Flexible and Wearable Antenna for Biomedical Application: Progress and Opportunity

In recent years, wearable antenna demand in biomedical applications has increased drastically. Several effective techniques and efficient structures of antenna were proposed to overcome the challenges with respect to the vicinity of the human body. In this paper, substrate and conductive material for the wearable antenna are reviewed thoroughly to ensure that it operates consistently and with flexibility. The design of wearable antennas becomes challenging when flexible substrates are examined, better conductive materials are employed during manufacturing processes and the issue exists in body-worn implementation. To address these issues in this paper, exhaustive literature is carried out based on validated characteristics such as Specific Absorption Rate (SAR), gain, and bandwidth by employing radiating structure, feed, ground, and slot optimization. Effective topologies for gain enhancement, bandwidth enhancement, and SAR reduction for various wearable antenna designs on biomedical applications were reviewed through qualitative study. This extensive study on flexible and wearable technology explores the various problems and challenges involved while designing suitable antennas are discussed. This review paper also highlights the advancements in 5G technology, applications on ISM band and IOT, with particular emphasis on wearable antennas and their potential applications in the biomedical field.

SECTION I. Introduction

The expanding field of wearable technology holds the promise of lowering medical errors and raising institutional healthcare standards [1]. Since the fourth and fifth generation of wireless mobile communication entered the industry, antennas have become increasingly important in the realm of wearable technology. Data collection and monitoring for multiple patients simultaneously was a critical task. Hence Wearable technology helps to monitor health parameters effectively such as blood pressure, heartbeat rate, stress, oxygen and fitness level [2]. Most widely used frequency bands for biomedical/ wearable technology are 2.4, 2.45, 3.5, 5.8 and 5.85 GHz as per the wireless, ISM and 5G standards [3], [4]. For applications such as wearable/ biomedical devices, the design of an antenna and the selection of the material are the important steps due to involvement of wearer’s comfortability, mechanical deformation, environment (conductivity/ resistivity of a medium), SAR, humidity and fabrication cost [5]. Even when the antenna is placed close to the body, the load of the body tissue effect rarely has an impact on the antenna’s performance [6].

In upcoming years, advanced communication through wearable technology can provide efficient and economical solutions in biomedical field. The important application is in smart health care systems for health monitoring and in telemedicine. Rectangular and circular patch with different type of slots is preferred antenna design for wearable antenna operating under ISM band due to the controlling of surface waves and improving impedance matching [7], [8], [9], [10]. Rogers duriod substrate is referred to as semi-flexible because of minimum thickness less than 1.6 mm. Due to its thinness and simplicity of manufacture, researchers suggest employing this material to construct lightweight antenna for biomedical application [9], [15], [17], [23]. The material utilised in the body-worn antenna should be flexible rather than rigid. When rigid material is attached to a body, it generates tiny cracks during bending, which lowers the bandwidth and gain of the antenna [11]. To overcome this issue, flexible material is chosen to design the antenna to have less weight, compactness, low cost and it also feasible for wearer’s comfortability [12]. Textile antenna is fabricated by using fabrics as flexible material in both conducting and substrate area to enhance the flexibility and the efficiency. Textile substrates with conductive fabric constructed of conductive threads like copper on flannel substrate are more flexible since they are lighter and thinner. To improve the performance of ultrawideband antennas for biomedical applications, these textile materials are recommended, along with other varieties of cotton fabric [13]. Cotton jean substrate is a cost-effective material employed in wearable antenna to sense the vital signs of the patient for health monitoring applications [14].

Body-centric devices linked to patients’ clothing can wirelessly alert medical staff to their health status, enabling them to care for patients more effectively and take necessary action right away. Wearable antennas, often known as embedded antennas, because these kinds of antennas are used in body centric devices to provide efficient information transfer [15]. Researchers have proven that impedance matching can be improved for a stretchable substrate like felt and Teflon by using defected ground structure (DGS) along with an antenna [16], [17]. Specific absorption rate (SAR) is one of the crucial parameters while measuring the absorbed radiation by any human body [18]. The standard SAR should be less than 1.6 W/Kg. Different shapes of metamaterial such as polygonal ring and omega shaped structure are embedded in antenna to provide better possibility of reduction of SAR [19]. To control the radiation efficiency and directivity, Artificial magnetic conductor (AMC) was studied and suggested due to its low-profile nature. It is attached with Styrofoam and placed below the substrate of an antenna to increase gain and simultaneously provides SAR reduction [20]. When compared to PEC, an AMC structure can offer a 50% vertical size reduction. The considered operating bandwidth for AMC unit cell is between +90° and -90° reflection phase. The resonance occurs at 0° phase shift where it is referred to as a center frequency.

Various slot configurations can be utilized in wearable antennas that allows for further height reduction without affecting antenna characteristics. It also provides broadband responsiveness and small. When slot is added in ground is referred to as defected ground structure which enhances wide bandwidth. If the slots are added in patch area, it provides good impedance matching and return loss [21]. Less antenna loss results from extending the sleeve than from reducing it and to obtain desired ISM band [22]. For further improvement of antenna gain and directivity can be increased while back radiation is decreased and refracted by embedding metasurfaces at backend and ensure that $- 90^{0}$ reflection phase occurs at targeted bands. The dispersion characteristics of surface waves or the attributes of reflection and refractive index are controlled by thin metasurfaces [23], [24], [25]. Less reflected back radiation can be achieved by using Electromagnetic Band Gap (EBG) technique that turns to yield low SAR value [28], [30]. When its reflection phase extends from -90° to +90°, the EBG behaves as an artificial magnetic conductor which can suppress the surface wave at specified operational frequencies [31], [32], [33], [34]. Researchers have suggested using a conductive cloth, Electromagnetic Band Gap (EBG) ground plane to increase antenna gain and efficiency while further reducing radiation towards human tissue [91].

