Emerging Trends in Smart Self-Healing Coatings: A Focus on Micro/Nanocontainer Technologies for Enhanced Corrosion Protection

Round 1

Reviewer 1 Report

Authors made quite an effort to summarize the up-to-date knowledge about smart self-healing coatings, especially focused on advancement in use of micro and nanocontainer technologies for the protection of metals. The paper is well written, and provides an adequate review of nowadays coatings, their mechanism of action, and its advantage in comparison to traditional anti-corrosive coatings. Also, authors stated the existing challenges regarding corrosion resistance during extended working periods. It would be useful to add the information about longevity and stability of materials covered with nowadays coatings, in comparison to the traditional one, when describing different coatings. To emphasize their advantages in practical usage. It would be useful information for the readers.

Author Response

Respond to comments provided by Reviewer 1

The authors extend their deep appreciation for the constructive feedback from the reviewer and for their insightful and valuable input, which was instrumental in refining their manuscript. This feedback was crucial in making the complex details of their research and methodologies clearer and more accessible to readers. Importantly, the improvements made in the manuscript not only facilitate a better understanding among current readers but also serve as a robust reference for future researchers aiming to replicate the study's findings. This contributes to the broader goal of advancing knowledge in the field.

As per the valuable feedback from the reviewers, the authors have embarked on an in-depth discussion of the proposed methodology, meticulously delineating both its advantages and disadvantages. Furthermore, they have expanded the scope of their analysis to include detailed information on the longevity and stability of materials treated with contemporary coatings, offering a comparative perspective against those treated with traditional coatings. This detailed exposition is structured to address each type of coating within the framework of the proposed methodology separately and with explicit clarity. Through this approach, the authors aim to provide a comprehensive evaluation that not only showcases the potential benefits and acknowledges the limitations of their method but also situates it within the broader context of advancements in material coatings. This enriched analysis seeks to contribute to a deeper understanding of how modern coatings enhance material longevity and stability, thereby offering valuable insights into the evolving landscape of coating technologies.

As per the valuable feedback from the reviewers, the authors have embarked on an in-depth discussion of the proposed methodology, meticulously delineating both its advantages and disadvantages. Furthermore, they have expanded the scope of their analysis to include detailed information on the longevity and stability of materials treated with contemporary coatings, offering a comparative perspective against those treated with traditional coatings. This detailed exposition is structured to address each type of coating within the framework of the proposed methodology separately and with explicit clarity. Through this approach, the authors aim to provide a comprehensive evaluation that not only showcases the potential benefits and acknowledges the limitations of their method but also situates it within the broader context of advancements in material coatings. This enriched analysis seeks to contribute to a deeper understanding of how modern coatings enhance material longevity and stability, thereby offering valuable insights into the evolving landscape of coating technologies.

Introduction

Advantages of Smart Self-Healing Coatings: The introduction of smart self-healing coatings represents a significant advancement in corrosion protection technologies. These coatings, which can be intrinsic or extrinsic, of-fer autonomous repair capabilities that are not found in traditional coatings. Intrinsic coat-ings, which utilize reversible bonds, can heal themselves without any external interven-tion, providing a continuous repair mechanism that can significantly extend the lifespan of metal surfaces. On the other hand, extrinsic coatings rely on micro/nanocontainers filled with healing agents that are released in response to environmental triggers, such as the presence of moisture or a change in pH, to repair damage. This targeted release ensures that healing agents are only used when necessary, reducing waste and potentially lower-ing long-term maintenance costs. The development of stimuli-responsive self-healing coat-ings, particularly through advancements in micro/nanocontainer technology, enhances the sensitivity and functionality of these coatings. This allows for the creation of coatings that can respond to multiple stimuli, further improving their effectiveness. Additionally, the in-corporation of multifunctional properties such as self-reporting and antimicrobial actions not only contributes to corrosion protection but also adds value by monitoring the health of the coating and preventing microbial-induced corrosion.

Research Gaps and Disadvantage and there Challenges: Despite the promising advantages, there are several challenges and disadvantages associated with smart self-healing coatings that need to be addressed. One of the primary limitations is the complexity involved in synthesizing and encapsulating the mi-cro/nanocontainers, which can be a costly and time-consuming process. This complexity also extends to the design of multi-stimulus-responsive systems, which require precise control over the material properties to ensure effective functionality. Another challenge is the scalability of these technologies; producing these advanced coatings on an industrial scale can be difficult, limiting their widespread application. The volume of healing agent that can be encapsulated within micro/nanocontainers is another critical factor that dic-tates the efficacy of the repair. There is a limit to how much agent can be stored and re-leased, which may not be sufficient for repairing larger cracks or damage. Furthermore, while the addition of multifunctional traits increases the coatings' applicability, it also adds to the complexity and cost of the coating systems. These factors contribute to the cur-rent gap between laboratory-scale success and industrial-scale application, necessitating further research and development to overcome these hurdles and fully realize the potential of smart self-healing coatings in corrosion protection.

2.   Exploring the World of Micro/Nanocontainers: Key to Advanced Extrinsic Self-Healing Coatings

                        The advantages of utilizing micro/nanocontainers in extrinsic self-healing coatings are significant, presenting a transformative approach in corrosion protection technologies. These containers excel in precisely housing and dispensing healing agents, such as corrosion inhibitors, directly into the cracks formed in the coating. This targeted release mechanism, driven by capillary action and environmental stimuli, ensures an efficient and responsive healing process. The use of microcontainers, which are optimal in size, allows for the storage of ample healing agents and ensures their rupture under mechanical stress to initiate the healing process. On the other hand, nanocontainers are particularly suitable for carrying corrosion inhibitors due to their smaller size, enhancing the coatings' protective capabilities. The separation provided by these containers between the inhibitors and the coating matrix is crucial for maintaining the coating's integrity and ensuring its long-term corrosion resistance. The diversity in container materials ranging from organic, in-organic, to hybrid offers a wide array of properties, including biocompatibility, robust-ness, and environmental friendliness, further broadening the applicability of these advanced coatings in various industrial sectors.

                        However, the methodology also presents certain disadvantages when compared to existing literature and technologies. The complexity and cost of synthesizing micro/nanocontainers can be significant, potentially limiting their widespread application. The precise control over the size, shape, and permeability of these containers, crucial for their optimal performance, requires advanced and sometimes costly fabrication techniques. Additionally, the long-term stability and environmental impact of these containers, especially those made from non-biodegradable materials, remain concerns that need further investigation. The interaction between the encapsulated agents and the polymer matrix during the healing process, as well as the potential for incomplete healing or re-lease of agents, are challenges that necessitate ongoing research to fully understand and mitigate. Despite these drawbacks, the innovative approach of micro/nanocontainers in self-healing coatings represents a promising direction in corrosion protection technology, warranting further development and optimization to overcome these limitations.

          2.1. Organic Micro/Nanocontainers: A Vital Component in Self-Healing Materials

Advantage on organic micro/nanocontainers have revolutionized the field of self-healing materials by offering high load capacity for active healing agents, which significantly enhances the efficiency of corrosion protection and material repair. Their compatibility with various polymer matrices and the ability to protect encapsulated substances from harsh environmental conditions stand out as critical benefits. This compatibility ensures that the healing agents remain potent and ready to act upon the occurrence of dam-age. The innovation in the synthesis and application of these containers, including methods like in-situ polymerization and Pickering emulsion, contributes to the precise control over encapsulation, enabling a consistent and effective healing response. Furthermore, the development of covalent organic frameworks (COFs) as a new class of micro/nanocontainers has opened up possibilities for enhanced dispersion and compatibility within organic matrices, further broadening the scope and efficiency of self-healing materials.

       Despite these advancements, the complexity of creating organic mi-cro/nanocontainers poses significant challenges. The multifaceted process involving polymerization, encapsulation of the active agent, and the subsequent removal of by-products and solvents can be resource-intensive and technically demanding. This complexity may limit the scalability and cost-effectiveness of producing these self-healing materials for broader industrial applications. Additionally, achieving the optimal balance be-tween the mechanical strength and the responsive rupture of the containers remains a critical challenge. Containers that are too robust may fail to release their healing agents when needed, while those too fragile may rupture prematurely or fail to withstand the operational environment, thus compromising the healing efficacy.

                        In comparison to existing literature, these methodologies underscore a significant leap in the targeted delivery and controlled release of healing agents, offering a more responsive and efficient approach to corrosion protection and material repair. However, the challenges highlight the need for ongoing research to refine synthesis techniques, improve cost-efficiency, and ensure the practical applicability of these advanced materials in real-world conditions. The continuous evolution of synthesis methods and the exploration of new materials like COFs and graphene oxide (GO) sheet hybrids for containers suggest a promising trajectory for overcoming these hurdles, pushing the boundaries of self-healing technology towards more sophisticated and widely applicable solutions.

          2.2 Inorganic micro/nanocontainers, focusing on their structure, applications, and limitations

Inorganic micro/nanocontainers offer a significant leap forward in the development of self-healing materials, particularly in the context of corrosion protection. These containers distinguish themselves through their stable, small, cavity-like structures capable of en-capsulating and subsequently releasing active agents like corrosion inhibitors in response to specific environmental triggers. This innovative approach facilitates the enhancement of thermomechanical properties and barrier performance of coatings, addressing the critical need for durable and efficient corrosion protection systems. The major advantages on the primary benefits of utilizing inorganic micro/nanocontainers include their enhanced stability and ability to undergo surface modification, which ensures compatibility and controlled release of active agents. This feature is pivotal in applications requiring precise targeting and release of functional species to mitigate corrosion effectively. Inorganic con-tainers like titanium dioxide and mesoporous silica simplify the encapsulation process and are inherently more stable, offering broad coverage and an easy encapsulation process. Their design for responding to external microenvironments leads to structural changes that help in the efficient release of functional species into targeted areas, thus significantly improving the self-healing capabilities of materials.

