Now, self-healing materials can mimic human skin, healing again and again
James E. Kloeppel, Physical Sciences Editor
217-244-1073;
kloeppel@uiuc.edu
Photo by L. Brian Stauffer
The
next generation of self-healing materials, invented
by researchers at the University of Illinois, mimics
human skin by healing itself time after time. The
researchers, clockwise
from front, graduate student Katie
Toohey; Nancy Sottos and Jennifer Lewis, both professors
of materials science and engineering; Scott White,
professor of aerospace engineering; and Jeffrey
Moore, professor of chemistry, pose in the robocaster
lab.
Released 6/11/2007
CHAMPAIGN,
Ill. —
The next generation of self-healing materials, invented by researchers
at the University of Illinois, mimics human skin by healing itself
time after time. The new materials rely upon embedded, three-dimensional
microvascular networks that emulate biological circulatory systems.
“In the same manner that a cut in the skin triggers blood flow
to promote healing, a crack in these new materials will trigger the
flow of healing agent to repair the damage,” said Nancy Sottos,
a Willett Professor of
materials
science and engineering, and the corresponding author of a paper
accepted for publication in the journal Nature Materials, and posted
on its Web site.
“The vascular nature of this new supply system means minor damage
to the same location can be healed repeatedly,” said Sottos, who
also is a researcher at the university’s
Beckman
Institute.
In the researchers’ original approach, self-healing materials
consisted of a microencapsulated healing agent and a catalyst distributed
throughout a composite matrix. When the material cracked, microcapsules
would rupture and release healing agent. The healing agent then reacted
with the embedded catalyst to repair the damage.
“With repeated damage in the same location, however, the supply
of healing agent would become exhausted,” said Scott White, a
Willett Professor of
aerospace engineering and
a researcher at the Beckman Institute. “In our new circulation-based
approach, there is a continuous supply of healing agent, so the material
could heal itself indefinitely.”
To create their self-healing materials, the researchers begin by building
a scaffold using a robotic deposition process called direct-write assembly.
The process employs a concentrated polymeric ink, dispensed as a continuous
filament, to fabricate a three-dimensional structure, layer by layer.
Once the scaffold has been produced, it is surrounded with an epoxy
resin. After curing, the resin is heated and the ink – which liquefies
– is extracted, leaving behind a substrate with a network of
interlocking microchannels.
In the final steps, the researchers deposit a brittle epoxy coating
on top of the substrate, and fill the network with a liquid healing
agent.
In the researchers’ tests, the coating and substrate are bent
until a crack forms in the coating. The crack propagates through the
coating until it encounters one of the fluid-filled “capillaries”
at the interface of the coating and substrate. Healing agent moves
from the capillary into the crack, where it interacts with catalyst
particles. If the crack reopens under additional stress, the healing
cycle is repeated.
“Ultimately, the ability to achieve further healing events is
controlled by the availability of active catalyst,” said Kathleen
S. Toohey, a U. of I. graduate student and lead author of the paper.
“While we can pump more healing agent into the network, ‘scar
tissue’ builds up in the coating and prevents the healing agent
from reaching the catalyst.”
In the current system, the healing process stops after seven healing
cycles. This limitation might be overcome by implementing a new microvascular
design based on dual networks, the researchers suggest. The improved
design would allow new healing chemistries – such as two-part
epoxies – to be exploited, which could ultimately lead to unlimited
healing capability.
“Currently, the material can heal cracks in the epoxy coating
– analogous to small cuts in skin,” Sottos said. “The
next step is to extend the design to where the network can heal ‘lacerations’
that extend into the material’s substrate.”
With Sottos, Toohey and White, the paper’s other co-authors are
Jennifer Lewis, the Thurnauer Professor of Materials Science and Engineering
and interim director of the
Frederick
Seitz Materials Research Laboratory, and Jeffrey Moore, a William
H. and Janet Lycan Professor of
Chemistry
and a researcher at the Frederick Seitz Materials Research Laboratory
and Beckman Institute. White, Sottos and Moore co-invented self-healing
plastic; Lewis and White pioneered direct ink writing of three-dimensional
microvascular networks.
The work was funded by the U.S. Air Force Office of Scientific Research
and the Beckman Institute.
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