Screen and inkjet printing are a preferred method for fabricating the textile wearable antenna with an excellent production precision for wearable applications [3]. The polyester/cotton material’s surface roughness was decreased using a screen-printed interface layer, which made it easier to print a persistent conducting surface. By eliminating the requirement for a screen and printing antennas directly from the final design, low-volume orders may be produced quickly while still allowing for custom designs. A frequently utilised technology, inkjet printing also allows very fine resolution for better fabrication process [38], [86]. When it comes to compactness, the antenna’s feeding mechanism is crucial. To further minimize the antenna, an asymmetric coplanar strip (ACS) feed structure has been implemented. Using the ACS-feeding strategy will result in a smaller size antenna. The ACS fed antenna configuration takes up less space than a normal CPW fed antenna owing to the half ground plane [48], [53]. The split ring resonator was etched with larger slits in the antenna to form as open split ring resonator (OSRR). It helps to achieve size reduction and multiband operational frequency. The large, distributed capacitance value of the OSRR antenna allowed it to be more compact than conventional antennas [51]. The folded shorted patch (FSP) antenna is proportional to the dominant (TM₀₁₀) mode, allowing the frequency to be tuned by varying the top layer size. The bandwidth at 400 MHz is rather narrow due to the folding process as well as the compactness of an antenna. However, by widening the gap between metallic layers, the BW can be raised while keeping the FSP at low profile [52].

In this review article, wearable antenna has been thoroughly discussed based on important applications such as 5G, WBAN, WLAN and WiMAX [16], [62], [64], [67]. Section II elaborates a comparative analysis of conductive and substrate materials for its flexibility. Properties of conductive material such as conductivity and thickness are discussed in Table 1. Several materials for substrate are defined based on their properties such as relative permittivity, tangent loss and thickness were studied and tabulated in Table 2. Antenna parameters play vital role in antenna design such as gain, SAR and bandwidth are compared based on their application with the usage of flexible materials were summarized in Table 3. The list of bands covered for specific field of application are reviewed and tabulated in Table 4. Section III represents the comparison of various antenna geometry and type of feeding to enhance the antenna performance and it is discussed thoroughly in Table 5. Section IV introduces different techniques of SAR reduction such as AMC and metamaterial are discussed in Table 6. Gain enhancement such as AMC, EBG and DGS were studied in detail and tabulated in Table 7. Section V covers the techniques for bandwidth enhancement based on the structure of an antenna, defected ground structure, AMC, EBG and slot loading. Finally, a comparative analysis is presented in Table 8. Section VI concludes with the comparison of this review work with the other recent review article based on challenges and direction for future research work.

TABLE 1 List of Flexible Conductive Material for Wearable Antenna

TABLE 2 List for Flexible Substrate Material for Wearable Antenna

TABLE 3 Summary of Flexible Material Based on Bandwidth, Gain and SAR

TABLE 4 List of Prominent Application Based on Standard Bands

TABLE 5 Comparative Analysis of Different Feeding Technique for Wearable Antenna

TABLE 6 A Comparative Analysis of Various SAR Reduction Techniques for Wearable Antenna

TABLE 7 A Comparative Analysis of Gain Enhancement Techniques for Wearable Antenna

TABLE 8 A Comparative Analysis of Bandwidth Enhancement Techniques for Wearable Antenna

SECTION II. Wearable Antenna

One of the crucial components of wearable electronics is flexible antenna design. This type of antenna is prominently used in a variety of wearable applications, including medical, military, entertainment, and other everyday usage of wearable devices [38]. The wearable antenna should be integrated into the human body for biomedical applications. However, the functioning of the antenna is impacted by the structure of the human body and the electrical characteristics of biological tissues. Low dielectric constant materials have become prominent to meet all these requirements because rigid materials with high dielectric constant do not comply with these requirements. The flexibility of the antennas and bandwidth increase both significantly relying on the low-dielectric materials. The different applications of wearable antenna are depicted in Fig. 1. The researcher has carried out and proposed several flexible antennas in the domain of medical gadgets, health monitoring, smartwatches for wireless application, stress level and heart rate level monitoring for mental health, use of Wi-Fi and Bluetooth for body-worn cameras, wearable athletic gear, tracking and navigation system for military application, etc. [6], [16], [38], [39], [53], [61], [83].

FIGURE 1.

Different application on wearable antenna.

Designing the antenna for wearable applications is based on three different sets such as typical wearable antenna, textile antenna and embroidered antenna. Novel patterns, shapes and geometrical structure in monopole and planar antenna, inverted F-antenna and microstrip patch antenna are included in conventional wearable antennas [12]. If both substrate and radiating part are fabricated with textile cloth, then it is referred to as textile antennas. Polyimide (PI) is utilised in the antenna for its flexibility and ease of fabrication with inverted L-shaped radiator patch to obtain further antenna miniaturization [27]. Conductive threads are used in the radiating portion of embroidered antennas by employing embroidery machines [87]. There are many challenges involved in designing the wearable antenna such as selection of material, light weight, desired radiation characteristics, compactness, desired frequency band operation and reliable performance under different bending and environmental conditions. Flow representation of different wearable antenna is represented in Fig. 2.

FIGURE 2.

Flow representation of different wearable antenna.

This section is divided into four sub-sections. section A classifies the most used conductive material for flexible antenna design with their important properties such as conductivity and thickness. Further in section B, widely used substrate material for flexible antenna design are based on relative permittivity, tangent loss and thickness. Section C elaborates the impact on the antenna performances due to the usage of both conductive and substrate material for different flexible wearable applications. In section D, devoted for the different bands used for several wearable applications.

A. Flexible Conductive Material for Wearable Antenna

In antenna, conducting material is placed in patch and ground area for radiating and grounding function. The usage of conducting material in the patch antenna are estimated by means of conductivity, resistivity, tensile stress, deformability, and their merging with the flexible material. Antennas require a low-loss dielectric material in addition to materials that are especially conductive for efficient electromagnetic radiation transmission and reception. When it comes to diverse applications that are concerned with wearable antenna requirements, conductive materials such as aluminum, conductive inks, polymers, and integrated conductive fibers are utilised. Some of the wearable antenna uses rigid material attached to the textile substrate due to its high conductivity and less expense but it would constrain its flexibility. Bending an antenna using rigid substrate degrades the performance of an antenna due to the deformation and minor cracks [63]. Therefore, few fabrics are utilized as non-rigid material to prevent degradation due to bending effect. Felt and shieldit material are applied in the antenna as substrate and conductive material to possess more flexibility and deformation while bending, without affecting the antenna performance [99].

Table 1 shows the widely used conductive materials for a wearable antenna such as copper tape, shieldIt and zelt due to its flexibility. When compared to other conductive material, Copper tape with a thickness of 0.035mm is frequently utilized in wearable antenna due to its cost effectiveness [10], [11], [12]. Textile material plays a major role in wearable antennas to employ in radiating part which has low conductivity and provides more flexibility [35], [100]. The usage of conductive fabrics and ink applies to many opportunistic innovations for a wide variety of wearable devices in the field of biomedical application.