The disadvantages with several challenges accompany the use of inorganic micro/nanocontainers. One of the most notable drawbacks is their relatively poor compatibility with polymer materials, which can lead to aggregation and the formation of micro defects within the coating matrix, potentially accelerating damage. Furthermore, the capacity of these containers to hold active agents is generally limited (usually less than 20%), which might constrain their effectiveness in providing long-term corrosion protection. Additionally, the manufacturing process, predominantly the sol-gel method, while mild and conducted at room temperature, may result in containers with limited compactness and reduced capacity for active agents. When compared to organic micro/nanocontainers, inorganic variants offer a contrasting profile of advantages and limitations. Organic containers are praised for their high compatibility with various polymer matrices and efficient protection against environmental degradation, whereas inorganic containers excel in stability and ease of encapsulation but struggle with compatibility issues and limited active agent capacity. Despite these challenges, the unique properties of inorganic micro/nanocontainers, such as their ability to enhance the thermomechanical properties and barrier performance of self-healing systems, underscore their potential in advancing the field of corrosion protection and beyond. Addressing the limitations related to compatibility, agent capacity, and manufacturing complexities remains a critical area of focus for future research to fully leverage the advantages offered by inorganic micro/nanocontainers in self-healing materials.

2.3. Micro/nanocontainers, their composition, manufacturing methods, and applications

The methodology involving organic/inorganic hybrid micro/nanocontainers represents a significant advancement in the development of self-healing materials, blending the multifunctionality and adjustable properties of organic containers with the physicochemical stability and mechanical robustness of inorganic ones. This hybrid approach allows for the creation of containers that offer both stability and flexibility, a combination that is particularly valuable in applications requiring controlled release mechanisms under specific conditions, such as in corrosion protection, drug delivery, and environmental remediation. The major advantages is one of the key strengths of these hybrid mi-cro/nanocontainers lies in their versatile functionality, which can be fine-tuned through the layer-by-layer (LBL) assembly technique. This method enables the precise control over the thickness, composition, and molecular organization of the containers, allowing for the incorporation of various nanoparticles to achieve desired properties. For instance, the integration of Fe3O4 and graphene oxide into silica-polymer hybrid capsules enhances their structural stability and expands their application range. Moreover, these containers' responsiveness to environmental stimuli such as pH and temperature changes enables the smart release of encapsulated agents, making them highly effective in self-healing applications where such controlled release is crucial.

The disadvantages on the development of organic/inorganic hybrid containers is not without its challenges. The integration of inorganic components can sometimes result in poor compatibility with polymer matrices, leading to potential aggregation issues. Moreover, the complexity of creating these hybrid containers, which involves sophisticated synthesis techniques like the LBL assembly, may pose scalability and cost-effectiveness concerns. Additionally, the performance of these containers heavily relies on the organic components' behavior within the hybrid, requiring careful consideration of dispersion within the coating to ensure optimal functionality. Compared to the existing literature, the proposed methodology of utilizing organic/inorganic hybrid micro/nanocontainers introduces a novel approach that leverages the synergistic effects of combining organic and in-organic materials. This results in containers with enhanced mechanical properties, chemical resistance, and multifunctionality over purely organic or inorganic containers. How-ever, addressing the aforementioned compatibility and manufacturing challenges is essential for fully realizing the potential of these advanced materials in practical applications. As research in this field progresses, further innovations and refinements in synthesis techniques and material selection are expected to overcome current limitations, paving the way for broader industrial adoption of these sophisticated self-healing systems.

3. Exploring the Dynamic Release Patterns of Encapsulated Agents from Micro and Nanocontainers

The methodology surrounding the encapsulation and release of active agents from micro/nanocontainers is pivotal in the advancement of materials science, especially for creating protective coatings against corrosion. This process, which leverages both physical and chemical interactions for encapsulation, is designed to release corrosion inhibitors precisely when and where needed, upon exposure to specific external stimuli. This strategic release mechanism aims to halt or decelerate the corrosion process effectively. However, the efficiency of this encapsulation and subsequent release is subject to various influencing factors such as the containers' loading capacity, dispersion within the coating, particle size, solubility of inhibitors, and the nature of the corrosion inhibitors themselves. These elements collectively dictate the release kinetics, which is crucial for the timely and effective deployment of inhibitors to combat corrosion.

The advantages on the use of micro/nanocontainers for encapsulating active agents presents a sophisticated approach to corrosion protection, allowing for a controlled and targeted release of inhibitors. This methodology benefits from employing several kinetic models (e.g., Zero-order, First-order, Higuchi, Hixson-Crowell, Korsmeyer-Peppas, and Hopfenberg models) to predict and understand the release behavior of encapsulated sub-stances, catering to various scenarios and encapsulated substance types. Such models are instrumental in optimizing release kinetics, ensuring that inhibitors are effectively deployed to prevent corrosion.

Despite the apparent benefits, challenges remain in accurately predicting and optimizing the release kinetics due to the complexity of the encapsulation and release processes. These processes involve multiple steps, each characterized by different and intricate physicochemical reactions, making it difficult to model the release behavior accurate-ly. Additionally, while kinetic models like the Korsmeyer-Peppas offer valuable insights into release phenomena, the reliance on R² values for model fit assessment can be misleading. This is because R² values tend to increase with the addition of more parameters, which may not always be relevant. To address this, the adjusted correlation coefficient (R² adjusted) is recommended for a more accurate representation of a model's applicability.

Compared to existing literature, this methodology underscores the intricate balance required in designing micro/nanocontainers for corrosion protection. The dynamic release patterns of encapsulated agents from these containers need to be thoroughly understood and optimized for the effective prevention of corrosion. While the models provide a foundation for predicting release kinetics, ongoing research and development are crucial to re-fine these models further, enhance the precision of release mechanisms, and overcome the limitations identified in current methodologies. This evolution is essential for advancing the field of protective coatings and developing new systems with more precise and con-trolled release behaviors, ensuring robust corrosion protection in diverse environments.

4. Stimulus-responsive coatings represent an advancement in material science

4.1. pH-responsive coatings

Stimulus-responsive coatings, particularly those incorporating micro/nanocontainers, represent a significant leap forward in the realm of material science, offering a dynamic approach to combating metal corrosion compared to traditional anti-corrosive coatings. These smart materials can adapt their physicochemical properties in response to environmental changes, providing targeted corrosion protection through the controlled release of encapsulated inhibitors. The major Advantages strength of these coatings lies in their ability to offer active corrosion protection. Unlike conventional coatings that serve merely as physical barriers, stimulus-responsive coatings can detect changes in environmental conditions (such as pH, temperature, and ions) and release corrosion inhibitors precisely where needed. This intelligent response system, particularly seen in pH-responsive coatings, leverages fluctuations in local pH levels due to corrosion processes to trigger the release of inhibitors, ensuring effective and localized protection against corrosion. This approach not only enhances the longevity of the metal substrates but also offers a more sustainable solution by minimizing the need for frequent reapplication or repair.

Despite the promising advancements, several disadvantages: remain. One of the primary concerns is achieving precise control over the stimuli-responsive release behavior, which is crucial for long-term effectiveness. Factors such as the compatibility and dispersion of micro/nanocontainers within the polymer matrix, and the variability in triggering pH values for different metals in diverse environments, can significantly impact performance. Additionally, the potential for uncontrolled release due to external pH fluctuations and the temporary nature of the self-healing effect presented by reversible release mechanisms highlight areas that require further research and development. Compared with existing literature, stimulus-responsive coatings offer an innovative approach to corrosion protection by integrating the benefits of both organic and inorganic materials through micro/nanocontainers. However, optimizing these coatings for a wider range of applications and environmental conditions remains a critical focus for ongoing research. The challenge lies in balancing the smart responsiveness of the coatings with practical considerations such as compatibility, longevity, and environmental impact. As the field progresses, it is expected that advancements in material science and coating technology will address these limitations, broadening the applicability and effectiveness of stimulus-responsive coatings in metal corrosion protection and beyond.

4.2. Redox-responsive coatings represent a sophisticated approach in the field of smart materials, particularly in the context of corrosion protection

Redox-responsive coatings represent an innovative advancement in smart material science, particularly for corrosion protection. These coatings are designed to react to changes in electrochemical potential, commonly associated with the corrosion process. The deployment of conducting polymers within these coatings, notable for their reversible redox properties, marks a significant stride in materials technology. These polymers, when reduced, facilitate the release of encapsulated healing agents, thus enabling a self-healing function that is critically responsive to corrosive environments.

The main advantage of redox-responsive coatings lies in their dynamic adaptability to the corrosive environment, offering a sophisticated, targeted approach to corrosion protection. By utilizing conducting polymers that undergo reversible redox reactions, these coatings can effectively control the release of encapsulated substances in response to specific electrochemical stimuli. This ensures that healing agents are released precisely where and when needed, directly at the sites of potential or active corrosion, enhancing the durability and lifespan of metal substrates. Moreover, the implementation of redox-sensitive nanovalves on the surfaces of micro/nanocontainers within these coatings has further bolstered their efficacy. These nanovalves, capable of controlling the release of encapsulated agents through reversible transitions under redox stimuli, represent a cutting-edge approach in the design of intelligent, stimulus-responsive self-healing coatings.

Despite these advancements, there are notable challenges to the widespread application of redox-responsive coatings. One significant issue is the complexity involved in engineering these materials to ensure precise control over the redox-triggered release mechanism. Additionally, the compatibility and dispersion of these micro/nanocontainers with-in the polymer matrix of the coatings must be carefully managed to prevent aggregation and ensure effective barrier properties. Furthermore, the specificity of the redox triggers and the potential for uncontrolled release under fluctuating electrochemical conditions present additional hurdles to achieving consistent, long-term corrosion protection. Com-pared to existing literature, redox-responsive coatings offer a more dynamic and adaptable solution to corrosion protection, moving beyond the passive barrier mechanisms of traditional anti-corrosive coatings. However, the challenges highlighted underscore the need for continued research and development. Addressing these issues is crucial for optimizing the performance of redox-responsive coatings and expanding their application across a broader range of environmental conditions and metal substrates. As the field progresses, it is anticipated that innovations in material synthesis, nanovalve design, and polymer technology will overcome current limitations, paving the way for more efficient and durable protective coatings.

