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Yusupov, et al.
cm (nanosecond laser and Au absorbing layer). In the change of the distance D still does not yield
2
practice, the starting values for the experiments can a “quality” droplet shape, then it is necessary to
be: E = 15 – 20 μJ and SS = 30 μm for a laser with analyze the operating regimes of jet formation.
a wavelength of 1064 nm and a pulse duration of It can be performed using high-speed shooting
~ 10 ns. Such values allow successful realization or, if it is unavailable, by the analysis of the
of laser transfer process with most hydrogel types. landed droplet images (Step 4, Figure 8). Further,
If no hydrogel transfer toward acceptor plate depending on the registered transfer regimes
occurs, one should gradually increase the laser (high-speed, turbulent, and plume), it is necessary
pulse energy and reduce the spot diameter and to select parameters according to the mentioned
gel layer thickness to initiate the transfer process. algorithm. In addition, high-speed shooting can
When the gel transfer is achieved, the next step help determine the optimal distance D between the
assesses the “quality” of the droplet shape by its donor and acceptor slides.
appearance. If the transfer is carried out with a On reaching the regime resulting in “quality”
droplet of an irregular shape or with several droplets droplets, further it may be necessary to adjust their
(splashing), then it is necessary to gradually adjust size to the required parameters of the experiment.
the experimental parameters. First, it is necessary At the first stage, for small adjustments, it will
to find the minimum laser pulse energy (Step 1, be sufficient to change the laser pulse energy.
Figure 8) sufficient for droplet transfer. Second, However, for significant adjustments, one should
it is advisable to increase the thickness of the gel simultaneously change several parameters, since
(Step 2, Figure 8), while raising the energy of otherwise, optimal transfer regime will be lost.
the laser pulse and spot diameter to achieve the
required droplet size. 5 Conclusion
If after the implementation of these steps the
quality of the droplet is still poor, then this is We investigated laser printing processes using three
probably due to suboptimal distance between most widely used hydrogels in bioprinting: 2%
the donor and acceptor slides. Usually, it occurs hyaluronic acid sodium salt, 1% methylcellulose,
if instead of a single droplet, the acceptor plate and 1% sodium alginate. For all gels, various
is reached by the forming jet, which leads to transfer regimes (no transfer mode, optimal jetting
its splashing. Therefore, in the next step, it is mode with single droplet transfer, high speed
advisable to increase the distance between the mode, turbulent mode, and plume mode) were
donor and acceptor slides. However, it will be determined depending on the change in laser pulse
wrong to conclude that for the implementation of energy. It was shown that in the optimal jetting
high-quality printing, a long distance is always regime, for all hydrogels, the size and volume of
necessary, since with its increase, the printing droplets increased almost linearly with increasing
quality will deteriorate. This deterioration is laser fluence. The effect of ambient temperature
associated with the horizontal velocity component on the viscosity of hydrogels was also evaluated.
of transferred droplets, which emerges due to It was shown that at room temperature (22 ± 2°C),
hydrogel heterogeneity and no axial symmetry in this change can be neglected. For the experiments
absorbing layer heating. In some cases, one can where no additional methods used for preventing
encounter a regime when the jet breaks up into ribbon hydrogel drying, we determined the
several droplets landing on the acceptor plate. In time periods which provide a stable controlled
this case, as the distance increases, the shape of the laser printing and which allow for neglecting
resulting droplet may distort thus demonstrating the drying-induced hydrogel thickness change.
a spraying effect . Therefore, we believe that it Moreover, we suggested a simple practical
[15]
is necessary to vary the entire range of distances algorithm for quick setting up of the LIFT printing
between the donor and acceptor plates to detect process to obtain “quality” bioink droplets with
the optimal transfer regime (Step 3, Figure 8). If the required parameters on a given experimental

International Journal of Bioprinting (2020)–Volume 6, Issue 3 89