A domestic research team has developed an artificial muscle that stretches like rubber but hardens like steel. Like human muscles that contract to lift heavy objects, this artificial muscle can also lift weight as it contracts, and the energy it can deliver is 30 times greater than that of human muscle.
A research team led by Professor Jeong Hun-ui in the Department of Mechanical Engineering at Ulsan National Institute of Science and Technology (UNIST) said on the 17th that it has developed a soft artificial muscle whose stiffness can be freely adjusted. The study was published online on the 7th in the international journal Advanced Functional Materials.
Soft artificial muscles can be used in robots that need to interact with people, wearable devices, and medical assistive equipment, but they have limits when it comes to lifting heavy objects. Their softness and flexibility hinder their role as muscles that exert actual force.
The soft artificial muscle developed by the team becomes rigid when it needs to support a load and becomes soft to contract when it needs to lift it. In the rigid state, this artificial muscle, which weighs only 1.25 g, can support a 5 kg load—about 4,000 times its own weight. In the soft state, it can stretch up to 12 times its length.
During the lifting process, the muscle showed an actuation strain in which 86.4% of its original length contracted, more than double that of human muscle (about 40%). Its work density reached 1,150 kJ/m³, 30 times higher than that of human muscle. Work density indicates how much work (energy) a 1 m³ volume of muscle can perform. The more deformable yet rigid the muscle is, the higher the work density, but in general those two conditions are at odds.
The team addressed this problem by designing a shape‑memory polymer material so that two types of bonding appear within the muscle. Chemical bonds in the muscle tightly link polymer chains with covalent bonds to maintain structural strength, while physical bonds break and re-form in response to thermal stimuli, making the muscle flexible and highly stretchable.
They also added magnetized particles with a specially treated surface to strengthen the physical bonds and enabled the muscle to move with an external magnetic field. They succeeded in an experiment that used a magnetic field to move the muscle and grasp an object.
Professor Jeong Hun-ui said, "This study overcomes a fundamental limitation of existing artificial muscles: when they stretch well, they are weak, and when they are strong, they do not stretch well," adding, "It could be used in a wide range of fields, including soft robots, wearable robots, and interfaces that allow people and machines to interact flexibly."
References
Advanced Functional Materials (2025), DOI: https://doi.org/10.1002/adfm.202516218