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Self-healing materials, which are inspired by the repair functionality of biological systems, increasingly incorporate microvascular components as an approach to mimic the autonomic healing abilities of biological organisms. Synthetic microvasculature addresses the limitations in healing agent availability within microcapsule-based systems in order to achieve multiple healing events at a particular damage location, and to repair large-scale fracture damage. Biological microvascular systems are briefly reviewed with respect to key design elements and considerations for functional and reliable synthetic microvascular systems. Subtractive and additive manufacturing techniques are discussed and their relative advantages and disadvantages compared within the context of structural and nonstructural parts. The performance of fabricated specimens and structures are presented with an emphasis on mechanical restoration. The healing efficacy is discussed within broad categories of mechanical loading and damage conditions, and includes quasi-static, fatigue, impact, and macroscale material loss. The field of microvascular self-healing has significant potential for continued growth; further inspiration from biology may yield in situ damage sensing, active fluid rerouting, improved computational optimization of vascular networks for multifunctional design objectives, and novel material chemistries for improved manufacture of engineered networks within large-scale structures.

Self-healing smart materials have been under extensive research and development during past decades. The recent developments in self-healing materials have resulted in a new generation of load carrying smart material systems with the ability to heal microscale to structural-scale damages. These materials are ranged from polymer matrix composites to ceramic matrix composites and from metal foams to biological materials. The self-healing agent might contribute to the load carrying capability of the system or it might only function as a glue. One may notice that the major emphasis in this field has been given to the experimental studies, and the modeling schemes for these systems are still in the development stage. To extend the applicability of the self-healing smart materials and deploy them into the industrial applications, the modeling techniques are of utmost important. Difficulties in manufacturing and testing of these new material systems can be compensated with incorporating modeling techniques such as finite element analysis. It is a state-of-the-art to accurately simulate the inelastic, damage, and healing responses of self-healing systems and then construct a virtual design laboratory for optimizing the performance of these material systems. The mechanical (ME) responses of self-healing materials can be categorized into elastic, plastic, damage, and healing deformation mechanisms. In the case of damage-healing mechanisms, the accumulation of microflaws forms damaged sites, and regardless of the type of embedded healing technology, the lost properties of the system in those damaged sites are recovered through the healing functionality. Thus physically consistent damage and healing models can be considered as basic tools to characterize the performance of self-healing materials. This chapter concerns the recent theoretical developments in the field of continuum damage-healing mechanics of self-healing materials. The mechanisms associated with the damage and healing are utilized to propose the damage and healing parameters. The evolution laws for the damage and healing processes are developed within the continuum damage healing mechanics framework and they are calibrated in accordance with the experimentally measurable properties such as changes in the elastic moduli.

Damage occurrence is an inevitable truth in natural or artificial materials. A variety of materials and methods have been introduced to deal with this concern, which self-healing phenomenon has been revealed to be one of the most promising and cost-effective methods. Self-healing property is a smart aptitude of materials to the autonomous or induced repair of damages. Self-healing property is typically accomplished through intrinsic or extrinsic mechanisms when the polymer matrix is comprised of a latent functionality that actuates repairing wounds by hydrogen bonding, thermally reversible reactions, ionomeric materials, or molecular diffusion and entanglement, it is called intrinsic self-healing. In the extrinsic mechanism, healing operant materials are added or implanted into the polymeric matrix, through different approaches such as fibers, capillaries or microcapsules. The mission of self-healing materials is to extend the efficient service life of the materials, by implementing the concept of spontaneous repair instead of increasing their initial performance. Ionomers are polymers with repeating units of ionic groups and their counterions. Ionomers are a type of polymers consisting of up to 20 mol.% ionic groups and their counterions which various new ionomers with different monomers and countercations. Discussion on the self-repairing property revealed by ionomers arising from their chemical arrangement is the subject of this chapter.

