Healing powers – self-repairing materials

Your skin can do it, and now scientists have developed self-healing materials that could repair damage to planes. Yi-Di Ng reports.

A time sequence shows how an impact-damaged material is repaired. The process begins with delivery of material through two isolated fluid streams (dyed red and blue) that infiltrate the cracks and wet the inner surface of the damage. The fluid transitions to a gel, which acts as a scaffold for the continuing delivery of restorative materials. After the process is completed (bottom right), the gel undergoes another transition to a rigid, structural solid. – Nathan Bajandas, Beckman Institute, University of Illinois

In 2002 China Airlines Flight 611 disintegrated in mid-air 20 minutes after take-off, crashing into the Taiwan Strait. The culprit? A tiny, 22-year-old scratch in the fuselage that ripped open into a big crack. Clearly the smallest scrapes can have catastrophic consequences for aeroplanes.

Now aerospace engineer Scott White and his team from the University of Illinois have developed a potential fuselage material that, like skin, rapidly heals itself. They report their invention in the May issue of Science.

Scientists have been working on self-repairing materials for the last few decades. Early on they had some success with microscopic wounds but even then the repairs required additional exposure to light or heat to trigger the healing reaction. White overcame that requirement 13 years ago, creating a polymer that could truly self-repair factures less than one tenth of a millimetre in size. Now his new material is ready to tackle serious tears. “What we showed was that you could repair and regrow damage up to 40 millimetres – that’s some orders of magnitude larger,” says White.

The key to White’s self-healing system lies in two unique fluids called “gelators”. When these fluids mix, a two-stage chemical reaction takes place. First, the mixture quickly coagulates into a gel with the consistency of honey, which spreads to fill the cavity. Then, within three hours, the gel hardens into a solid polymer. Tests showed the repairs are about two-thirds as strong as the original structure.

But timing is critical. The gel needs to spread and plug the gaps before the hardening reaction starts. Harden too soon and the cavity won’t get filled; too late and the gel loses its battle against gravity and bleeds out of the crack before any repair can take place. The challenge was to keep the gelators apart, and then release them to ensure they would mix when needed.

The team borrowed another idea from nature – our circulatory system. When we are wounded, both arteries and veins rapidly release healing compounds. The team created materials that contained a similar network of microchannels, each containing one of the two gelators. Like veins and arteries, these channels lie alongside each other. On impact, they rupture, releasing the fluid gelators like blood gushing from a burst vessel, and quickly supply enough healing agent to repair major damage.

CSIRO’s Russell Varley, a material scientist who has worked on self-healing polymers, is impressed. “This development tackles the important question of how to fill a cavity inevitably created by large damage, as opposed to small cracks that are much easier to repair,” he says.

But as White points out, the materials were subjected to controlled damage in the lab, rather than the challenges faced by an aircraft at 10,000 metres.

“The systems are still in their infancy and there is a long way to go for them to be used in practice,” Varley agrees.

Yi-Di Ng is a science writer based in Melbourne.