Functionally Graded Polymer Lattice Structures Enabled by Multi-Material Fused Deposition Modeling: Current Progress, Challenges, and Future Perspectives

Functionally graded lattice (FGL) structures can decouple stiffness, strength and energy absorption. Multi-material fused deposition modeling (MM-FDM) is an accessible route to lattices that vary in composition and topology within one print. This review examines MM-FDM polymer FGLs through a PRISMA 2020-informed screening of 610 records. Of the 126 screened references, 65 primary studies meet all eligibility criteria and form the core evidence, and 20 are supporting studies. Four grading strategies were identified: material-based zoning, geometry-induced grading, combined material–geometry grading and process-induced grading, applied mainly to strut-based, triply periodic minimal surface (TPMS) and hybrid plate–TPMS architectures. Pairing rigid thermoplastics (PLA, ABS, PETG) with hyperelastic TPU increased specific energy absorption by up to two- to threefold over single-material lattices, but the gain over the rigid constituent alone was smaller and, in several quasi-static studies, negative; it depends on configuration, baseline and loading rate. Graded lattices collapse layer by layer, yet their energy gain over uniform lattices of equal mass is often modest. TPMS architectures are reported to reduce nodal stress concentrations. Most evidence remains at the specimen level. The main limitations are weak interfaces between incompatible polymers, process-induced defects, models that neglect manufacturing effects and the lack of standardized test protocols.