Self-healing polymers look back on almost 40 years of history. Beginning in the early 1980s, many different examples were studied—for instance, the self-healing polymers based on microcapsules in 2001. These materials were able to close cracks, to heal damage, and to restore their (mechanical) properties almost like the big role model: nature. The chapter will provide a general discussion of the basic principles of self-healing polymers and will elucidate the underlying healing mechanisms. Thus it will go beyond a pure description of several aspects of self-healing polymers, and it will contain information on how a polymer must be designed to show self-healing. Within this context, a view on the mechanism of extrinsic and intrinsic self-healing polymers will be presented.

Self-healing polymeric materials are at the state-of-art of advanced materials for the last three decades. Among the three self-healing approaches, intrinsic, vascular, and capsule-based, the most easily applicable method is that of the encapsulation of the healing agent into distinct capsules and the subsequent incorporation of these capsules inside the matrix material. Capsule-based self-healing systems have already been studied for a variety of applications in polymers and composites. In this chapter, a detailed review of the research on capsule-based self-healing systems is presented. Beginning with the design of such systems, and ranging from the synthesis and characterization to the application of these advanced materials in polymer matrices, coatings, and composites. The chapter aims to give an overview of the state-of-art as well as to identify the possible gaps and provide suggestions for further research.

The combination of shape memory and self-healing in a polymer introduces the potential for materials to intrinsically close a crack followed by the application of a stimuli, such as heat, light, electromagnetic fields, or changes in the chemical environment to heal itself. In particular, shape memory behavior can assist the self-healing process of a material in a variety of different nonideal environments such as temperature, pressure, and humidity. The shape memory response can be introduced by intrinsic and extrinsic methods while self-healing uses either covalent or noncovalent interactions. Shape memory fixity, shape memory recovery, and self-healing efficiencies are the main characteristics for shape memory-assisted self-healing (SMASH) systems. This chapter overviews the state of the art in different SMASH polymer systems, the self-healing mechanisms and applications. The aim is to spurn on further work in this field to overcome the current challenges.

One of the vital properties of the human biological organs is repairing injured tissues without any external stimuli. Inspired from the natural systems, scientists have been trying to fabricate man-made materials, having self-healing properties. Self-healing is described as the ability of materials to recover or repair mechanical damages. Self-healing materials are a group of smart man-made materials that can possibly heal different kinds of damages such as fracture toughness, tensile strength, and surface smoothness with or without external stimulus. This chapter covers the design and generic principles of self-healing polymers with different approaches. In addition, recent research about self-healing materials in different applications such as coating, aerospace, conductive device, and biological and pharmaceutical applications is discussed and evaluated.

Coordination bonds are most often reversible. Using them to crosslink polymers into for example hydrogels can therefore be expected to result in reversible crosslinks, which in turn provides self-healing properties. Polyphenols such as the catechol group of the special amino acid DOPA ( l -3,4-dihydroxyphenylalanine), which is used in mussel byssal threads to form a self-healing coating and for adhesion, are excellent coordinators of hard metal ions like iron(III). We will discuss the use of polyphenols and metal crosslinking to form self-healing hydrogels. We also treat less specific effects of metal ions on hydrogel gel mechanical properties by describing Hofmeister effects of cations on hydrogel rheological properties.

Self-healing is most advantageous as it can prolong the service life of a material, which is considered to be the vital aspect in designing the structures. The phenomenon of revamping the damaged structures without any external aid is called self-healing. During the past decade, various self-healing polymers were manufactured using various techniques for a wide range of applications. Autonomic healing is the sole objective of repairing the composites. In fiber-reinforced polymers, damage is initiated at the fiber–matrix interface where initiation and propagation of microcracks is the most common occurrence. This reduces the properties and strength of the structures. In general, conventional method of repairing is carried out by bolting or binding the layers which follows steps such as disassembling the structure, repairing material collection, and restoring them. This chapter deals with the techniques of self-healing and repair of polymers with special attention toward structural materials. This survey describes the mechanism and chemistry that inducted self-healing, beginning from the inherent healing polymers to self-healing-enabled polymers that contain dispersed networks or vascular agents.

