Control your 3D Bioprinting Hydrogels

Aug 24, 2019

(Featured Image: Planar lattice structure and b)

Exciting things are happening in the fields of tissue engineering and regenerative medicine, all thanks to 3D bioprinting. The high precision and convenient operation allow 3D bioprinting to expand into new areas. Current research efforts all over the world are now leading to innovations in regenerative medicine, vascularized printed organs and replicating organ functionalization. However, to attain reproducibility or to print clinically relevant forms and sizes, material control is important. The implementation of bioprinting relies on the geometrical accuracy of replicating the design file. This, in turn, depends on printing parameters and material properties. Studying and understanding the correlation between hydrogel material characteristics and successful bioprinting is important to obtain functional 3d constructs to fulfill intended applications. Much research has been done on ink properties and printing optimization. However, little attention has been paid to the relation between hydrogel parameters and printing fidelity. Most of the users are uninformed about the properties of hydrogels that should be kept in mind for obtaining a good 3D printed construct for the intended application. There are many material parameters that influence the printing process and resolution directly and can be tuned to achieve a finely tuned process.

3D multilayer structure printed using Alginate bioprinting hydrogel (bioinks)
3D multilayer structure printed using Alginate hydrogel bioinks

Be mindful of Gelation Time

Not all hydrogels are created equal. Gelation time in hydrogel varies according to their crosslinking chemistry. This parameter has to be kept in mind when choosing a hydrogel for a particular application. Collagen has been found to be the slowest to gel so far [1]. 

How is the swelling?

Swelling is defined as the ratio of the mass of swollen hydrogel to the mass at the equilibrium. Swelling contractile characteristics can be of central concern for producing biological constructs especially for skin and wound applications. The swelling rate also governs the stability, as one would want the bioprinted hydrogel to be in place for a sufficient amount of time.

Against the gravity

Care has to be taken to avoid diffusion of hydrogels in one another when overlapping layers are printed. The diffusion phenomenon will make the printed pores to decrease in diameter when printing more layers. For a lattice structure, with an increase in line distance, the diffusion rate can be slowed down thus reducing the diffusion effect (fig 1).

Figure 1: Graph depicting the relationship between line distance and diffusion rate in a lattice structure (reprinted with permission from ref. 2)].

Figure 1: Graph depicting the relationship between line distance and diffusion rate in a lattice structure (reprinted with permission from ref. 2)]. 

Control your curves

Fabricating sharp angles through 3D bioprinting is a challenge. Sharp bends create material overlap resulting in non-uniform thickness, which eventually leads to printing failures. Most of the time the construct has to be re-designed to avoid such issues. An alternate way is to reduce the extrusion rate for such prints.

The Game of Viscosity

This material property is the most important and well-understood for printing. For hydrogels, the viscosity (η) should ideally lie between 300-30000 cps (centipoise) for most printers. Although attempts have been made to print hydrogels with η˃30000 cps using higher pressure systems, however, the printing is not reproducible and stable with time. 

References:

[1]. S. V. Murphy, A. Skardal, A. Atala, Evaluation of hydrogels for bio-printing applications, Journal of Biomedical Materials Research 101A (2013) 272-284.

[2]. Y. He, F. F. Yang, H. M. Zhao, Q. Gao, B. Xia, J. Z. Fu, Research on the printability of hydrogels in 3D bioprinting, Scientific Reports 6 (2016) 29977.

About the Author:

Shweta Agarwala is Assistant professor at Department of Engineering, Aarhus University (Denmark). Dr. Agarwala graduated in electronics engineering from Nanyang Technological University, Singapore and obtained her Ph.D. in the same field from the National University of Singapore. Her research is directed towards printed electronics for flexible devices and bioelectronics. She is pioneering new routes to put electronics on unconventional surfaces to enable future generation healthcare. 

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