Dr. Anke Nellesen [profile], a scientist at the Fraunhofer Institute for Environmental, Safety and Energy Technology in Germany, was fascinated by caoutchouc tree hevea brasiliensis and plants that conduct latex, such as the Weeping Benjamin.  

Millions of years of evolution yielded a highly specialized response to wounding in these trees.  When attacked by insects or suffering other mechanical damage, the trees emitted a thick mess of latex particles.  Mixed in with those particles were capsules of the protein hevein.  When the latex reached the wound, the hevein broke and was released.  Active, it links the latex, closing the wound.  In short, the mixture acted as a self-forming plastic.

In manmade plastics such as car parts, seats, tires, and more plastics can "break" after being overstressed and developing micro-cracks.  So Dr. Nellesen led a team that looked to create a self-healing rubbers and plastics inspired by nature's evolved mechanisms.

In Dr. Nellesen's lab, elastomers, the general term for rubbers and plastics, were strengthened by the addition of adhesive filled microcapsules that could plug minor cracks before they caused catastrophic failure [press release] [newsletter; PDF].

States the researcher, "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. If pressure is put on the capsules, they break open and separate this viscous material. Then this mixes with the polymer chains of the elastomers and closes the cracks. We were successful at making capsules stable to production, although they did not provide the self-healing effect we wanted."

Interestingly, even without the encapsulation, researchers found the polyisobutylene self-healed.  The trees also used ion-bonding to speed the formation of new bonds and self-healing, so the team also looked to dope the plastics with ions to make for speeder crack filling.  The results were an even greater success than the previous work with unencapsulated polyisobutylene alone.

The resulting self-healing material is a landmark discovery, according to Dr. Nellesen.  He states, "[T]here are already duromers with self-healing functions in the form of self-repairing paints in cars. We still haven’t developed elastomers that can close their cracks without interference from outside."

The current material cannot self-heal entirely independently, like natural systems, as it currently requires an injection of ions to be effective.  Of course this could be done via an automated process.  Such automated systems could eventually be worked into sensor feedback loops to create the manmade equivalent of nature's healing process.

While there's a multitude of possible industrial applications for the technology, the team is looking to initially target the automobile industry, given Germany's active role in it.  They are showing off a self-repairing muffler suspension at the Hannover Fair in Hannover, Germany from April 4-8 at the joint Biokon stand in Hall 2.

Self-healing materials are a topic of very active research, with scientists exploring other forms of materials like self-healing fabrics or concrete, as well.  U.S. researchers have been working on self-healing in composite materials, targeted at military aircraft.  Europe is currently working on developing a self-healing spacecraft.