There are many potential ways to injure cells during any bioprinting process, and each dispensing technology exposes the cells to varying degrees of these stresses. Some of these stresses include: Acceleration of the cells, pressure, thermal effects, shear forces, impact stresses, etc.
With pneumatic dispensing, the driving pressure and nozzle size can have an effect on the pressure (experienced by the cells) within the chamber, so using a lower pressure or a larger orifice could help lessen the damage.
Thermal effects should be minimized; avoiding biomaterials that require extreme temperature control or minimizing/localizing the thermal damage from the actual printing process (as in laser and inkjet systems).
Ensuring cells are provided nutrition/oxygen is also important, so if there are any long processes or delays, this could impact viability (for example, when cells are encapsulated within hydrogel layers that might require crosslinking, it could take some time).
Shear forces induced by the walls of the needles and impact forces should be reduced, and the surrounding matrix should provide enough structural support for cells yet still allow mobility.
These are just a few examples that will help increase viability in bioprinting methods.