B. Flexible Substrate Material for Wearable Antenna

Substrates provide mechanical stability of an antenna which helps to improve the radiation characteristics of an antenna. The dielectric substrate needs to be thick and have a low dielectric constant to achieve good antenna performance, greater antenna efficiency, a wider bandwidth, and better radiation [59]. Textile material is used as substrate instead of rigid material to overcome the problem of bending and stretchable with less loss [63]. The list of materials for substrate part are shown in Table 2. Most of the textile which is used as a flexible substrate possess low relative permittivity, low tangent loss and thickness that tends to achieve fringing effect at the edges. Therefore, flexible substrates are used to increase the performance of an antenna.

A human body’s curvature is made up of a stacking of folds in different directions. Textile fabrics respond well to these surfaces due to their excellent flexibility and elasticity. However, as the fabric bends and deforms in response to the structural paths, changes are made to its electromagnetic properties, which have an impact on the performance of the antenna.

The permittivity of the dielectric fabric and its thickness are impacted by its bending and stretching, which alters the antenna’s resonance frequency [69], [99]. The bending effect exhibits a slight shifting of frequency in the lower band with minimal fluctuation in S₁₁ values across the entire covered range of frequency [92]. While bending in the y-direction causes the resonance to shift, the bending in the x-direction does not really alter the resonant frequency. The resonance shifts to higher frequencies mostly because the current on the patch is along the y-direction, therefore any bending in that direction would shorten its effective length [96], [97]. Additionally, the antenna’s behaviors may alter due to variations in its resonance frequency caused by the geometric size of the textile components changing because of their stretching and compression.

C. Impact of Flexible Substrate and Conductive Material on Wearable Antenna

Flexible materials are used as a substrate in antenna to have minimum dielectric loss, low relative permittivity, less coefficient of thermal expansion and maximum thermal conductivity. It should be biocompatible, which is necessary for health monitoring applications. The substrate would be flexible depending on the density and thickness of the fabric material which should have stretchable and compressible properties. Fabrics having low dielectric loss which have less surface resistance and hence increase antenna performances such as impedance bandwidth and high gain. Table 3 shows the detailed list of substrate and conductive material with an improvement of antenna parameters such as specific absorption rate (SAR), gain and bandwidth for wide range of wearable applications. The planar structure, simple design, flexibility and light weight of textile material substrate makes it more appropriate preference for wearable antenna applications. It is more appropriate for both on-body and off-body applications due to flexibility and harmless [59]. Flexible substrates are used to control the antenna radiation, but it limits the gain and antenna efficiency owing to proximity to the human body [38].

Zelt is a flexible electro-textile material that offers better stretchability and stable radiation performance against deformation [1]. Polyamide laminate is suggested for good mechanical and thermal resistance which is suitable for firefighter and military applications. Fractal geometry was introduced which increases the electrical length and offers lower frequency resonance in turn miniaturization was possible [2]. Cotton jean as a fabric substrate is utilized in the antenna for wireless and health monitoring application due to its flexibility, deformation and easy integration in wearer’s clothing [7], [8]. Denim was suggested by a researcher due to its low cost and better flexibility for different wearable ISM band antenna [11], [12]. Impedance matching was improved by etching the radiating area and slot in the ground plane resulting in reduction of bandwidth with better return loss of -40 dB [28]. The effectiveness of a wearable microstrip antenna built on a substrate made of photo paper and denim has been compared by researchers. Researchers have observed that the high gain response of 8.71 dB is achieved in denim based microstrip antenna over photopaper microstrip antenna due to the conductivity of the silver fabric on the patch. Alternatively, because of the substrate’s thickness and roughness, photo paper-based microstrip antennas achieve lower return losses than denim-based microstrip antenna [45]. Denim was chosen for wearer’s comfort in circular patch antenna. Moreover, it provides multiband frequency due to insertion of elliptical slot and rectangular slot to have better directional radiation and better SAR reduction [59]. Further adding stubs and tapered edge on the patch can minimize the back radiation [65]. Open mesh structure was proposed for having several air cavities in the substrate which makes an antenna, a breathable product [77]. Polymer based substrate is used as polydimethylsiloxane (PDMS) for better flexibility, durability, and low cost [83]. Denim is used as a stretchable fabric mounted at the end of the conductive strip of the patch area. Two strips connected with plastic pins help to resonate dual frequency at 2.45 GHz and 5.4 GHz operational frequency where felt is used as substrate for better flexibility [98]. One technique for achieving triple band uses an iterative structure with nested-split rings that supports 4G, 5G, WLAN, and GPS applications. Three toroid-shaped ring patches are positioned above the polyimide substrate with a 0.1mm thickness for flexibility. The CPW feed uses an elliptical curve shape to increase the bandwidth of the triple band response [102]. One of the flexible substrates used in the antenna for transparency is polyethylene terephthalate (PET). For improved conductivity, the patch uses AGHT-8, an oxide-based conductive substance. To enable capacitive coupling, two circular rings on the patch are loaded with star-shaped slots. As a result, it improves the 40% bandwidth from 3.89 to 5.8 GHz that covers sub-6 GHz for 5G applications [104]. The artificial fingernail substrate in the patch antenna, designed to work at 15 GHz and 28 GHz for millimeter wave and microwave applications. This antenna is composed of 0.5 mm thick Acrylonitrile butadiene styrene (ABS) substrate to operate under mid 5G and higher frequencies. Aerosol jet technique is used to release conductive nanoparticle silver paint, which is used to create the microscopic features of an antenna. While the 28 GHz band antenna needs an extra copper plating process, the 15 GHz band antenna has already been tested after printing and curing [108]. It is evident from Table 3, flexible fabric such as denim, cotton and felt are more appropriate substrate material due to lower radiation. These fabrics influence the wearable antenna to improve its parameters such as bandwidth, SAR and gain for biomedical application.

The bending of an antenna plays a pivotal role on wearable antenna to analyze the shift in antenna performance due to the impact of flexible substrate. The bending effect is observed to examine the modest variation in S₁₁ response curve, indicating that suggested antenna can attain similar response of unbent antenna when mounted to a sloped surface.

The directional antenna, which is bent in either x- or y-orientation on a cylinder-shaped model with a 27 mm radius suitable for the arm of a person, is used in the simulation. From Fig. 3, the bending has no effect on the antenna’s functionality across the whole UWB frequency range [58].

FIGURE 3.

Antenna bending effect on X and Y orientation.

