Advanced Micro/Nanocapsules for Self-Healing Coatings

Abstract

The concept of intelligence has many applications, such as in coatings and cyber security. Smart coatings have the ability to sense and/or respond to external stimuli and generally interact with their environment. Self-healing coatings represent a significant advance in improving material durability and performance using microcapsules and nanocontainers loaded with self-healing agents, catalysts, corrosion inhibitors, and water-repellents. These smart coatings can repair damage on their own and restore mechanical properties without external intervention and are inspired by biological systems. Properties that are affected by either momentary or continuous external stimuli in smart coatings include corrosion, fouling, fungal, self-healing, piezoelectric, and microbiological properties. These coating properties can be obtained via combinations of either organic or inorganic polymer phases, additives, and pigments. In this article, a review of the advancements in micro/nanocapsules for self-healing coatings is reported from the aspect of extrinsic self-healing ability. The concept of extrinsic self-healing coatings is based on the use of capsules or multichannel vascular systems loaded with healing agents/inhibitors. The result is that self-healing coatings exhibit improved properties compared to traditional coatings. Self-healing anticorrosive coating not only enhances passive barrier function but also realizes active defense. As a result, there is a significant improvement in the service life and overall performance of the coating. Future research should be devoted to refining self-healing mechanisms and developing cost-effective solutions for a wide range of industrial applications.

1. Introduction

Self-healing coatings are considered an alternative approach for the efficient protection of materials [1]. These coatings usually embed microcapsules and nanocontainers that include self-healing agents, catalysts, corrosion inhibitors, monomers, and water-repellent liquids [2]. Automatic healing is defined as the self-healing function, according to which a structural material self-repairs its damage and regains its mechanical properties without any external intervention. The self-healing function requires the ability to sense damage, together with the healing ability. There are four different self-healing methods, namely, self-healing using remendable polymers, self-healing using additives and fillers such as microcapsules and nanocontainers, self-healing using solvents, and self-healing using thermoplastic polymers [3,4]. Several criteria can be taken into consideration for the classification of self-healing coatings, such as the healing mechanism, the materials used, and the type of damage they address. The main classifications are based on (a) the healing mechanism, which can be either intrinsic self-healing coatings or extrinsic self-healing coatings; (b) the type of damage, which can be addressed by either scratch-resistant coatings, corrosion-resistant coatings, or barrier coatings; (c) the materials used, which can be either inorganic coatings, polymeric coatings, or composite coatings; (d) the stimuli-responsive healing triggers, which can be either pH-sensitive, thermally activated, moisture-sensitive, or light-induced; and (e) the functional specificity and applications, such as protective coatings or functional coatings [5]. The classification of self-healing coatings is illustrated in Figure 1.

Application of self-healing coatings addresses the demand for sustainability, service-life extension, and low-maintenance surfaces in a range of industrial applications. The principle of self-healing derives inspiration from biological systems, which are endowed with an intrinsic ability to heal themselves, in order to guarantee the long-term reliability and efficiency of the material in the absence of external influence [6]. These coatings are developed to react to damage by triggering a self-healing process that assists in restoring the integrity of the material [7]. The inclusion of microcapsules and nanocontainers in these coatings is essential because these microscopic containers hold active ingredients like catalysts, repair monomers, and corrosion inhibitors that are released when they are damaged. Because of the flexibility in the production and composition of these micro- and nanostructures, customized responses to particular mechanical and environmental problems are possible [8].

Microcapsules and nanocontainers have attracted scientific and industrial interest. Their composition can be inorganic oxides and hydroxides, inorganic and organic polymers, natural polymers, and hybrid composites. Moreover, their shape can be stable, such as spherical, octahedral, and cubic, or vary according to the conditions applied and the mechanism of dynamic self-assembly. Some systems discussed in the literature are liposomes, micelles, polymersomes, solid nanocapsules, dendrimers, nanoparticles, nanotubes, and nanogels. Microcapsules and nanocontainers can have either a core–shell structure or they can be nanofunnels, nanospheres, nanobottles, nanotubes, or nanofibers. Microcapsules and nanocontainers can be fabricated via several different synthetic methodologies. Some of them include the self-assembly methods, precipitation-based synthetic methods, the layer-by-layer technique, ultrasonic techniques, the sol–gel technique, and emulsion-based processes [9,10,11].

This review article aims to demonstrate the advancements related to self-healing coatings with autonomous ability. Several studies and works are presented with respect to the fabrication of microcapsules and nanocontainers and their subsequent incorporation into organic coatings to improve corrosion protection and enhance the self-healing ability.

2. Self-Healing Coatings Based on Additives Including Corrosion Inhibitors

Corrosion inhibitors are chemical compounds that slow down or prevent the material degradation of metals [12]. In coatings demonstrating self-healing properties, these inhibitors can be incorporated into the coating. When damage occurs to the coating due to its interaction with the environment, the inhibitors protect the metal through three different procedures. The first one is cathodic or anodic protection, where the metal’s electrochemical potential is shifted, enabling it to be less prone to oxidation. Another procedure is the formation of a protective layer, where a passive layer is created onto the metal surface by the inhibitors, preventing further corrosion. The third process is the neutralization of the aggressive agents, such as sulfur or chloride compounds.

Several chemical compounds can be used as corrosion inhibitors in self-healing coatings (Figure 2). One category is organic inhibitors, which can be adsorbed onto the metal surface, blocking active corrosion sites [13]. The effectiveness of an organic inhibitor depends on the size of the organic molecule, the number and type of groups or bonding atoms in the molecule, aromaticity, the nature and surface charge, and the type of aggressive environment. Another type is inorganic inhibitors, which precipitate as insoluble salts or can create protective layers on the metal substrates [14]. Green inhibitors are specified as both organic and inorganic, demonstrating eco-friendly properties and reducing adverse effects on the environment [15]. Another category is smart inhibitors, which are activated only when corrosion occurs [16]. Smart inhibitors can be incorporated in microcapsules or embedded in a coating matrix that responds to environmental titillating, such as thermal or pH changes. Self-healing coatings, including corrosion inhibitors, are used in a variety of industries, such as marine, aerospace, automotive, and construction. The benefits include reduced maintenance, extended lifespan, and improved safety.

Yeganeh et al. performed experiments on the synthesis of mesoporous silica nanocontainers and functionalized and loaded them with the corrosion inhibitor sodium molybdate [17]. The mesoporous silica nanocontainers were fabricated by mixing cetyltrimethylammonium bromide as a surfactant and tetraethylorthosilicate as a silica precursor. Then, the produced mesoporous silica nanocontainers were functionalized with 1-(2-aminoethyl)-3aminopropyltrimethoxysilane, and a positive charge was obtained using anhydrous FeCl₃ in order to accomplish enhanced adsorption of molybdate (MoO₄²⁻) as a corrosion inhibitor on the functionalized nanocontainers. Epoxy composite coatings incorporating loaded nanocontainers were applied onto mild steel substrates by a film applicator. Evaluation of the produced loaded mesoporous silica nanocontainers displayed the pH dependence of molybdate release from the epoxy coating. The increase in the pH from 1 to 14 resulted in a 160-fold increase in the molybdate concentration in the solution. Electrochemical impedance spectroscopy and electrochemical noise analysis were used for the assessment of the corrosion behavior of the coatings. The results revealed enhanced corrosion resistance of the coatings, including the loaded nanocontainers, which confirmed the proper corrosion-inhibition effect of the released molybdate ions.

