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LIFT hydrogel printing: A defined route for highly controlled process
the donor ribbon and printed drops on the acceptor
plate, which is largely dependent on, besides the
temperature, the air humidity. The high drying
rate of the gel layer covered on the donor ribbon
can lead to a change in its thickness and viscosity
during the experiment, which will directly affect
the printing regimes and landed droplet sizes. The
rapid drying of the gel on an acceptor plate, in
particular, leads to the change of gel droplet shapes
resulting in the “coffee-ring” effect observed
[21]
within few minutes after printing.
In our experiments, at a relative humidity of
50%, an ambient temperature of 20°C, and with
the use of relatively thick hydrogel layers (200
Figure 7. Relationship between laser-induced ± 20 μm) within 30 min after applying the gel
forward transfer regimes and laser fluences layer to ribbon, a mass loss of more than 60% was
for three gel types. Vertical lines indicate the observed (Table 2).
corresponding boundaries of the optimal range. However, glycerol can be added to prevent
the hydrogel from drying; for example, in one
[21]
the optimal transfer regime, (2) the shape and size study introduced 10% (v/v) of glycerol into all
of the jet [54,55] , as well as (3) the resulting droplet printing solutions. Nevertheless, it should be borne
volume on the acceptor plate. in mind that an increase in glycerol concentration
[21]
In our experiments, the viscosities of hydrogels above 10% (v/v) is detrimental for cell survival .
were determined at temperatures ranging from Furthermore, glycerol should be added to
20 to 37 °C. These temperatures were chosen protein-containing solutions with caution, since
[57]
deliberately, since they covered the range from glycerin is considered to be a denaturing agent .
[58]
standard (room) temperature to standard cell Alternatively, in another study , the authors
culture temperatures. A temperature change had added methylcellulose to reduce the drying rate of
a similar effect on the viscosity of hydrogels the hydrogel layer.
based on sodium alginate and hyaluronic acid Thus, to ensure they stable controlled laser
sodium salt: An increase in temperature by 4°C printing, it is necessary to maintain a constant
led to a decrease in viscosity by an average of ambient temperature and consider the high drying
9%. In the case of methylcellulose, the viscosity rate of hydrogels (~ 8 μm/min) under standard
of the solutions also decreased with an increase in conditions during the experiment (Table 2). For
temperature for every 4°С by approximately 14% example, within 5 min, the mass of the hydrogel
(Table 1). Importantly, all the obtained viscosity (and, therefore, its thickness on the donor
values are suitable for LIFT: According to Hölzl ribbon) decreases by 20%, which is beyond the
et al. , LIFT technology utilizes hydrogels with measurement error (10%). Therefore, to obtain
[56]
a viscosity ranging from 1 to 300 mPa*s. droplets of the same size within the experiment,
Table 1 shows that with a change in temperature one should finish it in few minutes, or organize
in the range of 20–37° C, the viscosity of hydrogels additional donor ribbon moisturizing, or introduce
can reduce by almost 2 times, which must be the drying-preventing agents into the hydrogel.
considered in experiments. 4 Decision tree for controlled hydrogel transfer
In addition to controlling the ambient
temperature during the experiment, one should For more than 15 years, various printing methods
also monitor the drying of the hydrogel layer on for the transfer of biomaterials have been

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