Gellan Gum-based Inks: A step aside from the ordinary

3D bioprinting technology brought to the tissue engineering field the possibility of developing complex tissues with high reproducibility. Despite the acknowledged advantages of this technology, bioengineers are facing innumerous challenges on adapting the biomaterials previously optimized for tissue engineering, to inks for bioprinting. Up to now, collagen, fibrin or gelatin are the most used materials as inks for bioprinting owed to their rheological and biological cues. Nonetheless, issues of fast degradability are recurrently documented post-printing. Thus, herein we will introduce a new ink based on gellan gum (GG), its advantages and all the struggles overthrown throughout its adaptation for bioprinting. We aimed to develop a printable ink in which printed cells could adhere, proliferate and maturate to form a functional tissue surrogate. To attain this goal, we developed different ink formulations of GG combined with GG biofunctionalized with RGD^1,2. The printability of the inks was optimized by triggering a sol-gel transition right after printing, by tailoring ink temperature pre-extrusion and prompting a fast crosslinking with an ionic crosslinker post-extrusion. The printing speed, needle diameter and layer height were also optimized for each ink. The adaptation of the ink to different cell types isolated from the human skin tissue was also performed. It is well known that cells respond differently to hydrogels of different mechanical properties and this was confirmed with our developed inks, as human dermal fibroblasts (HDFBs), human microvascular dermal endothelial cells (HMDECs) and human adipose-derived stem cells (HASCs) preferred stiffer inks since they behaved better and similarly to control conditions. We acknowledged that cell density was a key factor for the communication of the encapsulated cells and hence it was tailored for each cell type. Interestingly, cell behavior also differed among populations obtained from different donors which highlights the need of a broader study to present reliable and reproducible results. HDFBs and HDMECs viability was not affected by printing but HASCs viability decreased by 10%. Overall, cells were capable of colonizing the hydrogel one-week post-printing without affecting the mechanical structure of the hydrogel, evidencing GG-based inks as suitable and advantageous inks for the bioprinting of different skin cell types.