Task-specific computational fluid dynamics evaluation of multi-outlet extrusion nozzles for bioprinting

INTRODUCTION: Extrusion bioprinting is transitioning from proof-of-concept demonstrations toward repeatable, manufacturing-style workflows, yet nozzle design remains a key bottleneck. Within compact, opaque channels, bioinks experience geometry-driven shear, pressure losses, and flow redistribution that directly affect cell viability, extrusion stability, and deposition uniformity. Multi-outlet nozzles offer increased throughput by splitting a single feed into parallel filaments, but often suffer from flow imbalance and junction-induced shear hotspots. As a result, nozzle selection is still largely guided by trial-and-error rather than quantitative design evidence. METHODS: This study presents a controlled, task-oriented comparison of two multi-outlet splitter archetypes-a radial 90° manifold and a branched Y-split-each implemented with two and four outlets. Three-dimensional computational fluid dynamics (CFD) simulations were performed using representative rheological models for common hydrogel bioinks (GelMA, MeHA, and alginate) under pneumatic actuation. Internal pressure, velocity, and wall shear stress fields were resolved and translated into practical performance metrics, including outlet flow balance and pressure-normalised throughput. RESULTS: The two-outlet 90° manifold consistently produced the most uniform flow distribution and lowest shear exposure across shear-thinning bioinks, establishing it as the most robust configuration for cell-laden and precision printing. The two-outlet Y-split achieved higher outlet velocities, supporting faster deposition, but introduced elevated shear at junctions and greater sensitivity to operating conditions. Increasing the outlet count to four significantly increased flow maldistribution across all geometries and conditions, while failing to eliminate junction-driven shear hotspots, particularly in Y-split designs. DISCUSSION: These findings demonstrate that nozzle geometry is a primary control parameter in extrusion bioprinting and cannot be reliably scaled by symmetry alone. The results establish practical, evidence-based guidelines for selecting nozzle architectures based on application requirements, including cell safety, precision, and throughput. The proposed framework enables more predictable nozzle design, improves reproducibility, and defines safer operating windows for bioprinting with living cells.