Advancements in Millimeter Wave and Sub-THz Flexible Devices for Industrial Applications: From Sensors to Intelligent Surfaces

Abstract

This paper presents a comprehensive investigation of a scalable manufacturing framework for advanced flexible radio-frequency (RF) devices based on screen-printing technology. The study systematically evaluates the electrical and electromagnetic performance of multiple conformal components spanning microwave to sub-terahertz frequency ranges. First, a low-loss flexible microstrip transmission line (FML) is designed and experimentally characterized from 1 GHz to 20 GHz under various bending conditions. The results demonstrate minimal degradation in S-parameters, with phase variation remaining below 10° at a 45° bend, confirming its suitability for high-speed digital and analog interconnects. Building upon this platform, a whispering gallery mode (WGM) resonator-based sensor is developed for liquid characterization, specifically for monitoring alcohol concentration in water. The sensor maintains stable operation under mechanical deformation, highlighting its applicability in confined and dynamically varying environments. Notably, this work demonstrates that phase variation provides a highly sensitive and robust sensing metric, offering improved immunity to environmental noise compared to conventional amplitude- or frequency-based approaches. Furthermore, the proposed fabrication process is extended to millimeter-wave applications through the realization of an ultrathin dual-polarized conformal transmissive band-stop frequency selective surface (FSS) operating at 79.25 GHz and 84 GHz, as well as reflective intelligent surfaces (RIS) at 30 GHz and 60 GHz enabling high-gain beam and beam redirection for enhancing signal-to-noise-ratio and field of view (FoV) in wireless communications. Furthermore, the FSS maintains functionality under extreme bending conditions up to 80°. Ultimately, to assess scalability toward higher frequencies, dielectric properties of candidate substrates are characterized using vector network analyzer (VNA)-based quasi-optical and terahertz time-domain spectroscopy (THz-TDS) methods. The simplicity, cost-effectiveness, and versatility of the proposed screen-printing process make it a promising candidate for next-generation flexible communication, sensing, and intelligent surface applications.