Waste Sulfur Transformed into Moving Robots... Korean Researchers Achieve First Circular 4D Printing [Reading Science]

Soft Robots Powered by Oil Refining Byproduct Sulfur
Circular Materials Enable Repeated Printing and Recycling

A domestic research team has developed a "circular 4D printing" technology that utilizes sulfur discarded from oil refining processes to create materials that can move autonomously and be recycled. This case of transforming industrial waste into advanced robotics material is being regarded as a new milestone, presenting fresh possibilities in the fields of soft robotics and smart materials.

A joint research team consisting of Dongkyun Kim, Principal Researcher at the Korea Research Institute of Chemical Technology (corresponding author), Jeongjae Wi, Professor at Hanyang University (corresponding author), and Yongseok Kim, Professor at Sejong University (corresponding author), announced on March 4 that they have developed, for the first time in the world, a 4D printing technology using sulfur polymers that respond to various stimuli such as temperature, light, and magnetic fields. The research findings were published in the November 2025 issue of the international journal 'Advanced Materials.'

Gold plastic synthesized with a loose network structure can be 3D printed, welded and assembled with light or heat, and has shape-memory functionality. Additionally, it can be recycled after use and reprinted into new structures. 4D printed robots made from this material respond to temperature, light, and magnets; the thread-shaped robot overcomes obstacles, the claw-shaped robot carries objects, and the capsule-shaped robot performs liquid release functions. Provided by the research team

Turning Waste Sulfur into Advanced Materials

The oil refining process generates large amounts of sulfur byproducts. According to the United States Geological Survey (USGS), global sulfur production in 2024 reached approximately 85 million tons. Most of this sulfur is a byproduct of the refining process, and because its uses are limited, ongoing concerns have been raised about disposal costs and environmental impact.

'Sulfur plastic' is emerging as a promising material to address this issue. This polymer material, synthesized based on sulfur, transmits infrared light well and has excellent heavy metal adsorption capabilities, unlike ordinary plastics. As a result, it has been suggested as a potential material for infrared camera lenses and water purification applications.

However, conventional sulfur plastics have a tightly cross-linked molecular structure, which makes them difficult to process and limits their application in 3D printing for producing complex shapes.

"Printing with Sulfur"… World's First 4D Printing with Sulfur Plastics

The research team designed the internal molecular structure of sulfur polymers to be loose, enhancing workability and enabling a new sulfur plastic that can be extruded through a nozzle when heated. As a result, they succeeded in 3D printing with sulfur-based materials for the first time in the world.

They further incorporated shape memory functions to realize "4D printing" technology. 4D printing refers to technology in which a 3D-printed structure autonomously changes form over time or in response to external stimuli.

This material deforms into a pre-designed shape when exposed to temperature or light, allowing the structure to move on its own without any external motors or electrical devices.

In particular, the research team also demonstrated a "laser welding" technique, in which the internal bonds of the material are temporarily broken and then reformed after about 8 seconds of laser irradiation. This makes it possible to connect multiple structures like LEGO blocks without adhesives, enabling the creation of complex 4D structures.

Selected as the cover paper of Advanced Materials November 2025 issue (I.F.=26.8). Provided by Korea Research Institute of Chemical Technology

Battery-Free Soft Robots… Even Recyclable

The research team also developed a composite material by mixing about 20% iron powder into sulfur plastic, making it responsive to magnetic fields. Structures made from this material function as ultra-small soft robots that move or change direction according to the movement of a magnet.

In experiments, thread-shaped robots about 1 mm thick were able to move underwater following a magnet or overcome obstacles. Additionally, structures were realized that automatically open a claw to release objects at a specific temperature or release a catalyst at a certain temperature for chemical experiments, such as capsule-shaped robots.

Notably, this technology enables a circular manufacturing structure of "printing→use→recycling→reprinting." Printed structures can be remelted and reused as raw materials, significantly reducing material waste.

Dongkyun Kim, Principal Researcher, stated, "This is the first case of upcycling industrial byproduct sulfur into advanced robotics material. Smart materials that can move autonomously and be recycled are expected to become core technologies for future soft robotics and automation industries."

This content was produced with the assistance of AI translation services.