# Flexible Antennas: A Review
## 3 Fabrication Techniques for Flexible Antennas
### Inkjet Printing  
Inkjet-printing technology has emerged as an alternative to conventional fabrication techniques such as etching and milling. It is an additive process so that the design is directly transferred on to the substrate without any masks and ensures less material wastage [90]. It is the preferred fabrication technique for polymeric substrates like polyimide, PET, paper due to its accurate and speedy prototyping fabrication method [90]. Figure 5 shows the overview of the printing process and the printer assembly. For printing purposes, nanoparticle metallic inks, graphene nanoflake inks, metal-organic inks are utilized. The printing technique can be sorted into two types: drop-on-demand (DoD) and continuous inkjet. Drop on-demand print heads apply pressurized pulses to ink with either a piezo or thermoelement in which drives a drop from a nozzle when needed [15]. New generation printers offer precise printing using picolitre volume cartridges. Printing quality is controlled by the jetting \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substrates** & **Dielectric Constant (\(\mathbf{t_{x}}\))** & **Dielectric Loss (tan\(\delta\))** & **Thickness (mm)** \\ \hline PET [63] & 3 & 0.008 & 0.140 \\ \hline PEN [70] & 2.9 & 0.025 & 0.125 \\ \hline Polyimide [61] & 2.91 & 0.005 & 0.2 \\ \hline PDMS-MCT [84] & 3.8 & 0.015 & - \\ \hline PDMS [73] & 2.65 & 0.02 & - \\ \hline PDMS with glass microsphere [73] & 1.85 & 0.014 & - \\ \hline PDMS with phenolic microsphere [73] & 2.24 & 0.022 & - \\ \hline PDMS with silicate microsphere [73] & 2.45 & 0.02 & - \\ \hline Paper (Kodak Photo paper) [79] & 2.85 & 0.05 & 0.254 \\ \hline Liquid Crystal Polymer (U.UTRALIM 3850) [62] & 2.9 & 0.0025 & 0.1 \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} Wearable antenna \\ substrates \\ \end{tabular} } &
\begin{tabular}{c} Fieece Fabric [85] \\ \end{tabular} & 1.25 & - & 2.56 \\ \cline{2-4}  & Cordura [86] & 1.1-1.7 & 0.0098 & 0.5 \\ \cline{1-1} \cline{2-4}  & Woolen felt [87] & 1.16 & 0.02 & 3.5 \\ \cline{1-1} \cline{2-4}  & Felt [88] & 1.3 & 0.02 & 1.1 \\ \cline{1-1} \cline{2-4}  & Cotton/Polyester [89] & 1.6 & 0.02 & 2.808 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Commonly used flexible substrates with their dielectric constant, loss tangent, and thickness. Figure 5: Overview of the inkjet printing process for antenna fabrication. (**a**) Flowchart of the inkjet printing process [91] and (**b**) Dimatix Materials Printer, DMP-2800 (FUJIFILM Dimatix Inc., Santa Clara, CA, USA), and the PC that is used to control the printer. waveform, the jetting voltage of the nozzles, the jetting frequency, the cartridge temperature, the platen temperature (where the substrate is placed), and the resolution of the pattern [91; 92]. After the printing of the antenna design, sintering is necessary for removing the solvent and capping agent and attaining electrical conductivity [93]. An example of inkjet printing is the use of silver nanoparticle ink to fabricate a wideband right-hand circularly polarized high-gain \(4\times 4\) microstrip patch array antenna on a PET substrate using Epson stylus c88 series printer [65]. Another example of an inkjet-printed antenna is epidermal antennas suitable for radio-frequency identification (RFID), and sensing on transparent PET film has been presented in an earlier study [66]. The unique feature of this antenna fabrication method is that no heat sintering is needed (printer model is Brother MFC-J5910 DW). A miniaturized fully inkjet-printed flexible multiple-input-multiple-output (MIMO) antenna for ultrawideband (UWB) application was proposed on Kapton polyimide substrate using a Dimatix DMP 2800 printer [59]. Using Dimatix DMP 2831 printer, a flexible, wearable, and reversibly deformable CPW fed antenna was designed on PET substrate using silver nanoparticles [94]. A high-gain, multidirector Yagi-Uda antennas for use within the 2.45-GHz ISM band were realized using silver and dielectric ink on LCP substrate using the same printer [95].