A material that has self-healing properties is capable of repairing damage such as cracks, punctures, and tears automatically, intrinsically, and independently. In a way similar to biological systems, self-healing materials can sense failure and repair it automatically. Such materials can be found in the form of fabric, concrete, ceramic, polymers, and metals- a number of examples are outlined below.

Self-healing materials like concrete and polymers may contain pockets of restorative chemistry that upon breaking, integrate into its host. If a material is stressed, tiny capsules of catalyst (such as an adhesive or a monomer that reconstitutes into its original polymer material) are broken and spread out to repair the damaged area. Coatings with embedded microcapsules of resin can restore scratched surfaces in the affected area. Longer-lasting architecture and infrastructure may be achieved with a limestone-producing microorganism embedded into the cement which becomes activated when water leaks into the structure.

Another type of self-healing material operates in a similar way to the human body’s vascular system, where a pressurized reactant is circulated through a network of tubes and released only where needed (and in larger quantities compared to the microcapsules). This can be effective in slowly deteriorating materials, where the healing solution has time to spread out to the affected area.

Memory shape alloys can also be used as a self-healing material. Exploiting its two distinct crystal structures, a memory shape alloy (commonly made from nickel and titanium) can be “programmed” to hold it’s shape with heat, be deformed, and repeatedly “remember” its original shape when the set temperature is recalled. Memory shape materials have widespread practical applications, such as in surgical instruments, where they can be used to expand and contract in minimally invasive procedures. Another example of a potential shape memory material is in a fiber optic application, where the broken tube can be repaired by the heat from the localized area that is leaking light, and correct its shape to its original form.

NASA has worked to develop puncture-repairing materials for use in protecting satellites and spacecraft. Thermoplastics designed to heal themselves find use in absorbing bullets. In this situation, the plastic is engineered so when it is punctured, the heat from the bullet wound breaks the polymer down into its constituent monomer. The hole closes as it restores itself back into the polymer while cooling down.

The area of self-healing materials is a relatively new field, and faces a number of engineering challenges. A main drawback of the microcapsule design is, once activated, those now empty cavities create a weaker structure. Could any solutions be found in pockets that also store a material that behaves like a hardening expanding foam? Or if the position or shape of the cavities were designed to support weight better, like the way foam or a seashell does? Another issue is once the initial repair has been made, the healing agent cannot replenish itself, so it is incapable of further repairs. Perhaps there are more biological alternatives like the limestone-producing bacteria that can regenerate itself. As the field advances, materials like self-healing glass, rubber, and paints are expected to be introduced, whose healing-systems more closely resemble those found in nature.