The team’s laser-assisted direct ink writing method allows microscopic metallic, free-standing 3D structures to be printed in one step without auxiliary support material.
In the team’s method an ink composed of silver nanoparticles is sent through a printing nozzle and then annealed using a precisely programmed laser that applies just the right amount of energy to drive the ink's solidification. The printing nozzle moves along x, y, and z axes and is combined with a rotary print stage to enable freeform curvature. In this way, tiny complex shapes made of silver wires less than the width of a hair can be printed in free space within seconds. The printed wires are said to exhibit excellent electrical conductivity, almost matching that of bulk silver.
When compared to conventional 3D printing techniques used to fabricate conductive metallic features, laser-assisted direct ink writing is not only superior in its ability to produce curvilinear, complex wire patterns in one step, but also in the sense that localised laser heating enables electrically conductive silver wires to be printed directly on low-cost plastic substrates.
According to Wyss Institute postdoctoral fellow Mark Skylar-Scott, the most challenging aspect of honing the technique was optimising the nozzle-to-laser separation distance.
"If the laser gets too close to the nozzle during printing, heat is conducted upstream which clogs the nozzle with solidified ink," Skylar-Scott explained. "To address this, we devised a heat transfer model to account for temperature distribution along a given silver wire pattern, allowing us to modulate the printing speed and distance between the nozzle and laser to elegantly control the laser annealing process 'on the fly.' "
The result is that the method can produce not only sweeping curves and spirals but also sharp angular turns and directional changes written into thin air with silver inks. According to the researchers, this opens up near limitless potential applications in electronic and biomedical devices that rely on customised metallic architectures.