Textile Antennas for Wearable IoT Applications
(M.E. E&TC), D.K.T.E. Society’s Textile and Engineering Institute, Ichalkaranji-416115, (M.S.), INDIA
(M. E.ETRX), D.K.T.E. Society’s Textile and Engineering Institute, Ichalkaranji-416115, (M.S.), INDIA
(M.TECH. CSE), D.K.T..E Society’s Textile and Engineering Institute, Ichalkaranji-416115, (M.S.), INDIA
(M. E. ETRX), D.K.T.E. Society’s Textile and Engineering Institute, Ichalkaranji-416115, (M.S.), INDIA
Received in revised form: 09/06/2019
In the technology circles today one of the most common buzzwords which one can come across is Internet of things (IoT). The term Internet of things generally refers to a network of devices which can sense, accumulate and transfer data using internet as a medium without any human intervention. Consumer products, Textiles, durable goods, automobiles, industrial and utility components, sensors and other everyday objects can be combined with internet connectivity and data analytic capabilities to transform the scenario of the world we live today. Recently, wearable devices are rapidly emerging and forming a new segment––“Wearable IoT (WIoT)” due to their capability of sensing, computing and communication. Future generations of WIoT promise to transform the healthcare sector, wherein individuals are seamlessly tracked by wearable sensors for personalized health and wellness information; body vital parameters, physical activity, behaviours, and other critical parameters impacting quality of daily life. The Internet of Things (IoT) scenario is strongly related with the advance of the development of wireless sensor networks (WSN) and radio frequency identiﬁcation (RFID) systems. This paper presents a review on design of different wearable textile antennas used for IoT applications.
Keywords: IoT, Textile Antenna, fabric, RFID, wearable
Nowadays, the Global technical development and lifestyle trends shows an increased consumption of technological products and processes, powered by emergent concepts such as the Internet of Things (IoT), where everything is connected in a single network . The development of smart objects for IoT applications, include the capacity of this objects to be identiﬁable, to communicate and to interact. In this context, wearable technology has been addressed to make the person, mainly through his clothes, able to communicate with, and be part of, this technological network . Wireless communication systems are made up of several electronic components, which, over the years, have been miniaturized and made more ﬂexible, such as batteries, sensors, actuators, data processing units, interconnectors, and antennas . In the systems for on-body applications, the antennas have been challenging, because they are conventionally built on rigid substrates, which makes it difficult to integrate antennas comfortably and efficiently into the garment. However, embedding antennas into clothing allows expanding the interaction of the user with some electronic devices, making them less invasive and more discrete. Thus, textile antennas that are designed combining the traditional textile materials with new technologies emerge as a potential interface of the human-technology-environment relationship. Textile antennas, thus, become an active part in the wireless communication systems aiming applications such as tracking and navigation, mobile computing, and others.
Wearable IoT architecture
Wearable IoT stands for the IoT devices which can be worn by a person or animal.
Figure 1 Three Layer Architecture of Wearable IOT 
To understand how it works let us first look at a three-layer architecture of wearable IoT as shown in fig.1 above. Internet of Things can be seen as simply an interaction between the physical and digital worlds using internet as a communication medium. For this IoT devices are equipped with embedded sensors, actuators, processors, and transceivers. Functions of three layers of IoT are explained below.
The Sensing layer
It is the physical layer, which has sensors worn by humans or animals for sensing and gathering information about the environment. It senses some physical parameters or identifies other smart objects in the environment.
The transport layer
The transport layer transfers the sensor data from the sensing layer to the processing layer and vice versa through networks such as wireless, 3G, LAN, Bluetooth, RFID, and NFC. The processing layer is also known as the middleware layer. It stores, analyzes, and processes huge amounts of data that comes from the transport layer. It can manage and provide a diverse set of services to the lower layers. It employs many technologies such as databases, cloud computing, and big data processing modules.
The application layer
It is responsible for delivering application specific services to the user. It defines various applications in which the Internet of Things can be deployed, for example, smart homes, smart cities, and smart health.
As shown above transport layer needs to send and receive data for further processing; this is where antennas are required. An antenna is a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. There are two basic types the receiving antenna, which intercepts RF energy and delivers AC to electronic equipment, and the transmitting antenna, which is fed with AC from electronic equipment and generates an RF field.
