Soft robotics has a longstanding problem: the materials that make robots flexible and lifelike are notoriously difficult to manufacture into precise, useful shapes. A team of researchers from the Korea Research Institute of Chemical Technology (KRICT) may have found a way around that, using one of the cheapest and most overlooked industrial waste materials available: elemental sulfur.
Their approach combines a sulfur-based polymer with 4D printing, a manufacturing method where printed objects can physically change shape over time in response to heat, light, or magnetic fields. The result is a platform capable of producing swimming robots, gripping arms, and self-opening capsules, all from a material that can be melted down and reprinted when no longer needed.
The research team, led by Dr. Dong-Gyun Kim of KRICT alongside collaborators from Hanyang University and Sejong University, describes this as the first demonstration of its kind. “This study represents the first example of upcycling industrial sulfur waste into advanced robotic materials,” said Dr. Kim. “Smart materials that can move autonomously and be recycled are expected to become key drivers of future soft robotics and automation technologies.”
From Industrial Waste to Printable Material
Elemental sulfur is a by-product of oil and gas refining that accumulates in enormous quantities worldwide. Here, it functions as the key building block in a new class of polymers. This structure enables an unusual combination of properties: the material can flow like a liquid when heated, solidify to retain a printed shape upon cooling, and later be reshaped on demand.
What makes this possible is the nature of the chemical bonds within the material. Unlike conventional plastics, whose molecular structure is permanently fixed, these sulfur-based polymers contain bonds that can break and reform. When heated, the material softens and flows through a printer nozzle. When it cools, it solidifies into whatever shape was printed. And when you want to change that shape or recycle the whole product, heat does the job again.
A Material That Remembers Its Shape
Each polymer variant has a specific temperature at which it transitions from stiff to flexible. Below that temperature it holds its shape. Above it, the material softens and snaps back to its original printed form. By combining three variants, each responding at a different temperature, roughly 14°C, 32°C, and 52°C, researchers built structures that move in sequence as they warm up, with different sections activating one after another from a single source of heat. No motors, no batteries, no wiring required.
A second trigger is also available: near-infrared light. Shining a low-power light source on a specific part of a structure heats only that spot, allowing one section to move while everything around it stays still. For applications where precision matters, a single joint opening, a cap releasing its content, this level of control is difficult to achieve with conventional materials.
When a device reaches the end of its useful life, it can be broken apart, ground back into powder, and fed into the printer again. The same chemistry that allows the material to be printed also allows it to be unprinted, a closed loop that addresses one of the more persistent criticisms of 3D printing, which typically generates waste that cannot be recovered.
Robots That Move Without Wires
By mixing iron oxide particles into the polymer, the researchers created a composite material that responds to magnetic fields. This enables fully untethered robots, devices that swim, spin, grip, and release with no cables or power connections of any kind, controlled entirely by a magnet moved beneath them.
Thin filament robots demonstrated three distinct swimming styles underwater and could navigate over physical obstacles. The addition of iron oxide does make the material slightly stiffer and less stretchy, but all versions retained enough flexibility for practical robotic use.
Where the Challenges Remain
The platform is novel, but it is still early-stage, and the researchers are transparent about its current limitations.
The conditions required to print the material are sensitive and shift every time the formulation changes. Adding more iron oxide, or adjusting the sulfur content, means recalibrating the printer from scratch. Printing too fast introduces tiny internal bubbles that are invisible to the eye but enough to throw a small robot off balance during motion.
The material also has two chemical weak points: it degrades in contact with tetrahydrofuran and chloroform, solvents that appear regularly in laboratory and industrial settings. And the amount of iron oxide that can be included is capped, push past 20 percent and the material becomes too thick to extrude, limiting how strongly the robots can respond to magnetic fields.
Scaling 4D Printing Toward Real Materials
The KRICT work lands at a moment when researchers are actively pushing 4D printing beyond demonstration and toward materials that are programmable, reusable, and manufacturable. The challenge has never been proving that shape-change is possible, it has been building materials that do it reliably.
Beyond the work at KRICT, other groups are also pushing the field forward. Researchers at Penn State developed a 4D printing method producing a hydrogel-based smart skin capable of dynamically changing its shape, texture, and appearance in response to stimuli such as heat, solvents, or mechanical stress, demonstrating that a single printed material can carry multiple programmable functions rather than a fixed set of properties.
Separately, researchers from the Universidad Politécnica de Madrid and IMDEA Materials Institute introduced a degradation-triggered approach, using the controlled breakdown of polyvinyl alcohol in water to gradually release elastic energy stored in a second polymer, shifting the actuation trigger from heat or light to the passage of time itself.
While both expand what 4D printing can do, neither resolves the recyclability question. The KRICT sulfur platform is notable for addressing that gap directly.
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Featured image shows Schematic illustrations of closed-loop and sustainable 4D printing of poly(phenylene polysulfide) networks (PSNs) and PSN-Fe3O4 composites (MPSNs) with shape-morphing capabilities for realizing multi-stimuli-responsive soft robots. Image via KRICT.
