Crumpled graphene forms artificial muscles

Engineers at Duke University engineers have layered atom-thick lattices of carbon with polymers to create unique materials with a wide range of uses, including artificial muscles.

The lattice, known as graphene, is made of pure carbon and appears under magnification much like chicken wire. Because of its unique optical, electrical and mechanical properties, graphene can be used in electronics, energy storage, composite materials and biomedicine.

Nevertheless, graphene is extremely difficult to handle in that it easily “crumples.”

Unfortunately, scientists have thus far been incapable of controlling the crumpling and unfolding of large-area graphene to take advantage of its properties.

Indeed, Duke engineer Xuanhe Zhao, assistant professor in Duke’s Pratt School of Engineering, compares the challenge of controlling graphene to the difference between unfolding paper and wet tissue.

“If you crumpled up normal paper, you can pretty easily flatten it out,” Zhao explained.

“However, graphene is more like wet tissue paper. It is extremely thin and sticky and difficult to unfold once crumpled. We have developed a method to solve this problem and control the crumpling and unfolding of large-area graphene films.”

According to Zhao, Duke engineers attached the graphene to a rubber film that had been pre-stretched to many times its original size. Once the rubber film was relaxed, parts of the graphene detached from the rubber while other parts kept adhering to it, forming an attached-detached pattern with a feature size of a few nanometers.

As the rubber relaxed, the detached graphene was compressed to crumple. But as the rubber film was stretched back, the adhered spots of graphene pulled on the crumpled areas to unfold the sheet.

“In this way, the crumpling and unfolding of large-area, atomic-thick graphene can be controlled by simply stretching and relaxing a rubber film, even by hands,” Zhao said.

Jianfeng Zang, a postdoctoral fellow in Zhao’s group, notes that this particular approach has opened avenues to exploit unprecedented properties and functions of graphene.

“For example, we can tune the graphene from being transparent to opaque by crumpling it, and tune it back by unfolding it,” he said.

In addition, the Duke engineers layered the graphene with different polymer films to make a “soft” material that can act like muscle tissues by contracting and expanding on demand. When electricity is applied to the graphene, the artificial muscle expands in area; when the electricity is cut off, it relaxes. As such, varying the voltage controls the degree of contraction and relaxation.

“The crumpling and unfolding of graphene allows large deformation of the artificial muscle.New artificial muscles are enabling diverse technologies ranging from robotics and drug delivery to energy harvesting and storage.

“In particular, they promise to greatly improve the quality of life for millions of disabled people by providing affordable devices such as lightweight prostheses and full-page Braille displays,” he added.