When we get scratched, our skin can repair itself. Nonliving coatings can't currently do the same thing, so we cannot put self-repairing surfaces on cars, cell phones, laptops, and many other items. As the commercial applications are numerous and the financial payoffs are potentially huge, material scientists have been actively developing polymers that can self-heal. Everything from nanoparticles to expandable gels have been tried. While some of the developments are certainly promising, nothing is quite at the stage where it's ready to be commercialized.

In today’s issue of Science, Biswajit Ghosh and Marek Urban from the University of Southern Mississippi present a new polymer design that can employ UV light from the Sun to activate a latent self-pair capacity. Their strategy involves using the combined functions of three chemical components.

At the core of their design is polyurethane, which is an elastic polymer that already has decent scratch resistance. To enhance its ability to withstand mechanical damage, Ghosh and Urban added two more components, OXE and CHI. OXE has an unstable chemical structure (a four-membered ring containing three carbons and one oxygen) that makes it prone to being split open. CHI is UV sensitive.

The idea is that, if the polyurethane gets damaged by a scratch, the unstable ring structure of OXE will open to create two reactive ends. Then, UV light can trigger CHI to form new links with the reactive ends of OXE and thereby fix the break in the polymer.

Ghosh and Urban purposefully created scratches in films of their polyurethane-CHI-OXE material and tested to see if it mended itself under UV light. When they placed the damaged film under a 120 W fluorescent UV lamp, the scratches became negligible within half an hour. This repair reaction can work under a variety of conditions, ranging from dry air to high humidity.

IR and optical images of a self-healing scratch. Credit: Marek Urban/Science

They estimated that the UV lamps used in the experiments generated about a 0.3 W/m2 per nm power density, which is only a little more than what the Sun gives off on average (0.25 W/m2 per nm). Thus, they propose that the repair would take about the same amount of time under sunlight. Of course, the time of the year would matter—the process could take much less time under the harsh summer sun or much longer on a gloomy winter day.

The ability to use natural sunlight for self-repair and the simple design are advantageous, but this polymer system still needs some work before it can be released commercially. For example, the authors must figure out what happens if a second scratch occurs directly where a previous scratch was mended. They also need to determine the shelf life of their three-component polymer.

Science, 2009. DOI: 10.1126/science.1167391

Listing image by Janet Sinn-Hanlon, Univ. Ill.