Multi-Material Droplet-Based Hydrogel Threads for Extrusion 3D Printing.

Multi-material 3D printing holds significant promise for fabricating complex structures, but is hindered by viscosity incompatibility and material cross-contamination. These limitations stem from the two dominant printing methods: extrusion and inkjet. Extrusion printing enables precise deposition of high-viscosity materials but suffers from cross-contamination. In contrast, inkjet printing effectively manages low-viscosity inks in distinct material compartments, but lacks precision, scalability, and accurate droplet placement. This study introduces a multi-material hydrogel thread fabrication technique that integrates the strengths of both methods. The threads consist of distinct, aqueous hydrogel droplets generated using a microfluidic chip within an oil stream and brought into contact through a continuous oil siphoning region. Phospholipids in the oil phase prevent droplet fusion while promoting adhesion by forming phospholipid bilayers between neighboring droplets. These assembled threads are then deposited using a 3-axis stage and cured into stable hydrogel structures. The technique's ability to achieve high-resolution structures is demonstrated by successfully printing Hilbert curve-based patterns. This printing approach for soft, multi-material structures enables precise material deposition, minimizes cross-contamination, and facilitates effective compartmentalization, thereby bridging the gap between extrusion and inkjet printing. It enables scalable production of complex structures with diverse properties for applications in tissue engineering, soft robotics, and biofabrication.