8.1 Introduction

Self-healing materials are no more an illusion; and due to their efficiency in diagnosing and autonomously healing the damage, such materials have been attracting rising interest of the research fraternity over the past decades. Frequent attempts have been presented every year heading toward the development of different self-healing systems as well as their assimilation to large-scale production with the best possible property–cost relationship. The day is not far when materials developed by researchers can restore their structural integrity in case of a failure.

8.2 Classification of Self-Healing Materials

Polymers are an everyday material in basic life, with products like rubbers, films, plastics, fibers, or paints. This massive demand has compelled the extension of their reliability and lifespan and the designing of a new kind of polymeric materials capable of restoring their functional property after damage or fatigue was envisioned. Usually, the incorporation of self-healing properties in man-made materials cannot perform the self-healing action without an external trigger. The classification of self-healing also was inspired by the self-healing in biological systems, which can be identified by two major classes: autonomic (without any intervention) [23] and nonautonomic (needing human intervention/external triggering) self-healing materials. The same concept applies to the categorization of self-healing materials into two different groups based on the way of the self-healing mechanism: extrinsic or intrinsic [8, 12].

1. Extrinsic: In this approach, the healing agents have to be preplanned for encapsulation into a (polymeric) matrix, enabling their release during a rupture event and thus self-healing.
2. Intrinsic: In this particular case, self-healing polymers apply an inherent material ability to self-heal, actuated either in combination with an external stimulus or during a damage event.

The self-healing polymers follow a three-step process which shows a resemblance to the biological response. The first step is triggering or actuation, and this response occurs almost immediately after undergoing damage. The second step is transport of materials to the affected area, which also happens instantly. The third step is the chemical repair process, which depends on the type of healing mechanism (e.g., entanglement, polymerization, reversible cross-linking). Modern self-healing composites can be classified into the following three groups [8, 12]:

1. 1. Capsule-based self-healing materials
2. 2. Vascular self-healing materials
3. 3. Intrinsic self-healing materials

Although in all three categories the processes are quite similar in some ways, the difference lies only in the ways that response is hidden or prevented until actual damage has occurred.

8.3 Conductivity in Self-Healing Materials

Researchers and engineers have been paying immense attention to the development of self-healing polymeric materials to improve their safety and lifetime [80]. Development of a material having a combination of properties like elasticity and mechanical, functional, and intrinsic self-healing ability is highly desirable. Electrically conductive polymer composites consist of a nonconductive polymer matrix and are widely used in various commercial applications due to their good manufacturability, light weight, corrosion resistance, and excellent electrical conductivity [81–88]. Since introduction of conductivity in polymeric systems will make the material suitable for electronic applications, electrically conductive healable materials are therefore highly fascinating and important for the development of various modern electronics. The conductivity in healable polymeric materials can transfer information on the structural reliability through electronic assessment, which might give an insight into the most challenging task of identify and quantifying microcracks. Materials having abilities like conductivity and self-healing capability are highly advantageous, especially in deep sea or space applications. Some researchers have been successful in enhancing the conductivity in self-healing polymer materials and have already set a milestone. Single-walled carbon nanotubes (SWCNTs) were used by Guo et al. [89] to design composites by connecting self-healing conductive composites poly(2-hydroxyethyl methacrylate) (PHEMA) and SWCNTs through host–guest interactions. This PHEMA–SWCNT composite provides bulk proximity sensitivity, humidity sensitivity, and electrical conductivity and is able to self-heal under surrounding conditions without an external impulse. A hybrid gel-based conductive material on self-assembled supramolecular gel and nanostructure polypyrrole was developed by Shi et al. [90], which synergizes the dynamic assembly or disassembly nature of the metal–ligand supramolecule and the conductive nanostructure of polypyrrole hydrogel. Synthesized self-healing conductive materials show features of high conductivity (12 S m⁻¹), meet mechanical and electrical self-healing properties without any external influence, and enhance flexibility and mechanical strength. Another idea introduced by Williams et al. [91], where the interaction between N-heterocyclic carbenes (NHCs) and transition metals is found to be reversible, has been well studied along with the electronic communication of these systems [92, 93]. Carbon nanotubes are very promising as electrically conductive fillers. Remarkable work has been done by Sandler et al. [81], who reported percolation thresholds below 0.01% in a carbon nanotube/epoxy system. Li et al. [94] fabricated electrically conductive self-healing films by depositing silver nanowires on top of healable polyelectrolyte multilayer films consisting of a layer-by-layer assembled branched poly-(ethylenimine) and poly(acrylic acid)–hyaluronic acid blend. Kitajima et al. [95] fabricated anisotropic electrically conductive polymer composites by applying a strong magnetic field to orient fillers. Incorporation of bulky N-alkyl moieties into carbenes reduced the viscosity upon depolymerization, which will boost its flow into the cracks. Higher conductivities (∼1 S cm⁻¹) should be achieved to have practical self-healing applications. Feller et al. have successfully incorporated multiwalled nanotubes (MWNTs) in glass fiber–epoxy composites [86]. Another material made up of silicone rubber nanocomposites with electrical conductivity activated by temperature and self-healing capabilities has been reported by Italian scientist Bittolo Bon and Valentini [96]. A combination of silicon rubber (SR) filled with graphite nanoplatelets (GNPs) and synthesized via a liquid mixing method has found promising applications for seals and hoses and in the automotive field. Another example of electrical conductivity healing was introduced by Tee et al. They prepared a composite having electrical and mechanical properties using a supramolecular polymeric hydrogen-bonding network with self-healing ability filled with chemically compatible micro-nickel particles with nanoscale surface features [97]. Palleau et al. [98] reported self-healing of electrical properties in stretchable wires by combining the self-healing Reverlink polymer produced by Arkema with liquid metal. Another example of a self-healing electrically conductive polymer composite was reported by Wang et al. using fabricated silicon microparticle (SiMP) anodes for high-energy lithium-ion batteries. In this work, they coated the material with a self-healing polymer composite consisting of a randomly branched hydrogen-bonding polymer matrix and carbon black nanoparticles [99].

