## 6 SAR for Antennas on Human Body
### Different Techniques to Reduce SAR Value
#### 6.1.4 Reduction Using Frequency Selective Surfaces (FSSs)  
Special stopband or passband features of frequency selective surfaces (FSSs) are utilized to reduce the SAR. To reduce the SAR and boost gain, a 120 \(\times\) 120 mm\({}^{2}\) FSS textile-based superstrate was used in a square wave loop, backed by a 50.5 \(\times\) 38 mm\({}^{2}\) compact monopole antenna [103]. The square-shaped modified monopole radiator with a defective ground structure radiated in the 2.45 GHz ISM frequency range with a bandwidth of 2.075 to 2.625 GHz. The achiral FSS provided consistent performance for both TE and TM polarizations, with a progressive increase in inductance [104]. A 120 \(\times\) 120 \(\times\) 30 mm\({}^{2}\) antenna system with an FSS had a maximum gain of 7.76 dB and a fractional bandwidth of 22.44 percent. The FSS reduced back radiation, resulting in a 95% reduction in SAR levels. Figure 15 depicts the cross-shaped structure of an FSS [93].  
Table 10 shows many SAR reduction methods for wearable antennas that work in different frequency bands and tissue conditions. The EBG approach can reduce the SAR from 7.18/7.96 W/kg to 0.31/0.42 W/kg at 2.45/5.8 GHz using Rogers Duroid and textile substrates [105]. Metamaterials in copper form reduce the SAR from 7.78 W/kg to 0.0283 W/kg at 2.4 GHz [106]. The AMC method, using Taconic CRE-10 PCB material, lowers the SAR to 0.7 W/kg at 1.97 GHz [107; 108]. By using silver nanoparticles on textiles, the FSS approach reduces the SAR from 5.1972 W/kg to 0.1641 W/kg at 2.45 GHz. Additionally, metal reflectors with aluminum shielding exhibit an SAR of 0.20 W/kg at 2.44 GHz [109]. Advanced materials like flexible PCBs and meta surfaces can lower the SAR to 0.0646 W/kg at 5.2 GHz when used with flexible conductive coatings [105]. These technologies and materials, such as flexible polymers, conductive coatings on textile substrates, and ceramic-based materials, have effectively improved user safety in wearable antennas [110; 111; 112; 113; 114; 115; 116]. This comprehensive study emphasizes the importance of material science and inventive design in wearable SARs.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Refs. No.** & **Techniques Used** & **Material Used** & \begin{tabular}{c} **SAR Without** \\ **Reduction** \\ **(W/kg)** \\ \end{tabular} & \
\begin{tabular}{c} **SAR with** \\ **Reduction** \\ **Technique (W/kg)** \\ \end{tabular} &
\begin{tabular}{c} **SAR per** \\ **1 or 10 gm Tissue** \\ \end{tabular} & **Frequency (GHz)** \\ \hline
[64] & Electromagnetic band gap & [PERSON] & 7.18/7.96 & 0.31/0.42 & 10 & 2.45/5.8 \\ \hline
[69] & Metamaterial & Textile & 7.78 & 0.0283 & 1 & 2.4 \\ \hline
[98] & Electromagnetic band gap & Denim & - & 0.364/0.165 & 1/10 & 2.4 \\ \hline
[103] &
\begin{tabular}{c} Artificial magnetic \\ connector \\ \end{tabular} & Taconic CRE-10 & 1.4 & 0.7 & 10 & 1.97 \\ \hline
[105] & Metal reflector & Aluminum & - & 0.20 & 1 & 2.44 \\ \hline
[110] & Electromagnetic band gap & Fabric-Based & 5.77/6.62 & 0.024/0.016 & 1 & 1.8/2.45 \\ \hline
[106] & Electromagnetic band gap & Textile & 5.41 & 0.0545 & 1 & 2.4 \\ \hline
[107] & Electromagnetic band gap & Roger’s Material & 2.36 & 1.77 & 10 & 2.4 \\ \hline
[108] &
\begin{tabular}{c} Frequency selective \\ surface array \\ \end{tabular} & Textile & 5.1972 & 0.1641 & 1 & 2.45 \\ \hline
[109] &
\begin{tabular}{c} Artificial magnetic \\ connector \\ \end{tabular} & denim & 46.9 & Less than 1.6 & 1 & 5.8 \\ \hline
[110] &
\begin{tabular}{c} Artificial magnetic \\ connector \\ \end{tabular} & Polymer & 2 & 0.29 & 1 & 2.45 \\ \hline
[111] & Electromagnetic band gap & PDMS & 6.56 & 0.0251 & 1 & 2.4 \\ \hline
[112] &
\begin{tabular}{c} Artificial magnetic \\ connector \\ \end{tabular} & Flexible & 4.2 & 1.24 & 10 & 24 \\ \hline
[113] & Meta surface & \(\mathrm{RT}/\mathrm{D}\mathrm{u}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{o} \mathrm{