# Flexible Antennas for a Sub-6 GHz 5G Band:
## 2 Materials for a Flexible Antenna
### Substrate Materials  
Substrate materials play a crucial role in the design of any flexible antenna. The substrate used in a flexible antenna should have a low relative permittivity, a low loss tangent value, and a high thermal conductivity. Since flexible electronics have gained greater application in body area networks, the choice of the flexible substrate should be in such a way that it should be very conformal to maintain the deformation criteria, such as bending, stretching, etc. Various substrate materials are seen in the literature for the development of flexible antennas, as shown in Table 3. Paper, fabrics, plastic, and polymer substrates are commonly used in the development of flexible antennas. Paper has many advantages to be used as a flexible substrate since it is cost-effective and easy to manufacture. Paper substrates are low \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Conductive Materials** & **Conductivity, \(\sigma\) (S/m)** & **Advantages** & **Disadvantages** \\ \hline Silver Nanoparticles & \(2.173\times 10^{7}\) & High electrical conductivity & Expensive \\ \hline PANI/CCo Composite & \(7.3\times 10^{3}\) & Good mechanical properties & Low conductivity \\ \hline Paper & \(4.2\times 10^{5}\) & Easy availability & Low Stability \\ \hline Meshed Fabric & \(2\times 10^{5}\) & Easy availability, Low profile & Low fabrication accuracy \\ \hline Nano Flakes & \(6\times 10^{5}\) & Good thermal stability & Less radiation efficiency \\ \hline Graphene Assembled Film & \(1\times 10^{6}\) & High electrical conductivity & Expensive fabrication \\ \hline Copper Nanoparticle/ & \(1\times 10^{6}\) & Good conductivity, good radiation characteristics & High oxide formation \\ \hline AgHT & \(2.2\times 10^{5}\) & Transparency & Low resistivity \\ \hline AgNW & \(8.1\times 10^{5}\) & Conformality & Low durability, high roughness \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Conductive materials for a flexible antenna. Reprinted from refs. [27; 28]. profile and lightweight and thus make them easier to be integrated into flexible devices. One of the main disadvantages of paper, as a substrate for the design of flexible electronics, is that when given continuous bending the paper becomes very fragile, thus degrading the antenna performance completely. Another disadvantage of the paper substrate is its high moisture absorption which in turn affects the antenna performance [33]. Textile or fabric substrates are seen in the literature for the design of flexible antennas. Fabrics are low in profile and can be easily integrated with the conducting materials, making them feasible to be used in wearable applications. However, fabrics have certain disadvantages, such as a high level of moisture absorption, and they are also prone to cracks after many bending cycles. This turns out to be a notable disadvantage while used in medical applications that involve the repeated bending of antennas around different radii [34]. In general, fabric materials possess an extremely low dielectric constant that further reduces the surface wave losses and improves the impedance bandwidth of the antenna. The measured dielectric values of the different textile fabrics are provided in [35]. The polymer-based substrate is used in the field of wearable electronics due to its stretchability, flexibility, and robustness [24]. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are utilized in several flexible antennas due to their outstanding mechanical properties as well as their resistance to moisture absorption [29; 36]. Liquid crystal polymer (LCP), which is a thin film substrate, is utilized in the development of a flexible antenna due to its very low loss tangent, low dielectric constant, and low thermal expansion characteristics. The low thickness of the LCP substrate reduces the antenna volume and also enables it to work in a bending state [37]. In [38], a flexible PIFA antenna is designed using a PEN substrate for 5G applications.