Thermally recoverable PLA/TPU hooks for durable 4D-printed mechanical interlocks

Abstract Reversible mechanical interlocking is widely used in fastening and attachment systems, yet polymer-based interlocks typically suffer from irreversible deformation and progressive loss of load-bearing capacity under repeated use. This work presents a 4D-printed laminate architecture that integrates stiff shape-memory polylactic acid (PLA) with compliant thermoplastic polyurethane (TPU) to enable recoverable, fatigue-resistant interlocking interfaces. By spatially organizing the two polymers into a controlled sandwich architecture, shape-memory functionality is decoupled from the stiffness of PLA and ductility of TPU, allowing mechanical performance and recoverability to be independently tuned. The thermomechanical response of the laminates was characterized under cyclic programming and recovery at two activation temperatures. Programming near the glass transition temperature resulted in architecture-dependent recovery stability, whereas activation near the cold-crystallization regime produced uniformly high shape fixity and high recoverability across all laminates. When implemented in bio-inspired hook geometries, the optimized laminates exhibited improved cyclic interlocking performance compared to monolithic PLA. While single-hook designs remained susceptible to fatigue due to stress localization, a double-hook geometry distributed load more effectively, suppressed interfacial delamination, and preserved both force and energy dissipation under repeated cycling with thermal recovery. These results establish a material and architecture framework for reusable, shape memory-enabled interlocking systems fabricated via multi-material additive manufacturing.