The interface is the part: designing functional, graded, and multi-material architectures by multi-polymer fused filament fabrication

Multi-polymer fused filament fabrication (FFF) has matured into a practical route to the capabilities central to virtual and physical prototyping: functional, multi-material, functionally graded, 4D, and lightweight architectures built in a single job. These hardware advances now place several polymers within one build, so that core–shell, layered, sandwich, graded, and lattice parts can be designed from function. Yet the promise of ‘any material, anywhere’ meets a stubborn reality: in dissimilar-polymer parts, mechanical failure frequently initiates at the interface rather than in the bulk. The interface – not the constituent materials – is therefore the true design object, and it decides whether a geometrically successful print is structurally reliable. This review reframes multi-polymer FFF around that principle. We organise the enabling hardware; synthesise the physics of interfacial bond formation (wetting, neck growth, reptation-controlled inter-diffusion, thermal mismatch, shrinkage, and miscibility); map polymer-pair compatibility as a design-for-AM aid; connect it to the mechanics of hybrid and graded architectures; and critically and quantitatively compare interface-engineering strategies, from interlocking, surface, and thermal treatments to compatibilizer and filler-based interphases. Written for researchers and the design and process engineers who choose polymer pairs, architectures, and treatments, we close with applications and the key standardisation, modelling, and materials-design gaps.