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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. 

Abstract Magnetoactive elastomers (MAEs) are capable of large deformation, shape programming, and moderately large actuation forces when driven by an external magnetic field. These capabilities enable applications such as soft grippers, biomedical devices, and actuators. To facilitate complex shape deformation and enhanced range of motion, a unimorph can be designed with varying geometries, behave spatially varying multi-material properties, and be actuated with a non-uniform external magnetic field. To predict actuation performance under these complex conditions, an analytical model of a segmented MAE unimorph is developed based on beam theory with large deformation. The effect of the spatially-varying magnetic field is approximated using a segment-wise effective torque. The model accommodates spatially varying concentrations of magnetic particles and differentiates between the actuation mechanisms of hard and soft magnetic particles by accommodating different assumptions concerning the magnitude and direction of induced magnetization under a magnetic field. To validate the accuracy of the model predictions, four case studies are considered with various magnetic particles and matrix materials. Actuation performance is measured experimentally to validate the model for the case studies. The results show good agreement between experimental measurements and model predictions. A further parametric study is conducted to investigate the effects of the magnetic properties of particles and external magnetic fields on the free deflection. In addition, complex shape programming of the unimorph actuator is demonstrated by locally altering the geometric and material properties.