Designs of BBB-on-a-chip. (A) Porous membrane separates blood components and brain components. As an example for this type, endothelial cells (ECs) forming vascular lumen are seeded in the lower chamber, and astrocytes and pericytes are seeded on the top of the porous membrane. Adapted with permission from Park et al. (39). Copyright (2019) Springer Nature. (B) Blood vessels form inside the matrix. As an example for this type, ECs can form blood vessels inside the matrix, with astrocytes and pericytes embedded around the blood vessel. Adapted with permission from Seo et al. (40). Copyright (2021) Wiley-VCH GmbH. (C) Self-assembled vasculatures of the BBB. As an example for this type, ECs are embedded in fibrin gel and vascular networks are formed in the matrix. Neurons and astrocytes are seeded in the adjacent channel by the side of the vascular networks. Adapted with permission from Bang et al. (41). Copyright (2017) Springer Nature. (D) 3D spheroid models of the BBB. As an example for this type, the BBB spheroids are formed with ECs, astrocytes, and pericytes in low adhesive microwells. Adapted with permission from Eilenberger et al. (44). Copyright (2021) Wiley-VCH GmbH.