When an antenna is bent on phantom, radiation pattern of an antenna diminishes at the YZ plane, it is marginally altered at the XY plane. This is due to the physical deformation of the antenna to fit onto the curving surface of the phantom as depicted in the Fig. 4 [60].

FIGURE 4.

Antenna bending effect on (a) Z-axis and (b) Y- axis.

D. Bandwidth Coverage in Biomedical Application

Normally, microstrip antenna will provide narrow bandwidth but nowadays as technology improves, the range of users has been increased. Wearable antenna became more impact on medical, sports and telemedicine. Novel techniques should be used to propagate the antenna in a wider bandwidth to cover essential operational frequency within a single, dual or multiband. ISM and 5G are the most noticeable bands to work for wearable biomedical and IOT based applications [46], [62]. The coverage of bandwidth is reviewed with a specific application provided in Table 4.

A Geometry of circular patch was designed with eight slots to work in dual polarisation at a resonating frequency of 2.45 GHz and 5.8 GHz on the mode of TM₁₁ and TM₀₂ [64]. Slots in the ground plane helps to improve the current distribution in the lower part of the antenna to enhance the bandwidth [66], [67]. Defected ground structure was modified at the ground to improve the bandwidth that covers from (4.8-9) GHz where impedance bandwidth was obtained as 60.86% [68]. The achievement of multiband features, compactness, and good impedance matching is due to the loading of a split ring resonator (SRR) and L-shaped branches by tapering the ground plane. ACS feed was utilized to reduce size reduction and that makes the antenna to be compact [74]. Mercedes Benz logo structure was designed at 2.45 GHz operational frequency where a conductor plate is attached with an antenna to function as transmitter and reflector to decrease SAR and to achieve better impedance bandwidth [79]. A Patch antenna loaded with many slots to operate dual resonance of lower and upper frequency such as 918 MHz and 2.45 GHz by using semi-flexible FR-4 substrate with 0.8 mm thickness [79]. A metamaterial structure with hexagonal radiator and partial ground are synthesized to produce ultra-wideband which provides low SAR [81]. Ultrawideband is obtained by usage of felt substrate material and partial ground in the hexagonal monopole antenna. It reduces the SAR up to 98.3% by loading the square shaped metamaterial [82]. Tapered patch, CPW extended ground and folded ground are utilized to resonate on UWB range. 3D structures are used as folding type to extend the bandwidth and improve impedance matching [84]. A round shaped button design made with Rogers 5880 is placed at the top of the textile substrate where it is capacitively connected with front patch which provide wide bandwidth and less SAR value [85]. Complementary split ring resonator is used to produce cross-polarisation effect which tends to resonate dual band where moderate bandwidth and gain is achieved [89]. The mu-zero resonance (MZR) is produced by capacitively loading the horizontal arms of the loop with slots to create MNG metamaterial unit cells. To further stimulate resonance and broaden the antenna bandwidth at lower frequencies, a strip patch is placed close to the loop. The antenna structure achieves a wide bandwidth of 52% (0.64-1.1 GHz) with a peak FBR and gain values of 12 dB and 3.2 dBi, respectively, while being 80% smaller than comparable mu-negative metamaterial (MNG) loaded loops [90]. The antenna consists of two strip of long and short where denim is used as stretchable fabric at the conductive strip to cover Wi-Fi and 4G-LTE bands [98]. A flexible polyimide substrate is employed since it is economical and ease of fabrication. The plus- shaped geometry on the patch is used to shift the frequency from 2.9 GHz to 3.1 GHz. To further shift the resonance frequency from 3.1 GHz to 3.3 GHz, the L-shaped structure is realized in the slotted L-arm shaped patch to provide capacitive coupling. Thus, it achieves sub-6 GHz band from (3.05-3.74) GHz for 5G applications [103]. Higher-order modes cannot be controlled by a single circular or elliptical form; only the first two modes (orthogonal modes) can be changed. Circular patch is utilized to determine the resonance frequency for excitation of the TM₁₁ dominant mode, separated orthogonal modes, and higher order TM₂₁ and TM₃₁ modes that contribute to the antenna radiation process. The technique included elliptical structure and four triangular arms which provided multiband response due to the presence of current distribution at low surface area. This resulted in multi-band operation for 2G, 3G, 4G and sub-6 GHz 5G applications [106].

In this section represents the comparison of different flexible substrate and conductive material based on SAR, gain and bandwidth. In section III, elaborates about the different feeding techniques and antenna geometry for different wearable applications.

SECTION III. Impacts on Feeding Techniques and Antenna Geometry

In the field of antenna design, the feeding method is essential. A feedline is the transmission line that connects the antenna to the radio transmitter / receiver. The microstrip patch antenna is fed using different types of feeding techniques such as microstrip feed, CPW feed and coaxial feed. CPW feed was suggested by several researchers to obtain wide bandwidth [92]. Different antenna geometry, ground plane modification, addition of different kind of slots are suggested for miniaturization of an antenna. Further, all these techniques were studied using flexible substrate materials and several designs has been proposed for biomedical wearable application.

In Table 5 discussed the survey related to the geometrical shape of the conducting patch and the type of feed used in a wearable antenna. The shape of a radiating patch also impacts the performance of an antenna such as bandwidth, gain and SAR value. The prototypes from (Fig. 5 –16) were taken from the journal related to wearable antenna for the literature review.

Semi-circular ring slot antenna provides low SAR due to EBG which is placed beneath the substrate. Wool felt and Nora-cell DR fabric as a substrate and a conducting textile material are utilised in the antenna for the wearer’s comfortability. The reflected radiation occurs in negative z- direction due to the current is increased in vertical direction which causes low back radiation. SAR reduction of 95.5% is achieved with EBG of 0.554 W/Kg when compared to without EBG of 13.5 W/Kg. High gain of 7.3 dBi is obtained with 70% of improvement is achieved with EBG placed in the antenna [4]. Slot loading in the flexible patch antenna provides wideband covers from (2.96-11.6) GHz. Size reduction was achieved by exciting the lower frequency resonance is due to increasing the electrical length for wearable antenna where denim jean is attached for the malleability to the user’s comfort [13]. The different flexible antenna has been proposed using cotton jean for mining area or underground area [14].

A dual band antenna resonating for WBAN application used semi-flexible Rogers RT-duroid 5880 substrate for better flexibility. Slit and two small meander lines are utilized in the antenna to improve the miniaturization by resonating lower frequency with the help of enhancing inductance and capacitance area. When compared to standard SAR value (1.6 W/Kg), less SAR value is achieved at 2.45 GHz and 5.85 GHz such as 0.919 W/Kg and 0.118 W/Kg. High efficiencies is obtained as 86% and 91% is due to better antenna radiation characteristics [15].

