Plasma modification of PDMS microfluidic devices for control of electroosmotic flow

Polydimethylsiloxane (PDMS) capillary electrophoresis microchips were modified using plasma-enhanced chemical vapor deposition (PECVD), resulting in modified electroosmotic flow (EOF) values. Octafluoropropane (C3F8) and acrylic acid (AA) plasmas were chosen as initial test systems for device modification. Argon plasma pretreatments were used to improve adhesion of the fluorocarbon (FC) and AA films. Contact angle measurements and X-ray photoelectron spectroscopy data demonstrated that the Ar/C3F8 plasma treatment of PDMS results in the deposition of a hydrophobic, crosslinked FC film, whereas the Ar/AA plasma treatment results in the deposition of a hydrophilic film with ionizable acid groups. The extent of plasma modification within the device channels was explored using scanning Auger microscopy and dye absorption measurements. EOF values were measured for plasma-treated chips as a function of pH, and aging studies were performed to determine the durability of the plasma treatments. Results show that EOF is decreased in Ar/C3F8 plasma-treated chips, and varies less with pH than untreated devices. Additionally, EOF measurements are constant for a minimum of 5 d. In contrast, EOF for Ar/AA plasma-treated devices is dependent on pH. EOF measurements of C3F8 and AA treated chips without the Ar pretreatment are less stable, particularly in the AA case. In addition to improving adhesion, the Ar plasma treatment results in a decreased hydrophobic dye absorption into the PDMS, which is attributed to the physical crosslinking of the polymer by the Ar plasma.