Northwestern University Invents Biomimetic Artificial Muscles Enabling Robot Limbs to Push, Lift, and Kick

In the near future, robots may soon possess much greater muscle strength. According to foreign media reports, engineers at Northwestern University have developed a soft artificial muscle, paving the way for untethered animal and human-like robots. This new type of muscle, also known as an actuator, provides the performance and mechanical characteristics needed to build robotic musculoskeletal systems.

Image source: Northwest University

To demonstrate the functionality of these artificial muscles, engineers implanted them into a life-sized human leg model, equipped with rigid plastic "bones," elastic "tendons," and even a sensor that allows the robot to "sense" its own movements. The leg uses three artificial muscles—the quadriceps, hamstrings, and calf muscles—to flex the knee and ankle joints. These muscles are flexible enough to absorb impact, yet still exert sufficient force and motion to kick a volleyball off its stand.

This new bionic material innovation could change the way robots walk, run, interact with humans, and navigate the surrounding world. The related research paper was published in the journal "Advanced Materials" on July 24th.

"Robots are typically made of rigid materials and mechanical devices that can precisely carry out specific tasks," said Ryan Truby, senior author of the study from Northwestern University. "But the real world is ever-changing and extremely complex. Our goal is to construct bio-inspired robotic bodies that are flexible, adaptable, and capable of handling the uncertainties of the physical world."

This not only includes incorporating practical artificial muscles, but also integrating components such as bones, tendons, or ligaments into robots. If we can achieve this, robots will not only become more resilient and adaptable but also improve efficiency by leveraging the mechanical principles of softer materials.

Copy the current challenges of muscles

At present, most robots are rigid and cumbersome, making it difficult for them to smoothly adapt to uneven terrain or to perform complex and delicate tasks without damaging other objects or injuring themselves.

"It is very challenging for robots that lack physical compliance to smoothly respond or adapt to external changes and interact safely with humans," said Dr. Kim, a postdoctoral researcher at the Truby Lab. "To enable future robots to move more naturally and safely in unstructured environments, we need to design them more like the human body—with both hard skeletons and soft, muscle-like actuators."

Recently, robotics experts have started developing soft actuators with muscle-like mechanical properties. However, current methods typically require large and heavy equipment to drive them. Even so, they are not durable enough to generate sufficient force to accomplish practical tasks.

"Designing soft materials that work like muscles is really difficult," Truby said. "Even if you can make a material move like artificial muscles, there are many other challenges, such as delivering enough power to exert sufficient force. Connecting them to rigid structures similar to bones poses even more problems."

Manufacturing artificial muscles

To overcome these challenges, the team referred to an actuator previously developed by the Truby Lab. The core of this actuator is a 3D-printed cylindrical structure called a "chiral shear auxetic" (HSA). The HSA has a complex structure capable of achieving unique movements and characteristics, such as stretching and expanding when twisted.

The twisting motion required to move the HSA can be generated by a small integrated motor. Kim developed a method to 3D print the HSA using inexpensive rubber commonly found in phone cases.

In the new design, the team encapsulates the HSA in a rubber origami bellows structure, allowing the rotary motor to drive the assembled actuator to extend and retract. Now, these actuators can push and pull with astonishing force, much like artificial muscles. These muscles can even dynamically stiffen under load, just like human muscles.

Each muscle weighs approximately the same as a football, slightly larger than a can of soda. It can stretch to 30% of its length, contract, and lift objects 17 times its own weight. Perhaps most crucial for their application in robots, these muscles can be powered by batteries without the need for bulky external devices.

A life-sized leg that can "kick" and "sense"

To demonstrate the practical application potential of these muscles, Truby, Kim, and their team used 3D printing technology to create a life-sized robotic leg. The team made the "bones" of the leg from hard plastic and used rubber to create connectors similar to tendons. The elastic tendons connect the quadriceps and hamstring muscles to the lower leg bones and connect the calf muscles to the foot structure. The tendons and muscles help to dampen motion and absorb impact, similar to a biological musculoskeletal system.

The team also added a flexible 3D-printed sensor that enables the leg to "sense" its own muscles. The sensor is designed in a sandwich structure, with a layer of flexible conductive plastic sandwiched between two non-conductive layers. When the artificial muscle moves, the sensor moves as well. As it stretches, its resistance changes, allowing the robot to sense the degree of extension or contraction of its muscles.

The final prosthesis is compact in structure and powered by a battery. A portable battery can provide enough energy on a single charge to bend the knee thousands of times within an hour. Achieving similar functionality using other soft actuator technologies would be very difficult, if not impractical.

“By designing new robotic materials with the performance and characteristics of biological musculoskeletal systems, we can create more resilient and durable robots to meet practical usage demands,” said Truby. “We are very much looking forward to seeing how these artificial muscles will drive new directions for humanoid and animal-like robots.”