A Comprehensive Review and Analysis of the Design Aspects, Structure, and Applications of Flexible Wearable Antennas

Abstract

This review provides a comprehensive analysis of the design, materials, fabrication techniques, and applications of flexible wearable antennas, with a primary focus on their roles in Wireless Body Area Networks (WBANs) and healthcare technologies. Wearable antennas are increasingly vital for applications that require seamless integration with the human body while maintaining optimal performance under deformation and environmental stress. Return loss, gain, bandwidth, efficiency, and the SAR are some of the most important parameters that define the performance of an antenna. Their interactions with human tissues are also studied in greater detail. Such studies are essential to ensure that wearable and body-centric communication systems perform optimally, remain safe, and are in compliance with regulatory standards. Advanced materials, including textiles, polymers, and conductive composites, are analyzed for their electromagnetic properties and mechanical resilience. This study also explores innovative fabrication techniques, such as inkjet printing, screen printing, and embroidery, which enable scalable and cost-effective production. Additionally, solutions for SAR optimization, including the use of metamaterials, electromagnetic band gap (EBG) structures, and frequency-selective surfaces (FSSs), are discussed. This review highlights the transformative potential of wearable antennas in healthcare, the IoT, and next-generation communication systems, emphasizing their adaptability for real-time monitoring and advanced wireless technologies, such as 5G and 6G. The integration of energy harvesting, biocompatible materials, and sustainable manufacturing processes is identified as a future direction, paving the way for wearable antennas to become integral to the evolution of smart healthcare and connected systems.

1. Introduction

In the early 1950s, the concept of microstrip patch antennas emerged. Printed circuit board (PCB) technology gained momentum during the 1970s. After this era, patch antenna technology further accelerated and advanced into diverse fields. To date, among the numerous types of antennas, patch-type antennas are the most prominent. People attribute their popularity to their lightweight construction, low-profile design, and cost-effectiveness, as well as their planar geometry, portability, and easy integration into modern electronic systems. In applications requiring compact efficient wireless solutions, microstrip antennas are chosen. They are easy to conform to, can be used in arrays, and are simple to fabricate and integrate with microwave monolithic integrated circuits (MMICs). Microstrip patch antennas are easy to construct using a variety of feeding techniques, such as offset, inset, and probe feeds. They come in diverse types of patch structural shapes, including hexagonal, square, circular, triangular, and rectangular. These antennas have been widely used in applications such as radio frequency identification (RFID), broadcast radio, mobile systems, global positioning systems (GPS), television, multiple-input multiple-output (MIMO) systems, vehicle collision avoidance systems, satellite communications, surveillance systems, direction finding, radar systems, remote sensing, missile guidance, and many more. In recent years, many countries have struggled with the rising cost of long-term healthcare and wellness services. According to the World Health Organization (WHO), the global population aged 60 and older has grown to 1.4 billion. Projections indicate that by 2050, this number will increase to 2.1 billion. Additionally, the population of people aged 80 and older is expected to triple between 2020 and 2050, reaching 426 million [1].

New developments in medicine and advancements in fighting disease are important milestones for society and medicine but pose a challenge to global healthcare systems. As demand for healthcare services is on the rise, the development of new strategies, regulations, and procedures is required for efficient resource utilization. New inventions in portable, wireless, and electromagnetic sensor technologies offer great promise for future research and therapeutic applications targeting various parts of the body [2]. These modern technologies have potential in different fields such as sports, gaming, and healthcare. Medically known as wearable or on-body antennas, their popularity had been boosted due to their easy attachment to the body and subsequent remote operation. Their structure is lightweight, thin, flexible, and able to twist; hence, they are adaptable to irregular human body surface topologies [3]. Future generations could take the form of compact, modular wearable sensors and signal processors that transmit health data to portable devices. Wearable antennas can serve as health monitoring systems. These systems enable individuals to closely track changes in their vital signs and provide feedback to help maintain optimal health conditions. When integrated into telemedical systems, wearable health monitoring systems can also alert medical staff to life-threatening changes in a patient’s condition [4]. Figure 1 illustrates the growing prevalence of wearables and offers insight into the future of wearable technology.

WBANs are widely utilized for various real-time health monitoring applications. These networks rely on wearable antennas to transmit and receive data for healthcare-oriented devices [5]. In recent years, significant advancements in wearable electronics have driven research into body-conformal antennas. It has become increasingly important to frequently monitor several key physiological indicators, such as heart rate, blood glucose levels, blood pressure, and electrocardiograms (ECGs), due to their critical role in human health. As a result, various sensors can be deployed on the human body to track these essential indicators. Body-worn sensors can store data related to these physiological metrics. Technology advancements have also introduced new tools to markets, including smart clothing, smart rings, and smart glasses [6].

According to the IEEE Std 802.15, the frequency range supported by WBANs must comply with the requirements of regulatory medical and communication organizations. These include the Industrial, Scientific, and Medical (ISM) (90–928 MHz, 950–958 MHz, 2360–2400 MHz, 2400–2483.5 MHz), ultra-wideband (3.1–10 GHz), and Medical Implant Communications Service (MICS) (402–405 MHz) frequency ranges. Recently, millimeter-wave (mm) bands have also been added to the list of frequency bands for development and implementation of WBAN communication systems [7,8]. The frequency bands of wearable antennas are described in

Table 1. Frequency range for ISM bands [9].

S. No.	Frequency	Applications
1.	402–405 MHz	Medical Implant Communications Service (MICS)
2.	402–405 MHz, 863–950 MHz	Wireless Medicals Telemetry Services (WMTS)
3.	902–928 MHz, 950–958 MHz, 2360–2400 MHz, 2400–2483.5 MHz	Industrial, Scientific, and Medical (ISM)
4.	3.1–10 GHz	Ultra-Wideband

.

This study discusses the use of flexible materials, including textiles and polymers, in the design of wearable antennas. The study emphasizes their use in a wide range of applications and their long-term durability, mechanical reliability, and performance under changing environmental stress conditions. Developing such knowledge is important for wearable antenna systems’ practical and challenging operation. Existing fabrication techniques, such as inkjet and screen printing, require additional advancements to enhance scalability, cost-effectiveness, and precision for the mass production of wearable antennas. A lot of research is being performed on ways to lower the specific absorption rate (SAR), but it is still hard to find the best balance between lowering the SAR and keeping an antenna’s efficiency and functionality when it is being bent and deformed in practical scenarios.

The major contributions of this review are as follows:

• We investigate wearable antenna design, materials, and structural adaptability to ensure compatibility with WBANs and real-time applications.
• We evaluate the impact of bending, deformation, and proximity to human tissues on critical antenna parameters, such as gain, return loss, bandwidth, and the SAR.
• We analyze the role of flexible materials like textiles, polymers, and conductive composites in improving antenna functionality while exploring fabrication methods like inkjet printing and screen printing for scalable production.
• We examine techniques such as metamaterials, EBG structures, and FSSs to reduce SAR levels while maintaining antenna efficiency and safety.
• We explore the potential applications of wearable antennas in healthcare, the IoT, and next-generation communication systems, emphasizing their role in enhancing real-time monitoring and wireless communication.

This study explores various aspects of wearable antennas, focusing on their development, manufacturing, testing, and applications.

