• alveolar barrier
• in-vitro model
• bioprinting
• core-shell spheroid

Epithelial barriers have several functions: compartmentalizing the internal environment, maintaining organ homeostasis by modulating the entry of nutrients and solutes, and defending our body from the entry of potentially toxic substances (Yeste et al., 2018). The alveolar epithelium is of particular interest as the object of numerous studies on permeability to drugs, nanoparticles, toxic substances, nutrient absorption, and diseases (cancer, infections, genetic diseases).
To date, the models used to study and investigate these aspects are both in vivo and in vitro. The first ones, however, are limited because of the fundamental differences in anatomy and physiology between humans and other animal species. In vitro models, on the other hand, offer controlled cellular environments that can be evaluated in real-time, are easily scalable and replicable, and are increasingly close to the future of personalized medicine (Devarasetty et al, 2018).
The standard model for biological barriers is the Transwell®. 3D models, such as spheroids and organoids, allow better replication of the 3D environment in vivo. However, they currently require time-consuming protocols for the formation of an internal lumen.
Therefore, this thesis aims at optimizing the fabrication of pre-structured alveolar spheroids using bioprinting engineering technologies. Using COSMIC (Core-Shell MIcrobeads Creator) bioprinter designed at the Research Centre 'E. Piaggio' in Pisa, core-shell structures with liquid core and an alginate-based shell were created. To replicate the alveolar barrier, the epithelial cell line A549 was used. In particular, cell densities and the combination of alginate with adhesive materials such as gelatin and collagen were optimized. The core-shell alveolar barrier model was analyzed with different biological investigation assays: observation of morphology (bright field imaging), cell viability assay (Alamar Blue assay), paracellular and transcellular permeability assay to test barrier integrity (FITC-DEXTRAN/ Rho 123 assay) and analysis of cell differentiation (Oil Red O Staining). It was possible to observe cells’ ability to dispose at the alginate/liquid interface, their contribution in modulating substance passage, and differentiation into a more pneumocyte-like phenotype.
In the future, the model can be extended to other barrier types, such as the intestinal barrier, and co-culture with fibroblasts will be implemented to make the model physiologically relevant and thus more similar to the complex in vivo microenvironment. The developed core-shell models can also be placed inside magneto-responsive gels to mimic the mechanical function of the extracellular matrix on cells and the physiological mechanism of lung ventilation and intestinal peristalsis.