Controlling inter-filament fusion in embedded 3D printing

Embedded 3D printing is an additive manufacturing technique wherein a nozzle extrudes continuous filaments into a viscoelastic support bath. Because the bath holds the form of the printed part, embedded 3D printing allows for softer materials than direct ink writing and enables freeform toolpaths, as opposed to layer-by-layer printing. The technique has been applied to biomaterials, cell-laden inks, soft functional materials, and intricate vasculatures via sacrificial printing. However, the bath can introduce barriers that prevent neighboring filaments from fusing together, or driving forces that cause filaments to over-fuse, distorting the shape of the printed part. Here, we use in-situ imaging experiments to investigate how to control inter-filament fusion using ink rheology, support rheology, interfacial energy, inter-filament spacing, and print speeds. We find that the spacing between filaments greatly impacts their ability to fuse, and that filaments must be printed closer together than the ideal space-filling distance in order to fuse together. Moreover, there is a trade-off in capillary number between fusion and capillary instabilities. At low capillary number, filaments over-fuse and contract towards a spherical shape or rupture into droplets. At high capillary number, filaments under-fuse but do not contract or rupture. A moderate capillary number is necessary to achieve accurate shape fidelity.