The review article is organized as follows: Section 2 discusses types of flexible wearable antennas using textiles, polymers, and microwave imaging antennas. Section 3 discusses the selection of material used for designing wearable antennas. Section 4 describes the bending effect of wearable antennas. Section 5 shifts focus to fabricating techniques such as screen printing, inkjet printing, and embroidery. Further, Section 6 discusses the SAR level and its effect on the human body and discusses the various methods to reduce the SAR level, while Section 7 represents various applications of flexible wearable antennas and Section 8 provides a summary and the future scope of this review scope. Section 9 encapsulates this study’s insights and its conclusion.

2. Types of Flexible Wearable Antennas

Flexible antennas are commonly used for ISM-band applications. Various designs including textile antennas, polymer-based antennas, and microwave imaging antennas are developed. These diverse types of wearable antennas are widely employed in body-centric communication systems, particularly in the healthcare industry, where they serve as wearable tools for detecting vital health issues in patients. They are used in recovery rooms, clinics, operation theaters, homes, and even during patient mobility [10,11]. Antenna designers for wearable devices face two critical challenges: assessing antenna performance under bending conditions and evaluating the absorption of radiation by the human body. Most of the wearable antennas reported in the literature are monopoly or patch designs [12]. Adaptability is achieved by mounting the antenna on flexible materials, which allows it to conform to the user’s body shape and posture. Key designs of flexible wearable antennas are discussed below.

2.1. Textile Antennas

This study introduces biomedical-optimized tiny patch antennas and a frequency-reconfigurable textile patch antenna for the 2.45 and 5.8 GHz ISM wireless bands [13]. Flexible textile antennas radiate strongly in UWB frequencies and beyond, even under bending conditions. Stable operation is demonstrated with copper-based antennas integrated into textile substrates, which are antennas displaying compatibility with both UWB and WBAN frequencies. Additionally, the bending performance of a dual-band circular textile patch antenna is explored, further validating its applicability for wearable and body-centric applications [14]. The integration of smart materials, e-textiles, smart garments, or garment tracking has enabled smart capabilities in health monitoring and garment tracking. Electrical textiles, leveraging the innovations in conductive polymers since the Nobel Prize-winning development in 1977, have played a pivotal role in the developing e-textile technologies. For wireless detection, recognition, processing, and control, smart garments use textile antennas [15]. Wearable electronics made by fusion, material science, and creative design utilize textile antennas for thermal protection and aesthetics. Copper sheets for conductivity and denim for the dielectric substrate in these antennas exhibit a relationship between the bending radius and resonance frequency, demonstrating the need for flexible, electrically conductive materials in portable antenna design [15]. Wearable antennas designed for WLAN compatibility emphasize form factors and high-performance characteristics. Compared to traditional button antennas, dielectric resonator antennas (DRAs) offer superior compactness, adaptability, enhanced performance, higher gains, and broader bandwidth [16]. A 5.8 GHz, flexible DRA with a dielectric resonator performed well in free space, with phantoms, and near a cell phone with a 5.4 dB gain. However, DRAs are larger than button antennas and exhibit superior radiation characteristics, making them highly suitable for wearable applications [17]. Textiles’ antennas have several shapes and sizes in medical applications, as seen in

Table 2. Textile-based antennas and their applications.

Ref.	Antenna Type and Shape	Dimension (λ)	Substrate Material	Frequency (GHz)	Gain (dB)	Application
[18]	Wearable Textile Antenna	0.24 × 0.24	Textile Materials	2.4–2.5	3.5	Healthcare and Communication
[19]	UWB Patch Antenna	0.21 × 0.26	Denim Substrate	3.1–10.6	6.2	Early Breast Cancer Detection
[20]	Dual-Band Patch Antenna	0.17 × 0.17	Poly-Cotton Textile	5.8 and 8.3	8.73 and 4.64	Wearable Monitoring
[21]	Frequency Selective Surface Textile Antenna	0.3 × 0.42	Textile-Based FSS Reflector	1.5–6.0	Not specified	Wearable Body Area Network
[22]	Dual-Band Textile Antenna	0.18 × 0.18	Conductive Textile Layers	0.868 and 2.4	Not specified	Military, Industrial, and Telemedicine

.

2.2. Polymer-Based Antennas

Flexible substrates were successfully implemented in circuits in the 1980s, which involved the development of the IoT and wearable polymer-based flexible antennas. This study investigates the radiation characteristics of flexible polymer antennas under bending and shaping conditions, testing five polymer-based substrates: polyimide (PI), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), and liquid crystal polymer (LCP). These polymers collectively constitute approximately 80% of polymer-based FSSs, flexible antennas, and RFID tags [23]. A powerful rectenna does not use impedance matching or body loss; instead, it uses a small, conformal tattoo–polymer loop antenna with a 3 × 3 FSS. This tattoo–polymer antenna enhances the performance of digital watches, enabling a 59% improvement in Wi-Fi and mobile hotspot charging efficiency [24]. Additionally, experiments are underway with new polymer composite antennas incorporating 45% to 50% conductive materials, demonstrating successful electromagnetic shielding at GHz frequencies. These antennas utilize a combination of composites, polymers, and conductive fabrics to optimize radiation properties. Testing confirms maximum gains of −10 dB and −5.5 dB across various frequency bands [25]. A compact, lightweight, flexible, and cost-effective patch antenna was also developed for smart glasses and wireless communication. Enhanced simulations using CST microwave studio software preceded its manufacturing and measurement stages. A robust theoretical and experimental reflection coefficient exists for 2.45, 3.5, and 5.20 GHz resonance frequencies, achieving gains of 1.91, 1.98, and 1.87 dB, respectively. Additionally, omnidirectional radiation patterns were observed for both bent and unbent configurations, with unbent patch antennas effectively conforming to cylinders with diameters of 40 mm, 25 mm, and 60 mm [26].

2.3. Microwave Imaging Antennas

A microwave imaging antenna uses microwave frequencies to image or map environments. Microwave imaging employs antennas to receive and broadcast microwave radiation to make images or identify things. Industrial testing, medical imaging, and security can utilize customized designs and technology. This antenna transmits and receives microwave signals for imaging and structure detection. Medical imaging, security screening, nondestructive testing, etc., are industrial uses. Frequency range microwaves typically range from 300 MHz to 300 GHz [27], with the selection of imaging frequency governed by the application’s resolution and functional requirements. Due to its non-ionizing radiation, ultra-wideband microwave imaging is becoming important for cancer diagnosis. System efficiency depends on antenna performance [28]. This article discusses advancements in microwave imaging systems’ high-directivity sensors. For cancer diagnosis, it emphasizes antenna designs near the body. Due to its benefits over conventional imaging methods, microwave imaging is popular. In-depth biological tissue investigations for cancer diagnosis are beneficial.

This study focuses on optimizing the material, size, bandwidth, and pulse propagation of UWB microwave imaging antennas, which form the core components of these imaging systems. Among the antenna designs examined, the Vivaldi antenna is highlighted for its efficacy in breast cancer diagnosis. The number of components affects array quality, and water and electrical properties distinguish healthy and cancerous tissues. Imaging systems often employ antipodal Vivaldi antennas (AVA) operating in the 2.4–12 GHz frequency range, known for their omnidirectional radiation patterns and suitability for adaptive radar applications. Balanced antipodal Vivaldi antennas (BAVA) and AVA antenna designs show energy flow density and performance [29]. Figure 2 shows how high-gain antennas and meta-surfaces can detect several breast cancers in microwave imaging. Microwave imaging safely and accurately detect breast abnormalities, making it a beneficial early breast cancer screening and treatment option. Microwave imaging is essential for breast health monitoring because normal and cancerous cells have different electrical characteristics. Microwave imaging is better than X-ray, ultrasound, and magnetic resonance imaging [30].