4.3. Ions-responsive coatings represent a novel class of smart materials that are tailored to respond to specific aggressive ions present in corrosive environment

Ions-responsive coatings mark a notable advancement in smart material science, specifically designed to counteract the effects of aggressive ions in corrosive environments. These coatings employ ion-responsive micro/nanocontainers capable of swiftly releasing active inhibitors to mend damaged areas on the surface. This capability is especially crucial for addressing microbiologically influenced corrosion, commonly seen in marine set-tings. Through the incorporation of innovations such as ZIF-8 nanovalves on mesoporous silica nanocontainers (MSNs), these coatings can detect and respond to specific ion fluctuations, such as S2- ions associated with sulfate-reducing bacteria activity, a prevalent cause of microbiological corrosion in marine environments. The primary advantage of ions-responsive coatings lies in their targeted response to specific ions, allowing for the direct and efficient mitigation of corrosion processes. This specificity is particularly valuable in environments like marine settings, where microbiologically influenced corrosion poses a significant threat. The integration of innovative materials like LDHs, with their unique layered structure and ion-exchange capabilities, further enhances these coatings' ability to trap corrosive ions while releasing protective inhibitors, thus providing a double-action protective mechanism against corrosion.

However, the application of ions-responsive coatings is not without its limitations. Their effectiveness is heavily reliant on the presence of specific ions, narrowing their applicability to environments where such ions are prevalent. Additionally, the sensitivity of these coatings to detect and respond to specific ion concentrations requires further improvement to ensure consistent and reliable corrosion protection. Moreover, while LDHs themselves offer some level of protection, the exploration of their synergistic effects with encapsulated inhibitors needs more extensive research to optimize the coatings' overall protective capabilities. Compared to existing literature, ions-responsive coatings represent a promising yet relatively niche solution to corrosion protection. The need for specific environmental conditions and the challenge in achieving broad-spectrum applicability highlight the importance of ongoing research. Future developments should focus on enhancing the sensitivity and range of ions these coatings can respond to, as well as exploring the potential of various LDHs and other ion-responsive materials for broader application. By addressing these challenges, ions-responsive coatings have the potential to be-come a more universally applicable solution for corrosion protection across diverse environmental conditions.

 4.4. Light-responsive coatings

Light-responsive coatings represent a cutting-edge development in smart materials, especially within the sphere of corrosion protection for metals. These coatings are ingeniously designed to react to light exposure particularly UV light by activating photo-chemical reactions that lead to the release of encapsulated corrosion inhibitors. This innovative approach not only promises enhanced corrosion resistance but also introduces the potential for self-repairing capabilities in the coatings. One of the primary benefits of light-responsive coatings lies in their ability to provide remote, targeted activation and a rapid response to environmental changes. The incorporation of photocatalytic substances like TiO2 into these coatings leverages the alteration in electron density under UV light, ingeniously modulating the corrosion protection efficiency. This feature allows for the on-demand release of inhibitors, precisely when the metal surface encounters corrosive threats, thereby offering a proactive approach to corrosion protection. Furthermore, the versatility of these coatings, demonstrated by their application in dual light and thermal responsive systems, showcases the breadth of their potential utility beyond mere corrosion resistance, encompassing self-repair and even scratch repair functionalities.

However, the deployment of light-responsive coatings also comes with its set of challenges. The reliance on specific wavelengths of light to trigger the release mechanisms might limit the coatings' effectiveness in environments with limited light exposure. Additionally, the complexity involved in integrating photocatalytic substances and ensuring their uniform distribution within the coating matrix poses significant technical hurdles. Moreover, the durability of these light-triggered mechanisms under continuous or pro-longed exposure to environmental conditions remains a concern, potentially affecting the long-term reliability of the corrosion protection offered. Compared to existing literature, light-responsive coatings mark a notable advancement by introducing an innovative mechanism for corrosion protection that extends beyond the passive barrier methods traditionally employed. These coatings' dynamic adaptability and self-healing capabilities represent a significant leap forward. However, addressing the challenges related to light dependence, material integration, and long-term durability is crucial for optimizing these smart coatings' performance. As research in this area progresses, further innovations in material science and coating technology are expected to enhance the practical applicability and effectiveness of light-responsive coatings in various industrial and environmental settings.

4.5. Thermo-responsive coatings

Thermo-responsive coatings, leveraging shape memory polymers (SMPs) and photo-thermal materials like graphene oxide, represent a significant leap forward in corrosion protection technologies. These innovative coatings are designed to respond to temperature changes, enabling them to repair damage autonomously, which is crucial for protecting metal substrates from corrosion. The primary advantage of thermo-responsive coatings is their ability to self-heal. Utilizing SMPs that can revert to their original shape upon heating allows these coatings to close cracks and repair damage without manual intervention, thereby preserving the integrity of the underlying metal. The incorporation of materials with photothermal conversion capabilities, such as graphene oxide, further enhances these coatings by enabling rapid self-healing upon exposure to specific light wavelengths. This dual mechanism of action thermal responsiveness for healing and photothermal ef-fects for rapid activation—provides a robust defense against corrosion. Furthermore, the addition of thermoplastic fillers to SMPs has shown to improve the healing efficiency, offering a 'close-then-heal' approach that extends the coatings' self-healing capability to larger defects.

However, several challenges accompany thermo-responsive coatings. Their effective-ness heavily relies on the specific materials used, their inhibitor loading capacity, and the precision in activating the self-healing process. Achieving uniform dispersion of micro/nanocontainers within the SMP matrix and ensuring their stable integration without compromising the coating's physical properties are critical yet complex tasks. Additional-ly, the dependency on external heat sources to trigger the healing process may limit the applicability of these coatings in environments where such control is not feasible. Com-pared to existing literature, thermo-responsive coatings offer a more dynamic and adaptable solution for corrosion protection. The innovative 'close-then-heal' strategy, combined with the integration of micro/nanocontainers, sets them apart from traditional corrosion protection methods, which are typically passive and do not possess self-healing capabilities. However, the need for further research to optimize the materials used, improve the efficiency of the healing process, and expand the applicability of these coatings is evident. Future developments in this field are expected to address these challenges, enhancing the performance of thermo-responsive coatings and broadening their use in various environ-mental conditions and applications.

4.6. Magnetic-field responsive coatings

Magnetic-field responsive coatings represent an innovative approach in the domain of corrosion protection, mirroring strategies from drug delivery systems to harness magnetic nanoparticles for targeted delivery and release of active substances. These advanced coatings are designed to respond to environmental changes, deploying encapsulated active agents to form a protective barrier over metal surfaces, thereby offering both active corrosion protection and self-repair capabilities. The primary advantage of magnetic-field responsive coatings lies in the precise control and targeted delivery of corrosion inhibitors. By leveraging magnetic nanoparticles, these coatings enable the manipulation of mi-cro/nanocontainers' location and orientation within the coating matrix through an external magnetic field. This specificity ensures that corrosion protection is not just effective but also efficient, focusing on areas most in need of repair or protection. Magnetic nanoparticles are celebrated for their high chemical stability, low toxicity, and ease of functionalization, making them an ideal choice for such applications. The ability to quickly maneuver magnetic microcapsules under a magnetic field shortens the transmission pathway of re-leased active substances, enhancing the coating's protective efficiency and extending its service life.

Despite their promising potential, magnetic-field responsive coatings face challenges that limit their widespread application. The requirement for an external magnetic field to activate the release mechanism may not always be practical or feasible in all operational environments. Moreover, ensuring uniform dispersion of magnetic micro/nanocontainers within the coating matrix to prevent agglomeration and maintain the integrity of the protective layer poses significant technical challenges. Additionally, the cost and complexity of integrating magnetic nanoparticles into coatings can be higher compared to non-magnetic systems, potentially hindering their adoption in cost-sensitive industries. Com-pared to existing literature, magnetic-field responsive coatings offer a more dynamic and targeted approach to corrosion protection, surpassing traditional methods that act as passive barriers. However, the practicality of applying external magnetic fields and the cost implications of using magnetic nanoparticles necessitate further research and development. Future innovations are expected to focus on overcoming these limitations, enhancing the practical applicability of magnetic-field responsive coatings, and broadening their use across various industries exposed to corrosive environments.

4.7. Multi-stimulus-responsive coatings

Multi-stimulus-responsive coatings are at the forefront of smart material technology, offering unparalleled advantages in corrosion protection by responding to various environmental triggers. These coatings, capable of reacting to changes in pH, temperature, and redox conditions, represent a significant leap from traditional single stimulus-responsive systems. Their development has been propelled by the need to address the multifaceted nature of real-world corrosive environments, leading to the innovation of coatings that offer active protection and self-healing capabilities through the integration of various responsive elements. The primary benefit of multi-stimulus-responsive coatings is their versatility. They can adapt to a wide range of environmental changes, offering targeted and efficient corrosion protection. For instance, coatings that incorporate hybrid nanotubes responsive to pH, temperature, and redox changes can demonstrate self-healing effects, thereby significantly extending the lifespan of metal substrates. Furthermore, the integration of conducting polymers and supramolecular valves into these coatings enhances their responsive nature, allowing for the controlled release of inhibitors and improving the coatings' self-healing capabilities. This intelligent design ensures that the coatings can provide superior active corrosion protection, adapting dynamically to varying conditions.

However, the development of these advanced coatings faces several challenges. The fabrication process of multi-stimulus-responsive micro/nanocontainers is complex and of-ten involves multi-step synthesis procedures, making the production process labor-intensive and potentially costly. Additionally, ensuring the compatibility of different responsive elements within a single coating matrix, without compromising the individual responsiveness of each element, presents a significant challenge. There's also the task of precisely controlling the release of encapsulated agents in response to multiple stimuli, which requires sophisticated design and testing to achieve optimal performance. Com-pared to existing literature, multi-stimulus-responsive coatings mark a significant advancement by offering a more comprehensive approach to corrosion protection. While past research has made considerable strides in single-stimulus systems, the dynamic and complex nature of real-world environments necessitates coatings that can respond to multiple triggers. Future research in this field should aim not only at innovating new types of stimulus-responsive materials but also at refining the synthesis processes to make the production of these advanced coatings more practical and scalable. By addressing these challenges, multi-stimulus-responsive coatings have the potential to revolutionize corrosion protection across various industries, providing a smarter, more adaptable, and efficient solution to metal preservation.

