Researchers at Northwestern University have developed a hybrid 3D printing method called direct ink additive lithography (DIAL) that is designed to print thick, particle-filled extrusion inks at speeds and resolutions closer to light-based processes. The work, led by Pranav Kaarthik, was published in Science Advances.
The development targets a longstanding trade-off. Digital light processing requires low-viscosity, weakly scattering resins, while direct ink writing can handle functional yield-stress inks but is slowed by filament deposition and limited by nozzle diameter. Ryan Truby, June and Donald Brewer Junior Professor of Materials Science and Engineering at Northwestern, said manufacturers have had to choose between needed materials and desired printing speed, resolution and scale. He said DIAL lays a foundation for eliminating that compromise.
A sheet-based hybrid process
For each layer, a slot die extrudes ink onto the substrate, and a doctor blade scrapes it into a sheet as thin as 25 microns. A 405-nanometer projector then travels over the sheet and crosslinks selected regions, leaving uncured ink in place as support. Because light only has to pass through a thin sheet, opaque, heavily filled inks remain printable. The projector's 50-micron pixel pitch sets the planar resolution.
This sequence separates material delivery from optical patterning. The ink can be highly filled and functional, while the projector defines features across each thin layer. Uncrosslinked material remains in place as support for subsequent patterning.
Demonstrations and throughput
Using a model ink of polyurethane-based resin thickened with fumed silica, the team printed a 60 mm by 40 mm log-pile structure in 30 minutes, compared with 124 minutes using direct ink writing. The process also produced fins about 150 microns thick. Rastered projection over overlaid ink sheets yielded a multimaterial lattice four times the projector's illumination area.
The researchers also printed interlocking chain mail with struts about 250 microns thick, 15 gyroid structural electrolyte lattices from an ionogel ink in a single 15-minute run, and hydrogel cell culture inserts containing 0.5-mm-deep inverted pyramids. These demonstrations combine structural, electrolyte and cell-culture geometries.
Limitations and scale-up path
The authors described their print times as conservative because tool changes had not been tuned for speed. They also said the absence of suction and peel forces seen in vat-based printing should ease expansion of build area and volume.
Kaarthik and Truby are listed as coinventors on a U.S. provisional patent application. Funding came from the National Science Foundation, the Air Force Office of Scientific Research and Northwestern's McCormick School of Engineering. Truby said rebuilding American manufacturing will depend on new ways to make advanced materials, and that DIAL's ability to print many complex, functional parts at once offers a path to scale from the lab to larger-volume manufacturing for robotics, aerospace, healthcare and other areas.