groups such as carboxyl and amino groups. Based on in-depth studies of hydroxyl and carboxyl groups, it was found that other oxygen-containing functional groups also exhibit hydrophilic properties, such as carbonyl and sulfonic acid groups. Hung et al. [17] functionalized graphene nanoplates (GNPs) with oxygen-containing groups, enriching their surfaces with hydrophilic groups such as hydroxyl, carboxyl, and carbonyl groups, thereby synthesizing a superhydrophilic surface with a water contact angle of 0° and exhibiting ultra-fast water penetration performance. For hydrophilic surfaces, constructing micro-/nano-structures, such as porous structures or micro-nano hierarchical structures, can enhance surface hydrophilicity. For example, Bastami et al. [18] used ZrO₂ films as raw materials to investigate how different temperatures affect surface roughness, verifying that increasing surface roughness within a certain range enhances the hydrophilicity of hydrophilic materials. Liu et al. [4] successfully prepared a robust superhydrophilic surface by spraying the mixture of PEI and TiO₂ nanoparticles onto an epoxy resin surface to alter surface roughness, thereby effectively enhancing the substrate hydrophilicity and stability.

3. Self-Healing Mechanism of Superhydrophilic Surfaces

It is difficult for superhydrophilic surfaces to withstand long-term physical wear or chemical erosion. To address this challenge, researchers have innovatively developed superhydrophilic surfaces with self-healing capabilities inspired by the healing mechanisms of biological tissues [19]. Upon damage, the pre-designed healing units are activated to facilitate autonomous self-healing via two reversible dynamic processes: (i) composition self-healing based on the composition recovery and (ii) structure self-healing based on the reformation of chemical bonds via polymer chain migration and reorganization. This innovative self-healing mechanism ensures the surface maintains superhydrophilic properties and interfacial function stability, even causing structure defects such as cracks, pores, and so on. Furthermore, the hydrogels are formed by hydrophilic polymer chain networks [20], and they can quickly absorb the liquid droplets added onto their surface, presenting a state of rapid spreading on the surface and having superhydrophilic properties [21]. Therefore, some self-healing hydrogels are classified as superhydrophilic surfaces for introduction in this paper.

3.1. Composition Self-Healing

The self-healing phenomena inherent to biological systems have inspired the development of self-healing materials. By facilitating the regeneration of surface compositions, these materials can restore the composition of the surface, recover functional properties, and reestablish surface wettability, thereby enhancing durability. Srubar et al. [22] significantly enhanced the self-healing performance of active building materials by combining the physical re-crosslinking of hydrogels with the mineralization effects of different microorganisms. When material surfaces are damaged, rapid healing of the damaged areas can be achieved through biomineralization using the CO₂ concentration mechanism of photosynthetic microorganisms or through the hydrolysis of urea by urease-producing bacteria to produce CaCO₃. To extend the application to underwater environments, Chen et al. [23] obtained a high-strength underwater superoleophobic surface through a synergistic combination of superspreading and biomineralization techniques. The wettability was found that could be recovered by the re-biomineralization process after mechanical wear.

Additionally, by employing the polymer-functionalized microgel spheres as the templates, Chen et al. [24] achieved surface hydrophilicity recovery through volumetric expansion of the microgel matrix coupled with interfacial reaggregation of hydrophilic polymer chains at the liquid/solid interface (Figure 3). Compared with the local self-healing of microgel beads, the self-polishing strategy can effectively address large-scale surface scratches and damage. For example, Kang et al. [25] developed a self-polishing multi-layer coating material based on dextran and chitosan, achieving interlayer bonding through imine bonds between dextran aldehyde and carboxymethyl chitosan. When the surface undergoes biofouling due to bacterial adhesion, it creates a local acidic microenvironment, which promotes the breaking of imine bonds to accelerate surface self-polishing and restore the coating’s anti-fouling and antibacterial properties. Additionally, self-polishing polymer/SiO₂ composite surfaces containing hydrolytically cleavable side chains were fabricated by molecular engineering strategies, as reported by Wang et al. [26]. Autonomous wettability restoration was demonstrated via marine environment-triggered hydrolytic cleavage of siloxane ester linkages at the composite–liquid interface.

However, once the surface suffers structure damage, the composition self-healing cannot effectively restore the damaged structure, leading to the loss of surface wettability. Therefore, how to heal surface structure damage is extremely important for maintaining the anti-fouling performance of superhydrophilic surfaces.

