Photolithographic Fabrication of Microfluidic Devices

The droplet-based microfluidic device was fabricated by a photolithography process, as previously described^{33 (link)}. Briefly, the microfluidic device was designed by AutoCAD software. Silicon wafers (Wangxing Silicon-Peak Electronics, China) were spin-coated with SU-8 50 photoresist (Microchem Corp., USA) at 1000 rpm for 60 s, resulting in a thickness of 100 μm. Afterwards, the coated wafers were exposed to UV light for 30 s through a mask film. Poly(dimethylsiloxane) (PDMS, Dow Corning, USA) was mixed in a ratio of 10:1 (monomer:crosslinker), degassed, poured into the fabricated molds, and cured in an oven at 80 °C for 1 h. The cured PDMS devices were carefully separated from the silicon wafers and bonded onto glass substrates (Marienfeld, Germany) using oxygen plasma treatment (Femto Scientific, Korea). Before use, the channels of the PDMS device were treated with Aquapel (PPG Industries, USA) for 10 min to increase the hydrophobicity.