UNIST Develops Artificial Muscle That Stretches Like Rubber and Hardens Like Steel

Professor Jeong Hoon's Team Develops Soft Artificial Muscle with Enhanced Elasticity and Stiffness
Performs 30 Times More Work Than Human Muscle, Published in Adv. Funct. Mater.

An artificial muscle has been developed that can stretch like rubber yet become as hard as steel.

Just as human muscles contract to lift heavy objects, this artificial muscle can also contract and lift weights. The energy it can generate in this process is 30 times greater than that of human muscle.

The research team led by Professor Jeong Hoon at the Department of Mechanical Engineering at Ulsan National Institute of Science and Technology (UNIST) announced on September 17 that they have developed a soft artificial muscle capable of freely changing its stiffness.

Research team, Professor Jeong Hoon (left) and researcher Somi Kim (first author). Provided by Ulsan National Institute of Science and Technology (UNIST)

Soft artificial muscles can be used in robots that interact with humans, wearable devices, and medical assistive devices. However, they have limitations when it comes to lifting heavy objects, as their softness and flexibility hinder their ability to perform as real muscles do in terms of strength.

The soft artificial muscle developed by the research team becomes rigid when it needs to bear a load, and softens to contract when it needs to lift. In its rigid state, this artificial muscle, which weighs only 1.25 grams, can support a load of 5 kilograms-approximately 4,000 times its own weight. In its soft state, it can stretch up to 12 times its original length.

During the lifting process, this muscle exhibited an actuation strain of 86.4% of its original length, which is more than twice that of human muscle (about 40%). Its work density reached 1,150 KJ/m³, 30 times greater than that of human muscle. Work density is an indicator of how much work (energy) a muscle of 1 m³ can perform. The more deformable and rigid the muscle, the higher the work density; however, these two properties typically conflict with each other.

The research team addressed this issue by designing a shape memory polymer material that exhibits two types of bonds within the muscle. Chemical bonds tightly link the polymer chains through covalent bonding, maintaining structural strength, while physical bonds break and reform in response to thermal stimuli, making the muscle flexible and stretchable.

Additionally, the team enhanced the physical bonds by incorporating magnetically responsive particles with a specially treated surface, allowing the muscle to move under an external magnetic field. They successfully demonstrated the muscle picking up objects using a magnetic field in actual experiments.

Demonstration of artificial muscle movement through a robotic arm system.

Professor Jeong Hoon stated, "This research overcomes the fundamental limitation of existing artificial muscles, where high stretchability comes at the expense of strength and vice versa. In the future, this technology could be used in various fields such as soft robotics, wearable robots, and interfaces that enable flexible interaction between humans and machines."

This research was published online in the international journal Advanced Functional Materials on September 7, 2025, and was supported by the National Research Foundation of Korea.

This content was produced with the assistance of AI translation services.