Algal cellulose and its nanoscale derivatives for sustainable polymer nanocomposites

Algae-derived cellulose has attracted significant attention as a renewable, bio-based material for biotechnology and sustainable engineering applications. Compared with terrestrial biomass, algae offer rapid growth, reduced land requirements, and simplified processing, making them a promising alternative feedstock for cellulose production. However, despite these excellent properties, pure cellulose remains limited in many applications, and researchers continue to face challenges in achieving controlled functionalization. This review assesses the process for extracting cellulose from algae and its conversion into cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs), with particular focus on their function as reinforcing nanofillers in polymer nanocomposites. Extraction strategies, including chemical, mechanical, enzymatic, and emerging green solvent approaches, are systematically evaluated in relation to nanocellulose yield, morphology, and crystallinity. This review establishes an integrated framework that links extraction conditions with nanoscale structure, composite performance, and processing feasibility. A key contribution is the structured evaluation of trade-offs among extraction severity, yield, and material properties, along with the correlation of nanocellulose characteristics with thermomechanical performance in polymer matrices. The comparison of different fabrication techniques, such as melt mixing, solution casting, electrospinning, and three-dimensional printing, suggests that hybrid processing is important for translating laboratory-scale developments into industrial applications. Beyond conventional structural composite applications, emerging applications in packaging, biomedical engineering, environmental remediation, and energy storage are also discussed. Despite significant progress, major challenges remain regarding biomass variability, process standardisation, large-scale production, techno-economic feasibility, and life-cycle benchmarking against terrestrial and bacterial cellulose sources. This review concludes that there is a need for species-specific biorefinery frameworks and integrated processing strategies to fully realise the industrial potential of algae-derived nanocellulose as a next-generation, sustainable reinforcement material for multifunctional polymer systems.