Fucoidan–Alginate Composite Nanogels and Hydrogels: Design Strategies, Gelation Mechanisms and Sustainable Applications

Fucoidan and alginate, two marine-derived polysaccharides, have recently attracted considerable interest in both biomedical and food applications due to their unique structural characteristics and multifunctional bioactivity. However, few studies have examined fucoidan–alginate composite nanogels and hydrogels, and a comprehensive review covering formulation strategies, gelation mechanisms and other aspects of these composite systems is still lacking. Therefore, this narrative review provides a critical overview of recent progress in fucoidan–alginate composite nanogels and hydrogels, especially in terms of formulation strategies, gelation mechanisms and their potential applications in food and biomedical fields. In these composite systems, alginate can be ionically cross-linked with divalent cations (e.g., Ca2+) to form three-dimensional networks that retain water and can encapsulate and release bioactive cargo, whereas fucoidan is proposed to modulate network structure and bioactivity. Combining sulfated, fucose-rich fucoidan with guluronic acid-rich alginate has been reported to produce complementary effects on mechanical properties, structural stability and bioactivity; however, whether these effects are genuinely synergistic has rarely been tested directly, and the level of evidence is therefore explicitly distinguished throughout this review. These properties suggest potential applications in drug delivery, wound healing and tissue engineering, although most available evidence is in vitro or preclinical. Key barriers to translation include the source-dependent variability of fucoidan, incomplete mechanistic understanding of the composites, safety considerations such as anticoagulant activity, limited in vivo and clinical evidence, and unresolved issues related to scalability and regulatory standardization.