Microwave imaging antennas have several medical diagnostic applications, as seen in

Table 3. Applications of microwave imaging antennas in medical diagnostics.

Ref. No.	Antenna Type	Frequency Range (GHz)	Medical Application	Key Characteristics	Advantages
[31]	Patch Antenna	3–10 (C/X band)	Breast cancer detection	Compact, low-profile	Low-cost fabrication
[32]	Slot Antenna	1–6 (S/C band)	Brain stroke detection	Wideband, linear, or circular slots	High gain, good directivity
[33]	Vivaldi Antenna	1–10 (S/C/X band)	Breast tissue imaging	Slot antenna with wide bandwidth	High sensitivity
[34]	Horn Antenna	0.3–30 (UHF/L/S band)	Lung disease diagnosis	Directional antenna	Deep penetration into tissues
[35]	Array Antenna	10–30 (K/Ku band)	High-resolution 3D imaging	Multiple smaller antennas	High-resolution imaging
[36]	Metamaterial Antenna	30–100	Cancer detection	Advanced control over electromagnetic waves	Compact and flexible

. The compact, cheap patch antenna (3–10 GHz, C/X band) aids breast cancer diagnostics. The slot antenna (1–6 GHz, S/C band) detects brain strokes with high gain and directivity [30]. The array antenna (10–30 GHz, K/Ku band) delivers high-resolution 3D imaging while the metamaterial antenna (30–100 GHz) is compact and flexible for wearable cancer diagnosis [31,32,33,34,35,36]. These antennas balance frequency, sensitivity, and integration to improve healthcare diagnostics. The breast tissue imaging Vivaldi antenna (1–10 GHz, S/C/X band) has high sensitivity and a wide bandwidth. Deep penetration makes the horn antenna (0.3–3 GHz, UHF/L/S band) suitable for lung disease diagnosis [33].

2.4. Patch Antennas

The patch antenna generates enhanced directivity and provides a unidirectional radiation pattern. It is important to determine the advantages of applications that call for focused communication because satellite communication, radar systems, and point-to-point communication are all examples of such applications. It is necessary to identify situations in which the application requires directed energy. Examples of such situations include microwave imaging, global positioning systems (GPS), and wireless local area networks (WLANs). Both patch antennas and monopole antennas radiate in their own unique ways. Patch antennas are better suited for applications that require high directivity and precision due to their unidirectional emission pattern.

2.5. Monopole Antennas

Monopole antennas emit energy in an omnidirectional pattern so that the coverage is achieved horizontally at 360 degrees. They are thus highly appropriate for applications requiring wide and uniform coverage, such as mobile communication, IoT networks, and vehicular systems. Monopole antennas are valued for their simplicity, cost-effectiveness, and ease of deployment; they are often used in scenarios where devices need to communicate with multiple nodes or cover wide areas. In the field of the SAR, monopoles have omnidirectional radiation. The antenna design may change to add slots or wings to deflect radiation from the body or use reflectors and directors to control the radiation. Electromagnetic shielding or low-SAR materials, such as ferrite sheets, can be used to absorb and redirect waves, and substrates with a low dielectric constant can be used to limit tissue energy absorption near the antenna. The SAR can be reduced by changing the antenna power with transmission power or proximity sensor-based adaptive power management. Antennas are placed away from the body or in insensitive areas to reduce the SAR.

For fair comparison,

Table 4. Comparison table of wearable antennas for health monitoring systems.

Refs.	Antenna Type	Dielectric Constant	Dimensions of Wavelength (λ)	Frequency (GHz)	Wavelength (mm)	Gain (dB)	Return Loss (dB)	Material	Application	Features
[37]	Planar antenna	1.7	0.2 × 0.2	2.5–20	12–120	3.17	−27	Textile substrate	UWB Medical	Broad UWB coverage
[38]	Patch Antenna	1.6	0.12 × 0.13	2.4 to 2.5	120–125	1.24	−36	FR4 substrate	ISM band	Body-centric environments
[39]	Patch Antenna	1.67	0.1 × 0.093	2–4	75–150	N/A	−31	RT/Duroid	Breast Cancer	High sensitivity for breast tissue
[40]	Loop Antenna	1.3	0.016 × 0.15	2.45	122.45	1.86	−34	Polymer substrate	Breath Monitoring	Compact design for monitoring
[41]	Dual-Band Antenna	3	0.136 × 0.146	2.45 and 5.8	51.72	3.74	−39	Textile substrate	WBAN	Dual-band operation
[42]	Dipole Antenna	1.44	0.86 × 0.9	2.45	122.45	N/A	−13.6	Conductive fabric	Wireless Comm.	Large area design
[43]	Monopole Antenna	1.44	0.33 × 0.33	1.0 to 5.0	60–300	N/A	−26	Textile substrate	Bone health Detection	Deeper tissue penetration
[44]	Patch Antenna	3.5	0.08 × 0.08	2.45	122.45	N/A	−30	Flexible polymer	Medical	Specific point-of-care [1] diagnostics
[45]	Planar Inverted-F Antenna	1.28	0.09 × 0.09	2.45–5.8	51.72–122	1.68	−26 and −19	Textile substrate	Medical	Data transmission in healthcare
[46]	Microstrip Patch Antenna	1.8	0.82 × 0.73	2.45	122.45	4.79	−34	Flexible polymer	WLAN	Wearable WLAN systems
[47]	Monopole Antenna	1.05	0.7 × 0.47	5.5–7	42.8–54.54	4.6	N/A	Flexible substrate	X-band	X-band communication
[48]	Patch Antenna	3	0.216 × 0.216	2.45	122.45	4.36	N/A	Textile substrate	Knee bone monitoring	Monitoring with minimal discomfort

selects a list of wearable antenna designs, including gain, return loss, dielectric constant, materials, and applications [37,38,39,40,41,42,43,44,45,46,47,48].

Return loss matches impedance well, with values as low as −34 dB [39]. Gain varies from 1.68 to 4.79 dB. for wearability, which favors textiles and polymers [45], while it uses rigid materials for specific applications. Wavelengths ranging from 42.85 mm at 7 GHz to 122.45 mm at 2.45 GHz [45,47] help to balance size and performance.

3. Selection of Materials for Wearable Antennas

The efficiency of wearable antennas depends on their smooth functionality when affixed to the human body. Wearable antenna technology predominantly utilizes WBANs. As a result, carefully selecting appropriate substrate material that possesses both conductive and non-conductive properties is an essential component during constructing these antennas. The employed substrate must be adequate to position the antenna near the human body. Several researchers have extensively researched the most suitable substrate material for this purpose [49]. This work introduces a small wearable antenna created from jeans and denim materials for designing an antenna for integration into smart wearable textiles, operating at a frequency of 3.4 GHz. Simulations and actual implementations assess the antenna’s performance. This study examines the impact of humidity on the resonance frequency of a microstrip textile antenna. It specifically focuses on the features of the substrate material and how environmental factors affect the antenna’s performance. This research looks at how well the ring resonator and quarter-wavelength stub resonator methods work for testing the dielectric properties of textiles made in Nigeria. It also verifies their suitability as substrates for designing wearable antennas [50]. This research examines the design of a small, adaptable, and slim ultra-wideband antenna for wearable devices, with a focus on its compatibility with WBANs and an analysis of its performance when bent [12,51,52]. Several different adaptable substrates and conductive materials are used in the construction of antennas. As shown in Figure 3, the substrate is selected due to its dielectric qualities, resistance to mechanical deformations (such as bending, twisting, and wrapping), compatibility with miniaturization, and longevity in situations exposed to the elements [53]. Alternatively, the selection of conductive material, which is determined by the material’s electrical conductivity, impacts the antenna’s performance, including the antenna’s radiation efficiency [54].

