# Advanced Micro/Nanocapsules for Self-Healing Coatings
## 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]. Microapsules 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. Microapsules 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.