Tunable Flexible Bandpass Filters Based on Spoof Surface Plasmon Polaritons

In this work, dual-mode and multimode tunable flexible bandpass filters (BPFs) based on spoof surface plasmon polaritons (SSPPs) are presented. In the designed circuit, the tunable characteristic of the filters is controlled by the RF switches—organic electrochemical transistors (OECTs). The operating state of the OECTs can be changed by applying a voltage to tune the center frequency of the filter. It is worth mentioning that OECTs are devices that facilitate the design of flexible circuits. Flexible zinc-ion batteries (FZIBs) are integrated into the filters to provide stable switching voltages for OECTs. Due to the integration of flexible batteries during the structural design process, tunable flexible filters operate without the need for an external power supply or microwave electronics. The results indicate that the center frequency of the dual-mode filter is tuned from 3.6 to 4.1 GHz when the OECTs change from on to off. The multimode filter with six tunable states can be digitally encoded. The maximum tuning range of the center frequency of the multimode filter can be adjusted from 2.91 to 3.59 GHz. Furthermore, the transmission characteristics of the filters were measured under different states, including flat, bent, and folded. The simulation and measurement results are in good agreement, showing that the designed tunable flexible filters exhibit excellent transmission performance, which can be widely used in fifth-generation (5G), wireless local area network (WLAN), industrial, scientific, and medical (ISM), and wearable systems.