Surface analysis, hydrophilic enhancement, ageing behavior and flow in plasma modified cyclic olefin copolymer (COC)-based microfluidic devices

Radio frequency (RF) argon and argon-oxygen plasma were used to improve hydrophilicity and increase wettability of the COC (Topas) surface to enhance its surface properties for applications in microfluidic devices. This is necessary because the hydrophobic nature of COC can lead to adsorption of specific compounds from biological fluids in the channel which can impair precise quantification and cause clogging. The significant loss in hydrophilicity during ageing was found to be associated with the decrease in –OH groups on the plasma modified surface. A correlation was found between the surface oxygen/carbon ratio and hydrophobicity of the treated surface. Plasma power ratings of 60 W and 100 W with exposure times from 15 s to 240 s were carried out. The surface roughness was characterized using atomic force microscopy (AFM) and wettability was determined using contact angle measurements. The oxygenated-argon plasma treatment improved surface wettability more effectively as compared to the argon treatment. The changes in surface properties due to treatment and during ageing were characterized using FTIR and XPS (X-ray photoelectron spectroscopy) revealing the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. The probable chemical surface mechanisms were identified. The study indicates that plasma modified COC-based microfluidic devices ought to be vacuum-packed for storage to ensure that their performance will not be impaired.