Investigation of the effects of nozzle geometry and fluid rheological properties on the reliability of piezoelectric inkjet bio-printers

A. Ahmadi, B. Enright, E. Cheng, H. Yu, K.C. Cheung
The University of British Columbia,
Canada

Keywords: inkjet, cell printing, bio-printer, hydrodynamics

Summary:

Piezoelectrically actuated inkjet nozzles have been used for printing of suspensions of living cells for numerous applications including tissue engineering and drug discovery. Although inkjet cell printing systems provide high precision droplet volume dispensing capability for a wide range of ink rheology, they do not offer high reliability in the number of printed cells per drop. In addition to non-uniform distribution of cells due to sedimentation and aggregation, the observed deviation from the expected trend is attributed to the unexpected hydrodynamic response of cells inside the nozzle. The hydrodynamics of cell motion inside the inkjet nozzle are different from that of smaller particles due to the comparable size of cells to the nozzle dimensions: smaller (sub-micron sized) particles tend to follow the fluid flow, whereas cells (with the average size of 12 µm) can slip with respect to the fluid flow. The relative velocity of the cells with respect to the fluid flow is partly caused by the interaction of high frequency traveling pressure wave and fluid velocity which have an important effect on the dynamics of particle motion inside the nozzle. Moreover, particularly closer to the orifice (with the average diameter of 80 µm), the motion of cells can cause disturbance in the fluid flow. As a result, it has been observed that cells within a specific region inside the nozzle (at the onset of drop ejection cycle) are pushed further away from the orifice during the ejection cycle. This behavior which was called cell reflection, is only observed for larger particles (such as cells). Cell reflection is hypothesized to be one of the reasons for inconsistent cell count per drop. Therefore, the effects of rheological manipulation of the suspension on inkjet cell printing reliability must be explored. In this paper, the hydrodynamics of cell motion inside the inkjet nozzle is studied, and the effects of rheological manipulation and nozzle geometry on the reliability of inkjet cell printing is investigated. Through our theoretical calculations, two main hydrodynamic reasons for cell reflection are identified: 1) the high velocity of retracting meniscus which returns the particles to the nozzle with high velocity; and 2) the phase difference between the fluid velocity and pressure field which changes the dynamics of particle motion. A thorough parametric study is performed to investigate the effects of nozzle geometry and dimensionless numbers including Reynolds, Womersley and Stokes numbers on the cell printing reliability. For instance, it is shown that as the Stokes number increases with the size of the particles, the rate of occurrence for cell reflection increases for larger cells. Moreover, the use of straight nozzles rather than tapered ones increases the reliability in the number of printed cells per drop. The findings of the present study conclude that selective design of the rheology of the cell suspension can improve the inkjet cell printing reliability for biofabrication applications.