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The rise of selfhealing materials

by R. Raghunandan
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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, 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 selfhealing 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!

Today’s relentless tech advances can often feel like something from a sci-fi film. But perhaps one development in particular feels distinctly like the conjuring of a Hollywood special effects department: the rise of selfhealing materials. Self-healing materials are artificial substances that repair themselves automatically – without human intervention. Some selfhealing materials are activated with external stimulus, like light or heat. They are an important breakthrough because materials generally degrade over time, which can change their properties. The benefits of selfhealing are profound, potentially increasing reliability, structural integrity and performance while reducing maintenance costs and increasing their lifespan.

Self-healing materials come in many forms including; materials embedded with tiny capsules that contain polymers that combine when released to seal up cracks, like glue; a network of tiny tubes that can pump healing agents to a damaged area; materials that have shape memory, with the drawback that heat must be applied to return the image to its original shape; and reversible polymers, which have highly reactive ends that join back up again sometimes through built-in electrostatic attraction.

A range of self-healing materials exist, including concrete which uses encapsulated bacteria that secrete calcium carbonate when released, to repair cracks. A recent breakthrough by Clemson University gives selfhealing qualities to polymers used in relatively inexpensive commodities, such as paints, plastics and coatings without having to build factories to manufacture them. This could, for example, be used with the paint on a car, to self-repair chips and scrapes.

What are self-healing materials?

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
  • Wear
  • Defects

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. 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 themself.

Types of self-healing materials

Self-healing materials come in four main kinds. Let’s look at each of these in turn

1. 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.

2. Microvascular materials : These materials 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.

3. Shape-memory materials:

Shape-memory materials behave differently. They’re strong, lightweight alloys (generally, mixtures of two or metals) with a very special property. They can be “programmed” to remember their original shape, so if you bend or squeeze them you can get that original shape back again just by heating them. This is called the shape-memory effect (or thermal shape-memory effect, since heat energy makes it happen). Some shape-memory alloys remember one shape when they’re hot and a different one when they’re cold, so if you cool them they spring into one shape and if you heat them they “forget” that shape and flex into a different one. This is known as the two-way shape-memory effect.

4. 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 re-bond 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.

Scientists hope that self-healing materials will improve the durability of everything from cars and electronics to bridges and aeroplane fuselages. If their research makes it out of the lab and into our handheld devices, it could alter how we think about everyday objects and products.

 

 



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