## 2 Non-FO Smart Materials: Metamaterials
### Metamaterial Designs
#### 2.2.1 Some Examples of Metamaterial Designs in SHM and Future Path
As stated earlier, the size of the metamaterial element reduces as we move from left to right in the EM spectrum, which implies extreme heat generations that affect the measuring devices. Thus, the designers should make their designs resistive to higher temperatures if they aim to build metamaterials for higher frequencies with smaller element sizes and subsequent smaller sample sizes. However, this enforces their application only in the far fields. The designs of Li group, or any small sized elements, can be adopted for monitoring of mechanical and aerospace engineering components or smaller civil engineering components in the far field setup. However, rugged civil infrastructures and most large mechanical or aerospace structures need open air acquisition as they cannot fit inside anechoic chambers. Thus, it is desirable to adopt a relatively larger element sizes for outdoor applications via near field setup using microwave range either in the Giga Hz or Tera Hz range. C) Design by [PERSON] et al. [34; 86; 87]  
The [PERSON] group developed a design which can be applied in a partial near field setup unlike the previous two cases. This is a comb-like nested SRR (or simply named as N-SRR) probe with metal strips, first proposed by the same group in 2010 [83]. This is not exactly a strain measurement design, but developed uniquely for tracking separation, another common engineering problem near the joints of engineering structures. This comb-like sensor consists of two identical parts separated by a distance and connected to each other via a thin flexible wire. These two parts are free to move with respect to each other. This N-SRR was fabricated over a footprint of 47 \(\times\) 47 mm\({}^{2}\) on a single-sided Rogers Duroid substrate with a relative dielectric constant of 3.2 and a thickness of 0.508 mm. A near field effect can be developed when this N-SRR probe is placed closer to a large antenna. The technical aspects of design can be obtained from their articles [34; 86; 87]. In their experiments, the input EM wave as well as the output were handled by the same antenna. This implies that the measurement can be obtained at the source/incident port 1 and hence \(S_{11}\) is obtained which was otherwise not possible in far field condition. However, their antenna is much wider with a feed line (to supply input) similar to the feedline of box/horn antenna commonly used in far field applications. Their experiment was conducted in steps, where a N-SRR probe was first attached to a standard steel rebar specimen. Later, a compressive force is applied linearly from 0 to 900 kgf where the elastic region and plastic region were monitored [86]. In the next stage, a 4-cm thick concrete block was placed between the antenna and the receiver probe. The receiver was found to function effectively even through a concrete block [34]. This design exhibited a relatively high resolution (<1 \(\upmu\)m) and sensitivity (>12.7 MHz/mm) over a wide distance of separation. Lately, this design was adopted to monitor a reinforced concrete beam in a simply supported condition. The N-SRR sensors were attached on the reinforcing bars inside the beam, and the antenna was placed outside the beam [87]. The results of the experiment show that the plastic deformation region strain/displacement can be detected wirelessly. The detailed study, including the sensitivity, resolution, and stress-strain results can be obtained from their work [34; 86; 87]. Path forward: The main problem with this N-SRR sensor is its dimension, which is much larger in the planar direction than the rebar specimen. Even then, it was adopted to monitor compressive load [34]. Such a situation should be avoided as the sensor is expected to be smaller than the engineering structure under investigation. The next problem is with the thin flexible wire, connecting two identical parts, which can break during the monitoring. However, a flaw/discrepancy in a deeper location up to 4 cm inside a structure can be detected.