The development of dynamic plastics that can self-repair materials properties on demand will provide tremendous leaps forward in the ability to reprocess commodity plastics on a global scale. To this end, the utilization of dynamic, supramolecular bonds as reversible cross-links has the potential for great impact. These supramolecular bonds have been implemented for a vast range of polymers and materials with differing levels of dynamicity depending on the interaction. Supramolecular interactions utilized in self-healing materials are described herein and include such dynamic bonds as host–guest complexes, ligand–metal coordination, hydrogen bonding, dynamic covalent bonds, and electrostatic interactions.

Self-healing materials have attracted the attention of many researchers over the last decades in order to provide satisfactory material properties and outstanding product durability. Self-healing has great potential to extend the service life of a material, and this capability has been regarded as an important strategy when designing a sustainable infrastructure. Preparing ultrahigh stretchable, tough, and self-healing materials is the key component for the artificial intelligence apparatus which can produce high mechanical properties and prolong the service life. This chapter presents a comprehensive summary of the state-of-the-art investigations concerning preparation, characterization, and enhancement of various electrically conductive self-healing materials. From the literature, it was found that the samples had an enhanced self-healing capability because of the electrical and thermal self-healing mechanisms.

We report the synthesis of controlled sized Urea-Formaldehyde (UF) microcapsules containing an epoxy healing agent via in situ emulsification polymerization for the study of self-healing epoxy systems. Scanning Electron Microscopy (SEM) confirmed that the capsules possessed rough external surface which enhanced mechanical interlocking. Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy were employed so as to determine the capsules thermal stability and decompositions and encapsulated healing agent percentage. To our knowledge it is the first time the solid-state NMR is used for the estimation of encapsulated healing agent. The obtained results clearly indicated that with decreasing capsule size, capsules remained thermally stable at high temperatures (approximetly up to 230 °C). Additionally, capsule size is for the first time directly correlated to both healing efficiency and the reduction of mechanical performance after self-healing system incorporation. Healing efficiency is proportional to capsule size with larger capsules resulting in 68% maximum load recovery. However, smaller capsules result to lower reduction of properties, i.e. 7% as oppose to 18% for larger ones. Although healing efficiency can be enhanced through the use of relatively large capsules, this is in expense of mechanical performance, i.e. there is an optimal capsule size.

The idea of self-healing materials for analyte sensing was inspired by the wound healing ability of the sensory organs such as nose for odors, tongue for tastes, or skin for pressure, temperature, and location. It is noteworthy that “self-healing” property is a new trend of bioinspired device technologies, specifically sensing platforms (e.g., electronic skin—e-skin, electronic nose—e-nose, and electronic tongue—e-tongue) or a combination of all. Their applications extend from healthcare to environmental sciences. This chapter is the state-of-the-art update focusing on the key chemistry of self-healing materials as well as their composites and the fabrication methodologies in order to integrate these materials into sensing devices including fluorescence sensors, chemiresistors, field-effect transistors, electrochemical sensors, etc. Furthermore, the strong and weak points in the development of each platform were sharply highlighted and criticized, respectively. Several ideas conducted led to the improvement of self-healing materials for analyte sensing.

Self-healing materials are a group of materials that are capable of reforming the broken bonds in response to applied stress. Recently, inspired by self-healing systems in nature and biological environment, the design of self-healable polymers and composites has attracted a lot of attention. Studies on nanoscale provide more accurate patterns to predict results from different healing methods. In this chapter, we are going to classify nanoscale self-healing systems on the autonomic and nonautonomic basis and offer such fabrication techniques and examples.

