4.8. Microfluidic device fabrication and passivation

All steps regarding the fabrication of microchannel patterns was performed using facilities within the cleanroom affiliated with the Vanderbilt Institute of Nanoscale Science and Engineering (VINSE). To make a microchannel mold, mr‐DWL_40 resist was cast on a clean silicon wafer and spin‐coated at 1000 rpm for 1 min, yielding a 60 μm‐thick resist layer. Then designed patterns were directly written into the photoresist using a laser writer (Heidelberg, μPG 101). The wafer with patterned resist was coated with a thin layer of TCPS to facilitate subsequent removal. To make a microfluidic device, liquid polydimethylsiloxane (PDMS) (Sylgard 184) was mixed in a 1:10 ratio of curing agent and PDMS resin, degassed in a desiccator and carefully poured onto the resist mold placed in a petri dish. After curing in an oven for 3 hr at 65°C, the PDMS layer was cut and peeled off the resist mold. Holes were punched at the inlet and outlet of the microchannels using a 8.5 mm internal diameter punch. A clean microscope glass slide was bonded to the patterned PDMS layer by exposing to an oxygen plasma for 4 min. Each well of the microfluidic device was then passivated with 150 μl of 20 mM mPEG‐silane (dissolved in 100% acetone) for 1 hr at room temperature to avoid nonspecific binding. The microfluidic device was cleaned with Milli‐Q water (18 MΩ) and dried with nitrogen. The passivated device was stored at −20°C in glovebox for future use.