Recent Trends in the Reconfigurable Intelligent Surfaces (RIS): Active RIS to Brain-controlled RIS

Reconfigurable Intelligent Surfaces (RIS) have risen as a promising technology for future wireless communications. RIS consists of a massive number of unit cells, each of which can be tuned individually to induce a manipulation of the wireless signal properties. These properties include the signal's phase, magnitude, frequency, and even polarization. In this manuscript, we briefly present the recent trends in RIS research. This manuscript covers the trends in the development of active RIS, various RIS control elements comparison (i.e., electronics, mechanics, and materials approaches), flexible and transparent RIS, RIS lens and RIS mirror, and the most-recent brain-controlled RIS paradigm. This review was derived from the recent high-quality meta-analyses, reviews, and original research papers related to the recent RIS works. We extensively searched online libraries such as IEEE, Nature, Scopus, and Google Scholar for those articles. There were no limitations on the year of the publications. In this manuscript, we consider any research study types written in English (excluding unpublished data, technical notes, and submitted manuscripts). As a result, we have gathered more than 50 references related to the recent RIS research. While RIS promised attractive capabilities for reconfiguring the wireless environment, several challenges are lying ahead and must be overcome. Among those, there are two major challenges: multiplicative fading effect and channel estimation problems.