Binder rheology and printability in direct ink writing: A framework for hierarchically porous structures

Abstract

Model single and dual-binder metal inks are developed and studied for direct ink writing (DIW) of hierarchical porous metal parts. This study establishes a generalized framework for tuning ink rheology in metal DIW by linking binder molecular conformation to shear-thinning behavior, printability, and hierarchical porosity formation. These aqueous-based ink formulations leverage the unique properties of xanthan gum (XG) and hydroxyethyl cellulose (HEC), both serving as binders and viscosity modifiers with distinct conformational structures, combined with stainless-steel micro particles. A novel printability assessment methodology is introduced, integrating rheological modeling with a figure of merit and processing maps to define effective printing windows. Helical-concentrated inks (XG) exhibit stronger shear-thinning behavior and limited effective printability regions due to increased shear rate dependence than their linear counterparts (HEC). After partial sintering at 1100°C, printed parts achieve 20–34 % porosity with pore areas ranging from 3–6 µm², which are present within the struts of the printed scaffold. The resulting scaffolds feature hierarchical porosity, with open millimeter-scale lattice pores and micropores in struts, achieving total porosity of 52–80 %. Experimental investigations show tensile elastic moduli of 3.84–5.13 GPa and compression moduli of 0.06–0.97 GPa. These findings provide fundamental insights into DIW ink formulation and hierarchical porosity development, offering a transferable strategy for processing porous metallic and ceramic scaffolds in biomedical and structural applications.