Polymers are very important in our daily lives. They can be biological or synthetic origin and are used in all the major industries including textile, automotive, household goods, medical devices, etc. They can be produced in a very wide spectrum changing from very hard to soft, from elastomeric to fibrous, from spongy to crystalline structures. Synthetic polymers can be prepared by using a variety of polymerization processes to yield products which are different in their molecular weight, organization of its constituents, stereoregularity and crystallinity. All these differences yield polymers with different physical and mechanical properties. Since the number of monomers available is very high, so is the variety of the possible polymers. Some polymers are viscoelastic and some are plastic. Polymers are not always linear like noodles but can be made to have branches either of the same monomer or by adding other monomers to change the chemistry and the crystallinity of the product. They can be further branched or crosslinked so that the polymer becomes insoluble and just swells in solvents creating the gels. A number of monomers can be used simultaneously to prepare macromolecules with properties that can be tailored to the needs of the application. All these play a deciding role on whether a polymer is suitable for load bearing applications or in replacing the soft tissues in the human body. In this article, the main approaches to polymer synthesis and the parameters that influence the properties of the final product are summarized.

The present chapter deals with the synthetic pathways for obtaining self-healing materials based on benzoxazine chemistry. Several strategies for self-healing systems involving the use of polybenzoxazine precursors as healing additives are presented. In the first approach, the additive poly(BA-dah)main contained in polysulfone (PSU) films were shown to faintly undergo a thermal ring opening reaction. Thus, thermal treatment at 160°C enables poly(BA-dah)main to chemically bind to PSU chains and form networks through the Friedel-Crafts reaction, demonstrating a novel self-healing behavior. In the second strategy, a self-healing behavior of poly(propylene oxide)s bearing coumarine-benzoxazine units (poly(Cou-PPOa)tele) through light induced coumarine dimerization reactions is described. Four different types of poly(propylene oxide) amines with molecular weights ranging from 440 to 5000 Da were reacted with formaldehyde and 4-methyl-7-hydroxycoumarin to yield desired polymers. The photo induced healing of the cured films was proven by UV-Vis spectral investigations. It was demonstrated that the toughness of the films were improved by employing allyl benzoxazine (P-ala) in the self-healable film formulation. Additional self-healing strategies for the poly(propylene oxide)s bearing benzoxazine units poly(BA-PPOa)main through supramolecular attractions are also described. The cross-linked polymer films from poly(BA-PPOa)main and additive carboxylic acid containing benzoxazine monomer (P-aba) showed autonomous self-healing capacity through the supramolecular interactions.

Self-healing is one of the most remarkable intrinsic capabilities of biological systems. A small cut on human skin can heal itself with the help of blood cells. Inspired by the self-healing of biological systems, several synthetic self-healing systems have been developed, such as microcapsule-based healing materials and microvascular healing materials. Similar to a major wound in human skin that cannot heal itself unless it is stitched up using medical sutures, synthetic structural materials face a similar problem when healing macroscopic damages. To heal macroscopic damages in synthetic material, a thermal-responsive fiber has been developed to function as the medical suture. The thermal-responsive fibers preembedded in composites can produce contraction force to close macroscopic cracks upon heating. At a second step, thermoplastic particles (TPs) that are predispersed in the matrix melt and bond the closed crack. The healing performance of the thermal-responsive fiber/TP composites is studied in 1-D, 2-D, and 3-D fiber composites. In the 1-D composites, fibers are aligned in one direction; in the 2-D composites, fibers are aligned in 2-D directions and perpendicular to each other; and in the 3-D composites, short fibers are dispersed in the composites in a random 3-D direction. The composites were able to heal wide-open cracks repeatedly. Cold-drawing programming of the fiber can improve the healing efficiency significantly.

This chapter mainly discusses reversible cross-linking polymer-based self-healing materials, which will help to eliminate hidden hazards, prolong the service life, and extend the application area of polymeric materials. The chapter first reviews automatically self-healing polymers with embedded microencapsulated healing agents. The chapter then discusses cross-linked healable polymeric materials based on reversible noncovalent bonds, including hydrogen bonds, π–π stacking interaction, and metal–ligand coordination. Finally, the chapter reviews cross-linked healable polymeric materials containing reversible covalent bonds (Diels–Alder adducts).