For Wearable IoT applications, embedding antennas into clothing makes the garments become a smart interface for the interaction between the user and the network. Wearable antennas should be thin, lightweight, of easy or no maintenance, robust, and resistant to washing cycles and usage and, moreover, must be low cost for manufacturing and commercializing. For obtaining these characteristics following features of textile materials which are to be used in designing wearable antennas become crucial 
The Dielectric Constant of the Fabrics
Dielectric constant ϵr is defined as the ratio of the electric permeability of the material to the electric permeability of free space ϵo (i.e. vacuum). The dielectric behavior of textile materials depends on the properties of the constituent fibres and polymers , and on the fibre packing density in the fibrous material [5,4]. However, textile fabrics are rough, porous and heterogeneous, having air in between the fibres, making their characterization difficult . In general, textiles present a very low dielectric constant as they are very porous materials and the presence of air approaches the relative permittivity to one. The low dielectric constant reduces the surface wave losses which are tied to guided wave propagation within the substrates. Therefore, lowering the dielectric constant increases spatial waves and hence increases the impedance bandwidth of the antenna, allowing the development of antennas with acceptable efficiency and high gain [8-10]. Again, one should note that the relative permittivity value changes with the moisture content of the substrate affecting the bandwidth of the antenna [11, 12].
Thickness of the Dielectric Fabrics
The bandwidth and efficiency performance of a planar microstrip antenna is mainly determined by the substrate dielectric constant and its thickness [13-14]. Change in permittivity may change the antenna bandwidth, but lowering the substrate permittivity can also increase the resonance frequency of the antenna. As textile materials present a quite narrow range of permittivity values, it is therefore their thickness that will mainly determine the bandwidth as well as the input impedance of the antenna and so its resonance frequency as there may be larger variation in thickness values. The thickness of the dielectric material is thus crucial in the design of antennas[13-14]. There are commercially available fabrics with a very diverse range of thickness values. Plus, nominal thickness values are given in any technical data sheet, allowing a careful choice of the material based on the required thickness. Moreover, accurate values of the thickness of fabrics under specified pressure are easily obtained by simple standard methods, such as ISO 5084:1996 and ASTM D374-99(2004) or with a Digimatic Indicator . Therefore, the thickness of the fabrics is a feature that may guide the search for suitable textile dielectrics.
The Electrical Surface Resistivity of the Conductive Fabrics
Being planar materials electrical behavior of Fabrics may be quantified by the surface resistance and characterized by the surface resistivity. The surface resistance is the ratio of a DC voltage to the current flowing between electrodes of specific configuration that are in contact with the same face of the material under test and its unit is (Ω), . The surface resistivity is the ratio of the DC voltage drop per unit length to the surface current per unit width. For the antenna design, the relevant parameter is the conductivity of the fabric, σ which is measured Siemens per meter (S/m). In general, these fabrics must have a very low electrical surface resistance in order to minimize the electric losses and thus increase the antenna efficiency.
The Moisture Content of the Fabrics
The fibres are continuously exchanging water molecules with air and always establish equilibrium with the temperature and humidity of air in contact with it. When water is absorbed by textile fibres, it changes the electromagnetic properties and increases its dielectric constant and loss [11, 16, 4, 17, 14]. Climatic changes as well as proximity of antenna to skin will cause the fabric to absorb moisture from skin and hence affecting the antenna performance.
Mechanical Deformations of the Dielectric and Conductive Fabrics
A curvature on a human body consists of a superposition of bends in arbitrary directions. The bending and elongation of dielectric fabrics, when it adapts to the surface topology, influences its permittivity and thickness, which further affects the bandwidth as well as the resonant frequency of the antenna. Woven and nonwoven fabrics are more stable fabrics than knitted fabrics, as they allow higher geometrical accuracies .
Textile Materials Used in Wearable Antennas
Conductive textile materials are required for the antenna patch and ground plane, whereas nonconductive textiles constitute the antenna substrate. Natural as well as manmade fabrics with different dielectric constant can be used for designing textile antennas the study  shows comparison of behavior and efficiency of different textile materials like Cotton, polyester, cordura, lycra, Fleece fabric , Woolen felt ; the study shows that manmade fabrics show better performance compared to natural fabrics.