In this paper, Smida et al., suggested that ground plane modification and slot loading in the patch area provides size reduction without affecting antenna performance. It tends to have good impedance matching, better efficiency of 93% and wide bandwidth of 59.7% with the help of hook-shaped stub resonator at the ground when compared to the conventional microstrip patch antenna [17]. Defected ground structure is added on the rectangular patch antenna to have better impedance matching where felt and teflon are utilised on the antenna to enhance antenna performance. Better gain of 6.38 dB is achieved if Teflon is used as a substrate for the antenna where 2.14 dB is obtained when felt substrate is utilised [18].

Antenna miniaturization is achieved by using rectangular loop structure that increases the length of the current path leads to frequency shifting from higher to lower resonance. Teflon fabric is used in the antenna for its flexibility and accuracy of an antenna for bio-implantable devices [26]. Polymer substrate with silver nanoparticle conductive ink was suggested by a researcher to provide better flexibility and low cost of the proposed antenna. Tuning the gap between T-resonator and fractal geometry can modify the resonance frequency. Minkowski fractal structure with metamaterial unit cell can react as a magnetic resonance that reduces the SAR by controlling the surface waves. This provides lower SAR value of 0.25 W/Kg and 0.33 W/Kg for MICS (Medical Implant Communication System) and wireless application respectively. The obtained SAR value is well below the standard SAR value of 1.6 W/Kg [47]. ACS feeding technique is applied to enhance the bandwidth and cover less area which makes the antenna to be compact. Slits are utilized in the antenna to reduce the size and increase the length of the current path. As a result, the resonating frequency changes to a lower range [48]. Probe feeding is utilized in the patch where parasitic switches are implemented. When the switch is OFF condition, it works as a convectional antenna at 2.4 GHz operational frequency. This antenna provides zeroth order resonance (ZOR) which exhibits $0^{0}$ phase delay, when switch is OFF condition. The radiation pattern resembles a monopole pattern because it can be viewed as emerging from an associated magnetic current that is moving around the rectangular area [96]. A “” Pane Structure is employed on the circular patch to provide multiband lower resonance by extending the electrical length of antenna. Four different resonant frequencies such as (1.58-1.77) GHz, (2.1-5.5) GHz, (3.61-4.09) GHz and (4.65-6.05) GHz were excited via providing slits at four corners of the patch. These frequencies encompass 4G LTE, 5G, GPS and WLAN bands [107].

The second section describes the antenna strucxture and feeding technique to improve the performance of an antenna with respect to return loss, bandwidth and gain. The CPW feed is used to control the low back radiation and enhances the bandwidth whereas the microstrip feed is used to improve the directivity and gain. Asymmetric coplanar strip (ACS) is introduced as novel feeding technique that helps to decrease 50% size of an antenna and allows better impedance matching. Therefore, CPW feed, ACS feed and microstrip feed are suitable for flexible antenna designing. In section IV, SAR and gain enhancement are discussed in sub-section A and B for biomedical wearable applications.

SECTION IV. Techniques for Specific Absorption Rate Reduction and Gain Enhancement

The key element of wearable technology is a wearable antenna, which functions as an essential link between the on-body wearable device and external digital devices [80]. Ground plane reduction is frequently used to miniaturize antennas. The radiation properties of the antenna can be impacted by spurious currents that flow in the ground plane because of a decreased ground plane. This can be overcome by the entire ground plane that shields the wearer’s body from harmful radiation, which lowers the SAR value even further. Different methods of SAR reduction techniques were studied and tabulated in subsection A. The gain of a wearable antenna plays an important role for stable radiation and directivity. Gain can be enhanced by different techniques addressed in subsection B.

A. Techniques for SAR Reduction

Normally Specific absorption rate is a standard method to measure the effect of radiation when exposed to the body. SAR, which is defined as the relationship between the transferred power and the body mass (kg/lb) where the values are being assessed, reflects the amount of EM radiation that a human body can sustain without posing any health risks. IEEE defined SAR of 1gm, and 10 gm average of human tissue have standard value of 1.6 W/Kg and 2 W/Kg. Different techniques for SAR reduction are summarized in Table 6.\begin{equation*} SAR=\frac {\sigma E^{2}}{m_{d}} \tag {1}\end{equation*} where,

AbbreviationExpansion

$\sigma =$	Conductivity of a material
E =	Electric field (V/m)
$ m_{d} =$	Mass density (Kg/m3)

The SAR reduction techniques are covered in this section. Artificial magnetic conductor is one of the SAR reducing method by shielding the patch antenna from the body [31], [34], [46]. To accurately describe the impact, author have introduced a novel method of ground truncation. In this method, by observing the electric field intensity, one can truncate the position of more electric field [71]. The presence of an antenna causes an increase in the number of excited currents on the EBG surface, indicating a stronger coupling between them that enhances the S11 in higher frequency bands. EBG loaded with antenna can provide low SAR by reducing the amount of radiation emitted at the human body [94]. Several techniques have been proposed for SAR reduction and it has been suggested that the exposure of EM radiation should be reduced at lower frequency for biomedical application [46], [73]. The human torso has multiple layers, including skin, fat, muscle, and organs. Therefore, it is used to simulate SAR value based on the antenna design [80].

B. Techniques for Gain Enhancement

Gain of an antenna is proportional to the effective area and frequency of operation. One can control the gain and directivity of an antenna by altering the physical dimension of an antenna. The gain, wavelength of a signal and effective area of the antenna were shown in equation 2 [109].\begin{equation*} Gain(dB)=10{log}_{10}4\Pi \frac {A}{\lambda ^{2}} \tag {2}\end{equation*} where,

AbbreviationExpansion

A =	Physical dimension
$\lambda =$	Antenna wavelength

The antenna gain and directivity unit is decibel and isotropic decibel are mathematically related as shown in equation 3 [109].\begin{equation*} dBi=dB \mathrm {+2.15} \tag {3}\end{equation*}

Table 6 and Table 7 summarize the detailed comparison of SAR reduction techniques and gain improvement methodology for wearable application.

