Self-healing Materials

Starting with reversible polymer networks based on the Diels–Alder reaction of furan and maleimide, a large variety of different self-healing polymers based on reversible covalent interactions have been developed in the last decade. These intrinsic self-healing polymers are mainly based on reversible addition reactions, exchange reactions, and condensations. The most prominent examples of such materials are based on Diels–Alder chemistry, photocycloadditions, disulfide reactions, acylhydrazones, and reversible radical reactions.

Supramolecular polymers are an intriguing class of materials with dynamic behavior as a result of the presence of non-covalent bonds. These bonds include hydrogen bonds, metallopolymers, ionomers, host–guest as well as π–π interactions. The strength of these supramolecular bonds can be tuned by varying the binding motifs. Their reversible and dynamic character can be utilized to engineer self-healing polymers. This review briefly presents the preconditions for design of self-healing polymers and summarizes the development of supramolecular self-healing polymers based on various non-covalent interactions. Furthermore, challenges and perspectives for the understanding of self-healing mechanisms and the preparation of novel materials with enhanced properties are discussed.

Over the last few years, several testing methods have been introduced for the detection and quantification of autonomous and thermally stimulated healing in polymers. This review summarizes some of the most prominent state-of-the-art techniques for the characterization of polymer healing occurring at the microscopic and macroscopic levels during the repair of damage such as scratches, cracks, or ballistic perforations. In addition to phenomenological investigation of the self-healing process, a range of physical characterization techniques have been explored for elucidation of the underlying healing mechanism at the molecular or polymer network level. The present state of visual methods, spectroscopic techniques, scattering techniques, and dynamic methods is described. A short outlook is provided, discussing the future challenges and expected new trends in the characterization of self-healing polymers.

Polymers and polymeric composites are used in many engineering applications, but these materials can spontaneously lose structural integrity as a result of microdamage caused by stress peaks during service. This internal microdamage is hard to detect and nearly impossible to repair. To extend the lifetime of such materials and save maintenance costs, self-healing mechanisms can be applied that are able to repair internal microdamage during the usual service load. This can be realized, for example, by incorporating microcapsules filled with monomer and dispersed catalysts into the polymeric matrix material. If a crack occurs, the monomer flows into the crack, reacts with the catalysts, and closes the crack.

This contribution focuses on the development of a thermodynamically consistent constitutive model that is able to describe the damage and healing behavior of a microcapsule-based self-healing material. The material under investigation is an epoxy matrix with microencapsulated dicyclopentadiene healing agents and dispersed Grubbs’ catalysts. The simulation of such a multiphase material is numerically very expensive if the microstructure is to be completely resolved. To overcome this, a homogenization technique can be applied to decrease the computational costs of the simulation. Here, the theoretical framework is based on the theory of porous media, which is a macroscopic continuum mechanical homogenization approach. The developed five-phase model consists of solid matrix material with dispersed catalysts, liquid healing agents, solidified healed material, and gas phase. A discontinuous damage model is used for the description of the damage behavior, and healing is simulated by a phase transition between the liquid-like healing agents and the solidified healed material. Applicability of the developed model is shown by means of numerical simulations of the global damage and healing behavior of a tapered double cantilever beam, as well as simulations of the flow behavior of the healing agents at the microscale.

Sol–gel pretreatments and coatings are an important class of passive coatings primarily used to increase adhesion of subsequent layers to metallic substrates herby improving the protection against corrosion. Sol–gel chemistry has been broadly used as an appropriate method for versatile implementation of extrinsic self-healing in coatings. Nevertheless, it is only recently that such a chemical approach to the development of intrinsic healing coatings has been proposed. In this review, we analyse existing literature describing the introduction of extrinsic and intrinsic self-healing routes into sol–gel coatings. The development of active sol–gel coatings capable of recovering their protective functionality is based on the broad chemical diversity of precursors and the low processing temperatures required for this type of chemistry.

Although the protection of metals from corrosion appears to be a Sisyphean work, promising developments have recently been proposed in the literature. We present here some new strategies, in which traditional methodologies employed for corrosion protection are married with the concept of self-healing. Efficient anticorrosion properties can be achieved by the encapsulation of corrosion inhibitors and/or monomers and catalysts for self-healing reactions. Nanocontainers for anticorrosion comprise a shell that is responsive to stimuli induced by corrosion and a core containing the healing substance. We present here the requirements for their design, synthesis, and application in coatings for metal substrates. The important factors to be taken into account and future directions are also discussed.

