Bonding of SU-8 to glass for gastight picoliter reactors allowing in situ optical observation

Experimental tasks in chemistry and biology require large experiment statistics, small solution volumes, and no gas or vapor exchange with the environment. Microfluidics devices built of the epoxy-based polymer SU-8 comply with these requirements. However, the additional constraints of reactor thickness of several micrometers, localized temperature control, and optical access for high-resolution observation provide a challenge. We developed a device consisting of top and bottom glass plates separated by a layer of cross-linked SU-8, with a resistive microheater deposited at the bottom. Picoliter reactor volumes are achieved by suspending aqueous solution droplets in silicone oil held in a channel with cross-section 5 × 40 μm ². The narrow channels lead to large capillary resistance and, in turn, high injection pressures. To achieve bonding of SU-8 to glass capable of withstanding such pressures, we optimized the oxygen plasma treatment of the polymer surface by monitoring the evolution of the treated surface with atomic force microscopy. We found that the plasma effect was fully determined by its power. The hydrophilicity of the treated surfaces was characterized by their contact angles with water. The treated surfaces were bonded to glass bottoms using a torque-controlled clamp with a 3 μm smoothness of the holding plates. The bonding pressure and temperature were chosen in the gap between those for the glass transition of the SU-8 layer and fracture of the glass bottom.