Page 214 - IJB-9-2

International Journal of Bioprinting Three-dimensional bioprinting in toxicological research

different subtypes: thermal inkjet bioprinting, piezoelectric the formation of Taylor cone, which is the result of a state
inkjet bioprinting, electrostatic inkjet bioprinting, and of equilibrium between the Maxwell forces present in the
electrohydrodynamic jet bioprinting [10,80,90,94,100-106] . molten liquid and the surface tension that maintains the
meniscus. The electric fields promote the accumulation of
(A) Drop-on-demand methods
charges that gives rise to the formation of sharper cone, and
Thermal inkjet bioprinting is based on a heat actuator when the surface charges exceed surface tension, the ink
located in chamber. It develops heat bubbles during printing could be ejected from the nozzle. The main characteristic
by heating the ink locally for several microseconds with is that the droplet is pulled out by electric field between the
high temperature (250 – 350°C). The brief but very high ink and the substrate plate and it cannot eject single droplet.
heating phase causes the vaporization of bioink, and then A great advantage of this technique is that the droplet
this heat bubble bursts, forming the driving force of droplet size could be variable. Due to the Taylor cone-dependent
ejection, since ink is forced toward the nozzle outlet to emit droplet emission, the droplet could be smaller than the
the droplet. This method is preferred because it is relatively diameter of nozzle, thus increasing printing resolution and
cheap and has fast printing speed as well as its extremely allowing printing of viscous hydrogels [10,80,90,94,100,102-108] .
brief heating phase only increases the material temperature
by 4 – 10°C above room temperature, so most of the 8.2.2. Laser-induced forward transfer (LIFT)
cells remain viable. Since droplet formation depends on The laser-LIFT method applies a laser generator, a laser
explosion of heat bubble, the optimization and maintenance path-adjustment module consisting of mirrors and focusing
of standard printing protocol is difficult to approach, and lenses, and a cell transfer module for bioprinting. In general,
shear stress could also affect the cells [10,80,90,94,100,102-107] . the cell transfer module contains a ribbon and a substrate
Piezoelectric inkjet bioprinting takes advantage of layer. Two setups of LIFT exist and require different ribbon
piezoelectric crystals located on the inner wall of chamber. constructs: matrix-assisted pulse laser evaporation direct
Piezoelectric actuator converts voltage into mechanical writing without energy absorbing layer and absorbing
energy. The pulse crystals undergo deformation in the layer-assisted LIFT (AFA-LIFT). For bioprinting, the AFA-
presence of voltage, thus squeezing the ink and promoting LIFT method is more suitable, since it has energy-absorbing
droplet formation. High viability rate was observed after layer to protect the cells. In this setup, the ribbon consists
printing although the mechanical shape formation caused of three layers: a transparent and thick supporting layer, a
the generation of ultrasonic waves and the shear stress nano-scale laser-absorbing sacrifice layer, and a bioink layer
could be harmful to the cells. Drop-on-demand method is containing the transferrable material. During printing, the
a popular technique because of a few features, such as easy laser generator launches the laser beam throughout the
control of droplet production, availability of a wide variety laser path-adjustment module, which directs and focuses
of nozzle sizes, and easy cleaning [10,80,90,94,100,102-107] . the beam to the desired spot on the upper side of sacrifice

Electrostatic inkjet bioprinting applies a pressure layer. When the laser beam reaches the bioink layer at
plate placed in chamber, which allows the squeezing of the irradiation point, a bubble is spawned and grows
ink during droplet formation. The pressure plate could until the bioink elongates. The expansion drops the inner
move between two positions because of static electricity pressure results in bubble collapse, and the high pressure
to influence the chamber volume. In circuit, the pressure in bubble pole drives the jet or droplet formation. Then,
plate is attracted to the electrode plate, thus increasing the substrate layer receives the apex of stretched bioink,
the holding capacity of chamber. When the circuit is thus a small portion of ink is printed. LIFT has excellent
disconnected, the attraction is immediately ceased and the features, including: (i) nozzle-free and very fast (5,000
plate returns to its original position. This action reduces the droplets/sec) printing speed; (ii) micro-scale resolution;
chamber volume, and the ink leaves the nozzle as droplet. (iii) ability to handle high cell density within bioink; and
It is a very safe method for printing cells, since the bioink (iv) highest viability rate among all bioprinting methods.
is not exposed to heat or sonication, but the only concerns Other than that, LIFT gives the opportunity to achieve in
are small nozzle diameter and shear stress [10,80,90,94,100,102-107] . situ bioprinting and can be combined with other printing
techniques. Despite its advantages, there are several flaws
(B) Non-drop-on-demand methods needs to be improved, especially in the aspects of efficiency,
Electrohydrodynamic jet bioprinting is different from productivity and building cost [10,80,84,90,94,102-105] .
previous methods because it uses electric fields for 8.2.3. Microvalve-based bioprinting
droplet emission. The bioink is mechanically pulled to the
aperture of nozzle, creating a meniscus between ink and The process of droplet formation applies electromechanical
substrate ground. The application of electric force causes micro-valves made up of plunge and solenoid coil, which is

Volume 9 Issue 2 (2023) 206 https://doi.org/10.18063/ijb.v9i2.663