Hydrogels Take Multiple Shapes

How light or moisture triggers a plant to curl its leaves or release its seeds has long inspired scientists to strive toward soft synthetic materials that similarly shape-shift in response to the surrounding environment. Researchers envision such substances being used to trigger the activity of drug delivery or robotic devices.

Some experimental materials have already been synthesized that deform because of a change in a single parameter, such as temperature, but they can take only one of two shapes. Researchers have now taken the next step: They’ve programmed a polymer hydrogel to adopt multiple shapes in response to multiple triggers (J. Am. Chem. Soc., DOI: 10.1021/ja400518c).

“We made a hydrogel take three shapes,” says the University of Toronto’s Eugenia Kumacheva, who led the team with Zhihong Nie of the University of Maryland, College Park. “But in general, with our method, we can make it take more.”

The work opens the door to more sophisticated shape-shifting materials than are now available.

To fabricate the shape-shifters, the team exposes a thin layer of gel made with one type of monomer to a solution containing another monomer as well as a cross-linker. The solution also holds a photoinitiator, a compound that jump-starts polymerization in response to light. After the gel absorbs these components, the team places a patterned mask over top of it and irradiates the assembly with ultraviolet light.

The researchers then rinse away unreacted reagents and repeat the process with a new mask and different monomers. The end result is a patterned hydrogel sheet with myriad microscopic patches that have different rigidities and swelling abilities.

When exposed to a high-pH solution, polymethacrylic acid-rich stripes on a hydrogel sheet swell, causing the entire film to curl into a long, thin tube. And at high salt concentrations, the team showed, poly(N-isopropylamide)-rich patches shrink, leading to a stubby, wide-bore tube.

Ryan C. Hayward, a polymer scientist at the University of Massachusetts, Amherst, said the work “demonstrates a route to patterning multiple different stimuli-responsive polymer networks within the same object.”