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About The Cover

Cover image expansion

COVER Cellular structures found in nature are often imitated in synthetic materials like polymeric and metallic foams. New technologies like four-dimensional printing are being used to create cellular structures with various characteristics that display unique mechanical, functional and thermal properties. These materials—known as metamaterials—generally do not respond to a changing environment because the material and architecture are fixed in time after their creation. Seeking a material that better adjusts to the stimuli in their environment, Jackson et al. incorporated magnetorheological (MR) fluid suspensions into three-dimensional-printed polymer tubes to create a new class of FRMMs. The MR fluids line up under a magnetic field, thus making them stiffer. The authors demonstrated their approach by printing cuboctahedron unit cells with controlled fluid delivery methods and testing them in a custom device in which the magnetic field strength could be specially regulated. They found that the average response time when hardening was less than a second and, when turned off to soften, was less than a quarter of a second. The researchers further developed a model to predict the responsiveness of their FRMMs in different conditions. [CREDIT: JULIE A. JACKSON]