Microfluidic chips made from polydimethylsiloxane (PDMS) were
prepared by using a mold created by soft photolithography mainly
according to the procedure described byWhitesides [24]. The channel
pattern was printed as a high-resolution (4000 dpi) photomask. In
order to get a thickness of around 100 μm (that is, much thicker than
usual in microfluidics), the negative type photoresist (SU-8 2025,
Microchem, Newton, MA) was spin-coated onto a 3″ silicon wafer
only with 500 rpm for 30 s.