Direct Ink Writing (DIW) of Mechanically Graded and Programmable Crosslinked Polymers with Seamless Interlayer Fusion

Direct ink writing (DIW) enables the fabrication of complex polymer architectures under mild processing conditions, yet the production of mechanically graded multimaterial structures remains limited by weak interfacial adhesion and by the restricted availability of printable thermoset feedstocks. Here, we introduce a high internal phase emulsion (HIPE)-based printing platform for the fabrication of mechanically programmable crosslinked polymers with seamless interlayer integration. Oil-in-water HIPE inks were formulated from methacrylated dextran (DexMA) in the continuous phase and mixtures of methyl methacrylate and hexyl methacrylate in the dispersed phase, yielding printable precursors that generate crosslinked networks spanning elastic moduli from below 1 MPa to above 150 MPa after curing. Rheological and morphological analyses demonstrated that the different formulations possess comparable printability despite their markedly different final mechanical properties. Photo-rheological experiments revealed a two-stage curing process, consisting of rapid DexMA crosslinking followed by slower polymerization of the monomer-rich phase, providing a temporal window that promotes molecular interpenetration across adjacent deposited filaments. This mechanism enables the formation of intermolecularly fused interfaces rather than conventional layer-by-layer contacts. Monomaterial and multimaterial constructs were fabricated and mechanically characterized under compression. While multimaterial architectures exhibited compressive moduli comparable to intermediate-composition controls, they displayed substantially enhanced toughness and enabled programmable deformation and recovery behaviors. The ability to sequentially combine mechanically distinct formulations within a continuous crosslinked network, while maintaining robust interlayer cohesion, establishes HIPE-based DIW as a versatile strategy for the fabrication of mechanically graded thermoset architectures whose performance emerges from the interplay between material composition and spatial design.