# Advanced Micro/Nanocapsules for Self-Healing Coatings
## 5 Types of Self-Healing Capsules
### 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.  
Figure 5: Schematic illustration of the distribution and effect of the healing agents and micro- and nanocontainers in the coating matrix.