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Ren, et al
1. Introduction with other types of cells. Recent bioprinting techniques
have been used to overcome some of these shortcomings.
An organoid is a three-dimensional (3D) model, similar Bioprinting is an additive manufacturing technology
to the source tissue or organ in vivo, of an in vitro cell that can design and selectively distribute cells, bioactive
culture system. . In addition, an organoid is a collection materials, and cytokines to construct 3D living organs
[1]
of organ-specific cells that are developed from pluripotent and tissues . Bioprinting defines two types of fabricated
[7]
stem cells (PSCs) or adult stem cells (AdSCs). They can structural properties or smart surface properties of cell-
self-form in a similar manner to the body through cell free constructs characterized by layering, which guides
sorting and spatially restricted lineage differentiation [2-4] cellular activity and cell-filled biological constructs .
[7]
(Figure 1). At present, animal models are typically Before the rise of biomanufacturing technologies, the
used for experiments, such as drug screening, disease fusion of developmental biology and cellular self-
modeling, and tissue regeneration and repair; however, organization has emerged as a new paradigm for
such models do not accurately reflect the physiological advancing tissue engineering. The formation of self-
characteristics of the human body . The new in vitro organizing multicellular modules is a key step in this
[5]
model organoid bridges the gap between animal models technology; however, the ability to assemble intermediate
and humans by replicating the cellular composition modules into larger tissue units in a controlled manner is
and behavior of a normal organism meticulously to a major challenge . Thus, the synergistic development
[8]
recreate the physiological structure of human organs. of bioprinting and cellular self-organization technologies,
The construction of organoids offers the advantages of working in tandem, can significantly facilitate the
individualization, short modeling times, high-throughput functionalization of organ tissues. Bioprinting can apply
genetic or drug screening, and the possibility of gene a specific spatial architecture design similar to actual
editing . The organoid compensates for the shortcomings organs for PSCs or AdSCs so that the specific structure
[6]
of simple cellular models and complex animal models, of organoids can be quickly printed with high-precision
and provides an important experimental basis for studying and high-throughput. Accordingly, they can self-organize
key functions of living organisms. Organoids have better and faster to form organoids. In addition, stem
become a current research hotspot with great theoretical cell suspensions can self-organize into millimeter-scale
significance and broad development prospects in basic structures, which contain only moderate complexity, and
biology research, drug testing, and molecular medicine . can be further printed into more complex tissues and
[6]
However, the construction of organoids poses certain organs, with the use of the resulting geometry to guide
limitations, such as the inability to fully simulate the organoid formation.
in vivo microenvironment, inadequate vascularization, Here, we review existing bioprinting methods
the slightly different size of the self-organization of and bioinks, highlight the recent success stories in the
organoids from that of normal organs and lack of precise field of organ bioprinting, and summarize strategies
spatial ordering, and unestablished co-culture system and possible directions for future organoid bioprinting.

Figure 1. Organoids can be identified based on PSCs or AdSCs that are created autotrophically. PSC-derived organoids need to pass through
the endoderm, mesoderm, or ectoderm, and then be induced and matured by applying certain growth signals and then differentiate into
the phenotype required for a particular organ. A few PSC-derived organoids may be determined from numerous distinctive germ layers of
cells. AdSC-derived organoids require segregation of tissue-specific stem cell populations, which are at that point implanted in extracellular
matrix (ECM) and engendered in combination with particular tissue development components.

International Journal of Bioprinting (2021)–Volume 7, Issue 3 19