## 2 Non-FO Smart Materials: Metamaterials
### Metamaterial Designs
#### 2.2.1 Some Examples of Metamaterial Designs in SHM and Future Path  
Different criteria are used to differentiate between different applications, for example, application in civil engineering is different from application in biology. The first criterion is the input/output wavelength (or frequency) and the other is the design (pattern and element sizes) of metamaterial. In the last 10-12 years, researchers and designers from material, electrical, and electronic engineering are trying to establish their designs in the CMA engineering market apart from the regular market in chemical, biological, and medical sciences. CMA engineers and scientists need to understand the pros and cons of these designs or sensors before recommending them. This section presents some such designs available in the literature for strain applications, and the path forward. A) Designs by [PERSON] et al. [79]  
Measuring and reporting strain in structural components using telemetric methods represents a significant engineering challenge. About two decades ago, initial efforts in developing of metamaterial designs for SHM of CMA came from [PERSON] group. They proposed metamaterial-based strain sensors, which are highly sensitive to deformation by improving their previous design architectures that were implemented for bio applications [4; 80]. They developed a 5 \(\times\) 5 SRR architecture which comprises of patterns along vertical and horizon directions similar to that shown in Figure 2b, but greatly reduced in dimensions. This metamaterial sensor comprises of a base silicon substrate of 0.1 \(\upmu\)m thick Si3N4, which was deposited by plasma-enhanced chemical vapor deposition method. On the top of this substrate, a pattern of 0.1 \(\upmu\)m thick 'Au' film was built using standard lithography, metal evaporation, and lift-off technique; to achieve an SRR structure with a 2220 \(\upmu\)m outer length and a 1500 \(\upmu\)m inner length. The unit cell length comprises of 2780 \(\upmu\)m, and hence a 5 \(\times\) 5 array of the SRR resulted in a total of 1.5 cm\({}^{2}\) sized metamaterial sensor. Their experiment comprised of far field setup in an enclosed anechoic chamber (Figure 1c). The SRR array sensor was surface bonded using epoxy on a polyamide stick specimen for strain monitoring. This specimen was then subjected to a compressive load from 0 to 300 kgf. Technically, the compression resulted in a decrease of the dielectric area of the base substrate and capacitance; whereas an increase in spacing between the Au metal elements or between the inner and outer SRRs in a unit cell resulted in an overall increase in the resonance frequency. As stated earlier, in a far field setup only transmission signals are possible, which are acquired in a narrow range of 12 to 12.6 GHz. They reported a sensitivity of 109 kHz/kgf or 5.148 kHz/microstrain with error of 200 microstrain and a frequency shift of 109 kHz for every 1 kgf compressive force, or 5.148 kHz shift for every microstrain increment. Later, the same research group proposed a flexible metamaterial design with increased sensitivity, suitable for attaching on any curved or nonplanar surface. In their previous design they used a silicon substrate, whereas in this design they used flexible substrate (Kapton) tape as base material. This is a vacuum tape of the polyimide type with a great heat resistance, which is generally used in fabrication and packaging. This tape contains silicone adhesive on the back side that does not leave any residue when the tape is removed [71; 81]. This new design has greater sensitivity and a more linear response to compressive loading compared to their previous designs. This exhibited a 6-fold increase in sensitivity and a 16-fold reduction in error. Frequency shifts by 0.292 MHz for every 1 kgf compressive force. Later, the same research group presented another design for compressive load monitoring.