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LIFT hydrogel printing: A defined route for highly controlled process
to create three-dimensional (3D) bioequivalents of parameters are configured each time according to
natural tissues [6-10] ; and (4) to transfer stimulatory a particular experimental condition.
factors for cell differentiation [6-10] . To implement the LIFT printing process, a
For the printing process, one should prepare the laser system with a set of basic optomechanical
ribbon (target, donor, or a source film). The ribbon components (3D stages and focusing optics) is
usually consists of a glass slide on the surface, of required. The main component of the system
which a nanolayer of laser-absorbing material (Au, is a laser source, which determines the main
Ti, etc.) is applied [8,11,12] , which is then covered characteristics of the laser pulse. The most
with a hydrogel-containing transferred objects. common, commercially available, and studied [19,20] ,
Laser pulses pass through the uncoated laser sources have nanosecond pulse length [21-24]
transparent side of the ribbon and then focus and a wavelength of λ = 1064 nm (near infrared
on a layer that absorbs laser radiation. The range). Near-IR radiation is well absorbed by the
partial absorption of the laser pulse energy by metal absorbing layer, while remaining transparent
the absorbing layer leads to the rapid heating of to living tissues and cells [25,26] . It is also important
its local area and of the adjacent thin layer of that IR radiation, due to the low energy of quanta,
hydrogel, which leads to the formation of the high- practically does not affect the physicochemical
pressure and high-temperature vapor bubble . characteristics of hydrogels and does not
[13]
[27]
The rapid expansion of this vapored region leads damage cells . It is worth noting that several
[25]
to the formation of jets of various types with the studies Zhang et al. [28,29] used laser sources of the
[14]
subsequent separation of one or more droplets and ultraviolet spectrum, and their radiation is well
their transfer to the acceptor surface . Finally, absorbed by the hydrogel material, which makes it
[15]
according to a predetermined design and with a high possible to avoid using the absorbing layer. On the
productivity of the process , a two-dimensional other hand, such short-wavelength laser radiation
[16]
(2D) and, if necessary, 3D structure with the can change the physicochemical characteristics
[18]
[17]
necessary concentration of transferred objects is of hydrogels, and moreover, this radiation is well
fabricated from these microdroplets. absorbed by living tissues and cells, which can be
To become a truly universal method, laser detrimental to them.
bioprinting applying the LIFT technique must When choosing a laser source and an radiation
be an accurate technology that can guarantee the optical focusing scheme, the following laser
transfer of a strictly specified volume of hydrogel factors should be considered (Figure 1): Pulse
with minimal negative effects on living objects in duration, laser pulse energy, and laser spot
the gel. The main difficulty in the work with this size, since these parameters directly determine
technology is the accurate selection of factors that the energy density (fluence) [16,21,30,31] , and peak
significantly affect the process of laser transfer. laser power. However, the vapor bubble growth
The characteristics of the laser printing dynamics and all LIFT laser transfer process [19,21]
process depend (Figure 1) on the (1) parameters depend on the pulse duration and absorbed fluence,
of the laser impact (wavelength, laser pulse which is, in turn, determined by the laser radiation
duration, focusing parameters, and energy); (2) absorption coefficient of the energy absorbing
ribbon factors (absorbing layer material, type of layer. The choice of material and thickness of the
transferred objects, hydrogel parameters, and ribbon absorbing layer are associated with the
hydrogel layer thickness); and (3) external factors need to provide the most efficient conversion of
(experiment configuration, environmental factors, a laser pulse with a given wavelength λ into heat,
and transfer distance). while minimizing transmitted radiation, which can
Red parameters, as a rule, are set by an adversely affect biological objects contained in the
experimental setup, and most often stay hydrogel layer. It is also necessary to consider that,
unchanged. Yellow parameters change, as a from a metal absorbing layer, laser irradiation can
rule, when printed objects are changed. Green produce nanoparticles which are then transferred

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