Researchers develop a recyclable sulfur-based polymer that enables 4D printing of soft robots with programmable shape changes and magnetic responsiveness.
(Nanowerk Spotlight) Materials designed to move, adapt, or respond to their surroundings are at the core of soft robotics. Yet the very features that make these systems functional—shape memory, resilience, and environmental responsiveness—often render them difficult to reprocess or recycle. Many are made from thermoset plastics, permanently crosslinked polymers that resist reshaping once set. Their functionality is embedded at the cost of flexibility and sustainability.
The contradiction is stark: soft robotic systems, meant to operate dynamically, are typically manufactured using static, resource-intensive materials that can’t be reused after a single deployment.
Overcoming this requires a new class of polymers—materials that are mechanically robust, chemically resistant, responsive to multiple triggers, and still printable into precise, complex geometries. Such materials must also be reconfigurable: able to undergo multiple cycles of programming and recovery, and recyclable without loss of integrity. This combination of properties has proven difficult to achieve.
Polymers that offer chemical durability are often unprintable. Those that can be printed tend to lack environmental resistance or cannot recover their shape. And materials that can be reshaped usually degrade with each cycle or fail under demanding conditions like exposure to solvents or heat.
Elemental sulfur, a low-cost byproduct of petroleum refining, offers a potentially disruptive route forward. When polymerized with organic crosslinkers, it forms sulfur-rich polymers (SRPs) featuring dynamic disulfide (S–S) bonds. These dynamic bonds allow network rearrangement under thermal or photothermal stimulation, enabling shape memory and self-healing. SRPs have already shown utility in batteries, optics, and environmental remediation, thanks to their high sulfur content, infrared transparency, and metal-binding capacity.
But despite these advantages, fully crosslinked SRPs remain difficult to process. Their poor solubility and rigid network structure have blocked efforts to fabricate intricate, functional forms through 3D printing. While some advances have been made using sulfur-containing inks or monomer precursors, these approaches often lead to inconsistent material properties or limited device performance.
To date, no platform has combined the mechanical strength and solvent resistance of crosslinked SRPs with direct 3D or 4D printability, shape programmability, and recyclability. New research presented by Hwang and colleagues in Advanced Materials (“Closed-Loop and Sustainable 4D Printing of Multi-Stimuli-Responsive Sulfur-Rich Polymer Composites for Autonomous Task Execution”) addresses this gap directly. Their work introduces a closed-loop 4D printing strategy using sulfur-rich poly(phenylene polysulfide) networks (PSNs) and their magnetically responsive composites (MPSNs).
By designing polymers that flow under shear, recover their shape with heat or light, and reassemble through dynamic bond exchange, the team demonstrates a new approach to soft robotics that unifies responsiveness, sustainability, and fabrication control.
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. a) 3D-printable PSNs and MPSNs for architectural assembly, featuring shape-programming, recovery, and recycling capabilities. b) Actuation of 3D-printed 1D-, 2D-, and 3D- structured multi-stimuli-responsive MPSN robots. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The PSNs developed in this work consist of loosely crosslinked networks made by reacting elemental sulfur with aromatic diiodide monomers. The resulting materials maintain a covalently bonded structure, but their S–S linkages allow internal rearrangement under thermal or photothermal conditions. This enables the polymer to behave as a shear-thinning fluid—one whose viscosity decreases under applied force—allowing it to be hot-melt extruded through a 3D printer nozzle. Once printed, the material solidifies into a durable shape-memory network.
By adjusting the sulfur content, the researchers produced a series of PSNs with varying thermal and mechanical properties. Samples containing 46%, 63%, and 76% sulfur by weight (PSN46, PSN63, PSN76) showed distinct glass transition temperatures (Tg) ranging from 14 °C to 52 °C. These differences allowed them to program sequential or localized shape changes in assembled structures based on temperature alone. A printed part made from PSN76 would deform and recover at room temperature, while a part made from PSN46 required higher temperatures to activate shape recovery.
