Shape memory behaviour of 4D-printed PLA-TPU core-shell structures under repeated programming-recovery cycles

Purpose This study aims to systematically investigate the cyclic shape memory behaviour of 4D-printed polylactic acid (PLA)-thermoplastic polyurethane (TPU) core-shell structures subjected to repeated programming-recovery cycles, with emphasis on understanding the influence of filament-level architecture and printing parameters. Design/methodology/approach Core-shell PLA-TPU filaments were fabricated using a dual-extrusion fused deposition modelling process and reprinted into shape memory specimens. Cyclic thermomechanical tests were conducted under repeating programming-recovery cycles at 70 °C recovery temperature. A Box-Behnken response surface methodology was used to evaluate the effects of TPU content (30%–50%), layer thickness (0.1–0.3 mm) and print temperature (210–230°C). Shape recovery after the third cycle and a cyclic stability index (CSI) were selected as response variables. Findings The results demonstrated that the core-shell 4D-printed specimens exhibit excellent shape fixity performance, with shape fixity ratios exceeding 98%–99%. Shape recovery also remained high after repeated programming-recovery cycles, ranged from 80.9% to 89.7% depending on processing conditions. The CSI, which serves as a measure of shape recovery retention over multiple programming cycles, was found to range between 84.9% and 92.2%. It was found that the PLA shell effectively preserved the programmed geometry, while the TPU core played a major role in elastic recovery. Increasing TPU content significantly improved both shape recovery and cyclic stability. Layer thickness exhibited a pronounced non-linear effect, with reduced performance at intermediate values (0.2 mm). Print temperature showed no statistically significant influence within the investigated range (210–230°C), indicating robustness of the core-shell architecture against moderate thermal variations during printing. Practical implications The proposed approach enables scalable and cost-effective production of cyclically stable 4D-printed structures without material blending or specialised co-extrusion systems, supporting applications requiring repeatable thermally activated deformation. Originality/value While core-shell architectures have been reported in the literature, existing studies have primarily focused on morphological and basic mechanical characterisation. Systematic investigations of functional behaviour, particularly cyclic thermomechanical shape memory performance of core-shell architectures, remain limited. This study addresses this gap by quantifying the influence of processing parameters on cyclic shape memory performance in 4D-printed PLA-TPU core-shell structures fabricated via a print-and-reprint approach.