3D printing method uses colored light to meld opposite material properties

PHOTO CREDIT: THE UNIVERSITY OF TEXAS AT AUSTIN

Inspired by how nature blends toughness and flexibility, such as the rigid structure of bone surrounded by pliable cartilage, all with elegant and precise geometric properties, The University of Texas at Austin researchers have developed a fast, precise 3D printing method seamlessly merging soft and hard properties into a single object using different colors of light.

This advance could pave the way for next-generation prosthetics, flexible medical devices, and stretchable electronics that move naturally with the body, like a human joint or ligament.

“What really motivated me and my research group is looking at materials in nature,” says Zak Page, an assistant professor of chemistry at UT Austin and corresponding author. “Nature does this in an organic way, combining hard and soft materials without failure at the interface. We wanted to replicate that.”

“This approach could make additive manufacturing more competitive for higher-volume production compared with traditional processes like injection molding. Just as important, it opens up new design possibilities,” says Keldy Mason, a graduate student in Page’s lab. “This gives engineers, designers, and makers more freedom to create.”

One of the biggest challenges in creating objects with vastly different physical properties is materials tend to fail at the interface, or the point they come in contact. Think about how the rubber sole of a running shoe will separate over time from the softer mesh cloth above it.

The new 3D printing method uses a custom-designed liquid resin and a dual-light printing system activating different chemical reactions depending on the color of light used. By shining violet light, the resin cures into a stretchy, rubber-like material. But in areas hit with higher-energy ultraviolet light, it becomes rigid and strong. The result is an object with distinct zones of softness and hardness crafted in a single print.

“We built in a molecule with both reactive groups so our two solidification reactions could ‘talk to each other’ at the interface,” Page says. “That gives us a much stronger connection between the soft and hard parts, and there can be a gradual transition if we want.”

The team demonstrated the system by printing a small but functional knee joint with flexible ligaments and rigid bones moving together smoothly. They also created a prototype stretchable electronic device with a gold wire mounted on a strip that could bend and stretch in parts, but with a more rigid section to prevent the circuit from breaking.

“Honestly, what surprised me most was how well it worked on the first try. That almost never happens with 3D printing resins,” Page says. “We were also shocked by how different the properties were. The soft parts stretched like a rubber band and bounced back. The hard parts were as strong as plastics used in consumer products.”

The process also works faster and with better resolution than previous approaches. And because the printer setup is relatively simple and affordable, the technology could become accessible to researchers, hospitals, and educators.

“It could be used to prototype surgical models, wearable sensors, or even soft robots,” Page says. “There’s so much potential here.”