Aging effects on oxidized and amine-modified poly(dimethylsiloxane) surfaces studied with chemical forced titrations: Effects on electroosmotic flow rate in microfluidic channels

Poly(dimethylsiloxane) (PDMS) has been used extensively as a substrate material for microfluidic devices because of its low cost and the ease with which micron-sized features can be molded. PDMS, however, has disadvantages compared to other materials including high hydrophobicity and minimal surface charge. Surface modification by Telsa coil oxidation can be utilized to augment these properties by yielding a hydrophilic surface that is capable of possessing a significant zeta potential at high pH's. However, the modification is only temporary because the hydrophilic surface begins to revert back to its original hydrophobic form, exhibiting a 75% decrease in flow associated with the surface modification in less than 24 h. We show that derivatizing the PDMS surface with (aminopropyl)triethoxysilane following oxidation yields a surface that is more hydrophilic than the native material. Using a combination of chemical force microscopy (CFM) and electroosmotic measurements, we show that this surface is still modified after more than 10 days. We also demonstrate that Tesla coil oxidation, with the inherent damage caused to the surface, is not the only means of oxidizing PDMS. Direct exposure to ozone is also capable of creating a similarly oxidized surface although its ability to support electroosmotic flow is reduced compared to the Tesla coil oxidized surface. The increased stability of the modified surface has ramifications for microfluidic devices constructed from PDMS by enabling the increased control of both magnitude and direction of electroosmotic flow within the device microchannels.