## 1 Introduction  
Wireless body area network (WBAN) applications are paving the way for a future in which doctors can have patient-specific information at their fingertips whenever they need it (Figure 1). Wearable devices have recently attracted the interest of researchers [1, 2]. However, body presence, robustness, as well as material choice, form factor, mechanical flexibility, and stretchability, all influence wearable devices design [3, 4]. Various types of design have been proposed in wearable applications; one of the first sensor antennas was published in [5]. In order to calculate the humidity content of sludge samples, a circular antenna was proposed. The sample was placed in a plastic beaker at the top of the proposed design. Based on the effective dielectric permittivity of the design, the Bottchermodel was utilized to find out the humidity content. The first sensor antenna dedicated to temperature measurement was reported in [6]. A shape memory polymer (SMP) paper was used to create the sensor, which was sandwiched between a radio frequency identification (RFID) tag and a metallic sheet. The SMP changed its relative permittivity as long as the temperature crossed the threshold, which could be determined by the RFID tag as it was switched on. Sensor antennas, which are fabricated on rigid materials, have been used in a variety of medical and non-medical applications. As an example, [7] describes a microstrip patch antenna-based sensor made on a Flame Retardant 4 (FR4) substrate. This sensor can be used to detect various percentages of sugar and salt by detecting the change in their dielectric properties. Rogers R03006 substrate was used in a microstrip patch antenna to create a sensor described in [8]. The sensor was used to detect the temperature by thermally cycling a patch antenna that was bonded to various metal bases. Rigid substrates are typically unsuitable for wearable applications because they are difficult to bend, stretch or adhere to textile materials [9]. In [10], bio-based waste material was used to create a biodegradable substrate for a microstrip antenna. The antenna was used to show the variation in the dielectric constant of high-permittivity materials such as water and wet soil. Wearable textile sensor antennas have recently garnered a lot of attention due to their ability to detect microstructure deformations and human movements, as well as to monitor a variety of vital signals in healthcare applications [11]. In [12], a wearable finger motion sensor based on a dipole antenna was developed with the dual purpose of detecting and communicating. In addition, the proposed design was attached to a glove to measure the influence of bending the fingers. To monitor human physiology, a planar inverted-F antenna (PIFA) was employed as an on-body wearable device [13]. The design is bulky, and the effect of the surrounding circuit was ignored. Table 1 compares a few of the recent publications related to sensor antennas. Wearable sensor antennas are often flexible. They are made from flexible materials with a shape that can be used as part of the clothing or garment. Various investigations  
Figure 1: The general architecture of WBAN in healthcare applications. have focused on the design approach and its realization. Some research activities are focused on the characterization and stability of the substrate as well as sensor antenna performance variations due to the tolerances in their substrate. Other investigations focus on the human body's influence on the sensor antenna's performance as well as the safety and power limitations [14; 15; 16; 17; 18; 19]. Wearable devices have found countless applications and each one of them calls for specific requirements. Wearable technology has been developed in the areas of tattoo and epidermal electronics [20], and flexible and stretchable antennas for bio-integrated electronics and biosensors [21; 22; 23].