Numerous materials have been employed in the surface modification of membranes, including different polymers and nanomaterials over the years. The pernicious challenges of membrane separation include the trade-off between water permeability and solute rejection as well as fouling when it is tested under liquid conditions. Zwitterionic polymers have attracted wide interest due to their unique properties of containing both cationic and anionic groups while maintaining electroneutrality and high hydrophilicity. These zwitterionic polymers have been utilized as coating materials or grafted layers not only on the surfaces of porous membranes but also as thin film composite membranes. To illustrate, the membranes modified with this class of materials are capable of resisting foulants through two distinct mechanisms, which are the hydration layer formation and steric repulsion effect, showcasing their unique chemical properties. To bridge the gaps of the recent development of zwitterionic membranes and their relationships with foulants for targeted separation such as ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), this critical review will first fundamentally classify the fouling into four categories according to their characteristics and fouling mechanisms, namely, inorganic foulants, nonmigratory foulants, spreadable foulants, and proliferative foulants. The antifouling strategies which are active antifouling and passive antifouling (e.g., fouling resistance and fouling release) will be summarized. Apart from that, the chemistry, morphology, and modification approaches of zwitterionic membranes including surface coating, surface grafting, and physical blending on targeted applications as well as the separation performance of the state-of-the-art membranes will be presented in detail. Lastly, the summary, outlook of major challenges and opportunities of the zwitterionic materials will be highlighted. It is anticipated that this review can generate a pathway to facilitate the next generation of zwitterionicbased membranes with superior separation performance for specific separation.

The aggravation of environmental problems such as water scarcity and air pollution has called upon the need for a sustainable solution globally. Membrane technology, owing to its simplicity, sustainability, and cost-effectiveness, has emerged as one of the favorable technologies for water and air purification. Among all of the membrane configurations, hollow fiber membranes hold promise due to their outstanding packing density and ease of module assembly. Herein, this review systematically outlines the fundamentals of hollow fiber membranes, which comprise the structural analyses and phase inversion mechanism. Furthermore, illustrations of the latest advances in the fabrication of organic, inorganic, and composite hollow fiber membranes are presented. Key findings on the utilization of hollow fiber membranes in microfiltration (MF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), pervaporation, gas and vapor separation, membrane distillation, and membrane contactor are also reported. Moreover, the applications in nuclear waste treatment and biomedical fields such as hemodialysis and drug delivery are emphasized. Subsequently, the emerging R&D areas, precisely on green fabrication and modification techniques as well as sustainable materials for hollow fiber membranes, are highlighted. Last but not least, this review offers invigorating perspectives on the future directions for the design of next-generation hollow fiber membranes for various applications. As such, the comprehensive and critical insights gained in this review are anticipated to provide a new research doorway to stimulate the future development and optimization of hollow fiber membranes.

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