The key to keratin’s shape-changing abilities is its hierarchical structure, said Luca Cera, a postdoctoral fellow at SEAS and first author of the paper. A single chain of keratin is arranged into a spring-like structure known as alpha-helix. Two of these chains twist together to form a structure known as a coiled coil. Many of these are assembled into protofilaments and eventually large fibers.

“The organization of the alpha helix and the connective chemical bonds give the material both strength and shape memory,” said Cera.

When a fiber is stretched or exposed to a particular stimulus, the spring-like structures uncoil, and the bonds realign to form stable beta-sheets. The fiber remains in that position until it is triggered to coil back into its original shape.

To demonstrate this process, the researchers 3D-printed keratin sheets in a variety of shapes. They programmed the material’s permanent shape — which it will always return to when triggered — using a solution of hydrogen peroxide and monosodium phosphate. Once the memory was set, the sheet could be re-programmed and molded into new shapes.

For example, one keratin sheet was folded into a complex origami star as its permanent shape. Once the memory was set, the researchers dunked the star in water, where it unfolded and became malleable. From there, they rolled the sheet into a tight tube. Once dry, the sheet was locked in as a fully stable and functional tube. To reverse the process, they put the tube back into water, where it unrolled and folded back into an origami star.

“This two-step process of 3D printing the material and then setting its permanent shapes allows for the fabrication of really complex shapes with structural features down to the micron level,” said Cera. “This makes the material suitable for a vast range of applications from textile to tissue engineering.”

“Whether you are using fibers like this to make brassieres whose cup size and shape can be customized every day, or you are trying to make actuating textiles for medical therapeutics, the possibilities of Luca’s work are broad and exciting,” said Parker. “We are continuing to reimagine textiles by using biological molecules as engineering substrates like they have never been used before.”

 This research is co-authored by Grant Gonzalez, Qihan Liu, Suji Choi, Christophe Chantre, Juncheol Lee, Rudy Gabardi, Myung Choi, and Kwanwoo Shin.

It was supported in part by the Harvard University Materials Research Science and Engineering Center (MRSEC), under grant DMR-1420570 from the Nation Science Foundation.