# Multi‐Material Droplet‐Based Hydrogel Threads for Extrusion 3D Printing

## TL;DR

A new 3D printing method uses hydrogel droplets separated by lipid layers to create complex structures without material mixing, combining the best of two printing techniques.

## Contribution

A novel multi-material hydrogel thread fabrication technique that prevents cross-contamination and enables precise, scalable printing of soft structures.

## Key findings

- Hydrogel droplets are separated by phospholipid bilayers to prevent fusion and enable precise assembly.
- The method successfully prints high-resolution structures like Hilbert curves.
- The technique bridges extrusion and inkjet printing limitations for soft, multi-material applications.

## Abstract

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.

This study introduces a novel approach to produce multi‐material threads for 3D printing by precisely arranging hydrogel droplets separated by lipid bilayers. By merging the precision of inkjet printing with the scalability of extrusion printing, this method prevents cross‐contamination, maintains material control, and enables intricate designs—unlocking new possibilities for advanced soft robotics, biomedical devices, and tissue engineering applications.