Researchers at Northwestern University have developed a hybrid additive manufacturing method called direct ink additive lithography (DIAL) that aims to print thick, particle-filled inks with speed and planar resolution closer to light-based processes. The work, led by Pranav Kaarthik, was published in Science Advances.
The method addresses a split between two established approaches. Digital light processing requires low-viscosity, weakly scattering resins. Direct ink writing can handle functional yield-stress inks, but filament deposition slows it down and nozzle diameter limits feature size. DIAL tries to retain the material range of extrusion while borrowing the parallel exposure of projection lithography.
How the process works
For each layer, a slot die extrudes ink onto a substrate and a doctor blade spreads 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 has to pass only through a thin sheet, opaque and heavily filled inks remain printable. The projector's 50-micron pixel pitch sets the planar resolution.
The build sequence separates material deposition from optical patterning. That separation is central to the claim: the ink does not need to be transparent enough for light to penetrate a deep vat, and the pattern does not depend on a nozzle tracing every feature. Each layer is spread mechanically and then exposed only where solid material is wanted. The unexposed ink remains as a support bed. This arrangement is intended to allow high particle loading and thick layers without the light-scattering constraints of vat photopolymerization.
Reported build results
With a model ink made from polyurethane-based resin thickened with fumed silica, the team printed a 60 mm by 40 mm log-pile structure in 30 minutes. The same structure took 124 minutes with direct ink writing. The researchers also produced fins about 150 microns thick. In another demonstration, rastered projection over overlaid ink sheets produced a multimaterial lattice four times the projector's illumination area.
Other parts included interlocking chain mail with struts about 250 microns thick, 15 gyroid structural electrolyte lattices printed from an ionogel ink in a single 15-minute run, and hydrogel cell culture inserts containing 0.5-mm-deep inverted pyramids. The range matters because it tests more than a simple geometric shape: the demonstrations cover a structural lattice, an interlocking architecture, an ion-conducting material and a hydrogel insert for cell culture.
Speed, resolution and trade-offs
The reported 30-minute build versus 124 minutes for direct ink writing is a direct comparison on the same structure, but the authors describe their print times as conservative because tool changes had not been tuned for speed. That caveat matters for interpreting the result. DIAL's advantage is not yet a production benchmark; it is a laboratory demonstration of a process route, with the timing comparison showing headroom rather than a fully optimized cycle.
The resolution claim also needs qualification. The projector's 50-micron pixel pitch sets planar resolution, while the doctor blade can spread layers down to 25 microns. Those are process parameters, not a guarantee that every feature or material will reach the same fidelity. The reported fins at about 150 microns and chain-mail struts at about 250 microns show the scale of features the team actually printed, not the absolute limit of the method.
Why the material flexibility matters
The method's main promise is the combination of thick, particle-filled inks with projection-based patterning. Digital light processing's need for low-viscosity, weakly scattering resins restricts formulation. Direct ink writing accommodates functional yield-stress inks but is serial at the nozzle and limited by nozzle diameter. DIAL's doctor-bladed sheet avoids both constraints in principle: the ink can be opaque and heavily filled, and the projector can expose a region rather than drawing every point with a nozzle.
The authors argue this could remove a longstanding trade-off. Ryan Truby, June and Donald Brewer Junior Professor of Materials Science and Engineering at Northwestern University, said manufacturers have had to choose between the materials they need and the printing speed, resolution and scale they want, and that DIAL is intended to lay a foundation for eliminating that compromise. That is a research-team claim about the method's direction, not an independently demonstrated manufacturing result.
Scaling questions
The researchers say the absence of suction and peel forces seen in vat-based printing should make it easier to expand build area and volume. That is a design argument: DIAL spreads ink and exposes a thin sheet rather than pulling a cured layer away from a vat floor. If the argument holds, the approach could be more forgiving as projectors, substrates and motion systems scale.
Truby also linked the work to advanced materials manufacturing. He said rebuilding American manufacturing would depend on new ways to make advanced materials and that DIAL's ability to print many complex, functional parts at once offers a path from the lab toward larger-volume production of architected materials for robotics, aerospace, healthcare and other applications. Those are potential application areas identified by the research team, not commercial deployments.
What the demonstration does and does not show
DIAL has been shown with a model ink and several functional formulations, with a measured comparison against direct ink writing and demonstrations of multimaterial and hydrogel structures. It has not been shown as a production process with optimized tool changes, arbitrary materials or guaranteed feature fidelity at the projector's pixel scale. The authors' caveat about conservative print times is a reminder that the speed gap could change once automation and tooling are tuned.
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. The work therefore sits at the transition from a laboratory proof of concept to a process that would need engineering development before it could be judged on manufacturing cost, throughput and reliability.