Fabrication of a SU-8-based polymer-enclosed channel with a penetrating UV/ozone-modified interior surface for electrokinetic separation of proteins

[[abstract]]This paper introduces electrokinetic separation inside fully cross-linked epoxy-based polymer channels that were batch modified on the inner surfaces using a penetrating UV/ozone treatment from the outside. The treatment can employ either a 254 nm UV source in an ozone-rich environment or a stand-alone 172 nm UV source to directly generate C=O hydrophilic functional groups on the embedded polymer channel wall surfaces. Short-wavelength UV radiation was employed to break polymer surface bonds inside the channel. Ozone generated directly from air or supplied externally oxidized the reaction site on the activated polymer surface to generate the desired functional groups. An epoxy-based photoresist compound, SU-8 (MicroChem, MA), which is widely used in microfluidic systems, was employed to demonstrate the surface modification. Fourier transform infrared spectroscopy (FTIR) and high resolution x-ray photoelectron spectroscopy (HRXPS) were employed to characterize the functional groups that formed after the UV/ozone surface modification and to confirm the formation of O-H functional groups from the phenol group covalently bonded to the SU-8 surface, attributed mostly to the surface hydrophilicity modification. Water contact angles on the modified surface ranged from 72 degrees to 12 degrees depending on the processing time, UV power and ozone concentration. These angles were retained for at least 4 weeks after the process. Finally, the inner wall surfaces of the SU-8-enclosed channels were successfully modified using this technology, and rapid water transportation and EOF pumping were visualized inside the channel after surface modification. Successful electrokinetic separation of 10 mM BSA and 10 mM anti-rabbit IgG labeled with FITC inside the channel was also carried out. The polymer channel revealed a surface charge density of 75% of the zeta potential on a microslide glass surface, indicating the potential for molecule separation using polymer channels instead of glass channels. This simple process provides a novel way to integrate surface modification into microfluidic structures after fabrication for fully polymer-based lab-on-a-chip systems