3D printing of hydrogels: a synergistic approach of rheology and computational fluid dynamics (CFD) modeling.

The development of printable hydrogel inks with optimized rheological properties is critical for advancing extrusion-based 3D printing. In this study, we present a systematic investigation of kappa-carrageenan (κCG) hydrogel inks formulated with and without 10 mM KCl and gold nanoparticles (AuNPs) to assess their printability, flow behavior, and structural performance. All formulations exhibited shear-thinning behavior, characterized using the Power Law model, while stress sweep measurements provided insights into viscoelastic moduli and yield stress. Inks without KCl displayed smooth flow and superior single-layer printability but lacked multi-layer stability, while KCl-crosslinked inks showed enhanced mechanical stability and structural retention in multi-layer constructs. The ink containing both KCl and AuNPs demonstrated the best results in multi-layer printing, combining the mechanical stability imparted by KCl with enhanced shear-thinning behavior from AuNPs. CFD simulations using ANSYS fluent were employed to estimate extrusion pressures and visualize shear rate distributions within the syringe-nozzle geometry. Using CFD results, the thixotropic protocol was modified to reflect actual shear conditions during printing. Inks without KCl showed high viscosity recovery, while those with KCl exhibited lower values, likely due to sample slippage at high shear. Despite this, KCl-containing inks showed excellent multi-layer printability, indicating effective structural recovery. Complementary FTIR, XRD, thermal, and FESEM analyses validated structural and morphological features of inks. This integrated experimental-CFD framework offers a predictive approach to understand hydrogel ink behavior, highlighting the interplay between formulation, flow properties, and print performance. The findings provide a foundation for next-generation bioinks for tissue engineering and soft material applications.