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International Journal of Bioprinting Multi-Cellular tissues/organoids manufacturing strategies

photopolymerization of orderly extruded multi-materials
(POEM) has been proposed, developed, and fully
characterized , and a novel workflow utilizing machine
[44]
learning to achieve precise control over the stiffness of
elastomeric scaffolds enables the fabrication of multi-scale
structures with MTOs applications . However, their
[45]
applications in bioprinting are limited due to the harmful
effects of UV rays (resulting in more DNA damage with
shorter wavelengths) and cytotoxic effects from increased
light intensity and photoinitiator concentration. To
mitigate DNA damage, the use of visible light sources and
biocompatible photoinitiators is necessary. The drawbacks
of VP-based bioprinting exceed its benefits. The inherent
characteristics of this technique render it inappropriate
for generating complete tissue constructs that meet the
requirements of regenerative medicine . Nevertheless,
[46]
this approach can be employed to achieve precise
patterning of multi-cellular microenvironments, such as
the tumor microenvironment, for the purpose of disease
modeling or drug testing .
[47]
The scaffold is used to provide material support and a
conducive cell growth environment for MTOs. An ideal
scaffold should possess sufficient mechanical stability and
biocompatibility for cell viability. Excessive cell aggregation Figure 4. Different forms and morphological structures of scaffolds for
leading to hypoxic necrosis is a major reason for the failure biomedical applications. Reprinted from Ref. [48] , Creative Commons
Attribution 4.0 International (CC BY 4.0).
of MTOs fabrication. Therefore, the use of porous structures
should be maximized to increase the cell attachment
surface area and maintain the good breathability of MTOs. in specific bioassembly techniques, akin to constructing
Additionally, non-toxic and safe biocompatible materials Lego bricks while incorporating a culture medium to
should be employed for the scaffold. Thin film and fiber support cell survival and fusion during the biofabrication
structures can complement the mechanical-support process [10,49] . The foundation of this strategy lies in cell–
stability and serve as plastic-support structures on the outer cell adhesion, a critical regulatory factor governing the
surface once MTOs are initially formed. Therefore, the morphology and functionality of MTOs both in vivo and
optimal scaffold structure should involve a combination of in vitro . Numerous studies have concluded that cell-
[50]
these four types of structures, with a particular emphasis aggregated spheroids exhibit excellent scalability and
on the rational use of porous structures. According to controllability. In the scaffold-free strategy, aggregate
the classification based on structural forms, the scaffolds spheroids serve as the smallest fabrication unit, mimicking
produced through 3D bioprinting can be categorized the cell–cell and cell–extracellular matrix communication
into the following types: porous, hydrogel, fiber, and film and interactions observed during embryonic development
scaffolds , as shown in Figure 4. Porous structures provide and MTOs construction .
[48]
[51]
a suitable environment for ECM secretion and nutrient
delivery, while hydrogel structures are highly biocompatible The distinguishing feature of the scaffold-free strategy
with controlled degradation. Fiber structures promote cell is that the manufacturing of MTOs does not rely on
behavior but require surface functionalization, and film scaffolds or only requires temporary scaffolds. Instead,
structures maintain natural ECM but necessitate partial it relies on the precise assembly of building units and
decellularization. subsequent cultivation of MTOs. Replicating complex
3D organ structures and tissue microenvironments using
3. Scaffold-free strategy with bioassembly conventional scaffold-based 3D bioprinting technologies
techniques is challenging due to the inability to precisely define the
spatial distribution of cells . Moreover, as scaffold-
[52]
The scaffold-free strategy represents an alternative based approaches advance, issues and limitations such as
approach for MTOs fabrication, where cell-aggregated immune responses, infection risks, and potential disease
spheroids and microtissue building blocks are assembled transmission associated with bioinks and biomaterials have

Volume 9 Issue 6 (2023) 206 https://doi.org/10.36922/ijb.0135