Multifunctional composites: A metamaterial perspective

Multifunctional composites offer the ability to increase the efficiency, autonomy and lifespan of a structure by performing functions that would have been considered by designers as mutually exclusive. In the present perspective paper, a subclass of multifunctional composites is considered: metamaterials. In this perspective, a multifunctional composite is defined as 'made of two or more materials that perform two or more functions in a manner that is constructive to the overall purpose of the structure' where there is no differentiation between structural or non-structural functions. Equally, we define metamaterials are a class of man-made structures that display properties that are opposite to those typically found in nature. These 'engineered' architected materials continue to revisit and extend the boundaries of traditional materials science, opening up a wealth of new opportunities impacting on all aspects of human life. In our work, multifunctional metamaterials are delineated: electrodynamic, acoustic and mechanical. We review the current progress in these types of multifunctional metamaterials in terms of their bandwidth, fabrication techniques and applicability; noting that lattice structures offer considerable potential across all three functionalities. It culminates in the discussion of three key challenges which are seen by the authors as critical in the development of the next generation of lattice-type multifunctional metamaterials; namely, bandwidth, fabrication technique and proof of applicability. Success by the scientific community in these areas will lead to 3D multi-scale and multimedia lattice frameworks capable of influencing all three types of waves instantly; such a system would be a major technological breakthrough and will redefine our concept and understanding of multifunctional metamaterials in the next 10-20 years.