Combination of Extrusion and Drop‐on‐Demand Bioprinting in One Process Enables the Local Placement of Cells or Signaling Factors Into (Bio) Printed Hydrogel Structures

Combining the volumetric fabrication of hydrogel constructs using extrusion bioprinting with highly precise drop-on-demand (DoD) bioprinting offers exciting opportunities in biofabrication. This technical report presents a technique in which a solenoid micro-pipette is operated as an additional tool in an extrusion (bio)printing system to deposit small volumes of bioinks into extrusion-printed hydrogel constructs. Using three exemplary approaches, we show that this enables the patterned placement of cells or growth factors within 3D constructs and thus influences developmental processes. Human cells within low-viscosity bioinks, deposited into extrusion-printed hydrogel constructs by filling inter-strand cavities or by injection into the hydrogel strands, maintained their viability and functionality up to 28 days. As demonstrated for salivary gland cells, the properties of the hydrogel matrix can influence the fate of the injected cells: In a stiff alginate (Alg)-based hydrogel, they formed aggregates, which is beneficial for organoid formation, and in softer hydrogels, they migrated to neighboring cell clusters. Locally injected signaling factors such as vascular endothelial growth factor (VEGF) attracted endothelial cells and fibroblasts, which migrated into previously cell-free hydrogel areas. The combination of extrusion and DoD bioprinting opens new approaches to integrate different cell types and functionalizations in one construct, facilitating the creation of more complex and dynamic models.