Evolution of self-healing elastomers, from extrinsic to combined intrinsic mechanisms: a review

The evolution of self-healing polymers has resulted in a myriad of healing designs that have given way to complex systems capable of supporting multiple cycles, among other features. This progression enables us to propose the implementation of a timeline that classifies self-healing polymers in generations based on the healing mechanism, and correlated with historical development. The first generation employed the encapsulation of external healing agents; the second one, based on intrinsic mechanisms, applied reversible chemistries; and the third generation was inspired by natural examples such as plants and human skin, in which the healing agent is embedded in vascular networks. Despite great efforts and, with a few exceptions, polymers with high healing efficiency and high mechanical performance are not common. To improve this situation, a combination of different mechanisms is currently emerging, giving birth to the fourth generation of self-healing materials. This article, focused on self-healing elastomers, provides a rigorous overview of this new generation, in which the combination of covalent bonds and non-covalent interactions provides an optimal balance between mechanical performance and repairability. The implementation of this concept leads to materials with real commercial potential in functional applications, such as coatings, sensors, actuators, controlled release of drugs, seals, gaskets, hoses, and even high-performance applications such as tires and railway components.