**3.1 Self-healing polymers and polymer matrix composites**

The self-healing concept has been most successful in the development of self-healing polymer-based systems [10, 11]. This emanates from the fast diffusion rate and high plasticity due to open-molecular structures in polymers, which facilitate diffusion of healing agents to fill and seal voids or micro-cracks [10] even at room temperature. Unlike metallic and ceramic systems, polymeric systems are light weight, chemically stable and can be easily processed [4]. These properties are exploited in developing efficient self-repairing polymers and polymer-based fiber-reinforced composites, which have applications in transportation, electronics, defense, biomedicine and construction industries [7, 12, 13].

Based on the strategies exploited to achieve self-repair, polymers are generally grouped into extrinsic and intrinsic self-healing systems [14]. Intrinsic self-repair is achieved by synthesis of smart polymers containing functional groups with the inherent ability to reversibly polymerize or cross-link their bonds in the presence of a stimulus like light or heat [15] and by so doing act as healing agents. The processes for obtaining extrinsic self-healing include (a) embedding microcapsules containing curable healing agents into polymer networks; and (b) incorporation of healing agents into polymer networks via microvascular channels [5, 16].

The microcapsule in (a) could be in form of capsule containing healing agent and catalyst or twin microcapsules each containing a monomer/resin and its hardener [5, 16, 17] while that in (b) can be in form of fibrous composite architecture impregnated with a microvessel filled with reactive healants [18]. Unlike extrinsic routes where healing agents are consumed during the curing process and are not replenished, intrinsic approaches have the advantage of multiple healing of damage in the same area owing to reversible polymerization [19]. Self-healing has been exploited and accomplished in thermoplastic, thermosetting and elastomeric systems.

The mode of damage often encountered in polymers and structural composites is in form of matrix micro-cracking, fiber breakage or delamination and fibermatrix debonding [7, 20, 21]. Self-healing mechanism or recovery or recuperation takes place when a damage/crack is encountered and is healed by intrinsic polymerization or polymerization of healing agent as crack ruptures the capsules as in (a) or by favorable reaction kinetics and post-polymerization as in (b).

#### **3.2 Self-healing metals**

When compared to other material systems, it is much difficult to achieve selfhealing in metals [22–24]. This is as a result of their high melting temperatures and strong atomic bonds, which limit diffusion of healing agents/solute atoms to sites of damage at low temperatures [22, 23]. There is also further restriction due to the relative small size and volume of the solute atoms. As a result, rate of mass transport to fill damage sites is intrinsically low at the usual low operating temperatures [23, 25].

The major factor limiting the useful life of metals is the occurrence of internal damage such as voids and cracks during processing or service. These defects usually initiate as nano- or micro-cracks in the bulk or on the surface, grow and propagate and eventually lead to failure. The self-healing process in metals in response to crack initiation follows the sequence of diffusion or release or transport of healing agents or atoms into the void or crack to fill and seal it, thereby restoring partially or fully the mechanical properties such as fatigue strength, stiffness and fracture toughness.

The approaches that have been proposed and attempted in developing selfhealing in metallic systems according to [22] include: (a) precipitation-induced self-healing approach at low and high temperatures [26]; (b) dispersion of nanoshaped memory alloy (SMA) in off-eutectic metal matrix [27–29]; (c) SMA-clamp and melt [30]; (d) solder tubes/capsules; (e) coating agent [31] and (f) electrohealing [32, 33]. Blazej Grabowski and C. Cem Tasan [22] classified these concepts into two based on the healing length scale as (i) healing of nanoscale voids (which includes approaches a and b) and (ii) healing of macroscale cracks (which includes approaches c, d, e and f) [22]. One (I) and two (II) above were earlier classified into damage prevention and damage management by Van der Zwag et al. [34], respectively. This implies that healing at nanoscale targets prevention of macroscale damage while healing at macroscale focuses on management of macroscale damage to prevent total failure. Manuel [30] further classified approaches a and b as solid-state healing; approaches c, d and e as liquid-state healing and approach f as electrolyte-assisted healing. The self-healing concepts in metals and metal matrices are summarized in **Figure 1** and details of the features of these concepts are available in Ref. [22].