In another study, Haddasi et al. evaluated the concentration of 2-mercaptobenzimidazole (MBI) in graphene-based carbon hollow spheres with respect to their active/barrier corrosion protective properties after their incorporation into epoxy coatings [18]. Several epoxy coatings, including either those loaded with inhibitor or empty hollow spheres, were applied on mild steel plates via a wet film roller applicator. Studies based on the release kinetics of MBI from the CHSs proved that deposition of the released MBI molecules onto the metal substrate could be achieved. The mechanical and electrochemical evaluation of the produced coatings after their exposure to corrosive environments revealed enhanced active/self-healing/barrier properties as well as wet adhesion strength of the coating onto the metal substrate.

Montemor and her team conducted experiments on the self-healing ability of epoxy coatings after incorporation of microcapsules loaded with highly reactive isocyanates [19] and polyethyleneimine [20]. Calcium carbonate microagglomerates, with an average diameter of 2.5 mm and consisting of several round-shaped nanoparticles, were synthesized and loaded with the corrosion inhibitor polyethyleneimine [20]. The pH-sensitive loaded microparticles were utilized as additives in epoxy coatings, which were applied to carbon steel via the dip-coating process. The obtained coatings were characterized with respect to their morphological, structural, and corrosion protective properties, and the results revealed that the modified coatings imparted enhanced corrosion protection as well as very stable barrier properties as the time elapsed. Moreover, the abovementioned coatings demonstrated improved self-healing properties. The corrosion inhibition effect was the result of the dissolution of calcium carbonate microparticles due to the pH variation, resulting in the subsequent release of the corrosion inhibitor. The outcome was the formation of a protective film on the steel’s surface.

Steel plate corrosion resistance after the application of epoxy-based coatings modified with microcapsules loaded with isophorone diisocyanate and with pH-sensitive cerium tri(bis(2-ethylhexyl)phosphate particles serving as inhibitors was also investigated by Montemor et al. [19]. The healing effects were assessed by a combination of several electrochemical techniques. The acquired results after the combination of both additives showed a multilevel protection effect that was characterized by more stable and enhanced coating barrier properties without any interfacial corrosion activity. The healing mechanism in the coating included hydroxyl ion accumulation, which significantly reinforced the corrosion protection conferred by the epoxy coating, providing stable corrosion protection over time.

Habib and his team reported the self-healing properties of epoxy-based polymeric nanocomposite coatings that were applied to steel substrates [21]. Both talc nanoparticles (TNPs) modified with sodium nitrate as well as microcapsules of urea–formaldehyde, including linseed oil, were embedded into these coatings. The efficient loading of NaNO₃ into TNPs, which can be attributed to the existence of a physiochemical adsorption mechanism, proceeded without affecting the TNPs’ parent lamellae structure. The characterizations showed that the release of the NaNO₃ corrosion inhibitor from TNPs was influenced by the variations in the solution pH as well as the immersion time. Additionally, the release of the self-healing agent, i.e., the linseed oil microcapsules, in response to the external damage was estimated to be a time-dependent procedure. Promising corrosion inhibition efficiency and self-healing properties were achieved and were ascribed to the combination of inhibitors, nanocontainers, and self-healing agents. In a similar work by the same team, epoxy coatings were reinforced by the incorporation of cerium dioxide nanoparticles loaded with dodecylamine and n-methylthiourea as organic corrosion inhibitors [22]. The potential self-healing properties of the produced coatings as well as the increase in their lifetime were estimated. The CeO₂ nanoparticles displayed a pH-sensitive and self-releasing behavior of the corrosion inhibitors. The release of the inhibitors resulted in the formation of a protective layer on the defect sites of the steel surface and the corresponding healing effect. The fabricated protective film prevented the aggressiveness of Cl⁻ in the solution, resulting in the existence of exceptional corrosion inhibition efficiency for the modified coating with the aforementioned loaded microcapsules.

In the study of Wen and his team, an intelligent coating for mild steel corrosion protection was developed [23]. Free radical polymerization was used for the synthesis of a hybrid hydrogel based on benzotriazole, polyethyleneimine, hydroxypropyl acrylate, and vinyltriethoxysilane. The prepared hydrogel was used in the fabrication of an intelligent coating that was applied to mild steel. Electrochemical characterizations were conducted for the corrosion protection assessment of the intelligent coating. The obtained results showed that the increase in the hydrogel in the coatings resulted in an improvement of the anti-corrosion capability of the coatings. The corresponding mechanism relied on the formation of an inhibitor-adsorptive layer onto the mild steel due to the release of benzotriazole.

In order to promote the application of dissimilar material blisks for advanced aeroengines built from Ti17 (α+β) and Ti17(β) alloys developed via linear friction welding (LFW), a thorough investigation was conducted on the failure mechanisms and performance enhancements of these joints. Using methods including in situ tensile testing with scanning electron microscopy and quasi in situ electron back-scattering diffraction combined with transmission electron microscopy for high-cycle fatigue analysis, the study concentrated on microstructural changes, including phase transformations and dynamic recrystallization. Microstructural changes during welding were determined via α dissolution and continuous dynamic recrystallization (CDRX) of the β-phase. According to the research findings, microcracks at α/metastable β borders on the Ti17 (α+β) side were linked to high-cycle fatigue failure, whereas tensile failure was associated with dislocation slip concentration in the thermomechanically impacted zone [24].

The integration of micro- and nanocontainers into self-healing coatings significantly enhances their functional applications in material science. Microscale containers, typically ranging from 50 to 200 μm, are designed to easily rupture under mechanical stress, thereby releasing healing agents stored within them, which is crucial for the efficient repair of damaged areas. This intelligent release mechanism is not solely dependent on mechanical stress; it can also be finely tuned to respond to various environmental stimuli, such as changes in pH or temperature, ensuring active agents are discharged only when necessary. The responsiveness of these containers to external stimuli facilitates the smart release of agents, improving the self-healing capabilities and corrosion protection of advanced coatings. Nanocontainers, due to their smaller size, are particularly effective for carrying corrosion inhibitors, allowing a more efficient delivery of active agents and enhancing the protective qualities of the coatings. The release of active agents from these containers through capillary action when cracks form aids in restoring the coating’s barrier properties. This combination of intelligent design and responsive behavior underscores the transformative potential of micro/nanocontainers in the development of advanced self-healing materials, marking a significant advancement in modern material science and technology [25].

The development, characterization, and validation of self-healing coatings, including corrosion inhibitors, is an active area of research that pays attention to enhancing the economic viability, environmental friendliness, and effectiveness of these advanced materials.

3. Self-Healing Coatings Based on Additives Including Healing Agents

One effective approach to obtaining coatings with self-healing properties is the use of additives, including healing agents. Healing agents refer to substances, compounds, or factors that initiate and encourage the healing process in the damaged coatings.

