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Yusupov, et al.
bubble becomes easier to expand along the optical onto an acceptor plate. With a further increase in
axis, but not across it. All these factors generate a the laser pulse energy, the length and velocity of
pressure gradient along the bubble wall leading to the transferring jet increase, which lead to the
the gel flows from the periphery to the optical axe transfer, besides the main droplet, of small satellite
center of the bubble top. The interaction of these droplets (high speed regime). On reaching the
flows leads at t = 2 to the appearance of a jet and energies leading to turbulent jets or the formation
a counter jet. of several jets simultaneously (turbulent regime),
As the bubble expands, the pressure inside it large satellite drops are transferred along with
drops monotonically and its expansion at the stages the main droplet. With a further increase in laser
t = 1 and t = 2 occurs by inertia. When the bubble energy, the energy stored in the expanding bubble
reaches its maximum size, the pressure inside it becomes so large that a rupture of the outer wall
decreases almost to zero and the bubble begins of the bubble (plume regime) occurs, leading to
to compress (t = 3) due to the pressure difference a chaotic transfer of hydrogel both in shape and
outside (≈10 Pa) and inside it. The bubble apex volume.
5
[48]
serves as the base of an elongated jet, from which The best printing quality achievable in laser
a microdroplet detaches due to Rayleigh–Plateau transferring can only be realized in a relatively
instability. Depending on the selected regime, the narrow range of laser energies corresponding to
jet can move back and be completely absorbed by the optimal regime and leading to the transfer of
the gel layer or it can separate and continue the a single droplet. It is worth noting that usually
independent movement toward the acceptor plate for describing the LIFT process, instead of the
or it can break up into several fragments. parameter “laser pulse energy,” the parameter
At stage t = 4, the microdroplet (not shown in “laser fluence” is used, which is determined by
the figure) continues to move toward the acceptor the ratio of laser pulse energy to laser spot size
plate or has already reached its surface. Having and measured in J/cm . Laser fluence is a more
2
collapsed, the bubble begins to expand again. universal parameter, since the main role in the
Moreover, since its center is substituted with by transfer processes is played by the radiation energy
fragments of the counter jet, its shape looks like density. In our experiments, when the energy
a donut. The maximum speeds of the outer walls changed from 7 to 25 μJ while maintaining a spot
of this “donut” are directed at a certain angle to size of 30 μm, the fluence range was 1 – 5 J/cm .
2
the optical axis. At sufficiently high laser energies, To assess the effect of laser fluence on the
hydrogel splashing can occur in these directions. jetting regime and jetting velocity, we used a high-

3.2 Regimes of jet formation speed shooting of the printing process. The LIFT
occurred between the donor ribbon and acceptor
In this part, we will discuss the regimes of jet plate located at a distance of 1 mm.
formation in more detail. The energy of a laser Figure 5 shows that using hyaluronic acid
pulse directly sets the amount of transferred energy sodium salt (2%) as an example, with small
which leads to the formation of a gas bubble . fluence (up to 1.7 J/cm ), the transfer process did
2
[47]
The effect of the kinetic energy transferred to the not occur. With an increase in laser fluence, there
jet/droplet on the printing process is well studied was a transition to the optimal jetting regime with
2
in the literature [14,15] . Figure 4B shows five basic single droplet transferring (2.0 J/cm ), then to the
laser transfer regimes that determine the amount of high-speed regime (2.5 J/cm ), and, finally, to the
2
transferred material and printing quality [14,15,45] . At turbulent regime (3.7 J/cm ).
2
low values of laser energy (subthreshold regime) a During the process of laser printing, the living
small jet forms and does not transfer the material. objects transferred inside the jet practically do not
When a certain energy threshold is overpassed experience any mechanical and thermal stresses.
(optimal regime), a part of the jet begins to They experience significant dynamic stresses
separate and transfer in the form of a single droplet only at the initial moment of bubble and jet

International Journal of Bioprinting (2020)–Volume 6, Issue 3 83