Double network granular elastomers (DNGEs) © Titouan Veuillet
EPFL researchers have discovered that a soft material originally optimized for 3D printing may solve a longstanding challenge in materials science: make 3D printable elastomers both tough and durable.
In 2024, researchers from the Soft Materials Laboratory (SMaL) in EPFL's School of Engineering introduced double network granular elastomers (DNGEs): rubber-like materials made of microscopic elastomer particles connected by a softer elastomer network. DNGEs were designed as 3D printing 'inks' for structures with finely tuned mechanical properties.
Now, the team has published a follow-up study in Science Advances showing that the same architecture that enables DNGEs to be 3D printed with unprecedented mechanical control also delivers an unexpected advantage: strong resistance to both fracture and fatigue. This is a rare combination, because elastomers that resist fracture typically accumulate damage with repeated mechanical stress, limiting their lifetime, while those that are fatigue-resistant are often prone to breakage if subjected to excessive stretching or shocks.
"Originally, our focus was on improving processibility, but once we had the granular structure, we discovered that these materials are also very tough," says Soft Materials Lab head Esther Amstad. "Then, we realized that a lot of this toughness came from repetitive energy dissipation mechanisms - the material could absorb energy over and over without irreversibly breaking."
Amstad explains that the DNGEs are able to overcome the typical trade-off between toughness and fatigue-resistance thanks to their uniquely varied internal structure.
"Essentially, the two different networks - one made of granular elastomer particles and one of soft elastomer - share mechanical strain between them, making the material stronger overall."
Longer-lasting advanced materials
In experiments, optimized DNGEs demonstrated fracture toughness values up to 15 times higher than comparable elastomers, and fatigue resistance values up to three times higher.
When stretched, the materials redistribute mechanical stress from the stiff microparticles into the softer regions between them. There, strain energy can be repeatedly dissipated through the sliding and rearrangement of polymer chains, rather than through the irreversible breakage of polymer bonds, limiting permanent damage.
The DNGEs' granular structure also changes how cracks move through them. Rather than following a straight path, cracks prefer to travel through the softer regions between the elastomer microparticles, producing a winding route that slows their growth and delays failure.
The findings suggest that the SMaL's material architecture, originally developed to enable advanced 3D printing, may also offer a new strategy for designing longer-lasting soft materials. Such materials could help extend the lifetime of soft robots, electronics and biomedical devices, where components are subjected to repeated stresses and deformations over long periods of time.
The team is already working on further optimizing their material for sustainability, for example by using biodegradable elastomers and those derived from recycled materials.
"Our aim is to implement more sustainable materials without compromising on mechanics," Amstad says. "By increasing the scope of materials we can use, we can not only reduce the DNGEs' environmental footprint, but also make them even more widely accessible to any lab with a commercial 3D printer."
