a) Schematic design of the microfluidic BBB-on-a-chip consisted of a set of cell insert and three 3D-printed parts: lid, chamber, and perfusion layer. b) Side view (top) and enlarged cross-sectional view (bottom) of the BBB system to show the arrangement of electrodes, cell layers, and fluid pathways. c) 3D microfluidic BBB demonstrated significantly higher TEER values by day 3 compared to BMEC or astrocyte monocultures, and the TEER remained high up to 10 days. d) BBB-on-a-chip demonstrated permeability to dextrans and drug molecules at various sizes consistent with in vivo data. Copyright Wiley, 2017.[25] Reproduced with permission.