3.2. Structure Self-Healing

The exposure of material surfaces to external mechanical stresses can induce structural damage, resulting in diminishing surface hydrophilicity. Therefore, a comprehensive understanding of surface restoration mechanisms and development of effective strategies for surface structure self-healing is of great necessity. The restoration of both micro-structural defects and macrostructural integrity of material surfaces has been demonstrated to effectively enhance its longevity, reducing maintenance frequency while improving structural safety and system energy efficiency [27].

Currently, the structural self-healing mechanisms of superhydrophilic surfaces can be classified into two categories, including extrinsic type and intrinsic type. The difference between these two categories lies in the need of adding external healing agents to complete the self-healing progress. The self-healing microcapsules are the primary type of extrinsic self-healing [28]. The intrinsic type, on the other hand, can be classified into electrostatic interaction reconfiguration, hydrogen bonding reorganization, host–guest recognition, and metal–ligand interactions according to the types of interactions.

3.2.1. Extrinsic Type

The self-healing microcapsule is accomplished by constructing a core–shell microcapsule containing a healing agent, and then the microcapsule ruptures upon reaching a specific environmental trigger such as when the local stress field reaches the critical strength, releasing the healing agent to accomplish autonomous self-healing for the damaged area [29]. For instance, Fu et al. [30] used chitosan and sodium tripolyphosphate as primary materials to fabricate a microcapsule, and the restorative agents of fluorosurfactant FS-60 and dopamine hydrochloride were encapsulated. When the material surface undergoes mechanical abrasion, washing, and chemical erosion, the surface superhydrophilicity can be restored by soaking the material with distilled water followed by heating to induce partial degradation of the microcapsule and release restorative agents. Since this coated fabric exhibits excellent performance in separating oil–water mixtures, it holds broad application prospects in the field of oil–water separation. However, limited by the spatial distribution density of the pre-embedded microcapsules, the healing performance of this method is greatly constrained, leading to a strong confinement on the times of localized self-healing and making it difficult to achieve multiple times of healing at localized areas (Figure 4). In addition, the introduction of larger-sized microcapsules can disrupt the overall homogeneous structure of the material, causing defects and thereby reducing its strength and resistance to impacts [31].

3.2.2. Intrinsic Type

The intrinsic-type self-healing surfaces rely on reversible bonding interactions within the material itself to achieve autonomous healing [32]. Such a type of self-healing strategy is theoretically capable of undergoing multiple healing cycles at the same location, enhancing the durability and longevity of the material [33,34,35]. Compared with extrinsic self-healing strategies, surfaces with intrinsic self-healing properties typically exhibit more homogeneous structure.

Electrostatic Interaction

Electrostatic interactions help the establishment of reversible crosslinks between polymer chains through ion–dipole or dipole–dipole interactions [36] (Figure 5a). When scratches or cracks emerged on the surface, external environmental stimulations can induce the happening of electrostatic effects leading to the reorientation of molecular chain segments or functional groups and repair the damaged structure through breakage-reassembly mechanisms of non-covalent bonds [37].

Hozumi et al. [38] prepared a hydrophilic composite membrane using amine-mineralized nanoclay particles (AMP-NCPs) and polyvinylpyrrolidone (PVP). Upon surface scratching, the electrostatic interactions between the positively charged amino groups on the AMP-NCPs and the negatively charged groups of PVP facilitate the self-healing process in humid environments, restoring the initial membrane performance. Furthermore, combining the synergistic effect of the nanoparticles, internal electrostatic interactions, and hydrogen bonding is also a good strategy to improve the mechanical strength and durability while maintaining surface self-healing capability [39] (Figure 5b).

Figure 5. (a) Schematic diagram of the electrostatic interaction self-healing mechanism [36]. (b) Internal structure of the superhydrophilic coating [39]. (a) Reprinted with permission from ref. [36]. Copyright (2022) (Elsevier). (b) Reprinted with permission from ref. [39]. Copyright (2019) (American Chemical Society).

Figure 5. (a) Schematic diagram of the electrostatic interaction self-healing mechanism [36]. (b) Internal structure of the superhydrophilic coating [39]. (a) Reprinted with permission from ref. [36]. Copyright (2022) (Elsevier). (b) Reprinted with permission from ref. [39]. Copyright (2019) (American Chemical Society).