5. Application of the micro/nanocontainers in functional coatings

5.1. Self-reporting and self-healing coatings

The integration of micro/nanocontainers into functional coatings signifies a substan-tial progression in materials science, offering innovative solutions for corrosion protection and beyond. These micro/nanocontainers are engineered to release active agents in re-sponse to specific environmental stimuli, thereby enhancing the durability and function-ality of coatings. One of the primary advantages of utilizing micro/nanocontainers in functional coatings is their ability to provide active and targeted corrosion protection. The stimulus-responsive release mechanism ensures that protective agents are dispensed pre-cisely when needed, minimizing waste and maximizing efficiency. Furthermore, the po-tential for multifunctionality, depending on the encapsulated substances, broadens the scope of applications for these coatings. From self-reporting capabilities for maintenance monitoring to antibacterial properties for maintaining hygienic surfaces, these intelligent coatings offer a versatile platform for addressing a wide array of material challenges.

However, there are challenges to the widespread application of these coatings. The effective dispersion of micro/nanocontainers within the coating matrix and ensuring their stability and compatibility are critical yet complex tasks. Additionally, the precise control over the stimulus-responsive release behavior to ensure long-term effectiveness poses a significant challenge. Moreover, the complexity involved in creating these sophisticated coatings can lead to increased production costs and complexities in scaling up for industrial applications. Compared to traditional coatings that act primarily as physical barriers, micro/nanocontainer-based functional coatings represent a leap forward by offering self-repairing and active protection functionalities. Previous studies have largely focused on single-stimulus systems, but the complexity of real-world applications demands coatings that can respond to multiple environmental factors. While significant advancements have been made, further research is needed to optimize the encapsulation and release mecha-nisms, improve the compatibility of containers with various coating matrices, and devel-op cost-effective manufacturing processes. Self-reporting and self-healing coatings, in particular, illustrate the innovative direction in which material science is headed, offering solutions that not only protect materials but also signal when repair is needed. These coatings incorporate indicators that change color or fluoresce in response to corrosion, providing an early warning system for maintenance. Yet, the application-specific nature of these indicators, their stability, and the reversibility of their signaling mechanisms present areas for further exploration to enhance their effectiveness and reliability. In summary, the ap-plication of micro/nanocontainers in functional coatings is a promising area of development in material science, offering smart solutions for corrosion protection and beyond. Future research will likely focus on overcoming current limitations and expanding the functional capabilities of these coatings to meet the diverse needs of various industries.

5.2. Anti-microbial and anti-fouling coatings

The development and application of anti-microbial and anti-fouling coatings for marine environments mark a significant advancement in combating microbial corrosion and biofouling, which are major causes of metal deterioration. These innovative coatings integrate anticorrosive properties with anti-microbial and anti-fouling capabilities, offering a comprehensive approach to protect metallic structures. The encapsulation of active agents within nanocapsules enables targeted release, enhancing the effectiveness of these coatings. The use of advanced materials like microcapsules with dual-function shells, ZIF-8 nanoparticles, and environmentally friendly polymers like chitosan reflects the potential of smart coatings to dynamically respond to environmental changes. Additionally, the shift towards bio-based materials emphasizes the industry's move towards environmentally sustainable solutions, reducing the ecological impact associated with traditional coatings.

However, the deployment of these coatings faces several challenges, including concerns about the environmental impact of releasing anti-microbial substances, the complexity of optimizing coating performance, and the potential increase in manufacturing costs due to the sophisticated design of these multi-functional coatings. Ensuring the long-term stability and effectiveness of these coatings in harsh marine conditions remains a critical area for further research and development. Despite these challenges, the advancements in anti-microbial and anti-fouling coatings signify a promising direction for protecting marine metal structures, aligning with sustainability goals and opening new possibilities for application in various sectors beyond marine environments. Future efforts will likely focus on addressing current limitations and expanding the scope of these innovative coatings.

5.3. Self-lubrication coatings

Self-lubrication coatings are a pivotal innovation in enhancing mechanical systems' efficiency and service life, drawing from bionic designs to create composites with automatic lubrication capabilities. Utilizing materials like polyetheretherketone and poly (melamine–formaldehyde) for their formability and processability, these coatings significantly reduce friction and wear rates, contributing to the improved tribological performance of various applications. The incorporation of nanosized lubricant capsules within an epoxy resin matrix, as demonstrated in early research, highlights the effectiveness of these coatings in evenly distributing lubricants across friction surfaces. This innovation not only extends the durability of mechanical components but also introduces the potential for multifunctionality, including self-healing and corrosion inhibition properties, thus broadening the scope of applications for these intelligent coatings.

However, the complexity of developing these composite materials and ensuring their mechanical performance poses challenges, including the need for precise control over capsule size and distribution. Additionally, the integration of additional functionalities, such as self-reporting and enhanced lubricating agents, while promising, increases the sophistication and potentially the cost of these coatings. Despite these challenges, the shift towards self-lubrication coatings marks a significant advancement over traditional lubrication methods, offering a more sustainable and efficient solution. Future research will likely focus on optimizing these coatings' material compositions and functionalities, aim-ing to reduce production costs and expand their applicability across various industries, thus contributing to the development of safer and more sustainable industrial applications.

Author Response File: Author Response.pdf

Reviewer 2 Report

The study reviews the self-healing coatings with a special focus on the micro/nano container technology. Synthesis and encapsulation processes of different micro/nanocontainers are also discussed. Although the study seems interesting, I have found some critical concerns while reviewing the article:

(1)    The research gaps in the existing literature are missing. For instance, are there any review articles on this topic? If so, what are the gaps that this study is addressing?

(2)   The authors failed to thoroughly discuss the advantages and disadvantages of the proposed methodology when compared with existing literature.

(3)   The methodology implemented to conduct the review process is missing. What is the inclusion and exclusion criteria?

(4)    The quality of the images should be improved. The text in some of the images is pixelated. The captions for the images and tables are extremely long.

Author Response

Respond to comments provided by Reviewer 2

The study reviews the self-healing coatings with a special focus on the micro/nano container technology. Synthesis and encapsulation processes of different micro/nanocontainers are also discussed. Although the study seems interesting, I have found some critical concerns while reviewing the article:

The authors express their profound appreciation to the reviewer for their astute and constructive feedback, which significantly contributed to the enhancement of the manuscript. Their expert advice was pivotal in improving the clarity and readability of the document, rendering it more comprehensible for a broader audience. Through careful consideration and integration of the reviewer's detailed observations regarding their research and methods, the authors have successfully refined the manuscript into a more concise and thorough scholarly work. This polished rendition not only highlights the extensive research undertaken but also facilitates future replication and experimentation by other researchers, thereby promoting continuous scientific inquiry and advancement in the field.

• The research gaps in the existing literature are missing. For instance, are there any review articles on this topic? If so, what are the gaps that this study is addressing?

Based on the insightful feedback from the reviewer, this study meticulously identifies and comprehensively addresses the specific research gaps that have been previously overlooked or inadequately explored in the current body of literature. By pinpointing these gaps, our research aims to fill the existing voids with empirical evidence and theoretical insights, thereby contributing to a more thorough understanding of the subject matter.

  Advantages of Smart Self-Healing Coatings:

The introduction of smart self-healing coatings represents a significant advancement in corrosion protection technologies. These coatings, which can be intrinsic or extrinsic, offer autonomous repair capabilities that are not found in traditional coatings. Intrinsic coatings, which utilize reversible bonds, can heal themselves without any external intervention, providing a continuous repair mechanism that can significantly extend the lifespan of metal surfaces. On the other hand, extrinsic coatings rely on micro/nanocontainers filled with healing agents that are released in response to environmental triggers, such as the presence of moisture or a change in pH, to repair damage. This targeted release ensures that healing agents are only used when necessary, reducing waste and potentially lowering long-term maintenance costs. The development of stimuli-responsive self-healing coatings, particularly through advancements in micro/nanocontainer technology, enhances the sensitivity and functionality of these coatings. This allows for the creation of coatings that can respond to multiple stimuli, further improving their effectiveness. Additionally, the incorporation of multifunctional properties such as self-reporting and antimicrobial actions not only contributes to corrosion protection but also adds value by monitoring the health of the coating and preventing microbial-induced corrosion.

Research Gaps and Disadvantage and there Challenges:

Despite the promising advantages, there are several challenges and disadvantages associated with smart self-healing coatings that need to be addressed. One of the primary limitations is the complexity involved in synthesizing and encapsulating the micro/nanocontainers, which can be a costly and time-consuming process. This complexity also extends to the design of multi-stimulus-responsive systems, which require precise control over the material properties to ensure effective functionality. Another challenge is the scalability of these technologies; producing these advanced coatings on an industrial scale can be difficult, limiting their widespread application. The volume of healing agent that can be encapsulated within micro/nanocontainers is another critical factor that dictates the efficacy of the repair. There is a limit to how much agent can be stored and released, which may not be sufficient for repairing larger cracks or damage. Furthermore, while the addition of multifunctional traits increases the coatings' applicability, it also adds to the complexity and cost of the coating systems. These factors contribute to the current gap between laboratory-scale success and industrial-scale application, necessitating further research and development to overcome these hurdles and fully realize the potential of smart self-healing coatings in corrosion protection.

• The authors failed to thoroughly discuss the advantages and disadvantages of the proposed methodology when compared with existing literature.

In response to the insightful comments provided by the reviewers, the authors have undertaken a detailed examination of the proposed methodology. This includes a thorough discussion of both the advantages and disadvantages associated with the methodology, ensuring that each aspect is addressed separately and articulated with clarity. By doing so, the authors aim to provide a balanced and transparent evaluation of their approach, highlighting its potential benefits while also acknowledging any limitations or challenges that may arise. This comprehensive analysis serves to enhance the understanding of the methodology's applicability and efficacy in the context of the study's objectives

2.   Exploring the World of Micro/Nanocontainers: Key to Advanced Extrinsic Self-Healing Coatings

                        The advantages of utilizing micro/nanocontainers in extrinsic self-healing coatings are significant, presenting a transformative approach in corrosion protection technologies. These containers excel in precisely housing and dispensing healing agents, such as corrosion inhibitors, directly into the cracks formed in the coating. This targeted release mechanism, driven by capillary action and environmental stimuli, ensures an efficient and responsive healing process. The use of microcontainers, which are optimal in size, allows for the storage of ample healing agents and ensures their rupture under mechanical stress to initiate the healing process. On the other hand, nanocontainers are particularly suitable for carrying corrosion inhibitors due to their smaller size, enhancing the coatings' protective capabilities. The separation provided by these containers between the inhibitors and the coating matrix is crucial for maintaining the coating's integrity and ensuring its long-term corrosion resistance. The diversity in container materials ranging from organic, in-organic, to hybrid offers a wide array of properties, including biocompatibility, robust-ness, and environmental friendliness, further broadening the applicability of these advanced coatings in various industrial sectors.

