Printing in microgel suspensions: Understanding the complex interactions between ink and bath fluid

Embedded printing relies on the deposition of ink in a bath of another fluid that provides support. The substrate allows the use of low-viscosity inks and extends the types of 3D printable structures. Yet, the interactions between the ink and support fluid can give rise to defects that constraint the fluid-based design. We focus on microgel substrates that are particularly promising for bio-printing applications. In particular, we work with carbopol suspensions, which are mixtures of water and polyacrylic-acid-based microgel particles that are a model shear-thinning fluid at low concentrations and a model yield stress fluid at high concentrations. After discussing the complex rheology of the microgel, we will describe the spreading of the dispensed ink behind the moving nozzle. By printing horizontal threads made of dyed water in baths of Carbopol suspensions, we demonstrate that the spreading can be attributed to the pressure field generated in the substrate by the relative motion of the nozzle. Both viscous and elastic stresses contribute to the spreading of the ink, which is mitigated by the yield stress. We will then present results on the time-dependence or aging of the printed threads in microgels. Our findings highlight the link between bath rheology and printing quality, and we propose strategies to stabilize the fluid structures in embedded 3D printing.