Pukyong National University

Pukyong National University Professor Dae-Seok Kim’s Team Develops High-Power Artificial Muscle by Modifying ‘Molecular Crosslinks’ in Liquid Crystal Elastomers

- ‘Anisotropic Crosslinker’ developed to reinforce artificial muscles at the molecular level

- More than 30% contraction with high power output … Electrically driven artificial muscle lifts a 1 kg load

- Findings published in ‘Angewandte Chemie International Edition,’ a world-renowned international journal in chemistry and materials science

Researchers at Pukyong National University have developed a high-power artificial muscle capable of maintaining strong force even at high temperatures through a new approach that modifies the structure of the “molecular connection points (crosslinks)” that make up Liquid Crystal Elastomers (LCEs).

Professor Dae-Seok Kim’s research team in the Major of Polymer Engineering at Pukyong National University developed a “mesogen-mimetic anisotropic crosslinker” that resembles the rod-shaped liquid crystal mesogens forming the main chains of LCEs. This approach significantly reduces mechanical softening―the tendency of a rigid material to become softer―at high temperatures, a major limitation of conventional liquid crystal artificial muscles.

Liquid crystal elastomers are soft and flexible like rubber while also being capable of contracting in response to stimuli such as heat or light, making them promising key materials for artificial muscles and soft robots. However, conventional LCEs tend to soften at the high temperatures where contraction becomes most active, making it difficult for them to sustain sufficient force.

To address this issue, the research team focused on the crosslinker, which until now had been regarded simply as a component that connects and fixes polymer chains. In conventional liquid crystal elastomers, elongated, rod-shaped liquid crystal mesogens in the main chains are aligned in a specific direction, while the crosslinkers connecting these chains are generally flexible or lack a distinct directional structure.

Starting from the idea, “If liquid crystal chains have directionality, why not design the crosslinkers connecting them to resemble liquid crystal mesogens as well?” the team designed a new crosslinker called PBB-TT, featuring an elongated, rigid core structure similar to that of liquid crystal mesogens. This transformed the crosslinks from simple “connection points” that hold chains together into “molecular reinforcement points” that work in harmony with the liquid crystal molecules to support and transmit force.

Importantly, the new crosslinker incorporates not only a rigid structure but also flexible linking segments, preventing the material from becoming excessively stiff and restricting the movement of the liquid crystals. As a result, the LCE maintained its mechanical strength at high temperatures while preserving the large contraction characteristic of liquid crystal elastomers.

The developed artificial muscle contracted by more than 30% even while supporting an actual load and maintained stable performance over repeated actuation cycles. An artificial muscle incorporating an electrical heating structure also successfully lifted a 1 kg load.

The research team also utilized the material’s remaining photoreactivity to develop a technique for directly bonding LCE films to one another without using a separate adhesive. Using this technique, the researchers fabricated not only electrically driven artificial muscles but also a soft gripper capable of grasping and releasing objects. They also confirmed that damaged artificial muscles could be rejoined to recover most of their original performance.

The significance of this study lies in redesigning the molecular structure of the liquid crystal elastomer itself, rather than adding separate reinforcing materials such as carbon nanotubes or graphene. While conventional LCE networks combine directionally aligned liquid crystal mesogens with non-directional crosslinks, the new approach designs the crosslinkers themselves to resemble the liquid crystal mesogens in the main chains, thereby integrating the directionality of the entire network and the transmission of force at the molecular level.

Professor Dae-Seok Kim explained, “The main chains of liquid crystal elastomers already contain directionally aligned, rod-shaped mesogens, but conventional crosslinkers have largely been limited to simply connecting the chains. The key to this study was to give the crosslinkers a directional structure resembling that of liquid crystal mesogens, transforming simple molecular connection points into reinforcement points that actually support and transmit force.”

He added, “For artificial muscles to perform actual work, they must not only produce large movements but also maintain sufficient force while moving. This study demonstrates the potential of high-power liquid crystal artificial muscles through a new molecular design strategy based on the structure and directionality of crosslinkers, and we expect the approach to be extended to a wide range of applications, including soft robots and wearable actuators.”

The paper presenting the findings, titled “Mesogen-Mimetic Anisotropic Crosslinkers for Load-Bearing Liquid Crystal Elastomer Artificial Muscles,” was published on September 8 in Angewandte Chemie International Edition (IF 17.6), a world-renowned international journal in the fields of chemistry and materials science. <Pukyong Today>