Failure is a potential occurrence in any human invention such as buildings, spaceships, electronic devices, and medical devices. All these products or structures are susceptible to damage due to many factors, one being environmental factors, such as environmental threats like mechanical stresses, chemical abrasion, ultra-violet irradiations, thermal loading, and pressure. Generally, materials tend to be damaged at the micro- or macrolevel. At the microlevel, cracking for instance, affects the material’s characteristics, while at the macrolevel, serious damage can occur from cracking as it can cause the material or structure to destruct, which, in turn, leads to more catastrophic impact [1].

Take for example a concrete structure subjected to microcracking due to the weakness of concrete in tension. Fatigue loading can reduce the concrete’s toughness, which leads to the increment of permeability, and hence, can ultimately lead to the reduction of the concrete’s structural integrity, durability, and life span. In medical application, microcracking, wear and tear, fatigue, and material deformation and loss often arise from cyclic loading or aggressive usage, which can cause mechanical failure in biomedical implants [2]. Surface scratching and microcracking in the coating system may promote the formation of cracks, which eventually leads to macroscopic damage, up to the point where the coating loses its aesthetic as well as its external protective function. Exposure to a high electric field may also have the potential to cause mechanical damage in dielectric and insulating elements, which can lead to the failure of electronic devices and electrical equipment.

Other examples of major material failure modes that are routinely encountered in composite materials in aerospace application are fiber-matrix debonding, matrix microcracking, and impact damage. These examples of damage initiation may lead to more serious damage or can also contribute to catastrophic failure. These catastrophic failures may cause significant impacts on repair, maintenance, and the rehabilitation of existing and new structures. Apparently, deterioration of products or structure is an ongoing and concerning problem. The consequential effects of damage may have far reaching implications if the damage is not controlled at an early stage. Thus, this catastrophic failure can have a significant influence on the economic, social, and environmental impacts. Therefore, preliminary action should be addressed at the microlevel to prevent serious damage from happening.

Self-healing methods, which refer to the ability of a system to recover after initial damage, have the potentials to address these challenges. Self-healing technology can mitigate the molecular level functionality, enabling the repair, impart reprocess ability, or longer lifetimes of products. The concept of self-healing was inspired from natural or biological process, for example, blood clotting or reparation of fractured bones. This concept was then incorporated at a wider range of engineering applications. In some cases, manual repair works can be difficult, especially if the cracks or damages are invisible, or their locations cannot be easily accessed.

Therefore, self-healing process provides the best solution because the system has the capability to repair itself as per the biological system, without the need for any human detection or repair by manual intervention. One of the strategies to address the microcrack or inaccessible damage sites is by using self-healing polymers. This is because self-healing polymers have high efficiency to recover the properties of the components on three basis stages; which are (1) triggering, (2) the healing material is transported to the damage site, and (3) remodeling by the chemical repair process [3]. With the application of the self-healing method in each system, the lifetime of the material may be extended, which in turn can reduce the cost of maintenance and improve product safety.

Self-healing technologies were progressively developed in various kinds of applications. Market research has portrayed the increase of self-healing products at a value of USD $381.65 M in 2019 and is expected to stretch to USD $943.38 M by 2027. The building industry segment held approximately 56.5% of the market, as this industry is the largest consumer for self-healing material. Other sectors, such as automotive and transportation, energy generation, electronics and semiconductors, and the medical sector, have also contributed to the increasing of the self-healing material market [4].

In developing self-healing technology, many innovations have been explored including research from the National University of Singapore (NUS) Institute for Health Innovation and Technology and NUS Materials Science and Engineering. They have developed a durable, low-power material for next-generation electronic wearables and soft robots. This unique material has very high dielectric permittivity and self-healing properties, which enables it to store more electronic charges at lower voltages and a higher brightness when used in a light-emitting capacitor device. The material is a transparent, elastic rubber sheet made up of a unique blend of Fluor elastomer and surfactant. This innovation is called the HELIOS (which stands for healable, low-field illuminating optoelectronic stretchable) device [5]. Another innovation developed at the University of Technology in the Netherlands is the self-healing “bio” concrete that uses nutrients (peptone and yeast extract) and Bacillus Subtilis without urea as a microbial adjuvant, and this could also be an effective way to promote microbial CaCO₃ precipitation in sufficient quantities to enhance sustainability and to promote self-healing of concrete [6].

To date, the number of publications with various applications of self-healing materials has risen dramatically [7,8,9,10,11,12]. For instance, B. Aissa et al. summarized the main technologies that are currently being developed specifically in thermosetting composites polymeric system [12]. Besides that, Qiang et al. also reviewed on recently developed self-healing conductive materials and their applications [7]. Most of the recent reviews reported the emerging of self-healing materials in specific applications with their self-healing mechanism. A comprehensive review on the self-healing mechanisms, approaches, polymeric materials used, and their applications is still limited. Thus, due to the remarkable and emerging self-healing products to date, this review paper summarizes the self-healing polymer technologies in different types of applications, such as in electronics, coating, aerospace, the medical field, and construction. We discuss thoroughly on their principles, interaction mechanisms, self-healing polymeric materials used, and their respective effectiveness. The different types of self-healing polymers for various types of applications are explored. In the successive section, the healing process is classified and described according to the self-healing mechanism (embedded, vascular, and intrinsic polymer-based system). Finally, in order to gain an overall insight into the self-healing materials, the advantages and disadvantages of the self-healing approach in every application and the current challenges of self-healing technologies are also discussed. Therefore, this review paper is expected to significantly give a broader idea of self-healing polymers, which in turn proves to be the solution to increasing the life cycle of materials, and also concurrently advocates a safer environment in all the applications involved.

Self-healing is described as a method which has the ability to perceive any unfunctional device or system without human intervention and make the necessary adjustments to restore itself back to its normal condition. The self-healing method is expected to detect and repair the microcracking in products or system that help to recover the mechanical or functional performance of the material. It represents the next generation of technology, which helps to greatly improve important performance of products, as it has been built in into various applications for structural, electronic, medical and aerospace products [13,14,15,16]. This approach is very significant in situations where repairing or inspection is difficult, hazardous, and costly [17].

Over the last decade, self-healing methods have attracted a lot of attention in the research community and numerous studies have explored them in order to develop various self-healing capabilities, particularly using polymeric materials. Polymeric materials are formed by the bonding of monomer and polymer chains and they are one of the most extensively researched materials in self-healing because of their versatility in polymer systems. For instance, a self-healing process that requires relatively low temperature can be easily achieved by the modification and functionalization of polymer system and can effectively fill the cracks because of the higher viscosity of polymer. Besides that, the self-healing polymers also can transform physical energy into a chemical and/or physical response to heal the damage. Polymers are in demand as they offer a light weight material with good processability and chemical stability [18]. In particular, efficient self-healing polymers and polymer-based fiber-reinforced composites are of high significance because these materials are utilized in numerous engineering fields, such as for cars, ships, spacecraft, electronics, and biomedicine [19,20,21].