by Chris Woodford. Last updated: September 19, 2018.
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!
Photo: Searching for damage: Testing for cracks in metal parts is time-consuming, expensive, and laborious. It's also prone to human error: undetected cracks in vital airplane components could cost lives. The "nondestructive testing" shown here involves immersing an object in a fluorescent dye, washing the surface, then shining ultraviolet (UV) light on it. If there are any cracks, some dye will stick in them and they'll show up as bright, visible lines under the UV. Wouldn't it be better if the material could automatically detect internal damage and heal itself up? That's the concept behind self-healing materials. Photo by Dana Hill courtesy of US Air Force.
What are self-healing materials?
Photo: Skin: the ultimate self-healing material. Despite suffering a blister, the skin on this toe is almost back to normal, thanks to a few days of rest and (automatic) repair. Wouldn't it be amazing if synthetic materials could do this too?
Nothing lasts forever, although some natural materials (such as stone) certainly do their best. Materials that we use everyday generally stop working for three different reasons:
- Aging: Most materials gradually decay, sometimes over a very long period of time (wood rots eventually when microorganisms or insects eat it away, and even plastics break down after a few hundred years—or sooner with help from heat and light).
- Wear: Most materials wear out gradually through constant use (friction is one of the biggest culprits; materials that are repeatedly moved back and forth will break through fatigue).
- Defects: Some materials break suddenly and very unexpectedly when applied forces (stresses and strains) make internal fractures (usually tiny cracks or other defects inside) spread rapidly.
To a materials scientist, the third problem—spontaneous failure—is the most dangerous and the hardest to tackle. With regular inspection and maintenance, it's easy to spot rotting wood or rusting iron; it's much harder to notice hairline cracks hiding in crucial components, themselves buried deep inside hot engines spinning at high speeds. Technologies such as nondestructive testing (including ultrasound scanning) make it easier to find potential problems during routine inspections, but they're not much use if failures occur while materials are actually in use.
What we really need are artificial materials that behave like the human body: sensing a failure, stopping it from getting worse, and then repairing it as quickly as possible, all by themselves. That's the basic concept of the "self-healing" material, which we'll define as an artificial (synthetic) substance that automatically repairs itself without any overt diagnosis of the problem or intervention by a human being.
Types of self-healing materials
The first self-healing materials were polymers (plastics made from long, repeating molecules) with a kind of embedded internal adhesive, reported in 2001 by Scott White, Nancy Sottos, and colleagues from the University of Illinois at Urbana-Champaign. Since then, a variety of other self-healing materials have been developed. Self-healing materials come in four main kinds: materials with embedded "healing agents," like those developed by Professor White; materials with a kind of internal "vascular" circulation analogous to blood; shape-memory materials; and reversible polymers. Let's look at each of these in turn.
Embedded healing agents
The best-known self-healing materials have built-in microcapsules (tiny embedded pockets) filled with a glue-like chemical that can repair damage. If the material cracks inside, the capsules break open, the repair material "wicks" out, and the crack seals up. It works in a similar way to a type of adhesive (glue) called epoxy, which is supplied in the form of two liquid polymers in separate containers (often two syringes). When you mix the liquids together, a chemical reaction occurs and a strong adhesive (a copolymer) forms.
Photo: Epoxy in action: This is how we usually see epoxy: two tubes of polymer are squeezed together, side by side, so they mix, react, and form a tough adhesive or sealant. Now imagine this process shrunk down to microscopic form and embedded inside a material so it can repair itself automatically. Photo by Quinton Russ courtesy of US Air Force.
Self-healing materials can use embedded capsules in a variety of different ways. The simplest approach is for the capsules to release an adhesive that simply fills the crack and binds the material together. In a slightly different approach, the main body of the material is a solid polymer, while the capsules contain a liquid monomer (one of the basic, endlessly repeated units that makes up the polymer). When the material fails and the capsules break, the monomer mixes with the polymer, more polymerization occurs, and the damage is healed effectively by creating more of the original material to replace the damaged area. Typically, a powdered chemical catalyst has to be embedded as well so the polymerization will happen at a relatively low, everyday temperature and pressure.
The main drawback with the encapsulation method is that the capsules have to be very small indeed or they weaken the material in which they're embedded; that limits the amount of damage they can fix (the size of the cracks they can fill). Another problem is that the capsules can only heal damage once: if the material fails again (more likely since it is almost certainly weaker after repair) it cannot heal itself twice.
Photo: How embedded microcapsules work: The material (1) contains tiny, embedded capsules of healing agent (2) and a catalyst (3). When a crack (4) starts to spread (the white line opening up from the left), it breaks open some of the capules (5), releasing the healing agent, which reacts with the help of the catalyst to form a polymer that fills the crack (6).
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.
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.
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 have been tested by firing bullets (up to 9mm in diameter) at them. The localized heat from the impact provides 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 imagine invaluable applications in fighter jets with bullet holes that rapidly seal up and disappear!
Photo: Bullets cut through metal like butter, threatening the safety of airplanes—but what if "wounds" like this could seal and heal themselves automatically? Picture by TSgt. Val Gempis courtesy of US Air Force.
What can we use self-healing materials for?
It's not difficult to imagine all kinds of applications for self-healing materials, from bridges and buildings that repair their own cracks to car fenders made from shape-memory polymers that automatically flex back to shape after low-speed collisions. The first self-healing materials we're likely to see in mass production will be paints and coatings that can better survive the weather and other kinds of surface wear-and-tear. (How about car paintwork that automatically seals up scratches?) More advanced self-repairing materials are likely to follow on, including things like self-repairing seals and gaskets for pipelines. One day, we might even have replacement parts for the human body that can heal themselves as well as their natural equivalents. At that point, the science of self-healing will truly have come round full circle. Trust scientists to reinvent nature!