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Yusupov, et al.
3.3 Effect of laser pulse fluence on printed measured the resulting jet velocities, the diameters
droplets and volumes of the droplets formed on the acceptor
plate. The measurement results for the all gel types
It is known that the key role in controlling the LIFT are represented in Figure 7.
process is played by laser fluence [19,30] . Changing From Figure 7, it follows that for all the gels
the laser fluence allows for controlling the diameter used in the experiment, the optimal laser fluence
and volume of the transferred droplets, which range for the formation of a single jet and stable
determine the resolution and throughput of printing transfer of hydrogel droplets is between 2 and
as well as the ability to transfer objects of various 3 J/cm . For optimal transfer regime, the widest
2
sizes from single cells to cell spheroids. Figure 6 fluence range is observed for hyaluronic acid
shows the influence of laser fluence on the diameter sodium salt, while the narrowest fluence range
and volume of droplets for different hydrogels. is observed for methylcellulose. That is, the use
The volumes of transferred droplets were of 2% solution of hyaluronic acid sodium salt
calculated by measuring their contact angles on provides the most stable transfer of gel droplets.
the acceptor plate, particularly, for the applied In this case, small changes in the parameters of the
hydrogels the contact angles were: 2% hyaluronic hydrogel, the laser fluence, and the parameters of
acid sodium salt −21.5° ± 0.4, 1% sodium alginate the absorbing layer of the plate will not approve the
−17.4° ± 0.3, and 1% methylcellulose −22.4° ± transition to other non-optimal printing regimes.
0.6. From the data obtained, it follows that for the
optimal jetting regime, the increase in the diameter 3.4 The environment-dependent parameters of
and volume of the transferred droplets has linear- the hydrogel
like relationship with laser fluence. In this case, This section discusses the influence of external
the smallest droplets are obtained for hyaluronic factors (experiment temperature and experiment
acid-based hydrogel. For other two hydrogels, the time) on the viscosity of the hydrogel layer and
obtained values of the diameter and volume of the its drying on the donor ribbon. The vast majority
transferred droplets are practically the same for of articles pay insufficient attention to the drying-
most of the fluences applied. induced change of the hydrogel layer parameters
For all the gels used in this work, various during the experiment. When the hydrogel layer
transfer regimes were defined depending on the dries, the changes in its viscosity and thickness can
change in laser pulse energy. It was realized by significantly impact the process of laser transfer.
conducting a series of experiments with a high- The viscosity of the hydrogel layer directly
speed shooting of transfer processes, where we the determines (1) the required laser fluence to provide

A B

Figure 6. Relationship between laser fluence, diameter, and volume of transferred droplets. (A) Droplet
diameters (inserted image illustrates the transferred droplets of 2% hyaluronic acid sodium salt, scale bar:
100 µm); (B) droplet volumes. For all relationships, linear trends are plotted with the error considered.
The measurements were carried out in the optimal transfer regime.
International Journal of Bioprinting (2020)–Volume 6, Issue 3 85