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International Journal of Bioprinting Three-dimensional bioprinting in toxicological research

located on the aperture of nozzle. The gas regulator keeps and bottom-up methods. In both cases, the elevator moved
the bioink under pneumatic pressure, thus contributing to fabrication platform is sunk into a photo-curable bio-resin
the ink ejection. The valve opening depends on micro-valve filled vat, and a scanning system coordinates the laser
parts and back-pressured bioink. On receiving voltage beam. Top-down printing approach applies a laser right
pulse, solenoid coil generates magnetic field, pulling the above the bio-resin vat, and the printing stage is descended
plunge upwards to unblock the orifice and eject the bioink. after every cured layer to build 3D structure. In contrast,
This technique applies variable nozzle sizes and low the bottom-up method uses a laser source located below
pneumatic pressure, which is favorable for avoiding cell the vat and printing stage is raised above each cured layer
damage, but it could not achieve high resolution printing through a peeling step. This technique significantly slower
because of the larger size of droplets [10,80,90,94,101-105,109] . than the top-down approach if peeling step is included.
Both methods require manually removable supports
8.2.4. Acoustic bioprinting to print 3D structures, which are built from the same
Acoustic bioprinting is a nozzle-free method, and the material as the printed construct. The appropriate cross-
droplet formation is based on acoustic waves during linking between fabrication stage and printed structure is
printing. The acoustic actuator is made up of interdigitated crucial, and finding the right cross-sectional area of each
gold rings placed on a piezoelectric substrate located printed layer is necessary to avoid damages during peeling
in a pool. The actuator is surrounded by the bioink in step. To perform photo-polymerization, the density of
microfluidic channels, with exits at the bottom. The radiation has to overcome the threshold to initiate curing
actuator generates gentle circular acoustic waves, which process, but excessive radiation could shrink bio-resin.
create acoustic focal points on the interface between bioink Stereolithography is a fast, flexible, and accurate printing
and air. The acoustic radiation-impinged force overcomes method, but this technique requires an expensive setup
the surface tension at the exit of channel, so that the and the fragility of constructs printed by this technique
bioink could be ejected. Since it does not use nozzles and may cause a problem [10,80,90,94,102-105,111,112] .
mechanical forces to form droplet, the cells avoid shear
stress, heat shock, high pressure and voltage, preserving the 8.3.2. Digital light processing
high rate of viability. However, viscosity of hydrogel and The set-up of digital light processing is very similar to that
cell density could be an obstacle of printing [10,80,90,94,101-105,110] . of stereolithography, and it utilizes properties of photo-
curable bio-resins but applies a digital micromirror device
8.3. Vat polymerization-based bioprinting instead of a scanning system. Digital micromirror device
Vat polymerization-based bioprinting can be divided into contains high number of rotatable micromirrors and
three categories: stereolithography, digital light processing, allows projecting an image on printing stage. This feature
and two-photon polymerization. Building process occurs facilitates the immediate solidification of an entire layer,
when the photo-curable/photo-activable liquid bio- achieving rapid printing speed. The layer thickness and
resin is radiated by a laser source and the cross-linking exposure duration need to be controlled, depending on
through photo-polymerization solidifies the material. the applied bio-resin, to ensure strong interface bonding.
Biocompatibility of vat polymerization is significantly Rapid printing speed and accuracy make this technique
lower than the above-mentioned techniques. Since the popular, but mechanical properties of built structure have
printing circumstances do not promote cell survival, to be improved [10,80,90,94,102-105,111,112] .
it is mainly used for creating tissue scaffolds made for
traditional cell seeding. In most of the cases, post-curing 8.3.3. Two-photon polymerization
is necessary and printing requires support material, which Two-photon polymerization is suitable for printing
has to be removed at the end of fabrication process, as complex high-resolution 3D micro- and nano-structures.
well as non-polymerized resin. However, these setups are In polymerization based on two-photon absorption, a
commercially available and suitable for rapid printing of molecule absorbs two photons within an extremely brief
large-volume, highly detailed structures [10,80,90,94,102-105,111,112] . time interval, converting its ground state to excited state.
The printing process occurs in a bio-resin-filled vat with
8.3.1. Stereolithography a glass slide at the bottom, and near-infrared femtosecond
Printing process is based on photo-polymerization of photo- laser radiates oil-immersion objective lens. The transparent
curable liquid bio-resin. A UV/visible light laser radiates liquid bio-resin acts as a photoresist material that contains
bio-resin with its specific curing wavelength; therefore, negative- and positive-tone photoresists. Through
polymers could be formed to print desired scaffold. There objective lens, the laser beam is focused precisely onto
are two possible ways to polymerize bio-resin: top-down the photoresist above the glass cover slip, thereby cross-

Volume 9 Issue 2 (2023) 207 https://doi.org/10.18063/ijb.v9i2.663