Metasurface Energy Harvesters: State-of-the-Art Designs and Their Potential for Energy Sustainable Reconfigurable Intelligent Surfaces

Metasurface Energy Harvesters (MEHs) have emerged as a prominent enabler of highly efficient Radio Frequency (RF) energy harvesters. This paper provides a comprehensive and in-depth review of recent advances in MEH technology, focusing on their working principles via both the transformer and circuit models, unit cell designs, and prototypes across various frequency bands, as well as state-of-the-art operational modes. Furthermore, the innate ability of MEHs to operate in simultaneous wireless information and power transfer systems is highlighted and the relevant state-of-the-art designs are discussed. Then, the recent advances made in 2D isotropic MEH designs are studied, showcasing their applicability in powering Internet of Things (IoT) devices. Motivated by the growing academic and industrial interest in Reconfigurable Intelligent Surfaces (RISs) for the upcoming sixth-generation (6G) wireless networks, we study the interplay between RISs and MEHs, aiming to develop self-powered RISs based on the MEH technology. We introduce a novel hybrid unit cell design capable of simultaneous energy harvesting and 1-bit tunable reflection, whose dual-functional response is validated via full-wave simulations. Additionally, we present a comparative collection of real-world measurements for ambient RF power levels and power consumption budgets of reflective RISs, highlighting the potential, as well as the requirements, for a self-sustainable RIS through ambient RF energy harvesting. The paper is concluded with an elaborative discussion on open design challenges and future research directions for MEHs and energy sustainable hybrid and multi-functional RISs.