Table 5.

Main challenges for the improvement of some specific organ-on-a-chip systems: Modified from [1,7].

Organ	Main challenges
Kidney	Formation of glomeruli with podocytes and entangled proximal tubules.
Lung	Generation of biomechanical ventilation without assistance. Further studies with inflammatory and immunological processes and tumors.
Heart	Establishment of validated studies for reproducible cardiotoxicity with various drugs.
Gut	Deepening knowledge of mesenchymal-epithelial interactions.
Placenta	Modelling with primary cells.
Adipose tissue	More knowledge of the functioning of brown fat in adipose tissue.
Brain	Standardization and reproducibility of brain organoids.