Wall shear stress during impingement at the building platform can exceed nozzle wall shear stress in microvalve-based bioprinting

It is well known that in microvalve-based bioprinting, the cells are subjected to wall shear stress, which can negatively affect their viability rate. We hypothesized that the wall shear stress during impingement at the building platform, hitherto not considered in microvalve-based bioprinting, can be even more critical for the processed cells than the wall shear stress inside the nozzle. To test our hypothesis, we used fluid mechanics numerical simulation based on finite volume method. In addition, viability of two functionally different cell types, HaCaT cell line and primary human umbilical vein endothelial cells (HUVECs), embedded in the cellladen hydrogel was assessed after bioprinting. Simulation results revealed that at low upstream pressure the kinetic energy was not sufficient to overcome the interfacial force for droplet formation and detachment. Oppositely, at relatively mid upstream pressure, a droplet and a ligament were formed, whereas at higher upstream pressure, a jet was formed between nozzle and platform. In the case of jet formation, the shear stress during impingement can exceed the wall shear stress in the nozzle. The amplitude of impingement shear stress depended on nozzle-toplatform distance. This was confirmed by evaluating cell viability which revealed an increase of up to 10% when increasing the nozzle-to-platform distance from 0.3 to 3 mm. In conclusion, the impingement-related shear stress can exceed the wall shear stress in the nozzle in microvalve-based bioprinting. However, this critical issue can be successfully addressed by adapting the distance between the nozzle and the building platform. Altogether, our results highlight impingement-related shear stress as another essential parameter to consider in devising bioprinting strategies.