No glue required: Broken polymer chains reform to repair a crack in this material when it is pressed together and exposed to UV light.
A polymer that mends itself could lead to medical implants or engine parts that fix themselves.
By Prachi Patel
A new polymer material that can repeatedly heal itself at room temperature when exposed to ultraviolet light presents the tantalizing possibility of products that can repair themselves when damaged. Possibilities include self-healing medical implants, cars, or even airplane parts.
The polymer, created by researchers at Carnegie Mellon University and Kyushu University, heals when a crack in the material is pressed together and exposed to UV light. The same treatment can cause separate chunks of the material to fuse together to form one solid piece.
The researchers were able to cut the same block into pieces and put them back together at least five times. With further refinement, the material could mend itself many more times, says CMU chemistry professor Krzysztof Matyjaszewski, who led the research team.
Currently, the polymer can only repair itself in an oxygen-free environment, so the researchers had to carry out the UV treatment in the presence of pure nitrogen. But they hope to develop polymers that heal under visible light and don’t require nitrogen, which should open up many practical applications, including products and components that heal after suffering minor damage. Such a material, Matyjaszewski says, "would be a dramatic improvement over what we’ve already done."
Self-healing materials have been made before, mainly polymers and composites. But most of those have relied on tiny capsules that are filled with a healing agent. When the polymer cracks, the capsules break open and release the healing agent, which becomes polymer solid and seals the crack and restores the material’s properties. But once the capsules are depleted, the material can no longer mend itself.
The new polymer relies on carbon-sulfur bonds within the material. "There are thousands of chemical bonds here, and even if you lose a small percent, one can think about potentially repeating the healing a hundred times," Matyjaszewski says.
The researchers found that even shredded bits of the polymer will join together to form a continuous piece when irradiated with UV light. This implies that the material could also be easy to recycle. The researchers presented the details of their experiments in a paper published online in Angewandte Chemie.
Some research groups, including Matyjaszewski’s, have made polymers that heal when exposed to heat or certain chemicals. But Michael Kessler, a materials science and engineering professor at Iowa State University, says light healing is a superior option. "I think that UV stimulus is particularly appealing as an external stimulus because it’s noncontact, it happens at room temperature, it’s pretty easy to acquire and handle, and, importantly, it’s limited to target areas where the damage occurs," Kessler says.
Kessler adds, however, that the new material suffers from two of the main drawbacks faced by other self-healing materials: it requires pressure, and the repair process takes hours.
Nonetheless, some self-healing materials are on their way to commercialization. Autonomic Materials in Champaign, Illinois, is readying corrosion- and scratch-resistant coatings containing microcapsules developed by Scott White, a professor at the Beckman Institute at the University of Illinois at Urbana-Champaign.
White’s colleague Nancy Sottos has made materials that mimic human skin and that heal themselves using underlying channels filled with healing agents. Sottos envisions the materials being used for structural applications such as airplane parts, car and spacecraft components, and for everyday products such as cell-phone and laptop cases.
UV-triggered healing won’t be suitable for all applications, says Sottos. That’s because the restructuring of carbon-sulfur bonds that allows the material to heal also requires that material to be rubbery and soft.
"You can make materials that are harder or softer," says Matyjaszewski. "Every self-healing material is somehow unique and has advantages over the other ones. It depends on the properties and area of application."