In the study of Neisiany and his team, a self-healing carbon/epoxy composite including healing agent loaded into core–shell nanofibers between carbon fiber fabric layers was fabricated [26]. A coaxial electrospinning method was used for healing agents based on low-viscosity epoxy resin together with its amine-precursor curing agent to be encapsulated into nanofibers, which consisted of copolymers of acrylonitrile and styrene. The encapsulation yield reached up to 97%. Mechanical studies confirmed that the core–shell nanofiber incorporation into the epoxy coating did not reduce the overall composite mechanical properties. On the other hand, the embodiment of the aforementioned nanofibers between carbon layers imparted a self-healing ability, resulting in the self-repairing and restoring of the composite mechanical properties.

Additionally, H. Li and coworkers examined [27] the efficient and facile encapsulation of a lubricant oil that acts as a core material into microcapsules of SiO₂ shell wrapped with polystyrene. The fabrication of the loaded microcapsules included Triton X-100 together with 3-isocyana-topropyl triethoxysilane and fumed SiO₂ nanoparticles to be used as emulsifiers for the formation of oil/water Pickering emulsions via the Pickering polymerization technique. The produced microcapsules were spheres with an average diameter of 3.3 μm and a shell thickness of roughly 900 nm. Moreover, the lubricant oil demonstrated improved thermal stability with a decomposition temperature of 250 °C. The obtained core–shell microcapsules were embedded into an epoxy coating, and the final self-lubricating coating was applied onto aluminum substrates. The produced coatings composed of SiO₂-wrapped PS microcapsules/epoxy composites demonstrated improved self-lubricating properties, which were ascribed to the synergetic effect between lubricant oil and SiO₂ nanoparticles, making a positive contribution to enhancing the tribological properties of the polymer coatings.

A very interesting concept was developed by W. Li and his team [28]. Solid wastes, such as fluorinated silica gel waste and polystyrene (PS) foam waste, were recycled and reused for the fabrication of a hydrophobic coating with self-healing properties. First, silica gel waste and PS foam waste were used as raw materials for the superhydrophobic coating fabrication. Second, near-infrared-responsive microcapsules were synthesized using fluorinated carbon nanoparticles via the method of electrostatic adsorption onto the surface of microcapsules. Then, the obtained particles were used as fillers with hydrophobic properties together with the polystyrene foam waste that acted as a coating binder. The produced superhydrophobic coating was applied onto several different substrates via a doctor blade and presented robust durability and enhanced self-healing properties.

Further studies have evaluated the mechanism of the microcapsules that are applied to the interlaminar interface during the synthesis of carbon fiber composites [29]. Microcapsules with enhanced thermal and mechanical properties composed of a high-boiling-point organic solvent and a diglycidyl ether bisphenol A-based epoxy resin were fabricated via in situ polymerization of formaldehyde and urea and applied to carbon fiber composite structural materials in order to improve their self-healing performance. The results showed enhanced self-healing properties when the aforementioned microcapsules were used together with Lewis acidic catalysts due to the encapsulated epoxy monomer polymerization.

In a similar study, also by Montemor and coworkers, emulsion polymerization followed by interfacial polymerization was used for the fabrication of chemically and thermally stable isophorone diisocyanate microcapsules [30]. The obtained microcapsules were embedded into an epoxy coating for carbon steel corrosion protection. Electrochemical characterization results showed an increase over time of the barrier properties of the modified coatings. Moreover, the produced coatings demonstrated self-healing properties that were attributed to the ability of the capsules to heal damaged areas in the coating and the formation of a protective polymeric barrier layer. The healing mechanism encompassed several reactions involving amine group formation due to partial conversion of isocyanate groups in the presence of water, the formation of polyurea due to an amine reaction with isocyanates, and finally the formation of a polyurethane layer.

In another study, Chen et al. reported the preparation of a smart coating with autonomous self-responding and self-healing functions without external intervention based on a simple integration microcapsule composed of one component into the matrix [31]. For this purpose, microcapsules were fabricated, including a hexamethylene diisocyanate solution, under photopolymerization. The formatted coatings were evaluated with respect to their morphology and thermal, corrosion, and mechanical resistance. The microcapsule-embedded coatings demonstrated improved self-repairing ability.

Additionally, in the work of Kouhi et al., healing agent-filled capsules with mean diameters as small as 450 nm and as large as 6 μm were fabricated by in situ polymerization [32]. The produced core–shell micro/nanocapsules consisted of urea–formaldehyde as the shell and drying linseed oil as the core. Then, the micro/nanocapsules were embedded in a polymer in a content range of 91–94%. The obtained coatings were assessed with respect to their mechanical and corrosion behavior properties and showed improved performance in self-healing applications. The outcome of the experiments demonstrated a release of the healing materials from the micro/nanocapsules in paint films when cracks were formed, resulting in efficient crack healing with satisfactory protective properties.

In situ polymerization of aniline onto CeO₂ nanoparticles was used for the fabrication of polyaniline/cerium dioxide nanocomposites by Lei and coworkers [33]. The prepared nanocomposites were embedded into epoxy coatings and applied to carbon steel substrates, revealing excellent corrosion resistance. The enhancement of corrosion protection performance of the modified epoxy coatings was correlated to the synergetic protection and enhancement of the protective barrier and the hindrance of the diffusion of aggressive ions of the nanoparticles. Moreover, the self-healing protection improvement was associated with the redox behavior of polyaniline. Thus, the hybrid polyaniline/CeO₂ nanoparticles were considered the best route to enhance the protection performance of epoxy coatings on carbon steel.

A very interesting concept was developed by Ma et al. in order for microcapsules with high encapsulating capacity to be obtained [34]. These microcapsules had enhanced self-healing efficiency. For this purpose, graphene oxide (GO)-modified double-walled polyurea microcapsules were fabricated using 1,6-diaminohexane as the inner core and a prepolymer based on isophorone diisocyanate as the outer core. The synthetic process included as a first step the preparation of GO-modified prepolymer emulsion; then, the emulsion of 1,6-diaminohexane was prepared; after that, initial single-walled microcapsules were fabricated through interfacial polymerization between the two aforementioned emulsions; and finally, double-walled microcapsules and coating of the second core material were developed. The outcome of this study revealed that microcapsules that had been modified with GO demonstrated a spherical shape with a mean diameter of 0.5 μm. The obtained microcapsules had excellent thermostability as well as good mechanical properties with respect to Young’s modulus, hardness, and load. Finally, the corresponding epoxy coating, including the above microcapsules, presented improved anticorrosive self-healing efficiency. The reason for the enhanced anticorrosive and barrier properties was attributed to the GO nanostructure, which hindered the penetration of aggressive species through scratches and defects.

Palazzo et al. considered the effect of supercritical emulsion extraction technology in order to fabricate microcapsules based on polymethylmethacrylate/diglycidyl ether of bisphenol A [35]. These microcapsules were able to act as a health-monitoring element. They had a spherical shape together with a smooth surface and a mean size of 220 nm and enhanced encapsulation efficiency. The health-monitoring liquid epoxy formulation, used in microcapsule preparation, was accomplished by homogeneously mixing a reactive diluent with a bifunctional epoxy resin. Then, a dye was added to this mixture under stirring. The results revealed that the produced capsules could be broken under stress, resulting in the release of the dye from the microcapsules.

