4D printing of TPU/PCL/Fe 3 O 4 composites exhibiting multi-stimuli responsive shape memory behavior

Abstract This work presents the development and characterization of multiresponsive shape memory polymer composites based on thermoplastic polyurethane (TPU), poly( ϵ -caprolactone) (PCL), and Fe 3 O 4 particles, targeting applications in 4D printing. The TPU/PCL blend provides mechanical strength, flexibility, and a pronounced shape memory effect, while Fe 3 O 4 enables remote activation under near-infrared (NIR) light and microwave radiation. Composites containing 15, 20, 25, and 30 phr of Fe 3 O 4 , derived from the TPU/PCL (50/50) blend, were compared to the unloaded blend. The shape memory behavior assessed via dynamic mechanical thermal analysis (DMTA) revealed excellent shape fixity (∼100%) and recovery (∼80%), with a slight decrease in recovery ratio (from 83% to 79%) as Fe 3 O 4 content increased. Under NIR and microwave stimuli, recovery efficiency improved with higher Fe 3 O 4 loading, reducing total recovery time (60–46 s) and activation time (22–11 s), with saturation above 25 phr. Self-healing properties were also evaluated, showing nearly complete repair in 120 s under NIR and microwave stimulation, while conventional oven heating required longer times (180 s) and was less effective. Additionally, adhesion tests demonstrated that after bonding, the samples retained 50% of their original tensile strength, and failure occurred outside the bonded region, indicating strong interfacial adhesion. Overall, the composites exhibited excellent multistimuli shape memory behaviour, rapid remote actuation, and strong self-healing, highlighting their potential for advanced smart materials and 4D-printed functional structures.