High-throughput fabrication of vascularized spheroids for bioprinting

Overcoming the problem of vascularization remains the main challenge in the field of tissue engineering.As three-dimensional (3D) bioprinting is the rising technique for the fabrication of large tissue constructs, small prevascularized building blocks were generated that can be incorporated throughout a printed construct, answering the need for a microvasculature within the small micron range (< 10 µm).Uniform spheroids with an ideal geometry and diameter for bioprinting were formed, using a high-throughput non-adhesive agarose microwell system.Since monoculture spheroids of endothelial cells were unable to remain stable, coculture spheroids combining endothelial cells with fibroblasts and/or adipose tissue derived mesenchymal stem cells (ADSC) as supporting cells, were created.When applying the favourable coculture ratio, viable spheroids were obtained and endothelial cells spontaneously formed a capillary like network and lumina, as shown by immunohistochemistry and transmission electron microscopy.Especially the presence of ADSC led to a higher vascularization and extracellular matrix (ECM) production of the microtissue.Moreover, spheroids were able to assemble at random in suspension and in a hydrogel, creating a macrotissue.During at random assembly, cells reorganized, creating a branched capillary network throughout the entire fused construct by inoculating with capillaries of adjacent spheroids.Combining the advantage of this natural capacity of microtissues to selfassemble and the controlled organization by bioprinting technologies, these prevascularized spheroids can be useful as building blocks for the engineering of large vascularized 3D tissues.