Cutting-edge materials like graphene—a thin sheet of carbon just one atom thick—are getting lighter, stronger, and easier to produce each day, offering new potential to transform industries from water desalination to solar cells and disease detection.
But our man-made materials still lack one much-desired quality that occurs naturally in the roots of plants and human skin: the ability to heal themselves.
A team led by Scott White at the University of Illinois at Urbana-Champaign has set out to change that by adding an artificial vascular system to plastic. The idea is to fill the material’s pseudo-veins with chemically reactive liquids so that when the plastic is ripped, the substances can combine and solidify like clotting blood, protecting the object from further damage.
In a demonstration video, the team tests the technique on a plastic block, pumping two liquids through separate channels into the object before puncturing the material with a 4-millimeter drill. The drill wound created cracks that released the liquid channels, but thanks to the vascular system, the liquids oozed into the hole and cracks, in 20 minutes forming a thick gel that stopped the damage from spreading. The gel solidified in a matter of three hours, eventually repairing itself to be about 60 percent as strong as the original material, according to the team.
Researchers envision using the technology to protect everything from military equipment to construction materials—potentially saving time and manpower in emergency situations or hard-to-reach work sites.
The chemical mixing and solidifying process may sound familiar to anyone who has ever used epoxy resin purchased from a hardware store. But Brett Krull, a co-author of the research, says the team has moved away from epoxies, largely because of their slow reaction times.
Although it produces an effect similar to epoxies, the new plastic helps repair damage must faster, Krull says.
The fundamental difference:
"We designed our system to undergo two different transitions," whereas epoxy resin works differently, Krull says. "Two chemical reactions initiate as soon as mixing occurs, but they occur on much different timescales."
Krull says the first reaction turns the mixture into a soft gel within 30 seconds. This keeps the chemicals in place inside the damaged area while still allowing the delivery of more fluids into the hole or crack until it has been filled. The second reaction, which turns the chemicals into a solid, happens afterward, at a rate that can be controlled by changing the composition and concentrations of the chemicals.
“Our chemistry does not approach the complexity of a natural system," Krull says, “but we have designed a system with a time-dependent response to damage.”
White and his team have demonstrated the ability to heal microscopic cracks in a different way in the past, using epoxy and embedded microspheres. But the new vascular approach allows for repair on a much larger scale. The technique could be used to repair a gash in the side of an underwater drill, for instance, or a pockmark on a spacecraft that collides with a meteor.
Researchers still face challenges as they continue to develop the self-hearling materials, including how to increase the effectiveness of the vascular networks in the material (plastic in this case) without significantly reducing its strength or performance. The team also wants to give the material the ability to heal from multiple "wounds" over time.
The chemicals will also likely have to be adjusted to handle larger areas of damage. According to New Scientist, holes in the material that were larger than 8mm caused the chemicals to sag. The team thinks using foam in the channels instead of fluid will allow the material to heal larger areas, though researchers have yet to test that option.
Krull says they’ll also look to make the material effective in different environments, like extreme temperatures, underwater or in space. (So far, testing has primarily been done in the lab).
While the technology may one day make its way to consumer products, don’t expect these self-healing materials to magically repair the backside of your iPhone or your car’s bumper quite yet. The technology is still in the early stages of development, Krull says. And because the research is funded by the U.S. Air Force, it’s likely to be used on fighter jets, tanks, or spacecraft first, along with devices that are difficult to repair, like underwater drilling equipment.
But that's just the beginning of what the material might be able to do, Krull says.
“The current version is more like a scar since the healed material isn’t quite as good as the original,” Krull says. “Our long-distance goal is to develop a truly regenerative polymer where material lost by a damage event can be replaced with material of the same composition.”