Novel stirring-rod-inspired mixer-integrated printhead for fabricating gradient tissue structures

Abstract

Conventional methods for creating gradient tissue structures using extrusion-based 3D printing (E3DP) systems involve using mixers to create homogeneous materials. However, commercially available mixers can generate high fluid pressure and shear force that harm cells. Hence, a new mixer-integrated printhead was developed, inspired by stirring-rod-assisted mixing processes. The as-designed mixing system was validated through computational fluid dynamics (CFD) simulations using input parameters including ink viscosity, density, and velocity vector and output parameters including material volume fraction, average fluid pressure, and shear force. Six-stir element mixer units were used to comprehensively analyze material mixing while considering the fluid volume fraction after flowing through the mixers. Printheads integrated with multiple discrete mixers with consistent spiral directions, performing well when introducing gaps, were used to reduce the cell pressure and shear force. The optimal mixer design, with two 1.62-mm gaps and six-stir element mixer units sharing the same spiral structure, reduced the average fluid pressure and shear force by 3.196e+4 Pa and 0.368e+2 Pa, respectively, compared with a commercial static mixer. Finally, a mixer-integrated printhead was manufactured, matching well with the original design. This research could improve E3DP systems and serve as a design reference for 3D printing structures of varying stiffness.