CLEMSON — Annoyed by that crack on your phone’s screen? You wouldn’t be if the screen could heal itself, like a body heals a cut. That day is coming. Research and development in this area is so popular now that today’s advice to Benjamin Braddock would be two words: self-healing plastics.
The scientific search for materials that mend on their own has mostly focused on creating new polymers, or plastics, with only modest success. But material scientists at Clemson University have taken a different tack that could soon lead to commercial products, from scratch-free cars and nail polish, to robots with soft and flexible “skin” that heals, to airplane wings that repair microscopic cracks and industrial surfaces that improve and prolong performance.
Rather than starting from scratch, Marek Urban, the J.E. Sirrine Foundation Endowed Chair and Professor at Clemson, changed the process by which existing polymers are made. In tests of these materials, a scratch about as deep as the width of a human hair, 100 micrometers, “healed” itself in 10 minutes.
Publishing in the journal Cell Press, Urban and his team explained how they drew inspiration from a showy little perennial plant, the Delosperma cooperi. Also known as the ice plant, this warm-weather succulent with fuchsia flowers can heal a tear to a stem or a leaf in about an hour.
Scientists attribute this phenomenon to a step in the growth process called “phase separation.” Chemical building blocks arrange in such a way as to store elastic energy, like compressing a coil. The same process occurs in plant or bone growth. When a cut or a break occurs, that stored energy is released, pushing the edges of plant or bone together to fill the gap so healing can occur.
Urban’s lab, led by senior scientist Ying Yang, made two batches of a common thermoplastic polymer — a plastic that responds to temperature changes — and induced phase separation in each using two different methods. The results were a material with the same chemical composition that reacted in different ways.
One batch of the polymer was stretched into fibers using a “cold draw” technique, which affected the material on a micrometer scale. (a micrometer is one-millionth of a meter, or about one-fifth the diameter of spider silk). The other batch was stretched with a “melt draw” technique, which affected the material at the nanoscale, one one-thousandth of a micron.
During nanoscale separation the chain of chemical building blocks “slip”, like allowing a coil to relax without built-up energy. But at the microscale level, the much larger blocks prevented slippage, creating tension that stored energy, like winding a coil tight.
The fibers were cut, then warmed to take advantage of their shape recovery response. When heat was applied to the fibers made with the cold draw technique, the stored energy released, like a coil unwinding, sending the chain of blocks back to their original shape. The material made with the melt-draw process didn’t have energy to release.
Urban suspects his is the only lab looking to existing materials for self-healing properties, probably because the process described in his paper is difficult.
“It’s very easy to say I’m going to make something, but actually making it, and making it functional so others can use it, is not trivial to do,” says Urban, who published one of the first papers on self-healing materials in the journal Science in 2009.
“There needs to be an interplay of different backgrounds — not just chemistry, but physics and engineering,” Urban says. “This is what my team can do. We start with fundamental chemistry and process it.”
Self-healing properties add value to existing materials by making them more durable, more functional and more environmentally sustainable. But that reduces the need to sell more materials. Still, the ability to add value and create new products from existing materials without major investments will appeal to industry leaders focused on innovation.
“This is not for companies that want to stay stagnant,” Urban says.
Tweaking the manufacturing process to create new properties also eliminates the use of additives, which are currently used for that purpose. These small-molecule additives, and not the original polymers, are often the culprit in harmful plastics, Urban says.
The ice plant (Delosperma cooperi) heals itself due to energy that’s created and stored at the molecular level. When cut, the energy unwinds, pushing the edges together so healing can occur.
Clinton Colmenares is with Clemson University Relations.