Revolutionary Artificial Muscle Can Switch From Soft To Rigid Like Steel, Offering Unprecedented Power And Flexibility

Abstract

Soft artificial muscles offer transformative potential in robotics, wearable electronics, and biomedical devices due to their light weight, mechanical compliance, and multidirectional actuation. However, their broader utility is hindered by an intrinsic trade-off between stretchability and energy output, often resulting in limited work densities. Here, a high-performance magnetic composite actuator is presented that addresses this limitation through an optimized dual cross-linked polymer network comprising covalent bonds and dynamic physical interactions. The actuator incorporates a stiffness-tunable polymer matrix embedded with surface-functionalized magnetic microparticles, enabling reversible, on-demand stiffness modulation and reprogrammable actuation. This composite architecture achieves exceptional deformability (elongation at break of 1274%) and programmable stiffness switching from 213 kPa to 292 MPa (switching ratio of 1.37 × 103), with shape fixation exceeding 99%. Together, these properties yield a work density of 1150 kJ m−3 and an actuation strain of 86.4%, representing one of the highest values reported for soft artificial muscles. It also supports loads exceeding 4000 times its own weight, demonstrating a powerful and reconfigurable platform for next-generation soft actuation.

A research team, affiliated with UNIST has unveiled a new type of artificial muscle that can seamlessly transition from soft and flexible to rigid and strong-much like rubber transforming into steel. When contracting, this innovative muscle can lift weights over 30 times its own weight, delivering energy output far surpassing that of human muscles.

Led by Professor Hoon Eui Jeong in the Department of Mechanical Engineering at UNIST, the research team announced that they have successfully created a soft artificial muscle capable of dynamically adjusting its stiffness.

While soft artificial muscles hold tremendous promise for applications in robotics, wearable devices, and medical assistive technologies that require human-like interaction, their widespread use has been limited by an inherent trade-off: they tend to be either highly flexible or capable of exerting significant force, but not both simultaneously.

Their breakthrough addresses this challenge by engineering a composite muscle that becomes stiff when bearing heavy loads and softens when it needs to contract. Remarkably, in its stiffened state, this tiny artificial muscle-weighing just 1.25 grams-can support up to 5 kilograms, roughly 4,000 times its own weight. When softened, it can stretch up to 12 times its original length.

During contraction, the muscle achieves a strain of 86.4%, more than double the approximately 40% strain typical of human muscles. Its work density reaches 1,150 kJ/m³, which is 30 times higher than that of human tissue. The work density indicates how much energy per unit volume the muscle can deliver, and achieving high values alongside high stretchability has been a longstanding challenge.

Figure 1. Schematic image, illustrating the dual cross-linking strategy and thermomechanical actuation mechanism of the magnetic artificial muscles.

The key innovation lies in the dual cross-linked polymer network designed by the researchers. The muscle's chemical bonds-formed through covalent linkages-provide structural strength, while physical interactions-formed and broken by thermal stimuli-grant it exceptional flexibility. Additionally, surface-treated magnetic microparticles embedded within the muscle enable external magnetic fields to precisely control its movement, which was demonstrated in successful experiments lifting objects using magnetic actuation.

Professor Jeong explained, "This research overcomes the fundamental limitation where traditional artificial muscles are either highly stretchable but weak or strong but stiff. Our composite material can do both, opening the door to more versatile soft robots, wearable devices, and intuitive human-machine interfaces."

The study was published online in Advanced Functional Materials on September 7, and was supported by the National Research Foundation of Korea (NRF).

Journal Reference

Somi Kim, Sang-Woo Lee, Hyukjoo Kwon, et al., "Soft Magnetic Artificial Muscles with High Work Density and Actuation Strain via Dual Cross-Linking Design," Adv. Funct. Mater., (2025).