3D-Printed Permanent and Impermanent Magnetic Elastomer Architectures—Identifying Key Structure and Processing Parameters for Enhanced Magnetoactive Performance

Magnetic elastomers are of interest for soft robotic applications where remote contactless activation is desired. These composite smart materials are made of magnetic particulate embedded in an elastomeric matrix and respond to applied magnetic fields with mechanical deformation, termed magnetoaction. It’s been shown that anisotropic structures and properties within these materials leads to increased magnetoactive performance. Magnetic annealing is the main processing technique that has been used to increase magnetoactive performance through the introduction of local anisotropy. For further enhancements to magnetoaction, the 3D-printing technique, Fused Filament Fabrication (FFF), can be used to create additional anisotropy in the structures and properties of magnetic elastomers. FFF architectures create anisotropic sub-structures within a part by extruding 1D lines of the molten material (termed "infill") in 2D patterns that build up to a 3D part. This infill pattern is entirely customizable with respect to orientation and percentage filling allowing for one to readily tune the properties of the resulting part. While it’s been clearly shown that these parameters significantly influence the resulting mechanical and magnetic properties of FFF-printed parts, their influence on magnetoaction (the confluence of mechanical and magnetic responses) has yet to be reported.<br/><br/>Here, we investigate how a variety of parameters on multiple length-scales influence magnetoaction in FFF-printed magnetic elastomers. In particular, we study how FFF <i>infill percentage </i>and <i>infill orientation</i> relative to applied magnetic field influences magnetoaction of the printed part. We also study a variety of <i>soft vs. hard magnetic particulates</i> in order to understand how these distinct magnetic material properties interact with the various 3D printed structural features.<br/><br/>A custom setup and approach has been developed to meticulously measure resulting magnetoactive effects by isolating the deflection and rotational components of any complex motion observed. For both modes, the sample is suspended from a fixed point and a transverse DC magnetic field is applied up to 0.4 T. The degree of magnetoaction is quantified as the angle of deflection or rotation of the sample for a given applied field. The data is captured visually in digital images that are then processed in order to determine the quantitative deflection or rotation angle for a given applied field. Supporting magnetic, mechanical and structure (SEM) studies are utilized to investigate the origins of the differences in magnetoaction.<br/><br/>For the FFF architectures studied, we find that the greatest magnetoaction is found for a combination of lower infill percentages and infill oriented perpendicular to the applied magnetic field. These effects arise from both maximizing anisotropy and limiting infill crosslinking within the structure, which greatly affects mechanical stiffness and resistance to motion. Moreover, we find that the soft magnetic particulates (Fe and Fe₃O₄) give the greatest magnetoaction, but only simple, noncomplex motion; meanwhile, the hard magnetic particulate (SrFerrite) gives complex magnetoactive responses involving motion in multiple dimensions and is intricately dependent on a host of variables studied. All of these results together highlight a number of key structure and processing parameters that lead to enhanced magnetoactive performance in 3D-printed magnetic elastomer architectures.