Time to heal: The materials that repair themselves

Guitar smash The possible applications are endless


They will need to be replaced - at a cost.

But what if we made things from materials that can heal themselves - like a plant or animal heals a wound?



Smartphone solution

Cracked iPhone screen

One much discussed application of self-healing materials is for smartphones or tablet devices with scratched or cracked screens. In this case, restoring optical clarity to the screen might present an extra consideration, says Ian Bond.

His 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.

The University of Bristol professor 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."

He adds: "I'm not aware of anyone doing it, but with consumer stuff no one's going to tell you what they're doing. If they're about to sell an everlasting [smartphone] screen, they're going to make a packet out of it."

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.


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.



The Illinois group is already commercialising 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," he told BBC News.

Dr Richard Trask of the University of Bristol demonstrates a prototype material that is designed to repair itself when damaged (from BBC Four's How It Works).

Start Quote

One of the fuel hauler tanks has over 600 bullet holes and, we understand, is still delivering fuel”

End Quote Russ Monk Co-founder, HIT-USA

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 asset, 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 $500,000 (£312,000) or $1m (£624,000) a day if an asset is out of service."


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.


Boeing 787 Aerospace is a vibrant area of research for re-healable materials



Each of the three 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".


Self-healing systems
Self healing materials Capsules containing the healing agents and other chemicals are distributed throughout the material. If a breakage occurs, the capsules release their contents casuing a chemical reaction to "heal" the breakage.
Self healing materials Vascular systems use networks of refillable channels to deliver the healing agent and polymerisers to the breakage point.
Self healing materials Intrinsic systems use the reversible nature of certain chemical bonds to incorporate healing properties directly into the material.

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  Credit: HIT-USA

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 told BBC News: "One of the fuel hauler tanks has over 600 bullet holes and, we understand, is still delivering fuel."

The composite materials extensively used in the structural elements of aircraft "are inherently damage prone", says Prof Bond, adding: "You often can't see it even though it can have a serious knock-down effect on performance."

The Bristol team is targeting known areas of stress build-up inside the skin of a plane. "Self-healing in those sorts of areas is potentially very attractive because you know you're going to get cracks there," he explains. The challenge is likely to be in convincing aviation authorities of the technology's value and safety.

So Prof Bond is working to overcome some of the hurdles facing vascular systems. The step up from microcapsules to a network in two or three dimensions, for example, presents a significant manufacturing challenge. Fluid flow - getting the healing agent through the material - represents another problem.

Then there is the issue of controlling when healing happens. "If you think of blood, it doesn't clot until it's outside the vessel," he says. "You want something like that, because the danger with simple chemistries is that the whole network, once healing has been triggered, will just solidify."

Despite this, says Scott White, vascular networks offer exceptional healing efficiency and vast possibilities. "In some of the laboratory tests we've done, we've been able to show we could heal something 15, 20, 30 times in a row," he says.

Prof Stuart Rowan at Case Western Reserve University in Cleveland, Ohio, has developed a polymer-based material that repairs itself in response to an intense beam of ultraviolet light. He says: "What you can imagine is essentially a paint coating on your car that you can heal whenever someone has rubbed a key down the side of it."

Self-healing fibres  Credit: Benjamin Blaiszik The microcapsules release a healing agent when the material is damaged

Unlike conventional polymers, which are composed of long, chain-like molecules, this material (an example of an intrinsic system) is composed of smaller molecules. They are assembled into chains with metal ions acting as "glue" between them. UV light causes these bonds to weaken, turning the solid into a liquid. When the light is switched off, the material quickly solidifies.

Prof Rowan told BBC News: "Having proved the concept, we are working on the next generation of films that utilise such concepts for photohealing but where the materials exhibit better properties better designed for a specific application."

Meanwhile, Henk Jonkers and Erik Schlangen at Delft University of Technology (TU Delft) in the Netherlands want to bring self-repair to the world's most used building material: concrete. Concrete has a serious flaw: it is prone to cracks.

Henk Jonkers with self-healing concrete  Credit TU Delft Limestone-producing bacteria are embedded, along with food "parcels", in the concrete mix


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.


Self-healing provides a case study in the way that biological systems can drive advances in materials, but Ian Bond says: "There's a lot more we could do with what we have… the way we currently make composites is with flat layers and fibres all pointing in the same direction - it's that simple.

"We're only beginning to understand how nature does what it does with such basic materials."


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