Hydrogen Bond

Hydrogen bonding interactions arise from the attraction and repulsion between molecules, including electrostatic, polarization, charge transfer, dispersion, and exchange mutual exclusion [40], mainly resulting from the interaction between hydrogen atoms and electronegative heteroatoms (F, O, and N) that possess lone electron pairs. As a highly efficient supramolecular interaction, it plays a critical role in the self-healing progress of superhydrophilic surfaces, particularly owing to its reversible and reconstructive nature.

Fan et al. [41] used polyvinyl alcohol (PVA) as the matrix material and salicylic acid (SA) as the cross-linking agent to establish a hydrogen-bonding network to achieve self-healing of damage. However, the healing rate of the coating is slow. It requires a small amount of water existing in the damaged location at the temperature of 85 °C for 15 min to complete the healing progress.

To address this issue, Lai et al. [42] fabricated a self-healing superhydrophilic coating with anti-fog and anti-freezing properties by spinning a mixed solution of polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and Fe (III). The interaction between the carboxyl group of PAA and the hydroxyl group of CMC forms a hydrogen bond structure, which can achieve rapid self-healing of scratches within 1 min.

You et al. [43] developed a composite coating by mixing surfactant Triton X-100 into the epoxy resin/amine hardener. This coating exhibits exceptional resistance against a series of environmental challenges, including acids, alkalis, salts, high temperatures, and ultrasonic waves. Moreover, this surface could effectively heal the damages caused by scratches, cuts, and exposure to polar solutions such as tetrahydrofuran (THF), which thus successfully addressed the long-standing limitation of application scenarios for conventional superhydrophilic surfaces. The robust performance is rooted in the hydrogen bonding interactions established between the hydroxyl groups of Triton X-100 and the ether group of the epoxy resin (Figure 6).

Host–Guest Interaction

Generally, the host molecules form inclusion complexes with guest molecules, which include polymers, organic molecules, and biomolecules. The formation of these complexes is typically driven by non-covalent interactions, such hydrophobic interactions, hydrogen bonding, and π–π stacking [44,45]. Zhang et al. [46] used Fe₃O₄@cucurbituron (CB) nanoparticles as the cross-linking agent to fabricate nanocomposite hydrogels. The gelation happened under the interaction of the CB cavity with the polymer formed by acrylamide and guest molecules of 1-benzyl-3-vinylimidazole. Nie et al. [47] demonstrated that the host–guest interactions between β-cyclodextrin-modified silk fibroin (CD-SF) as the host polymer and adamantane-modified silk fibroin (AD-SF) as the guest polymer, combined with hydrogen bonding interactions involving quaternary ammonium chitosan (QCS), enable rapid self-healing of damaged hydrogels following cutting injuries. Additionally, functionalized MoS₂ can be utilized to significantly improve the mechanical properties of hydrogels without losing self-healing properties [48] (Figure 7).

Metal–Ligand Interaction

The formation of coordination bonds typically accompanied the increase in entropy and the decrease in enthalpy. Hence, the metal–ligand interaction is a dynamic noncovalent bond with high efficiency and capable of spontaneous self-healing without any external stimulations [49] (Figure 8). Recent studies have demonstrated that both zinc (II) and nickel (II) ions serve as effective central metals in complexes of self-healing superhydrophilic surfaces. Notably, pyridine analogs exhibit superior coordination abilities with zinc(II) compared with imidazole or histidine, leading to the formation of more stable and more effective self-healing chelating ligands, while the nickel (II) in comparison to zinc (II) has even better performance [50,51]. Furthermore, Pourjavadi et al. [52] developed interpenetrating network (IPN) hydrogels by polymerizing acrylates and acrylamides in gelatin with the addition of the cross-linking agent of Fe (III)). Such hydrogel exhibits superior mechanical strength, along with controlled degradation and sustained self-healing efficiency even after being cut.

The aforementioned four mechanisms could all facilitate the self-healing of damage at the same location for multiple times. However, it should be noted that they exhibit significant differences. Electrostatic interactions respond fast while they are susceptible to the environmental factors. Hydrogen bonding has a simple structure, which allows for easy synergy with a variety of interactions to improve the surface stability, while it is easy to be influenced by water molecules. The metal–ligand interactions are fast but severely affected by other ions to disrupt the healing progress. Part of the host–guest interactions are insensitive to the water, but most of the surfaces with host–guest molecules have some problems such as low mechanical properties due to the low bonding energy; sensitivity to the healing environment in terms of temperature, pH, and so on; a low healing rate limiting the dynamics; a complex synthesis route; and high cost.

