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Ng, et al.
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Figure 8. Cell proliferation profile over time. (A) Schematic drawing of the various steps that influence the cell viability during the DOD
bioprinting process. (B) Representative fluorescence images of the printed primary HDFs (30 nL droplet volume per spot) printed using the
optimal parameters of 4 million cells/mL cell-laden bio-inks printed within 2-min printing duration over a period of 7 days; scale bar = 200 µm.
(C) Analysis of the cell proliferation profile using normalized RFUs from the PrestoBlue assay at different time intervals (day 1, 3 and 7).
®
4. Conclusions thermal inkjet print-heads) with nozzle diameter of 80 µm
is between 1 – 4 million cells/mL, and a change in the cell
This work pioneers the investigation of droplet impact
velocity and droplet evaporation on viability of printed concentration (1 – 4 million cells/mL) has no significant
primary human cells during the DOD thermal inkjet effect on the viability of printed cells during the printing
bioprinting process. It provides a better understanding on process. Next, the evaluation of droplet velocity profile
the different factors that affect the viability of printed cells using the high-speed camera revealed that an increase in
in sub-nanoliter droplets. A systematic approach was used the cell concentration leads to significantly slower droplet
to first determine the influence of cell concentration on the impact velocity. A slower droplet impact velocity helps to
bio-ink’s physical properties (viscosity, surface tension, mitigate droplet splashing, improve the printing accuracy
and density) and printability. The printability range of and significantly enhance the viability of printed cells
cell concentration using a thermal inkjet printer (HP within the sub-nanoliter droplets. Furthermore, the PBS
D300e Digital Dispenser) and its cell-printing cassettes solution serves as a baseline to understand the influence
(specially-designed C-8 cassettes with 8 embedded of droplet evaporation on cell viability and a minimum

International Journal of Bioprinting (2022)–Volume 8, Issue 1 37