Design aspects and characterization of hydrogel-based bioinks for extrusion-based bioprinting

3D-bioprinting has become a valid technique for tissue and organ regeneration, as the printing of living cells is allowed while the hydrogel-based ink material provides them mechanical and structural support. Self-healing shear-thinning hydrogel inks can be considered most promising ink materials for extrusion-based bioprinting (EBB), because the ink can be extruded due to the decrease in viscosity under shear, and self-healed after removing the shear, which ensures safe printing of cells and shape fidelity after bioprinting. To achieve the best final bioprinting result, some printing technique, ink material and biological aspects of bioprinting need to be considered. In addition, the versatile characterization of pre- and post-printing properties of the inks helps to improve the final bioprinted constructs. However, despite the great advances in 3D-bioprinting, ink related challenges such as opposing characteristics, and lack of controllable micro-environment, or technological challenges such as the need to increase printing speed and print resolution must be resolved. In terms of ink characterization, more standardization is also needed. In addition, the computational modeling would help to improve the performance of the bioprinted construct. Thus, the future of 3D-bioprinting is going towards larger multifunctional tissue/organ constructs with multi-scale vascularization and innervation. Multiple printing techniques are probably combined, but also completely new techniques are needed. Further, multimaterial printing would enable heterogeneity and gradients to the construct. On the other hand, using 4D-bioprinting, the dynamic nature of complex organs could be added to the construct. By combining bioprinting with microphysiological platforms (tissue- or organ-on-a-chip systems) the development of functional tissues and organs intended for implantation would go forward. The translation of EBB into clinical practice is still in the early stages, but EBB has a great potential in regenerative medicine after the challenges, such as biomimicry, reproducibility or up-scaling related issues have been overcome. In this review, the design aspects related to extrusion-based bioprinting technique, the property requirements for ideal bioink, the biological aspects of 3D-bioprinting, and the characterization of the pre- and post-printing properties of bioinks are presented. Also, the challenges and future prospects of 3D-bioprinting are discussed.