Pgmea

The devices were fabricated from PDMS using the widely used method of photolithographically defined SU-8 on a silicon master, as described elsewhere [24 (link),25 (link)]. Briefly, SU-8 masters were produced by spin coating Si wafers with SU-8 (SU-8 2025 MicroChem) and exposed to UV light through a mask to initiate cross-linking, which was enhanced by baking. The non-cross-linked SU-8 was removed using developer solution (PGMEA, MicroChem Inc., Newton, MA, USA), leaving a mould which can be used to produce multiple identical devices. Liquid PDMS (Sylgard 184, Dow Midland, MI, USA) mixed in a 10:1 elastomer/curing agent ratio was poured onto the master and baked in a 60 °C oven for 3 h before being cut out and access holes punched using a biopsy puncher. The device and PDMS base were treated with an air plasma apparatus (Zepto, Diener, Ebhausen, Germany) and brought into contact to seal. Then, they were placed in an oven at 110 °C for at least fifteen minutes. On cooling to room temperature, the inner surfaces of the devices were treated with PicoGlide (Sphere Fluidics, Cambridge, UK) by flushing the solution into the channels, which was ideally left overnight and then cleared with compressed air to render the surfaces hydrophobic.

Fabrication of the microfluidic device was completed using a previously described method [53 (link)]. Briefly, SU-8 2025 epoxy-based photoresist (MicroChem) was poured on top of a 3-in. silicon wafer at 95 °C and left overnight. A photomask was applied over the wafer before exposure with a 200-W UV lamp for 2 h. Propylene glycol methyl ether acetate (PGMEA) (MicroChem) was used to dissolve the unpolymerized photoresist, and polydimethylsiloxane (PDMS, # 2065622, Ellsworth) was used to create negative molds from the silicon master. Similarly, positive stamps made of PDMS were cast to create the microfluidic channels on glass coverslips. Before gel fabrication, the hydrogel reservoir was filled with 5 M sulfuric acid (#258105, Sigma) for 90 min, washed several times with distilled water, and then filled with 50 µg/mL collagen type I and sterilized with UV light. The schematic design of the microfluidic device is shown in Fig. 2A, B.Fig. 2

3D blood–brain barrier model. A Schematic of a microfluidic device with the location of inlet/outlet ports and ports to fill the hydrogel reservoir. B Picture of the PDMS device. Scale bar, 1 cm. C Permeability testing with 4 kDa Dextran-FITC configuration. D TEER measurements