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
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
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.