Sustainable Valorization of Agri-Food By-Products Through 3D Printing: A Review of Advances, Multisectoral Applications, and Circular Economy Challenges

Agri-food by-products and residues are increasingly being studied as raw materials for three-dimensional (3D) printing, especially to convert secondary streams into useful, more sustainable products. This review discusses recent advances in the use of these materials, focusing on how their origins, compositions, pretreatments, processing behaviors, and final applications are interconnected. Plant-, animal-, industrial-, and post-consumer-derived residues are considered, particularly when they provide fibers, proteins, polysaccharides, lipids, bioactive compounds, or biopolymers with technological value. The review also examines the main printing approaches used in this field, including extrusion-based printing, inkjet printing, sintering, and direct ink writing. Particular attention is given to factors that determine successful printing, including rheological behavior, viscosity, particle size, moisture content, structural stability after deposition, interlayer adhesion, and shape fidelity. Current applications range from functional foods and biodegradable or active packaging to biomaterials, controlled-release systems, biocomposites, construction materials, energy-related devices, textiles, and agricultural products. Although the reviewed studies show clear potential, most developments still face important barriers, including raw material variability, safety requirements, sensory acceptance, mechanical performance, scale-up, and regulatory uncertainty. Future progress will depend on moving beyond printable prototypes toward reproducible formulations, real-use validation, and clearer comparison criteria, in line with Sustainable Development Goal 12 and the transition toward higher-value circular production chains.