Five different synthetic fabric materials namely fleece, upholstery fabric, vellux, synthetic felt, and Cordura were used in the study  which was focused on the comparison of different synthetic fabrics suitable for circular polarized antennas. Special attention was paid to GPS antenna performance. It has been shown that all the textile materials are suitable for circular polarized antennas. However, the mechanical elasticity can cause problems in order to maintain antenna's electrical performance parameters. It shows that Cordura maintains its mechanical dimensions. In addition, its high-water column resistance makes it very practical smart clothes. Therefore, it might be a widely used fabric antenna substrate in the near future.
Along with proper choice of textile substrate and conductive material, selection of appropriate manufacturing technique also plays crucial role in performance of textile antennas . The manufacture process of wearable antennas should guarantee good agreement with the design and simulation results and should lead to robustness and repeatability of the wearable antenna. This arises from the fact that the wearable antenna is supposed to be embedded on the garments and it will operate under different conditions (movement of the wearer, weather conditions, temperature, bending and crumpling conditions). In addition, washability is a requirement in the case of embroidered textile antennas where the goal is that these antennas should be part of the clothes. Embroidered antennas are a great opportunity to connect wearable antenna technology with the industry of textiles. In this section, the manufacturing techniques used so far on wearable antennas are described for two categories: (1) rigid and (2) flexible (often textile) antennas.
Rigid Wearable Antennas
The manufacture process of rigid wearable antennas follows the conventional techniques of printing or constructing antennas for example by using etching. Rigid antennas worn on the body can be miniaturized so as to minimize the inconvenience to the user as conventional antennas are not practical. These rigid antennas sometimes need to be manufactured in a curved contoured shape so as to meet the wearable requirement . Another difficulty is the small size of these antennas and their complex structure .
Flexible Textile Wearable Antennas
Textile antenna manufacturing techniques can be divided into the following categories:
Thin and uniform metallization layers (i.e., copper or silver tape, foil) attached to the non-conductive textile fabric ;
The use of conductive textile yarns to weave or knit the conductive patterns of the antenna and then attach or stitch them onto the non-conductive textile substrate ;
Inkjet and screen-printed printing on non-conductive textile materials. Inkjet and screen printing  can also be used to create conducting sections of wearable antennas. However, the substrates are generally paper or Kapton while textiles are not ideal printing substrates.
Screen printing requires a mask to be made for each design and therefore is less practical for different individual designs [23-24]. RF transmission lines have been previously screen printed onto a cotton substrate .
Inkjet printing does not require a mask and designs can be created within minutes of receiving the computer file containing the geometry and hence enables great manufacturing flexibility. Typically, silver nanoparticles in solution are used to make thin conducting lines which are approximately 1 µm thick. Therefore, printing on rough surfaces such as textiles is very challenging.
Wearable antennas must be designed to operate adequately in the 2.4–2.4835-GHz industrial-scientiﬁc-medical bandwidth. Many Researchers have designed various wearable Antennas which are shown in Fig. 2 below:
A Truncated corner patch planar antenna in a ﬂexible protective pad foam suitable for ﬁreﬁghter garments is presented in . Its construction is shown in fig.2.a above. Flexible pad foam is commonly available in protective clothing which provides a uniform, stable, and sufﬁcient thickness; its cellular structure and properties such as ﬂame retardance and water repellence make it an excellent substrate material for the integration of antennas into protective garments; shock absorbing foam with a thickness of 3.94 mm was used to achieve a nearly circularly polarized antenna with a bandwidth of more than 180 MHz even when the antenna was compressed or bent.
Louis Vuitton logo antenna for wireless communication and anti-theft applications for fashionable wearable devices such as luxury bags is presented in  as shown in fig.2.b. Shielded conductive metalized nylon fabric (Zell) with a thickness of 0.1 mm (surface resistance = 0.02 Ω/square) was used as conductive layer and the sheepskin leather which is a textile material (εr = 2.5) with thickness 0.7 mm, is used as a five-layered substrate (3.5-mm thickness). The SMA