In‐Mold Wettability Patterning of Microfluidic Devices Without Back‐End Surface Modification

Selective surface energy patterning has remained a significant challenge in microfluidic device fabrication. The dominant methods of surface modification rely primarily on physical or chemical approaches, all of which require additional back‐end processing, thereby increasing complexity, cost, and overall cycle time. To tackle these challenges, we present a novel approach for simultaneous structuring and wettability patterning of both open and closed microfluidic systems. This is achieved using microreaction injection molding of off‐stoichiometry thiol‐ene‐epoxy (OSTE+) systems without additional monomers. Employing multi‐material mold assemblies with dissimilar surface energies, we fabricated replicas with heterogeneous wettability, achieving a contact‐angle contrast exceeding 30° without any post‐processing modifications. We demonstrate the potential of our approach through two representative applications: (i) in situ structuring and wettability patterning of hydrophilic‐in‐hydrophobic well arrays in open microfluidic devices, enabling efficient and robust droplet seeding and (ii) fabrication of microchannels with built‐in heterogeneous wettability in enclosed microfluidic devices, validated by successful capillary action of the hydrophilic channels. These results demonstrate the feasibility of generating selective and high‐precision wettability contrast in microfluidic devices without the need for back‐end surface treatment, which holds significant promise for digital microfluidics and bioassays.