Polygonal split ring metamaterial (MTM) is placed (8mm) above the monopole antenna to maximize the gain as well as SAR reduction simultaneously. SAR reduction of 84.5% (1.645 W/Kg) was obtained using metamaterial compared to without metamaterial inclusion. In comparison to the gain of 2.7 dB without metamaterial, a gain improvement of 30% was attained with metamaterial. Further miniaturization is achieved due to the slot placed in the ground area. It helps to shift the frequency response to lower resonance. The stable radiation and gain of this antenna are more useful for wireless communication devices [19].

Felt material is used as substrate in the antenna design for better flexibility and low-cost fabrication. $\Omega $ - shaped grounded metamaterial (MTM) with the patch antenna were suggested to reduce the SAR value and improve the gain by utilizing $\mu $ -very large (MVL) property of the metamaterial. Maximum SAR of 0.405 W/Kg was achieved at 2.45 GHz that occurs within the standard SAR of 1.6 W/Kg. A peak gain of 7.79 dB was achieved using loaded metamaterial is better than the unloaded metamaterial in the antenna [20].

Denim jeans were utilized as a substrate to provide better flexibility and radiation pattern was controlled using the frequency selective surface (FSS). Further, miniaturization can be achieved due to the gap between the FSS unit and capacitance can be modified by varying the gap width. The resonance frequency would be shifted to lower side by controlling the capacitance. The average gain enhancement of (4-5) dBi can be achieved using frequency selective surface. Researchers have reported that 94.8% of SAR reduction can be obtained by using FSS [28].

Square ring-shaped artificial magnetic conductor (AMC) ground plane was proposed to maximize the gain of the antenna and leather substrate material was used to get better flexibility. P. Saha et al., studied the AMC effect as a ground plane to enhance the gain as well as the reduction of SAR [29].

Hexagonal copper shaped EBG and the gap between the unit cells optimization can be used to control the inductance and capacitance that helps to minimize the back radiation. This effect tends to reduce the SAR by 98.5% when compared to the antenna without EBG. Further it enhances the gain of 4.85 dB that is better than the unloaded EBG [30].

The cross-shaped EBG array is attached with the CPW-fed antenna to obtain wide bandwidth that can cover from (1.44 - 2.75) GHz. It ensures the SAR level within the safety limits and provides a low SAR of 0.022 W/Kg with EBG at 2.4 GHz. Current coupling can be maintained due to the small gap between the monopole antenna and the extended CPW ground. The currents of the oval-shaped monopole sum up with those on the surface of the enlarged CPW semi-circular ground plane since they flow in the same direction. It tends to enhance the achieved gain of the proposed monopole antenna by 1.5 dB. An average gain of 6.584 dBi is obtained by using this technique [32].

Electromagnetic band gap (EBG) cells in triple band antenna used as a reflector which helps to overcome the high radiation exposure to the human body. The patch is expanded by adding four perpendicular rectangular stubs to generate different metallic sectors that are accountable for multi-band features. Loaded EBG provide 91.5% of SAR reduction is better than the antenna with unloaded antenna. The gain improves from 2.88, 3.53, and 4.01 dBi of the antenna without EBG to 5.11, 6.43, and 7.41 dBi at 2.45, 3.5, and 5.8 GHz respectively [33].

Square-shaped AMC is utilised for the enhancement of gain by controlling inductive characteristics. It tends to achieve high gain of 9.37 dBi and 6.64 dBi at the resonant frequencies of 3.5 GHz and 5.8 GHz respectively. At 15 mm from the human tissue model, the antenna kept its dual-band resonance with good impedance matching. SAR reduction at the maximum of 99% is achieved due to minimal back radiation provided by AMC [34].

The J-slot EBG is placed beneath the J-slotted antenna where jean cloth substrate is employed. Surface waves are suppressed due to the presence of reflected EM radiation between the radiator and ground area of the J-slot EBG. This effect makes an antenna to lower the back radiation up to 0.85 W/Kg and average gain of 2.44 dB at 2.45 GHz [35]. A Patch antenna designed with reactive impedance surface (RIS) operates at 2.45 GHz resonant frequency with the usage of felt substrate and conductive part as ShieldIt for better flexibility. It provides 20% of gain enhancement of 6.072 dB when compared without RIS is measured as 4.67 dB [43].

When the C-shaped antenna is positioned over an AMC with symmetrical and square slots, gain is increased while the SAR value is reduced. It enables enhancing gain and efficiency by controlling impedance matching. Minimization in size is provided by shifting the phase to the lower frequency by increasing the effective path length of the current. The gain of 6.05 dB is achieved due to the loaded AMC is better than the unloaded AMC. Further, better SAR is achieved with AMC as 0.649 W/Kg is compared to the antenna without AMC [46].

Monopole U-shaped antenna was placed over an AMC that operates at dual band. The resonances can be tuned by controlling the length of the patch branches. AMC integration provided gain improvement of 4.0 dB compared to conventional antenna gain. The reduction of back radiation was achieved by controlling the capacitive coupling between inner and outer rings of AMC cell. When compared to an unloaded antenna, it provides reduced SAR values of 0.018 W/Kg and 0.015 W/Kg at 2.45 GHz and 5.8 GHz respectively [49]. The gain of a microstrip patch antenna can be increased by changing the effective dielectric constant of the dielectric plate. The gain of 2.55 dB was attained at 5.8 GHz, which is 79.9% more than the conventional antenna [55].

Regina et. al, suggested the use of two substrates such as Talconin TLY bonded to the patch and felt substrate was placed above the Shieldit conductive part to prevent propagating EM waves to the human body. The straight-line strip attached in folded ring that enables to resonate low frequency such as 2.45 GHz and 3.45 GHz due to increase in effective path length. The gap between the two straight lines affects the capacitance coupling to maintain in lower resonant mode. This antenna can be used for wearable application because of reduction in SAR value up to 0.1 and 0.04 W/Kg and broad gain of 5.1 dB and 8.6 dB [56]. A dual band antenna resonates at 0.99 GHz and 3.5 GHz frequency by using leather substrate for better flexibility. It is placed with a split ring resonator to filter the unwanted signals and to maintain the lower mode resonance. SAR value reduction was also achieved using the split ring unit cell. The maximum SAR obtained was 0.016 W/Kg and 0.64 W/Kg within the limit of standard SAR of 1.6 W/Kg [57].

Simple patch antenna comparing with different types of EBG structure such as slotted, mushroom-type and spiral were used to suppress the surface waves. It helps to achieve SAR reduction of 78.06% with SAR of 1.75 W/Kg which is less than unloaded EBG of 7.95W/Kg. Gain can be also enhanced due to increased current coupling using EBG. One can achieve better gain by using mushroom EBG structure with conventional antenna. 1.07 dB gain improvement was obtained using mushroom EBG [70].

