Artificial muscles created for soft robotics

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have developed an ultrathin, artificial muscle for soft robotics. Their work has been described in the journal Science Robotics and demonstrated via a robotic blooming flower brooch, dancing robotic butterflies and fluttering tree leaves on a kinetic art piece.

The robotic equivalent of a muscle that can move is called an actuator. The actuator expands, contracts or rotates like muscle fibres using a stimulus such as electricity. Engineers around the world are striving to develop more dynamic actuators that respond quickly, can bend without breaking and are very durable. Soft, robotic muscles could have a wide variety of applications, from wearable electronics to advanced prosthetics.

The team from KAIST’s Creative Research Initiative Center for Functionally Antagonistic Nano-Engineering developed a very thin, responsive, flexible and durable artificial muscle. The actuator looks like a skinny strip of paper about an inch long. They used a particular type of material called MXene, which is a class of compounds that have layers only a few atoms thick.

The chosen MXene material (T₃C₂T_x) is made of thin layers of titanium and carbon compounds. It was not flexible by itself; sheets of material would flake off the actuator when bent in a loop. That changed when the MXene was ionically cross-linked — connected through an ionic bond — to a synthetic polymer. The combination of materials made the actuator flexible while still maintaining strength and conductivity, which is critical for movements driven by electricity. The actuator also responded very quickly to low voltage, and lasted for more than five hours moving continuously.

To prove the tiny robotic muscle worked, the team incorporated the actuator into wearable art: an origami-inspired brooch mimics how a narcissus flower unfolds its petals when a small amount of electricity is applied. They also designed robotic butterflies that move their wings up and down, and made the leaves of a tree sculpture flutter.

“Wearable robotics and kinetic art demonstrate how robotic muscles can have fun and beautiful applications,” said Professor Il-Kwon Oh, lead author on the paper. “It also shows the enormous potential for small, artificial muscles for a variety of uses, such as haptic feedback systems and active biomedical devices.”

The team next plans to investigate more practical applications of MXene-based soft actuators, as well as engineering applications of MXene 2D nanomaterials.

Pictured: Actuator-based butterfly robots on tree branches with different wing positions. Initially, the butterflies were stationary; after applying 2 V of AC input with a frequency of 0.2 Hz, they behaved like live butterflies. Image courtesy of the study authors.

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