Table 1. Comparison of healing performance of materials with different healing mechanisms.

Type	Raw Materials	Types of Damage	Triggering Mechanism	Healing Time	Healing Efficiency	References
Self-healing microcapsules	CS, sodium tripolyphosphate, FS-60	Abrasion	Heat distilled water to 100 °C after moistening	30 min	Basically fully recovered	[30]
Electrostatic interaction	AMP-NCPs, PVP	Severe damage of 20 microns	Placed in a humid environment	36–48 h	>80%	[38]
Electrostatic interaction and hydrogen bond	Sulfobetaine methacrylate and 2-hydroxyethyl methacrylate	Shallow grooves formed after scraping	Soak in deionized water for 2 s, then place in a room temperature environment	2 min	90%	[39]
Hydrogen bond	PAA, CMC, and Fe (III)	Cut	Water vapor wetting	Within 1 min	Full restoration of transparency	[42]
	Epoxy resin, Triton X-100	Scratches, cuts, and THF solution	Heat separately at 60 °C, 140 °C, and 100 °C	3 min, 3 min, and 30 min	Basically restored to its original appearance	[43]
Host–guest interaction	β-Cyclodextrin, MoS2, and adamantyl segments	Cut	30 °C	10 min	Fully healed	[48]
Metal–ligand interaction	Pyridine and Zinc (II)-based complexes	Scratch damage with a width of 10 ± 3 μm and a depth of 100 μm	Heat 120 °C	16 h	Healed but with scars remaining	[50]
	2,6-diethynyl pyridine, PEG-diazide (L2), Ni2+	Cut	Continuous contact	2 min	Fully healed	[51]

summarizes different researches under various self-healing mechanisms in recent years.

Reversible Dynamic Covalent Bond

Dynamic covalent bonds are a class of chemical bonds that can undergo reversible exchange under certain conditions (e.g., light, heat, humidity stimulation, etc.) [54]. Due to their reversibility and condition-responsive nature, they exhibit significant application potential in fields such as materials science and biomedicine. As classic types of dynamic covalent bonds, the hydrazone bonds, imine bonds, borate ester bonds, and disulfide bonds are widely studied. Zhou et al. [55] utilized the reaction between the aldehyde group of aldehyde hyaluronic acid (AHA) and the imide group of 3,3′-dithiobis (propionyl hydrazide) (DTP) to form dynamic covalent imide bonds, which constitute the multiple types of reversible interactions of the hydrogel network. When the hydrogel is damaged by external forces, the hydrazone bonds break due to mechanical stress, leading to local network disintegration. After removing the external force, the aldehyde groups of AHA and the hydrazone groups of DTP recontact through molecular diffusion, rapidly reforming the hydrazone bonds at room temperature to achieve network healing.

Some dynamic covalent bonds, such as hydrazone bonds, are unstable and can reversibly form and break under acid catalysis. Therefore, in certain environments, such as pH changes or exposure to moisture, unintended bond breakage may occur, potentially affecting the material’s long-term stability and the reliability of self-healing. However, some dynamic covalent bonds are relatively stable, such as disulfide bonds, whose breaking and reformation typically require specific conditions such as redox reactions, thermal activation, or UV light irradiation. Therefore, they exhibit relatively high stability and are unlikely to break unexpectedly in non-catalyzed environments, effectively addressing the instability issue of hydrazone bonds.

For example, Jeon et al. [56] prepared a polysulfide bond hydrogel using 2,3-dithiol-1-propanol, racemic 2,3-dithiolbutanedioic acid, and hydrobromic acid as raw materials. When subjected to cutting damage, the hydrogel can achieve complete fusion at the cutting interface in approximately 5 s in air and approximately 6 s underwater, approaching the initial mechanical properties. Based on this theory, researchers have expanded the application areas of this self-healing hydrogel. For example, Guo et al. [57] used thioglycolic acid containing disulfide bonds as a green cross-linking agent to design and construct an in situ self-healing cross-linked polymer gel system with a dynamic covalent adaptive net-work and dynamic disulfide bonds, endowing the hydrogel with excellent toughness and self-healing performance, with a healing efficiency of up to 88% after cutting damage, making it suitable for high-temperature, high-salinity reservoir environments.

