# Flexible Antennas for a Sub-6 GHz 5G Band:
## 2 Materials for a Flexible Antenna
### Conductive Materials  
The choice of conductive materials for flexible antennas involves materials with a good electrical conductivity that provides high gain, bandwidth, and efficiency. Moreover, good conductive material will have resistance to degradation concerning mechanical deformation. The stiff architecture of the wearable antennas, which are created using a variety of conductive materials, limits their flexibility. Several smart textiles have been used for flexible antennas which provide a high level of flexibility. However, stretching and crumpling of the antenna will lead to the deterioration in the antenna's performance [15].  
Wearable antennas that are durable and frequency-tunable are created using conductive fibers which are embedded in a polymer substrate. However, this type of antenna is vulnerable to stretch because of the deformation effects in the flexible materials [17]. To overcome this, highly stretchable materials have come to the frontline such that deformation due to the high stretchability of the antenna can be well managed.  
Stretchable, doped materials, such as silver-integrated rubber [18], stretchable fabric [19], polymer composite (MXene ink) [20], etc. are integrated for wearable applications. Several new materials are used to increment the performance of stretchable materials, including indium, meshed wires, and transparent fabrics. A new Ti3C2 Tx/PVDF meta composite with a negative permittivity has gained greater attention in recent days due to its unique properties and thus finds applications in the field of the flexible antenna [21]. Several conductive textile materials for flexible antennas are shown in Figure 3.  
Applications such as wireless body area networks, telemetry, etc., utilize conductive materials to sustain repetitive pressure when continuously bent [22]. Therefore, these materials should be selected in such a way that they will not degrade the antenna's performance.  
The high electrical conductivity of the nanoparticles makes them a suitable choice for the fabrication of flexible antennas. Silver and copper-based nanoparticles are commonly  
Figure 3: Different textile conductive materials for flexible antennas. Reprinted from ref. [15].  
used. Silver nanoparticles exhibit a good electrical conductivity, adhesion, and visibility [23]. A carbon nanoparticle integrated with polyaniline (PANI) shows good mechanical and electrical properties and silver nano wire (AgNW) shows a good conformality [24].  
Copper-based conducting materials such as copper mesh are also seen in the literature for the fabrication of flexible antennas due to their good electrical properties, easy availability, and good radiation properties [25]. Nickel-embedded metallic mesh is used as a conductive electrode which shows a good mechanical stability and a good electrical conductivity than the traditional metallic mesh [26]. Meshed fabric and nanoflakes are also used as the conductive materials for the design of flexible antenna due to their good mechanical and thermal stabilities [27; 28].  
Transparent conductors also have come into the frontline along with the transparent substrates for the design of flexible antennas. Some transparent conductors that are reported in the literature are indium-tin oxide, silver coated polyester film (AgHT), aluminum zinc oxide, etc. However, such conductors exhibit a low conductivity even though they provide a good visual clutter [29; 30]. Two dimensional materials such as MXenes and graphene have come into the frontline especially for communication devices that need a high electrical conductivity. MXenes possess a high electrical conductivity and graphene has a good chemical stability. In [31], the conductivity of the graphene assembled film (GAF) is improved by applying a high temperature annealing process. Graphene-based conductors with larger sizes and areas increase the electrical conductivity of the conducting material [32].  
Table 2 shows the various conductive materials utilized for the design of flexible antennas and their characteristics [27; 28].