## 2 Non-FO Smart Materials: Metamaterials
### Metamaterial Designs
#### 2.2.2 Our Near Field Metamaterial Designs  
Apart from the above discussed designs of SRR, I, crossed I, and N-SRR, there exist several other designs built using SPPs and localized surface plasmons (LSPs) materials which were validated for several applications including a few specially in SHM [67; 70; 82; 90; 91; 92]. Even these plasmon designs comprise of elements made of metal or plastic inscriptions on materials such as Rogers RT using PCB technique [68; 93; 94; 95; 96]. When these designs are subjected to EM waves in the microwave range, the output in the form of either progressive or surface waves is observed in the far or near fields. These designs can be used for various studies, including health diagnosis of biological and engineering structures [12; 73]. However, it is not always possible to attach or embed such metamaterial sensors to every engineering structure under inspection. Furthermore, huge and continuous structures such as bridges, airplanes, machines, MRT stations, and trains, can be neither monitored in isolation chambers nor by big antennas in public places. Hence, near field experimental setup, which does not require big antennas and anechoic chambers need to be developed. This method of application (see Figure 1b) can monitor any on-site engineering structure such as a bridge, MRT station, or train. Apart from the strain and separation measurements, we can extend metamaterials to various studies such as crack, damage, corrosion, and static and dynamic load monitoring. They can co-exist with FO based technologies that are mostly adopted for large engineering infrastructures [10; 14; 97]. Applications of metamaterial sensors or metamaterial inspired sensors in sciences, especially in biology and medical are well established, but they are still in the preliminary stages in CMA engineering, even after a decade of research efforts. Furthermore, each metamaterial sensor or design is discrete and as a multiple-metamaterial (network) sensing technique is not yet explored. 'Reshape and rethink' is a phrase attributed to metamaterial designs by [PERSON] et al. group [98]. Thus, considering all the aspects studied earlier, we developed a few simple near field based LSP designs inspired by the simplistic design of [PERSON] group [91]. We considered factors such as their resemblance, shape, size, properties, sensitivity, and their performance in comparison to other leading smart material sensors, especially the piezoelectric type of sensors which are already in use for SHM [12; 99]. We developed a radial-LSP design and a'microwave near field' software [100; 101]. Recently, a new longitudinal-LSP design is also introduced successfully by supplying near field EM microwavesto a force sensing resistor which generated diagnosable output surface waves. These types of LSP designs are highly non-fragile as they are manufactured as bi-material composites with metal inscriptions on flexible and ultra-thin dielectric or piezoelectric materials such as Rogers (RT5880), using PCB technique. They are bendable and twistable ranging from linear (0\({}^{0}\)) to double bend (180\({}^{0}\)) shapes [77; 102]. Figure 3a shows a photo of our radial-LSP sensor, which comprises of periodic grooves with period of 1.256 mm and 0.628 mm air gap between neighboring strips (radial elements), etched on an 18 um thick copper disk. This is engraved in a circular fashion on a 254 um thick Rogers RT5880 substrate [77]. It is symmetrical in the X-Y plane and offers EM leakage-free application in the near field. Figure 3b shows a photo of our longitudinal-LSP sensor, which comprises of longitudinal metal strides and provides a 38 \(\times\) 38 mm sensing area. Figure 3: _Cont._  
The working principle of our designs involves application of a pair of dipole antennas where one is a point source dipole for input excitations and the other is a receiver dipole for output probing/receiving (see Figure 1b). Furthermore, Z-axis is normal to the X-Y/sensor plane, which starts from the interface of metal and air.