3.1. Flexible Conductive Material for Wearable Antennas

In wireless applications, achieving conductive patterns with excellent electrical conductivity ensures optimal gain, efficiency, and bandwidth. Furthermore, the conductive material should also possess resistance to degradation caused by mechanical deformation. Nanoparticle (NP) inks, such as silver and copper, are commonly used for making antennas that are flexible because they have a high level of electrical conductivity. Silver nanoparticle ink surpasses copper nanoparticles because it forms oxides at a slower pace [55]. Nevertheless, high conductivity and stretchability are not compatible attributes. One possible approach to address is that this issue involves the implementation of one-dimensional nanostructures, including metal nanowires and carbon nanotubes affixed to a stretchable fabric [56].

Currently, the conductivity values under significant strain continue to be insufficient to meet the demands of practical applications. In [57], a conductive composite material composed of rubber fibers and silver nanoparticles that enables the creation of exceptionally stretchable circuits via a fabrication method is presented. Figure 4 shows the properties of choosing conductive materials, considering the physical features of the pattern, such as adherence to the substrate, and the desired physiochemical properties of the inks, such as compatibility with the printing process. One example is the restrictions placed on the size of the printing nozzle for nanoparticles, as well as problems with aggregation, stability, rheology, and electrical and mechanical properties [52].

Table 5. Conductive materials used in wearable antennas for the ISM band [55].

Conductive Materials	Conductivity (S/m)	Thickness (mm)
Egaln Liquid	2.5 × 105	0.08
Polyleurethene–Nanoparticle Composite	1.1 × 106	0.0065
Zoflex + Copper	1.93 × 105	0.175
Silver Flakes Fluorine Rubber	8.5 × 104	-----
AgNW/PDMS	8.1 × 105	0.5
PANI/CCo Composite	7.3 × 103	0.075
Copper Coated Taffeta	3.4 × 106	0.15
Graphene	3.3 × 104	0.01

describes the various flexible conductive materials utilized in wearable antennas for the ISM band, exhibiting properties such as thickness, conductivity, and specific material composition. These materials are selected based on criteria including high conductivity, low resistivity, deformability, weather resistance, and high tensile strength, tailored to the design and application requirements of the wearable antenna [57].

The wearable antenna employs both a ground and top radiator. Conductive materials can be assessed based on their resistivity, conductivity, malleability, resistance to weathering, tensile strength, and compatibility with flexible materials. Several inflexible conductive materials have been employed for wearable antennas. These materials possess excellent conductivity, are inexpensive, and may be incorporated into textile substrates utilizing adhesive laminated materials, eliminating the need for embroidery and stitching techniques. However, their inflexible structure limits their applicability for flexible applications [58].

3.2. Flexible Substrate Material for Wearable Antennas

To improve the radiation characteristics of an antenna, substrates are used to provide the antenna with mechanical stability. A thicker dielectric substrate with a low dielectric constant is recommended to achieve ideal antenna performance, including increased efficiency, a broader bandwidth, and enhanced radiation. Rather than using stiff materials as a substrate, textile material is employed since it is more flexible and can reduce signal loss while also mitigating concerns associated with bending and stretching [50]. Most textile materials that are used as flexible substrates have a low relative permittivity, a low tangent loss, and an adequate thickness to cause a fringing effect at the margins. As a result, flexible substrates are applied to enhance the effectiveness of antenna technologies [59]. An antenna operates at two specific frequency bands used in wireless network communications and the substrate material is used to enhance the performance of the antenna [60]. Figure 5 demonstrates how flexible antennas are created by combining non-conductive materials like cotton, wool, and silk with different polymer materials and metallic conductive fibers [54].

These materials are used to create antennas, and the relative permittivity is determined by the fabric’s thickness and the type of fabric itself and whether it is knitted or woven. Polymers are highly resistant, non-conductive materials that are monomers which are repeatedly occurring hydrocarbon subunits that comprise the structure [42]. Because they are thin, affordable, and very flexible, these materials also have a low loss tangent. Paper, on the other hand, is a material that is both environmentally friendly and inexpensive. It can be modified to be a fire retardant, making it appropriate for use as a substrate for flexible antennas.

Table 6. Flexible substrates’ material for flexible wearable antennas.

Ref.	Materials	Dielectric Constant	Frequency Range	Loss Tangent	Substrate Height
[61]	Flannel	1.7	2–20 GHz	0.025	--
[62]	Polyester	1.44	2.6–5.8 GHz	0.0045	2.85 mm
[47]	Polyurethane	1.28	2.38–6 GHz	0.016	2.4 mm
[63]	Fleece	1.60	2.45 GHz	0.0400	3 mm
[64]	Felt	1.38	2.6–3.95 GHz	0.023	3.5 mm
[65]	Tween	1.69	2.6–3.95 GHz	0.0084	--
[66]	Perspex	2.57	2.6–3.95 GHz	0.008	-
[57]	Moleskin	1.51	2.6–3.95 GHz	0.02	3 mm
[55]	Cordura/Lycra	1.5	2.6 GHz	0.0093	1.6 mm
[67]	Quartzes fabric	1.95	2.6 GHz	0.0004	1.3 micron
[68]	Cotton	1.58	2.4 GHz	0.115	0.7mm
[69]	Jeans	1.68	2.4 GHz	0.05	2.5mm
[68]	Silk	1.5	2.45–5.8 GHz	0.012	0.254 mm
[32]	PTEE	2.1	2.6–5 GHz	0.0004	6 mm
[70]	Panama	2.12	2.4–5.8 GHz	0.018	1.6mm
[71]	Rogers Duroid RO3003TM	3	2.45 GHz	0.001	2.4 mm
[72]	PDMS	2.65	2.45 GHz	0.02	2.5 mm

presents several different types of flexible materials that are frequently employed as substrates in flexible wearable antenna, highlighting their extensive use in applications of this kind.

Since research has been conducted, many patterns in different sizes are accessible. Other frequencies, gains, and bandwidths separate these systems. For testing purposes, an author designed and constructed an antenna from denim, excluding the SMA connector. The antenna, loaded with right rectangular slots and strip lines, reduced its size by 75% compared to the reference antenna. The antenna consistently achieved a radiation efficiency of 79% and a bandwidth of 15% [73]. For an author’s jeans-based UWB MIMO, the proposed construction employed denim. Two inverted unformed stubs were attached to the partially etched ground on the antenna’s rear to improve port isolation. A 50 × 35 mm² universal wideband multiple-input multiple-output antenna operated from 1.83 to 13.82 GHz [74]. An author proposed a flexible PDMS antenna where the antenna covered 3.43–11.1 GHz with a 59.9% bandwidth. Changing bending angles changed the bandwidth by less than 1% [75]. It implied a flexible, wide-band antenna. The first coplanar waveguide-fed planar monopole on a Kapton polyimide substrate worked at 2.27 to 5.27 GHz and was 56 mm wide, 56 mm tall, and 0.11 mm thick [69]. For a possible ultra-wideband (UWB) denim textile antenna, the design antenna covered the FCC’s UWB range (3.1–10.6 GHz) and had a 118.68 percent bandwidth (2.96–11.6 GHz). Frequently, its 3.5 × 10 × 1 mm³ dimension yielded a 5.47 dB peak gain [68] which operated at 7.3 GHz. This antenna had a maximum average SAR of 10 g in open space, and on flat ground, the 2.45 GHz ISM band was 90 percent below the standard [32]. The antenna measured a mere 51 × 45 × 0.785 mm. A 10 × 10 mm rectangular slot enhanced return loss and gain. The substrate materials and design frequencies were subject to variation with a 2.45 GHz semi-flexible wearable antenna for industrial, scientific, and medical band applications [44,53,72,73].