                        However, the methodology also presents certain disadvantages when compared to existing literature and technologies. The complexity and cost of synthesizing micro/nanocontainers can be significant, potentially limiting their widespread application. The precise control over the size, shape, and permeability of these containers, crucial for their optimal performance, requires advanced and sometimes costly fabrication techniques. Additionally, the long-term stability and environmental impact of these containers, especially those made from non-biodegradable materials, remain concerns that need further investigation. The interaction between the encapsulated agents and the polymer matrix during the healing process, as well as the potential for incomplete healing or re-lease of agents, are challenges that necessitate ongoing research to fully understand and mitigate. Despite these drawbacks, the innovative approach of micro/nanocontainers in self-healing coatings represents a promising direction in corrosion protection technology, warranting further development and optimization to overcome these limitations.

          2.1. Organic Micro/Nanocontainers: A Vital Component in Self-Healing Materials

Advantage on organic micro/nanocontainers have revolutionized the field of self-healing materials by offering high load capacity for active healing agents, which significantly enhances the efficiency of corrosion protection and material repair. Their compatibility with various polymer matrices and the ability to protect encapsulated substances from harsh environmental conditions stand out as critical benefits. This compatibility ensures that the healing agents remain potent and ready to act upon the occurrence of dam-age. The innovation in the synthesis and application of these containers, including methods like in-situ polymerization and Pickering emulsion, contributes to the precise control over encapsulation, enabling a consistent and effective healing response. Furthermore, the development of covalent organic frameworks (COFs) as a new class of micro/nanocontainers has opened up possibilities for enhanced dispersion and compatibility within organic matrices, further broadening the scope and efficiency of self-healing materials.

       Despite these advancements, the complexity of creating organic mi-cro/nanocontainers poses significant challenges. The multifaceted process involving polymerization, encapsulation of the active agent, and the subsequent removal of by-products and solvents can be resource-intensive and technically demanding. This complexity may limit the scalability and cost-effectiveness of producing these self-healing materials for broader industrial applications. Additionally, achieving the optimal balance be-tween the mechanical strength and the responsive rupture of the containers remains a critical challenge. Containers that are too robust may fail to release their healing agents when needed, while those too fragile may rupture prematurely or fail to withstand the operational environment, thus compromising the healing efficacy.

                        In comparison to existing literature, these methodologies underscore a significant leap in the targeted delivery and controlled release of healing agents, offering a more responsive and efficient approach to corrosion protection and material repair. However, the challenges highlight the need for ongoing research to refine synthesis techniques, improve cost-efficiency, and ensure the practical applicability of these advanced materials in real-world conditions. The continuous evolution of synthesis methods and the exploration of new materials like COFs and graphene oxide (GO) sheet hybrids for containers suggest a promising trajectory for overcoming these hurdles, pushing the boundaries of self-healing technology towards more sophisticated and widely applicable solutions.

          2.2 Inorganic micro/nanocontainers, focusing on their structure, applications, and limitations

Inorganic micro/nanocontainers offer a significant leap forward in the development of self-healing materials, particularly in the context of corrosion protection. These containers distinguish themselves through their stable, small, cavity-like structures capable of en-capsulating and subsequently releasing active agents like corrosion inhibitors in response to specific environmental triggers. This innovative approach facilitates the enhancement of thermomechanical properties and barrier performance of coatings, addressing the critical need for durable and efficient corrosion protection systems. The major advantages on the primary benefits of utilizing inorganic micro/nanocontainers include their enhanced stability and ability to undergo surface modification, which ensures compatibility and controlled release of active agents. This feature is pivotal in applications requiring precise targeting and release of functional species to mitigate corrosion effectively. Inorganic con-tainers like titanium dioxide and mesoporous silica simplify the encapsulation process and are inherently more stable, offering broad coverage and an easy encapsulation process. Their design for responding to external microenvironments leads to structural changes that help in the efficient release of functional species into targeted areas, thus significantly improving the self-healing capabilities of materials.

The disadvantages with several challenges accompany the use of inorganic micro/nanocontainers. One of the most notable drawbacks is their relatively poor compatibility with polymer materials, which can lead to aggregation and the formation of micro defects within the coating matrix, potentially accelerating damage. Furthermore, the capacity of these containers to hold active agents is generally limited (usually less than 20%), which might constrain their effectiveness in providing long-term corrosion protection. Additionally, the manufacturing process, predominantly the sol-gel method, while mild and conducted at room temperature, may result in containers with limited compactness and reduced capacity for active agents. When compared to organic micro/nanocontainers, inorganic variants offer a contrasting profile of advantages and limitations. Organic containers are praised for their high compatibility with various polymer matrices and efficient protection against environmental degradation, whereas inorganic containers excel in stability and ease of encapsulation but struggle with compatibility issues and limited active agent capacity. Despite these challenges, the unique properties of inorganic micro/nanocontainers, such as their ability to enhance the thermomechanical properties and barrier performance of self-healing systems, underscore their potential in advancing the field of corrosion protection and beyond. Addressing the limitations related to compatibility, agent capacity, and manufacturing complexities remains a critical area of focus for future research to fully leverage the advantages offered by inorganic micro/nanocontainers in self-healing materials.

2.3. Micro/nanocontainers, their composition, manufacturing methods, and applications

The methodology involving organic/inorganic hybrid micro/nanocontainers represents a significant advancement in the development of self-healing materials, blending the multifunctionality and adjustable properties of organic containers with the physicochemical stability and mechanical robustness of inorganic ones. This hybrid approach allows for the creation of containers that offer both stability and flexibility, a combination that is particularly valuable in applications requiring controlled release mechanisms under specific conditions, such as in corrosion protection, drug delivery, and environmental remediation. The major advantages is one of the key strengths of these hybrid mi-cro/nanocontainers lies in their versatile functionality, which can be fine-tuned through the layer-by-layer (LBL) assembly technique. This method enables the precise control over the thickness, composition, and molecular organization of the containers, allowing for the incorporation of various nanoparticles to achieve desired properties. For instance, the integration of Fe3O4 and graphene oxide into silica-polymer hybrid capsules enhances their structural stability and expands their application range. Moreover, these containers' responsiveness to environmental stimuli such as pH and temperature changes enables the smart release of encapsulated agents, making them highly effective in self-healing applications where such controlled release is crucial.

The disadvantages on the development of organic/inorganic hybrid containers is not without its challenges. The integration of inorganic components can sometimes result in poor compatibility with polymer matrices, leading to potential aggregation issues. Moreover, the complexity of creating these hybrid containers, which involves sophisticated synthesis techniques like the LBL assembly, may pose scalability and cost-effectiveness concerns. Additionally, the performance of these containers heavily relies on the organic components' behavior within the hybrid, requiring careful consideration of dispersion within the coating to ensure optimal functionality. Compared to the existing literature, the proposed methodology of utilizing organic/inorganic hybrid micro/nanocontainers introduces a novel approach that leverages the synergistic effects of combining organic and in-organic materials. This results in containers with enhanced mechanical properties, chemical resistance, and multifunctionality over purely organic or inorganic containers. How-ever, addressing the aforementioned compatibility and manufacturing challenges is essential for fully realizing the potential of these advanced materials in practical applications. As research in this field progresses, further innovations and refinements in synthesis techniques and material selection are expected to overcome current limitations, paving the way for broader industrial adoption of these sophisticated self-healing systems.

3. Exploring the Dynamic Release Patterns of Encapsulated Agents from Micro and Nanocontainers

The methodology surrounding the encapsulation and release of active agents from micro/nanocontainers is pivotal in the advancement of materials science, especially for creating protective coatings against corrosion. This process, which leverages both physical and chemical interactions for encapsulation, is designed to release corrosion inhibitors precisely when and where needed, upon exposure to specific external stimuli. This strategic release mechanism aims to halt or decelerate the corrosion process effectively. However, the efficiency of this encapsulation and subsequent release is subject to various influencing factors such as the containers' loading capacity, dispersion within the coating, particle size, solubility of inhibitors, and the nature of the corrosion inhibitors themselves. These elements collectively dictate the release kinetics, which is crucial for the timely and effective deployment of inhibitors to combat corrosion.

The advantages on the use of micro/nanocontainers for encapsulating active agents presents a sophisticated approach to corrosion protection, allowing for a controlled and targeted release of inhibitors. This methodology benefits from employing several kinetic models (e.g., Zero-order, First-order, Higuchi, Hixson-Crowell, Korsmeyer-Peppas, and Hopfenberg models) to predict and understand the release behavior of encapsulated sub-stances, catering to various scenarios and encapsulated substance types. Such models are instrumental in optimizing release kinetics, ensuring that inhibitors are effectively deployed to prevent corrosion.

Despite the apparent benefits, challenges remain in accurately predicting and optimizing the release kinetics due to the complexity of the encapsulation and release processes. These processes involve multiple steps, each characterized by different and intricate physicochemical reactions, making it difficult to model the release behavior accurate-ly. Additionally, while kinetic models like the Korsmeyer-Peppas offer valuable insights into release phenomena, the reliance on R² values for model fit assessment can be misleading. This is because R² values tend to increase with the addition of more parameters, which may not always be relevant. To address this, the adjusted correlation coefficient (R² adjusted) is recommended for a more accurate representation of a model's applicability.

Compared to existing literature, this methodology underscores the intricate balance required in designing micro/nanocontainers for corrosion protection. The dynamic release patterns of encapsulated agents from these containers need to be thoroughly understood and optimized for the effective prevention of corrosion. While the models provide a foundation for predicting release kinetics, ongoing research and development are crucial to re-fine these models further, enhance the precision of release mechanisms, and overcome the limitations identified in current methodologies. This evolution is essential for advancing the field of protective coatings and developing new systems with more precise and con-trolled release behaviors, ensuring robust corrosion protection in diverse environments.