Dual meander line linked to a T-shaped antenna and inverted U-shaped truncated ground tends to reduce the SAR value by E-field cancellation technique without any secondary unit. This effect helps to get a lowered SAR of 0.98 W/kg with a 25.5% reduction in SAR when an antenna is placed 15 mm away from a hand model phantom [71]. The slot’s high impedance is matched to the $50~\Omega $ CPW feed line by the stub. The stub is asymmetrical in relative to the feed line, which causes the current to be deflected on the stub, producing a dipole-type radiation. The length of the stub can be altered to change the folded slot antenna’s configuration. This can be done by using a PIN diode and using a basic bias circuitry to turn it ON and OFF. The dual-band, polarisation is dependent on the AMC surface, has a considerable impact on lowering the SAR level. For the slot and stub configurations, the antenna’s gain is increased around 3.6 dB and 2.4 dB, respectively [72]. EBG-FSS is positioned beneath the c-shaped antenna to produce low back radiation. It provides 0.055 W/Kg as compared to the SAR value of 5.41 W/Kg without EBG-FSS, resulting in a maximum SAR reduction of 95%. Due to the capacitance characteristics reacts in EBG-FSS, a gain of 4.4 dB was obtained which is better than the conventional antenna [73]. Antenna and wrist tissue are isolated by the H-slot in the EBG structure. Therefore, it reduces the SAR value by 0.364 W/Kg under 1gm of tissue when the antenna loaded with the EBG structure [75].

Meta structure of five circular disc provides isolation and reduce the SAR up to 0.264W/kg by truncating the circular elements in the meta surface [76]. A squared metal patch with slots, a dielectric substrate with a ground layer used to make the AMC unit cell structure. The increased flow of current on the AMC surface and the inserted slots broaden the operating frequency band near the resonance. SAR reduction of 38.2% was obtained with the loaded AMC than the convectional antenna [93]. The proposed antenna is kept near to the skin and hence has a low profile owing to the in-phase reflection properties of the introduced AMC reflector, which also shields the human body [95]. Square shaped Metamaterial structure was placed beneath the patch to achieve the low SAR value due to lower back radiation. SAR reduction of 74.6% and 86.3% was obtained when the antenna loaded with the metamaterial. Besides, it ensures to enhance the gain due to filtering of electromagnetic radiation [97].

In this section IV, the methods for designing the wearable antenna to increase gain and reduce the value of SAR are summarized in Table 6 and Table 7. Both AMC and EBG design play a crucial role for reducing the SAR value more efficiently than FSS and MTM. This technique also achieves a better percentile in SAR reduction. In technical comparison of gain, AMC have better gain response (>6 dB) than EBG and FSS. As a result, approaches like metamaterials, AMC, EBG, FSS, and Metamaterial (MTM) are thought to be effective for reducing SAR and enhancing gain performance. In section V, enhancement of bandwidth was discussed for wideband wearable applications.

SECTION V. Bandwidth Enhancement Techniques

The range of frequency over which an antenna can operate efficiently is known as the bandwidth. It is mathematically expressed as given in equation 4 and 5 [109].\begin{align*} BW& = \frac {F_{H -}F_{L}}{F_{C}} \tag {4}\\ BW (\%)& = \frac {F_{H -}F_{L}}{F_{C}}\times 100 \tag {5}\end{align*} where,

AbbreviationExpansion

$F_{H } =$	Higher frequency
$F_{L} =$	Lower frequency
$F_{c} =$	Center frequency of the band

In the literature review, several techniques such as use of low dielectric constant substrate, probe feeding and insertion of slot and notch in the antenna design were discussed to enhance the bandwidth. Furthermore, because of the bandwidth optimization methods, other antenna parameters like SAR, gain, and efficiency were decreased. Several articles have been studied and compared based on the methodology used to improve bandwidth and some effective techniques for wearable antenna using flexible substrates are discussed in Table 8.

A semi-circular patch antenna was integrated with square ring EBG resonator, which helped to enhance 14.7% of bandwidth compared to the convectional antenna [4]. Truncation methodology was implemented to obtain circular polarization without affecting the overall size of any antenna. Further impedance matching can also be controlled using suggested method. The four slits are introduced near to each corner of the antenna to excite lower frequencies at 1.575 GHz and 2.45 GHz respectively due to an increase in electrical current path. One can improve the bandwidth by combining all the resonances [16]. In the next article, two loaded strips having different lengths were added to the feed to provide resonance at 2.45 GHz resonant frequency. These loaded strips can provide another distinct resonance and it is combined with patch resonant frequency. This technique results in enhancing the bandwidth of 120 MHz from (2.358-2.52) GHz. Better isolation between two ports more than 15 dB was achieved by adjusting the distance between the microstrip patch and loaded strip from 5.5 to 8.5 mm [36]. A series of patch antennas are fed by two orthogonal microstrip feed lines for providing dual polarization in elevation and azimuth planes. Therefore, the transmission line length will maintain uniform phase due to multiple waveguide wavelength. Thus, it achieves wider bandwidth at port-2 than port-1about 300 MHz from (5.6-5.9) GHz [37].

In the next paper, multiple slots loaded in microstrip patch antenna have been proposed to increase the current path that helps to enhance bandwidth. Adding complementary rhombus resonator (CRR) at the backside of an antenna is used for additional enhancement of bandwidth due to magnetic resonance that provides up to 238 MHz [40]. In the next paper, bandwidth is improved by separation of two layers of conducting part by via holes of 0.3mm radius shorting pins between the patch and the ground plane. In this antenna, polyester is used in the substrate layer for flexibility. Extended current path is provided due to slot loading of open stub on the patch to enhance 85% of bandwidth of an antenna [41]. Increase of capacitive couplings between the patch edges and the mushroom-EBG cells array which is fed by proximity coupled where large amount of electric field is observed. This tends the patch antenna to co-excite at TM₁₀ and TM₃₀ mode through normal incident plane that provides enhancement of bandwidth up to 32% from (2.4-3.4) GHz [42]. Reactive Impedance Surface (RIS) is used to produce less coupling between the patch antenna and ground plane which results in improved bandwidth, high gain, and efficiency [43].

