The goal was to build a material that could be used to stimulate muscles if a person’s nerves were damaged or lost. It was one part of a broader effort in the large Massachusetts Institute of Technology laboratory run by Robert Langer to engineer devices and tissues that could be useful in biomedical applications. But science doesn’t always follow directions.
In April 2011, Mingming Ma, a postdoctoral researcher, picked up a a thin, black film that he had just synthesized to take a closer look. To his surprise, it started writhing around.
Ma had accidentally designed a polymer that changed shape when exposed to trace amounts of moisture, such as a sweaty palm. He walked into Langer’s office, moistened a small patch of the black film, and placed it in Langer’s hand. It proceeded to do the equivalent of a gymnastics routine.
“It started doing somersaults and stuff like that,” Langer said. “We knew this was something very different.”
The team reported Thursday in the journal Science the details of its water-responsive film, which curves, buckles, and backflips as it absorbs water, which then evaporates.
Although the researchers freely admit they don’t exactly know how such a material could ultimately be useful, they put forth all kinds of possibilities. In the paper, they use a circuit that converts mechnical energy (such as a polymer that squirms) into electricity to create a tiny amount of energy that might be useful in powering miniature devices. They also think it could be used as a sensor that detects water, or to do mechanical work, like an artifical muscle.
Christian Santangelo, a physicist at the University of Massachusetts Amherst who has worked on designing polymers that change shape, said this sort of behavior used to be seen as imperfections.
“Most people think of that kind of process as being the gunk you get on the edge of a sample. It wrinkles, the shape changes at the edge of a sample, and people haven’t been focusing on it,” Santangelo said. “It turns out that polymer chemistry has advanced to a point where we can control how the growth happens and use that to actually tune the shape of the stuff you get.”
The polymer Langer’s team described responds to moisture faster than the folding polymer Santangelo has worked on, and he said it’s unclear to him, too, what the best use of such a material would be. In his own work, the potential application that captures his interest is creating 3-D objects that automatically fold into the right shapes, from flat sheets. It might be a cheaper or more efficient way to create objects than to use a 3-D printer, Santangelo said.
Langer said that although his laboratory has demonstrated using the polymer as a way of harvesting energy that comes from having differences in the amount of moisture in the environment, he’s not sure that will be the best use. He’s looking forward to seeing what other people propose after reading his paper.
“I doubt that we have figured out the best application,” Langer said. “When people read it, they’ll think of other applications.”