With the surge of theoretical work investigating Reconfigurable Intelligent Surfaces (RISs) for wireless communication and sensing, there exists an urgent need of hardware solutions for the evaluation of these theoretical results and further advancing the field. The most common solutions proposed in the literature are based on varactors, Positive Intrinsic-Negative (PIN) diodes, and Micro-Electro-Mechanical Systems (MEMS). This paper presents the use of Liquid Crystal (LC) technology for the realization of continuously tunable extremely large millimeter-wave RISs. We review the basic physical principles of LC theory, introduce two different realizations of LC-RISs, namely reflect-array and phased-array, and highlight their key properties that have an impact on the system design and RIS reconfiguration strategy. Moreover, the LC technology is compared with the competing technologies in terms of feasibility, cost, power consumption, reconfiguration speed, and bandwidth. Furthermore, several important open problems for both theoretical and experimental research on LC-RISs are presented.

翻译：随着可重构智能表面（RIS）在无线通信与感知领域理论研究的蓬勃兴起，亟需硬件解决方案来验证这些理论成果并推动该领域进一步发展。现有文献中最常见的解决方案基于变容二极管、正-本征-负（PIN）二极管和微机电系统（MEMS）。本文提出利用液晶（LC）技术实现连续可调的超大规模毫米波RIS。我们回顾了LC理论的基本物理原理，介绍了两种不同的LC-RIS实现方式（即反射阵列和相控阵列），并重点阐述了对系统设计和RIS重配置策略具有关键影响的核心特性。此外，从可行性、成本、功耗、重配置速度和带宽等方面对LC技术与竞争技术进行了比较。最后，提出了LC-RIS理论与实验研究中若干重要的未解决问题。