Towards Complex Shape Actuation: An Investigation of Local and Global Magnetoactive Gradients in 3D-Printed Multi-Stimuli Responsive Shape Memory Polymer Composites

Abstract In this research, we investigate multi-stimuli responsive multimaterial structures by combining shape memory polymers (SMPs) with magnetoactive fillers. Our objective is to design 3D-printed composites with local and global magnetoactive filler gradients, which exhibit complex shape actuation under magnetic and thermal fields. We first carry out a rheological study of SMP dispersions containing surface-treated magnetic particles to understand the effect of magnetic particle surface treatment, additives content, and shear rate on the complex flow behavior. Our findings reveal that dispersions filled with surface-treated magnetic particles exhibit enhanced shear thinning behavior and shape integrity compared to unfunctionalized dispersions. The improved rheological behavior and shape integrity are important results that indicate that PEG-functionalized SMP composites are promising candidates for direct ink printing. To create complex actuation, a 3D printing system is designed in a way that the magnetic particle-SMP dispersions are oriented using both shear and an external magnetic field, enabling a local angular gradient of magnetic particles. In addition, a global gradient is designed-in by varying the volume fraction of magnetic particles in the SMP suspensions. By adjusting the local and global gradients of magnetic particles within the SMP, different actuation patterns can be achieved. SEM analysis confirms the presence of the global gradient in iron oxide particles and their alignment along the magnetic field direction post-printing. Vibrating Sample Magnetometry (VSM) studies reveal an improved mass magnetization along the length of the printed samples, moving away from the printing origin. In addition, the iron oxide weight percent in the samples increases from 2.5 wt.% at the printing origin to 12.5wt.% at the end, creating a pronounced Fe3O4 global gradient. These findings contribute to the development of advanced stimuli-responsive materials with tunable properties for various applications where complex shape actuation is required, including soft robotics, and biomedical devices.