Lexington, Virginia • January 16, 2009
It’s almost the stuff of science fiction — bullet holes that seem to repair themselves magically. But it turns out that a type of polymer (plastic) can do just that.
That's what Stephen Kalista is trying to figure out.
Kalista, a visiting instructor of physics and engineering at Washington and Lee University, is investigating this emerging area of materials research and has recently contributed a chapter titled “Self-Healing Ionomers” in the book Self-Healing Materials: Fundamentals, Design Strategies and Applications (Wiley-VCH, Weinheim, 2008).
“There’s a ‘Wow!’ factor to this area,” Kalista said. “I get a big response to my lecture I call ‘Self-Healing Plastics and the Governor of California.’ In The Terminator II movie, my students call the bad guy, the T-1000 played by Robert Patrick, the liquid-metal man because he can morph into different shapes and heal cybernetic ‘life-ending’ injuries. It’s the perfect example of what self-healing materials could potentially do, at least in a Hollywood film, and it really grabs people’s imagination.”
While self-healing materials aren’t even close to matching the capabilities of the T-1000, they do hold the potential to revolutionize the way materials are engineered for different applications.
“Researchers are investigating the use of self-healing concepts in metals, polymers, concretes and other materials,” said Kalista. “There are concretes which automatically repair cracks using embedded calcite-producing bacteria, metals and coatings that produce a self-healing corrosion-proof barrier for sea vessel hulls, and polymers which incorporate healing-agent containing microcapsules to autonomically repair cuts and cracks.”
Kalista’s own focus is on materials that can heal themselves without the aid of a catalyst or even any chemical means. He’s working with a certain class of poly(ethylene-co-methacrylic acid) copolymers (EMAA). “This type of plastic is relatively cheap to produce and has this unique and exciting ability to heal itself,” he said. “Other systems are engineered to heal small cracks, but this plastic can heal itself even after a bullet puncture.”
The possible applications include combat aircraft fuel tanks, orbiting space probes and medical devices. At the moment, only rifle range targets use this type of self-healing plastic. He has fired projectiles at sheets of EMAA under a range of different temperatures and speeds to try and determine the mechanism behind the event.
“I would shoot a hole through the material and then immediately pick it up and already see the healed scar,” he said. “It’s fast. We tried to capture the healing process using a high-speed camera, but the event happens almost instantaneously. But this gives us a clue as to what’s happening. During impact, the material is heated until it melts, but it retains sufficient elasticity for the hole to close. In other words, the material snaps back into place, and the heat generated by the pellet going through the material seals it back together.”
But why this occurs is still a mystery. “The specific origin of the healing response remains the most elusive question,” Kalista said. “Initially, we believed the healing occurred as a result of discrete ionic (charged) groups in the material re-establishing chemical bonds, but the data didn’t support that hypothesis. A larger array of non-ionic materials has shown the same self-healing response.”
So the research continues. “I’m exploring other materials with similar chemistry which may form different molecular arrangements—and which may or may not heal. Once we understand what is going on, then we can design new materials that take advantage of this unique response.”
Kalista graduated from W&L in 1999 with a B.S. in physics-engineering, has gone on to earn an M.S. in engineering mechanics from Virginia Tech and is completing his Ph.D. in macromolecular science and engineering at Virginia Tech. He currently serves on the Scientific Committee for the Second International Conference on Self-Healing Materials to be held in Chicago this summer and is co-organizing a symposium on thermally-activated/thermoplastic self-healing as part of that conference.