4. Bending Effect on Wearable Antennas

Different substrate materials have an impact on how wearable antennas flex. Figure 6 illustrates how the application determines the placement of wearable antennas on the body. Geometric or physical antennas’ orientation or form modification is called the ‘bending of antennas of various positions’. Several factors can affect antenna behavior. When bent, antenna radiation patterns show energy dispersion in space during transmission or reception. Bent antennas can yield unexpected lobes and nulls. Bending helps wearable, flexible, or conformable antennas fit in tight areas or curves. Their structure makes these antennas bend-resistant. Most antenna designs allow for deliberate bending or reconfiguration to change the antenna’s properties, such as pattern shaping or frequency tuning. When discussing bending antennas in different locations, a wide range of factors are considered, ranging from mechanical and physical features to electrical properties, including impedance, polarization, and emission patterns. Effective antenna design and deployment depend on an awareness of the substantial influence that any physical modifications to an antenna can have on its performance [76].

4.1. Primary Planes

Bending an antenna in the E- and H-planes changes electric and magnetic fields. Bends or distortions in the E- and H-planes can impact the antenna’s signal output. Authors tested antennas with 45, 35, 55, and 15 mm radii. The simulation and understanding of antenna performance with varied bending degrees were achievable. Bending an antenna alters its radiation bandwidth and return loss, allowing for the study of frequency effects. It indicates the antenna’s versatility and longevity. The durability and flexibility of antennas were strictly tested through rigorous test protocols. One of the crucial design aspects is that the phase difference had to be 90 degrees between synchronized elements of the antenna. Bending affects performance and impedance matching, as return loss models describe the degradation in signal reflection under various bending conditions.

These challenges allow the development of conformal and reliable antenna designs suitable for real-world applications. As shown in Figure 7, the antennas were bent along both principal planes with bending radii of 45 mm, 35 mm, 25 mm, and 15 mm. The results provide an understanding of the mechanical flexibility of the antennas and the stability of their performance. These tests give a way to optimize the function of antennas under practical usage scenarios [11].

4.2. Distinct Effects on the E- and H-Planes

This section explores how E- and H-planes’ form and bending affect the reflection coefficient in these orientations, as shown in Figure 8. This matters because antenna orientation affects performance and behavior in each plane. E- and H-plane bending influences S₁₁ differentially depending on antenna geometry. If the reflection coefficients alter dramatically, bending may impair the efficiency, range, signal loss, and frequency. Engineers can solve problems and improve antenna design for realistic applications by altering the antenna’s shape during bending with S11 in the E- and H-planes. Figure 8A,B illustrate the virtual return loss and the ability of antennas to bend with a changing main plane radio. It show how the E- and H-planes bend material at 0 (flat), 15, 25, and 35 mm. Bendable antennas operate in the target band without frequency detuning [77].

Table 7. Comparison of various bending angles used for wearable antennas.

Ref.	Substrate	Radius	Bandwidth (GHz)	Application
[59]	Zelt	15, 25, 35	2.35	ISM band
[73]	Jeans	-----	3.09–3.94 4.23–5.65	Wi-Max, 5G lower band
[78]	Cotton	X-, Y-bent condition	2.45	ISM band
[79]	Felt	30, 35, 50	0.915	UHF-RFID Tag
[27]	Synthetic fabric	Z-, Y-axis	1.198–4.055	UWB
[80]	Shielding conducting fabric	35, 40, 50, 70	2.4	ISM band
[81]	Denim	10°, 60°, 90°, 150°	3.37	Wi-Max
[70]	Polyester textile	0, 10, 20, 30, 40, 50, 60	7–28	WBAN, UWB
[79]	Felt	40°, 60°, 80°	2.45, 5.8	LTE, WLAN
[79]	Thin planner textile	0°, 45°, 90°	2.45	WBAN
[80]	Denim	45°	2.45	ISM band
[77]	Polyester	X-, Y-bent condition	1.575	GPS
[27]	Zelt	Crumping condition	2.43	ISM band
[80]	Fleece	(20, 40, 70)	2.45, 5.8	ISM band
[81]	Woven cotton	Crumping condition	2.45	ISM band
[82]	Pl substrate	X-, Y-bent condition	2.4	ISM band
[83]	Flexible felt	10°, 20°, 35°	5.8	ISM band
[84]	Jeans	X-, Y-bent, crumping condition	3.5, 5.8	Wi-Max, ISM band

summarizes the results of our analysis of bending at different angles, and it can show the changes in the resonant frequency and radiation efficiency. Various bending angles are used for the application of wearable antennas and these bending angles are based on different criteria such as the radius, radiation, and some other factors that play an important role in design. However, the effect of S₁₁ variations in impedance caused by the antenna’s changing form may result in different degrees of signal reflection. This impacts S₁₁, which gauges the amount of signal bounced back from the antenna. An antenna’s impedance characteristics might alter when it is physically bent or deformed from its initial shape, changing its reflection coefficient (S₁₁).

There is also a wider radiation pattern when the antenna is placed on the body because human cells are lossy. This means that a wider bandwidth is achieved. As a result, the power is spread out in all directions, creating a diffuse radiation pattern across both bands.

5. Various Fabricating Methods for Flexible Antennas

The way a flexible antenna is manufactured (which varies between substrates) dictates its efficacy. Wet-etching, inkjet printing, screen printing, and various other specialized techniques are frequently employed in the production of flexible wearable antennas. Former publications contain a comprehensive summary of the various fabrication methods used to create flexible antennas. In response to the growing demand for antennas that may be worn, researchers and antenna designers have proposed a variety of different fabrication techniques.

This section provides an overview of the many approaches that are now available for the manufacturing of flexible and wearable antennas. The discussion includes a summary of the methods, as well as the benefits and disadvantages of each strategy. The following is a list of some of the most well-known and commonly utilized approaches to the fabrication of flexible wearable antennas [53,82,83].

• Screen printing;
• Inkjet printing;
• Sewing/stitching and embroidery processes.

5.1. Screen Printing

Screen printing is an inexpensive and simple manufacturing method for producing or printing electronics. This method uses a woven screen with varied thread densities and thicknesses. A squeegee blade presses the screen on the substrate to print a design. This approach enables the creation of lightweight, affordable antennas that are also flexible and wearable. This method utilizes a squeegee to move downward and entails applying pressure to the ink as it passes through a screen and onto the substrate. Screen printing forcibly releases ink onto the substrate, creating the desired pattern. This ejects ink through the screen’s exposed portions onto the substrate, creating the desired design. Most of this technology uses polyester and stainless steel [49].

