Complex Magnetoactuation of 3D-Printed Hard Magnetic Elastomers—Effects of Infill Percentage and Poling Orientation

Magnetic elastomers are composite smart materials made up of a flexible polymer and magnetic particulate which mechanically articulate in response to an applied magnetic field (H). They allow for highly configurable and untethered actuators. To enhance configurability, 3D-Printing allows for great control of the object’s meso and macro structure, both in terms of shape geometry and property anisotropy. Specifically, Fused Filament Fabrication (FFF) is an additive manufacturing process by which material is extruded in lines (infill) to gradually build up an object through layer-by-layer extrusion. One of the clear advantages of FFF is that it requires no curing time after the piece has been printed. Magnetic elastomers with hard magnetic particulate, due to the presence of large remnant magnetization, have the unique advantage of performing more complex actuation, such as rolling, twisting, and folding. SrF is an excellent hard magnet candidate given its lower price and greater accessibility compared to Neodymium Iron Boride. Poling allows for consistency of the object’s remnant magnetization, giving more control and magnitude to its motion. In this study, we explore how the FFF structural parameter of infill percentage, sample orientation relative to H, and poling orientation affect the magnetoactive response of printed elastomer structures made up of SrF particulate suspended within a thermoplastic urethane polymer matrix. Rectangular sample beams are 3D-printed with dimensions of 5mm x 5mm x 60mm.Prior to testing, the samples are first poled in a certain direction, and then the magnetoactive properties are measured by a custom setup where each printed sample beam is suspended vertically and optically tracked for a transverse H up to 0.4 T. In order to isolate different components of the complex movement to see how our variable change affect magnetoaction, multiple orientations relative to H are studied. Sample sets are either studied for rotational motion or deflection, and within each set the sample itself is rotated 90 degrees each new testing cycle for four cycles. A custom MATLAB program overlays the digital images for increasing H in order to quantify the magnetoactive deformation that results. The magnetoactive responses exhibited by the various sample types highlight clear design goals for tailoring large and complex magnetoactive actuation modes. In addition to the traditional transverse bending seen from typical (magnetically soft) magnetic elastomers, we also observe complex twisting and torquing of the beams in multiple dimensions based upon the printed structure and the sample orientation relative to H. These effects are greatest for the orientations and structures that maximize magnetic anisotropy and minimize the infill crosslinking. This work demonstrates the clear potential to tailor complex magnetoactive responses from FFF-printed hard magnetic elastomers, opening the door to highly complex actuators that can be readily and accessibly designed and created.