Yield-stress support bath-enabled three-dimensional (3D) printing has been widely used in recent years for diverse applications. However, current yield-stress fluids usually possess single microstructures and still face the challenges of on-demand adding and/or removing support bath materials during printing, constraining their application scope. This study aims to propose a concept of stimuli-responsive yield-stress fluids with an interactive dual microstructure as support bath materials. The microstructure from a yield-stress additive allows the fluids to present switchable states at different stresses, facilitating an embedded 3D printing process. The microstructure from stimuli-responsive polymers enables the fluids to have regulable rheological properties upon external stimuli, making it feasible to perfuse additional yield-stress fluids during printing and easily remove residual fluids after printing. A nanoclay-Pluronic F127 nanocomposite is studied as a thermosensitive yield-stress fluid. The key material properties are characterized to unveil the interactions in the formed dual microstructure and microstructure evolutions at different stresses and temperatures. Core scientific issues, including the filament formation principle, surface roughness control, and thermal effects of the newly added nanocomposite, are comprehensively investigated. Finally, three representative 3D structures, the Hall of Prayer, capsule, and tube with changing diameter, are successfully printed to validate the printing capability of stimuli-responsive yield-stress fluids for fabricating arbitrary architectures.

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Microfluidic devices made from polydimethylsiloxane (PDMS) have diverse biomedical applications. However, due to the poor printability of PDMS, current three-dimensional (3D) printing techniques are rarely used to fabricate microfluidic devices. This study aims to investigate a fumed silica-PDMS suspension that can function as a matrix bath for embedded 3D printing (e-3DP) purposes, making it technically feasible to print microfluidic chips with complex embedded channels via low-cost extrusion 3D printing. The rheological properties, mechanical properties, transparency, and filament fidelity of the fumed silica-PDMS suspension have been systematically studied. It is found that the addition of fumed silica particles can effectively change PDMS from a viscous solution to a yield-stress suspension with suitable rheological properties for e-3DP. Also, the mechanical properties of the crosslinked fumed silica-PDMS are enhanced with an increased concentration of fumed silica. Although the transparency of PDMS has been lessened by mixing it with fumed silica particles, the visibility of the printed microfluidic chips is still acceptable. The filament fidelity has been studied by embedded printing filaments using a sacrificial ink in the fumed silica-PDMS suspension. Finally, two representative microfluidic chips for biomedical applications have been successfully printed to validate the effectiveness of the proposed fumed silica-PDMS suspension-enabled e-3DP method.