can start autonomously upon a crack formation due to mechanical rupture, whereas in others an external stimulus is applied to initiate a chemical reaction, providing thermal-, photo-, pH- or electrical fi eld-induced healing [13] .
An approach to self-healing based on stress stimulated cross linking or polymerization has been proposed [25, 32, 33] . Coatings with microcapsules or microvascular networks of hollow channels fi lled with a specifi c healing agent that are embedded in a polymer matrix serve as an alternative to self-healing coatings with an intrinsic healing system [4–7, 34, 35] .
Self-healing of materials with intrinsic mechanisms can be based on an inherent reversibility of chemical bonds induced by thermally reversible reactions, on hydrogen bond formation, reactions between ionomers, or meltable thermoplastics [5] . Self-healing with a reversibility of chemical bonding induced by heating is one of the most common approaches [13, 20, 23] . To increase the speed and the effi ciency of healing coating defects, the recovery of a coating can be induced by UV light. An example of such materials is reported by Burnworth et al. [36] . The materials are based on metallosupramolecular polymers that can be healed by exposure to UV light. The absorbed energy due to local exposure to UV light is converted into heat, which leads to a rearrangement of the metal ligand and to a reversible decrease in the polymer molecular weight and viscosity, resulting in healing of the defects.
Microcapsules or hollow channels with a healing agent that are incorporated in polymer materials were used in some of the fi rst attempts to introduce self-healing systems [16] . This approach is inspired by the natural phenomenon of biological systems to heal wounds in order to prolong their life. Imitating capillaries, veins or arteries transporting blood in a biological system, capsules and networks of capillaries or hollow channels incorporated in a polymer system contain a healing or a functional agent which is released if they break at the damaged area of the polymer material or coating [5, 24, 26] . The structure of the network of hollow channels can vary from single tubes up to three-dimensional networks of channels. Based on the type of healing or functional agent, self-healing of such materials/coatings can provide recovery of mechanical strength or restoration of specifi c functions. The network of hollow channels can be refi lled and used for several healing possibilities, whereas capsules can serve only for a single local healing [5] .
5.3 Corrosion and other functions of coatings
applications, or wear resistance; they can, for example, have load transferring recovery capability, as well as other functions.
Corrosion protection is one of the broadest and most common applications of self-healing coatings [3–8, 37, 38] . The fi rst self-healing materials based on encapsulated healing agents incorporated in polymer-containing dispersed catalyst were introduced in 2001 [39] ; however, anticorrosion coatings containing corrosion inhibitors that can be released from coatings by leaching have been known and applied for a long time [37] . Self-healing coatings for corrosion protection can contain corrosion inhibitors or active healing agents incorporated directly in the polymer matrix, or can include capsules or microvascular systems embedded in a polymer matrix that are fi lled with polymerization catalysts, corrosion inhibitors [6, 38] , monomers [39] , drying oils [40, 41] , or other healing/functional agents (Fig. 5.3 ).
The release of a healing agent can be caused by a mechanical rupture of hard polymer capsules or hollow channels [6, 42, 43] , by abrasion or stress [9] , by shell degradation that can be induced by changes in pH [44, 45] , humidity [7, 46–48] , ionic strength, temperature or by exposure to UV light [45] (see Fig. 5.4 ).
A comparison of several methods of preparation of microcapsules for self-healing anticorrosion coatings has been provided by Nesterova et al.
5.4 Mechanisms of releasing of healing agent in self-healing materials and coatings (a) and the crack after healing (b).
Healing agents Activation by
internal sources:
changes in pH or ionic strength Activation by external
sources:
UV light, temperature, humidity, abrasion or stress
Healing defect
(a) (b)
5.3 Self-repairing coatings for corrosion protection containing different healing agents.
Coating
Encapsulated healing agents (polymerization catalysts, corrosion inhibitors, monomers or drying oils)
Healing agents in polymer matrix Metal
substrate
[6] . Examples of preparation of smart coatings containing nanocontainers with corrosion inhibitor are given by Shchukin et al. [34, 49] and Lamaka et al. [50] . A signifi cant number of publications are dedicated to self-healing systems based on the mechanical stimulus, considered to be one of the most realistic approaches to self-healing of coatings [4, 6] . An example of nanoreservoirs that possess controlled permeability sensitive to pH values is given by Calle et al. [44] .
Corrosion inhibitors can be released by the effect of humidity during exposure to the atmosphere [46–48] . Penetration of water into a coating promotes diffusion and release of an inhibitor. UV radiation can serve as an external stimulus for activating reactive functional groups promoting self-healing effects [7] . In other approaches to self-healing induced by UV radiation, the effect of photo-plasticity is applied [51, 52] . The application of the reversible Diels–Alder reactions for healing cracks, achieved with increasing temperature, has ten demonstrated by Nesterova et al. [6] , and Chen et al. [18] .
Adhesion of a coating to a substrate is a crucial factor for successfully performing functional coatings. Defects in coating adhesion can lead to moisture and air occurring at the interface between coating and substrate, which can promote corrosion. This can result in delamination of coatings and in corrosion degradation of coatings, devices and constructions. To prevent this, it is necessary to develop coatings capable of healing defects at the coating–substrate interface, to maintain and recover coating adhesion.
