Researchers at Lawrence Livermore National Laboratory (LLNL) have developed a new method to 3D print sturdy silicone structures that are bigger, taller, thinner and more porous than ever before. The team's two-part "fast cure" silicone-based ink for direct ink writing mixes just before printing and sets quickly at room temperature, allowing for longer print times, simplifying the fabrication process, and ensuring structures will not collapse or sag, even in complex shapes and configurations.
Their research appears on the front cover of the October issue of Advanced Materials Technologies.
"There are other methods for silicone direct-ink writing, but this is the simplest solution and the bulletproof one," said Anna Güell Izard, a postdoc in the Materials Engineering Division (MED) and the paper's first author. "There is nothing extra to worry about; you can just print."
Printing made easy
Silicone is a widely used polymer known for its flexibility, resilience and biocompatibility, making it ideal for protective materials, biomedical devices - especially implants and prostheses - flexible electronics, soft robotics and more. 3D printing, a type of additive manufacturing, makes the material even more versatile by fabricating hollow or porous structures that traditional manufacturing cannot.
Direct ink writing (DIW) - where material is forced (extruded) through a nozzle and selectively deposited layer by layer - works best for printing highly viscous materials like silicone. Still, this technique is limited to only manufacturing relatively simple, planar designs due to the low self-supporting capabilities of silicone-based "inks," making it impossible to print tall, overhanging or thin-walled structures.
For a successful print, the machine's accuracy, nozzle quality and ink properties also all need to be perfect, and the structures need to remain stable enough to support their own weight throughout printing and thermal curing in an oven. Printing also needs to finish before the ink begins to set, making it difficult to extrude. The team looked to address these challenges with a novel ink formulation and refined printing process.
Silicone inks contain a catalyst (which speeds up a chemical reaction) and a crosslinker (a substance that chemically joins molecules together) that slowly "gel" together to bind structures. Like a two-part epoxy, the team's Fast Cure" (FC) ink separates the cross-linker and catalyst until just before extrusion, when they are continuously mixed as they flow through the nozzle in a process called inline mixing. The chemicals then gel quickly and solidify right away, eliminating print time restrictions, as well as the need for extra steps to harden or cure the materials.
"Since the ink is kept separate, you don't have to worry about the print time because it is not going to solidify in the syringes," said Güell Izard. "It's also sturdier because the layers are gelling as you're printing, so your structure will not start sagging."
Printing the impossible
The FC formulation allows researchers to control the mix-to-print-to-cure time, resulting in increased self-supportive capabilities and improved shape retention. Compared with a one-part baseline, the FC ink had similar rheological (flow) properties and printed two types of woodpiles (a common shape in silicone DIW) that held their shape better and sagged roughly half as much.
Excited with these results, Güell Izard and the team "went crazy" with their discovery and printed configurations that were previously unattainable with silicone such as tall and slender structures, acute and unsupported overhangs and shell-based lattices like gyroids and cubic octets with porosities (empty spaces or voids) of up to 90%.
"These beautiful cubic octets and overhangs turned out of the oven exactly as I printed them with the walls totally firm, while the baseline ink structures collapsed," she said. "It made us feel like we were onto something; that all this magic chemistry was doing its job."
The next steps are testing the limits of the technology and leveraging the two-part setup to fine-tune the ink's properties. For example, the team noted that gel time depends on the amount of catalyst present, so the chemical may have an important role in determining a structure's height and overhanging features. Güell Izard and the team also hope to apply the concept to other materials and unlock a similarly wide range of new structures.
Other LLNL authors include Eric Duoss, Lemuel Pérez Pérez, Todd Weisgraber, Ilse Van Meerbeek, Melody Golobic and Jeremy Lenhardt.
- Noah Pflueger-Peters and Shelby Conn