4. Stimulus-responsive coatings represent an advancement in material science

4.1. pH-responsive coatings

Stimulus-responsive coatings, particularly those incorporating micro/nanocontainers, represent a significant leap forward in the realm of material science, offering a dynamic approach to combating metal corrosion compared to traditional anti-corrosive coatings. These smart materials can adapt their physicochemical properties in response to environmental changes, providing targeted corrosion protection through the controlled release of encapsulated inhibitors. The major Advantages strength of these coatings lies in their ability to offer active corrosion protection. Unlike conventional coatings that serve merely as physical barriers, stimulus-responsive coatings can detect changes in environmental conditions (such as pH, temperature, and ions) and release corrosion inhibitors precisely where needed. This intelligent response system, particularly seen in pH-responsive coatings, leverages fluctuations in local pH levels due to corrosion processes to trigger the release of inhibitors, ensuring effective and localized protection against corrosion. This approach not only enhances the longevity of the metal substrates but also offers a more sustainable solution by minimizing the need for frequent reapplication or repair.

Despite the promising advancements, several disadvantages: remain. One of the primary concerns is achieving precise control over the stimuli-responsive release behavior, which is crucial for long-term effectiveness. Factors such as the compatibility and dispersion of micro/nanocontainers within the polymer matrix, and the variability in triggering pH values for different metals in diverse environments, can significantly impact performance. Additionally, the potential for uncontrolled release due to external pH fluctuations and the temporary nature of the self-healing effect presented by reversible release mechanisms highlight areas that require further research and development. Compared with existing literature, stimulus-responsive coatings offer an innovative approach to corrosion protection by integrating the benefits of both organic and inorganic materials through micro/nanocontainers. However, optimizing these coatings for a wider range of applications and environmental conditions remains a critical focus for ongoing research. The challenge lies in balancing the smart responsiveness of the coatings with practical considerations such as compatibility, longevity, and environmental impact. As the field progresses, it is expected that advancements in material science and coating technology will address these limitations, broadening the applicability and effectiveness of stimulus-responsive coatings in metal corrosion protection and beyond.

4.2. Redox-responsive coatings represent a sophisticated approach in the field of smart materials, particularly in the context of corrosion protection

Redox-responsive coatings represent an innovative advancement in smart material science, particularly for corrosion protection. These coatings are designed to react to changes in electrochemical potential, commonly associated with the corrosion process. The deployment of conducting polymers within these coatings, notable for their reversible redox properties, marks a significant stride in materials technology. These polymers, when reduced, facilitate the release of encapsulated healing agents, thus enabling a self-healing function that is critically responsive to corrosive environments.

The main advantage of redox-responsive coatings lies in their dynamic adaptability to the corrosive environment, offering a sophisticated, targeted approach to corrosion protection. By utilizing conducting polymers that undergo reversible redox reactions, these coatings can effectively control the release of encapsulated substances in response to specific electrochemical stimuli. This ensures that healing agents are released precisely where and when needed, directly at the sites of potential or active corrosion, enhancing the durability and lifespan of metal substrates. Moreover, the implementation of redox-sensitive nanovalves on the surfaces of micro/nanocontainers within these coatings has further bolstered their efficacy. These nanovalves, capable of controlling the release of encapsulated agents through reversible transitions under redox stimuli, represent a cutting-edge approach in the design of intelligent, stimulus-responsive self-healing coatings.

Despite these advancements, there are notable challenges to the widespread application of redox-responsive coatings. One significant issue is the complexity involved in engineering these materials to ensure precise control over the redox-triggered release mechanism. Additionally, the compatibility and dispersion of these micro/nanocontainers with-in the polymer matrix of the coatings must be carefully managed to prevent aggregation and ensure effective barrier properties. Furthermore, the specificity of the redox triggers and the potential for uncontrolled release under fluctuating electrochemical conditions present additional hurdles to achieving consistent, long-term corrosion protection. Com-pared to existing literature, redox-responsive coatings offer a more dynamic and adaptable solution to corrosion protection, moving beyond the passive barrier mechanisms of traditional anti-corrosive coatings. However, the challenges highlighted underscore the need for continued research and development. Addressing these issues is crucial for optimizing the performance of redox-responsive coatings and expanding their application across a broader range of environmental conditions and metal substrates. As the field progresses, it is anticipated that innovations in material synthesis, nanovalve design, and polymer technology will overcome current limitations, paving the way for more efficient and durable protective coatings.

4.3. Ions-responsive coatings represent a novel class of smart materials that are tailored to respond to specific aggressive ions present in corrosive environment

Ions-responsive coatings mark a notable advancement in smart material science, specifically designed to counteract the effects of aggressive ions in corrosive environments. These coatings employ ion-responsive micro/nanocontainers capable of swiftly releasing active inhibitors to mend damaged areas on the surface. This capability is especially crucial for addressing microbiologically influenced corrosion, commonly seen in marine set-tings. Through the incorporation of innovations such as ZIF-8 nanovalves on mesoporous silica nanocontainers (MSNs), these coatings can detect and respond to specific ion fluctuations, such as S2- ions associated with sulfate-reducing bacteria activity, a prevalent cause of microbiological corrosion in marine environments. The primary advantage of ions-responsive coatings lies in their targeted response to specific ions, allowing for the direct and efficient mitigation of corrosion processes. This specificity is particularly valuable in environments like marine settings, where microbiologically influenced corrosion poses a significant threat. The integration of innovative materials like LDHs, with their unique layered structure and ion-exchange capabilities, further enhances these coatings' ability to trap corrosive ions while releasing protective inhibitors, thus providing a double-action protective mechanism against corrosion.

However, the application of ions-responsive coatings is not without its limitations. Their effectiveness is heavily reliant on the presence of specific ions, narrowing their applicability to environments where such ions are prevalent. Additionally, the sensitivity of these coatings to detect and respond to specific ion concentrations requires further improvement to ensure consistent and reliable corrosion protection. Moreover, while LDHs themselves offer some level of protection, the exploration of their synergistic effects with encapsulated inhibitors needs more extensive research to optimize the coatings' overall protective capabilities. Compared to existing literature, ions-responsive coatings represent a promising yet relatively niche solution to corrosion protection. The need for specific environmental conditions and the challenge in achieving broad-spectrum applicability highlight the importance of ongoing research. Future developments should focus on enhancing the sensitivity and range of ions these coatings can respond to, as well as exploring the potential of various LDHs and other ion-responsive materials for broader application. By addressing these challenges, ions-responsive coatings have the potential to be-come a more universally applicable solution for corrosion protection across diverse environmental conditions.

 4.4. Light-responsive coatings

Light-responsive coatings represent a cutting-edge development in smart materials, especially within the sphere of corrosion protection for metals. These coatings are ingeniously designed to react to light exposure particularly UV light by activating photo-chemical reactions that lead to the release of encapsulated corrosion inhibitors. This innovative approach not only promises enhanced corrosion resistance but also introduces the potential for self-repairing capabilities in the coatings. One of the primary benefits of light-responsive coatings lies in their ability to provide remote, targeted activation and a rapid response to environmental changes. The incorporation of photocatalytic substances like TiO2 into these coatings leverages the alteration in electron density under UV light, ingeniously modulating the corrosion protection efficiency. This feature allows for the on-demand release of inhibitors, precisely when the metal surface encounters corrosive threats, thereby offering a proactive approach to corrosion protection. Furthermore, the versatility of these coatings, demonstrated by their application in dual light and thermal responsive systems, showcases the breadth of their potential utility beyond mere corrosion resistance, encompassing self-repair and even scratch repair functionalities.

However, the deployment of light-responsive coatings also comes with its set of challenges. The reliance on specific wavelengths of light to trigger the release mechanisms might limit the coatings' effectiveness in environments with limited light exposure. Additionally, the complexity involved in integrating photocatalytic substances and ensuring their uniform distribution within the coating matrix poses significant technical hurdles. Moreover, the durability of these light-triggered mechanisms under continuous or pro-longed exposure to environmental conditions remains a concern, potentially affecting the long-term reliability of the corrosion protection offered. Compared to existing literature, light-responsive coatings mark a notable advancement by introducing an innovative mechanism for corrosion protection that extends beyond the passive barrier methods traditionally employed. These coatings' dynamic adaptability and self-healing capabilities represent a significant leap forward. However, addressing the challenges related to light dependence, material integration, and long-term durability is crucial for optimizing these smart coatings' performance. As research in this area progresses, further innovations in material science and coating technology are expected to enhance the practical applicability and effectiveness of light-responsive coatings in various industrial and environmental settings.

4.5. Thermo-responsive coatings

Thermo-responsive coatings, leveraging shape memory polymers (SMPs) and photo-thermal materials like graphene oxide, represent a significant leap forward in corrosion protection technologies. These innovative coatings are designed to respond to temperature changes, enabling them to repair damage autonomously, which is crucial for protecting metal substrates from corrosion. The primary advantage of thermo-responsive coatings is their ability to self-heal. Utilizing SMPs that can revert to their original shape upon heating allows these coatings to close cracks and repair damage without manual intervention, thereby preserving the integrity of the underlying metal. The incorporation of materials with photothermal conversion capabilities, such as graphene oxide, further enhances these coatings by enabling rapid self-healing upon exposure to specific light wavelengths. This dual mechanism of action thermal responsiveness for healing and photothermal ef-fects for rapid activation—provides a robust defense against corrosion. Furthermore, the addition of thermoplastic fillers to SMPs has shown to improve the healing efficiency, offering a 'close-then-heal' approach that extends the coatings' self-healing capability to larger defects.

