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International Journal of Bioprinting 3D bone: Current & future

cartridge. However, the force required for droplet all bioprinting methods. However, the cost of these
formation is not generated from within but between bioprinters is high and has an approximately small
the nozzle and the surface of the build plate. build volume.
45
Additionally, pressure is applied to the hydrogel (v) Acoustic bioprinting: Acoustic bioprinting is an
using a piston similar to extrusion techniques by ultrasound-based nozzle-free droplet printing
electrohydrodynamic bioprinters. As a result, a technique. The hydrogel is loaded into a microfluidic
constant meniscus of the hydrogel is formed on the reservoir with piezoelectric rings that generate
aperture of the nozzle. A high DC voltage power circular ultrasound waves. These waves form an
source is applied to the collector substrate while the acoustic focal plane at the air–ink interface at the
nozzle is grounded, creating a strong electromagnetic opening of the channels. The channels are open
field between the substrate and the nozzle and from the bottom, but the bioink drips only when the
leading to an accumulation of electrical charges in force of the ultrasound exceeds the surface tension.
the meniscus. When the electrical charges exceed A substrate, closely placed under the holes, collects
the surface tension, a drop leaves the nozzle through the droplets. As this technique does not apply any
the wide-operated electrohydrodynamic jet printers. electric charge, heat, excessive pressure, shear stress,
Therefore, a single drop is produced by low electric
voltage, field strength, and flow rate. Likewise, a or shocks, it can avoid nozzle damage to the cells.
continuous stream can be achieved with high electric However, the timing of the droplets may be affected
field strength and flow rate via electrospinning. by external movements or forces due to the wide gap
39,46
Due to the strong driving force and ability to create between the substrate and the fluid surface.
droplets smaller than the nozzle size, this technique 4.1.3. Vat polymerization-based bioprinting
enables single-cell printing. 43 Vat polymerization-based bioprinting utilizes a vat of
(iii) Microvalve-operated bioprinting: Similar to inkjet photocurable liquid to construct a 3D model layerwise
and electrohydrodynamic bioprinting, the bioink with a digitally controlled light source (i.e., ultraviolet
is filled in a cartridge and pushed out through a [UV] or blue light as a focused beam or projection). The
nozzle, and microvalve-operated bioprinting also only physical movement present is the table along the Z
uses drop-on-demand type printers. However, axis. The bioresin can be polymerized via top-down and
droplet formation does not occur due to force on the bottom-up methods. However, vat polymerization-based
hydrogel, but instead with a mechanical valve. The bioprinting requires additional materials for printing
valve comprises a solenoid coil and a plunger that and post-processing. Moreover, the number of suitable
blocks the aperture of the nozzle. An applied voltage materials is limited, and cell viability is reportedly lower.
creates a magnetic field in the coil that pulls the Despite the limitations, the technology features a high
plunger upward. Similar to the electrohydrodynamic resolution and may not require a support system (Table 4),
method, pressure is applied to the hydrogel to leave suggesting its application in printing cell-free hard
the nozzle as a droplet when the valve opens. The supporting scaffolds and experimental devices, such as
main advantages of this printing technique are the microvascular vessels and channels. 47,48 Vat polymerization-
adjustable droplet size and high cell viability. 39,44 based bioprinting techniques can be further categorized
depending on the photo-crosslinking mechanism:
(iv) LAB: LAB is based on the laser-induced forward stereolithography (SLA), digital light processing (DLP),
transfer method. The main parts of laser-assisted and two-photon polymerization (Figure 10). These types
bioprinters are the optical system, donor ribbon, and of vat polymerization-based bioprinting are discussed
receiving substrate. The ribbon is typically covered as follows:
with an absorbing metal layer on the upper side and
an adhesive layer with bioink on the lower side. The (i) SLA: SLA uses a laser source to cure bioresins via
optical system produces and focuses a laser beam on bottom-up or top-down order. In top-down printers,
the top of the donor substrate, resulting in localized the optical system is located above the vat, and the
evaporation and bubble formation. Subsequently, the building platform is submerged in the bioresin
high-energy bubble ejects a droplet from the hydrogel from below. The laser unit directs a UV beam to a
layer that lands on the substrate. Laser-assisted movable mirror, where a Galvo motor system steers
bioprinters are advantageous in many aspects, such the focused beam to the resin surface, drawing the
as print speed and high resolution, and they can layer by scanning. After a layer is imaged, an elevator
handle high cell-content bioinks. In addition, this system moves the building platform down, and a
technique provides the best cell viability rate among recorder blade moves across the platform to apply

Volume 10 Issue 3 (2024) 157 doi: 10.36922/ijb.2056