# Flexible Antennas: A Review
## 1 Introduction  
The availability of high speed, massive capacity, and low latency 5G network has enabled the 'Fourth Industrial Revolution' [1]. Every sector will benefit from 5G networks ranging from 3-D imaging, advanced health care, streaming services, and smart cities, to name a few [2]. Further, a strong 5G network is essential to the proper functioning of the Internet of Things (IoT) devices [3, 4]. A visual representation of the interconnection between IoT things and the 5G network is shown in Figure 1.  
One of the critical barriers to technological advancements of next-generation IoT related devices is inflexibility stemming from form factor and weight considerations. While there have been orders of magnitudes of advances in miniaturization, flexibility is a feature that is hard to conquer. Recent innovations in engineered materials have been leveraged to augment the field of flexible electronics. Flexible electronic devices are often lightweight, portable, less expensive, environment friendly, and disposable [6]. The flexible electronics market is expected to reach 40.37 billion in revenue by 2023 [7,8]. Figure 2 shows the various applications of flexible electronic devices. Flexible electronic systems require the integration of flexible antennas operating in specific frequency bands to provide wireless connectivity, which is a necessity in today's information-oriented society. The markets for flexible wireless devices are rapidly increasing partly due to the demands in wearable and implantable devices for health-monitoring systems and daily-life wireless devices  
Figure 1: Connection architecture between 5G and Internet of Things (IoT). Reprinted with permission from Ref [5]. Figure 2: Application areas for flexible electronics. Reprinted with permission from Ref [9]. (e.g., cell phones, laptop computers, etc.). For this reason, the need for flexible printed antennas has increased in recent years, especially for biomedical applications [10; 11]. Specifically, flexible antennas are a major component in the implementation of in vivo monitoring of vital signs, regulation of organ functions, neural interfaces, continuous gait analysis, intracranial sensors, drug delivery systems, and countless other functions [12; 13]. To integrate the devices onto the human body characterized by curvilinear surfaces and dynamically changing motions, the device must be conformal and physically flexible or even stretchable. Because the bending stiffness of a thin film structure that characterizes its resistance against bending deformation roughly scales with the cubic of its thickness thinning down the thickness of the structure represents an effective means to enable flexible/bendable antennas. Apart from biomedical applications, there is shared interest between Federal agencies, industry, and academia in developing a flexible antenna for extreme conditions. Federal agencies' applications for high-temperature flexible devices include: the new gas-cooled nuclear reactor (high-temperature H\({}_{2}\) safety monitors) and strict automotive emissions control requirements (tailpipe sensors) from the Department of Energy (DOE); requirement of flexible antennas and antenna radomes with extreme thermal shock resistance for missile applications and high-temperature substrates for hypersonic antennas for the Department of Defense (DoD); communication solution for beyond-line-of-sight communications on small- to medium-scale unmanned aircraft systems (UAS) for the National Aeronautics and Space Administration (NASA); and Wireless Physiological and Environmental Monitoring (WiPEM) system requirement (for first responders) from the Department of Homeland Security (DHS). Several recent investigations have reported that provide extensive review on materials, fabrication, and applications of different flexible and wearable antennas [14; 15; 16; 17; 18; 19]. A practical guideline for designing and fabricating both nontextile and fully textile wearable antennas has been reported in [14].