'Living' Plastic: Are Self-Healing Plastics a Possibility for O&P?

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." <img src="https://opedge.com/Content/OldArticles/images/2007-12_02/2007-12_02-01.jpg" alt="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." /> 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. 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

‘Living’ Plastic: Are Self-Healing Plastics a Possibility for O&P?

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