Harnessing Chaotic Advection to Improve the Capabilities and Capacity of Multi-Material Direct Ink Writing Nozzles

Multi-material additive manufacturing incorporates multiple species within a single 3D printed object to enhance its mechanical properties and functionality. Recent advances in multi-material direct ink writing, for example, allow users to selectively embed stimuli-responsive material in printed parts and program macroscopic deformations with exquisite control. In comparison to other soft actuator manufacturing techniques, direct ink writing (DIW) offers high templating precision and flexibility in architectural design. However, DIW of soft actuators suffers from some of the same drawbacks as encountered in layer-by-layer additive manufacturing. Sequential deposition of layers requires long build times, limits the resolution to the diameter of the print nozzle, and excludes ink materials that exhibit poor interfacial adhesion. While these manufacturing constraints may be perfectly manageable during actuator prototyping, they limit generalizability and capacity on larger production scales.<br/><br/>To circumvent some of these manufacturing bottlenecks, we leverage principles of chaotic advection to structure multi-material filaments in the printhead prior to deposition. Inspired by static mixers optimized to layer polymeric melts, we design modular millifluidic nozzles that force disparate streams through serpentine splitting, rotation, and recombination junctions. These junctions multiply the incoming 2D composition field across the cross-sectional area while preserving its relative spacing and orientation. Serial repetition of junctions compounds multiplication, allowing the heterogeneous distribution to be efficiently shrunk before it is dissipated by diffusive mixing. The ‘advective assemblers’ extrude precisely structured multi-material filaments, which can then be arranged into objects along a conventional 3D printing path or extruded into a support bath. Preassembling the lower levels of hierarchy in flow overcomes intrinsic challenges in layer-by-layer deposition, including improving interlayer adhesion and maintaining high volumetric throughput while maintaining fine layer resolution.<br/><br/>Unlike self-assembly or directed-assembly, advective assembly is dictated by rheology rather than chemistry. Granular inks with sufficiently high yield stress flow as a plug and stabilize streamlines even through abrupt flow junctions. Provided that the flowing streams are rheologically matched, materials with different chemistries can be predictively structured and extruded in a single processing step. We exploit this universality to manufacture hydrogel actuators sensitive to different environmental triggers, specifically water, salt, heat, and light. Gel precursors containing poly(ethylene glycol) diacrylate and poly(n-isopropyl acrylamide) are first dispersed with a poly(acrylic acid) microgel suspension. The microgels serve as a viscoplastic carrier fluid that preserves the fidelity of patterned concentration maps. After polymerization, the distribution of stimuli-responsive components causes gel filaments to swell differentially when triggered, giving rise to shape changes that persist over tens of centimeters. Deformation is predictively programmed by changing the concentration density map by simply adjusting the relative flow rates of incoming streams. The unique structures produced by advective assembly nozzles, and the geometrically dictated, chemistry-agnostic operating principles used to achieve them, provide an efficient route to fabricate designer functional materials.