Hydrophilicity and optic property of polyethylene glycol coating on polydimethylsiloxane for fast prototyping and its application to backlight microfluidic chip

The polydimethylsiloxane (PDMS) is widely applied to prototyping of various categories by coating or molding due to bio-compatibility, transparency, and flexibility. However, the native hydrophobicity and short-time recovery after O-2 plasma treatment for the microfluidic chip are the defects in self-driven capillary pumping and long-term stability. Good optical transparency is necessary for backlight experiment observation. In addition, the self-bond in dual-tone PDMS-to-PDMS casting leads to a separation problem during demolding, which prevents rapid prototyping application. In this article, the coating of polyethylene glycol (PEG) with molecular weights (MW) of 6000 and 1000 denoted as PEG6000 and PEG1000 on PDMS is investigated as an anti-stiction layer for rapid prototyping as well as for the surface modification on the hydrophilicity and backlight optic property of PDMS for the microfluidic chip application. The contact angles of both PEG6000 and PEG1000 coated PDMS after 21 days are 42.6 degrees and 41.7 degrees, respectively, for good hydrophilicity; however, the pure O-2-PDMS without PEG coating after 1.5 h has hydrophobic recovery to 78.7 degrees. After PDMS-to-PDMS demolding, the PEG6000 coated PDMS has some broken area on the surface, while the PEG1000-PDMS keeps a whole smooth surface. Regarding transparency, the PEG1000-PDMS has a higher transmittance of about 55-70% near to the PDMS of 65-78% in visible wavelength, but the PEG6000-PDMS one is vague about 15%. It is attributed to PEG1000 of much smaller molecules with a lower light scattering and absorbance compared to PEG6000. The PEG1000-PDMS of good hydrophilicity, transparency, and anti-stiction properties makes it a good candidate for the capillary-driven microfluidic chip application.