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LIFT hydrogel printing: A defined route for highly controlled process
2.5 Visualization of jetting and measurement of subsequent transfer to an acceptor plate . The
[15]
jetting velocity processes associated with the formation and
collapse of a gas bubble, as well as possible regimes
The printing and transfer processes of the material of jet formation are represented in Figure 4.
were visualized and investigated using a high- At the first stage of the LIFT process (t = 0.01
speed Fastcam SA-3 camera (Photron, Japan) a.u.), the metal energy absorbing layer, the nearest
(shutter 1/60000, FPS 10000-60000) with a
long-focus microscopic lens providing a shooting thin layers of the glass slide, and the hydrogel
field of up to 5 × 3.5 mm. The illumination was are heated to high temperatures due to the partial
performed using a K150 Laboratory Illuminator absorption of the laser pulse energy by the energy
(Optics Co, China). absorbing layer. In the applied laser printing
regimes, the achieved temperature significantly
3 Results and discussion exceeds the melting temperature of the material
of the absorbing layer (for Au T≈1337 K) . On
[13]
3.1 General observations reaching a temperature of ~ 0.95 from the critical
The LIFT process is based on local heating temperature (for water T = 647 K), the water
c
[34]
occurring during the interaction of a laser pulse contained in the gel boils explosively , which
with an absorbing layer, which leads to explosive converts the water into steam compressed to a high
boiling of water in a gel , while more intensive (around critical) pressure (Pc ~ 22.5 MPa). As a
[13]
heating can lead to explosive boiling of the result, a rapidly expanding vapor bubble forms in
absorbing layer material . As a result, a high- the gel in the area of laser exposure.
[45]
pressure bubble is formed [46-48] , the expansion of At t = 1, the size of this bubble already exceeds
which leads, depending on the initial parameters , the thickness of the gel. Due to the hydrogel
[15]
to the formation of various types of jets , and viscosity, the presence of a solid surface of the
[49]
the separation of one or more droplets and their donor plate and the limited thickness of the gel, this

A

B

Figure 4. (A) The main stages of the jet and droplet formation during laser printing. Red arrows indicate
the direction of fluid movement (B) changes in jetting regimes with the increase of laser energy.

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