A Material that Mends Itself

The vascular network allows the plastic to heal and heal again. Credit: Dorothy Loudermilk, UIUC

Wednesday, June 13th, 2007--

A new polymer material, modeled after human skin, can heal itself when it breaks, according to a study published in the journal Nature Materials this week. Embedded in the polymer is a microvascular network, like a system of capillaries, that transports a “healing agent” (a monomer called dicyclopentadiene) to the damaged area.

When the surface of the material cracks, the surface layer pushes down into the vascular network underneath. The healing agent is naturally pulled into the crack through a force called the "capillary effect." The healing agent reacts with a catalyst—called Grubbs’ catalyst—on the surface of the material, forming another polymer. Over the course of about ten hours, the mixture hardens into a solid, filling the crack.

Nancy Sottos, a professor of engineering at the Beckman Institute at the University of Illinois at Urbana-Champaign and an author of the study, says that the healing doesn’t have to take ten hours: “We could make it go faster depending on the concentration of catalyst and the choice of the monomer that we use. But faster isn’t always better.” The healing agent needs time to mix with the catalyst and fill in the crack; faster healing can lead to patchiness, Sottos says.

Self-healing materials are not entirely new: a precursor to this polymer was designed to store both the healing agent and the catalyst in capsules at the surface of the material. When the material cracked, the capsules ruptured, the ingredients mixed, and material repaired itself. The drawback of this system is that the material could only be damaged and healed once "because all of the available healing agent will have been used up,” Sottos says.

The new self-healing material developed by Sottos and colleagues Kathleen Toohey, Jennifer Lewis, Jeffrey Moore and Scott White, all of the Beckman Institute, can be damaged and healed as many as seven times in the same place, although it does become weaker: Sottos says that after a repair the polymers "recover sixty to seventy percent of the toughness on average."

To create the vascular system in the material, Sottos and her colleagues piped a wax-like substance, called fugitive organic ink and referred to by Sottos as “glorified toothpaste,” into the plastic. The ink is then be melted and sucked out, leaving three-dimensional tubes that act as the vascular network and carry the healing agent. “Everywhere there was wax you are left with these nice round channels,” says Sottos.

“Self healing materials have not really hit the commercial market yet,” Sottos says. (The closest thing you can find are self-healing tires, which have a viscous layer of gel between the inner tube and the outer tire. If you puncture the tube the gel will fill the hole.) Sottos and her colleagues are investigating finer scale applications; imagine paint that can fill its own cracks or outdoor furniture that can re-waterproof itself.

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--Flora Lichtman