# Recent Developments and State of the Art in Flexible and Conformal Reconfigurable Antennas
## 2 Frequency-Reconfigurable Antennas
### Mechanically Frequency-Reconfigurable Antennas
#### Microfluidic Antennas  
Recent development in the composite structure manufacturing creates the ability to embed microvascular networks in flexible materials and enables conductive or dielectric fluid to be injected into these materials. To reconfigure the antenna's frequency, the conductive fluid is used as a switch [27], the reactive loading effects of the fluid metal are used in [64; 65], antenna fabricated by injecting alloys is elongated via stretching [66; 67], the permittivity range of substrate is changed [28; 29]. In microfluidic antennas in which conductive fluid is used as a switch, PDMS and styrene ethylene butylene styrene (SEBS) are usually used to produce microfluidic channels. The examples of liquid metals are mercury and eutectic gallium indium alloy (EGaIn). There is no need for electrical actuation switches in these types of the antenna which leads to enable efficient power handling. In the first microfluidic reconfigurable antennas with simple geometry, the liquid metal conductor is used in the PDMS channel [26]. In the presented frequency reconfigurable dipole antenna, for the microfluidic channels, SEBS with the \(\varepsilon_{r}\) = 2.3 and loss of 0.07 is employed and the channels are filled with EGaIn. The proposed antenna reconfigures from 2 GHz to 5.5 GHz by increasing strain up to 120% [68]. In Reference [27], the liquid metal is employed as a shortening switch. The frequency of the proposed antenna shifts between 4.6 GHz, 3.84 GHz and 5.34 GHz by emptying and filling the liquid metal to the microchannel to the lengths of \(\lambda/2\) and \(4\lambda\). The antenna is placed on an S-glass composite substrate with \(\varepsilon_{r}\) = 3.4 at 4.6 GHz. A copper-tape and an EGaIn is used for the ground plane and the fluidic conductor, respectively. In other research, the reactive loading effect of the fluid metal is used for tuning of a coplanar waveguide (CPW) folded slot antenna and miniaturization as well [64; 65]. Two pairs of microfluidic channels are used as a switch to reconfigure between 2.4 GHz, 3.5 GHz, and 5.8 GHz [64]. The antenna is placed on Taconic TLY substrate with \(\varepsilon_{r}\) = 2.2, tan \(\delta\) = 0.0009, with a thickness of 1.53 mm and two separate PDMS structures are used for two channels. The same approach is employed in [65], where liquid metal is also used as a reactive load on top of the antenna to achieve tuning and miniaturization as well. In other types of microfluidic antennas, the frequency is tuned mechanically by elongating the antenna via stretching [66; 67]. In Reference [66], a dipole antenna is fabricated by injecting EGaIn into microfluidic channels placed in PDMS substrate. The antenna frequency is tuned from 1850 MHz in a relaxed position (54 mm length) to 1600 MHz in the elongated position (66 mm length). In Reference [67], a stretchable frequency reconfigurable antenna is fabricated by injecting liquid metal alloy into a flexible substrate. Galinstan and silicone TC5005 are used for alloy and substrate, respectively. Instead of a microchannel, a square reservoir is fabricated in the flexible substrate. The frequency of the antenna is tuned from 1.3 GHz to 3 GHz by stretching up to 300% and varying the electrical length of the patch from 31 mm to 87 mm. Despite the presented works [26; 27; 64; 65; 66; 67] in which conductive fluid is used for radiating element, in [28; 29], a microchannel is employed for adding different dielectric liquid materials to the antenna. The frequency shift is achieved in [28; 29], by changing the permittivity range by inserting different fluids (air, acetone, and de-ionized (DI) water) into the existing channels. As mentioned previously, fluidic-based antennas resist various or long time deformation compared to conventional antennas. In addition, the higher conductivity of conductive liquid compared to conductive fabrics, makes the design of an antenna with higher efficiency possible. On the other hand, there is a limitation for realizing the complex patterns for patch with this method.