# Direct-Write Spray Coating of a Full-Duplex Antenna for E-Textile Applications
## Materials and Methods
### Antenna Fabrication  
Nonwoven Evolon(r) fabric (30 wt.% polyamide and 70 wt.% polyester) was obtained from Freudenberg Performance Materials. This nonwoven fabric is an excellent choice for electronics printing due to the numerous condensed fibers exhibiting a high surface area and low surface roughness compared to conventional textiles. Single jersey-knit fabrics composed of 100% polyester (PET) with a basis weight of 145 g/m\({}^{2}\) were obtained from Hanesbrands Inc. Both the nonwoven and knit fabric thicknesses were \(\sim\)0.3 mm, as shown in Table 1. Prior work has demonstrated that stacking the nonwoven layers can result in a good substrate for antenna fabrication [23]. The commercial knit structure is generally too flexible to keep the antenna stable. Therefore, a combination structure with alternating layers of printed knit fabrics and nonwoven fabrics was chosen for the antenna fabrication. The structured device was assembled with six layers of printed knit fabrics adhered together by a porous TPU web (0.057 mm). The layered knit fabric and TPU structure were heat laminated at 150 \({}^{\circ}\)C for 5 min. The TPU web softens above 65 \({}^{\circ}\)C and melts under higher temperatures. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fabrics** & **Material** & **Thickness (mm)** & **Weight (g/m\({}^{2}\))** \\ \hline Nonwoven Fabric & 70 wt.\% Polyester/30 wt.\% Polyamide & 0.33 & 95 \\ Knit Fabric & Polyester & 0.31 & 145 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specification of the fabric for antenna design. An Asymtek C-341 conformal coating machine fitted with an EFD-781 s series spray valve was used to print the BT dielectric ink on the knit textile to modify the dielectric constant of the substrate. The BT dielectric ink was loaded into a syringe and capped with a plunger. The dispensing pressure was 20 psi, and the air-assist pressure was set to 60 psi. These settings enabled uniform ejection of ink from the spray valve. The processing included a dispensing velocity of 40 mm/s and a gap of 1 mm between the nozzle and substrate. Figure 2 shows the assembly process of the textile-based antenna. The first step in the fabrication process was to heat press six knit fabrics adhered to TPU webs. Then, the BT dielectric ink was spray coated onto the knit fabric composite to enhance its dielectric constant and was matched to the nonwoven fabrics (Figure 2b). Finally, the fabric coated with the BT dielectric ink was cured under a UV radiation machine for 300 s to make sure the photopolymer was completely cured. The patch and ground patterns were screen printed using a DuPont 5064 H conductive screen-printing silver ink on individual nonwoven fabrics (Figure 2a). The nonwoven Evolon(r) fabric had a low surface roughness (Ra = 18 um) and a high surface area (2.19 mm\({}^{2}\) mm\({}^{-2}\) of fabric area), allowing the ink to penetrate evenly into the fiber bulk due to the strong capillary wicking force. The patch design requires very high resolution for the transmission lines (~1.2 mm) and the gaps (~1.5 mm), which can be challenging for fabrication on a textile surface. To achieve the high resolution, we used a vinyl mask placed onto the nonwoven fabric and applied uniform pressure and velocity during screen printing to ensure a uniform printed area for the patch design. The printed nonwoven fabric was then cured in an oven at 130 \({}^{\circ}\)C for 15 min. The same process was repeated for the ground design (i.e., conductive backing). To form the final antenna assembly, the nonwoven patch design, nonwoven ground design, and dielectric printed knit fabrics were heat laminated together with the TPU webs (Figure 2c). Figure 2: Fabrication of the textile-based antenna. (**a**) Screen printing of the Ag paste from Dupont onto nonwoven fabrics to fabricate the patch and ground.