Biofabrication techniques provide the opportunity to construct tissue equivalents with defined size and geometry, combined with accurate positioning of cells and matrix components. In regenerative medicine, those procedures allow the patient-specific production of tissue replacement, for example bone substitutes. Still, the search for materials that are processable with the used printing technique and at the same time support the function of the tissue specific cells is ongoing. Therefore, bio-inks based on methacrylated gelatin (GM) were developed that either support the osteogenic differentiation of encapsulated stem cells as well as the osteogenesis in the hydrogels, or allow the formation of capillary-like structures, a prerequisite for tissue vascularization.
Both developed bioinks – one for softer hydrogels to support vascularization and a hydroxyapatite-laden one for osteogenic differentiation - were shown to be suitable for the printing of relevant hydrogel geometries with a microextrusion system. After culturing human adipose-derived stem cells (hASCs) and human dermal microvascular endothelial cells (hDMECs) in a co-culture layout for 14 days, the analysis of osteogenesis in the co-cultured hydrogels showed an increased matrix formation compared to the hydrogels cultured in monoculture, recognizable by an increase in elasticity and an intensified staining of bone-specific proteins. In the hydrogels containing hDMECs, the formation of capillary-like structures and networks was far more pronounced and more stable in the hydrogels cultured in close proximity to the osteogenically differentiating stem cells, while in monoculture the network formation was limited.
The results show that the developed hydrogel materials could be suitable matrices for the printing of vascularized bone tissue equivalents. The printing of relevant geometries was possible, as well as the proof of osteogenesis and vasculogenesis in the hydrogels. The synergistic effect of osteogenic differentiation and blood vessel formation onto each other could be shown and reveals the benefit of co-culturing the different types of cells for the eventual formation of functional tissue equivalents.