A South Korean Team Is Turning Sulfur from Refineries into Self-Repairing Soft Robots

Every year, the oil industry generates a surplus of sulfur that no one quite knows what to do with. Removing it from crude oil is required by environmental regulations, but its main industrial use (the production of sulfuric acid) is already at capacity. In 2024 alone, global production reached 85 million metric tons, according to the U.S. Geological Survey. A team from KRICT, Hanyang University, and Sejong University in South Korea has found a solution: converting it into a polymer suitable for 4D printing to manufacture soft robots capable of moving, grasping objects, and repairing themselves.

The material, called PSN, is a poly(phenylene polysulfide) network that until now had posed an obstacle to 3D printing. Its internal structure was too tightly intertwined to flow properly through an extruder. The team managed to stretch that network enough to print complex geometries without losing the material’s properties.

Diagram of the circular 4D printing cycle using PSN and its magnetic version, MPSN. A) The process of printing, shaping, recovering, and recycling the material. B) Soft robots in 1D, 2D, and 3D that move in response to heat, light, or magnets. (Credits: J. H. Hwang, S. Won, J. M. Lee, et al.)

The result is a shape-memory structure that reacts to heat or light, without the need for motors, wiring, or an external power source. By incorporating 20% iron powder, they have created a magnetic version with which they have built robots less than one centimeter in size that respond to an external magnetic field. The robots climb over obstacles, drop a payload at a specific point, or release a catalyst to trigger a chemical reaction when needed.