**3. Preparatory routes and mechanisms of self-healing**

*Advanced Functional Materials*

ferent materials classes.

**2. Self-healing materials**

encountering in-service damage [3, 7].

seal them automatically.

Several techniques have been employed to evaluate and quantify self-healing capacities and effectiveness in each of above material classes. Nevertheless, challenges arise in the characterization of self-healing materials as evaluation methods are not only materials and application specific, but also depend on the mechanism of the self-healing process. More so, self-healing processes take place on a very small length scale requiring sophisticated experimental procedures and equipment to unravel the mechanism of self-healing. Furthermore, it is important to establish a uniform testing procedure and standard for different material groups for better understanding of the concept [4]. This chapter surveys the main techniques applied to reveal the damage-restoring mechanisms in some material classes, but begins with a brief survey of preparatory routes and mechanisms of self-repair in the dif-

Self-healing is the capability of a material to recover from any kind of damage automatically without any external intervention as obtainable in biological systems or with external stimulation such as heat, light, electrical stimulus and solvent. The materials that exhibit self-healing without any external intervention or stimuli are said to be autonomic while self-healing that involves human or external influence to induce healing is said to be nonautonomic in nature [5, 6]. One of the major problems encountered in the use of materials in diverse fields is how to ensure their durability and minimize structural failures [7]. A self-healing material is therefore an artificial material designed with built-in ability to detect failure and respond automatically to restore partial or full properties or function of the structure after

This in-service damage, which is usually in form of micro-scratches, surface and internal cracks, voids or other defects [8, 9], is majorly responsible for failure in materials systems. Over time, these micro-cracks accumulate and grow until catastrophic failure of the entire product or system occurs. Since this source of failure normally initiates at the nanoscale level and progresses subsequently to the micro- and macroscale levels until failure occurs, an ideal self-healing material would without any external influence prevent initiation of failure at these small length scales or repair already nucleated damage, thereby restoring the original material properties in a shortest time [3, 7]. Since the greater of the in-service damage encountered in material systems is usually in form of micro-cracks, voids or other defects, the objective of designing self-healing materials is to impart them with the capabilities to prevent the initiation of micro-cracks and voids or fill and

For so many years now, the strategies of fabricating synthetic materials with the capability to self-heal like a biological system or as envisaged above have been exploited greatly. This huge interest is anchored on benefits of self-healing in materials. These benefits include enhancing materials' service lifetime, reduction in replacement costs and improvement in product safety [7]. Great advances have been witnessed in creation of self-healing materials since the birth of the concept. The concept has been exploited in almost all materials classes including polymers, metals, ceramics, cements, coatings and composites [3]. The design strategies and processing routes involved in creating self-healing capabilities in these material classes are different, just like their self-healing reactions to damage encountered during their lifetimes. The next subsection takes a look at the creation of selfhealing abilities in these systems, the prevalent mode of failures and mechanisms of

**216**

self-repairing.
