A team of Lawrence Livermore National Laboratory researchers have demonstrated the 3D printing of shape-shifting structures that can fold or unfold to reshape themselves when exposed to heat or electricity. The micro-architected structures are fabricated from a conductive, environmentally responsive polymer ink developed at the lab.

Scientists and engineers revealed their strategy for creating boxes, spirals, and spheres from shape memory polymers (SMPs), bio-based "smart" materials that exhibit shape changes when resistively heated or when exposed to the appropriate temperature. While the approach of using responsive materials in 3D printing, often known as 4D printing, is not new, LLNL researchers are the first to combine the process of 3D printing and subsequent folding (via origami methods) with conductive smart materials to build complex structures.

The researchers create primary shapes from an ink made from soybean oil, additional co-polymers, and carbon nanofibers and "program" them into a temporary shape at an engineered temperature determined by chemical composition. Then the shape-morphing effect is induced by ambient heat or by heating the material with an electrical current, which reverts the part's temporary shape back to its original shape.

"It's like baking a cake," said Jennifer Rodriguez, a postdoc in LLNL's Materials Engineering Division. "You take the part out of the oven before it's done and set the permanent structure of the part by folding or twisting after an initial gelling of the polymer."

Ultimately, Rodriguez said, researchers can use the materials to create extremely complex parts. "If we printed a part out of multiple versions of these formulations, with different transition temperatures, and run it through a heating ramp, they would expand in a segmented fashion and unpack into something much more complex."

Through a direct-ink writing 3D printing process, the team produced several types of structures: a bent conductive device that morphed to a straight device when exposed to an electric current or heat, a collapsed stent that expanded after being exposed to heat, and boxes that either opened or closed when heated.

The technology, the researchers said, could have applications in the medical field, in aerospace (in solar arrays or antennae that can unfold), as well as flexible circuits and robotic devices.