# Flexible Antennas: A Review
## 9 Challenges and Future Prospects of Flexible Antennas  
Recently, the research on flexible wireless devices has attracted much attention because of its nature to comply with the requirements of biomedical applications, vehicular navigation systems, wearables, and so on. An antenna is one of the key components in this whole system, and for ensuring the device conformality, it should be flexible and stretchable. The first step towards this goal is to replace the conventional rigid substrates with flexible materials like textiles, paper, or elastomeric such as polydimethylsiloxane (PDMS) [241], PEN, PET, and PI. Thus, it can be said that the very first challenge of designing a flexible antenna is finding a suitable substrate. In comparison to the traditional substrates like FR4 or Rogers that have a dielectric constant around 3-10 and a loss tangent of 0.001-0.02, typical flexible substrates have low dielectric constants. Even though this low value of dielectric constant helps to achieve larger bandwidth and radiation efficiency, it creates a problem (antenna performance) when miniaturization is needed. For flexible textile antennas, the uneven thickness is another problem to deal with. The electro textile substrate is prone to crumble and susceptible to fluid absorption. Paper-based flexible antenna faces similar problems with relatively high loss factor [90], which causes low antenna efficiency and impedance mismatch. In an earlier work [154], an organic paper-based UWB antenna was presented. Although it is a low-profile antenna, it is not the right choice for applications that require high levels of bending and twisting because of discontinuities and lack of robustness. A polymer-based substrate is an excellent option to solve these problems. For example, an earlier report [176] studied a compact polyimide-based antenna where Kapton polyimide film was used because of its low loss tangent (\(\tan\delta=0.002\)) for broadband frequency operation with physical and chemical flexibility. This substrate had a temperature rating up to 400 \({}^{\circ}\)C and tensile strength of 165 MPa at 73 \({}^{\circ}\)F that confirms the robustness of the Kapton polyimide film. Furthermore, polyimide and Kapton are not very expensive because of roll-to-roll mass production while being transparent and bendable. There are many other polymer-based designs reported in the literature [238; 240; 242; 243; 244; 245; 246]. One problem that can arise from the polymer-based antenna is excessive bending or twisting that might result in micro-cracks in the substrate. This will affect the electrical conductivity of the antenna and raise the risk of breakdown. Further, low glass transition temperatures of polymer make them unusable in high-temperature applications. Ceramic substrates can be an alternative which can withstand high temperature and can be used in flexible applications [247]. Such a limitation can be overcome by embedding very thin metallic nanowires on the surface of elastomers like PDMS to make it highly conductive and stretchable [37]. Because of the fabrication and design complexity, it is not very suitable for low-cost, flexible applications. Instead of solid metal wire, if liquid metal (LM) is used in the microfluidic channel created by elastomers, it will give the antenna reconfigurability, an exciting feature of antenna that is needed in many applications. PDMS is the most popular commercial elastomers to make the microfluidic channel for the flexible antenna. Different liquid metals such as mercury, carbon nanotubes (CNT), Galinstan, gallium indium (GaIn), and eutectic gallium indium (EGaIn) are injected into the channel to form the antenna [248; 249; 250]. Besides PDMS, EcoFlex silicone rubber [251] and thermoplastic polyurethane (TPU) based NinjaFlex [114] are also used as an elastomer for creating microfluidic channel, and are usually 3-D printed to realize a specific pattern.