However, several challenges accompany thermo-responsive coatings. Their effective-ness heavily relies on the specific materials used, their inhibitor loading capacity, and the precision in activating the self-healing process. Achieving uniform dispersion of micro/nanocontainers within the SMP matrix and ensuring their stable integration without compromising the coating's physical properties are critical yet complex tasks. Additional-ly, the dependency on external heat sources to trigger the healing process may limit the applicability of these coatings in environments where such control is not feasible. Com-pared to existing literature, thermo-responsive coatings offer a more dynamic and adaptable solution for corrosion protection. The innovative 'close-then-heal' strategy, combined with the integration of micro/nanocontainers, sets them apart from traditional corrosion protection methods, which are typically passive and do not possess self-healing capabilities. However, the need for further research to optimize the materials used, improve the efficiency of the healing process, and expand the applicability of these coatings is evident. Future developments in this field are expected to address these challenges, enhancing the performance of thermo-responsive coatings and broadening their use in various environ-mental conditions and applications.

4.6. Magnetic-field responsive coatings

Magnetic-field responsive coatings represent an innovative approach in the domain of corrosion protection, mirroring strategies from drug delivery systems to harness magnetic nanoparticles for targeted delivery and release of active substances. These advanced coatings are designed to respond to environmental changes, deploying encapsulated active agents to form a protective barrier over metal surfaces, thereby offering both active corrosion protection and self-repair capabilities. The primary advantage of magnetic-field responsive coatings lies in the precise control and targeted delivery of corrosion inhibitors. By leveraging magnetic nanoparticles, these coatings enable the manipulation of mi-cro/nanocontainers' location and orientation within the coating matrix through an external magnetic field. This specificity ensures that corrosion protection is not just effective but also efficient, focusing on areas most in need of repair or protection. Magnetic nanoparticles are celebrated for their high chemical stability, low toxicity, and ease of functionalization, making them an ideal choice for such applications. The ability to quickly maneuver magnetic microcapsules under a magnetic field shortens the transmission pathway of re-leased active substances, enhancing the coating's protective efficiency and extending its service life.

Despite their promising potential, magnetic-field responsive coatings face challenges that limit their widespread application. The requirement for an external magnetic field to activate the release mechanism may not always be practical or feasible in all operational environments. Moreover, ensuring uniform dispersion of magnetic micro/nanocontainers within the coating matrix to prevent agglomeration and maintain the integrity of the protective layer poses significant technical challenges. Additionally, the cost and complexity of integrating magnetic nanoparticles into coatings can be higher compared to non-magnetic systems, potentially hindering their adoption in cost-sensitive industries. Com-pared to existing literature, magnetic-field responsive coatings offer a more dynamic and targeted approach to corrosion protection, surpassing traditional methods that act as passive barriers. However, the practicality of applying external magnetic fields and the cost implications of using magnetic nanoparticles necessitate further research and development. Future innovations are expected to focus on overcoming these limitations, enhancing the practical applicability of magnetic-field responsive coatings, and broadening their use across various industries exposed to corrosive environments.

4.7. Multi-stimulus-responsive coatings

Multi-stimulus-responsive coatings are at the forefront of smart material technology, offering unparalleled advantages in corrosion protection by responding to various environmental triggers. These coatings, capable of reacting to changes in pH, temperature, and redox conditions, represent a significant leap from traditional single stimulus-responsive systems. Their development has been propelled by the need to address the multifaceted nature of real-world corrosive environments, leading to the innovation of coatings that offer active protection and self-healing capabilities through the integration of various responsive elements. The primary benefit of multi-stimulus-responsive coatings is their versatility. They can adapt to a wide range of environmental changes, offering targeted and efficient corrosion protection. For instance, coatings that incorporate hybrid nanotubes responsive to pH, temperature, and redox changes can demonstrate self-healing effects, thereby significantly extending the lifespan of metal substrates. Furthermore, the integration of conducting polymers and supramolecular valves into these coatings enhances their responsive nature, allowing for the controlled release of inhibitors and improving the coatings' self-healing capabilities. This intelligent design ensures that the coatings can provide superior active corrosion protection, adapting dynamically to varying conditions.

However, the development of these advanced coatings faces several challenges. The fabrication process of multi-stimulus-responsive micro/nanocontainers is complex and of-ten involves multi-step synthesis procedures, making the production process labor-intensive and potentially costly. Additionally, ensuring the compatibility of different responsive elements within a single coating matrix, without compromising the individual responsiveness of each element, presents a significant challenge. There's also the task of precisely controlling the release of encapsulated agents in response to multiple stimuli, which requires sophisticated design and testing to achieve optimal performance. Com-pared to existing literature, multi-stimulus-responsive coatings mark a significant advancement by offering a more comprehensive approach to corrosion protection. While past research has made considerable strides in single-stimulus systems, the dynamic and complex nature of real-world environments necessitates coatings that can respond to multiple triggers. Future research in this field should aim not only at innovating new types of stimulus-responsive materials but also at refining the synthesis processes to make the production of these advanced coatings more practical and scalable. By addressing these challenges, multi-stimulus-responsive coatings have the potential to revolutionize corrosion protection across various industries, providing a smarter, more adaptable, and efficient solution to metal preservation.

5. Application of the micro/nanocontainers in functional coatings

5.1. Self-reporting and self-healing coatings

The integration of micro/nanocontainers into functional coatings signifies a substan-tial progression in materials science, offering innovative solutions for corrosion protection and beyond. These micro/nanocontainers are engineered to release active agents in re-sponse to specific environmental stimuli, thereby enhancing the durability and function-ality of coatings. One of the primary advantages of utilizing micro/nanocontainers in functional coatings is their ability to provide active and targeted corrosion protection. The stimulus-responsive release mechanism ensures that protective agents are dispensed pre-cisely when needed, minimizing waste and maximizing efficiency. Furthermore, the po-tential for multifunctionality, depending on the encapsulated substances, broadens the scope of applications for these coatings. From self-reporting capabilities for maintenance monitoring to antibacterial properties for maintaining hygienic surfaces, these intelligent coatings offer a versatile platform for addressing a wide array of material challenges.

However, there are challenges to the widespread application of these coatings. The effective dispersion of micro/nanocontainers within the coating matrix and ensuring their stability and compatibility are critical yet complex tasks. Additionally, the precise control over the stimulus-responsive release behavior to ensure long-term effectiveness poses a significant challenge. Moreover, the complexity involved in creating these sophisticated coatings can lead to increased production costs and complexities in scaling up for industrial applications. Compared to traditional coatings that act primarily as physical barriers, micro/nanocontainer-based functional coatings represent a leap forward by offering self-repairing and active protection functionalities. Previous studies have largely focused on single-stimulus systems, but the complexity of real-world applications demands coatings that can respond to multiple environmental factors. While significant advancements have been made, further research is needed to optimize the encapsulation and release mecha-nisms, improve the compatibility of containers with various coating matrices, and devel-op cost-effective manufacturing processes. Self-reporting and self-healing coatings, in particular, illustrate the innovative direction in which material science is headed, offering solutions that not only protect materials but also signal when repair is needed. These coatings incorporate indicators that change color or fluoresce in response to corrosion, providing an early warning system for maintenance. Yet, the application-specific nature of these indicators, their stability, and the reversibility of their signaling mechanisms present areas for further exploration to enhance their effectiveness and reliability. In summary, the ap-plication of micro/nanocontainers in functional coatings is a promising area of development in material science, offering smart solutions for corrosion protection and beyond. Future research will likely focus on overcoming current limitations and expanding the functional capabilities of these coatings to meet the diverse needs of various industries.

5.2. Anti-microbial and anti-fouling coatings

The development and application of anti-microbial and anti-fouling coatings for marine environments mark a significant advancement in combating microbial corrosion and biofouling, which are major causes of metal deterioration. These innovative coatings integrate anticorrosive properties with anti-microbial and anti-fouling capabilities, offering a comprehensive approach to protect metallic structures. The encapsulation of active agents within nanocapsules enables targeted release, enhancing the effectiveness of these coatings. The use of advanced materials like microcapsules with dual-function shells, ZIF-8 nanoparticles, and environmentally friendly polymers like chitosan reflects the potential of smart coatings to dynamically respond to environmental changes. Additionally, the shift towards bio-based materials emphasizes the industry's move towards environmentally sustainable solutions, reducing the ecological impact associated with traditional coatings.

However, the deployment of these coatings faces several challenges, including concerns about the environmental impact of releasing anti-microbial substances, the complexity of optimizing coating performance, and the potential increase in manufacturing costs due to the sophisticated design of these multi-functional coatings. Ensuring the long-term stability and effectiveness of these coatings in harsh marine conditions remains a critical area for further research and development. Despite these challenges, the advancements in anti-microbial and anti-fouling coatings signify a promising direction for protecting marine metal structures, aligning with sustainability goals and opening new possibilities for application in various sectors beyond marine environments. Future efforts will likely focus on addressing current limitations and expanding the scope of these innovative coatings.

5.3. Self-lubrication coatings

Self-lubrication coatings are a pivotal innovation in enhancing mechanical systems' efficiency and service life, drawing from bionic designs to create composites with automatic lubrication capabilities. Utilizing materials like polyetheretherketone and poly (melamine–formaldehyde) for their formability and processability, these coatings significantly reduce friction and wear rates, contributing to the improved tribological performance of various applications. The incorporation of nanosized lubricant capsules within an epoxy resin matrix, as demonstrated in early research, highlights the effectiveness of these coatings in evenly distributing lubricants across friction surfaces. This innovation not only extends the durability of mechanical components but also introduces the potential for multifunctionality, including self-healing and corrosion inhibition properties, thus broadening the scope of applications for these intelligent coatings.

However, the complexity of developing these composite materials and ensuring their mechanical performance poses challenges, including the need for precise control over capsule size and distribution. Additionally, the integration of additional functionalities, such as self-reporting and enhanced lubricating agents, while promising, increases the sophistication and potentially the cost of these coatings. Despite these challenges, the shift towards self-lubrication coatings marks a significant advancement over traditional lubrication methods, offering a more sustainable and efficient solution. Future research will likely focus on optimizing these coatings' material compositions and functionalities, aim-ing to reduce production costs and expand their applicability across various industries, thus contributing to the development of safer and more sustainable industrial applications.

(3)   The methodology implemented to conduct the review process is missing. What is the inclusion and exclusion criteria?

