## 5 RIS Implementation Structures
### Toward Liquid Crystal (LCs) Based RIS  
Recent developments using liquid-crystal-loaded unit cells demonstrate promising metasurfaces with adjustable reflection phase distribution [91]. The phase tunability of the metasurface reflector is achieved via the dielectric anisotropy of nematic LCs in the reconfigurable metasurface. The effective dielectric constant and, thus, phase differential at various points of a metasurface can be changed by adjusting DC voltages on microstrip patches of liquid-crystal-loaded unit cells. The concept combines aspects of a metasurface with the unique attribute of electronically adjustable LCs. In literature, there are many electrically sensitive materials, but nematic LCs are particularly well-known due to rapid advancement in optical display technology. Nematic LC molecules reorient themselves in response to the bias electric field, resulting in voltage-dependent birefringence. Furthermore, their liquid nature can be infiltrated into various metasurface structures, resulting in substantial refractive index modulation for operation in the microwave, terahertz, and optical regimes. Nematic LCs have been used in multiple prominent metasurface applications, including tunable absorbers and cloaks with real-time invisibility control [92; 93]. A large number of metamaterial cells are loaded with a thin layer of LCs in the proposed metasurface. Due to the anisotropic features of the nematic LCs, the effective dielectric constant and resonant frequency of such unit cells can be modified individually by adjusting the external electrostatic field. Because the frequency of the incoming wave in relation to the resonance frequency determines the reflection phase, such a metasurface can be changed in real-time to obtain the desired reflected wave direction by providing a suitable DC voltage distribution across the surface [94]. The metasurface is a multilayered printed circuit board surface with a high impedance. The metasurface's bottom layer is a solid ground plane, while the top layer is a gridded wire mesh utilized for DC grounding of the top microstrip patches. An array of little microstrip patches can be located in the central layers. These microstrip patches are approximately a quarter of a wavelength in size. Between the two microstrip patch layers is an LCs mixture with a tunable dielectric constant. As a result of the anisotropy of the LCs, the dielectric constant between the microstrip patches at each unit cell can be controlled individually by altering the electrostatic field between the patches. A RIS, in particular, is made up of a large number of low-cost and passive reflecting elements (REs) that are not connected by radio frequency chains. RIS wireless systems can achieve system gain by modifying the phase shifts and amplitudes of the REs so that the desired signals can be combined constructively at the receiver. However, a RIS often has limited signal processing capabilities and cannot do active transmitting/receiving in general, posing significant issues in RIS wireless system physical layer design. From the literature, it is clear that LCs technology has been identified as a potential solution for the improved design of passive and reflectarray-based RIS. Therefore, it plays a significant role in the future RIS structure design. Additionally, a flat reflector made of liquid-crystal-loaded metasurface can be used for beam-steerable antenna applications by utilizing the dielectric anisotropy of LCs. Changing the DC voltages of LC-loaded unit cells in real-time to give the desired phase distribution throughout the metasurface can be used to create a beam steering IoT system. In addition, the proposed unit cell structure can be used in dual linearly polarized applications. In existing research, the dependence of LCs on the voltage applied has been utilized to design efficient RIS with control over a huge range of wavelengths.