Surface treatment of polymers for the fabrication of all-polymer MEMS devices

Polymer microelectromechanical systems (MEMS) have received considerable attention due to their low cost, remarkable biocompatibility, and high flexibility when compared to glass and silicon devices. However, the fabrication process of all-polymer MEMS devices can be complicated and often requires a special group of techniques. In particular, different types of polymers possess different properties in terms of surface chemistry and hydrophilicity, making device assembly a challenging task. In this paper, we demonstrate the fabrication of an all-polymer microfluidic device through the investigation of the essential surface treatment methods. An SU-8–SU-8-polydimethylsiloxane (PDMS) sandwiched structure is used in this research because SU-8 enables alignment capabilities during the fabrication process. It can also be easily extended to other lithography-compatible processes. Both untreated SU-8 and PDMS are hydrophobic and they have different surface chemistry properties, so surface modifications are necessary. Three critical surface treatment steps are used in our process. The first step is to treat the first SU-8 layer with low-power (10 W) oxygen plasma, making its surface hydrophilic. This step enables the uniform coating of the second SU-8 layer. The next surface treatment is on the second SU-8 layer. Both oxygen plasma (40 W) etching and diluted 3-aminopropyltriethoxysilane (APTES, a silane solution) coating are needed. APTES introduces amine (SiNH2) groups on the surface. The last treatment step is to introduce silanol (SiOH) groups on PDMS using oxygen plasma. These surface treatment steps are critical in the fabrication process and can determine the quality of the final device. The knowledge obtained from this research can enable the development of novel all-polymer MEMS devices for biomedical applications.