Cut yourself and—if you're lucky—your skin will heal with no trace in as little as a week. Crash your car into a wall or scratch its paintwork and you won't be so fortunate; you'll need to drive it to a repair shop for horribly expensive correction. Skin, bone, and the stuff of life is truly amazing: it can sense damage, stop it getting any worse (using cunning mechanisms such as pain), and repair itself automatically with little or no help from us. It's incredible! If only metals, plastics, composites, and other everyday materials were half as smart. Soon they could be: in the early 2000s, scientists began developing self-healing materials that could repair internal damage all by themselves. Before long, we'll see self-healing paints and coatings—maybe even self-healing cars, bridges, and buildings! So how exactly do these wonder materials actually work? Let's take a closer look!

self healing materials

Self-healing materials are artificial or synthetically-created substances which have the built-in ability to automatically repair damage to themselves without any external diagnosis of the problem or human intervention. Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal, electrical, and acoustical properties of materials, and the propagation of cracks can lead to eventual failure of the material. In general, cracks are hard to detect at an early stage, and manual intervention is required for periodic inspections and repairs. In contrast, self-healing materials counter degradation through the initiation of a repair mechanism which responds to the micro-damage. Some self-healing materials are classed as smart structures, and can adapt to various environmental conditions according to their sensing and actuation properties

Type of self healing materials   

Shape-memory materials

 

Most of us know shape memory materials through relatively trivial everyday applications such as eyeglasses, made from alloys like nitinol (nickel-titanium), that flex exactly back to shape when you bend and then release them. Usually, shape memory works in a more complex (and interesting) way than this (read all about it in our detailed article on shape memory); typically you need to heat (or otherwise supply energy to) a material to make it snap back to its original, preferred form. Self-healing shape-memory materials therefore need some sort of mechanism for delivering heat to the place where damage has occurred.

In practice, that might be an embedded network of fiber-optic cables similar to the vascular networks used in other self-healing materials except that, instead of pumping up a polymer or adhesive, these tubes are used to feed laser light and heat energy to the point of failure. That causes them to flip back into ("remember") their preferred shape, effectively reversing the damage. How do the tubes know where to deliver their light? If the material cracks, it also cracks the fiber-optic tubes embedded inside it so the laser light they carry leaks out directly at the point of failure. Although you might think fiber-optic tubes would weaken a material, they can actually strengthen it by turning it into a fiber-reinforced composite (effectively, they serve as the fibers you'd get in something like fiberglass, or like the steel "rebar" rods in reinforced concrete). Systems like this are sometimes known as autonomous adaptive structures and have been pioneered by materials engineer Henry Sodano.

Reversible polymers

Polymers don't always need sophisticated internal systems, such as embedded capsules or vascular tubes, to repair internal damage. Some of them break apart to reveal what we might think of as highly "reactive" ends or fragments that naturally try to join up again. Energized by either light or heat, these stray fragments naturally try to rebond themselves to other nearby molecules, effectively reversing the damage and repairing the material. Some break to expose electrically charged ends, which give the broken fragments a built-in electrostatic attraction. When damage occurs, electrostatic forces pull the fragments together, enabling the material to self-repair.

Sometimes, all you need to repair damage is a little heat. Plastics come in two main kinds. Some (known as thermoplastics) are relatively easy to melt down, recycle, and mold into new forms; PVC (polyvinyl chloride), polyethylene, and polypropylene are typical examples. Others (known as thermosets or thermosetting plastics) work a different way: if you heat them, they degrade before they melt so you can't heat them to reshape them; melamine and bakelite are good examples. This suggests that we might be able to use thermoplastics (but not thermosets) as self-healing materials. We'd simply need them to melt under stress so the long polymer chains inside could rearrange themselves back into a strong, new form.

How would that happen in practice? Thermoplastics can be designed so that if they're cracked or damaged, and then heated, the polymers from which they're made will break down into their monomers (the repeating molecules from which they're built). When they cool down, the original polymer reforms, reversing the damage. This method does rely on a convenient supply of heat—but sometimes that's readily available. Materials like this haovides enough energy (a temperature rise in the damaged area of maybe a couple of hundred degrees) for the polymer to reseal the hole and completely bind the material together again. It's easy to ive been tested by firing bullets (up to 9mm in diameter) at them. The localized heat from the impact prmagine invaluable applications in fighter jets with bullet holes that rapidly seal up and disappear!

Microvascular materials

 

Embedded healing agents are simple and effective, but they do have a drawback: interrupting the structure of the material with capsules can actually weaken it, potentially increasing the risk of failure—which is the very problem we're trying to solve! Now the human body doesn't fix damage this way with makeshift repair materials waiting inside every bit of skin and bone in case we happen to cut ourselves or fall over. Instead, our body has an amazingly comprehensive vascular system (a network of blood vessels of different sizes) that transport blood and oxygen for energy and repair. If damage occurs, our blood system simply pumps extra resources to the places where they're needed, but only when they're needed.
Materials scientists have been trying to design self-healing materials that work the same way. Some have networks of extremely thin vascular tubes (around 100 microns thick—a little thicker than an average human hair) built into them that can pump healing agents (adhesives, or whatever else is needed) to the point of failure only when they need to do so. The tubes lead into pressurized reservoirs (think of syringes that are already pushed in slightly). When a failure occurs, the pressure is released at one end of the tube causing the healing agent to pump in to the place where it's needed. Although this method can seal cracks up to ten times the size that the microcapsule method can manage, it works more slowly because the repair material has further to travel; that could pose a problem if a crack is spreading faster than it's being repaired. But in something like a skyscraper or a bridge, where a failure might appear and creep (spread slowly) over months or years, a system of built-in repair tubes could certainly work well.