# Flexible Antennas: A Review
## 6 Flexible Antennas for Implantable Applications  
In recent years, the continuous advancing and revolutionizing of health care systems towards the advancement of an efficient system to increase the quality of life as well as implementing future IoT in medical sector. An implantable antenna system transmits and stores the recorded physiological parameters, conditions for real-time communication. So, flexible antennas play a huge role in implantable antenna applications and they are receiving significant attention to the researchers and thus have become a current research focus. Flexible antennas are quite necessary as most of them are from polymeric substrate which can be biocompatible in nature.  
For designing an implantable antenna, the basic requirements are the small size along with proper placement inside the human body, larger bandwidth, flexibility, and low specific absorption rate (SAR). It is also challenging due to the different dielectric constant of various tissues and organs of the human body. A flexible folded slot dipole antenna embedded in PDMS for implantation into the human body was pursued in a prior study [222] (Figure 13a).  
The antenna performance and SAR measurements were done using a liquid mimicking the human muscle tissue varying dielectric nominal values mimicking different tissues.  
The EM characteristics of the antenna were found stable for various properties of the surrounding tissues. Flexibility tests were done by bending the antenna over two different radii curvatures, which shifted the resonance frequency slightly within the bandwidth. An implantable wideband low  
Figure 12: Miniaturized antennas. (**a**) Miniaturization of EBG-backed monopole antenna with reflection coefficient [212], (**b**) fabricated cylindrical conformal array antenna with photonic bandgap (PBG) lattice [221], and (**c**) Prototype of an M-shaped printed monopole antenna and slotted Jerusalem Cross (JC)-artificial magnetic conductor (AMC) [208].  
Figure 13: Implantable antennas. (**a**) Implantable slot antenna, (**b**) embedded in PDMS [222], (**c**) Geometry of low specific absorption rate (SAR) antenna and its prototype [223], and (**d**) geometry of flexible implantable loop antenna with complementary split ring resonator (CSRR) [224].  
SAR antenna on a flexible PDMS substrate was proposed earlier [223] (Figure 13b). The unique design feature of the antennas was responsible for achieving low SAR. Pork loin and muscle mimicking gels were used for experimenting with the antenna features. Bending analysis of the antenna showed a slight variation in frequency from the flat condition.  
An implantable ring-slot antenna in the ISM frequency band was proposed using the grounded metamaterial technique in [61]. This antenna was validated inside the human tissue-mimicking gel and a chicken tissue sample. The average SAR values for the antenna were found in the safe range for using a multilayer metamaterial structure. The antenna showed better fractional bandwidth and gains in comparison to recent other implantable antennas. The antenna's return loss was found entirely unchanged during the bending test. The SAR values for the flexible antennas are relatively higher than typical implantable antennas. So designing flexible implantable antennas is quite challenging. Multiband implantable applications in the MedRadio and ISM bands were covered in an earlier article [224] (Figure 13c). A complementary split-ring resonator (CSRR) was introduced in this antenna to decrease the antenna efficiency and gain. The antenna prototype was fabricated using a copper sheet, and the performance was measured using a pork phantom.