Three elliptical slots added to a rectangular radiator that form a pre-fractal antenna that helps to increase bandwidth up to 131 MHz and result in a size decrease of 26.85%. This method also aids in achieving lower resonance due to an increase in current path that covers whole ISM band from (2.386-2.517) GHz [50]. Researchers have suggested inbuilt copper e-thread to be sewed on dipole and loop antenna. It provides a wide bandwidth of (2.19-3.44) GHz and (2.35-2.81) GHz due to the selection of substrate and conductive material in terms of thickness, relative permittivity and conductivity of a copper thread [54]. A rectangular patch is truncated into a Y-shaped patch radiator to wideband response from (4.5-10) GHz. Beveled edges are modified near to the feedline that transforms ultrawideband (3.1-10.6) GHz from wideband response. The inter-digital lines in the fork slotted EBG are introduced to achieve wide band stop response without changing the EBG size. It also enhances gain, low SAR, and efficiency [58]. The polyester substrate and the copper taffeta are both woven using the plain weaving technique. This ensures that the external factors won’t significantly degrade its performance. Tapering the lower edge, the patch at lower side helps to enhance bandwidth up to 109%, Parallel slots are introduced to increase the electrical current path that covers the frequency from (1.198-4.055) GHz [60]. The inverse E-shaped antenna has a few slits of different length provide 15% of bandwidth (330 MHz) from (2.23-2.59) GHz. Four T-shaped stripline in EBG conducts more inductance which is placed below the antenna. This method tends s to widen up the bandwidth up to 27% (660 MHz) that covers (2.17-2.83) GHz and able reduce the backward radiation [61]. Floating EBG technique is adopted to improve the bandwidth by varying the distance between the ground and EBG from (1)–​(5) mm distance. This overcomes the effect of detuning when placed on the human body. It also controls the frequency shifting of resonance at 2.45 GHz [75]. To achieve a smaller electrical volume, Interdigital capacitor loading reveals a lower resonance frequency where the reflection phase passes zero. This makes the antenna improve 15% of impedance bandwidth than conventional antenna [88]. One of the methods to widen the bandwidth is to embed the metamaterial in the antenna. The DNG (Double-negative) metamaterial ($\varepsilon _{\mathrm {r}}\lt 0$ , $\mu \lt 0$ ), which is the basis for the loaded slots in the patch, offers two wideband frequencies due to increase in current path length and inductance. It covers 960 MHz and 2760 MHz of wide bandwidth from (1.6-2.56) GHz and (4.24-7) GHz, respectively. Researchers hypothesized that optimizing the height of the antenna attains better wideband response in both lower and higher resonance [101]. Kapton substrate has been recommended by researchers for wearable medical devices due to its thinness, flexibility, and simplicity of integration. Trimming the CPW ground inwards to the feed that spans the frequency range of (2.05 to 14) GHz led to a 150% improvement in bandwidth. Dual notch band is achieved by further adding a square slit ring DGS resonator in both sides of the CPW ground. The capacitance of the DGS resonator falls with an increase in width while its inductance increases with a decrease in overall length, which form notches at WLAN and WiMAX bands. Therefore, this flexible antenna is appropriate for 3G, 4G, and 5G applications [105]. The Ultralam 3850 flexible laminate material with a thickness of 0.1 mm is selected due to high-frequency characteristics and its capacity to withstand temperature and frequency variations. It has relatively thin composition, it can be bent easily for applications that require flexibility and conformability. The dimensions of the disc on the patch are used to determine the antenna’s first resonance. The rectangular slots and the slit on the disc also provide additional resonance. The succeeding harmonics may therefore be calculated with ease because they are essentially the higher-order resonances of the fundamental frequency. Here, the fundamental and higher-order resonances that are near to one another overlap. This close spatial proximity of many harmonics across the whole spectrum allows the antenna to achieve extremely broad bandwidth from (1.74-100) GHz [110]. Surface wave suppression is achieved by placing the EBG unit cells across the MIMO antenna operating at 60 GHz. The capacitance and inductance provided by the gap and slots in the cross-shaped EBG unit cell are used to block the surface current at the resonance frequency results in low back radiation. The Z-shaped slot in the ground, which lengthens the current path more than the horizontal slot, provides the antenna resonance mode matched at 60GHz (S$11\lt -47$ dB) with wide bandwidth (5.9GHz) [111].

This fifth section briefly discusses the improvement of bandwidth. The technique of widening the bandwidth by using slots of different shapes embedded in a rectangular patch antenna [50], [78]. From the comparison study, the slits and slots provide low bandwidth of ($\lt =1$ GHz). This issue can be overcome by the technique of truncation of the patch at the corners achieves bandwidth efficiently from lower to higher than other bandwidth enhancement methods [58], [60]. A journal appropriate to the wearable antenna on related prototypes (Fig. 33–46) was used as the basis for the literature study.

SECTION VI. Summary of the Review Work

In the review article, the parameters selected for the analysis are size, gain, SAR, bandwidth, material characterization, and antenna geometry. Although size, gain, and SAR were used in the study [5], material features, and material properties were discussed in the article [38], [105]. Bandwidth, material characterization, and antenna geometry that plays crucial role in the wearable antenna design were not included. To address the missing parameter a detailed study was carried out and presented in this section. Moreover, the novelty of this review article over other published review work is, it produced a detailed study on material properties, gain improvement, SAR reduction and bandwidth enhancement techniques. Detailed comparison is presented in Table 9.

TABLE 9 A Comparative Analysis of Proposed Review Article With Recent Published Review Article

SECTION VII. Conclusion

While designing flexible antenna for wearable and biomedical applications, the conformal nature, low loss, and better flexibility plays a vital role to obtain high gain and low SAR value. So, detailed study of several materials such as denim, cotton jeans, felt and polyester with low tangent loss and better flexibility were suggested in this study. This material also offered wide bandwidth, high gain, and better SAR reduction. SAR reduction (>90%) and high gain (~7.5 dB) were obtained using flexible material such as leather, denim, and cotton jean. Different non-rigid conductive materials were studied where copper adhesive, zelt, shieldIt and conductive ink were suggested due to their effective flexibility. Thorough comparison of geometrical structure and the feeding type of wearable antenna was performed based on the operational frequency and the antenna performance. Here, coplanar waveguide feed is suggested with AMC ground to improve the gain and SAR reduction. Different metamaterial structures such as AMC, EBG and DGS were studied, and comparison was presented based on detailed study of gain and bandwidth improvement techniques. Finally, the work carried out in this article is compared with a recently published review article based on flexible material for designing of wearable antenna.