A fabric mesh screen is used, with a layer of emulsion applied to the area that does not contain the picture, exposing the image area. This technology’s versatility enables the printing of designs on a wide variety of substrate materials, including polyester and stainless steel, amongst others. The production of wearable antennas is now accomplished using three primary screen-printing techniques: rotary, flatbed, and cylinder screening. The flatbed approach is the one that is utilized the most frequently among these. Figure 9 shows this technique’s screen-printing process and wearable antenna prototypes.

5.2. Inkjet Printing

Because of its dependability and the speed with which it can design, inkjet printing has become a preferred and cost-effective approach for the fabrication of flexible antennas [84]. Using an additive process, the design is transferred directly onto the substrate without the use of masks, which contributes to a reduction in material waste. It is the primary method of production for polymeric substrates such as polyimide, PET, and paper [85] because of its ability to produce precise prototypes rapidly. The figure below provides an overview of the printing process as well as the assembly of the printer. When it comes to printing, the quality of the ink is dependent on factors such as its viscosity, surface tension, and particle size [86]. The topology of the substrate surface, the temperature of the platen, and the properties of the print head are all especially important.

Unlike photolithography, which is a subtractive approach since it entails eliminating unwanted patterns from the metallic or conductive side of the substrate, inkjet printing, on the other hand, deposits a regulated amount of ink droplets from the nozzle to the designated place. As a result, there is no waste or by-product produced, which results in a solution that is not only affordable but also clean and quick. Figure 10 illustrates the process of printing using an inkjet printer.

5.3. Stitching, Sewing, and Embroidery

This method is used to fabricate antennas on textile clothing material like jeans and cotton. The traditional embroidery process generates shapes that are both aesthetically pleasing and functional shapes using colorful threads on a base textile material. When antennas are embroidered onto the base material, specialized conductive threads are utilized for the embroidery process. Technology has improved to the point that it is now possible to directly embroider a digital image using a computer-aided embroidery machine, even though the fundamental principles remain the same [87].

6. SAR for Antennas on Human Body

The SAR measures bodies’ RF energy absorption from phones, antennas, and wireless devices. This statistic covers electromagnetic field safety issues. Pacemakers, implanted sensors, and wearable antennas for health monitoring are used for wireless data transfer. The absorption of electromagnetic energy by human tissues necessitates the careful design of wearable antennas, often made from flexible materials like textiles or polymers, to ensure compliance with SAR safety standards. Dielectric materials affect the SAR and EMF more than rigid substrates. The effects of body distance and antenna flexibility on the SAR can be due to various aspects; some of them are mentioned below:

• Most antennas are placed near the skin and the SAR rises near the source because tissues absorb energy. High-performance, low-SAR antennas are crucial. A better antenna dissipates body energy.
• Antenna design greatly affects the SAR. And, it varies by antenna size, shape, and frequency. Higher-frequency antennas increase the SAR because the body absorbs more radiation. For antenna efficiency and the SAR, designers must optimize location, ground planes, and materials. The international SAR monitors wearable antenna safety. Architects must consider regional codes and SAR-certified flat, bending, and twisting antennas.
• Flexible antennas are susceptible to environmental conditions such as heat, humidity, and perspiration, which can degrade their performance and influence SAR levels. Design strategies and robust testing protocols are necessary to ensure consistent SAR compliance under varying conditions of use.
• The complex geometries and mobility of wearable antennas present significant challenges for SAR prediction and testing. Accurate SAR evaluation requires sophisticated algorithms and realistic human body models to simulate the interaction between antennas and biological tissues effectively.

This study analyzes the various field properties of a recommended antenna along with gain, return loss, and the radiation pattern in a flat state with and without a body. When the body is not present, the return loss of the microstrip patch antenna is different, and when it is present, the return loss is different. The wattage per kilogram, or Watts/kg, is the unit of measurement used in the SAR. The SAR can also be conceptualized as an “absorbed rate,” which is related to the electric fields present in a specific area, i.e.,

$$SAR={\displaystyle \frac{\sigma \left|E\right|}{\rho }}\left({\displaystyle \frac{W}{Kg}}\right)$$

(1)

where E is the root mean square (r.m.s) of the electric field strength (V/m), and σ is the conductivity of the tissue (S/m) and ρ is the mass density of the human tissue. In certain fundamental studies, the SAR can be calculated as the rate of temperature rises in a specific location. The system’s SAR distribution was for different antenna configurations. It was shown that in each of the four scenarios, the SAR was higher than necessary and needed to fall within the safer limit of 1.6 W/kg specified by the IEEE C95.1-1999 standard. According to the data, one gram of human body tissue could absorb more radiation than ten grams of other tissue. Although the conductivity of human body tissue was higher, at 5.8 GHz, than 2.45 GHz, the latter was subjected to more electromagnetic radiation. Additionally, the peak SAR for 1 g of human body tissue was 17.50 w/kg.

Table 8. SAR value summary for 0.5 watts of input power.

SAR (W/Kg)	2.45 GHz	5.8 GHz	SAR (W/Kg)
1 g	17.5	9.3	1 g
10 g	8.99	4.08	10 g

shows that the antenna’s SAR, at 2.4 and 5.8 GHz frequencies, exceeded regulatory criteria, requiring extra measures to lower it to the safe limit [77].

Wearable flexible antennas were used to test the simulation results experimentally in various locations and motions. Proximity to the body reduced the antenna signal. Figure 11 shows space and flat-body antenna testing on phantom human muscle, fat, and skin at 2.4 GHz. SAR estimations can vary, but the sample volume average upper limit is 1.6 W/kg [88]. Human tissue absorbs electromagnetic field radiation, according to the SAR. Test phones, wearables, and wireless connectivity are required for electromagnetic field security. Several factors can affect the SAR. Some of them are the distance from the “phantom,” the antenna volume, and tissues. The size of the phantom, its separation distance from the antenna, and the types of tissues it represents all influence SAR values. This impacts SAR evaluation and EMF equipment security. Regulating these features creates safe, compliant equipment.

Table 9. SAR values in W/Kg are compared amongst several wearable antenna types.

Ref.	Antenna Size	Size (mm2)	Operating Frequency (GHz)	SAR 1 gm (W/Kg)	SAR 10 gm (W/Kg)
[78]	Rectangular patch antenna with inset feed line	51 × 45	2.45	0.54	0.486
[79]	T-shaped rectangular patch	70 × 70	2.45	0.0323	0.0153
[80]	Rectangular patch antenna with PIN diodes	113 × 99	4.85	0.024	0.353
[81]	Rectangular patch antenna	90 × 90	2.45	0.332	0.0234
[89]	Circular patch antenna	100 × 100	2.45, 5.8	0.042	0.09
[90]	E-shaped antenna	50 × 50	3.37	1.14	1.4
[91]	Folded ring-shaped antenna	60 × 60	2.45/3.45	0.2	0.1
[92]	L-shaped rectangular antenna	42 × 13	2.45, 5.8	0.028	0.68
[93]	Planar inverted-F antenna	60 × 26	1.82–3.72	0.03	1.6018
[80]	Patch antenna	101 × 96	2.4	0.24	0.003
[94]	C-shaped etching slot	18 × 19	0.09–3.94 4.23–5.65	1.58	0.91
[95]	Patch antenna	64.1 × 50	2.45	0.5	1.06
	Circle-shaped patch	40 × 40	0.01–5.30 8.12–12.35	0.12	0.07

provides safety- and performance-focused wearable antenna designs with their size, operation frequency, and SAR for 1 g and 10 g tissue models. The rectangular patch antenna with an inset feed line of 51 × 45 mm² operating at 2.45 GHz has SAR values of 0.54 W/kg 1 gm and 0.486 W/kg 10 gm, whereas the T-shaped 70 × 70 mm² antenna has values of 0.0323 W/kg (1 gm) and 0.0153. The reverse U&T slot patch antenna with dimensions of 58 × 63 mm² has low SAR values of 0.012 W/kg 1 gm and 0.23 W/kg 10 gm. The 100 × 100 mm² circular patch and 60 × 60 mm² folded ring-shaped antennas provide safe SAR levels for 2.45 GHz and 5.8 GHz dual-frequency applications. The 60 × 50 mm² ground coplanar waveguide antenna has a 0.25/0.7 W/kg SAR at 7–28 GHz. SAR-compliant wearables provide compact, efficient, and safe healthcare and IoT solutions with wireless connectivity.

