## 2 Non-FO Smart Materials: Metamaterials
### Metamaterial Designs  
In this world, there exist several naturally formed materials or organized materials or gases which create illusions by deflecting EM radiations due to their negative permeability and permittivity such as mirages [36]. The initial contributions to understanding such a behavour were made about five decades ago by [PERSON] who predicted that the materials with both negative permittivity and permeability are theoretically possible to be constructed in future [37]. However, the major contribution only came three decades later from [PERSON] group, especially towards the development of various far-field based metamaterial designs. They proposed periodically arranged wired based TW structural design which depicts the negative value of effective permittivity [38]. It was shown that the structure had a lower plasma frequency than the wave in the microwave regime. Similar behavior was achieved using an array of SRR design [39]. In 2000, [PERSON] et al. demonstrated a new LHM which produced simultaneously negative permittivity and permeability, and carried out microwave experiments [40]. Later, [PERSON] et al. showed negative refraction experimentally for the first time using a metamaterial design comprising of repeated SRR unit cells and copper strips [41; 42; 43]. In 2005, [PERSON] et al. proposed three types of SRR designs, viz., symmetrical ring, omega, and S shaped sensors [44]. Furthermore, these designs have smart cavities or spacings between the elements, which are capable of altering EM waves by blocking, absorbing, enhancing, or bending. Such designs can be considered as part of a 'family of smart materials' useful for SHM. Figure 2a shows an example of a typical SRR design consisting of vertical patterns where each pattern has three-unit cells. Each cell consists of an inner metal square with a split on one side, embedded in an outer square with a split on the other side. The SRRs are on the front surfaces of the square grid as shown, whereas the single vertical wires are on the back surfaces [35; 40; 45]. All types of SRR single cells generally have a pair of enclosed loops with splits in them, at opposite ends, as shown in the figure. The loops are made of non-magnetic metal like copper with a small gap between them and they can be concentric, square, rectangular, or gapped as needed. Figure 2b shows an example of an embedded copper element of 7.6 mm \(\times\) 7.6 mm on a circuit board sample of about 11 cm \(\times\) 5 cm \(\times\) 2 mm, known as inclusions. Various other designs are possible by merely changing the element shape and size [46]. Figure 2: Typical examples (**a**) various orientations of SRR on a circuit board, and (**b**) rectangular metamaterial sample. Most of such designs and their hybrids [39; 47; 48] gained importance academically, especially in the UK ([PERSON] group at Imperial college), which expanded from USA (especially [PERSON] group at Duke University) to Asia (especially in China, Singapore, and Japan) for applications in electronics and sciences; but negligible for SHM. They were applied in super-lenses [48; 49], slow light [50; 51; 52], data storage [53], optics [54], cloaking/invisibility [55; 56; 57; 58], sub-wavelength imaging [59], gradient negative index lenses [60], perfect absorbers [61], metasurface [62], photonic topological insulators [63], and so on. Until now, a variety of metamaterial designs have been successfully developed for a fairly wide range of the EM spectrum (Figure 1a) such as microwave, infrared and optical frequencies [64; 65; 66]. However, they were mostly adopted for non-CMA structures, and their latest entry for SHM of CMA structures [67; 68; 69; 70; 71; 72] started attracting engineering groups like ours. This is because of their swift output variations to the geometrical changes in the unit element or assemblies of elements [73], making them suitable for monitoring of'strain', a common engineering problem for most CMA structures.