Fabrication of Micropillar Substrate: Cylindrical micropillars (100 μm diameter and 50 µm height) were fabricated on a PDMS substrate in a 10 × 10 array with 200 μm center-to-center intervals (Figure ). Square and rectangular structures of 50 μm height were arranged near the sides of the substrate as spacers to determine the height of the fluid volume. The master mold was fabricated on a two-inch silicon wafer using DRIE process (iPB-400, Samco, Japan).