Direct Parylene Bonding for the Flexible Microfluidic Device Resistive Against the Absorption of Small Molecules

Flexible microfluidic devices are expected to be realized as sweat and blood sensors integrated into next-generation wearable devices directly attached to the skin. Dimethylpolysiloxane (PDMS), a silicone material, is commonly used in flexible microfluidic channels due to its excellent transparency, flexibility, and biocompatibility. Micro-scale grooves are created on PDMS using techniques like soft lithography, and the flexible microfluidic channels are fabricated by bonding thin film polymers or PDMS using adhesives or plasma modification methods.<br/>While this method allows for convenient and low-cost fabrication, it also faces limitations regarding material-related issues, which restrict its usage. For instance, PDMS tends to adsorb low-molecular-weight materials and proteins, and it swells in the presence of organic solvents. Moreover, its gas permeability can lead to changes in the concentration of analytes. Modifying PDMS or covering it with other materials is a beneficial way to prevent these issues. In particular, a non-reactive and gas-barrier polymer called parylene, commonly used as an encapsulation film for organic photovoltaics, is a promising material to prevent these PDMS-related problems. However, directly bonding parylene-coated PDMS is challenging due to the high-temperature bonding process (above 160°C), which causes PDMS to deform easily. On the other hand, the method of first plasma-bonding PDMS and then depositing parylene inside the channels using chemical vapor deposition (CVD) proves difficult for complex channel shapes or narrow channels.<br/>This study developed a low-temperature direct bonding method below the glass transition temperature of parylene using water vapor plasma treatment and steam heating. This method enabled the fabrication of microfluidic channels with parylene-coated PDMS by simply bonding them together, suppressing the adsorption of low-molecular-weight compounds. Straight microchannels with a width of 50 μm were created on PDMS, and a 1 μm-thick parylene film was deposited on the entire PDMS surface, including the lid PDMS. The bonding surfaces were hydrophilized using water vapor plasma treatment and aligned with clips to prevent misalignment. The flexible microfluidic channels were created by storing the assembled samples in a steam chamber for 24 hours. Then direct parylene bonding occurred. To evaluate the absorption of low molecular weight materials in microfluidic channels, Rhodamin was introduced into parylene-coated and PDMS-exposed microfluidic channels. After 15 hours of introducing Rhodamine, the width of Rhodamine was measured in each microfluidic device. As a result, the detection width of Rhodamine was 1.3 times wider in the PDMS channel without parylene coating. On the other hand, no diffusion was observed in the parylene-coated microfluidic channel developed in this study. Therefore, the absorption of Rhodamine was suppressed by the parylene-coated PDMS microfluidic channels. By fabricating and evaluating more complex channel patterns, it is possible to achieve a conveniently fabricated flexible microfluidic device that can be integrated as a sensor in next-generation wearable devices.