To prevent the adhesion of the PDMS to the etched master mold, a releasing agent (FLUOROSURF FG-5084, FluoroTechnology Co., Ltd., Japan) was coated onto the silicon mold and was dried on a hotplate at 120 °C for 20 min. A PDMS resin (SILPOT 184 W/C, Dow Corning Inc., USA) that was mixed with its curing agent at a weight ratio of 10:1 was poured into the master mold. After curing at 80 °C for 180 min, the PDMS substrate was obtained by peeling the mold off.