A research team led by Professor Park Yoon-seok from Dept. of Advanced Materials Engineering has successfully developed a magnetically responsive mechanical metamaterial whose stiffness can be precisely tuned in real time. This joint effort was conducted with a team led by Dr. Seol Seung-kwon from the Korea Electrotechnology Research Institute (KERI). This achievement was published in Advanced Materials (IF=29.4), a world-renowned journal in the field of materials science. The research was supported by the National Research Foundation of Korea under the Ministry of Science and ICT and was conducted in partnership with KERI.



Background and Limitations of Existing Research

Metamaterials are artificially engineered substances designed to possess properties not found in nature. There is a rapidly growing need for "intelligent materials" that can autonomously alter their physical or chemical properties in response to complex and dynamic environments. This is especially applied in fields like soft robotics, biomedical devices, and adaptive structures. Prof. Park explained that the research was motivated by the goal of advancing humanoid systems capable of performing tasks in hazardous environments originally designed for human workers.

The research team identified a critical limitation in existing metamaterial studies. They were largely confined to two-state systems. These systems could only alternate between two discrete states—such as hard or soft—which severely limited their ability to respond with precision to complex environments.

To overcome this technological barrier, the team drew inspiration from the fundamental structure and operating principles of biological muscle, utilizing its characteristic rapid response to external magnetic signals. As a result, they developed a mechanical metamaterial that is structurally similar to human muscle and allows for high-speed stiffness control.

Structure of the Metamaterial, Inspired by Biological Muscle

The team focused on the sarcomere, the basic contractile unit of biological muscle. They aimed to create a metamaterial that mimics muscle's ability to become stiff when flexed and remain flexible when relaxed. To achieve this, they developed a composite ink that blends neodymium (Nd-FeB) microparticles, which exhibit powerful magnetic properties, with a highly elastic and flexible Styrene-Isoprene-Styrene polymer. The team then used this composite ink with three-dimensional printing technology to fabricate the metamaterial's fundamental structure.

To solve the limitations of two-state operation, the team designed the material to enable three-stage stiffness control (soft, medium, and hard) in response to an external magnetic field. This allows the material to adapt to its surroundings more actively than the previous two-state limit.

Furthermore, the team designed a three-dimensional structure by creating a three-dimensional array of stacked unit cells. This design simplifies control, as the entire structure can be modulated simultaneously with a single external magnetic field signal, rather than requiring individual control of each unit. Consequently, the team achieved a stiffness variation of over 390% between its weakest and strongest states, a range comparable to the difference between a common sponge and a hard eraser.

Application, Significance, and Future Work

To validate their findings, the team demonstrated a stiffness Tunable Wheel constructed from the mechanical metamaterial. The results showed the wheel deforming softly to traverse uneven terrain while remaining firm on flat surfaces, enabling more stable locomotion with reduced vibration compared to a conventional wheel. Notably, the significant difference between the stiffness states was maintained even at high temperatures. If applied to automobiles, this technology could reduce the jolting from bumpy roads, allowing for a smooth ride regardless of the road's condition or temperature.

This demonstration highlights the material's potential for enhancing stability and efficiency in humanoid robots, autonomous driving systems, and artificial muscles, all of which must operate in complex and adaptive environments.

However, challenges remain to be addressed in follow-up studies. Key issues include the stable management of magnetic field signals when scaling up to larger structures and ensuring the metamaterial's properties are maintained over long-term use. Further research is also necessary to evaluate the energy efficiency of the magnetic signaling and to confirm that the magnetic fields pose no harm to users.

Prof. Park's team emphasized that designing stable magnetic field signals for large-scale metamaterials is a crucial next step, and they plan to conduct further research to address this. He also revealed that research is underway to integrate optical sensors, paving the way for intelligent materials that can autonomously assess their environment and alter their physical properties in real-time.

Prof. Park's research team has demonstrated the potential for a metamaterial that is not only more variably responsive to complex external environments than existing materials but is also easier to apply to real-world scenarios as a three-dimension structure.

If such research continues to advance, it will lead to the development of effective materials for situations where human intervention is difficult—such as fire and mountain rescue scenarios—or where immediate environmental adaptation is critical, as in automotive tires. In an era of increasing environmental complexity and diversity, this research shows the potential to meet critical needs.

Prof. Park stated, "This research is a cornerstone toward intelligent materials that go beyond simple deforming materials to autonomously change their mechanical properties in response to external stimuli. It also marks significant progress in the development of humanoids, which can reduce the risks inherent in hazardous environments and help prevent human casualties."