8.4 Current and Future Prospects

The principle of conductive approach of self-healing materials has already been proved. To boost conductivity, electrically conductive fibers appear to be much more efficient than nonfibrous fillers. When practical tests are conducted successfully, this method can be applied commercially in about 5 years from then.

Self-healing materials are efficient, as fewer raw materials will be necessary per unit product. This forms a perfect match with our sustainable investment approach.

Jan Willem Hofland–Managing Director, Triodos MeesPierson

In future, self-healing materials are expected to be used in places where dependability and/or stability play a key role; we can conclude them in the following six major points:

1. 1. The places that are difficult to access to repairs, such as underground (piping) or under the water surface (cables and piping), at high altitude (high buildings, wind turbines at sea).
2. 2. Structures which have to last very long (several decades), such as in large infrastructural applications as dams, dikes, and tunnels.
3. 3. Applications where large repairs result in a lot of inconvenience in society, such as repairs of roads and in energy supply.
4. 4. Applications where dependability and security are key issues, even during overload or unfavorable circumstances: airplanes, spacecraft, or long-term storage of nuclear waste.
5. 5. In products which need to have a smooth surface from a visual or shielding point of view over a long period of time, such as painted surfaces or coatings that have to protect against corrosion or high temperatures. Other examples are cars, optical systems or windows, and so on.
6. 6. High-tech equipment for the manufacture of high-quality products; machines which have to be in function 24/7, where off time should be minimized.

8.5 Conclusions

Over the past decade, research on self-healing polymers and composites has set up elevated performance levels for multiple materials system surroundings by a wide diversity of damage approaches and self-healing concepts. Continued progress in the field will give new healing chemistries that occupy greater stability, higher reactivity, and faster kinetics. Although this new field of research has made some great advancement over the past several years, many technical challenges still remain, which require persistent research efforts to address several areas of concern. The long-term sustainability of conductive self-healing materials on environmental exposure is still an unsolved mystery. Advanced environmental testing of self-healing systems is crucially required. However, the good majority of research on self-healing in response to fracture has focused on pseudostatic performances. Self-healing designs will most likely carry out into the targeted and localized distribution of self-healing components in vast applications to enhance efficiency while diminishing cost with deciding effects to the matrix material.

Current efforts for design and optimization of engineering models are still lacking in potential. Anticipating the life-cycle performance of a self-healing polymer composite is also beyond the capability of available analytical tools. However, so far the commercial successful demonstration of self-healing is achieved in the short term in all industrial applications and their modest mechanical performance requirements. However, speedy efforts are needed to transform laboratory demonstrations into constructive and practical applications across a broader platform of industries.

Facelift of properties such as conductivity may be highly favorable for many applications in the microelectronics industry, in which the current solution is usually chip replacement. Williams et al. [91] synthesized an organometallic polymer with semiconductor-level conductivity and the ability to self-heal with applied heat. Capsule-based or vascular approaches using conductive healing agents also show impact for restoring electrical conductivity. Many researchers [35, 118–120] have modeled and experimentally observed nanoparticle segregation to material defects, which may be applied to conductive particles in conductive substrates. Revival of optical properties may also be an encouraging path for self-healing research. Intrinsic self-healing of optically relevant materials may also abolish scattering by healing any generated cracks. The full range of mechanical, optoelectronic and chemical properties of materials, from dielectric character to optical transmission to chemical stability, may be met using self-healing concepts.

Self-healing concepts may lead to enhanced utility of materials. All potential applications for self-healing concepts benefit by longer life, fade-resistant fabrics, safer self-healing batteries, resealing tires, and anti-tamper electronics. Biological systems provide a way out for potential research developments. Many crucial biological materials, like bones, regenerate and remodel in a new way. The day is not far off when materials may be capable of responding to damage in a more synchronized manner so that remodeling and regeneration take place over the lifetime of the material in response to mechanical loading. Efforts toward these targets, as well as exploring the potential of distribution of these materials in many of the aforementioned applications, are currently under way. Collectively, we discussed that self-healing characteristics were scrutinized in materials with electrically conductive properties. However, many challenges remain before their potential is fully utilized. Most importantly, their dependency on solvent vapor to facilitate healing must be eliminated.

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