Self-healing coatings, particularly those incorporating various healing agents, represent a significant advancement in the realm of material science, promising to mitigate the detrimental effects of environmental factors and enhance long-term durability. These coatings are designed to address the pervasive issue of metal corrosion, which has far-reaching economic and environmental implications. By integrating smart self-healing mechanisms, the coatings can autonomously repair damage, thereby extending the lifespan of the protected materials and reducing the frequency of maintenance interventions [25,36]. Notably, the efficiency of these coatings is heavily influenced by the type and functionality of the healing agents used. For instance, microencapsulated healing agents in organic coatings have shown considerable promise in autonomously restoring the integrity of the material upon damage [37]. This self-repair capability not only enhances the durability of the coatings but also contributes to the sustainability of the structures they protect by minimizing resource consumption and waste generation [38]. However, despite these advantages, the application of self-healing coatings is not without limitations. Factors such as the compatibility of healing agents with the coating matrix and the potential long-term stability of the healing system under various environmental conditions remain areas of ongoing research and development [39]. Therefore, while the current advancements in self-healing coatings are promising, further studies are essential to optimize their performance and ensure their effectiveness in diverse real-world applications.

Self-healing coatings based on healing agents are considered a promising route to acquire more reliable and durable surface protection. The technology is still under development, with ongoing research paying attention to improving the effectiveness, efficiency, and economic feasibility of the aforementioned systems.

4. Classification of Self-Healing Coatings Based on the Polymeric Matrix

The self-healing coatings are classified into three different categories, namely, thermoset polymers, thermoplastic polymers, and elastomers, depending on the polymeric matrix. Figure 3 illustrates the types of coatings.

4.1. Thermoset Polymers

Thermoset polymers are characterized by their ability to permanently harden after curing. In general, this type of polymer cannot be remelted. The healing mechanism is based on rearrangements or reversible chemical reactions, such as disulfide bonds and Diels–Alder reactions, hydrogen-bonded connections, and ionic polymers, where the matrix network can reconstruct bonds upon heating.

4.1.1. Epoxy-Based Self-Healing Coatings

Epoxy stands out as a highly effective and dependable polymer matrix and has been employed in self-healing polymer coatings for several years. Its exceptional thermal, mechanical, and adhesion properties set it apart from other polymer matrices. In the extrinsic self-healing mechanism, healing agents or inhibitors are typically encapsulated within micro/nanocarriers alongside the self-healing polymer matrix. When employing this micro/nanoencapsulation technique, it is crucial to consider that the self-healing agents or inhibitors should possess low viscosity to facilitate seamless encapsulation and timely release as needed [40]. Zhang et al. [41] formulated a multifunctional anticorrosion coating system using waterborne epoxy and nanofillers, specifically vanadium oxide modified by the polyaniline polymer PANI and tannic acid (V₂O₅@pani@TA-Fe). In this system, the polyaniline-modified vanadium oxide particles play a dual role, providing both corrosion passivation and inhibition effects. Additionally, the tannic acid contributes to the inhibition activity by actively releasing inhibitors in response to pH changes. The combined dual-action capability of the system significantly enhances its self-healing anti-corrosion properties. The self-healing performance was assessed using electrochemical impedance spectroscopy (EIS) and salt spray tests (SST). The results demonstrated that the inhibition value of the water-based epoxy reinforced with V₂O₅ modified by polyaniline and tannic acid complex (WEP/VPC) coating remained higher than 10⁸ X/cm² even after 80 days of immersion, surpassing the performance of pure water-based epoxy (WEP) coatings [42]. Water-based epoxy coatings were fabricated by the team of A. Sardari and demonstrated anti-corrosion and self-healing properties [43]. These coatings had several additives, such as organic microcapsules, including 8-hydroxyquinoline as a corrosion inhibitor. Robust superhydrophobic coatings combing chemically reactive epoxy groups and low surface-energy materials were produced by Dan Zhang et al. [44]. In the work of Soares and his group, epoxy coatings, including montmorillonite modified with ionic liquids and zinc-based salts acting as anti-corrosion additives, were synthesized and applied onto steel substrates [45]. In the study of Song et al. [46], a wear-resistant epoxy coating was fabricated that demonstrated self-lubricating properties and displayed self-healing action under photothermal effect.

However, more work is needed, including the introduction of novel healing techniques, engineering design optimization, cost reduction, and a faster transition from laboratory work to useable real-world applications in a variety of fields and industries. Additionally, the creation of long-lasting self-healing polymeric coatings is essential for prolonged environmental exposure.

4.1.2. Polyurethane-Based Self-Healing Coatings

Polyurethane stands out as a highly sought-after polymer type in the contemporary era due to its versatile characteristics that are suitable for a wide range of applications, including anti-corrosion coatings. Traditionally, polyurethane has been utilized to enhance the aesthetics, scratch resistance, lifespan, and corrosion resistance of various metallic objects. In the realm of polymeric coating systems, polyurethane (PU) takes a prominent position thanks to its numerous properties, including being eco-friendly, non-volatile, cost-effective, and sustainable. The utilization of polyurethane (PU) as a polymer matrix for anti-corrosion coatings has advanced through the development of various self-healing systems incorporating PU. In a study by Jinson Wang et al. [47], a smart-lignin-based green coating was formulated with notable anti-corrosion performance and the capability to swiftly release the corrosion inhibitor (nicotinic acid, a green inhibitor) upon mechanical damage. The assessment of anti-corrosion performance, conducted through electrochemical impedance spectroscopy (EIS), revealed an increase in the corrosion potential value from 481 (uncoated sample) to +187 mV. The released corrosion inhibitor facilitated the formation of a passive layer on the metal substrate’s surface in response to mechanical damage, underscoring the self-healing ability of the lignin polyurethane (LPU) coatings.

4.1.3. Polyimide-Based Self-Healing Coatings

Polyimides are a category of high-performance polymers that demonstrate thermal chemical resistance, mechanical strength, and stability. Nowadays, there is a remarkable interest in synthesizing polyimide-based self-healing coatings because of the demand for long-lasting and resistant materials that can be used in various applications, such as protective coatings, electronics, and aerospace.

Yong Chae Jung et al. developed a new type of colorless polyimides with a self-healing function and resolved the trade-off between functionality and durability. These coatings have the ability of self-healing, rapidly restoring any damage caused by external stress [48]. The team of Yanlian Xu et al. fabricated two-dimensional lamellar polymer composite coatings based on polyimide/cardanol together with benzoxazine copper. The obtained coatings exhibited excellent anti-corrosion properties and enhanced mechanical performances [49]. Kuang-Lieh Lu et al. synthesized durable high-performance materials using polyimides, revealing enhanced corrosion resistance properties, an anticorrosion effect against high temperatures, and long-term robustness [50]. Joseph Lichtenhan and his team investigated combinations of polyimide-based nanomaterials together with polyhedral oligomeric silsequioxane in order to produce coatings that exhibit improved stability in mechanical properties. The self-healing mechanism was assigned to the creation of a silica thin layer [51].

4.2. Thermoplastic Polymers

This type of polymer exhibits healing through thermal processes. Thermoplastic polymers are characterized by their ability to be reheated and reprocessed. The healing mechanism is based on the ability of the polymer matrix to soften upon heating, allowing the defected areas to flow and merge, resulting in coating recovery.

