Fig. 2: The interplay among gel, spheroid, and bioprinting process parameters and its role in spheroid viability and shape.

a, b Rheological properties of Carbopol at different concentrations and 0.5% alginate microparticles. The gels showed shear-thinning properties indicated by decreasing viscosity with shear rate. c The bioprinting accuracy of the yield-stress gels (with respect to spheroid size) (n = 5; *p < 0.05 and **p < 0.01). The positional precision for 0.8%, 1.2%, and 1.6% concentrations of Carbopol and 0.5% alginate microparticles were observed to be ~97%, 22%, 12%, and 34%, respectively. The colors correspond to the legend of panel (a). d, e Confirmation of the theoretical approach using the experimental validation for spheroids ranging from 150 to 450 μm in radius bioprinted in 1.2% Carbopol and 0.5% alginate microparticles. Note that human mesenchymal stem cell (MSC) spheroids were utilized in all experiments. The theoretical relation was plotted according to Eq. (6). f Cell viability of MSC spheroids in different yield-stress gels over 3 days (note that free standing MSC spheroids were used as a positive control, n = 3; **p < 0.01 and ***p < 0.001). g–j Cell viability and circularity of MSC spheroids at different bioprinting speed and aspiration pressure in alginate microparticles (n = 3; *p < 0.05, **p < 0.01 and ***p < 0.001). Increasing the bioprinting speed from 0.5 to 2.5 mm s⁻¹ did not reduce the cell viability when the aspiration pressure was maintained constant. However, increasing the aspiration pressure from 70 to 170 mm Hg decreased the cell viability. On the other hand, increase in the bioprinting speed did not significantly change the circularity of spheroids under a given aspiration pressure, whereas, increasing the aspiration pressure from 70 to 170 mm Hg increased the deformation of the spheroids and reduced their circularity. Error bars were plotted as mean ± standard deviation.