'Living' Plastic: Are Self-Healing Plastics a Possibility for O&P?
By Miki Farley When you get a cut in your skin, it marvelously starts to heal
itself—and it can do this repeatedly. Now imagine a plastic that
can mimic this ability.
Self-healing plastic is not a figment of the imagination. It's a
reality that was first developed by researchers at the University
of Illinois at Urbana-Champaign (UIUC) in 2001; other research
groups have since created different versions. However the new
generation developed in 2007 at UIUC can heal damage to itself
multiple times.
Could this material have an application in prosthetics and
orthotics?
"Absolutely!" says Chris Bielawski, PhD, assistant professor of
chemistry, University of Texas at Austin, whose research group is
working on new technology and materials for self-healing
electronics. "What's even better is that materials used in these
self-healing systems should hold up to the fabrication processes
currently used to make prosthetic and orthotic devices. There may
be some manufacturing issues that arise along the way, but the
concept is there."
Nancy Sottos, PhD, professor of engineering, Department of
Materials Science and Engineering & Beckman Institute for
Advanced Science and Technology at UIUC, agrees but expresses more
caution regarding the manufacturing process. "There is a potential
if the self-healing components can survive the molding process [for
prosthetic and orthotic applications]," she says. Sottos' team made
the recent landmark breakthrough in self-healing plastics that can
repair themselves multiple times without any external
intervention.
Researchers could use the same concept with other resin and
catalyst combinations to form different polymers, Sottos said in a
Technology Review article, published March 26, 2007, by
the Massachusetts Institute of Technology (MIT).
One component of some self-healing materials is a plastic
material known as polydicyclopentadiene, or poly-DCPD. "This is a
very strong material that can be rendered bulletproof," Bielawski
explains. "It ultimately comes from dicyclopentadiene, which is a
petroleum distillate, so it's relatively cheap. Over the years,
various ways were developed to convert this byproduct into a
useful, hard plastic. The University of Illinois team took it a
step further and used one of these processes as a foundation for
adding strength to materials when and where they are damaged and,
most recently, over multiple cycles." It would even be possible,
Bielawski says, to use it as a replacement for metal parts in
prosthetic and orthotic componentry—tough but lighter than metal,
and impervious to water and mud.
The plastic itself is not "stronger than steel," says Sottos.
"However, plastic composites such as carbon fiber-reinforced
polymer matrix composites can be stronger than steel. We can make
structural composites like carbon fiber/epoxy or glass fiber/epoxy
self-healing, so they might be good candidates to replace metal
parts."
The material in its liquid monomer form can be colored with any
organic dye in any shade desired, and the dyed liquid would
permeate through the material with uniform color when polymerized,
says Bielawski.
"The self-healing technology is straightforward," Bielawski
says. "The big advantage is that exotic methods are not needed to
impart self-healing properties. It's just very clever chemistry and
engineering."
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Cracks in a brittle coating are healed autonomously via a three-dimensional macrovascular network embedded in the underlying substrate. The network contains a healing agent (red) which polymerizes after contacting the catalyst (purple) in the damaged regions. Image courtesy of the Autonomic Materials System Group at the Beckman Institute, University of Illinois Urbana-Champaign. Image by Janet Sinn Hanlon, UIUC. |
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How It Works
The new material has been designed to mimic human skin,
according to the Technology Review article. When human
skin is cut, the skin's inner layer delivers nutrients to the cut
through a "dense network of tiny blood vessels" to help the outer
layer heal. "The self-healing material consists of an epoxy polymer
layer deposited on a substrate that contains a three-dimensional
network of microchannels. The epoxy coating contains tiny catalyst
particles, while the channels in the substrate are filled with a
liquid healing agent."
Self-healing materials would be useful for a number of reasons.
Structural polymers are susceptible to cracks, which form deep
within the structure where they are hard to detect and almost
impossible to repair. Cracking leads to a host of problems, such as
mechanical degradation of fiber-reinforced polymer composites,
which in turn can lead to electrical failure in microelectronic
polymeric components—not the kind of problem you want in a
computer circuit board in such hard-to-reach locations as space
satellites, submarines voyaging deep in the ocean, or medical
implants. Micro-cracking induced by thermal and mechanical fatigue
is a long-standing problem in polymer adhesives. "Regardless of the
application, once cracks have formed within polymeric materials,
the integrity of the structure is significantly compromised,"
according to experts at UIUC. "Engineering this self-healing
composite involves the challenge of combining polymer science,
experimental and analytical mechanics, and composites processing
principles."
To create self-healing materials, the researchers first build a
scaffold using a robotic deposition process called direct-write
assembly, explains James E. Kloeppel, UIUC physical sciences
editor. The process employs a concentrated polymeric ink, dispensed
as a continuous filament, to fabricate a three-dimensional
structure, layer by layer.
When the scaffold has been produced, it is then 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.
"An additional unique feature of our healing concept is the
utilization of living polymerization (that is, having unterminated
chain-ends) catalysts, thus enabling multiple healing events," UIUC
experts point out.
"To test the material, the researchers bend it and crack the
polymer coating," the Technology Review article explains.
"The crack spreads down through the coating and reaches the
underlying microchannel. This prompts the healing agent to 'whip
through the channels and into the crack,' Sottos says. There, it
comes into contact with the catalyst and, in about ten hours,
becomes a polymer and fills in the crack. The system does not need
any external pressure to push the healing agent into the crack.
Instead, the liquid moves through the narrow channels just as water
moves up a straw."
Thus, similar to human skin when it is cut, the damaged-induced
triggering mechanism provides site-specific autonomic repair. But
how durable is the repair? According to UIUC experts: "Our fracture
experiments yield more than 90 percent recovery in toughness."
According to Kloeppel, the healing process stops after seven
healing cycles in the current system. "This limitation might be
overcome by implementing a new microvascular design based on dual
networks, the researchers suggest," he says. "The improved design
would allow new healing chemistries—such as two-part epoxies—to
be exploited, which could ultimately lead to unlimited healing
capability."
Sottos says that is the direction the research is headed.
"Currently, the material can heal cracks in the epoxy coating -
analogous to small cuts in skin," says Sottos. "The next step is to
extend the design to where the network can heal 'lacerations' that
extend into the material's substrate."
Editor's note: Some of the research for
this article was obtained from the University of Illinois at
Urbana-Champaign website (www.uiuc.edu). For more
information, visit www.chemeng.uiuc.edu/dept/news_07/self_heal.php
and www.technologyreview.com/read_article.aspx?id=18841 
Table Of Contents - December 2007
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