Fluid Leak Detector for Cyclo-Olefin Polymer Microchannels Using Low-Temperature Bonding by Water Vapor Plasma

Cyclo-olefin polymer (COP) has attracted much attention as a base material for microfluidic channels because of its optical properties, excellent processability, and lower cost than glass-based materials. We have previously reported surface modification by water vapor plasma at low pressure as a method for bonding COP substrates [1]. This phenomenon is caused by OH radicals in the water vapor plasma. The OH radicals form functional groups -COOH and -OH on the COP surface that contribute to hydrogen bonding. Therefore, the surface of COP after plasma treatment becomes superhydrophilic, and when the base materials are bonded together, they are strongly bonded even at room temperature. However, it was not clear how effective this water vapor plasma bonding method would be for microfluidic channels using COP substrates. In this study, we investigated the practicality of this bonding method by process a microfluidic channel that can confirm the water pressure resistance performance by fluid leak detector.<br/>A straight channel with a width of 50 μm and a depth of 20 μm was prepared. A "fluid leak detector" with a depth of 2 μm was placed on both sides of this channel at intervals of 10-500 μm. If there is a defect in the bond or the bond strength is weaker than the pumping pressure of the fluid, the fluid would flow into the detector and the presence of a leak can be observed. The microchannel mold was fabricated with Silicon on Insulator (SOI) and Silicon (Si). The microfluidic channels were formed by imprinting this mold on a COP plate (ZEONEX 690R, Zeon Corporation). The plate with the transferred microchannels and the lid plate with the fluid inlet/outlet holes were processed with a water vapor plasma system (AQ-500, Samco Inc.). The plasma-treated surfaces were coated with water and the two plates were bonded together at room temperature (25 °C) and left for drying for 48 hours. After bonding, water was pumped into the straight channel at a maximum pressure of 200 kPa to examine any leakage.<br/>In order to achieve low temperature bonding without water leakage, surface flatness is required. In the present prototype, the etched bottom surface of Si mold is not flat near the edge, which caused unbonded area around the channel. On the other hand, in the case of SOI molding, the bottom surface is flat and the entire surface was bonded. In addition, the demolding temperature is especially important for imprinting on COP. Burrs at the edge of the flow channel were prevented from forming by demolding at slightly below the glass transition temperature.<br/>The room temperature-bonded microfluidic channels were pumped by capillary flow. This is thought to be due to the hydrophilic nature of the microchannel. In the fluid leak detector, the leaks appeared more transparent than in the dry channel and could be visually detected with an optical microscope. At the channel spacing of 50-500 μm, no water leakage was detected even when the channel was flushed with 200 kPa of water pressure. This water pressure resistance was higher than the previously reported values of glass leak detector [2]. Furthermore, this resistance is twice as high as that required for micro-scale channels, indicating that this bonding method has practical applications.<br/>References<br/>[1] Terai H, Funahashi R, Hashimoto T, Kakuta M. Electro. Engi. Jpn. 2018; 205:48-56.<br/>[2] Funano S, Ota N, Sato A, Tanaka Y. Chem. Commun. 2017; 53: 11193-11196.