Formed by noncovalent bonds, supramolecular polymer materials could form a dynamic network and then exhibit the ability to heal their damages. Actually, as an emerging and fascinating field, supramolecular self-healing materials have been developed for a decade. Because of the characteristics of noncovalent interaction, such as reversibility and self-repairing, these materials are being employed for a range of applications. In this chapter, some recent advances of supramolecular self-healing materials based on noncovalent interactions, including hydrogen bonding, π–π stacking, metal–ligand complexes, and ionomers are enumerated. Furthermore, the self-healing properties and mechanisms of these materials are described. Finally, their defects and prospects are also discussed.

Self-healing smart materials have been under extensive research and development during the past decade. Recent developments in self-healing materials have resulted in a new generation of load-carrying smart material systems with the ability to heal microscale to structural-scale damage. These materials have ranged from polymer matrix composites to ceramic matrix composites, and metal foams and others. The self-healing agent might contribute to the load-carrying capability of the system, or it might only function as a healing agent. One may notice that the major emphasis in this field has been given to experimental studies, and modeling schemes for these systems are still in the development stage. In order to extend the applicability of self-healing smart materials and deploy them in industrial applications, modeling techniques are of utmost importance. Difficulties in manufacturing and testing these new material systems can be compensated for by incorporating modeling techniques such as finite element analysis. It is state of the art to accurately simulate the inelastic, damage, and healing responses of self-healing systems and then construct a virtual design lab for optimizing the performance of these material systems. The mechanical responses of self-healing materials can be categorized into elastic, plastic, damage, and healing deformation mechanisms. In the case of damage-healing mechanisms, the accumulation of microflaws forms damaged sites, and regardless of the type of embedded healing technology, the lost properties of the system in those damaged sites are recovered through the healing functionality. Thus, physically consistent damage and healing models can be considered as basic tools to characterize the performance of self-healing materials. This chapter concerns the recent theoretical developments in the field of continuum damage-healing mechanics of self-healing materials. The mechanisms associated with the damage and healing are used to propose the damage and healing parameters. The evolution laws for the damage and healing processes are developed within the continuum damage-healing mechanics (CDHM) framework, and they are calibrated in accordance with experimentally measurable properties such as changes in the elastic moduli.

In the past few years, extrinsic self-healing based on microencapsulated healing agents has received more and more research interest. Self-healing of a variety of materials (including thermosets and thermoplastics, rigid and elastomeric) has been demonstrated by using this technique, which enables recovery of not only mechanical properties but also nonstructural functional properties. This chapter presents a comprehensive review of the recent progress in this field from the viewpoint of material design and preparation. The trends and challenges are presented at the end of the chapter. It is hoped that the reader will be provided with an understanding of the achievements to date and an insight into future developments.

This chapter deals with the recycling of synthetic polymers, another important topic of the future. High-temperature and supercritical water is an excellent reaction medium for decomposition of synthetic polymers, since it does not introduce compounds that contaminate the products and water is easy to remove from the decomposition products. In this chapter, examples are discussed that illustrate the principles of depolymerization reactions with water. Polymers from condensation polymerization processes, such as polyethylene terephthalate, nylon, and polyurethane, are readily depolymerized to their monomers in supercritical water. Polymers from addition polymerization processes, such as phenol resin, epoxy resin, and polyethylene, can also be depolymerized in high-temperature and supercritical water. Composite polymer materials can be separated into useful fractions. Chlorinated polymers can be treated to remove halogens, beside the conversion of the polymers to useful degradation products. From biodegradable polymers, such as poly(lactic acid), the monomer can be recovered in high purity and yield.

Infrastructures cover a very broad spectrum of different materials. This paper focuses on civil engineering structures, concrete and asphalt in particular. The public demand for such infrastructures is high level of service and performance, high durability and minimum negative ecological impact. New emerging self healing materials science provides solutions to the problem. An overview is given of new developments obtained in research on self healing of cracks in cement based materials and asphalt concrete. At Delft University various projects are running to study self healing mechanisms. The first project that is discussed is Bacterial Concrete, in which bacteria are mixed in concrete, that can precipitate calcite in a crack and with that make concrete structures water tight and enhance durability. In a second project hybrid fibre reinforced cementitious materials are studied that can mechanically repair cracks when they occur. The last project described in this paper is on the ravelling of porous asphalt concrete and how to heal this damage by incorporating embedded microcapsules or steel fibres. The state of the art results in all projects show that self healing is not just a miracle, but materials can be designed for it.

