Future robots could soon have a lot more muscle power.
Northwestern University engineers have developed a soft artificial muscle, paving the way for untethered animal- and human-scale robots. The new muscles, or actuators, provide the performance and mechanical properties required for building robotic musculoskeletal systems.
To demonstrate the artificial muscle's capabilities, the engineers implemented them into a life-size humanoid leg, complete with rigid plastic "bones," elastic "tendons" and even a sensor that enables the robot to "feel" its movements. The leg used three artificial muscles - a quadricep, hamstring and calf - to bend its knee and ankle joints. The muscles are compliant enough to absorb impacts but still can apply enough strength and motion to kick a volleyball off a pedestal.
The new bioinspired materials innovation could change how robots walk, run, interact with humans and navigate the world around them.
The study was published on Thursday (July 24) in the journal Advanced Materials.
"Robots are typically constructed from rigid materials and mechanisms that enable precise motion for specific tasks," said Northwestern's Ryan Truby, the study's senior author. "But the real world is constantly changing and incredibly complex. Our goal is to build bioinspired robotic bodies that can be flexible, adaptable and embrace the uncertainty of the physical world. This includes bringing together not only practical artificial muscles, but also bone- and tendon- or ligament-like components to robotics. If we can do that, then robots won't just become more resilient and adaptable. They will be able to harness the mechanics of softer materials to become more efficient."
Truby is the June and Donald Brewer Junior Professor of Materials Science and Engineering and Mechanical Engineering at the McCormick School of Engineering, where he directs The Robotic Matter Lab. Taekyoung Kim, a postdoctoral scholar in Truby's lab, is the study's first author.
Current challenges in replicating muscle
Stiff, rigid and clunky, most current robots have difficulty smoothly adapting to uneven terrain or performing complex, delicate tasks without breaking other objects or injuring themselves.
"It's difficult to make robots without physical compliance smoothly respond or adapt to external changes and safely interact with humans," Kim said. "To make future robots move more naturally and safely in unstructured environments, we need to design them more like human bodies - with both hard skeletons and soft, muscle-like actuators."
More recently, roboticists have started developing soft actuators with muscle-like mechanical properties. But current approaches often need large, heavy equipment to power or drive them. And, even then, they are not durable enough and cannot generate enough force to complete real tasks.
"It's really difficult to engineer soft materials to perform like muscle," Truby said. "Even if you can make a material move like an artificial muscle, there are many other challenges like transmitting sufficient force with enough power. Interfacing them with rigid bone-like features presents even more problems."
Making the artificial muscle
To overcome these challenges, the team looked to an actuator previously developed in Truby's lab. At the heart of the actuator is a 3D-printed cylindrical structure called a "handed shearing auxetic" (HSA). The HSA has a complex structure that enables unique movements and properties, such as extending and expanding when twisted. The twisting motion needed to move the HSA can be generated by a small, integrated electric motor. Kim developed a method of 3D-printing HSAs from a common, inexpensive rubber often used in cellphone cases.
In their new design, the team encased the HSA in a rubber origami bellows structure that enables the rotating motor to drive the assembled actuators' extension and contraction. The actuators now push and pull with impressive strength, performing as artificial muscles. The muscle can even dynamically stiffen when actuated - just like a human muscle.
Each muscle weighs about as much as a soccer ball and is slightly larger than a can of soda. It can stretch up to 30% of its length, shrink and lift objects 17 times heavier than itself. Perhaps most crucial to their use in robotic bodies, the muscle can be battery powered, bypassing the need for heavy, external equipment.
A human-scale leg that can 'kick' and 'feel'
To demonstrate the muscle's real-world potential, Truby, Kim and their team used 3D printing to build a human-sized robotic leg. The team constructed the leg's "bones" from rigid plastic and tendon-inspired connectors from rubber. The elastic tendons connect the quadricep and hamstring muscles to the shank bone and the calf muscle to the foot structure. The tendons and muscles helped dampen movements and absorb shocks, similar to a biological musculoskeletal system.
The team also added a flexible, 3D-printed sensor that allows the leg to "feel" its own muscle. Designed like a sandwich, a conductive layer of flexible plastic is squished between two non-conductive layers. When the artificial muscle moves, the sensor does too. As it stretches, its electrical resistance changes, allowing the robot to sense how much its muscle extends or contracts.
The resulting leg is compact and battery powered. A single charge from a portable battery supplied enough energy to allow the leg to bend its knee thousands of times in an hour. Achieving similar capabilities with other soft actuator technologies would be difficult if not impractical.
"By engineering new materials for robotics with the performance and properties of biological musculoskeletal systems, we can build robots to be more resilient and robust for real-world use," Truby said. "We're excited to see how these artificial muscles can drive new directions for humanoid and animal-like robots."
Other authors on the study are Eliot Dunn, a high school student research intern in the Robotic Matter Lab, and Melinda Chen, a participant in the Research Experience for Undergraduates (REU) program run by the Northwestern University Materials Research Science and Engineering Center (NU MRSEC). The work was supported by 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.
