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 (<10 wt%) can increase tensile strength by approximately 10–40% while improving UV shielding and antioxidant performance. However, excessive SCG content often reduces elongation at break, increases film brittleness, and compromises optical transparency, highlighting the importance of filler optimization and matrix compatibility. Therefore, this review provides a comprehensive and application-oriented perspective on the multifunctional utilization of SCG as a sustainable filler, highlighting matrix-dependent advantages, structure property relationships, and performance trade-offs that enable its tailored use in active, biodegradable, and functional packaging as well as related film-based applications.