Floating beyond limits: A review on engineered floatable hydrogel platforms and emerging sustainable applications

Hydrogels, featuring tunable porous structures and efficient mass transport, are promising materials for floatable platforms that improve light absorption, enable continuous operation at gas-liquid interfaces， and protect encapsulated materials and enhance recycling efficiency. Floatable hydrogels notably maximize the use of sunlight at the air-liquid interface, significantly improving solar energy capture and efficiency in photocatalytic and photothermal processes. This review systematically summarizes recent advancements in floatable hydrogels, highlighting three major fabrication strategies: chemical cross-linking, pore structure regulation, and surface engineering. Chemical cross-linking is the predominant method, offering strong stability and versatility through monomer selection and cross-linking conditions, though quantitative buoyancy control remains challenging. Pore-structure regulation, including mechanical foaming, agent-based foaming, and bubble locking, enables precise buoyancy control but may compromise structural integrity. Surface engineering usually serves as a complementary strategy by modulating hydrophobicity or hydrophilicity. Advanced fabrication methods such as 3D printing offer promising opportunities. Multifunctional applications of floatable hydrogels are comprehensively reviewed, covering photocatalysis, solar-driven water purification, photothermal energy conversion, wastewater treatment, environmental remediation, and renewable energy harvesting. Emerging fields, including photo-electrocatalysis, electrochemical energy storage, and biomedical engineering, are also discussed. Finally, analytical challenges regarding floating durability, accurate buoyancy tuning, anti-fouling properties, and scalability are critically examined, providing strategic insights for future research directions.