Updated: Apr 7, 2021
3D bioprinting has emerged in the field of tissue engineering as a powerful tool to enable the creation of complex tissue and organ structures with different components and intrinsic microvasculature. However, there are still several challenges to overcome in order to obtain these structures, such as the optimization of the biomaterial, or bioink, biocompatibility and printability. The printability of a bioink is dictated by the printing technique and technology: overall, the biomaterial needs to satisfy several essential criteria to achieve long-term shape constancy and reproduce complex micro-architecture of native tissues in vitro [1,2]. Different bioinks require specific printing parameters, which vary significantly among biomaterial types and 3D printing technology. Each bioprinting technology is compatible with certain gelation processes and also requires bioinks that show specific mechanical and physical properties.
The printability is usually determined regarding the controllable formation of well-defined droplets/jets/filaments and by the shape fidelity and morphology of deposited building blocks . In the case of extrusion-based 3D printing, for instance, parameters such as nozzle gauge, shear stress, fabrication time and speed, as well as bioink viscosity, gelation time and network properties, define the suitability of the biomaterial for the desired application.
The assessment of bioink printability is crucial for selecting ideal printing parameters and bioink formulation . A paradigm named "the biofabrication window" defines the relation between printability and the ability to encapsulate cells and maintain cell viability. High concentrations of some polymeric bioinks provide higher degree of crosslinking and stiffness to the printed material, but these networks often hinder cell migration, proliferation and matrix deposition. At the same time, less dense networks may collapse after printing, leading to poor shape fidelity. Nevertheless, networks with a lower degree of crosslinking and/or polymer concentration exhibit a good cell migration and proliferation rate. This paradigm affects the ideal compromise between the bioink for 3D printing and cell culture .
Figure 1: Key aspects to assess printability .
He et al. have systematically investigated the printability of alginate/gelatin hydrogels in 3D structures with a special focus on the accurate printing. They analyzed the influence of factors such as air pressure, flow rate, and printing distance on the printing quality of the expected 3D structures. The authors observed that the printing resolution can be affected by the diffusion and fusion of the bioinks, which could be avoided by reducing the extrusion rate or accelerating the moving speed for the material deposition. The cell viability after printing was tested and compared to materials obtained by casting. The results showed that this bioprinting method had almost no extra damage to the cells .
Figure 2: Assessment of the printability of alginate/gelatin hydrogel scaffolds by 3D printing. (a) The digital model of the 3D scaffold; (b) The outline of the first layer of hydrogel scaffold; (c) The first layer of scaffold shape after filled the blank area; (d) A view of printed hydrogel scaffold from three different angles .
As already mentioned, highly viscous hydrogels, consisting of dense polymeric networks, present a very good retention of shape after printing, but decreased cell viability due to the higher shear stresses to which cells are exposed during extrusion. Gao et al. established a quantitative technique to measure printability using extrudability, structural integrity, and extrusion uniformity of gelatin-alginate composite bioinks. The impact of rheological properties on a bioink performance were also demonstrated. Gelatin and alginate were mixed at various concentrations to obtain formulations with a wide range of storage and loss moduli. The study showed how a higher loss tangent (ratio of loss to storage modulus) can improve extrusion uniformity while a lower loss tangent can improve structural integrity. The ideal loss tangent window found for the gelatin-alginate concentrations evaluated may not be the same at all other hydrogel-based bioinks, however the framework presented in this study may serve for future progress in bioink development, allowing the fabrication of more complex, cell-based, soft tissue constructs in the future .
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 Zhang, Zhengyi, Yifei Jin, Jun Yin, Changxue Xu, Ruitong Xiong, Kyle Christensen, Bradley R. Ringeisen, Douglas B. Chrisey, and Yong Huang. 2018. “Evaluation of Bioink Printability for Bioprinting Applications.” Applied Physics Reviews 5 (4): 041304.
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 He, Y., F. F. Yang, H. M. Zhao, Q. Gao, B. Xia, and J. Z. Fu. 2016. “Research on the Printability of Hydrogels in 3D Bioprinting.” Scientific Reports. https://www.nature.com/articles/srep29977.
 Gao, Teng, Gregory J. Gillispie, Joshua S. Copus, Anil Kumar Pr, Young-Joon Seol, Anthony Atala, James J. Yoo, and Sang Jin Lee. 2018. “Optimization of Gelatin–alginate Composite Bioink Printability Using Rheological Parameters: A Systematic Approach.” Biofabrication 10 (3): 034106.