# A Printed Wearable Dual-Band Antenna for Wireless Power Transfer
## 1 Introduction  
Wearable devices have been of interest due to an increase in their applications, such as the Internet of Things (IoT), biomedical sensors, and body area network. For these particular applications, a reliable and suitable power source is required, especially a method of power transfer that does not rely on batteries or wired power sources. Therefore, wireless power transfer (WPT) has been of interest for recharging wearable devices. Typically, the WPT systems are divided into two categories, far-field [1] and near-field [2, 3] systems. A WPT system requires a transmitting unit connected to the main source of power and the transform the electrical power into an electromagnetic field. One or multiple receivers convert the electromagnetic field to electrical power. WPT technology is also used for energy transfer in wireless sensor networks (WSNs). In these applications, WSN is limited by the battery lifetime [2]. Flexible and textile implementations of WPT have been proposed in literature [4, 5]. In [6], the authors designed a wireless power transfer system for a wearable and wireless neurotransmitter sensor recording system. They used a multi-antenna system as a power transmitter to resolve lateral and angular misalignments of the receiver antennas and claimed that it provided better effective coverage. In [7], the authors designed a wearable resonant WPT system for biomedical applications. This system achieved a power transfer efficiency (PTE) of 5.4% transferring at least 570 mW of power. Nonetheless, low power transfer efficiency and poor received power stability are the main drawbacks of WPT systems used in biomedical and wearable application [3]. A wearable textile antenna embroidered on fabric for wireless power transfer systems is presented in [8]. The authors used a planar spiral coil generated with the conductive thread on a cotton substrate. This system operates at 6.78 MHz, providing \(-\)5.51 dB transfer efficiency and 12.75 mW power transmission at a distance of 15 cm. The authors in [9] designed a single band antenna operating at 2.45 GHz intended for wearable and flexible applications. They took advantage of a thermal deposition technique to fabricate antenna structures on a Kapton Polyimide platform. In this case, thin-film deposition was obtained by evaporating a source material in a vacuum allowing vapor particles to travel to the substrate covered with a mask outlining the desired structure. In the work presented in [10], a low-cost inkjet printing method for antenna fabrication on apoly-ethylene terephthalate (PET) substrate is utilized. Their proposed co-planar waveguide (CPW) fed Z-shape antenna that was operating at 2.45 GHz. The authors measured radiation efficiency of 62% and the gain of 1.44 dBi at 2.45 GHz. An office inkjet printer was used to print silver nanoparticle ink on the PET substrate to fabricate the antenna prototypes. The authors in [11] fabricated a flexible dual-band dipole antenna operating at 900 MHz/2.44 GHz. They printed their antenna on Kapton through screen-printing technology using a polymer-silver conductor. This antenna is considered in its receiving mode and is connected to a rectifier. This radio frequency (RF) energy harvester was tested in a wireless power transfer scenario. In this dual-band configuration, the system provides 1 V DC voltage for a power density of 0.7 \(\frac{mW}{m^{2}}\) at 900 MHz and 1.1 \(\frac{mW}{m^{2}}\) at 2.44 GHz. The efficiency of WPT systems depends on different factors, such as the geometry of transmitting and receiving elements, misalignment, bending, and the distance between transceivers. Some of these were investigated in [12; 13; 14; 15; 16; 17; 18]. There are multiple challenges that should be addressed in using the wireless power transfer method as a source of power for wearable devices. One of the challenges is managing and distributing power between multiple wearable devices [19]. In [19], a WPT technique that distributes power from a single or a few sources between items of clothing is proposed.