Interfacial heterogeneity tunes mechanical and functional performance of FDM-printed magnetically responsive PLA/Asp/Fe₃O₄ composites

Magnetically responsive shape memory polylactic acid (PLA) composites are promising for 4D printing due to their ability to undergo remote, non-contact actuation, yet the incorporation of magnetic fillers often compromises mechanical reliability in fused deposition modeling (FDM), where load transfer is strongly governed by interlayer interfaces. Here, a PLA/aspartic acid (Asp)/Fe₃O₄ composite system was developed to elucidate how filler and heterogeneity introduced during FDM printing affect thermal behavior, chain dynamics, interfacial properties, mechanical performance, and biocompatibility. The addition of Asp and Fe₃O₄ preserves PLA’s intrinsic thermal transition pathway while progressively lowering the glass transition temperature, indicating localized modulation of chain mobility. Tensile testing reveals reduced strength and ductility at medium-to-high Fe₃O₄ loadings, indicating that mechanical degradation is more closely associated with interfacial heterogeneity than with filler aggregation. Molecular dynamics simulations clarify the underlying mechanisms: single-layer models attribute T_g depression to altered segmental dynamics, whereas bilayer models show that interfacial heterogeneity, manifested by local stiffness–compliance mismatch and restricted cooperative chain deformation, causes stress concentration and premature failure. The composites exhibit the expected magnetic and shape memory functionalities, and preliminary in vitro assays confirm good cytocompatibility. These results provide insight into the multiscale structure–property–function relationships, emphasizing the key role of interfacial effects in balancing functional actuation, mechanical stability, and biocompatibility in FDM-based 4D-printed PLA composites.