Self-healing materials | Definition, Types, Mechanism, and Applications

• Self-healing materials respond to and work on micro-damage and micro-cracks. (Before material failure or fatigue).

The simplest and best type known of Self-healing materials, particularly those employing built-in microcapsules. These materials contain tiny embedded pockets known as microcapsules, which are filled with a glue-like chemical capable of fixing damage. When the material experiences a crack, the capsules rupture, releasing the repair material, which then seeps into the crack, effectively sealing it. This mechanism is remind us of the process used in epoxy adhesives, where two liquid polymers are mixed together to form a copolymer adhesive.

Self-healing materials can utilize embedded capsules in various ways. The simplest method involves the capsules releasing an adhesive that fills the crack and binds the material together. Alternatively, the material itself may be a solid polymer, with the capsules containing a liquid monomer—the basic repeating unit of the polymer. Upon failure and capsule rupture, the monomer mixes with the polymer, initiating further polymerization, effectively healing the damaged area by replenishing the original material. To enable polymerization at everyday temperatures and pressures, a powdered chemical catalyst is usually embedded within the capsules.

However, one limitation of the encapsulation method is that the capsules must be very small to avoid weakening the material in which they are embedded. This restricts their ability to fix larger damages or cracks. Additionally, the capsules can only heal damage once. If the material fails again, it cannot self-heal a second time, making it more vulnerable after repair.

Despite these challenges, self-healing materials using microcapsules remain a compelling avenue for research and development. Scientists continue to explore ways to optimize the healing capabilities and address these limitations, paving the way for more advanced and resilient self-repairing materials in the future.

Embedded healing agents are indeed effective but have the drawback of potentially weakening the material due to the interruption of its structure with capsules. The goal is to avoid increasing the risk of failure, which defeats the purpose of self-healing materials. Instead, scientists have turned to the human body's regenerative processes for inspiration.

The human body does not rely on makeshift repair materials waiting inside every part of our skin and bones. Instead, it has a remarkable vascular system—a network of blood vessels that transport blood and oxygen to provide energy and repair damaged tissues. When damage occurs, the blood system delivers extra resources precisely where they are needed, but only when needed.

In the quest for self-healing materials, scientists have been working to replicate this approach. Some materials now feature networks of extremely thin vascular tubes, approximately 100 microns thick (slightly thicker than an average human hair), integrated within them. These tubes act like microvascular systems capable of pumping healing agents, such as adhesives, to the point of failure when necessary. The tubes lead to pressurized reservoirs, functioning like pre-loaded syringes. When a failure occurs, the pressure is released at one end of the tube, causing the healing agent to pump into the damaged area precisely where it is needed.

While this vascular approach can seal cracks up to ten times the size that the microcapsule method can manage, it may work more slowly since the repair material has a greater distance to travel. This could become a concern if a crack is spreading faster than it can be repaired. However, for structures like skyscrapers or bridges, where failures might appear and creep over extended periods (months or years), a well-designed system of built-in repair tubes can be highly effective.

Some polymers possess innate self-healing capabilities without the need for sophisticated internal systems. These polymers have reactive ends or fragments that naturally attempt to rejoin when exposed to energy, either in the form of light or heat. When damaged, these fragments seek nearby molecules to bond with, effectively reversing the damage and repairing the material. In some cases, polymers break to expose electrically charged ends, leading to built-in electrostatic attraction that pulls the broken fragments together for self-repair.

In particular, certain thermoplastics offer promising potential as self-healing materials. Thermoplastics can be melted down, recycled, and molded into new forms, making them amenable to self-healing properties. When these thermoplastics experience damage, they can be designed to break down into their monomers—the repeating molecules from which they are built—when heated. Upon cooling, the original polymer reforms, effectively reversing the damage and restoring the material's strength.

Practically, this self-healing process can be triggered by applying heat to the damaged area. For instance, when thermoplastic materials are cracked or damaged and then subjected to localized heat (e.g., generated from an impact like a bullet), the polymers break down into their monomers and reseal the hole. The material binds together again, restoring its integrity.

This ability to self-heal through heat opens up fascinating possibilities, particularly in applications where heat sources are readily available. For instance, materials with such self-healing properties could be highly beneficial in high-stress environments like fighter jets, where bullet holes could rapidly seal up and disappear, ensuring continued performance and safety.

Shape memory materials are familiar in everyday applications like nitinol-based eyeglasses, which return to their original shape when bent and released. However, their operation is more complex and fascinating than this simple example. Typically, shape memory materials require energy input, usually in the form of heat, to snap back to their preferred shape. When it comes to self-healing shape-memory materials, delivering heat to the damaged area is crucial for their restoration.

One method to achieve this involves embedding a network of fiber-optic cables, similar to vascular networks in other self-healing materials, into the material. Instead of pumping up a polymer or adhesive, these tubes serve as channels to deliver laser light and heat energy precisely to the point of failure. The heat causes the material to revert to its original shape, effectively reversing the damage.

To determine where the laser light needs to be delivered, the fiber-optic tubes are designed to crack along with the material. As a result, when the material itself cracks, the fiber-optic tubes inside it also crack, causing the laser light to leak out directly at the point of failure.

Despite the initial concern that fiber-optic tubes might weaken the material, they can actually strengthen it by transforming it into a fiber-reinforced composite. This is similar to how fiberglass gains strength from embedded fibers, or how reinforced concrete incorporates steel "rebar" rods. The use of fiber-optic tubes enables the material to have improved mechanical properties.

These innovative systems, known as autonomous adaptive structures, have been pioneered by materials engineer Henry Sodano and represent an exciting advancement in the field of self-healing materials. By combining shape memory properties with self-healing capabilities, such materials offer tremendous potential in creating stronger, more resilient, and adaptable structures and products.

These materials respond to mechanical stress or external stimuli, triggering a self-healing process when damage occurs. The stress activates the healing mechanism, allowing the material to repair itself.

Inspired by natural biological processes, these materials imitate the regenerative abilities of living organisms. They might use biochemical reactions or mimic biological systems to heal themselves when damaged.

In this type, the material employs electrostatic forces to facilitate healing. When damage occurs, electrostatic interactions help bring the separated surfaces back together, promoting self-repair.

Some self-healing materials can be triggered to repair themselves when exposed to specific wavelengths of light. The light initiates chemical reactions that enable healing.

How do self-healing materials work?

Self-healing materials often contain tiny capsules or microcapsules dispersed throughout the material. These capsules are filled with a healing agent, such as a resin or adhesive. When the material experiences damage, the capsules rupture, releasing the healing agent, which then flows into the crack or damaged area. The healing agent then solidifies and bonds the separated surfaces, effectively healing the material.

Some self-healing materials use reversible chemical reactions to repair damage. When the material is damaged, specific chemical bonds within the material break. However, under the right conditions, these broken bonds can reform, effectively healing the material.

The exact mechanisms and efficiency of self-healing materials can vary depending on the specific material composition and the type of damage they are designed to heal. Researchers continue to explore and develop new self-healing strategies to enhance the performance and applicability of these innovative materials across various industries.

Advantages of Self-Healing Materials