Thank you for raising important questions regarding the methodology of our review process on smart self-healing coatings. Clarity in the research methodology, including detailed inclusion and exclusion criteria, is crucial for ensuring the reliability and reproducibility of our findings. Our review was carefully structured to encompass the most recent and relevant developments in smart self-healing coatings, with a special emphasis on micro/nanocontainer technology and its myriad applications. A thorough literature search was conducted across multiple scientific databases such as PubMed, Scopus, Web of Science, and Google Scholar, focusing on publications from the last decade. This approach ensured our review captured cutting-edge research in the field.

Our inclusion criteria were designed to hone in on studies directly related to the synthesis and application of micro/nanocontainers in self-healing coatings, particularly those offering innovative insights into the development of systems responsive to single or multiple stimuli. We sought research that not only detailed the integration of multifunctional traits like self-reporting, antimicrobial, anti-fouling, and self-lubrication into coatings but also those published within the last ten years to reflect the field's latest advancements. Conversely, our exclusion criteria filtered out studies that didn't directly contribute to our research scope, lacked empirical data, were not in English, or were inaccessible in full-text form. This methodological rigor allowed us to extract significant insights into the current and future landscape of smart self-healing coatings, identifying both advancements and areas ripe for further exploration. We acknowledge the oversight in not detailing this methodology in our initial submission and appreciate the opportunity to clarify our approach, aiming to provide a comprehensive overview that underscores the thoroughness of our review process.

(4)    The quality of the images should be improved. The text in some of the images is pixelated. The captions for the images and tables are extremely long.

Thank you for the feedback on image quality and the detailed nature of our captions. In response, we've enhanced all images to a high-quality resolution of 300 dpi to address the issue of pixelation and ensure that all text within the images is crisp and legible. Simultaneously, we've revised the captions for our images and tables, aiming for brevity while retaining the necessary information to make them informative yet succinct. These adjustments are made to improve the manuscript's clarity and presentation quality, aligning with the expectations for high-quality scholarly work. We hope these changes adequately address your concerns, contributing to the manuscript's overall effectiveness and readability.

Figure. 1. Illustrates various stimulus-responsive micro/nanocontainer coatings and their uses across fields, illustrating how they adapt to environmental changes like temperature or pH for applications in medicine, materials science, and environmental protection.

Figure 2. The diagram details two ways active agents are incorporated into coatings: (a) embedded directly in the coating for uniform functionality and (b) enclosed in mi-cro/nanocontainers for protection until released by stimuli like temperature or pH changes. This smart encapsulation allows for precise agent release, enabling applications in drug delivery, corrosion prevention, and self-healing materials.

Figure 3. The figure categorizes various synthesis techniques for micro and nanocontain-ers, crucial in nanotechnology and materials science for precision and control. Techniques range from self-assembly to complex methods like layer-by-layer assembly, sol-gel pro-cessing, and nano-precipitation, each chosen for specific advantages like size, permeabil-ity, and environmental responsiveness. The comparison highlights the best strategies for different applications, such as biomedical or industrial, guiding researchers and engineers in selecting the appropriate method.

Figure 4. Shows the creation of dual-stimuli-responsive microcapsules with multiple compartments. Starting with poly-N-isopropylacrylamide particles known for their tem-perature sensitivity, these are then combined with Nile Red, a fluorescent marker, to moni-tor encapsulation and release. The mixture is used as a Pickering emulsifier for a stable emulsion, forming the basis of the microcapsules. This enables them to react to both tem-perature changes and chemical triggers, allowing for precise content release, crucial for targeted drug delivery and smart materials

Figure 5. Illustrates the synthesis of microcapsules from bisphenol A cyanate ester and polyglycidyl methyacrylate using solvent evaporation. This process involves dissolving polymers in a volatile solvent, creating an emulsion, and then evaporating the solvent to form a polymer shell that encapsulates active agents. The figure highlights the capsules' morphology, showcasing spherical shapes with varying surface textures. The combina-tion of materials provides thermal stability, mechanical strength, and chemical resistance, making these microcapsules ideal for high-temperature industrial applications like con-trolled-release coatings. The figure explains the synthesis process and the characteristics of the microcapsules, emphasizing the factors that determine their structure and proper-ties.

Figure 6. Shows a schematic of the self-healing process in a Layer-by-Layer (LBL) polyelec-trolyte coating, designed for metal corrosion prevention. It details how localized pH changes from initial corrosion trigger the release of corrosion inhibitors (Inh) from layers of positively and negatively charged polyelectrolytes. These changes cause the polyelectro-lytes to reconfigure, releasing inhibitors that neutralize corrosive agents and repair the coating, restoring metal surface integrity. The figure highlights the coating's dynamic re-sponse to corrosion, demonstrating its application in durable metal protection across in-dustries.

Figure 7. Illustrates how intelligent coatings protect against microbiological corrosion, specifically from sulfate-reducing bacteria (SRB). It shows the process where sulfide ions from SRB activity break down ZIF-8 structures in the coating, a Metal-Organic Framework known for encapsulating substances. This breakdown releases biocides that diffuse through the coating to neutralize SRBs on the metal surface, preventing further corrosion. The figure highlights the coating's ability to detect microbial activity and release biocides, maintaining metal integrity.

Figure 8. Presumably illustrates the crystalline structure of Layered Double Hydroxides (LDHs), materials known for their brucite-like layered configuration. The diagram would depict stacked layers of octahedrally coordinated metal cations (e.g., magnesium, alumi-num, zinc) with hydroxide ions, carrying a net positive charge. The interlayer space, filled with anions, water molecules, and sometimes organic compounds, is negatively charged, balancing the structure. It may also illustrate hydrogen bonding and anion exchange ca-pabilities, crucial for LDHs' uses in adsorption, catalysis, and material synthesis. Annota-tions could detail layer distances, metal ion arrangement, and other structural characteris-tics essential for understanding LDHs' properties.

Figure 9. Outlines the corrosion protection offered by LDH-based coatings, showing their ability to exchange interlayer ions with corrosive anions (e.g., chloride) from the environ-ment, thus removing these agents from the metal surface. It also illustrates the release of 8-hydroxyquinoline (8-HQ) from LDH layers, which forms a chelate complex with metal ions at corrosion sites, blocking further corrosion. This dual-action mechanism—ion ex-change and chelation—highlights the advanced, self-healing properties of LDH coatings in preventing corrosion.

Figure 10. The illustration shows how azobenzene molecules on Mesoporous Silica Na-noparticles (MSNs) change shape under different light wavelengths through photoisomer-ization. Under UV light (365 nm), azobenzene shifts from trans to cis configuration, open-ing MSN pores for controlled cargo release. Visible light (450 nm) reverses this to trans, closing the pores and stopping release. This mechanism, depicted with molecular struc-tures and pore changes, highlights the smart, externally controllable delivery system.

Figure 11. Illustrates the molecular mechanisms in Shape Memory Polymers (SMPs), pol-ymers that return to their original shape when exposed to stimuli like heat. Part (a) shows a multiblock copolymer's structure with alternating polymer segments, each providing unique thermal and elastic properties for shape memory. Part (b) depicts a covalently cross-linked polymer, with cross-link junctions (black dots) and molecular chains repre-sented by blue lines (low mobility below transition temperature, T_trans) and red lines (high mobility above T_trans). Part (c) displays a polymer network deformable below its melting point (T_m) and recoverable above T_trans, highlighting SMPs' shape recovery process and underlying molecular transitions.

Figure 12. Represents a Shape Memory Polymer (SMP) returns from a temporary to its original shape upon reaching its glass transition temperature (Tg), where it shifts from a hard state to a more ductile one. Initially depicted in a deformed state below Tg with fro-zen polymer chains, the figure illustrates the SMP gaining mobility as temperature rises above Tg, enabling it to 'self-close' back to its memorized shape. Annotations explain tem-perature stages, polymer chain configurations, and SMP interactions during this trans-formation, highlighting its use in self-adjusting or repairing systems in fields like aero-space and biomedical devices.

Figure 13. Indicates a self-sensing SMP coating with a 'close-then-heal' mechanism. Trig-gered by NIR irradiation, the coating incorporates polydopamine and graphene oxide, which convert NIR to thermal energy, prompting the SMP to revert from deformed to orig-inal form, closing superficial damage like scratches. Then, 1,10-phenanthrolin-5-amine mobilizes into damaged areas during the shape memory response for self-sensing, indi-cating repair. This dual-action process, combining self-repair and damage detection, is crucial for applications requiring continuous integrity monitoring, such as structural health systems.

Figure 14. Illustrates double-walled hybrid nanotubes with an inner silica layer for stabil-ity and an outer polymer layer for responsive properties to temperature, pH, and specific chemicals or biological agents. The polymer's responsiveness allows the nanotubes to physically or chemically adapt, changing their diameter or charge properties for applica-tions like controlled drug release, chemical sensing, or microfluidic actuators. Visual cues in the schematic show how the nanotubes respond to stimuli, highlighting their potential in high-tech applications.

Figure 15. Shows a schematic representation of a material with self-healing and self-reporting functions. The self-reporting is marked by an intense red color and reduced fluo-rescence at damage sites due to the Phen-Fe complex formation, signaling the need for healing. The schematic details the Phen-Fe complex's molecular structure and optical sig-nature. Self-healing occurs through dynamic hydrogen bonds in the material's matrix, al-lowing it to mend autonomously. This process is reversible and repeatable, enhancing du-rability. The diagram highlights intermolecular interactions and the healing process, un-derlining the material's autonomous restoration capability, valuable in aerospace, elec-tronic skins, or smart coatings.

Figure 16.  Outlines the synthesis of microcapsules starting with a linseed oil core, modi-fied by a silane coupling agent, dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammoni-um chloride, for a strong bond and durable outer layer. The schematic shows the micro-capsule formation and stepwise addition of the silane agent, creating a shell that encapsu-lates linseed oil and adds properties like hydrophobicity or antimicrobial activity. It de-tails the reactions and modifications at each stage, highlighting applications in self-healing coatings or controlled-release protective treatments [96].

Table 1. Presents an overview of micro and nano-sized containers responsive to specific stimuli like temperature, pH, light, magnetic fields, or chemicals. These containers release contents like drugs or dyes in a targeted manner, useful in medical drug delivery and smart coatings. The table categorizes them by response mechanism, composition, size, and applications, offering insights into their properties and practical uses.           

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Authors addressed all the concerns..