Transforming Porous Membranes into Dual-Gradient Janus Structures by Directional Asymmetric Modification

Janus membranes are essential in applications such as water purification, biomedical diagnostics, and energy systems owing to their tunable pore structures and surface properties, which enable efficient fluid and ion transport. Here, a scalable strategy is presented for transforming conventional porous membranes into dual-gradient Janus structures through a parylene-plasma-porous (PPP) treatment that enables simultaneous and aligned control of porosity and wettability within a single porous substrate. This process integrates asymmetric parylene C deposition and O₂ plasma treatment to create vertically asymmetric structures and surface wettability, resulting in gravity-independent, self-pumping fluid transport driven by capillary forces. The dual gradients are validated through morphological and chemical characterization, and the versatility of the method is further demonstrated by its successful application to both glass fiber (GF) and cellulose membranes. Functionally, the resulting Janus membrane achieves efficient plasma separation from microliter-scale whole blood, exhibiting low hemolysis and high protein recovery without external power. This approach provides a broadly applicable platform for fabricating functional gradient membranes tailored for advanced microfluidic and diagnostic systems.