6.1. Different Techniques to Reduce SAR Value

There have been several different approaches created to reduce SAR levels to be as low as possible while still maintaining a higher level of efficiency. An antenna that does not have a ground plane is usually known as a dipole and it has substantial SAR values. This is because the SAR of on-body antennas is dependent on near-field coupling to the body. There are methods for reducing rear lobe radiation and increasing gain called SAR reduction techniques, which are shown in Figure 12.

Various regions of the body were analyzed to determine SAR levels. The methods for SAR computation were evaluated systematically and compared. Without mitigation techniques, it was found that the value of the SAR peaked at 46.9 W/kg. When EBG structures were incorporated into the design, the SAR values significantly decreased to a minimum of 0.016 W/kg.

These results confirm the significant potential of the techniques used here, namely the EBG, the AMC, and metamaterial-based techniques, to reduce SAR values. The techniques thus form potential solutions to minimize the electromagnetic energy absorbed by human tissues for the better safety and performance of wearable and implantable antenna systems [96].

6.1.1. Reduction Using Electromagnetic Band Gap (EBG)

The EBG’s material properties include the stopband and delayed reaction of the regular structure. EBG structures use high-impedance surfaces in a frequency band to limit surface wave propagation and change radiation patterns. The EBG structure decreases the surface electromagnetic radiation on the body. EBG architecture improves PMC antennas [97]. The peak specific absorption rate (SAR) is 0.0545 W/kg with EBG structural geometry at 2.4 GHz, compared to 5.41 W/kg without an EBG [98]. EBG structures’ periodic dimension is half the stopband wavelength, limiting their size for low-frequency applications [92]. This work introduces a small dual EBG. Two circular rings with slots and inner patches are used in the design. Simulations provide the reflection phase and coefficient, which are crucial design performance indicators. Figure 13 shows HFSS SAR values for the EBG antenna on a human arm and male torso model. The EBG array arrangement reduces the SAR to tolerable levels in simulations [99].

Advantages: EBG structures offer a significant advantage in reducing the specific absorption rate (SAR) by effectively suppressing surface waves and channeling electromagnetic radiation away from the body. EBG structures offer a compelling choice for wearable electronics as they can reduce the energy consumed by human tissues.

Disadvantages: EBG structures are susceptible to environmental conditions such as humidity, temperature, and mechanical stress. These factors might change the electromagnetic properties of the electromagnetic band gap (EBG), which might affect its ability to lower the specific absorption rate (SAR) and keep the antenna working well.

6.1.2. SAR Reduction Using Metamaterials

Metamaterials are frequently discussed for their unique physical properties and novel uses. Significant electromagnetic surface impedances in AMC structures can hinder surface current transfer and behave as a perfect magnetic conductor (PMC) at certain frequencies. In [100], to lower the SAR of the suggested PIFA antenna, the author employed an artificial magnetic conductor (AMC) with an implanted square metal patch, a dielectric substrate, and an underlayer. Figure 14 depicts an AMC designed with a 54 mm × 24 mm × 1.57 mm Taconic CRE-10 with a 3 × 7-unit cell array. The SAR decreased by 43.3% after adopting the AMC framework [101].

The design of a dual-band coplanar waveguide (CPW)-fed antenna was improved by using a metamaterial-based structure consisting of three layers: a separate metal layer acting as the ground plane beneath and a square metal ring surrounding a square metal patch on a dielectric substrate. At 2.45 and 5.8 GHz, respectively, this design produced a gain enhancement of 9.3 dB and 5.37 dB and an increase in the radiation efficiency of 48.4% and 35.7%. This structure led to a 70 percent reduction in the SAR [77].

Advantages: Metamaterials can alter electromagnetic waves, which can potentially decrease the SAR by redirecting energy away from the body. They offer innovative solutions tailored to specific needs.

Disadvantages: Metamaterials frequently entail higher costs and greater manufacturing complexities. Incorporating them into wearable devices could result in added mass, diminished flexibility, and elevated manufacturing expenses. Furthermore, it may be necessary to perform additional studies to investigate the long-term stability and performance of the subject under real-world conditions.

6.1.3. Reduction in Using Conductive Materials

A reduced specific absorption rate is important in antenna design because radiation affects tissues. Adsorptive and conductive materials such as metals or carbon-based compounds can reflect electromagnetic waves and lower the SAR. By placing a conductive layer next to the antenna, the SAR can be decreased, directing radiation away from the user’s head or body. Conductive coatings applied to an antenna to minimize SAR values can also reduce the electromagnetic field’s interaction with surrounding objects. However, employing conductive materials could impact the antenna’s functionality, changing its gain and emission pattern, including conductive foam, mesh, and fabric. Wire–mesh and multilayered shields can lower the SAR by shielding back radiation which does not affect the human body. Shields can be made of nickel, copper, or nickel–nickel [102].

6.1.4. Reduction Using Frequency Selective Surfaces (FSSs)

Special stopband or passband features of frequency selective surfaces (FSSs) are utilized to reduce the SAR. To reduce the SAR and boost gain, a 120 × 120 mm² FSS textile-based superstrate was used in a square wave loop, backed by a 50.5 × 38 mm² compact monopole antenna [103]. The square-shaped modified monopole radiator with a defective ground structure radiated in the 2.45 GHz ISM frequency range with a bandwidth of 2.075 to 2.625 GH. The achiral FSS provided consistent performance for both TE and TM polarizations, with a progressive increase in inductance [104]. A 120 × 120 × 30 mm² antenna system with an FSS had a maximum gain of 7.76 dB and a fractional bandwidth of 22.44 percent. The FSS reduced back radiation, resulting in a 95% reduction in SAR levels. Figure 15 depicts the cross-shaped structure of an FSS [93].

Table 10. Comparative analysis of various parameters of several recent techniques.

