Composition Optimization of Additively Manufacturable Magnetic Composite Silicones for Soft Robotic Applications

Magnetic composite silicones, soft silicone matrixes with magnetic micro particles dispersed within, are highly functional materials used in a variety of applications. Their stimuli responsivity to controllable external magnetic fields is especially useful in soft robotics used for biomedical applications, remote actuation, inductive heating, magnetostriction, and more. 3D printing of these magnetic composite silicones further enhances their functionality as it enables preprogramed actuation through magnetic alignment, as well as freeform manufacturing of intricate designs and patterns. Rheological modifiers such as fumed silica are necessary to enable direct ink writing of liquid solutions. This results in a composite soft material of two micro/nano additives in a soft polymeric matrix with components that have complex interactions between themselves and with each other. We studied the rheological, mechanical, and magnetic properties of additively manufacturable magnetic composite silicones with all possible composition permutations. We found that incorporating fumed silica into uncured silicones significantly increases viscosity at low shear while exhibiting shear thinning behaviour, both of which are necessary for 3D printing. On the other hand, incorporating magnetic particles into uncured silicone increases viscosity. However, adding magnetic particles to silicone solutions that already contain fumed silica decreases viscosity instead. Maximizing the amount of magnetic particles in silicone is optimal for magnetic actuation, but the necessity of incorporating additional fumed silica significantly increases the stiffness of the composite materials that are desired to be soft. Understanding the interactions between micro neodymium magnetic particles, nano fumed silica particles, and the silicone matrix enabled us to find the optimal material for soft robotic applications. We then demonstrate the utility of this material through 3D printing of intricate auxetic structures which expand when exposed to an external magnetic field, further advancing the capabilities of magnetic soft robotics found in the literature.