Characterization of SU-8 for electrokinetic microfluidic applications

The characterization of SU-8 microchannels for electrokinetic microfluidic applications is reported. The electroosmotic (EO) mobility in SU-8 microchannels was determined with respect to pH and ionic strength by the current monitoring method. Extensive electroosmotic flow (EOF), equal to that for glass microchannels, was observed at pH >= 4. The highest EO mobility was detected at pH >= 7 and was of the order of 5.8 x 10(-4) cm(2) V-1 s(-1) in 10 mM phosphate buffer. At pH <= 3 the electroosmotic flow was shown to reverse towards the anode and to reach a magnitude of 1.8 x 10(-4) cm(2) V-1 s(-1) in 10 mM phosphate buffer ( pH 2). Also the zeta-potential on the SU-8 surface was determined, employing lithographically defined SU-8 microparticles for which a similar pH dependence was observed. SU-8 microchannels were shown to perform repeatably from day to day and no aging effects were observed in long-term use.