  Metasurface Energy Harvesters (MEHs) have emerged as a prominent enabler of
highly efficient Radio Frequency (RF) energy harvesters. This survey delves
into the fundamentals of the MEH technology, providing a comprehensive overview
of their working principle, unit cell designs and prototypes over various
frequency bands, as well as state-of-the art modes of operation. Inspired by
the recent academic and industrial interest on Reconfigurable Intelligent
Surfaces (RISs)for the upcoming sixth-Generation (6G) of wireless networks, we
study the interplay between this technology and MEHs aiming for energy
sustainable RISs power by metasurface-based RF energy harvesting. We present 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. Then, we conduct a comparative collection of real-world
measurements for ambient RF power levels and power consumption budgets of
reflective RISs to unveil the potential for a self-sustainable RIS via 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 RISs.
