The self-healing superpower of the comic book character Wolverine is no longer a piece of imagination. Researchers around the world are working on self-healing materials for a variety of applications ranging from corrosion-resistant coatings to artificial muscles .
Many elements of the natural world are found in today’s modern technologies. The class of self-healing materials is inspired by the biological systems which have the inherent ability to heal. The method of healing and reform in organisms is initiated by a chemical signal. This triggers inflammation followed by the wound closure. Thus, the process first takes place at a molecular level supported by the tissue level . A similar approach is developed in case of the engineered materials mimicking this functionality.
A self-healing property in engineered materials is defined as “the ability to repair damage and restore lost or degraded properties using resources inherently available to the system .” All materials are bound to fail eventually as wear and damage is a part of their application. Self- healing materials offer a way forward on the path to safer and longer lasting products.
Types of self-healing materials
In 2001, Scott White and his colleagues from the University of Illinois at Urbana-Champaign reported the first self-healing material . The material, which utilised internally embedded adhesives to heal, belonged to the materials class of polymers. Since then, many research groups around the world have developed self-healing materials that follow distinctive approaches to achieve the same functionality. These can be classified broadly into the following types:
- Polymers: These are one of the most widely used families of materials due to the wide range of properties they offer. However, under normal ageing and wear during their use, they tend to develop small cracks. This wear leads to degradation of mechanical properties and eventually renders the material useless. This led to the development of self-healing polymers and hydrogels.
- Coatings: These are usually applied to protect the surface from wear and tear. Self- healing coatings have already found commercial applications, a corrosion resistant coating by Nissan being one such example.
- Ceramics: Self-healing is sometimes present in structural ceramics to restore mechanical properties. Various distinct approaches of self-healing can be used in the case of ceramics.
- Metals: The property of self-healing in the case of metals is not as developed as with other classes of materials. Researchers are currently studying the process computationally and developing models for possible designs of self-healing metals.
Approaches to self-healing
Several methods can be followed to achieve self-healing in engineered materials. These approaches differ by the mechanism used to seize the healing functionality. The extent of damage that can be healed, the repeatability of the process and the recovery rate also depend on the type of approach followed. Approaches to self-healing can be broadly classified into two categories:
- Intrinsic self-healing: When no external agent is required for the material to heal;
- Extrinsic self-healing: When a foreign agent is required to trigger the healing process.
The discussion in the subsequent sections will focus on self-healing polymers and hydrogels.
Polymers sometimes have inherently reversible bonds present in the matrix. These trigger the healing process when the material is damaged and this approach is called intrinsic self- healing . It can be achieved through the following pathways:
- Thermally reversible reactions
- Ionomeric coupling
- Molecular diffusion
In the extrinsic self-healing approach, microcapsules or vascular systems are used to separate the healing agent from the polymer matrix. When the material is damaged, these agents are released to facilitate healing and restoration of properties of the object . These systems can be sub-divided in the following categories:
1. Capsule-based self-healing: In some polymers, the healing agent is packed in discrete capsules. When the material is damaged, the contents of the capsule are released and the healing process begins. Many encapsulation techniques are available as the production of these capsules is easily industrialised. For the material to heal up to an acceptable level, sufficient healing agent should be present to fill the crack. The weight fraction and the size of the capsules are determined based on the approximate crack size that the material is likely to undergo. Other critical factors involved in capsule-based self-healing are:
- Adhesion between the healing agent and the matrix
- Concentration of the healing agent
- Rate of release and polymerisation
- Shell thickness of the capsules
2. Vascular self-healing: In this approach, the healing agent is stored in an interconnected network of capillaries or hollow channels. These capillaries are usually brittle and damage forces the healing agent out of them. It is a necessary condition for the healing agent to be fluid around the operating temperature of the parent material. Vacuum-assisted techniques are used to fill the interconnected vascular system with the healing agent. Several parameters such as tubing material, thickness, and spatial distribution are critical to ensure a high healing efficiency.
Assessment of healing performance
The main aim of developing self-healing materials was to overcome the damage a material undergoes during its usual operation. This is achieved by filling up of damaged volume and reformation of broken bonds to restore the properties, particularly mechanical properties.
- Quantification of healing efficiency: a number of parameters can be used to quantify the effectiveness of healing. The ratio of change in the material property (mostly mechanical properties) is one of the most common factors used.
- The mechanical properties of the parent material are characterised with techniques such as Dynamic Mechanical Analysis (DMA) and Atomic Force Microscopy (AFM). The test specimen is fractured and the mechanical properties are further characterised and recorded. These are then used to quantify the healing efficiency of the material.
Applications and future prospects
One can easily imagine all kinds of applications for self-healing materials. Engineers and designers around the world are slowly recognising their benefits and are starting to use them for a variety of applications. They are currently being used in applications ranging from Nissan’s scratch-resistant paint to self-healing concrete.
Inspired by the osteoblast cells which help in the formation of new bone, Dutch scientists at the Delft University of Technology developed the self-healing concrete . They embedded the concrete with capsules containing bacteria capable of producing limestone. Initiation of cracks exposes the bacteria to air and moisture which in turn commences the production of calcite. This seals off even the micro-cracks, thus preventing their possible growth. This is just one of the many examples where the self-healing property of material could solve a longstanding problem.
The potential next-generation applications of these materials are limitless. They could be used in robots for self-healing after a mechanical failure, extend the lifetime of batteries and much more. Still, many challenges remain before we will see them being used for such advanced applications. With increasing knowledge in this domain, one day we might even develop a self-healing material as versatile as the fictional comic book character Wolverine.
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