Compared with dynamic non-covalent bond networks based on weak interactions, dynamic covalent network structures connect polymer chains via reversible covalent bonds, showing superior mechanical strength and chemical stability to the material. Additionally, since dynamic covalent bonds and dynamic non-covalent bonds exhibit complementary advantages during the healing process, recent research has focused on combining them together to ensure both mechanical strength and rapid response after damage, significantly improving the material’s healing efficiency.

Lu et al. [58] utilized the synergistic effect of dynamic acylhydrazone bonds and host–guest interactions to achieve rapid healing of damaged hydrogels while also conferring hydrolysis resistance and functional stability for over 30 days. Li et al. [59] utilized the synergistic effect of dynamic disulfide bonds and multiple supramolecular interactions to accelerate the healing process. Disulfide bonds participate in the reconstruction of the network structure via reconnecting of broken disulfide bonds, ensuring the stability of the hydrogel, while supramolecular interactions assisted in driving molecular chains closer and reconnecting in the damaged region, thereby realizing the self-healing process of damaged hydrogels.

In recent years, superhydrophilic surfaces have gradually become a hot topic of research. However, the service life of surfaces is greatly reduced due to issues such as damage caused by external impacts, severely limiting their application. The self-healing superhydrophilic surfaces can effectively address this issue. With their unique wettability and durability, they have broad application prospects in areas such as self-cleaning coatings, biomaterials, oil–water separation, microfluidic devices, textiles, and anti-corrosion coatings.

4. Self-Healing in Air and Water

The environment plays a key role in the self-healing property, even determining whether it will happen or not. Most of the self-healing superhydrophilic surfaces demonstrate remarkable healing efficiency of damage in ambient environments. However, their self-healing performances are found to degrade significantly when placed in water, which leads to inferior interfacial stability and lowering the cyclic numbers of self-healing. Hence, the development of superhydrophilic surfaces with stable self-healing efficiency in water has attracted extensive research interest.

4.1. Self-Healing in Air

The superhydrophilic surface is usually healed in the ambient environments, where the presence of water molecules does not interfere with the reconstruction of the dynamic bonding network, thereby allowing the autonomous self-healing process of damaged surfaces. For example, Fu et al. [60] designed a nano-composite hydrogel based on a dual-network structure. At room temperature, simply keeping the two parts of the slice in contact allows for self-healing within 30 min under the dynamic action of borate bonds. To further reduce the healing time, Wang et al. [61] prepared a self-healing hydrogel based on 3D printing technology. Through the bonding and interaction of dynamic boron–diol bonds and hydrogen bonding interactions, the cut sample can be rapidly repaired within 30 s at room temperature without any external stimulation. Sun et al. [62] utilized the imine bonds between 4-formylphenylboronic acid (FPBA) and lysine (Lys) to construct a dynamic borate–imine–imine–borate bond structure between the PVA chains, which endowed the hydrogel with excellent tensile properties and ultra-fast self-healing capabilities without external stimulation.

4.2. Self-Healing in Water

The self-healing progress of conventional superhydrophilic surfaces faces three key challenges in water environments: (i) the mobility of water molecules limiting the transport of healing motifs; (ii) the imbalance of dynamic hydrogen bonding networks due to competitive hydration effects; and (iii) the presence of metal ions in water, which interferes with the chelation of metal–ligand bonds. With the growing demand for self-healing superhydrophilic surfaces in biomedicine, underwater protection devices, and other fields, researchers are conducting more in-depth studies on underwater self-healing strategies [63,64,65,66].

For example, researchers have found that energy can be delivered to the vicinity of the crack using NIR irradiation, which could melt the surface coating and triggers the movement of the surrounding polymer chains toward the crack [67]. The reconnection of the fracture site can also be achieved by exploiting the attraction of charges of the equal and opposite sign between dipoles [68]. Instead of reversible bonding, strong dipole interactions can effectively avoid the interference of water molecules, such as saturation of hydrogen bonding, coordination of metal cations, and solvation of ions. It could maintain stable performance, even in the flowing water environment, for a long time. Jiang et al. [69] established a dual network structure relying on hydrogen bonding for crosslinking, which maintains excellent self-healing performance in both air and underwater environments. Jia et al. [70], inspired by the hydrophilic shell and hydrophobic core structure in nature, realized rapid self-healing in various environments such as air, water, seawater, and sweat by constructing a hydrophilic–hydrophobic synergistic network.

