Smart membrane technologies for dynamic separation: materials innovation, functional mechanisms, and future directions

Traditional membrane technologies for water purification, biomolecular separations, environmental remediation, and industry applications are widely employed; yet their fixed pore structures and sensitivity to fouling impose restrictions on their adaptability under dynamic operation conditions. Stimuli-responsive membranes represent novel membrane technologies that allow to regulate the membrane permeability, selectivity, wettability, and mass transport in response to changes in temperature, pH, light, electric, magnetic fields, ions, or molecular interactions. In this review, the development of smart membranes is critically discussed from the points of view of stimuli-response mechanisms, functional materials, membrane fabrication techniques, characterization approaches, performance and industrial applicability. Specific attention is paid to the use of stimuli-responsive polymers, two-dimensional nanomaterials, molecular recognition devices, and hybrid systems, as well as the fabrication methods like coating, grafting, blending, electrospinning, and nanoscale assembly. Recent literature shows that smart membranes are able to demonstrate stimuli-dependent transport, self-cleaning properties, wettability modulation, selective ion transport, and molecular gating. Yet, many published results are obtained under laboratory conditions that do not necessarily take into account the effect of long-term stability, fouling in real feeds, response kinetics, material leaching, interference of multiple stimuli, complexity of fabrication, and scaling-up. As opposed to the existing reviews that address a single type of stimuli or functional materials or applications, this review establishes a relationship between stimuli-response mechanisms and fabrication techniques, discusses performance limitations, application-specific challenges and paths towards industrial implementation. Promising trends, such as multi-functional membranes, sustainable fabrication, advanced two-dimensional materials, data-driven design, and manufacturing scalability are critically discussed.