Design of bioink containing endothelial cell-seeded microcarriers for three-dimensional bioprinting of microvasculature

The vascularization of tissue-engineered constructs remains one of the critical challenges in tissue engineering (TE), limiting the viable size of constructs due to hypoxia, nutrient insufficiency, and waste accumulation. Three-dimensional (3D) bioprinting has emerged as a powerful fabrication strategy for TE. This strategy is based on depositing cells, bio-materials, and supporting components with controlled microstructure to fabricate functional tissue. Current challenges in 3D bioprinting of vascularized tissue are mainly due to limitations in printing resolution, especially regarding the creation of blood capillaries. To overcome these limitations, we design a smart bioink made of endothelial cells (EC)-seeded on microcarriers (MCs) loaded with angiogenic growth factors (AGFs) that are embedded within the bulk hydrogel. A droplet-based microfluidic system was utilized for fabricating alginate-RGD/sulfate MCs containing AGFs. The MCs were characterized by diameter, core and surface microstructure, and as a substratum for EC attachment to their surface. When embedding small (5-30 μm in diameter) EC-seeded-MCs within fibrin-based bioink, the MCs function as seed nuclei from which sprouting occurs much faster than in the control group that contains suspended ECs without MCs. The constructs displayed greater capillary area and density, greater average capillary length, and fewer capillary endpoints, meaning high connectivity between vessels, as determined by Angiotool®. The superiority of this strategy is due to the establishment of cell-cell contacts and actin cytoskeleton reorganization of ECs on MCs surface before and during bioprinting and to the proximity of cells to AGFs, enabling effective usage.