Page 540 - IJB-10-2

International Journal of Bioprinting Bottom-up and top-down VAT photopolimerization

and channels, in which cells can expand and grow, and generally uses ultraviolet (UV) light to selectively solidify
finally, build functional organ parts in vitro [6-8] . However, a light-sensitive bioink layer by layer. It can produce 3D
native tissues and organs possess inherent heterogeneity in structures with good resolution and can be combined with
physical, mechanical, and biological properties . In order hydrogel-based bioinks, allowing significant control over
[9]
to capture this heterogeneity, the combination of multiple the resulting matrix properties [24,25] . Similar to SLA, DLP
materials with distinct mechanical properties as well as is a technology based on the layer-by-layer crosslinking of
different cell types is necessary. Recent advancements in photosensitive inks by exposure to a projection mask .
[26]
tissue engineering evidenced the fabrication of multi-tissue The projection of light can be achieved via liquid crystal
models, which can closely imitate organ-to-organ interfaces, displays (LCDs) and digital micromirror devices (DMDs),
in order to investigate the complex interactions and monitor which focus the light as square voxel patterns on the surface
the dynamic responses of multiple organs to pharmaceutical of the ink . Due to the non-contact nature of light, SLA
[27]
compounds. Multi-tissue microfluidic platforms have been and DLP are attractive bioprinting methods because they
developed to model lung , liver , intestine , kidney , do not have the problems associated with contact-based
[12]
[13]
[11]
[10]
brain , and heart . Thus, it becomes apparent that the use bioprinting (e.g., clogging, shear stress) [22,28] .
[14]
[15]
of 3D bioprinting for the recreation of microphysiological
systems of tissue–tissue interfaces can expand the Current bioprinting methodologies are mostly limited
[29]
capabilities of the existing tissue models [16-18] . by printing single material tissue models , and the
available multi-material are less cost-effective and time-
The bioprinting technologies currently used for the consuming . Additionally, they are not able to bioprint
[30]
fabrication of living tissue models can be divided into three cell-laden constructs with multi-material components
broad categories, namely extrusion- and jetting-based in relevant clinical dimensions. Extrusion bioprinting
bioprinting as well as VAT photopolymerization. Extrusion was demonstrated for multi-material human umbilical
bioprinting uses pressure generated by a pneumatic or vein endothelial cells (HUVECs) . Likewise, 3D human
[31]
mechanical system, or a combination of both, to deposit renal proximal tubules were printed for replicating
cell-loaded bioinks . It is a relatively inexpensive process a human kidney-on-a-chip and retained viable for 8
[19]
that allows high printing speeds and provides precise weeks . The bioprinting time of the methodologies is
[32]
control over the porosity and mechanical properties of in the range of a few hours, indicating that extrusion 3D
the final constructs, making it an attractive method for bioprinting lacks acceptable printing time and resolution of
producing harder tissue scaffolds. Extrusion has been microtissue material fabrication; therefore, multi-material
used at high cell densities, but the choice of available methodologies able of using multiple bioinks for the rapid
biomaterials is relatively limited, as the rheological manufacture of hydrogel-based constructs are still required.
properties of the biotins are critical to cell viability.
Apart from the properties of the bioinks, the printing More recently, it has been demonstrated that a multi-
speed directly affects the shear stress to which the cells material DLP-based bioprinter was developed to fabricate
[26]
are subjected during printing. Therefore, the resolution high-resolution microtissues . In addition, Miri et al.
of the final scaffold may need to be limited to avoid cell used a multi-material DLP bioprinting of hydrogel-based
[28]
apoptosis . On the other hand, inkjet bioprinting is based microfluidic chips , while Zhu et al. applied a similar
[20]
on continuous (continuous inkjet, CI) or discrete (drop- DLP bioprinting for making a pre-vascularized (channels
[33]
on-demand, DOD) deposition of a liquid biomaterial seeded by HUVECs) tissue models . These studies
onto a substrate. This bioprinting method has been used suggest the capacity of DLP-based bioprinting for creating
to produce cell-loaded scaffolds with high resolution and multi-material microtissues [34,35] .
good shape fidelity, which are good candidates for soft Most light-based bioprinters are based on a “top-
tissue bioprinting . However, the major disadvantage of down” printing approach, where a UV light source located
[21]
inkjet bioprinting is the risk of needle clogging, which can below the bath of the photosensitive biomaterial is used
interrupt the printing process and expose the cells to high to selectively polymerize it to generate a 3D structure in
shear stress, negatively affecting cell viability. a layer-by-layer fashion . The “top-down” configuration
[36]
VAT photopolymerization bioprinting technologies has been shown to work well with hard materials, resulting
include light-based methods, namely laser-based in scaffolds with enhanced mechanical properties that can
[37]
bioprinting, stereolithography (SLA), and digital light provide sufficient support . However, processing softer
processing (DLP) . Laser-based printing offers good materials with this method is still challenging. Recent
[22]
resolution and high fabrication speeds but has not been studies reported an alternative approach, by placing the
successfully combined with cells because of the problems light source for the projection of the printing pattern above
associated with using laser light with living material . SLA the photosensitive material [38,39] , introducing a “bottom-
[23]
Volume 10 Issue 2 (2023) 532 doi: 10.36922/ijb.1017