4.2.1. Polyacrylate-Based Self-Healing Coatings

Polyacrylate (PAL) and its derivative-based materials (polymers) have gained widespread recognition in the coating industries due to numerous attributes, including low cost, high transparency, excellent adhesion, processability, and durability, particularly under ambient conditions. One particularly intriguing aspect of PAL (polyacrylate) materials is the ability to tailor their physical properties through the judicious selection of monomers for the formation of the polymer network. Numerous studies have been conducted to enhance the recurrent self-healing characteristics, typically relying on reversible bonds like covalent bonds, hydrogen bonds, coordinate covalent bonds, ionic interactions, and host–guest interactions [52,53]. The use of van der Waals interactions aims to eliminate both chemical and physical interactions, allowing for multiple recoveries under mechanical stress without the need for any external triggering agent. Wenpeng Zhao et al. [54] investigated the mechanical and self-healing characteristics of anisotropic composites by incorporating multiple reversible bonds, specifically strong coordination bonds and weak hydrogen bonds. Through FTIR and combined 2D FTIR analysis, it was determined that the rapid self-healing capability (within 30 s with a healing efficiency of approximately 95%) of the designed system was attributed to the weak hydrogen bonds. Meanwhile, the robust mechanical properties (12.6 MPa) were primarily a result of the strong coordinate interactions. Such systems can be utilized as a soft actuator that is further employed for the manufacturing of smart self-healing polymer materials for many applications, especially in the medical/biomedical sectors.

4.2.2. Polyvinyl-Based Self-Healing Coatings

Self-healing coatings based on polyvinyl pay attention to the fabrication of protective layers that enable automatic damage restoration. The utility of these coatings is to enhance the longevity and durability of various surfaces, ranging from plastics to metals, through the incorporation of mechanisms that permit the coating to heal itself after damage such as cracks or scratches. Polyvinyl compounds, like polyvinyl acetate, polyvinyl chloride, and polyvinyl alcohol, are polymers that can be used in several applications. The modification of these materials in self-healing coatings to include healing agents results in the demonstration of self-healing properties.

Kwan-Young Han et al. [55] synthesized and examined self-healing polyvinyl, including functional polymers that react to ultraviolet irradiation and thermal energy. The obtained material exhibited improved thermal stability, processability, chemical resistance, and mechanical strength because of the functional molecule introduction at the brush end joined to the polyvinyl chain. In the work of Chun Cheng et al. [56], high-performance self-healing coatings were prepared from a mixed solution of hydrolyzed poly(styrene-co-maleic anhydride) and polyvinyl alcohol. The coatings revealed enhanced antifogging performance in a humid, warm environment as well as a refrigerator room. An improved self-healing ability was detected that was ascribed to hydrogen bonding self-assembly. Hydrogel coatings based on borax and polyvinyl alcohol were produced and applied in rigid polyurethane foam by Xiaodong Qian et al. [57]. The fabricated coatings were investigated with respect to their self-healing, fire-retardant, and recycling properties. The reversible and dynamic cross-linked networks based on the hydrogen bonds and borate ester bonds imparted the hydrogels with enhanced recyclability, repairability, and elasticity. Self-healing organic coatings based on polyvinyl butyral formulations were synthesized by Mittal et al. [58]. Polypyrrole–carbon black additives were incorporated as an inhibiting pigment. The exposure of the composite coatings in aggressive environments imparted a self-healing effect that was assigned to the diffusion barrier nature and redox properties of polypyrrole. A stable passive layer was created on the metal surface because of the interaction of organic sulfonic acid dopant that was released from the polypyrrole with metal iron oxide. Mai M. Khalaf et al. [59] prepared, via the dip-coating process, protective systems with self-healing properties in order to defend carbon steel corrosion in an acidic medium environment containing chloride. Polyvinyl chloride was introduced as an organic healing agent into formulations that contained TiO₂, ZnO, and ZnO-TiO₂ nanocomposites.

4.3. Elastomers

Elastomers are characterized by highly elastic properties, indicating their ability to stretch and return to their original shape. The healing mechanism occurs through ionic interactions, physical entanglement, or reversible cross-links that can reconstruct after damage.

4.3.1. Silicone-Rubber-Based Self-Healing Coatings

Coatings that impart a self-healing effect based on silicone rubber are a very interesting area of material science. Because of the flexibility and durability of silicone rubber, these coatings are able to repair themselves after damage, such as cuts, scratches, or cracks, that enhances their performance and durability in various applications. Briefly, silicone rubber exhibits excellent elasticity and flexibility, even at elevated temperatures; provides good resistance to UV radiation, many chemicals, and weathering; and has thermal stability.

Elastic silicone rubbers based on polydimethylsiloxane-co-nickel(II)-pyridinedicarboxamide copolymers were developed by the team of Islamova et al. [60]. The self-healing capability was attributed to the reversible metal–ligand interactions, and it was estimated at room temperature. The Ni²⁺ ions acted as cross-links between the polydimethylsiloxane chains through the coordination of Ni–O and Ni–pyridine bonds. Yumin Wu et al. [61] prepared self-healing silicone rubber cross-linked by the multiple hydrogen bonds with ethylene carbonate and α,ω-aminopropyl poly(dimethylsiloxane) based on the nonisocyanate reaction. The several hydrogen bonds between the imino and carbonyl groups, together with the generated hydroxyl groups, finally produced a hydrogen bond cross-linked network imparting enhanced thermal-induced self-healing efficiency as well as good cyclic self-healing ability.

Zhang et al. [62] prepared a self-healing coating consisting of polydimethylsiloxane-based polyurea together with an organic antifouling agent. The coating demonstrated good adhesion to the substrate, which was assigned to the breaking and reorganization of the urea group hydrogen bonds. The coating exhibited enhanced self-healing capability in both artificial seawater and air at room temperature. The team of Lee et al. [63] synthesized a polysiloxane-based polyurethane elastomer via a triboelectric effect. The elastomer consisted of urea–oxime–carbamate units in order for the molecular chain to present improved fluidity to enhance reconstruction after damage. Reversible dissociation and reconnection occurred due to the imparting of high mobility to the molecular chains, revealing a self-healing effect. Rao et al. [64] developed a self-repairing polydimethylsiloxane elastomer based on the metal–ligand interaction. The chain of the elastomer had a structure based on 2,2′-bipyridine-5,5′-dicarboxamide. The metal–ligand interaction created by the addition of Zn²⁺ exhibited kinetic instability, enabling the elastomer to have self-healing ability at room temperature.

4.3.2. Poly1,3-Diene-Based Self-Healing Coatings

Polybutadiene and polyisoprene are both types of synthetic rubbers with enhanced flexibility and elasticity. Polyisoprene can exhibit self-healing properties that are assigned to reversible covalent bonds, such as the incorporation of Diels–Alder or disulfide bonds into the polymer structure, physical cross-links such as the presence of ionic interactions or hydrogen bonds, and molecular mobility at elevated temperatures, promoting a healing effect. The self-healing properties of polybutadiene are ascribed to microphase separation, creating domains that repair damage via the mobility of the rubbery phase, reversible networks of metal–ligand interactions or hydrogen bonds, and plasticization [65,66,67].