Self-healing materials have been intensively investigated in recent decades, whereby the healing process was mostly based on the restoration of mechanical properties after mechanical damage. However, self-healing functional polymeric materials have now become the focus of research. In recent years, several approaches have been developed for self-healing of conductivity as well as the restoration of optical properties. In contrast to the healing of mechanical properties, such as stiffness and strength, the self-healing of functional materials focuses on the restoration of functionalities after damage caused by harmful environments (e.g., high temperatures or irradiation). The ultimate goal is the investigation or mimicking of a multifunctional self-healing system (e.g., biological material). In this review, the current state of the art in self-healing functional polymeric materials is summarized. In particular, we discuss self-healing conductive materials, healable optoelectronics, and functional coatings.

Self-healing technology is a new field within material technology. It represents a revolution in materials engineering and is changing the way that materials behave. Incorporating self-healing technology into the road design process has the potential to transform road construction and maintenance processes by increasing the lifespan of roads and eliminating the need for road maintenance. By decreasing the unnecessary premature ageing of asphalt pavements, self-healing asphalt can reduce the amount of natural resources used to maintain road networks, decrease the traffic disruption caused by road maintenance processes, decrease CO₂ emissions during the road maintenance process and increase road safety. In addition to environmental savings, self-healing materials have the potential to deliver significant cost savings for road network maintenance across the EU. There are three main self-healing technologies available for asphalt pavement design: nanoparticles, induction heating and rejuvenation. This chapter reviews all three options and outlines the future development of self-healing asphalt technology.

Damage and fatigue are ever-present facts of life. Given enough time, even the most robust material, whether man-made or natural, succumbs to the deleterious effects of cracks, fissures, and defects during normal use. Traditionally, materials engineers have approached this problem by creating damage-tolerant structures, intensive quality control before use, vigilant inspection during use, and designing materials to function well below their theoretical limit. Living organisms, on the other hand, routinely produce materials that function close to their theoretical limit as a result of their remarkable ability to self-heal a range of non-catastrophic damage events. For this reason, many researchers in the last 15 years have turned to nature for inspiration for the design and development of self-healing composites and polymeric materials. However, these efforts have so far only scratched the surface of the richness of natural self-repair processes. In the present review, we provide an overview of some paradigmatic and well-studied examples of self-repair in living systems. The core of this overview takes the form of a number of case studies that provide a detailed description of the structure–function relationships defining the healing mechanism. Case studies include a number of examples dependent on cellular action in both animals (e.g., limb regeneration, antler growth, bone healing, and wound healing) and plants (e.g., latex-based healing, plant grafting, and wound closure in woody vines and succulent plants). Additionally, we examine several examples of acellular self-repair in biopolymeric materials (e.g., mussel byssus, caddisfly silks, and whelk egg capsules) that are already inspiring the development of a number of self-healing polymers.

Cracks are intrinsic concrete characteristics. However, cracking can endanger the durability of a structure, because it eases the ingress of aggressive gasses and liquids. Traditional practices tackle the problem by applying manual repair. Scientists inspired by nature have created self-healing concrete able to self-repair as a result of the metabolic activity of bacteria. Various research groups have studied bio-based self-healing concepts over the last decade. Although the metabolic pathways of different bacteria can vary, the principle is essentially the same: a bio-based healing agent is incorporated into fresh concrete and when a crack appears in hardened concrete the bacteria become active, precipitate limestone and seal the open crack. Bio-based self-healing concrete technology targets the recovery of the original performance of concrete by regaining water tightness lost by cracking. Along these lines, bio-based repair systems have also been developed to protect existing structures by applying materials that are more concrete-compatible and environmentally friendly than existing repair materials. All these innovative concepts have shown promising results in laboratory-scale tests. Steps have been taken towards the first full-scale outdoor applications, which will prove the functionality of this new technology.

Designing self-healing in metals is a challenging task. Self-healing concepts successfully applied in polymers cannot be directly transferred because of different energetics. This has detained the field of self-healing metals, as evidenced by absolute publication numbers. Yet, relative publication numbers indicate a rapidly increasing interest in recent years triggered by the potential economic impact of advanced metallic materials. This chapter reviews all currently available self-healing concepts in bulk metallic materials. We provide a classification into two conceptually distinct routes: (1) autonomous self-healing of nanovoids at the nanoscale, aiming at a prevention of large-scale damage and (2) non-autonomous self-healing of macrocracks by an external trigger such as heat. The general idea of each self-healing concept is comprehensibly introduced, relevant publications are reviewed, and the characteristics of the concepts are compared. Finally, we discuss current constraints and identify the most promising concepts.