Light-powered artificial muscles for underwater robots with reversible, high-stroke actuation

A Korean research team has developed a light-powered artificial muscle that operates freely underwater, paving the way for next-generation soft robotics.

The research team—Dr. Hyun Kim at the Korea Research Institute of Chemical Technology (KRICT), Prof. Habeom Lee at Pusan National University, and Prof. Taylor H. Ware at Texas A&M University—successfully developed artificial muscles based on azobenzene-functionalized semicrystalline liquid crystal elastomers (AC-LCEs) that actuate in response to light.

Traditional soft robotic actuators driven by electricity, heat, or pressurized air and liquids (pneumatic and hydraulic systems) are often challenging to operate in underwater environments due to the exposure of complex components like batteries, motors, wires, or pumps to water.

While photothermal materials have been proposed, achieving shape changes underwater remains challenging due to concurrent cooling effects, restricting their effective use. Existing photochemical actuators have also been mainly reported to simple bending motions, as molecular-level structural changes only occur near the surface.

To overcome these limitations, the team designed AC-LCEs with enhanced stiffness and controlled structures. By incorporating azobenzene molecules into a specifically engineered liquid crystal elastomer, they created materials that contract or expand when irradiated with UV or visible light, respectively. Unlike most thermal systems (photothermal or electrothermal), these materials can temporarily retain their deformed state even after the light is turned off, enabling a “latch-like” locking mechanism that allows for sequential and spatial control of motion.

The AC-LCEs were fabricated into both linear and ring-shaped spring structures and integrated into underwater robotic prototypes. These actuators demonstrated actuation strains more than three times higher than previous azobenzene-based actuators and generated work capacity exceeding those of mammalian muscle by a factor of two. Furthermore, by controlling the chirality (homochiral vs. heterochiral) of the coiled springs, the direction of actuation could be reversibly designed.

Using these artificial muscles, the team demonstrated fully untethered underwater soft robots that can grip and release objects or crawl through pipes—without any batteries, wires, or pumps. These systems were repeatedly operated over 100 light cycles with reliable performance.

The team aims to commercialize this technology by 2030 through further research on material scalability and system integration. According to the researchers, this innovation represents a meaningful step forward in the development of untethered, intelligent actuation systems suitable for diverse environments.

This work was published as a back cover article in the February 2025 issue of Small (IF: 13).

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KRICT is a non-profit research institute funded by the Korean government. Since its foundation in 1976, KRICT has played a leading role in advancing national chemical technologies in the fields of chemistry, material science, environmental science, and chemical engineering. Now, KRICT is moving forward to become a globally leading research institute tackling the most challenging issues in the field of chemistry and engineering and will continue to fulfill its role in developing chemical technologies that benefit the entire world and contribute to maintaining a healthy planet. More detailed information on KRICT can be found at https://www.krict.re.kr/eng/.

The study was conducted with support from KRICT’s basic research fund and the National Research Foundation of Korea.