3D bioprinting is a promising technology to fabricate complex tissue replacements layer by layer through the deposition of cells and biomaterials in a predefined path. It allows us to fabricate complicated geometries that are impossible to obtain through conventional manufacturing methods . Before printing, a computational design for the intended tissue construct needs to be generated together with a corresponding path file. The bioprinter follows the provided printing pattern to create the desired product from the prepared material, e.g., a hydrogel. During the fabrication process, the printability of the hydrogel plays an important role to achieve high accuracy and a good agreement between the printed structure and its computational design. However, it is often challenging to appropriately control the printability of hydrogels.
In this study, we have introduced a cooling step and carefully adapted the nozzle temperature to optimize the printability of hydrogels. We have used an alginate-gelatin hydrogel system of 2 % (w/v) alginate - 5% (w/v) gelatin as it provides a cell-friendly environment and is easy to prepare and use. We examined both the mechanical properties and printability of the hydrogel. Due to the long gelation time and the constant change in viscosity as well as printability during fabrication, it was initially impossible to print a well-defined structure precisely. Through a cooling step before the bioprinting process, we induced early thermal gelation and assessed its effect on the hydrogel properties. Moreover, we studied the effect of nozzle temperature on both mechanical properties and printability to identify a suitable temperature for printing.
Our results show that the cooling process stabilizes the hydrogel viscosity at different temperatures. In addition, we identified an appropriate cooling time for optimal gel stability. By decreasing the nozzle temperature, we could also decrease the viscosity variation, but it resulted in lower extrudability and uniformity of the gel. In conclusion, the cooling step is beneficial for the bioprinting of gelatin-based hydrogels, and it is key to identify the appropriate temperature to achieve precise printing patterns. Our presented approach will also be applicable for cell printing studies. Moreover, the printability of other concentrations of alginate-gelatin hydrogels could be improved by using the introduced process. Finally, the presented analyses form the basis for computational simulations in the future that will allow us to numerically tune the mechanical properties of the tissue constructs towards those of native tissues .