Future robots could soon be equipped with more advanced muscle power, thanks to a development by engineers at Northwestern University. They have created a soft artificial muscle that promises to enhance the capabilities of untethered animal- and human-scale robots. These new muscles, or actuators, offer the necessary performance and mechanical properties for constructing robotic musculoskeletal systems.
To showcase the potential of this innovation, engineers integrated these artificial muscles into a life-size humanoid leg. This robotic leg features rigid plastic "bones," elastic "tendons," and a sensor that allows it to "feel" its movements. The leg uses three artificial muscles—quadricep, hamstring, and calf—to move its knee and ankle joints. These muscles are designed to absorb impacts while providing enough strength to kick a volleyball off a pedestal.
The research was published in the journal Advanced Materials on July 24. Ryan Truby, senior author of the study and professor at Northwestern's McCormick School of Engineering, explained: “Robots are typically constructed from rigid materials and mechanisms that enable precise motion for specific tasks.” He added that their goal is to create bioinspired robotic bodies capable of flexibility and adaptability.
Truby's lab has been working on overcoming challenges associated with replicating muscle function in robots. Taekyoung Kim, first author of the study, noted: “It’s difficult to make robots without physical compliance smoothly respond or adapt to external changes.”
The team addressed these challenges by developing an actuator based on a 3D-printed cylindrical structure called a "handed shearing auxetic" (HSA). This structure allows unique movements when twisted by an electric motor. The actuator is encased in rubber origami bellows that enable impressive strength as artificial muscles.
Each muscle weighs about as much as a soccer ball but can lift objects 17 times heavier than itself. Notably, these muscles can be powered by batteries rather than heavy external equipment.
In addition to Truby and Kim, other contributors include Eliot Dunn and Melinda Chen from Northwestern University’s Research Experience for Undergraduates program. The project received support from the Office of Naval Research (grant number N00014-22-1-2447) and Leslie and Mac McQuown through Northwestern’s Center for Engineering Sustainability and Resilience.