Recent advances in the roles of spent coffee grounds as multifunctional fillers in biopolymer-based composites: a review

• Utilized SCG as multifunctional fillers that improved the performance of biofilms • Comparing biopolymer matrices reveals varied interactions shaping their properties • Lignin, cellulose, and phenolics in SCG enhance strength, UV-shielding, and antioxidant properties • Applying circular bioeconomy principles highlights SCG as a sustainable low-cost filler Spent coffee grounds (SCG), a prevalent byproduct of the global coffee industry, represent a significant lignocellulosic resource with considerable economic potential. Although the use of SCG as a filler in biopolymer-based films is increasing, a comprehensive evaluation of its multifunctional activities in diverse polymeric matrices is limited. This review aims to critically integrate current studies to elucidate how SCG composition and film preparation strategies govern the structural and functional performance of biopolymer films, demonstrating that SCG can perform multiple roles as a reinforcing, barrier-enhancing, and bioactive filler across a wide range of biopolymer matrices, enabling diverse functional and application-specific performances. SCG incorporation in biopolymer matrices such as chitosan, pectin, carboxymethyl cellulose (CMC), starch, gelatin, and polylactic acid (PLA) is discussed with emphasis on correlations between SCG composition, film processing, and resulting structural and functional properties. The findings indicate that SCG acts as a dual-function filler by enhancing film structure while imparting bioactive functionalities such as antioxidant activity and UV-blocking capability. Reported data show that SCG loadings (