# Flexible Antennas: A Review
## 3 Fabrication Techniques for Flexible Antennas
### 3-D Printing  
Recently, additive 3-D printing techniques for flexible antennas are gaining popularity with a myriad of commercially available printing materials and processes. It exhibits several advantages like in-house-made, fast fabrication of complex 3-D structures with various materials, and the capability to change the density of the printed object [106; 107; 108]. The flexibility to realize complex 3-D shapes from bulk materials and 3-D printing of flexible materials like polymers, metals, ceramics, and even biological tissues make it attractive for antenna design [109]. Polymers, such as thermosets and thermoplastics, are used as 3-D printing materials for flexible antenna applications. The common printing techniques of the polymers are Fused Deposition Modeling (FDM), Stereolithography (SLA), Direct Light Processing (DLP), and Material Jetting (MJ) [110]. The most common 3-D printing technology is FDM. In FDM, the filament is fed through to the extrusion head of the printer, and the motor of the heated-nozzle drives the filament melting it. The printer then lays down the melted material at a precise location, where it cools down and solidifies. The process repeats by stacking up the part layer-by-layer [110].  
One of the first examples of the exploitation of 3-D-printing in microwave components and antennas fabrication was presented in an earlier article [107]. NinjaFlex, a new 3-D printable, flexible filament, has been adopted for manufacturing a 3-D printed patch antenna. FDM technique has been used to realize 3-D printing substrate. A linearly polarized patch antenna was designed and implemented on the NinjaFlex substrate 100% infill at 2.4 GHz operating frequency (Figure 7a). Experimental verification under nominal and bending conditions showed good agreement with the simulation. A 'button-shaped' compact RFID tag fabricated by the combination of 3-D printing and inkjet printing technologies was reported in [111] for wearable applications (Figure 7c). The antenna showed a good performance with a measured maximum reading range of 2.1 m in the RFID Federal Communications Commission  
Figure 6: Schematic of the screen-printed antenna. (**a**) Fabrication process for parasitic beam-switching millimeter-wave antenna array [104] and (**b**) screen printed graphene flakes based wideband elliptical dipole antenna prototype [105].  
band (902-928 MHz). A proof-of-concept of the fabrication and performance analysis of a flexible and stretchable wearable antenna on a 3-D printed NinjaFlex substrate was presented in [112].  
The radiator of the antenna was brush-painted from the stretchable silver conductive paste (Figure 7b). The antenna's wireless performance under flat and bending conditions was satisfactory. Specific absorption rate (SAR) simulations validate its use for wearable applications. The antenna showed impedance bandwidth of 990 MHz (1.94-2.93 GHz) with a peak gain of \(-\)7.2 dB at 2.45 GHz. A bow-tie antenna with a CPW feed structure was fabricated using a desktop 3-D printer [113]. Polylactic Acid (PLA) and Acrylonitrile butadiene styrene (ABS) filaments were used as the dielectric and conductive parts of the antenna, respectively, which showed wide bandwidth, flexible structure, lightweight, and small size. Recently, a 3-D flexible, miniaturized inverted-F antenna for wearable applications was designed, manufactured using the Galinstan liquid metal to realize the radiating element, the NinjaFlex flexible plastic to realize dielectric substrate through a 3-D FDM printing process (Figure 7d) and the electro-textile copper constituting the antenna ground plane [114]. The performance of the antenna in several bent configurations and in the presence of the human body was found satisfactory.