Refs. No.	Techniques Used	Material Used	SAR Without Reduction (W/kg)	SAR with Reduction Technique (W/kg)	SAR per 1 or 10 gm Tissue	Frequency (GHz)
[64]	Electromagnetic band gap	Roger Duroid	7.18/7.96	0.31/0.42	10	2.45/5.8
[69]	Metamaterial	Textile	7.78	0.0283	1	2.4
[98]	Electromagnetic band gap	Denim	-	0.364/0.165	1/10	2.4
[103]	Artificial magnetic connector	Taconic CRE-10	1.4	0.7	10	1.97
[105]	Metal reflector	Aluminum	-	0.20	1	2.44
[110]	Electromagnetic band gap	Fabric-Based	5.77/6.62	0.024/0.016	1	1.8/2.45
[106]	Electromagnetic band gap	Textile	5.41	0.0545	1	2.4
[107]	Electromagnetic band gap	Roger’s Material	2.36	1.77	10	2.4
[108]	Frequency selective surface array	Textile	5.1972	0.1641	1	2.45
[109]	Artificial magnetic connector	denim	46.9	Less than 1.6	1	5.8
[110]	Artificial magnetic connector	Polymer	2	0.29	1	2.45
[111]	Electromagnetic band gap	PDMS	6.56	0.0251	1	2.4
[112]	Artificial magnetic connector	Flexible Conductive	4.2	1.24	10	24
[113]	Meta surface	RT/Duroid	6.6/11.7	0.0646/0.0268	1	5.2,5.8
[114]	Electromagnetic band gap	Polymer-Based	8.72	0.036	1	2.4
[115]	Flexible printed circuit board	Flexible Polymer	-	0.474/0.11	1/10	5.8
[116]	Metamaterial	Polymer	6.27	0.0671	10	UWB
[117]	Artificial magnetic connector	Textile	-	0.903/0.338	1	3.5/5.8
[118]	Planar antenna	Flexible Metallic Coating	-	0.919/0.517	1	2.45/5.85
[84]	Metasurface	Textile	7.51/8.64	0.0257/0.0358	1	2.45

shows many SAR reduction methods for wearable antennas that work in different frequency bands and tissue conditions. The EBG approach can reduce the SAR from 7.18/7.96 W/kg to 0.31/0.42 W/kg at 2.45/5.8 GHz using Rogers Duroid and textile substrates [105]. Metamaterials in copper form reduce the SAR from 7.78 W/kg to 0.0283 W/kg at 2.4 GHz [106]. The AMC method, using Taconic CRE-10 PCB material, lowers the SAR to 0.7 W/kg at 1.97 GHz [107,108]. By using silver nanoparticles on textiles, the FSS approach reduces the SAR from 5.1972 W/kg to 0.1641 W/kg at 2.45 GHz. Additionally, metal reflectors with aluminum shielding exhibit an SAR of 0.20 W/kg at 2.44 GHz [109]. Advanced materials like flexible PCBs and meta surfaces can lower the SAR to 0.0646 W/kg at 5.2 GHz when used with flexible conductive coatings [105]. These technologies and materials, such as flexible polymers, conductive coatings on textile substrates, and ceramic-based materials, have effectively improved user safety in wearable antennas [109,110,111,112,113,114,115,116]. This comprehensive study emphasizes the importance of material science and inventive design in wearable SARs.

7. Applications for Flexible Wearable Antennas

With current monitoring systems, users need wearable gadgets more. They are rising rapidly in healthcare, which will benefit society. Physiological signs can detect any disease early. Intelligent wearables can reduce health issues including illness control and cost. Clinical instruments are usually non-wearable for measurement, so researchers have concentrated on smart designs that enable low-profile, sturdy, and high-performance wearable devices. Standard body-centric communications applications are recognized by IEEE 802.15 [119]. Figure 16 illustrates the numerous applications of wearable antennas in different industries. Transparent antennas, along with antennas that are worn, display several uses, including but not limited to IoT applications, integrated circuits, solar energy systems, smart city initiatives, digital displays, vehicle-to-vehicle communications, and the healthcare sector [29].

Wearable antennas have important uses in a collection of sectors, including Body Area Networks (BANs) and wearable gadgets. Additionally, these antennas are meant to be incorporated into clothing, implanted in the body, or even tattooed on the skin, which provides a broad variety of options for communication [119]. For example, a crescent-shaped wearable antenna that was created for the ISM band at 5.4 GHz offers comfort and flexibility because it uses denim fabric as a substrate. There is also an example of a tiny, conformal textile antenna that operates in the 2.45 GHz ISM band. This antenna is appropriate for wristband applications and has an optimal performance even when it is in close contact with the human body [29]. These antennas have been designed to fulfill characteristics, such as having the factor of a compact form, being biocompatible, being flexible, and experiencing reliable performance. As a result, they are perfect for applications that include wearable technology [62].

8. Discussion and Future Scope

This overview of this paper covers several important topics of wearable antennas, all essential for comprehending their problems, applications, and design. Flexible and conformal, wearable antennas can be integrated into various technological products, including fitness trackers, smart watches, and medical equipment. The flexibility and conformability of an antenna should be adaptable enough to stretch, twist, and bend in response to the wearer’s movements. Wearable antennas are often made from flexible substrates such as fabrics, polymers, or other non-conductive materials, as described in this work. Copper, metallic inks, and conductive textiles are among the complex materials used to make antennas [56,72]. When assessing antenna performance, variables including radiation efficiency, gain, and bandwidth are essential. Antennas should adhere to safety regulations concerning the body’s absorption of electromagnetic radiation. Wearable antennas must resist deterioration from washing, perspiration, and other environmental conditions. The development of compact antenna designs prioritizes minimizing size while maintaining optimal functionality, ensuring their seamless integration into wearable applications. These antennas are engineered to be lightweight and unobtrusive, enabling incorporation into clothing or other wearable devices without causing discomfort or impeding the user’s activities. It should be ensured that antennas may be worn on clothes or other wearables without being uncomfortable or in the way. Metamaterials can be used to attain compactness and enhance antenna performance. Processes such as screen printing and inkjet printing are utilized to create antennas on flexible substrates. Antennas’ variable frequency or other characteristics accommodate various uses.

Future trends in wearable antennas will concentrate on flexible, wearable, robust, and biocompatible materials. Scalable and precise techniques of fabrication, such as 3D printing and screen printing, will also be explored. The size, the specific absorption rate, and the durability against bending and environmental effects will be minimized. There will be an integration of 5G/6G technology with the IoT and AI-driven systems to further improve performance for applications in healthcare, smart textiles, and augmented reality.

Advanced antennas featuring energy-harvesting functionalities will drive innovation forward, while sustainability objectives will emphasize biodegradable materials and a minimum environmental impact. Interdisciplinary research and international standardization will ensure the safe, efficient operation of wearable antenna technologies.

9. Conclusions

This comprehensive review highlights the transformative potential of wearable antennas in the realm of wireless communication, particularly for WBANs. These antennas, characterized by their flexibility, lightweight design, and adaptability, are pivotal in addressing the growing demands of healthcare, the IoT, and next-generation communication systems. By leveraging advanced materials, including textiles, polymers, and conductive composites, alongside innovative fabrication techniques such as inkjet printing and screen printing, wearable antennas achieve enhanced performance while conforming to complex human body topologies.

Despite their promising applications, significant challenges remain, including optimizing SAR levels, ensuring mechanical durability, and maintaining performance under environmental stress and deformation. Advanced solutions like metamaterials, EBG structures, and FSSs show considerable promise in mitigating these issues, further enhancing antenna efficiency and safety. Future advancements in this field are expected to integrate 5G/6G technologies, energy harvesting capabilities, and biocompatible materials to create sustainable and efficient systems. These developments, coupled with scalable and precise manufacturing methods, will pave the way for wearable antennas to revolutionize healthcare, smart textiles, and communication systems, ensuring a seamless blend of functionality, user comfort, and technological innovation.