Evaluation of polydimethylsiloxane (PDMS) surface modification approaches for microfluidic applications

Motivated by stabilizing a hydrophilic surface condition of polydimethylsiloxane (PDMS) and improving its zeta potential, several reported methods for treating PDMS surface were evaluated using two or more of the following techniques: capillary flow in dryly stored channels, static contact angle measurement, current monitoring technique and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The surface treatment methods that were evaluated include four groups: pre-doping PDMS with acrylic acid, extraction of uncured PDMS (oligomer), and monomer grafting using 2-hydroxyethyl methacrylate (HEMA) and monomer grafting using polyethylene glycol (PEG). This study also proposed a modified PEG grafting method. It was observed that the extraction and monomer grafting methods were capable of changing hydrophobic PDMS to the stable hydrophilic state. However, none of the evaluated methods were able to increase the zeta potential of PDMS to a value higher than that treated with a one-step plasma treatment based on the current monitoring measurement, where 1X Tris–Borate–EDTA (TBE) buffer was used as the liquid sample. It was found that combining extraction and one-step plasma treatment was the best approach to maintain the stable hydrophilic state of PDMS and gain a relatively high zeta potential. In addition, it was found that the extraction method provided the most consistent outcome in modified PDMS surfaces compared to other methods.