Bioink rheology controls secondary jetting and printed droplet formation in laser-assisted bioprinting

Predictive control of bioink jetting and voxel formation is essential for improving Laser Induced Forward Transfer (LIFT) as a reliable bioprinting tool. This study demonstrates that LIFT bioprinting is affected not only by apparent viscosity, but also by the time-dependent rheological response of the bioink during rapid jet formation. Newtonian glycerol-water and shear-thinning sodium alginate cell-free model bioinks were compared under identical LIFT conditions using high-speed imaging, dual LED flash shadowgraphy, rheological characterization, and dimensionless analysis. Glycerol solutions exhibited a viscosity dependent transition during laser bioprinting from two jet to single jet ejection, with high viscosity formulations suppressing secondary jetting and producing confined, low volume deposits. In contrast, alginate bioinks preserved two sequential jets across the measurable transfer conditions, even at apparent viscosities overlapping with or exceeding glycerol conditions that produced single ligaments. This transition was captured by the Weissenberg number, which separated the Newtonian regime from the polymer timescale dominated alginate regime. Early time imaging further showed that high viscosity alginate produced faster and wider jets than high viscosity glycerol, resulting in larger printed voxel volumes. These results establish secondary jetting as a rheology-controlled transfer mechanism in LIFT bioprinting. By relating bioink rheology, jet multiplicity, and deposited volume, this study provides a practical framework for selecting bioinks for either precise low volume patterning or larger volume hydrogel deposition in the field of biofabrication. The results provide a mechanistic framework for selecting and optimizing model bioink formulations for future cell-laden LIFT bioprinting studies and applications.