Bionic robotics now with ossified artificial muscles

Robotics

Technological Innovation Website Editorial Team - September 8, 2025

A prototype robotic leg—with integrated artificial muscles—bending at the ankle and knee. [Image: Ryan Truby/Taekyoung Kim/Northwestern University]

Bionic artificial muscle

A new type of soft, biomimetic artificial muscle promises to pave the way for building animal- and human-scale robots.

The new muscles - or actuators - provide the performance and mechanical properties needed to build robotic musculoskeletal systems, including prosthetics for medical use.

These new bioinspired materials could change the way robots walk, run, interact with humans, and navigate the world around them.

To demonstrate the capabilities of the artificial muscle, Taekyoung Kim and colleagues at Northwestern University in the US built a full-size humanoid leg, complete with rigid plastic "bones," elastic tendons, and even a sensor that allows the bionic prosthesis to sense its movements.

The leg uses three artificial muscles—a quadriceps, a hamstring, and a calf—to flex the knee and ankle joints. The muscles are flexible enough to absorb impact, yet capable of applying enough force and movement to kick a ball.

"It's difficult to make physically non-compliant robots respond or adapt smoothly to external changes and interact safely with humans," Kim said. "For future robots to move more naturally and safely in unstructured environments, we need to design them more like human bodies—with rigid skeletons and soft, muscle-like actuators."

In addition to robots, the technology will find widespread use in biomedical prosthetics. [Image: Taekyoung Kim et al. - 10.1002/adma.202501290]

Soft actuators

Soft actuators, with mechanical properties similar to those of muscles, have been the subject of great interest because they allow for the construction of lighter equipment, with lower energy consumption and more compatible not only with coexistence with humans, but also with integration into the human body.

The team's approach uses a 3D-printed cylindrical actuator, a structure called a "hand-shear auxetic," which allows for unique movements and properties, such as extending and expanding when twisted. The twisting motion required to move the structure can be generated by a small, integrated electric motor.

Kim also developed a method for 3D printing these structures using a common, inexpensive rubber often used in cell phone cases. The material was then used to build a bellows-shaped structure, which allows the rotary motor to drive the extension and contraction of the actuators.

These actuators push and pull with impressive force, acting like artificial muscles. The muscle can even dynamically stiffen when actuated, just like a human muscle. This was a substantial improvement over the team's previous prototypes, which have been working on artificial muscles for robots for several years.

Details of the bionic leg implementation. [Image: Taekyoung Kim et al. - 10.1002/adma.202501290]

Ossified artificial muscle

To demonstrate the muscle's true potential, the team used 3D printing to mimic leg bones using rigid plastic. Tendon-inspired connectors made of rubber connect the quadriceps and hamstring muscles to the shin bone, and the calf muscle to the foot structure. The tendons and muscles helped cushion movement and absorb shock, similar to a biological musculoskeletal system.

Each musculoskeletal system weighs about the same as a soccer ball and is slightly larger than a soda can. It can stretch up to 30% of its length and lift objects 17 times heavier than itself.

But perhaps most crucial to its use in robotic bodies and prosthetics is the fact that the muscle can be powered by a battery, eliminating the need for heavy external equipment.

"By developing new robotic materials with the performance and properties of biological musculoskeletal systems, we can build more resilient and robust robots for real-world use," said Professor Ryan Truby. "We're excited to see how these artificial muscles could propel new directions for humanoid and animal-like robots."

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