Preflow and postflow delay are two parameters used in pneumatically driven bioprinting. When bioprinting viscous gels, there is a delay between the initial application of pressure and the start of flow. 

Preflow delay starts flow by pausing printhead movement for a set time after applying the initial pressure, allowing the bioink to begin flowing before resuming movement.

Postflow delay prevents dripping and excess material deposition by turning off pressure before the printhead stops moving.