It is often difficult to improve on a definition provided by everyone’s prima-facie source on science and technology. The Free Encyclopedia describes metamaterials thus:
Metamaterials (from the Greek word "meta-", μετά- meaning "to go beyond") are smart materials engineered to have properties that have not yet been found in nature. They are made from assemblies of multiple elements fashioned from composite materials such as metals or plastics. The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable to manipulate electromagnetic waves: by blocking, absorbing, enhancing, bending waves, to achieve benefits that go beyond what is possible with conventional materials. …. Metamaterial research is interdisciplinary and involves such fields as electrical engineering, electromagnetics, classical optics, solid state physics, microwave and antennae engineering, optoelectronics, material sciences, nanoscience and semiconductor engineering.
I have commented on microlattice structures very recently but the topic takes no rest and rather continues to heat up as German scientists at the Karlsruhe Institute of Technology have now built the smallest human-made truss to date, featuring single strut lengths <1 μm and strut diameters of 200 nm made of no composite but of mere glassy carbon – five times smaller than previously known and comparable metamaterials. This dimension achieves hitherto unprecedented strength-to-density ratios that, at the most preliminary consideration, suggest applications in electrodes, filters, or optical elements. Now, it is fairly common knowledge that light and partially hollow materials such as bone and wood may be found just about everywhere in nature. So proof of concept is old news. These materials typically combine high-load capacity with low weight and serve as a biomimetic model for man-made mechanical metamaterials with a structure planned and produced to have mechanical or optical properties that unstructured solids cannot match as a matter of principle. Think stealth features that direct light, sound or heat around objects, auxetic materials that react counter-intuitively to pressure and to shear, or nanomaterials featuring high specific strength (force per unit area and density). The metamaterial now created in Karlsruhe by 3D laser lithography is extremely stable and its strength in relation to its specific density of up to 3GPa is surpassed only by diamond.