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International Journal of Bioprinting Application and prospects of 3D printable microgels

structures. Simply put, microgels, which are formed from simulate the in vivo structure of neural tissue. Microgels
hydrogel–microgel with certain rigidity, are materials that have the characteristics to solve the above difficulties,
can smoothly transition from solid state to liquid state, and so they have great potential in 3D bioprinting of neural
vice versa. Therefore, under the impact of printing injection tissue. The regeneration of cartilage tissue is also a focus
ports, the microgel liquefies and quickly solidifies after of 3D bioprinting, as cartilage has limited regenerative
the external force is removed, forming the desired shape. capabilities and can be broadly categorized into transparent
This microgel printing method eliminates the influence of cartilage, fibrous cartilage, and elastic cartilage, which have
surface tension, gravity, and particle diffusion, enhancing the structural and physiological differences. Osteoarthritis
design freedom of the printed structure. Bhattacharjee et al. and rheumatoid arthritis are the most common diseases
utilized microgel support baths to print a range of shapes that lead to damage in joint cartilage, and the number
with high aspect ratios, demonstrating the precision and of patients suffering from these conditions is increasing
versatility of this method [124] . This type of microgel-based annually [156,157] . Normal joint cartilage is an inhomogeneous
printing strategy is versatile and can be extended to various layered tissue, and this layering is crucial for maintaining
fields in biomedicine. Compaan et al. utilized this strategy the structure and function of cartilage [158-160] . Thus, the
to print the fundamental structure of a vascular system, particle heterogeneity of hydrogels and higher printing
namely well-connected branching channels formed by fork precision can ideally replicate this layered structure for 3D
printing paths [123] . Jeon et al. employed this strategy to print bioprinting. In summary, the recent progress of hydrogels
high-resolution complex 3D structures for the purpose for 3D bioprinting has been impressive, and it can be
of repairing bone defects or cartilage injuries [153] . Taken anticipated that this field will continue to grow and evolve
together, microgel support baths enable precise and high- in the coming years, providing solutions for reconstructing
resolution printing, providing a new strategy for printing intricate and complex organs and replicating specificity of
complex organ structures such as vascular and neural tissues. functional tissues.

One aspect of 3D bioprinting that has rapidly developed The use of microgels in 3D bioprinting is still in
in recent years is the research on new bioinks. Microgels, as the initial stages of exploration, with many different
bioinks, are able to replicate specific ECMs while maintaining approaches being tested, evaluated, refined, integrated, or
high cell viability and activity, and induce organ-specific abandoned. In this review, we focus on the use of microgels
cell behavior. As a new type of bioink, microgel not only in extrusion-based 3D printing, a technology that we
have the advantages of common hydrogels, but also make believe offers unique advantages in terms of simplicity,
up for their deficiencies, making them more suitable for versatility, and performance. The future trend of extrusion-
cell survival and ensuring the performance of biological based 3D printing is to print multi-layered, high-precision,
functions. In addition to maintaining cell viability, microgel biologically functional tissue structures, and to achieve
can enhance mechanical properties by adjusting the this goal, extrusion-based 3D printing in the form of
particle size and assembly mode, thus meeting the stability coagulation and support baths has been proposed in earlier
requirements of various organ structures. The complexity studies, and even further coaxial extrusion systems have
of organ structure is also a difficulty in 3D bioprinting. been proposed. Despite the limitations related to materials,
By using a microgel support bath, higher degrees of microgels are perfectly adapted to the support bath system.
freedom can be achieved for precise printing, providing Therefore, in the future, the extrusion-based 3D printing
an effective solution for printing precise tissue structures. approach should probably focus on the support bath system
Although the research prospects of microgel as a new of microgels. Second, the development of heterogeneity of
type of biological ink are very attractive, many challenges microgels is the focus of future development. Based on the
still need to be overcome in order to achieve clinical complexity of the structure and function of cells, tissues
translation. To achieve ideal 3D bioprinting, it is necessary and organs, the interconnection of multiple cells, growth
to recreate the complex structure and biological function factors and cellular microenvironment should be the focus
of tissues and organs. For example, vascular networks, of 3D bioprinting in future. The availability of a diverse
nervous tissue, and cartilage tissue all have complex array of microgels enables 3D bioprinting involving
structures and functions [125,154,155] . At present, vascularized multiple cells or growth factors from the bottom up. In
tissues or independent microvascular network structures addition, microgels with micron-sized pores are more
can be printed through the use of microgel support baths. suitable for simulating cellular microenvironment.
There are two difficulties in printing neural tissue: (i) Acknowledgments
the printing medium must have good cell viability and
sufficient pores for nerve cells to extend and communicate; We are grateful for the support of Life Science Institute of
and (ii) it is necessary to print fine structures that can Chongqing Medical University.

Volume 9 Issue 5 (2023) 103 https://doi.org/10.18063/ijb.753