Task-Specific Computational Fluid Dynamics Evaluation of Multi-Outlet Extrusion Nozzles for Bioprinting
Extrusion bioprinting is moving from proof-of-concept demonstrations toward repeatable, manufacturing-style workflows, but the nozzle remains a persistent bottleneck. Inside compact, opaque channels, bioinks experience strong geometric forcing that sets the local shear environment for cells, the pressure demand on the printer, and the uniformity of deposition. Multi-outlet nozzles are attractive because they promise higher throughput by splitting one feed into parallel filaments, yet in practice they often suffer from uneven outlet delivery and junction-driven shear hotspots. As a result, nozzle selection and scaling are still largely guided by trial-and-error rather than quantitative design evidence. In this work we provide a controlled, task-oriented comparison of two novel multi-outlet splitter archetypes-- a compact radial 90{degrees} manifold and a branched Y-split--implemented with two and four outlets. Using three-dimensional computational fluid dynamics with representative rheology for three common hydrogel bioinks (GelMA, MeHA, and alginate) under typical pneumatic actuation, we resolve internal pressure, velocity, and wall shear stress fields and translate them into practical decision metrics for outlet balance and pressure-normalised throughput. The results show that the two-outlet 90{degrees} manifold is the most robust "default" geometry, consistently delivering the most uniform outlet splitting for shear-thinning inks while maintaining the most conservative shear footprint, making it the safest option for cell-laden and precision printing. The two-outlet Y-split achieves higher outlet speeds and is therefore better suited to fast deposition of acellular or support materials, but it concentrates elevated shear at the primary junction and is more sensitive to operating regime for weakly shear-dependent inks. Scaling from two to four outlets markedly increases the risk of maldistribution across all materials and pressures, and does not eliminate junction-anchored shear hotspots in the Y-split, indicating that passive geometric symmetry is insufficient at higher outlet counts. Overall, this study converts an informal design choice into evidence-based nozzle selection rules, enabling practitioners to match nozzle architecture to printing task (cell safety, precision, or throughput) and to anticipate when multi-outlet scaling will require active balancing or flow control. The proposed framework accelerates nozzle development, improves print reproducibility, and helps define safer operating windows for bioprinting with living cells.