Don’t Force It: Heat, light, or sound is all it takes to turn these objects into something new – College of Arts and Sciences News Archive

These are the first nonliving objects that can shift back and forth between two shapes without any external force being applied.

Bend it, stretch it, squish it flat. To make something into the shape you want, you have to use force. Maybe a lot of force, like chopping firewood with an axe—or maybe just a little, like pressing your thumb into wet clay. Either way, you’re using force.

Not with the stuff in UNC chemist Sergei Sheiko’s lab. For the first time, his team has figured out how to make objects shift back and forth between two programmed shapes, over and over again, without applying any kind of force. This technology, he says, could someday change how we do surgery, pack things for shipment, and even how we fly planes.

http://vimeo.com/104432862

These polymers are “simply smaller molecules connected in a linear chain,” Sheiko says. Because polymers are made up of molecular chains instead of rigid structures, they can be highly flexible, easily adopting various shapes without fracturing.

Left side: Semi-crystalline polymers, cooled. Right side: Polymers warmed up, melted, and ready to form a new shape.

First, Sheiko says, you mold polymers into a certain shape, like a tube. Second, you warm the tube up to melt the microscopic crystals inside it. The tube becomes soft, like a rubber band, and you bend it into the shape you want, such as a coil. Third, you decrease the temperature, so the molecules crystallize and cement the new shape.

Now, whenever you increase the coil’s temperature, melting some—but not all—of its semi-crystalline structure, the coil returns to the tube form. Decrease the temperature, and the remaining crystals serve as a template to change to the coiled form. The lab of Department of Chemistry Chair Valerie Ashby worked on the chemical composition of the polymers to let the scientists choose which temperatures would make the shapes change form.

Using this idea, scientists could make a surgical stent, for example. Start out with a narrow tube. Then raise it to 98.6 degrees—the temperature of the human body—and bend it into a round shape to shore up the walls of an artery. Bring it back down to room temperature, and it’s a tube again.

Because the stent starts out as a narrow tube, a surgeon could insert it in a minimally invasive way. Then the patient’s body heat would bring it back to the stent shape once it was inside.

http://vimeo.com/104336981

The U.S. military is interested in shape-shifting, Sheiko says, for all kinds of reasons. Sheiko grabs a magazine off his desk. “Imagine I can put a piece of paper under your door and increase the temperature or shine some light on it, and it assembles into some kind of a device,” he says.

Or imagine a plane with shape-shifting wings. “If you look at birds like hawks or eagles, they change the geometry of their wings,” Sheiko says. “If they want to fly and look for prey, their wings have one geometry, but the geometry changes when they attack.”

And imagine if all these complex shape transformations were trigged remotely. Instead of using temperature, the lab has also used certain wavelengths of sound, magnets, and light waves to trigger a material to change.

Sheiko thinks that soft-matter science—the study of polymers and other soft materials like liquid crystals, gels, and foams—will be the nuclear physics of the twenty-first century: a new frontier of scientific discovery. There are fundamental principles we don’t yet understand about how molecules such as polymers self-organize and are held together by weak forces to work in a system.

“The ultimate example of this,” Sheiko says, “is the human body.”

Sergei Sheiko is the George A. Bush Jr. Distinguished Professor in the Department of Chemistry in the College of Arts and Sciences. Valerie Ashby, chair of the Department of Chemistry, was a coauthor of the paper describing shape-shifting polymers. OtherUNC coauthors were chemistry postdoc Jing Zhou, chemistry grad student Sarah Turner, physics grad student Qiaoxi Li, and former chemistry research associate Sarah Brosnan.

Their work is supported by the National Science Foundation. Their collaborators at the Brookhaven National Laboratory are supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

All images and videos provided courtesy of the Sheiko Group.

By Susan Hardy, Endeavors

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