Rheological properties of gellan gum for bioprinting

Gellan gum is linear anionic polysaccharide derived from bacteria fermentation. Its powders can be hydrated at a high temperature where the polymer assumes a random coil conformation. During cooling, the transition to double-helix leads to the self-assembly of the helices in clusters of anti-parallel structures joined together by untwined parts of the polymer. Gellan gum is liquid a room temperature if lower concentration than 1.5% w/vol in water are used. This way, cells can be suspended in the solution and can be entrapped in the hydrogel that forms when crosslinking ions are present. Differently from the more commonly used alginate that can be dissolved in Dulbecco Phosphate buffer or 0.9% Sodium Chloride, gellan gum immediately forms gels or thick solutions in the presence of any positive ions. In water, cells are susceptible to osmosis thus, if cells are resuspended in water-based polymeric solutions for prolonged time like in some processing techniques such as bioprinting or microfluidics, that phenomenon is particularly important. In our previous work (1) ⁠, we showed that sucrose could be used to protect cells from osmotic damage when isotonic solutions cannot be used during processing. In this present work, we investigated in deep the rheological properties of these gellan gum solutions of different concentration and how these can be affected by cells in suspension. Gellan gum solutions showed a marked thixotropic shear thinning character with an apparent yield stress due to their tendency to form microstructures at room temperature. At room temperature, the presence of cells acted as an obstacle to the flow of the fluid but at high deformation rates cells deformed resulting in a less marked effect. At a physiological temperature, cells were even more deformable and their presence did not affect the viscosity of the solution in all the conditions tested. Furthermore, we modeled the viscosity of gellan gum comparing different mathematical models to predict the pressure and force needed to flow the solution through a capillary. These results are important to design appropriate extrusion processes for the delivery and manipulation of GG and for further processing cell-containing GG hydrogels. Acknowledgments: Consolidator Grant Project “ECM_INK” ERC-2016-COG-726061 (APM).