## 2 Non-FO Smart Materials: Metamaterials
### Metamaterial Designs
#### 2.2.1 Some Examples of Metamaterial Designs in SHM and Future Path
Three sensors made of this design were surface bonded one each on three different structures made of a hard material (cast polyamide), semi-hard material (derlin), and a soft material (polyamide). Compressive loading tests from 28 to 271 kgf were carried out for all these specimens [82]. The hard material (high Young's modulus) shown a lower gradient in the frequency shift, whereas the soft material (low Young's modulus) shown larger gradient. The sensitivity of the metamaterial sensor was reported as 0.0543, 0.0577, and 0.119 MHz respectively for every 1 kgf compressive force. A one microstrain change resulted in 2.348 \(\times\) 10\({}^{-3}\) MHz, 2.224 \(\times\) 10\({}^{-3}\) MHz, and 3.224 \(\times\) 10\({}^{-3}\) MHz, respectively. Path forward: The work carried out by the [PERSON] group [79; 80; 81; 82; 83] are preliminary in nature and needs further investigations. They tested their designs on simple specimens in far field setup; which should be extended for real CMA engineering specimens. These designs should be adopted to monitor metals like steel with significantly high Young's modulus and non-metals like concrete or timber with lower Young's modulus. Furthermore, without massive commercialization, development of such sensors for individual research groups is costly, as it requires facilities such as chemical vapour deposition, metal evaporation, lithography, and lift-off techniques, apart from building expensive anechoic chamber. The frequency range adopted falls in the Giga Hz, on the right side within the microwave spectrum (Figure 1a). This can be useful for NDT adaptable in isolation, but people using CMA structures may wish to use the structures even during the period of monitoring. However, researchers can modify the size and shape of SRRs which forms the characteristic basis for highly sensitive designs. As stated earlier, lack of commercialization can enforce CMA engineers/scientists to opt for alternative designs, which they may have to fabricate based on complexity of the engineering structure under inspection, and they could be adopted in open air based near fields so that people can still use the engineering structure even while monitoring it. B) Design by [PERSON] et al. [76; 84; 85]  
Two metamaterial designs were built by [PERSON] et al, which were subjected to frequency excitation of lower order Giga Hertz (>10 GHz) that resulted in output signals suitable for strain monitoring. They preferred to call these designs as \"metamaterial inspired\" strain sensors rather than metamaterial strain sensors as their designs do not resemble the well-established SRR designs. Nonetheless, the concept adopted is the same as discussed in the previous designs. Instead of SRRs these researchersused simple I-shaped unit cells, suitable for single-axis strain sensing, and the other is a crossed-I (or +) shaped unit cell for dual-axis sensing. The elements of unit cells are made of Au, and the substrate adopted is PDMS. Both these designs comprise of several such unit cells that are directly surface bonded to a PDMS layer to make a metamaterial sensor which can then be attached to any test specimen. Compared to the silicon substrate (base material) as mentioned previously by [PERSON] et al. [4; 79; 80; 81; 82; 83], the PDMS substrate is not only cost effective, but it is a self-adhesive tape with high flexibility and excellent elastic properties. Furthermore, the sensitivities of these sensors are high, owing to the high surface tension of PDMS substrate. Both of these sensors have a maximum sensitivity of 107 GHz per microstrain under the applied stress. These sensors can even be attached to smooth surfaces without any additional adhesive, which facilitates direct strain transfer from engineering structure under inspection to the sensor to measure more direct strain. Path Forward: The designs of Li group are smaller compared to the earlier SRR designs. Furthermore, they can be varied by changing the dimensions of I-shaped and crossed I-shaped unit cells and the spacing between them.