Enhancing interfacial toughness of 3D printed bi-material polymers via mechanical interlocking and engineered fiber bridging

Combining materials with distinct properties enables the fabrication of complex structures unattainable with a single material. Hybrid structures, such as those combining stiff and flexible materials, have potential applications in morphing structures, medical prosthetics, and sports goods. The performance of these structures relies heavily on the bonding at the material interface, especially with flat interfaces. Poor bonding can lead to structural failure. This study investigates the bi-material 3D printing of Polylactic Acid (PLA) and Thermoplastic Polyurethane (TPU). PLA is chosen for its environmental friendliness and mechanical properties, while TPU is selected for its flexibility and deformability. The main problem is the weak bonding between PLA and TPU in Fused Filament Fabrication (FFF), often causing failure in the final object. We introduce a methodology to emulate and control the 'fiber bridging' effect occurring in Fiber-Reinforced Polymers (FRP), which enhances interfacial strength. By designing specific patterns and strategically sequencing materials, we create robust mechanical bonds between PLA and TPU. These interface designs significantly increase toughness, improving both bi-directional and unidirectional composites by up to two orders of magnitude. Furthermore, the proposed approach holds potential for application in other multi-material systems, making it a promising strategy for a broader range of material combinations.