How are self-healing materials reshaping the structure of things?

Gradual wear is another certainty for most materials due to continual use. Friction is the number one wear-and-tear culprit–materials rub together repeatedly, moving back and forth and breaking because of fatigue.

Another issue that limits the lifespan of materials is the presence of defects. Many materials will suddenly break. Sometimes, applied forces (strains and stresses) can cause internal fractures in the form of tiny cracks (or other inner defects) to spread quickly.

According to many materials scientists, spontaneous failure is the most challenging and dangerous threat to materials. Regular maintenance and inspections help easily spot rotting wood, rusting iron, etc. Still, undetectable hairline cracks are buried in crucial components already deep within high-speed scorching-hot engines.

Nondestructive testing methods (e.g., ultrasound scanning) help discover possible issues during inspections. However, they don’t help much during usage failures.

Enter an innovation known as self-healing materials. These artificial components treat various objects to react like the human body. On their own, these materials sense a breakage or failure and prevent it from worsening while repairing it ASAP.

By definition, self-healing material is an artificial substance that repairs itself without any human diagnosis or invention. In another sense, these materials provide a version of automation since they perform an automatic function. The differentiating factor here, though, is that this automation occurs beyond the realms of digital transformation.

Polymers are plastics consisting of long, repeating molecules. These were mixed with an embedded internal adhesive to form the first self-healing materials, as was reported in 2001 by a team from the University of Illinois at Urbana-Champaign.

Since then, many other forms of self-healing material innovation have been introduced.

One of the best-known examples of self-healing materials is an embedded capsule. These have built-in microcapsules (minuscule embedded pockets) filled with a restorative glue-like chemical. If the material cracks inside, the capsules burst, releasing the repair material into the cracks and sealing them up.

Embedded capsules work similarly to epoxy, a type of adhesive glue. Epoxy is supplied as two liquid polymers, usually in two syringes. After mixing the liquids, a chemical reaction occurs, forming a strong adhesive.

Self-healing materials have several ways to use embedded capsules, two of which are detailed below:

• Filling the cracks and binding the material with an adhesive released by the capsules.
• Using a solid polymer as the material’s main body. Additionally, a liquid monomer (a basic, constantly repeating unit making up the polymer) is contained within the capsules. The monomer and polymer mix when the capsule breaks after the material fails. This causes more polymerization, healing the damage by replacing the damaged area with the original material. This method might also require the embedding of a chemical catalyst.

Other self-healing materials have internal "vascular" circulation. There are also shape-memory materials and reversible polymers.

Despite the many associated benefits, a few common problems have hampered many self-healing materials since their inception. A new innovation in the space aims to rectify that issue, which we’re examining below.

Recently, a new self-healing composite innovation was developed, allowing structures to repair themselves without needing to be removed from service. Developed by engineering researchers, this state-of-the-art technology solves two problems forever plaguing self-healing materials, adding more lasting power to structural components (e.g., aircraft wings and wind turbine blades).

The two quintessential hurdles typically hindering self-healing materials are as follows:

1. For materials to heal, they must regularly be removed from service (e.g., oven hearing is often a necessity, which can’t be done with larger components or while a part is in use).
2. Self-healing is only effective for a limited time. For instance, the material can often heal several times, but the self-repairing properties would suffer.

This newly innovated composite rectifies the above issues. It will also ensure all affected structural components maintain optimal healing strength and performance.

Below, we’ll examine the nuts-and-bolts details of the innovative self-healing fibre-reinforced composites introduced above:

Fibrous reinforcement layers (like bonded-together carbon fibre and glass) help construct laminated composites. Typically, damage occurs when these layers’ binding agent (or “glue”) starts to delaminate and peel away from the reinforcement.

The research team's first step to address this issue was 3D printing a thermoplastic healing agent pattern onto the reinforcement material. They also embedded the composite with a thin heating layer, melting the healing agent. As needed, the healing agent flows into the composite’s microfractures or cracks and then repairs them.

This process can be repeated 100 times (at least) while maintaining the integrity of the self-healing properties. That said, the upper limit isn’t known.

Inherent fracture resistance increases by up to 500% once fortified with the printed thermoplastic. Thus, delaminating will require much more wear and tear and energy.

Another boon in this innovation is how the heater and healing agent layers all consist of everyday materials that are reasonable in price.

Incorporating this design into composites will be marginally more costly upfront than other self-healing materials. However, the resulting savings from the extended lifespans of the healed material more than offset those initial expenses.

If incorporated into aircraft wings, the heating elements of this new technology allow the airlines to stop using chemical agents to remove ice from the wings of grounded aircraft. Also, airlines could de-ice while in flight.

Self-healing materials have many potential use cases. For instance, it can help buildings and bridges repair cracks independently.

Another use case for self-healing materials is how shape-memory polymers can automatically flex car fender cracks back to shape after low-speed collisions.

What about mass production, though? What will be the first readily-available self-healing materials?

Experts believe coatings and paints meant to stave off the rigours of inclement weather or surface wear-and-tear will be sold to consumers before anything else.

More advanced self-healing materials will soon follow, such as self-healing gaskets and seals to repair pipelines.

Who knows what’s to come as self-healing materials become more advanced? Already, they’re changing the structure of “things.” Perhaps, a structural shift in society is next.