Microgel-based bioink for extrusion-based 3D bioprinting and its applications in tissue engineering

Extrusion-based 3D bioprinting is being increasingly adopted as a versatile biofabrication method for making biomimetic constructs in tissue engineering. However, the lack of ideal bioinks continues to limit its broader application. Conventional hydrogel-based bioinks typically possess a densely crosslinked nanoporous structure that hinders their ability to fully support cell behavior. Microgel-based bioinks have recently emerged as a promising alternative due to their enhanced printability and functionality. This review will begin with the evolution of the "bioink" concept, followed by a discussion on bioink categories and the requirements of ideal bioinks. It will then introduce hydrogel-based bioinks and their limitations, followed by a definition of microgels and microgel-based bioinks and a discussion of their key properties, highlighting their differences compared to conventional hydrogel-based bioinks. Topics on microgel-based bioinks are then presented in order of the printing process: pre-printing (fabrication of microgels and formulation of microgel-based bioinks), during printing and post-printing (microgel assembly kinetics). Uniquely, this review will examine the various applications of microgel-based bioinks in tissue engineering, summarizing their advantages and limitations. Finally, the current challenges and future perspectives of using microgel-based bioinks are discussed. This review comprehensively examines microgel-based bioinks for 3D bioprinting, highlighting their potential to overcome current challenges and setting the stage for their future applications in creating complex, functional tissue engineering scaffolds.