Encoding hydrogel actuation with flow-templated architecture
Date
2024
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Recent advances in additive manufacturing have accelerated the fabrication of designer soft actuators. In particular, multi-material direct ink writing allows users to selectively embed stimuli-responsive material in 3D 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 generalization and capacity on larger production scales. To circumvent some of these manufacturing bottlenecks, here we developed a continuous processing scheme that exploits viscoplastic, laminar flow to build and program the deformation of soft actuators. Inspired by techniques used to structure polymeric melts, we design custom millifluidic devices that force disparate streams through serpentine splitting, rotation, and recombination elements. These elements multiply the incoming 2D composition field across the cross-sectional area while preserving its relative spacing and orientation. Serial repetition of elements 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. 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. ☐ 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 elements. Provided that the flowing streams are rheologically matched, materials with different chemistries can be predicatively structured and extruded in a single processing step. We exploit this universality to manufacture hydrogel actuators sensitive to different environmental triggers, specifically temperature and light. We formulate multiple hydrogel precursor inks by combining two polymers, N-isopropylacrylamide (NIPAM) and poly(ethylene glycol) diacrylate (PEGDA), with viscoplastic poly(acrylic acid) (PAA) microgel particles and photo-sensitive iron(II,III) oxide nanoparticles. 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 predicatively programmed by changing the concentration density map by simply adjusting the relative flow rates of incoming streams. Modulating the flow rate over the course of the print creates smoothly varying, intermediate structures that bend in multiple directions. The unique structures achieved, and the geometrically dictated, chemistry-agnostic operating principles used to achieve them, provide a new means to fabricate actuators for a variety of applications.
Description
Keywords
Actuators, Additive manufacturing, Advective assembly, Hydrogel precursor, Soft robotics