Damage to coatings, leading to the formation of scratches, cracks or other structural defects on the surface or inside a coating, can result in decreased mechanical strength and reduced functional performance. Coating defects can lead to an increased penetration of humidity and oxygen, and, over the long term, cause degradation of coatings and substrate materials, and the devices and constructions underneath. To maintain the functional performance of coatings it is benefi cial to apply a self-healing component to functional coatings.
5.2.1 Self-healing approaches in nature
The self-healing properties of coatings are inspired by smart phenomena that occur in nature. Biological systems demonstrate impressive principles of self-repair that are based on several mechanisms. Repairing mechanisms arise from traditional life cycles of substances in nature, and are essential to respond to external damage to support the survival of living organisms and plants [13, 14] .
To develop self-healing functional materials and coatings, designers are keen to discover the mechanisms found in nature that allow biological organisms to recover their different functions. An essential approach created by nature is based on the ‘bleeding’ mechanism for healing wounds [15, 16] . The sophisticated vascular networks in living organisms can feed, remove waste, control internal temperature and provide self-healing [15] , serving as a model for artifi cial circulatory systems applied in a number of self-healing functional materials and coatings [16] .
Another smart mechanism mimicked from nature is blood clotting; a mechanism applied in autonomous self-healing by the auto-assembly of nanoparticles [17] . Fusion of failed surfaces is imitated in self-healing
materials by restoring broken bonds, for instance, with the application of heat [15, 18] .
An example of a smart approach to recovering hydrophobic properties of a surface can be demonstrated by the lotus, amongst other plants [11] . The leaves of the lotus plant have a special structure formed by a protective wax coating. Interestingly, the leaves of such plants are able to regenerate wax to heal voids on their surface, thus maintaining their superhydrophobic properties. The regeneration of wax takes place in a concentric fashion, creating an amount of wax proportional to the area to be healed [11, 19] . The wax regeneration mechanism of lotus leaves can be applied in materials science to develop coatings with a capability to heal and to maintain the surface ’ s superhydrophobicity, imitating lotus leaves [11] .
These multidisciplinary investigations, spanning biology, chemistry, physics, materials science and engineering, generate ideas to be adopted and adjusted to design new functional polymer materials and coatings.
5.2.2 Self-healing mechanisms in coatings
There are several classifi cations of self-healing mechanisms of fi lled polymer materials and coatings proposed by materials scientists [5, 13, 20] . Approaches to self-healing can be based on auto-assembly of nanoparticles [13, 17] , swelling of a polymer material or of inorganic particles incorporated in the polymer [20] , shape memory effects [20–22] and chemical reactions or reversible bonding in response to a damaging process [13, 20, 23] . Self- healing can be based on intrinsic mechanisms or involve the incorporation of encapsulated healing and functional materials or microvascular healing network systems [5, 24–26] (Fig. 5.1 a–b).
Auto-assembly of nanoparticles is demonstrated by their ability to migrate to a damaged area [16, 20, 27] . An approach to self-healing related to auto-assembly of nanoparticles incorporated in a polymer composition is based on the idea of fi lling cracks by the particles dispersed in the
5.1 Approaches to design of self-healing materials based on incorporation of an encapsulated healing agent (a) and hollow channels fi lled with a healing agent (b).
Coating
Metal substrate
(a) (b)
Coating
Metal substrate
polymer matrix (Fig. 5.2 ). This arises from the tendency to increase conformational entropy of polymer chains in a fi lled polymer system by repulsion of particles to the solid material–air interface [13] . Spontaneous atomic rearrangements at the interface take place in order to decrease the free surface energy that agrees with the Gibbs law [28] , which leads to recovery possibilities for underlying layers [13] . This approach does not usually involve chemical reactions of polymer chains during recovery, but refers to migration of nanoparticles.
Several researchers have performed computer simulations of these processes [16, 18, 20] and demonstrated the effect of surface functionalization of nanoparticles on the self-healing ability of the fi lled polymer materials and coatings [20, 29] . Intrinsic principles of self-healing are demonstrated in the approach of Micciche et al. [30] . The self-healing mechanism of barrier coatings in this approach is induced by moisture which causes swelling of montmorillonite particles and a ‘pushing up’ effect [30] .
Self-healing with shape memory materials is based on the capability of such materials to ‘memorize’ and recover their initial shape with the application of an external stimulus after being exposed to temporary deformations [21] . The term ‘shape memory’ was introduced fi rst in 1941 by Vernon and Vernon [31] and has been applied to assist self-healing to repair damaged surfaces [21] .
Self-healing of coatings with shape memory materials is primarily based on heating a material to achieve recovery of its surface by relaxing the polymer chains followed by further cooling of the material. Other external stimuli can also be used to promote shape recovery, such as UV light, electric or magnetic fi elds and others [21] . Affected by conformational entropy, self-healing of shape memory materials can be controlled and programmed [21] . Potential applications of this type of coating, based on polyurethane for anti-corrosion, have been demonstrated [22] .
An approach to self-healing based on chemical reactions or reversible physical bonding in response to a damaging process has been relatively widely discussed [13, 20, 23] . In some types of materials, chemical reactions
5.2 Self-healing of materials and coatings involving migration of nanoparticles at a crack.
Coating
Metal substrate
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] .