Mechanobiological effects of 3D-printed hydrogel-calcium phosphate composite materials on multipotent human mesenchymal stromal cells

The stiffness of a hydrogel has a significant role on the biological cell response but can be tuned only within a limited range. We hypothesized that the addition of calcium phosphate particles in the hydrogel can increase the stiffness as well as promote the osteogenesis of human mesenchymal stem cells (hMSCs), applicable for 3D bioprinting. Beta-tricalcium phosphate (β-TCP) microparticles and hydroxyapatite (HA) nanoparticles were incorporated at 0.5 mg/ml and 5 mg/ml concentrations into agarose and agarose-collagen and were characterized based on stiffness, viscosity, degradation, cell morphology, viability, and osteogenesis. The results showed that the addition of calcium phosphate particles yielded mechanical and rheological changes in pure hydrogels with good printability, and hMSCs in composites with low concentration of 0.5 mg/ml β-TCP and HA showed more pronounced osteogenic differentiation compared to pure blends and composites with higher concentration of 5 mg/ml β-TCP and HA, as proved by ALP activity and quantitative polymerase chain reaction. In the future, this technique of tuning the stiffness could be further used to study the effects of extracellular matrix stiffness on different cell types towards various differentiation lineages such as adipogenesis and chondrogenesis.