One much discussed application of self-healing materials is for smartphones or tablet devices with scratched or cracked screens. Prof Ian Bond's team has carried out a feasibility study to look at a similar problem affecting bullet-proof glass. "The windshield takes the bullet, but it shatters so that you can't see through it," he explains. Prof Bond looked at whether it was possible to flood the windshield with some substance that would fill the cracks. "That same principle you could potentially apply on a smaller scale," he says.
Bond says an intrinsic system with a trigger - sunlight perhaps - might also work: "Soldiers boil their goggles up in a billycan to heal any scratches or cracks on the surface. You can't do that with your phone, but you could perhaps put it by the window in the sunshine for 24 hours, and you might be able to heal up some cracks."
But it was a 2001 study led by Scott White from the University of Illinois at Urbana-Champaign, that really helped to kick-start the field. The group infused a plastic-like polymer with microscopic capsules containing a liquid healing agent. Cracking open the material caused the capsules to rupture, releasing the healing agent. When the agent made contact with a catalyst embedded in the material, a chemical reaction bonded the two faces of the crack together. The polymer recovered some 75% of its original toughness.
In the last decade, the team has developed and refined its capsule-based systems, recently demonstrating an electrical circuit that healed itself when damaged. Microcapsules in the gold circuit released liquid metal in response to damage, swiftly restoring electrical conductivity, and bringing self-repairing electronic chips a step closer.
Co-author Dr Benjamin Blaiszik, now at Argonne National Laboratory, explained that the self-healing circuitry could find uses in a military setting where it would be exposed to extreme stresses or in long-term space applications. He adds: "Imagine if there is a mechanical failure of a microchip on the Curiosity rover, due to thermomechanical stresses, or if there had been an interconnect failure during the landing phase. There is obviously no way to manually repair nor replace the probe."
The Illinois group is already commercializing their work via a spin-out company, Autonomic Materials, which has raised about $4m (£2.4m) of investment. Its chief executive, Joe Giuliani, told me the first applications of microcapsule systems are likely to be in coatings, paints and adhesives for environments where corrosion poses a challenge. "Worldwide, corrosion costs over $500bn (£312bn) a year, so it's a huge problem".
Oil and gas is a key area. Re-healable products are likely to find uses on platforms - where the ability to heal drilling parts would be highly desirable - in pipelines and in refineries. They would potentially last several years longer than their conventional counterparts, lengthening the periods between maintenance."Over the life of that equipment, there would be huge savings," says Giuliani. "It is out of commission for a lot less time too, which in the oil and gas business is huge. It can cost them $1m a day if an asset is out of service."
Military vehicles, cars and ships are other targets for self-healing coatings. The firm has about 30 products in testing and development and expects to fulfil its first commercial orders in the next six months. Some manufacturers might not welcome the idea of products that last years longer than usual. But paint and coatings producers "know they can get more per gallon of paint they're selling," says Mr Giuliani, "the customers have shown us they're willing to pay the up-charge."
Scott White, from Illinois University's Beckman Institute, says that healing structural damage in sports equipment or aircraft components, for example, represents a "mid-term target" for scientists.
He says the whole area of self-healing has seen an explosion of interest in the last decade, with some 200 academic papers published on the topic last year alone. And scientists are working on everything from re-healable polymers and composites (materials made from two or more different ones) to self-repairing metals and ceramics.
Each of the approaches has advantages and limitations that come into play when considering applications. Microcapsules are finite: as they get used up, the material loses its healing properties. And intrinsic systems need a stimulus - such as heat or light - to trigger healing, which can be good or bad depending on the application.
If the amount of damage is microscopic, capsule-based or intrinsic systems may be the best option. But, says Prof White, "if it's a large damaged volume, then neither of those approaches are going to work and you have to go with a vascular-based system". This is because they allow large amounts of healing agent to be transported to the breached area. But the sheer complexity of vascular networks presents a daunting challenge.
Prof Ian Bond and his colleague Dr Richard Trask at Bristol University are developing vascular networks based on hollow fibres that transmit healing agent through polymer composite materials. "A self-healing aeroplane is the idea," Prof Bond tells me.
Re-healable fuel tanks
Battlejacket is a spray-on coating that can be applied to fuel tanks to prevent or minimise leakage if they are hit by small-arms fire. The system, built by Oregon-based High Impact Technology (HIT-USA), has already proven its use to the US military in Iraq.
After the invasion in 2003, insurgents targeted fuel tankers in military convoys, which would leak flammable fuel, posing a hazard to troops and contractors. When a fuel tank is treated with Battlejacket it will quickly seal up bullet holes.
There are three layers to the coating: the inner and outer layers are made from a specially formulated plastic which stretches around the projectile. Between the plastic is a special layer containing absorbent beads. When these beads come into contact with fuel, they swell into a solid, sealing off the entrance and exit holes made by the bullet. HIT's co-founder Russ Monk said: "One of the fuel hauler tanks has over 600 bullet holes and we understand it is still delivering fuel.
Self Healing Concrete
Small cracks are a routine outcome of concrete hardening. But over time water and chemicals get inside the fractures and corrode the concrete.
The solution developed at TU Delft could improve the service life of the structure - promising considerable cost savings. Harmless calcite-producing bacteria, along with nutrients, are embedded in the concrete mixture. When water activates the dormant spores, the microbes feed on the nutrients to produce limestone, patching up cracks and small holes.
Longer-term, Scott White envisages materials that respond in a more complex way to damage or wear, renewing themselves over their lifetimes, in much the way that bones do.
"We're only beginning to understand how nature does what it does with such basic materials."