4D-printed shape memory alloy–polymer hybrid composites with tunable bidirectional actuation

This study presents a 4D-printed shape memory alloy (SMA) – thermoplastic polyurethane (TPU) soft composite actuator. Fabricated by co-extruding two independently addressable Nitinol wire circuits (0.3 mm ⌀) within a compliant TPU matrix, producing a single monolithic structure capable of reversible bidirectional bending. A constrained thermomechanical training protocol was used to induce two-way shape memory behaviour. Response surface methodology was applied to evaluate the effects of electrical current (1–3 A) and training cycles (20–40) on actuation performance. Applied current was found to be the dominant parameter (R² = 0.96) controlling normalised curvature difference, which reached a maximum of approximately 36% at 2.5 A, while the number of training cycles was not statistically significant within the investigated range. Under repeated actuation at 2.5 A, the normalised curvature difference decayed logarithmically over 50 cycles (R² = 0.99). The embedded wires were also assessed as intrinsic resistance-based sensors. Cycle-resolved analysis showed that the resistance–curvature relationship is unstable during the initial cycles but stabilises after approximately 10–15 conditioning cycles, with the median Spearman correlation exceeding 0.9 in both actuation directions. The proposed manufacturing-integrated approach demonstrates the potential of additively manufactured SMA–polymer composites for compact actuating and feedback-capable soft structures.