The integration of self-healing agents into coatings has emerged as a transformative approach to enhancing their durability, particularly in combating environmental degradation and extending their service life. Foundational studies have demonstrated the potential of these coatings to autonomously repair damage, thereby mitigating the adverse effects of environmental factors such as moisture, UV radiation, and pollutants [37]. Recent advancements have refined the design and efficiency of these self-healing systems by incorporating various healing agents, such as microcapsules, vascular networks, and intrinsic healing polymers [68]. These developments have not only improved the mechanical properties and longevity of the coatings but have also shown significant promise in reducing the environmental impact of maintenance and repair activities [69]. For instance, self-healing coatings have been particularly effective in addressing metal corrosion, which has substantial economic and environmental consequences [25]. By autonomously repairing microcracks and preventing the propagation of damage, these coatings reduce the frequency of manual interventions and associated resource consumption, thereby offering a sustainable solution for long-term protection. Moving forward, it is crucial to continue optimizing these systems to enhance their healing efficiency and broaden their applicability across different materials and environments.

5. Types of Self-Healing Capsules

The development of self-healing coatings has led to the development of multiple self-healing capsule types, each tailored to meet specific requirements and overcome obstacles in different applications. These capsules are designed to encapsulate healing agents and release them when damaged, thereby enabling the self-healing process. The common types of self-healing capsules are inorganic capsules, polymer microcapsules, hollow glass microcapsules, core–shell nanocapsules, vesicles/liposomes, and nanocontainers with controlled release (Figure 4) [70].

One of the main types of self-healing capsules is based on microencapsulation technology. Microencapsulation involves encapsulating the core drug in a protective shell distributed throughout the material. If the material is damaged, the capsules rupture and release the healing agent to repair the crack or break. This process not only prevents further damage but also extends the life cycle of the material. Microencapsulation technology has found widespread use in self-healing materials, from concrete to polymers, and offers a promising solution for improving the durability of various structures.

Chemical self-healing capsules play a crucial role in the development of advanced micro/nanocapsules for self-healing coatings [71]. These capsules are designed to encapsulate various active agents capable of repairing damage within coatings, offering a sustainable solution for enhancing material longevity and performance. The introduction of chemical self-healing capsules marks a significant advancement in the field of material science, particularly in the realm of protective coatings for various applications. By understanding the mechanism of action of these capsules, researchers and engineers can harness their potential for creating innovative self-healing materials. The mechanism of action of chemical self-healing capsules involves the controlled release of healing agents in response to damage or external stimuli. When a coating containing these capsules experiences a scratch, crack, or other forms of damage, the capsules rupture, releasing the encapsulated healing agents. These agents then react with the surrounding environment to initiate the healing process, effectively repairing the damage and restoring the coating’s integrity [39]. This self-healing mechanism offers a dynamic and autonomous approach to maintaining the functionality and aesthetics of coatings over time, reducing the need for frequent repairs or replacements. Chemical self-healing capsules include urea–formaldehyde (UF) capsules, which have been prepared using an in-situ encapsulation method. These capsules demonstrate a significant size reduction, enhancing their efficiency in delivering healing agents within coatings [72]. Additionally, organic shell nano/microcapsules such as poly(urea–formaldehyde), polystyrene, and polyurethane have been synthesized for self-healing applications, showcasing the versatility and adaptability of chemical self-healing capsules [73]. The development of microcapsule-type self-healing coating systems highlights the potential for these capsules to self-heal cracks and maintain the structural integrity of coatings over extended periods [74]. Through continued research and innovation, the utilization of chemical self-healing capsules is poised to revolutionize the field of protective coatings, offering sustainable and cost-effective solutions for enhanced material performance and longevity.

Biological self-healing capsules are a fascinating innovation in the realm of self-healing coatings, offering a unique approach to repairing damage inspired by natural biological processes. These capsules are designed to encapsulate self-healing agents and release them when needed to repair defects in coatings, such as those used in anti-corrosion applications. The concept of self-healing coatings inspired by biological systems is rooted in the idea of mimicking the regenerative capabilities found in living organisms, enabling materials to repair physical damage autonomously. The introduction of biological self-healing capsules into coatings has opened up new possibilities for enhancing durability and functionality in various industrial applications by harnessing the power of nature for self-repair [75]. The mechanism of action behind biological self-healing capsules involves a carefully orchestrated process that triggers the release of healing agents upon detecting damage in the coating [71]. These capsules are designed to respond to specific stimuli, such as changes in pH or temperature, which are indicative of damage or corrosion. Once the capsules rupture or degrade in response to the stimuli, the self-healing agents are released, filling in cracks or gaps in the coating to restore its integrity. This self-healing process not only repairs the visible damage but also helps prevent further deterioration, prolonging the lifespan of the coating and reducing maintenance requirements in various applications. Examples of biological self-healing capsules include micro/nanocapsules fabricated using different types of micro/nanocontainers. These capsules can be loaded with a variety of self-healing agents, such as polymers, resins, or other active materials capable of repairing damage in coatings. The incorporation of biological self-healing capsules into coatings has been shown to enhance their anti-corrosion properties and improve overall performance in harsh environments. By leveraging the versatility and efficacy of these innovative capsules, researchers and engineers are advancing the field of self-healing coatings, paving the way for more durable and sustainable materials in various industries.

Physical self-healing capsules are innovative materials that play a crucial role in the development of self-healing coatings. These capsules are designed to contain healing agents that can be released when damage occurs, promoting the repair of the coating without human intervention. The concept of physical self-healing capsules is inspired by biological systems that possess the remarkable ability to repair themselves in response to damage. By incorporating these capsules into coatings, researchers aim to enhance the durability and longevity of various materials, ranging from automotive paints to protective coatings for infrastructure. The mechanism of action of physical self-healing capsules involves a proactive response to damage, triggered by external stimuli such as mechanical stress or environmental factors. When a coating containing these capsules experiences a crack or scratch, the capsules rupture, releasing the healing agent into the damaged area. The healing agent then interacts with the surrounding material, catalyzing a self-repair process that restores the integrity and functionality of the coating. This autonomous healing process demonstrates the potential of physical self-healing capsules to revolutionize the field of protective coatings and materials engineering.

5.1. Nanocapsules in Self-Healing Coatings

Nanocapsules play a crucial role in the development of self-healing coatings, offering innovative solutions for enhancing the durability and longevity of various materials. These micro/nanocapsules are designed based on different types of micro/nanocontainers and are at the forefront of cutting-edge research in material science. The synthesis of micro/nanocapsules involves effective parameters that influence their structure and performance, leading to the fabrication of self-healing coatings that exhibit remarkable capabilities [76,77]. By encapsulating active agents within these capsules, such as healing agents or corrosion inhibitors, self-healing coatings can autonomously repair damage and protect surfaces from environmental factors, extending the lifespan of coated materials [40].

