Self-repairing elastomers inspired by the natural world

Here’s a question: Superman aside, how many things can you name that are indestructible? Not many, right? And even Superman had to come from another planet.  

Turns out, though, you don’t have to go nearly that far to find something with at least the potential to be that tough. A project currently being carried out by researchers at Germany’s Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen aims to create self-healing elastomers that can repair themselves autonomously, making them almost impervious to damage.

The challenge is to eliminate microcracks, often too tiny to be seen, that develop and spread quickly, leading to failure in even the best-engineered component part. To this end, the Fraunhofer scientists have borrowed a principal from the plant world – specifically plants such as the caoutchouc tree hevea brasiliensis, which conduct latex that contains capsules with a protein called “hevein”. If a caoutchouc tree is damaged, the latex escapes and the capsules break open to release hevein, which also links the latex particles in the latex to form – voila! – a wound closure.

You can see where this is going. “We loaded microcapsules with a one-component adhesive (polyisobutylene) and put it in elastomers made of synthetic caoutchouc to stimulate a self-healing process in plastics,” said the Fraunhofer’s Dr. Anke Nellesen. “If pressure is put on the capsules, they break open and separate this viscous material, which then this mixes with the polymer chains of the elastomers and closes the cracks.”

To date, the Fraunhofer researchers have obtained good results by putting the polyisobutylene component into the elastomer uncapsulised. “Various test bodies from different synthetic caoutchoucs indicated clear self-healing properties, since the restored tension expansion was 40 per cent after a healing period of 24 hours,” Nellesen said.

An even more promising outcome has been achieved by supplying elastomers with ions. “The hevein proteins that are released when there is damage link up to each other through ions and stick in this process so that the crack closes,” Nellesen continued. “Therefore, if the elastomer material is damaged, the particles with opposite charges are looking for a new bonding partner.”

So how many applications could benefit from a self-healing plastic? They probably don’t make numbers that large. The auto industry is an obvious example, where the slings and arrows of outrageous drivers can take a heavy toll on polymer parts. Perhaps for this reason, the first public showing of the Fraunhofer scientists’ project was a self-repairing muffler suspension on display at the recent Hannover Fair in Germany.

As Nellesen conceded, there’s still a long way to go in the quest for indestructibility – “we still haven’t developed elastomers that can close their cracks without interference from outside,” she noted – but it might just be that an important corner has been turned. Today a muffler, tomorrow – who knows?