From permanent bonding to programmable separation: AI-enabled circular design of multimaterial polymer 3D-printed structures

Multimaterial polymer additive manufacturing integrates rigid, compliant, conductive, reinforced, active, and sacrificial regions into a single part, but it has largely been governed by one overriding design objective: optimization of interfacial adhesion. Multimaterial polymer additive manufacturing integrates rigid, compliant, conductive, reinforced, active, and sacrificial regions into a single part, but its interfaces have mostly been designed with one goal in mind, which is maximum adhesion. That goal is essential for service reliability, yet it works against end-of-life recovery. Permanent interfaces limit repair, force replacement of whole components, hinder selective recovery of high-value materials, and can prevent closed-loop recycling. This critical review conceptualizes the printed interface as a lifecycle-controlled component and introduces programmable separation as an interface that remains mechanically reliable within a defined service envelope yet predictably releases in response to a deliberate mechanical, thermal, electrical, optical, solvent, moisture, shape-memory, or dynamic-chemical trigger. The review relates polymer wetting, chain interdiffusion, rheological compatibility, thermal history, residual stress, reinforcement, and interface topology to both fracture resistance and subsequent separability. It then assesses mechanically releasable joints, sacrificial interlayers, localized heating, photoresponsive systems, shape-memory attachments, supramolecular networks, dynamic covalent polymers, and chemically recyclable feedstocks. Emphasis is placed on recovery quality, including contamination, molecular-weight retention, reprintability, and performance over multiple cycles, rather than on mass recovery alone. Finally, an artificial-intelligence framework is proposed for simultaneous optimization of material, process, geometry, ageing, trigger, recovery, cost, and environmental descriptors using uncertainty-aware models and sequential experimentation. The evidence shows that no single release mechanism suits every case. The most realistic near-term route is to co-design specific materials, local interfacial chemistry, trigger delivery, and testing that covers both service and separation. Circular multimaterial printing therefore requires interface designs that not only carry load, but also release selectively and preserve material value at the end of service life.