Self-healing capsules play a crucial role in the restoration and maintenance of materials by releasing mending agents upon crack formation. These capsules are evenly dispersed within the material matrix, with the volume fraction typically below 5% of the matrix to ensure effectiveness [78]. The capsules, assumed to act independently and not cluster, rupture when a microcrack emerges, releasing reactive agents that flow to the fracture plane and form a polymer network, effectively sealing the crack [7,79,80]. Different structures of self-healing capsules have been explored, including micro/nanocapsule embedment, spherical capsules, and storage in polyurethane microcapsules, each designed to enhance the healing process and maintain material integrity [76,78,79]. Additionally, hollow fiber embedment has shown promising results in improving compressive strength and reducing permeability in cement matrices, highlighting the diverse applications of self-healing capsules in various material systems [76,78]. Through innovative designs and strategic distribution, these capsules release healing agents efficiently, enhancing the self-healing capabilities of materials and extending their lifespan [78,80].

5.2. Mechanisms of Self-Healing Capsules

Self-healing capsules exhibit a variety of release mechanisms based on their design and components. For instance, some capsules release the healing agent upon contact with the matrix itself, while others require contact with a second component present in the matrix to trigger the release. Additionally, self-healing capsules can be engineered to release the healing agent upon exposure to moisture, air, or heat, providing versatility in application based on environmental conditions [77,81]. The efficacy of the release mechanism is influenced by factors such as the crack area, capsule concentration, and crack volume relative to the healing agent available in the capsules. Moreover, the shape and size of the capsules play a significant role in the controlled release of the healing agent. Spherical capsules are known to offer a more controlled and enhanced release compared to cylindrical capsules, which suffer from inferior release upon cracking due to suction effects exerted by closed ends [81]. Furthermore, the reliability of self-healing systems is dependent on the number of capsules included to prevent cracks from propagating and fracturing them, highlighting the importance of proper capsule distribution within the matrix. Overall, the design and deployment of self-healing capsules are critical in ensuring effective healing of cracks and enhancing the durability of cementitious materials.

Categorizing self-healing capsules according to their structure and release mechanisms offers numerous advantages in the field of concrete repair and maintenance. By dispersing healing agents within a matrix or encasing them in matrix aggregates, the capsules can effectively deliver the mending materials where they are needed most. Moreover, integrating self-healing capsules during concrete production streamlines the repair process and enhances the structural integrity of the material [81]. One key benefit of this classification system is the mitigation of the suction effect caused by sealed cylindrical capsules, thereby improving the overall healing efficiency of the capsules. Encapsulating healing agents within coated hollow plant fibers allows for the controlled release of the mending substances when cracks develop in the concrete, ensuring targeted and efficient repair mechanisms [81]. Additionally, the systematic categorization of self-healing capsules based on their structure and release mechanisms enables researchers to assess their effectiveness across different structural components, facilitating the development of more resilient and durable concrete structures. This approach not only enhances the practical application of self-healing capsules in the construction industry but also contributes to the advancement of innovative solutions for sustainable infrastructure maintenance. Figure 5 illustrates the release of the healing agents through either core–shell capsules or cylindrical capsules after a crack (defect) formation onto the coating matrix of a pipeline substrate. The release of the healing agents results in crack sealing.

5.3. Application-Based Classification of Self-Healing Capsules

One of the key factors to consider is the application. Based on their structure, microcapsules can be classified into several categories. For instance, single-walled single nuclei and single-walled multiple nuclei microcapsules offer unique advantages in dual-targeted drug delivery via sonochemical methods [82]. These innovative structures allow for precise and efficient delivery of healing agents to the damaged areas, enhancing the overall healing process. Moreover, the utilization of microcapsule self-healing technology has shown promising results in the field of cement-based materials [83]. Researchers are exploring the potential of these capsules for future applications, indicating a growing interest in this advanced technology.

Another crucial aspect of self-healing capsules is the triggering mechanism that initiates the healing process. Extrinsic self-healing composites stand out for their automatic recovery capabilities when damaged, drawing inspiration from biological systems that exhibit natural healing properties [84]. Within this category, there are two major groups: autonomic and non-autonomic self-healing materials. Autonomic materials, in particular, have the remarkable ability to heal themselves without the need for external intervention [85]. This self-sustainability makes them highly desirable for various applications where continuous maintenance is impractical. By understanding the different triggering mechanisms, researchers can tailor self-healing capsules to specific requirements, maximizing their effectiveness.

The material compatibility of self-healing capsules plays a significant role in determining their performance and durability. Inorganic capsules, such as those based on magnesium oxide, have shown great promise in enhancing the healing potential of various materials [86]. Researchers have developed innovative techniques for preparing and encapsulating powder mineral pellets, further expanding the applications of inorganic capsules [86]. By focusing on the mineralogy of the inner core and granule strength, scientists can optimize the compatibility of these capsules with different matrices, ensuring long-lasting and effective healing properties [86]. This attention to detail in material compatibility is essential for advancing the field of self-healing technology and unlocking new possibilities for sustainable infrastructure development.

The classification of self-healing capsules based on the application method, triggering mechanism, and material compatibility provides a comprehensive framework for understanding their diverse functionalities. By exploring the nuances of each type of self-healing capsule, researchers can continue to innovate and develop cutting-edge solutions for a wide range of industries. The potential of self-healing capsules to revolutionize material maintenance and restoration is vast, making them a vital area of research and development in the modern era.

The integration of self-healing agents into coatings has emerged as a revolutionary technique with substantial implications for both environmental sustainability and long-term durability. Among the key areas of focus, the ability of these coatings to autonomously repair damages plays a crucial role in mitigating corrosion, which not only prolongs the lifespan of metal structures but also significantly reduces maintenance costs and environmental impact [25]. For instance, advancements in self-healing polymeric materials have demonstrated significant potential across various industries by enhancing the durability of protective applications, thus reducing the frequency of replacements and the associated waste. Furthermore, the development of highly optimized self-healing designs in organic coatings has shown marked improvements in performance, reflecting the ongoing evolution from foundational research to advanced practical applications. The cumulative effect of these innovations is profound, offering a sustainable solution that aligns with environmental conservation goals while simultaneously ensuring the structural integrity of diverse materials. Therefore, continued research and development in this field are imperative to fully realize the benefits and address the remaining challenges associated with these advanced self-healing systems.

5.4. Optimization of Micro/Nanocapsule Design for Self-Healing Coatings

To optimize the design of micro/nanocapsules for enhancing the lifespan of self-healing coatings, researchers focus on various factors such as responsive release mechanisms, container size, and encapsulation processes. Microcontainers, typically ranging from 50 to 200 μm, are identified as optimal for self-healing coatings due to their ability to rupture easily under mechanical stress and provide sufficient storage space for healing agents, thus initiating the healing process effectively [25]. On the other hand, nanocontainers, being smaller in size, are more suitable for carrying corrosion inhibitors in self-healing coatings, thereby enhancing the coatings’ protective capabilities. The authors emphasize the importance of maintaining the coating’s integrity and long-term corrosion resistance by utilizing the separation provided by micro/nanocontainers between the inhibitors and the coating matrix. Furthermore, the design of micro/nanocapsules can be optimized by incorporating pH- or wettability-responsive properties to enhance the lifespan of self-healing coatings and by developing green and innovative capsules from sustainable energy sources [71]. By employing kinetic models to understand the release behavior of encapsulated substances, researchers can predict and optimize the release kinetics, ensuring effective deployment of inhibitors for corrosion prevention [19]. Ultimately, the controlled and targeted release of inhibitors through micro/nanocontainers plays a crucial role in extending the longevity and stability of materials covered with self-healing coatings.

