Sol–gel based fabrication of hybrid microfluidic devices composed of PDMS and thermoplastic substrates

Fabrication of microfluidic devices using both rigid and flexible plastic substrates offers benefits for making pressure-actuated membrane valves, mechanically active components, and low-cost but highly functional 3D microchannel networks. Here we present a simple and versatile process for bonding flexible polydimethylsiloxane (PDMS) and rigid thermoplastics like poly(methyl methacrylate) (PMMA), by utilizing the sol–gel method. The silica sol, obtained by oligomerizing tetraethoxysilane monomers, was spin-coated on a thermoplastic plate and further polymerized to form a thin silica layer (silica gel) with a thickness of 140–300 nm. The silica-coated surface could be covalently and strongly bonded with an O2-plasma-activated PDMS plate, just by bringing them into contact. We applied the presented process to preparing multi-layer PDMS–PMMA microdevices having 3D crossing channels or pneumatically controlled membrane valves, and demonstrated the parallel flow distribution, mixing, and droplet generation. In addition, bonding strengths between PDMS and various thermoplastics, including PMMA, polyvinyl chloride (PVC), polycarbonate (PC), and polypropylene (PP), were examined.