Microfluidic device was fabricated according to procedures reported elsewhere.34,35 (link) The glass slides were cleaned in a boiling 7× cleaning solution (MP Biomedicals, Solon, OH) for 1.5 h, rinsed copiously with purified water, and then dried with high purity nitrogen gas. The photolithographically patterned photoresist (Microposit S1813, Shipley Corp., Marlborough, MA) was deposited on the glass surface. A patterned glass slide was HF-etched to form a model glass slide and washed with acetone. PDMS was degassed under vacuum for at least 1 h and was then poured onto a clean model glass slide and cured in incubator at 70 °C for 3 h. Small holes were poked with a needle in PDMS mold, serving as inlet and outlet ports for flowing liquid. The PDMS mold and glass slide were placed in a plasma cleaner (PDC-32 G, Harrick, Pleasantville, NY) for 1 min of plasma treatment. Immediately after plasma treatment, PDMS mold and glass slide were pressed together, and annealed at 108 °C for 1 min. PDMS well setup was fabricated by placing a PDMS film (∼0.5 mm) with a 6 mm diameter hole in the middle.