Thermoresponsive Reconfigurable Intelligent Electromagnetic Surfaces Enabled by VO ₂ and Wood-Derived Nanocellulose, Suberin, and Biocarbon

Reconfigurable intelligent surfaces (RISs) are key enabling technologies for next-generation wireless telecommunication systems, offering dynamic control over electromagnetic (EM) wave propagation. However, most existing RIS demonstrations rely on conventional electronic or metallic platforms, raising concerns about resource availability, recyclability, and environmental sustainability. In this study, hybrid nanostructured RIS prototypes (Prototypes I-III) were designed and fabricated using sustainable, wood-derived materials, namely, cellulose nanofibers (CNFs), suberin, and biocarbon, in combination with thermoresponsive vanadium dioxide (VO₂) nanoparticles. The EM performance of these RIS architectures was first optimized through full-wave simulations and then validated experimentally by the cast-layer deposition of VO₂/CNF-suberin functional layers onto printed circuit board (PCB) substrates. Among the tested designs, Prototype I, comprising a functional layer of 95 wt % VO₂, 2.5 wt % nonderivatized CNF, and 2.5 wt % suberin, exhibited the most pronounced thermal response, showing resonance frequency shifts of up to 19 MHz at a 5 GHz center frequency and phase shifts of 83° with temperature variation. Prototype II, containing cationic CNFs, demonstrated improved mechanical stability but reduced electrical continuity due to microstructural cracking, whereas Prototype III, modified with biocarbon, displayed diminished conductivity arising from its lower VO₂ content. Degree of linear polarization (DOLP) analysis revealed early stage phase transitions that occurred prior to complete conductive pathway formation. Overall, the hybrid RIS architectures developed from VO₂ and wood-derived materials through a sustainable processing route exhibited highly tunable, temperature-triggered EM modulation, with sensitivity ranging from low to high, depending on the material composition and assembly configuration.