Only a limited number of approaches have been made to address the problem of healing defects in the adhesion of coatings to substrates [37, 53, 54] . The possibility of using adhesion promoters that can be released upon adhesion failure has been demonstrated by Lane et al. [55] . Mardel et al.
[53] have reported an improvement in adhesion with the application of inhibitors such as cerium dibutyl phosphate in the primer coating due to development of an interfacial oxide at the metal–primer interface under the inhibitor-containing fi lm. New self-healing concepts of coating adhesion still need to be developed and investigated.
For several coating applications it is necessary to provide antimicrobial and antifouling functions in combination with their anticorrosion performance. The incorporation of encapsulated oxidizing agents or organophosphates can provide coatings with antifouling properties that have been used for inhibition of bacterial or fungal growth [7, 44] .
Szabo and co-workers have developed coatings with incorporated capsules containing corrosion inhibitor and slow-release microspheres fi lled with an antifouling agent [9] . Microcapsules for anticorrosion properties incorporated in coatings break upon a crack formation, releasing the fi lm- forming material and corrosion inhibitors. To obtain antifouling performance of coatings, special permeable microspheres containing a silver compound
used as an antifouling agent were applied in the same coating system.
Permeability of the microspheres allows slow release of the antifouling agent that is continuously dissolved in the polymer matrix. This results in a prolonged inhibition of adhesion of microorganisms to the coating surface [9] . Development of paint compositions containing microcapsules with a controlled-release mechanism antifouling agent is demonstrated by Nordstierna et al. [56] . Coatings containing core–shell nanoparticles based on silver as a core material and silica as a shell for antimicrobial corrosion protection under water have been reported by Le et al. [57] . The structure of the shell of the nanoparticles provides a controlled release of the active antimicrobial agent [57] .
One of the most important characteristics of a coating is its surface energy. A hydrophobic surface possesses a number of benefi ts, including the potential for water-repulsion and, for some types of coatings, repulsion of oil as well. Hydrophobic surfaces usually have a low coeffi cient of friction, good chemical resistance, and antimicrobial properties, due to low adherence of bacteria and fungi to the surface. To maintain these advantages, an implemented self-healing capability of hydrophobicity by a surface coating would be an advantage for developers, producers and fi nal customers of such coatings.
One of the possible solutions to enable self-healing of a surface ’ s hydrophobicity is demonstrated by Dikic [58] . It was proposed to introduce perfl uoroalkyl groups to the polymer, connected to a long polymer chain spacer. This structure enables the mobility of fl uorinated species of the polymer that are supposed to move to the newly created surface of a scratch or a crack. This would minimize the surface tension of the coating [58] .
Development of self-repairing functional coatings that are able to reduce their wear rate during or after being exposed to sliding or abrasion wear is one of the fi elds open to investigation. Such coatings can contain a lubricant incorporated in microcapsules that can be leached during wear of the surface [10] . Coatings that are able to recover antifriction properties have been demonstrated [34, 59] . Incorporation of oil-loaded hollow silica nanoparticles in the polymer matrix is one potential method to produce coatings with a combination of corrosion inhibition and wear resistance properties [34, 59] .
Electrically conductive materials and coatings are applied in a wide range of devices, such as electrical circuits, solar cells, photovoltaics, electrodes for batteries and novel fl exible electronic devices in the fi elds of defence, aerospace and healthcare. Maintenance of mechanical durability of such materials and coatings is highly important, as it affects the electrical conductivity of the materials and coatings and, therefore, the functional performance of the electronic devices.
Considerable attention has been paid by the research community and industry to the development of electronic materials with higher mechanical durability to provide suffi cient conductive functional performance. Damage of the structure of such materials due to microcrack formation would lead to a failure in the functional performance due to a loss of connectivity between the particles and, therefore, a decrease in electrical conductivity.
Although several investigations have been dedicated to improving the mechanical durability of conductive materials and coatings, designing a built-in capability for repairing electrical conductivity by self-healing coatings has been considered in only a limited number of publications to date [60–63] .
Self-healing of electrical conductivity can be achieved by incorporating core–shell microcapsules containing conductive materials in the polymer matrix [60, 61, 63] . Such systems have been demonstrated with the application of encapsulated suspensions of carbon nanotubes in organic solvents [60] . Carbon nanotubes are released upon mechanical damage and heal the electrical conductivity of the materials. It is possibile to restore the conductivity by incorporating encapsulated non-conductive repairing agents [61] . The encapsulated agents form a conductive charge transfer crystalline salt in situ upon a capsule rupture [61] .
An interesting approach to developing a self-healing conductive ink was proposed by Odom et al. [62] . They designed a conductive material that can be healed by releasing a solvent from microcapsules upon damage. The solvent dissolves the polymer binder locally, causing redistribution of conductive particles in the composition of the ink. As the solvent dries, the electrical conductivity is restored [62] .