Researchers from the Universidad Politécnica de Madrid and the IMDEA Materials Institute, in collaboration with German medical technology company Meotec, have developed a degradation-triggered 4D printing method that allows gradual, time-controlled shape changes using standard fused filament fabrication (FFF). Published in Additive Manufacturing, the study shows that hydrolytic degradation of polyvinyl alcohol (PVA) can be used as a programmed trigger to release elastic energy stored in polyethylene terephthalate glycol-modified (PETG), creating a new option for biomedical implants and adaptive structures.
Using degradation as an actuation mechanism
Four-dimensional (4D) printing refers to 3D printed structures that change shape over time in response to internal or external stimuli. Most existing systems rely on heat, light, electrical fields, or humidity to trigger transformation. In contrast, this research explores degradation itself as the trigger.
“This strategy introduces degradation as a powerful, yet underexplored, mechanism for triggering actuation in 4D-printed systems,” said Dr. William Solórzano, one of the authors behind the publication.
The approach combines PVA, a water‑soluble polymer traditionally used as a sacrificial support material in FFF, with PETG, a structural polymer capable of storing elastic energy and exhibiting shape recovery behavior.
In the printed multi-material system, PVA functions as a temporary mechanical constraint. When immersed in water, it gradually degrades, reducing its stiffness. As this constraint weakens, PETG releases stored elastic energy, producing a controlled and progressive shape transformation. Unlike rapid stimulus-driven systems, this mechanism enables slow, time-dependent actuation, which is particularly relevant for biomedical applications where gradual adaptation is required.
Mechanical degradation and exponential stiffness decay
The team conducted tensile and flexural testing to measure how PVA’s mechanical properties evolve during hydrolytic degradation. Initial tensile tests showed that dry PVA is stiff (elastic modulus around 5 GPa) and brittle. After immersion in water, samples exhibited an approximately 36% reduction in elastic modulus within 80 minutes, along with lower yield and tensile strength. The material also shifted from brittle to more ductile behavior. The degradation followed an exponential curve, consistent with molecular weight reduction during hydrolysis.
The study further found that, in solid samples, infill pattern did not significantly affect degradation rate. Instead, erosion speed depended on the surface area-to-volume ratio. This means designers can tune degradation timing by adjusting geometry.
PETG remained mechanically stable under immersion, though stress relaxation accelerated due to plasticization effects. The researchers modeled this behavior using a Prony-series viscoelastic model to predict long-term mechanical response.
Cytocompatibility for biomedical use
Alongside mechanical performance, the study assessed biological response using human mesenchymal stromal cells (hMSCs). Results showed that PVA dissolved quickly without cytotoxic effects, while PETG maintained high cell viability (greater than 89%) after six days. Both materials demonstrated compatibility with primary human cells. These findings indicate that PETG–PVA systems may be suitable for biomedical devices, particularly where controlled degradation and gradual actuation are required.
Proof-of-concept 4D printed shock absorber
To demonstrate the concept, the researchers fabricated a 4D printed actuator based on a mechanical shock absorber. The device pairs a helical PETG spring that stores elastic energy with a serpentine PVA spring that serves as a temporary mechanical restraint. During assembly, the PVA part keeps the PETG spring compressed, locking in the stored energy.
When placed in water, the PVA slowly breaks down, which lowers its stiffness and removes the restraint on the PETG structure. The PETG spring then extends over time, creating a controlled, time-dependent change in shape.
Actuation occurred in two stages. At first, PVA surface erosion produced an almost linear response as stiffness dropped gradually. Later, as water reached the inside of the structure, bulk erosion sped up the loss of mechanical strength, causing faster, exponential softening and quicker extension.
The researchers described the behavior with a two-spring model, linking the time-based drop in PVA stiffness to the release of elastic energy stored in the PETG spring. The authors suggest the same approach could be used for bone distraction devices and other biomedical implants requiring gradual, controlled shape change over time.
Expanding the design space of 4D printing
Recent coverage of 4D printing research shows the field expanding beyond traditional shape-memory systems. Researchers are investigating new responsive materials, multi-material fabrication strategies, and programmable approaches to controlling transformation. Advances in photothermal actuation, smart composites, and multifunctional adaptive structures continue to broaden the capabilities of 4D printing.
For example, researchers at Penn State recently demonstrated a 4D-printed hydrogel smart skin capable of dynamically changing shape, texture, and appearance in response to external stimuli, highlighting how programmable materials are moving toward soft robotics and biomedical applications.
In contrast, the PETG–PVA system uses a different actuation strategy. Instead of using heat or light to trigger a change, it uses controlled hydrolytic degradation to create gradual, time-dependent behavior. By treating degradation as a programmable material parameter, the study shifts 4D printing toward applications that need slow, predictable change.
The researchers say that combining PETG’s elastic energy storage capacity with PVA’s controlled degradability offers a cost-effective and scalable route for prototyping advanced 4D actuators using widely available FFF materials. The work was supported by the European Union’s Horizon Europe programme through the BIOMET4D project.
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Featured image shows key frames of the 4D-printed shock absorber during shape morphing. Image via Universidad Politécnica de Madrid / IMDEA Materials.
