Rheology Indicators for Assessing Bead Spreading of Hydrogels with Functional Rheology Modifiers for Direct Ink Writing: A Case Study for Chitosan–Graphene–Titanium Dioxide

The rheological behavior of a model system composed of chitosan (CS) hydrogels with graphene and titanium dioxide (TiO₂) as functional fillers is studied to define a design space between rheology modification and bead spreading in direct ink writing (DIW) additive manufacturing (AM). Rheological results are combined with printed bead studies to determine how each formulation component affects extrudability and bead shape retention, identifying tan delta as the strongest indicator of good shape retention. Samples with tan delta >1 exhibited ink spreading postdeposition, while those with predominantly solid-like behavior (tan delta ≤ 1 at low angular frequencies) and low-to-intermediate TiO₂ concentrations extruded well and maintained their shape in the time scale of printing. Further increasing functional particle content (25 wt % TiO₂) led to inconsistent extrusion that hindered adequate extrusion fidelity. Moreover, the time-dependent structural recovery of the formulations was strongly influenced by composition, with an increasing CS content being less detrimental to storage modulus recovery than the addition of fillers. Finally, the stress required for the inks to transition from a solid-like to a liquid-like state did not correlate with inconsistent extrusion, which were instead linked to high particle concentrations. This work provides insights into the effects of rheology modification using functional particles in DIW AM of hydrogel composites, ultimately helping to improve the efficiency of producing polymer-based hydrogel inks.