This artificial muscle can support 4,000 times its own weight and stretch 12 times its length

A research team — led by Professor Hoon Eui Jeong at UNIST — has engineered a new type of soft artificial muscle that can seamlessly go from soft and flexible to rigid and strong, overcoming a key trade-off that has limited the capabilities of soft robotics. This remarkable muscle — detailed in the journal Advanced Functional Materials — has high work density. A 1.25-gram sample was able to support 4,000 times its weight in its rigid state and stretch 12 times its length in the flexible state.

The new actuator is designed to solve a fundamental challenge in soft robotics — artificial muscles tend to be either highly flexible but weak or strong but stiff. The UNIST team achieved both by engineering a “dual cross-linked” polymer network. This design combines strong, permanent covalent bonds for mechanical integrity with dynamic physical interactions that can be formed and broken using thermal stimuli to provide flexibility.

The scientists also embedded magnetic microparticles within the polymer, enabling precise control of the muscle's movement with the use of an external magnetic field. This dual-responsive design enables the material to exhibit a wide range of stiffness — from 0.213 MPa to 292 MPa.

While contracting, the muscle achieves an actuation strain of over 86.4% and a work density of 1,150 kJ/m³, an impressively high value for a soft artificial muscle and roughly 30 times that of a human muscle tissue. Should the new polymer be successfully developed and integrated, we could see much stronger and flexible humanoid robots.