# Flexible Antennas: A Review
## 10 Flexible Antenna for Future Wireless Solutions
In addition to antennas being small, flexible, and capable of operating in a broad verity of wireless standards, some applications require devices to be battery and wire-free. Energy scavenging using a rectifying antenna (rectenna) can be leveraged to develop autonomous devices. A rectenna works by foraging RF energy emitted by radio transmitters. In another study [177], wireless power transfer was used in an RF-powered leadless pacemaker. The authors proposed a novel wideband numerical model (WBNM). Tissue simulating liquid (TLS) was used to design the wideband numerical model. The model was validated experimentally and analytically using a microstrip patch antenna. Furthermore, a novel metamaterial-based conformal implantable antenna operating in the frequency of 2.5 GHz was developed. The authors validated the antenna as a potential candidate for future RF harvesting leadless pacing applications. A dual band printed planar antenna was proposed for microwave power transfer (MPT) in an earlier study [274]. The antenna was suitable for wearable applications and operated at ultra-high frequencies of 2.5 and 4.5 GHz. The antenna's area is 15 mm \(\times\) 14 mm \(\times\) 0.17 mm, and Kapton polyimide-based flexible substrate and FR-4 substrate was used for the receiving antenna and the transmitting antenna, respectively. Safety is considered where the antenna meets the specific absorption rate (SAR) requirements. A planar inverted-F antenna (PIFA) and a rectifier circuit for far-field wireless power transfer were proposed in [275]. The compact implantable rectenna can operate at a frequency of 2.45 GHz. To increase the power level, a parasitic patch attached to the human body was used to enhance the directivity of the rectenna. Safety is considered where the Federal Communications Commission (FCC) limits for radiating antennas, Specific Absorption Rate (SAR), and temperature increase were tested to assure compliance of the rectenna with international safety regulations. In a prior report [276], a hybrid energy harvesting circuit combining a solar cell and electromagnetic energy (EM) harvesting rectenna is proposed (Figure 22a). A flexible polyethylene terephthalate (PET) substrate together with a flexible amorphous silicon solar cell was used to achieve both low cost and conformal structures. The antenna measures 13 mm \(\times\) 6.5 mm. Both wideband and multiband topologies are  
Figure 22: (**a**) Hybrid solar and electromagnetic energy (EM) energy harvesting antenna on PET substrate [263] and (**b**) reconfigurable antenna with AMC surface [271]. presented. To achieve energy harvesting using a variety of communication standards, the antenna is capable of operating in frequencies between 800 and 6 GHz. Millimeter-wave communication systems are anticipated to resolve the problems of congestion, lower bandwidth, and high latency in the current wireless systems. The upcoming 5G technology is expected to address these problems and offer higher channel capacity with wider bandwidth that ensures a higher data rate. Many smart devices are expected to have a high-speed uninterrupted internet connection and have irregular shapes. For this type of application, flexible and stretchable antennas are required that can be mounted on a conformal structure. A PET-based flexible T-shaped mm-wave antenna is proposed in [12] that is covering frequency from 26 to 40 GHz. Defected ground plane (DGS) is used to extend the bandwidth of the antenna by combining multiple resonant points. Impedance matching is not good at lower frequency range when the heat sintering has been done, and the performance of the antenna in bending condition has not been experimentally validated. In [277], the authors designed a transparent and flexible PET-based Y-shaped antenna that operates from 23 to 29.5 GHz depending on the angle of the arm of Y shape. For ensuring the conformity of the antenna, AgHT transparent conductive material is used in this research.