To enable magnetic responsiveness, iron oxide particles were mixed into the PSN powder, forming MPSNs. These composites retained the printability and shape-memory behavior of the PSNs, while gaining the ability to move in response to magnetic fields. A particle loading of 20% by weight proved optimal—high enough to enable actuation, but low enough to avoid clogging during printing. Rheological tests confirmed that MPSNs exhibited shear-thinning behavior and could be printed reliably under optimized temperature and pressure conditions.
The printed materials could be assembled into larger structures through welding at the interfaces. This was achieved by exploiting the photothermal effect of the materials: when exposed to near-infrared (NIR) light, the S–S bonds became active and facilitated bond exchange, enabling permanent joining of the parts. This approach allowed the team to build complex, modular architectures, including a multilayered structure inspired by the Sagrada Familia. Different PSN and MPSN components with tailored Tg values were assembled into a single object that could undergo spatially controlled, sequential transformations upon heating.
These structures were not only programmable but also recyclable. When no longer needed, parts could be disassembled by heating or NIR exposure, reprocessed into powder, and reprinted into new forms. The team demonstrated this by disassembling the original structure and reprinting its components into a retractable-roof stadium. The roof, made from MPSN63, was programmed to close at moderate temperatures and reopen upon NIR exposure. The ability to switch between shape states using heat or light and to reprint new architectures from the same material supports a closed-loop model of fabrication.
To evaluate mechanical performance, the researchers printed films of both PSNs and MPSNs and subjected them to tensile tests. Incorporating magnetic particles increased tensile strength and stiffness but reduced elongation at break, consistent with the stiffening effect of rigid fillers. Even so, MPSNs retained enough flexibility for use in soft robotic applications, with most samples showing elongation above 100%.
To illustrate potential functions, the team fabricated 1D, 2D, and 3D robots from MPSNs. One-dimensional filaments, when exposed to rotating magnetic fields, demonstrated swimming behavior through tumbling, rolling, and spinning motions. These movements were influenced by the filament’s aspect ratio and its position relative to the field. Robots with specific geometries could climb over submerged obstacles or rotate along curved trajectories, illustrating the degree of control achievable through design and actuation parameters.
In two dimensions, the team printed a cross-shaped gripper from MPSN76. After programming the arms to close around a spherical object and fixing the shape at low temperature, they showed that heating the gripper triggered reopening and cargo release. In three dimensions, a capsule composed of MPSN63 (cap) and MPSN46 (body) served as a temperature-triggered container. The cap was programmed to remain closed at room temperature but opened rapidly under NIR irradiation, releasing its contents.
This behavior was extended to chemical processing by using the capsule as a magnetic stirring bar loaded with a catalyst. When placed in a mixture of diol and isocyanate in chloroform-d, the MPSN capsule initially remained closed and stirred the solution. Upon heating above the cap’s Tg, the capsule opened, releasing the catalyst and triggering carbamate formation. Nuclear magnetic resonance (NMR) analysis confirmed that the catalyst remained encapsulated until release, and that the reaction proceeded as expected after exposure. This demonstrated the feasibility of autonomous, temperature-controlled reagent delivery in liquid-phase synthesis.
The combination of chemical resilience, modular printability, programmable actuation, and recyclability positions PSNs and MPSNs as a versatile platform for next-generation material systems. Their performance in dynamic environments—especially under solvent exposure—adds further utility for applications in chemical engineering, microfluidics, and automated synthesis. Because the base material is derived from elemental sulfur, this approach also supports sustainability goals by valorizing an abundant industrial byproduct.
Rather than sacrificing performance for reusability, this work demonstrates how dynamic bond chemistry and targeted molecular design can reconcile functionality with circularity. The printed devices not only move and reshape in response to external cues—they can also be disassembled, reformed, and repurposed, extending their utility beyond single-use tasks. This sets a precedent for designing high-performance, recyclable soft robots using materials tuned for both actuation and sustainability.
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