# Recent Developments and State of the Art in Flexible and Conformal Reconfigurable Antennas
## 1 Introduction  
Reconfigurable antennas have received a lot of attention in modern communication systems due to their versatile applications by offering extra functionality. A reconfigurable antenna is defined as a single structure that is capable of switching between one and a combination of characteristics such as frequency, pattern, and polarization [1]. These structures with their flexible and multi-operation characteristics offer compact and cost-effective antennas for modern communication systems such as cellular radio systems, satellite communication, airplane, health care systems, and unmanned airborne vehicle (UAV) systems [2; 3; 4]. There has been extensive research and substantial advancements demonstrated in the various types of reconfigurable antennas made from rigid and conventional materials in the last few decades. New and modern communication systems' are moving toward wearable technology and system [5]. Wearable technology and systems include flexible electronics and displays, conformal consumer electronic gadgets and devices for body area networks (BANs) [6; 7; 8].  
The requirements for flexible and reconfigurable antennas as a key component of the wearable systems have been increased because wearable devices must cope with the dynamics of the various surfaces.  
Antenna reconfigurability is achieved by changing the antenna's current path, physical structure, or electrical properties. As Figure 1 shows, there are various methods to achieve reconfigurability in the four main categories. The progress in the development and realization of the flexible reconfigurable antennas was not as fast as their rigid counterparts. There are various existing challenges for the realization of these types of antennas. Figure 1 presents the existing reconfiguration techniques, which include: the PIN diode, the microelectromechanical systems (MEMS) switch, the varactor from the electrical category, the microfluidic-based method and the origami-based antenna. The photoconductive switch is made of semiconductor materials and uses bias lines instead of metallic wires [9]. The reason for not having a flexible reconfigurable structure with this switch is because of the challenges of integrating it with unconventional and flexible materials. Moreover, the limited range of flexible materials and permittivity restricts researchers in this area in employing reconfiguration techniques based on smart materials. These are based on reconfiguring the substrate of the antenna through changing its permittivity or permeability under different levels of voltage [10].  
A comprehensive review papers focusing exclusively on simulation [11], a specific reconfigurability method [10, 12], antenna types [2, 13], and other [14, 15, 16] of rigid antennas have been presented. Despite great efforts for comprehensive reviews of rigid antennas, there are no comprehensive review papers for the flexible reconfigurable antennas. In this review paper, we summarize recent developments in flexible pattern, frequency, and polarization reconfigurable antennas, as well as antennas with two feature reconfigurability. The purpose of this paper is to review the reconfiguration techniques, the challenges and limitations for design and fabrication, the achieved results and a comparison of the materials used in flexible and rigid structures. This leads to the recognizing of the current gap in this field and identifying of the possible future work in this area.  
In the following sections, we discuss reconfigurable antennas in different categories:  
* Frequency-reconfigurable antenna;
* Pattern-reconfigurable antenna;
* Polarization-reconfigurable antenna;
* Multiple reconfigurable features in an antenna.