This chapter reviews the original classification of polymers and shows why it is no longer generally applicable. It summarizes the current accepted meaning of the terms addition and condensation polymers, and then turns to a useful, alternative classification which focuses on polymerization processes rather than the products of such processes. There are many possible ways to classify polymers. Each may be useful depending on the interests of the classifier. Examples include typing according to the source of the product (naturally occurring polymers, entirely synthetic macromolecules, or those derived by chemical modification of naturally occurring polymers), chemical structure (polyolefin, polyamide, et cetera.), polymer texture during use (rubbery, glassy, partially crystalline), area of application (adhesive, fiber, etc.), and so on. An important classification divides macromolecules into addition and condensation polymers. This distinction was made by W.H. Carothers, who invented nylon-6, 6 and made many fundamental contributions to our knowledge and control of polymerizations. A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. In addition polymers, by contrast, the recurring units have the same structures as the monomer(s) from which the polymer was formed. Examples are polystyrene (1-1), polyethylene (1-3), styrene-maleic anhydride copolymers (1-26), and so on.

The self-assembly of molecules by supramolecular interactions into highly ordered macromolecular architectures has evolved from a chemical oddity to a highly versatile tool in modern polymer and materials research. Following the basic concepts of polycondensation/polyaddition mechanisms, a diversity of supramolecular polymers, based on, for example, metal-to-ligand coordination, ionic interaction, or hydrogen bonding, has been reported. In this chapter, the synthetic strategies leading to these materials as well as their potential applications are discussed using a selection of descriptive examples.

Polymers can be of biological and synthetic origin, and are very important in our daily lives. They are used in all the major industries including textile, automotive, household goods, medical devices and products, etc. The significant difference between the requirements of these applications necessitates the availability of a large variety of polymers to choose from. Synthetic polymers can be prepared by addition and condensation polymerization using a variety of polymerization processes to yield polymers with differences in their stereoregularity, organization of their constituents, molecular weight, and crystallinity. All these differences yield polymers with different mechanical properties. Since the number of monomers available is very high, the variety of the polymers theoretically possible is also high. Some become viscoelastic and some are just plastic. Polymers are not always linear like noodles but can be made to have branches either of the same monomer or by adding other monomers to change the chemistry and the crystallinity of the product. They can be further branched or cross-linked so that the polymer becomes insoluble, and just swells in its linear polymer's solvents creating the gels. Or, a number of monomers can be used simultaneously to prepare macromolecules with properties that can be tailored to the needs of the application. All these play a deciding role on whether a polymer is suitable for load-bearing applications or in replacing the soft tissues in the human body. In this chapter, the main approaches to polymer synthesis and the parameters influencing the properties of the final product are discussed.

Copolymers of ethylene and methacrylic acid that can be prepared by control of polymerization conditions are known as ionomers because of their ionic nature. In general, ionic polymers (ionomers) have a hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. Another group with a much lower number of ionic pendant groups (up to about 10 mol %) exhibits high extensibility and low permanent set––that is, the characteristics of elastomers. Such elastomers exhibit properties of vulcanized rubber can be processed as thermoplastics. They have all the advantages of thermoplastic elastomers, that is, they can be processed by a variety of methods (extrusion, blow molding, thermoforming, injection molding), can be heat welded, and their scrap can be recycled. They require little compounding and their properties can be easily set by changing the ratio of components. However, they also have a number of disadvantages typical for many thermoplastic elastomers, such as softening and melting with increasing temperature, showing creep on extended use. In addition, unlike many thermoplastic elastomers, they deteriorate in the presence of water.