Self-healing elastic polymers developed with the help of a tree
By Ben Coxworth
March 7, 2011
When the caoutchouc tree is damaged, liquid latex containing capsules of the protein hevein escapes from inside of it. Those capsules rupture, releasing the hevein, which links the latex particles together and ultimately closes up the wound. The whole bursting/sealing-microcapsules thing is obviously a pretty good idea, as it has been put to use in human technology such as self-healing concrete, electronics, paint and aircraft epoxy resin. Now, German researchers have copied the caoutchouc tree's modus operandi to create a self-sealing elastic polymer.
Scientists from the Fraunhofer Institute for Environmental, Safety and Energy Technology loaded microcapsules with the adhesive polyisobutylene, then put those capsules into synthetic caoutchouc sap elastomers. The idea was that when pressure was put on the elastomers to the point of cracking, the capsules would rupture and mix with the elastomers' polymer chains, thus sealing the cracks.
As it turned out, the capsules didn't cooperate. When the polyisobutylene was added in an uncapsulized form, however, a self-healing effect did occur – the elastomers' tension expansion was restored by 40 percent after a 24-hour healing period.
The scientists further copied the caoutchouc tree, by charging the elastomer with ions. In the case of the tree, the hevein proteins link up with one another (and in the process, with the latex) via ionic bonding. By charging the synthetic elastomer, a similar effect could be achieved with the polyisobutylene.
Fraunhofer will have a self-healing muffler made from the experimental material on display at the Hannover Fair from April 4th to 8th. A self-healing biorenewable polymer is also being developed at Iowa State University.