Self-healing materials (also referred to as autonomous healing materials or self-healing/self-repair materials) are a class of materials capable of repairing damage autonomously, without human intervention, after sustaining mechanical or structural degradation.¹ While most research has focused on polymers—including thermoplastics, thermosets, and elastomers—as well as composite materials, self-healing concepts have also been successfully applied to a wide range of materials, including asphalt, concrete, ceramics, and metals. As such, self-healing materials represent an important category within the broader field of smart materials*.
The development of self-healing materials originated from the concept of biomimicry, which seeks to replicate characteristics found in nature and living organisms in inanimate objects and materials. In this context, the objective is to “heal” (repair) a component after it has suffered “injury” (damage or failure), drawing inspiration from principles used in medicine and pharmaceuticals through physico-chemical mechanisms. Naturally, the healing rate depends on both the material itself and the surrounding environmental conditions.
The first generation of self-healing materials, developed between the 1950s and 1980s, was capable of repairing damage only once. However, decades of scientific advances—particularly since the early 2000s—have led to the development of materials that can heal repeatedly, in some cases almost indefinitely. This has been made possible through the use of reversible crosslinks based on intermolecular interactions or reversible chemical reactions, often relying on principles of supramolecular chemistry.
Self-healing materials generally follow a three-stage healing process, analogous to a typical biological healing response.
The first stage is damage detection and activation, which occurs almost immediately after damage is sustained. The second stage involves the transport (diffusion) of a healing agent to the affected area and is typically also rapid. The third stage consists of the physico-chemical repair process, which may involve a variety of mechanisms, including polymerization, interchain interactions, reversible crosslinking, chain entanglement (chain diffusion), or other molecular rearrangements.
This final stage is generally the rate-limiting step, as it is the slowest phase of the healing process.
Based on their healing mechanisms, autonomous self-healing materials can generally be classified into three categories: extrinsic, intrinsic, and hybrid systems.
Extrinsic systems rely on an external healing agent, which is typically released from microcapsules dispersed within the matrix or delivered through vascular networks such as channels, pores, or hollow fibers, as illustrated in Figure 1. In these materials, the healing process is usually triggered by the formation of a microcrack, after which the healing agent is released and polymerization or crosslinking begins, often at room temperature. The first generation of self-healing materials was primarily based on these extrinsic approaches.
In intrinsic systems, healing occurs through the restoration of reversible chemical bonds (covalent or ionic), intermolecular or supramolecular interactions, or molecular interdiffusion processes, as shown in Figure 2. Because these mechanisms are built into the material itself, intrinsic systems can often undergo multiple healing cycles.
More recently, hybrid systems have been developed, particularly for elastomeric applications. These combine several intrinsic healing mechanisms, including both covalent processes—such as Diels–Alder chemistry, disulfide/diselenide exchange, and transesterification—and non-covalent interactions, including hydrogen bonding, π–π interactions, ionic interactions, metal–ligand coordination, and shape-memory effects.
A revised classification of self-healing materials was proposed in 2020, based on the chronological evolution of the field rather than solely on the underlying healing mechanism. In this framework, self-healing materials are categorized into four generations:
- First generation: Extrinsic systems based on embedded microcapsules.
- Second generation: Intrinsic systems relying on reversible chemical or physical interactions.
- Third generation: Extrinsic systems incorporating vascular networks to deliver healing agents.
- Fourth generation: Hybrid systems, primarily intrinsic, combining multiple covalent and non-covalent healing mechanisms.
If a material requires the application of an external stimulus—such as heat, pressure, light, pH changes, or another environmental trigger—to initiate the repair process, it is still considered self-healing, but not autonomously self-healing (see Figure 2).
The self-healing approach offers significant advantages by substantially extending the service life of components, making it closely aligned with the principles of durability and sustainable development that increasingly guide modern engineering. It also enhances the safety and reliability of structures by preventing catastrophic failure through the repair of microcracks, which are often difficult to detect and costly to repair, as well as by mitigating premature wear under demanding service conditions. A typical example is the increased lifespan of components subjected to cyclic loading, such as mechanical or thermal fatigue.²
In addition, self-healing materials can significantly reduce maintenance requirements, minimizing both downtime and repair costs over a component’s lifetime.
Finally, vitrimers, as discussed in my previous article,³ also exhibit self-healing capabilities due to their dynamic covalent network, whose ionic or exchangeable bonds can reorganize over time following localized damage. These unique characteristics open the door to a wide range of applications, enabling the development of components that are more durable, damage-tolerant, and capable of maintaining their performance over extended service lives.
* Did you know? A material is considered smart if it exhibits properties that are responsive, adaptive, and capable of evolving in response to changes in its environment.⁴ Many polymers and composite materials fall into this category, particularly when they can respond to an external stimulus. These stimuli may include changes in temperature, humidity, pH, or mechanical stress, as well as exposure to electric or magnetic fields, or various forms of electromagnetic radiation, including visible light, infrared, ultraviolet, and X-rays.
By Denis Rodrigue, Special Contributor
Denis Rodrigue, P.Eng., Ph.D., is a Professor and Researcher in the Department of Chemical Engineering at Laval University (Denis.Rodrigue@gch.ulaval.ca).
This article has not undergone thorough linguistic review. It may therefore contain some spelling and syntax errors. Rest assured that the nature of the content and information presented is not affected.
(1) https://fr.wikipedia.org/wiki/Mat%C3%A9riau_auto-cicatrisant
(2) O.A. Cioffi et al., A review on self-healing polymers and polymer composites for structural applications, Polymer Composites, 43, 7643 (2022).
(3) Rodrigue, Vitrimères : une nouvelle génération de polymères révolutionnant recyclabilité et durabilité, Polymères Québec, 2(2), 14-16 (2024).