Table 2. Comparison of healing performance and stability of underwater self-healing superhydrophilic surfaces.

Main Ingredients	Types of Damage	Self-Healing Properties	Durability and Self-Healing Properties	Stability	References
SiO2@PDA@Ag particles	Scratch	NIR (2.00 W) irradiation for 15 s can almost completely heal	Can exist in strong acids, strong alkalis, or salt solutions of varying concentrations	The coating maintains stable hydrophilic and oil-repellent properties	[67]
Acrylonitrile, acrylamide, methyl acrylate	Cut	Completely re-bonded after 60 min of contact, no scratches	Self-healing in air, underwater, and in seawater	All maintain stable high healing performance	[68]
Agarose, PVA	Cut	Contact 10 s tensile stress recovery 94.7%	Ultrafast self-healing in air and underwater	Agarose networks improve the strength and stability of hydrogels	[69]
Methacrylamide	Cut	Place in water and gently squeeze for 8 s. Once healed, both ends can withstand strong tensile forces.	Self-healing in deionized water, air, seawater, sweat, alkaline, and acidic aqueous solutions	Excellent healing efficiency in air and underwater but significantly reduced healing efficiency in seawater and acid–alkaline solutions	[70]

compares the contents of several studies on underwater self-healing superhydrophilic surfaces in recent years.

5. Conclusions

This article systematically reviews the latest research progress and self-healing mechanisms of superhydrophilic surfaces from the perspectives of composition self-healing and structure self-healing in recent years. For composition self-healing, superhydrophilic surfaces mainly achieve the restoration of wettability through strategies such as composition regeneration, re-biomineralization, re-enrichment of hydrophilic composition on surfaces, and self-polishing. However, the composition self-healing cannot effectively restore the surface wettability once the structure of the superhydrophilic surface is damaged, and the structure self-healing is highly desired. Structure self-healing of superhydrophilic surfaces is mainly divided into extrinsic and intrinsic types. Extrinsic structure self-healing mainly utilizes microcapsule technology to release the healing agent and restore the surface wettability, while intrinsic structure self-healing mainly utilizes dynamic non-covalent bonds, supramolecular interactions, and dynamic covalent bonds.

Although research in the field of self-healing superhydrophilic surfaces has made some progress, there are still many challenges ahead, with the following three areas being particularly prominent.

First, during the healing process, healing in air is mainly used, and how to heal in water remains a challenge, especially in complex underwater environments such as strong acids, strong alkalis, salts, and low temperatures. This is mainly because the healing conditions are crucial to the composition healing process. Once the healing conditions change, it is likely to lead to the failure of composition self-healing. For structure self-healing, healing conditions are equally crucial. This is mainly because the conventional dynamic healing process mainly relies on the reversible dynamic bonds, which are easily affected by pH, ions, water, etc., resulting in low healing efficiency or even failure. The environmental tolerance of healing can be enhanced by designing strong reversible dynamic bonds with anti-interference properties, such as dipole interactions, π–π interactions, hydrophobic interactions, etc., to reduce the influence of water molecules and various ions on the healing process. Combining multiple interactions such as hydrogen bonds and disulfide bonds to form dynamic crosslinking networks, the self-healing efficiency and stability of the surface in complex environments can be improved.

Second, the healing process is also prone to be limited by spatial conditions, and it is difficult to carry out the healing process in restricted spaces. By using various stimulus-responsive healing systems, such as light, heat, and electric fields, the spatial operability of the healing process can be improved. In addition, designing superhydrophilic surfaces with high strength and self-healing properties is also necessary, which can further enhance the service life of the surface. It will broaden the application of superhydrophilic surfaces in fields such as marine engineering and underwater equipment [71] by exploring multi-stimulus-responsive collaborative healing strategies, realizing the healing in restricted spaces, and achieving stable self-healing under extreme conditions.

Finally, composition self-healing can restore the surface composition, while structure self-healing can heal internal damage within the surface. By combining them together, full-scale surface healing can be achieved, restoring the surface wettability and mechanical properties and extending its service life.