# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.3 Wearable FSSs  
Recently, modulated FSSs have been proposed as transponders for RFID applications. Earlier researches show the possibility of these transceiver devices for localization use in the UWB radars [116] or frequency modulated continuous wave radars [178]. Another work [179] presented a semi-passive RFID arrangement having low power utilization modulated FSSs that could be attached to the body for wearable applications based on back-scattering communication in 2.45 GHz ISM band. This also includes a temperature sensor that works on modulation of the back-scattered field of FSS as an initial proposal. It displays a reading range of greater than 3 m for near-line-of-sight situations. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**FSS Ref.** & **Type of AFSS** & \
\begin{tabular}{c} **Switching/Tuning** \\ **Element Used** \\ \end{tabular} & **FBW\%** & **Tuning Range \%** &
\begin{tabular}{c} **Polarization** \\ **Insensitivity** \\ \end{tabular} \\ \hline
[169] & Switchable & PIN diode &
\begin{tabular}{c} 2 (-20 dB), \\ (fr = 12.6 GHz) \\ \end{tabular} & – & Yes \\ \hline
[151] & Switchable & PIN diode & 40 (\(-5\) dB) & – & No \\ \hline
[156] & Switchable & PIN diode & 79.5 (\(-10\) dB) & – & Yes \\ \hline
[154] & Switchable & PIN diode & 39.1 (\(-3\) dB) & – & No \\ \hline
[155] & Switchable & PIN diode & 7.3 (\(-15\) dB) & – & Yes \\ \hline
[157] & Switchable & PIN diode & 65 (\(-10\) dB) & – & No \\ \hline
[23] & Switchable & PIN diodes &
\begin{tabular}{c} 17 (\(-5\) dB), \\ (fr = 2.63 GHz) \\ \end{tabular} & – & No \\ \hline
[171] & Tunable (Storphand) & Varactor diode & \(12.5\) (\(-10\) dB) & 32 & No \\ \hline
[177] & Tunable Absorber & Varactor diode & \(4\) (\(-5\) dB) & 5 & No \\ \hline
[176] & Tunable (Storphand) & Varactor diode & \(81.4\) (\(-10\) dB) & 69.5 & No \\ \hline
[172] & Tunable & Varactor diode & \(152.4\) (\(-20\) dB) & 129 & Yes \\ \hline
[168] & Tunable (Storphand) & \begin{tabular}{c} High-permittivity \\ ceramics \\ \end{tabular} & \
\begin{tabular}{c} 25.8 (First state), \\ 8.4 (Second State (\(-10\) dB) \\ \end{tabular} &
\begin{tabular}{c} 22.9 \\ (1 st Resonance) \\ \end{tabular} & No \\ \hline
[139] & Tunable Absorber & PIN diode & \(91\) (\(-10\) dB) & 31.6 & Yes \\ \hline
[162] & Tunable (Passband) & Chip capacitor &
\begin{tabular}{c} 5.9 (\(-5\) dB), \\ (fr = 8.5 GHz) \\ \end{tabular} & 30 & No \\ \hline
[159] &
\begin{tabular}{c} 3D Tunable AFSS \\ (Storphand) \\ \end{tabular} & Graphene micro-ribbons & 5.5 (\(0.5\) 521 max) & 10 & No \\ \hline
[164] & Tunable Absorber & PIN, Varactor diode & \(113\) (\(-10\) dB) & 47.3 & No \\ \hline
[175] & Tunable 3D AFSS & Varactor diode & \(14\) (\(-3\) dB) & 65 & No \\ \hline
[163] & Tunable (Passband) & Varactor diode & \(11.5\) (\(0.5\) 521 max) & 24.6 & No \\ \hline \hline \end{tabular}
\end{table} 
Table 9: An overview of types of AFSS with the comparison of their FBW and tuning range (TRs). Diversity is anticipated as a distinctive property for transponders based on FSS because numerous FSS devices can be modulated at the same time, while sharing the modulated signal from a sensor at low-frequency by use of wires integrable into the fabrics. The inherent characteristics of the larger BW of FSSs offer robustness against the de-tuning originated by the variation of the electrical features of the body [179; 180]. The FSS can be directly in contact or a bit spaced with the body/skin for tiny textile stuff without any additional adjustments (fine change of dipole length). The FSS device can be positioned in different parts of the body or by using different surfaces, as required by the application. For example, an optical heart rate monitor that is designed with an LED and a photodiode can be attached to the finger, as seen in [181]. The interior analog to digital converter (ADC) of the microcontroller is used to digitize the photodiode output, and later the blood flow modulates its amplitude. From the samples of ADC, the heart rate is acquired by time measurement between two successive peaks.