Time to heal: The materials that repair themselves

By Paul Rincon
Science editor, BBC News website

image sourceGetty Images
image captionThe possible applications are endless

At some point in the near future you'll wear out those running shoes, break that squash racket, drop your smartphone and crack the screen.

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?

According to experts, the first products with truly self-healing properties may be just around the corner.

Serious proposals for this technology go back at least as far as the 1960s, when Soviet researchers published theory papers on the topic.

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 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.

media captionDr 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).

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."

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 told BBC News that 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.

image sourceGetty Images
image captionAerospace is a vibrant area of research for re-healable materials

Since 2001, two new approaches have joined microcapsules as approaches to self-repair.

Taking the circulatory system as their inspiration, vascular methods rely on a network of channels (like capillaries, veins and arteries) within the material to deliver healing agent to the site of damage. Intrinsic systems, meanwhile, exploit the reversible nature of certain chemical bonds to incorporate healing abilities directly into the material.

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".

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.

Self-healing systems

image captionCapsules 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.
image captionVascular systems use networks of refillable channels to deliver the healing agent and polymerisers to the breakage point.
image captionIntrinsic 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.

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."

image captionThe 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.

image sourceTU DELFT
image captionLimestone-producing bacteria are embedded, along with food "parcels", in the concrete mix

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.

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."