Materials that heal themselves are going bigtime. Scientists have cooked up a chemical concoction that can patch a 9-millimeter-wide hole in a sheet of plastic, a self-repair orders of magnitude larger than ever demonstrated before. The finding could lead to new kinds of airplane wings and spacecraft components that can repair themselves midflight.
“It’s exciting; I think it’s a big step forward in being able to autonomously heal structures without intervention,” says University of Michigan, Ann Arbor, mechanical engineer Ellen Arruda. She calls the researchers’ scheme “the polymer equivalent of a blood clot.”
Complex life could never have evolved without the ability to heal itself. When an animal suffers a puncture wound, for example, compounds flow from blood vessels to the wound site, where they feed the growth of new tissue to fill the damaged area. The process, however, requires a vascular system to deliver the needed components. Because most nonliving materials lack this complexity, repair typically requires human intervention.
Recently, though, scientists and engineers have begun designing materials that can patch up small defects. In one such scheme, narrow channels similar to animal blood vessels deliver compounds that seal small fissures in a material made of fiberglass and polymer resins. So far, such systems have only repaired fractures so small that opposite sides of the wound nearly touch. When the damage site is larger—say, the size of a bullet hole—liquid compounds tend to leak out before they can form a seal.
Now, scientists and engineers from the University of Illinois, Urbana-Champaign, report a two-part solution to fill such big voids. In the first step, the researchers filled two parallel 330-micrometer-diameter channels with two mixtures of organic molecules known to combine to form solid and semisolid structures; the mixtures were dyed red and blue. The researchers then embedded the channels in a 3-millimeter-thick plastic sheet, laid the sheet flat and punctured it, causing the red and blue mixtures to flow into the nearly circular hole and mingle with each other. A catalyst joined the two compounds into cross-linked fibers, resulting in a semisolid gel that filled the hole from the outside in.
The gel’s fibers formed a netlike scaffold that set the stage for the second step in the healing by supporting a third compound that flowed in from the channels. This compound reacted to form solid crisscrossing polymers, which filled in the hole and surrounding cracks with a cloudy, purplish substance. The polymer made a seal with the original clear plastic and restored most of the material’s strength.
“What we did here was what I like to call repair by regrowth,” says chemist Jeffrey Moore, a research team member. He says the critical insight was choosing chemicals that react at different rates, so the net could form before polymerization started. “Timing is everything here,” Moore says.
Moore and his colleagues report online today in Science that their scheme can repair a hole nearly 1 centimeter in diameter, with cracks radiating over an area 3.5 centimeters in diameter. This is about 100 times larger than any previously self-repaired defect in a nonliving material, Moore says. A system like theirs could someday be part of self-healing airplane wings or spaceship components that include composite materials made of multiple constituents. Humans cannot easily repair such components during flight.
But Arruda says that unlike in a human wound, which is eventually repaired with the same kind of tissue that was originally lost, the makeup of Moore’s team’s polymer differs from that of the original plastic. As a result, the repaired material is somewhat weaker than an intact sheet; in tests, it can absorb only about 62% as much energy from an impact. The researchers will also need to show that their repairs hold up under a range of real-world conditions like varying humidity and extreme temperatures, she says.