Rheological properties of GelMA-CBP biomaterial inks. GelMA is a popular biomaterial in the tissue engineering and bioprinting fields because it is derived from a natural biopolymer, contains cell adhesion (RGD) motifs, is biodegradable, biocompatible, affordable, and the presence of methacrylate groups allows fast and controllable photo-crosslinking. However, the direct extrusion of inks containing low concentrations of GelMA at physiological temperature is challenging without introducing high concentrations of rheological modifiers (Figure ). In this work, we focused on developing a simple strategy to formulate GelMA-based bioinks that overcomes these challenges while achieving high printability and cell viability. The first step was the formulation of GelMA-CBP inks with low total solids concentrations that exhibited adequate rheological properties for extrusion 3D bioprinting at 37 ºC. To do so, the concentration of the photoinitiator (LAP) was fixed at 0.5% and the contents of GelMA and CBP were varied (Figure ). As expected, the GelMA inks without CBP were liquid at 37 ºC at all the tested concentrations. Although the viscosity significantly increased with the addition of 0.1% CBP, such inks were flowing liquids and, therefore, inadequate for 3D extrusion bioprinting. However, when CBP percentages were increased to 0.25% and 0.5%, most of the formulations behaved as gels that did not flow after inverting the vials, indicating they were promising candidates for extrusion bioprinting. The only exception was the ink containing 3% GelMA and 0.25% CBP, where the viscosity was not high enough prevent the gel from slowly flowing after inverting the vial. It is worth noting that 0.25% CPB dissolved in water behaves as a gel; however, when 0.5% LAP is included, the increase in the ionic strength partially disrupts the gel structure and causes it to flow (Figure , panel 3, vial 1). Nevertheless, we observed that at 0.25% CBP, the inks containing GelMA showed a higher viscosity than CBP-LAP alone and passed the inversion test. The fact that both GelMA-0.5%LAP and 0.25%CBP-0.5%LAP solutions are liquids that flow at 37 ºC, but the GelMA-0.25%CBP-0.5%LAP inks were gels, suggests that there are GelMA-CBP interactions that favor gelation. We hypothesize that the free amino groups of GelMA could electrostatically interact with the carboxylic acid groups present in the CBP microparticles, leading to an increase in the viscosity of the formulation prior to any photo-initiated crosslinking. (I) G' and G" of crosslinked pucks made from 3 (blue), 5 (red), and 7% (green) GelMA inks with 0.25% CBP. (J) G' and G" of crosslinked pucks made from 5% GelMA inks with 0.1, 0.25, and 0.5% of CBP. All the experiments were conducted at 37 ºC. Data is presented as the mean and standard deviation for n ≥ 3 independently prepared samples.