# Flexible Antennas: A Review
## 2 Materials for Flexible Antennas
### Substrates
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials Types** & **Conductive Materials** & **Conductivity, \(\sigma\) (S/m)** \\ \hline Metal nanoparticles & Ag nanoparticle [43] & \(2.173\times 10^{7}\) \\ \cline{2-3}  & Cu nanoparticle [44] & \(1\times 10^{6}\) \\ \hline \multirow{3}{*}{Conductive Polymers} & PEDOT:PSS [45] & 100–1500 \\ \cline{2-3}  & Polyaniline (Pani) [45] & 5 \\ \cline{2-3}  & Polypyrrole (PPy) [45] & 40–200 \\ \hline \multirow{3}{*}{Conductive Polymers with additives} & C nanotube [46] & 4000–7000 \\ \cline{2-3}  & PANI/CCo Composite [47] & \(7.3\times 10^{3}\) \\ \cline{2-3}  & AgNW/PDMS [23] & 8130 \\ \cline{2-3}  & Ag flakes + Fluorine Rubber [24] & \(8.5\times 10^{4}\) \\ \hline \multirow{3}{*}{Graphene Based materials} & Nanoflakes [48] & \(6\times 10^{5}\) \\ \cline{2-3}  & Paper [31] & \(4.2\times 10^{5}\) \\ \cline{2-3}  & Meshed Fabric [49] & \(2\times 10^{5}\) \\ \hline Liquid Metal & Eutectic GaIn [40] & \(3.4\times 10^{6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Conductive materials and their electrical conductivity values. been reported in a prior article [3]. Felt, fleece, silk, and Cardura, off-the-shelf (electro) textile materials, and standard apparel are a few examples of substrates that have been used for wearable/flexible antennas. The use of polydimethylsiloxane (PDMS) polymer as a substrate has been emerging because of its low Young's modulus (<3 MPa) suggesting high flexibility/conformality [72]. However, the development of a flexible antenna is limited on the PDMS substrate due to the weak metal-polymer adhesion. Nevertheless, some solutions to this issue have been found in literature such as implanting carbon nanotube sheets [73] or different microspheres like glass, phenol or silicate [73] or nanowires (AgNWs) [37; 74], injecting liquid metal [75], and oxygen plasma treatment on the PDMS surface [76; 77; 78]. Paper substrate has been preferred for flexible antennas due to the cost-effective nature and ease of manufacturing. A coplanar waveguide (CPW)-fed flexible UWB antenna operating from 3.2-30 GHz (161% fractional bandwidth (FBW)) for IoT applications on photo paper has been presented in [79]. [PERSON] et al. [80] have demonstrated a paper substrate-based flexible antenna design for intrabody telemedicine systems in the 2.4 GHz industrial, scientific, and medical radio (ISM) bands. Liquid crystal polymer (LCP) is a flexible-printed-circuit like thin-film substrate and regarded as attractive for high-frequency flexible antennas due to low dielectric loss, lower moisture absorption, resistant to chemicals, and can withstand temperatures up to 300 \({}^{\circ}\)C [81]. A flexible millimeter-wave (mm-wave) antenna array on a thin film of flexible liquid crystal polymer for the fifth-generation (5G) wireless networks operating at Ka-band (26.5-40 GHz) was presented in [82] and a dual sensed liquid crystal polymer-based metamaterial loaded circularly polarized flexible antenna on LCP substrate is proposed in [83] working in Worldwide Interoperability for Microwave Access (WiMAX) and wireless Local Area Network (WLAN) band. Table 2 shows commonly used substrates for flexible antenna fabrication along with their dielectric constant, dielectric loss, and thickness values. Figure 4 shows flexible antenna prototypes on different substrates. It can be concluded without a doubt the choice of substrate material is of paramount importance in the realization of flexible antennas. Due to their conformal behavior and operational suitability, flexible materials have gained immense interest. These flexible materials need to be chosen carefully to withstand the physical deformation conditions such as bending, stretching, and even twisting while maintaining its functionality. Flexible antennas require low-loss dielectric materials as their substrate and highly conductive materials as conductors for efficient EM radiation reception/transmission.