One of the most significant challenges in the commercial scale-up of self-healing coatings is the issue of durability. Although self-healing materials show great promise in laboratory settings, their long-term performance in real-world applications remains uncertain, necessitating extensive future research and development to ensure their reliability over extended periods. Additionally, scalability presents another substantial hurdle; many self-healing materials are still at the experimental stage and have yet to demonstrate their capability to meet industrial production demands. It’s essential to develop scalable methods for producing these materials to cater to the growing demand from various industries, ensuring that the transition from laboratory to large-scale production is feasible. Furthermore, the current production costs of self-healing materials are significantly higher than those of traditional coatings, which hampers their adoption in cost-sensitive markets. Therefore, developing cost-effective manufacturing processes is crucial to making self-healing coatings economically viable for widespread use. Addressing these interconnected domains—durability, scalability, and cost—will be critical for the successful integration of self-healing coatings into existing technologies and their broader market adoption.

Table 1. Types of self-healing capsules along with their description, applications, and self-healing mechanisms.

Types of Self-Healing Capsules	Description	Self-Healing Mechanisms	Applications
Single-walled capsules	These capsules consist of a single shell that encases the healing agent.	When the material cracks, the capsule breaks, releasing the healing agent directly into the damaged area.	Used in polymers, coatings, and composite materials
Double-walled capsules	These capsules have two shells, with the healing agent contained within the inner shell and an activator or a secondary component within the outer shell.	Damage causes both shells to rupture, allowing the healing agent and the activator to mix and initiate the healing process.	Used in applications requiring delayed healing response or a more controlled response, such as in aerospace materials.
Core–shell microcapsules	These capsules have a core that contains the healing agent, surrounded by a protective shell.	Upon shell rupture, the core releases the agent to fill cracks or gaps.	Used in structural materials, protective coatings, and smart textiles.
Multicore capsules	These capsules contain multiple cores within a single shell, each filled with different healing agents or components.	Upon damage, multiple healing agents are released sequentially or simultaneously to enhance the repair process or to address different types of damage.	Used in complex systems where multiple types of damage might occur.
Encapsulated liquid metals	These capsules are filled with liquid metals, which can flow and solidify to repair damage.	When a crack forms, the liquid metal flows into the damaged area, filling the voids, and solidifies to restore the material’s integrity.	Used in batteries, electronic circuits, and advanced materials requiring rapid and robust healing.
Temperature-responsive capsules	These capsules are triggered by changes in temperature.	When the material is exposed to a certain temperature range, the capsule releases its healing agent to address thermal damage.	Used in high-temperature environments, such as in automotive or aerospace applications.
pH-responsive capsules	These capsules are designed to release their healing agent in response to changes in pH levels.	When the material’s environment becomes more acidic or basic due to corrosion or damage, the capsules respond by releasing the healing agent.	Used in materials exposed in harsh environments and corrosion-resistant coatings.
Polymer capsules	These capsules contain polymer precursors that can form a solid polymer when released.	Upon capsule rupture, the polymer precursors are released, and through chemical reactions, a solid polymer is created that heals the crack.	Used in adhesives, coatings, and structural polymers.
Emulsion-based capsules	These capsules contain emulsions that can release healing agents when triggered.	The emulsion can release the healing agent either through a gradual diffusion process or by capsule rupture.	Used in paints and self-healing coatings, where gradual healing over time is desired.
Time-delayed capsules	These capsules are designed to release their healing agent after a specific time delay.	The capsule slowly degrades or reacts over time, eventually releasing the healing agent.	Used in systems where immediate repair is not necessary or where gradual healing is needed.

and

Table 2. Key types of self-healing capsules.

Types of Self-Healing Capsules	Description	Self-Healing Mechanisms	Advantages	Disadvantages
Microcapsules	Tiny capsules embedded in the material, typically filled with liquid healing agents like adhesives or resins.	When a crack propagates, it ruptures the capsules, releasing the healing agent.	Localized healing, simple manufacturing, broad application.	Finite healing capacity, potential weakness, limited control.
Vascular networks	This system mimics biological vascular networks, where channels or hollow fibers filled with healing agents are embedded in the material.	When damage occurs, the healing agent flows from the network to the damaged area.	Extended healing capability, controlled release, self-refilling.	Complex manufacturing, potential for leakage, bulkier systems.
Nanocapsules	These capsules contain healing agents at the nanoscale, allowing them to be embedded in materials without significantly affecting their properties.	When a crack propagates, it ruptures the capsules, releasing the healing agent.	Minimal impact on material properties, versatility, and cost-effective ease of incorporation.	Slow healing process.

summarize the key types of self-healing capsules, demonstrating the various mechanisms by which self-healing capsules operate, together with their advantages and disadvantages.

6. Conclusions

This review paper highlights the significant advances in self-healing coatings, especially those containing various types of self-healing capsules. These capsules, including microcapsules and chemical, biological, and physical self-healing capsules, play a vital role in autonomously repairing damage and improving the durability of materials. Incorporation of these capsules into coatings has been shown to significantly improve corrosion resistance and performance, especially in aggressive environments. The mechanism of action, release behavior, and material compatibility of these capsules have been thoroughly studied and provide valuable insights into their effectiveness and potential applications. Despite significant progress, challenges remain in optimizing self-healing technologies, reducing costs, and accelerating the transition from laboratory research to practical applications.

Developing self-healing polymeric coatings that are highly durable and able to resist long-term environmental exposure is key to widespread adoption. In brief, advancements in self-healing coatings stand as significant achievements in enhancing material functionality to ensure longevity. It is imperative that advanced studies focus on these technologies, improving methods, exploring new healing agents innovatively, and coming up with economically viable solutions to achieve the maximum utilization of self-healing coatings in various industrial fields. This field can offer a bright future through continuous advancement with no compromise to innovation or optimization, which will completely overhaul the traditional approach of material maintenance towards achieving sustainability. It is very important that long-lived self-healing polymer coatings are developed after regular exposure to the environment for a considerable amount of time.

In conclusion, the development of self-healing coatings, especially through the development of micro- and nanocontainers, represents a quantum leap forward in improving material performance and durability. Further research into these technologies is imperative, and new treatment approaches must be investigated, while cost-effective applications must not be neglected. In this way, we can realize the full potential of self-healing coatings in various industrial sectors. The prospect of a future revolution in material care through continued dynamic innovation and optimization bodes well for the adoption of sustainable practices in the industry. The development of durable, aging-resistant polymer coatings that can withstand long-term environmental impacts is suitable for the widespread use of such materials. In conclusion, the development of self-healing coatings—especially through micro- and nanocontainers—undoubtedly represents an important step towards improving material performance and extending service life.

In future investigations, these areas need to be brought to perfection, and new healing agents need to be explored without cost concern in order to find cost-effective solutions that can unleash the full potential of self-healing coatings for all industrial spheres. The dynamics and ongoing fine-tuning within this area promise the transformation of approaches towards maintaining materials with an eye on green sustainability far into the future.