# Direct-Write Spray Coating of a Full-Duplex Antenna for E-Textile Applications
## 1 Introduction  
In the past decade, wireless body area networks (WBANs) have garnered significant attention as a way to wirelessly communicate between wearable devices placed at various locations across the human body [1; 2; 3]. These wearable devices can sense physiological signals, such as electrocardiogram (ECG), electroencephalogram (EEG), and electrooculogram (EOG) [4]. Moreover, these devices, when coupled with WBANs, create wearable health monitoring systems that enable a paradigm shift away from traditional, in-person healthcare towards remote telemedicine. A primary challenge to WBANs is that they primarily rely on conventional stiff and heavy antenna designs to wirelessly relay information between devices, which limits practical applications of WBANs in wearables. Having a wearable antenna that is flexible and lightweight, with the same efficiency as a conventional, rigid antenna,is critical for realizing WBANs for telemedicine applications [5; 6]. It is also important that there is a pathway to fabricating multifunctional wearable antennas, which typically require more complex geometrical features. The antenna is a necessary wireless communication device for receiving and transmitting relevant signals at specific frequencies [7]. Various wearable antennas with flexible properties exist in the literature, using a range of polymer- and paper-based substrates with permittivity values often ranging from 1.5 to 13 [8; 9]. Textile materials, made of fibrous polymer materials, are ubiquitous in our daily lives, and they offer a light and flexible substrate for wearable systems and devices. The textile antennas [10; 11; 12] can be light, flexible, and readily integrated into smart clothing, communicating with other wearable electronics and transmitting the body health data [1]. Design criteria, such as durability, functionality, and usability, need to be met in order to design convenient and safe textile antennas [13]. The safety impact of a textile antenna is defined by its specific absorption rate (SAR) [14], which measures the rate of absorption of radio-frequency (RF) electromagnetic radiation per unit mass by a human body to evaluate the RF dosimetry [15]. Ideally, all wearable antennas should exhibit low SARs to ensure a low negative impact on human tissues [16; 17]. With these design criteria in mind, researchers have explored multiple textile designs, materials, and manufacturing methods to realize textile-based antennas. For example, researchers have used conductive yarns embroidered into non-conductive woven fabrics to create wearable patch antennas [18]. These embroidered textile antennas are easy assemble and integrate into wearable systems. However, embroidery can be challenging for large-scale manufacturing processes. In addition, these embroidered antennas show low patch resolution and low wash durability, as the conductive yarns are susceptible to degradation during washing and handling. Nonwoven textiles serve as an alternate material substrate for textile-based antennas [19]. However, these types of antennas employ copper tape as the patch and ground, which make them very fragile, and they are easily broken after a few bending cycles [20]. Printing methods, such as screen printing, are currently the best processes to design textile-based antennas with excellent wash and mechanical durability [20; 21; 22]. In our previous work, we reported the use of a nonwoven fabric assembly laminated with a thermoplastic polyurethane (TPU) web to create a porous, flexible, and washable antenna [19]. Employing the breathable TPU web in the screen-printed antenna can work as a protection layer by mechanically entrapping the dielectric materials to enhance their durability [19]. Moreover, the micro-fiber TPU web with an open gap structure enabled breathability. The TPU web encapsulated silver printed on textiles, protecting the conductive area of the antenna from being washed and mechanically deformed [19; 21].