A low-voltage three-state flexible tunable bandpass filter using organic electrochemical transistors for 5G NR n79 and Wi-Fi 6E applications

Smarter Wireless for Bendable Gadgets

As phones, watches, and medical patches bend and wrap around our bodies, the radio hardware inside them still tends to be rigid and power-hungry. This paper introduces a flexible, low‑voltage radio filter that can smoothly select different wireless channels used in today’s 5G and Wi‑Fi 6E networks, bringing us closer to fully soft, wearable communication devices.

Why Future Wearables Need New Filters

Every wireless gadget relies on small components called bandpass filters to pick out the right slice of the radio spectrum while rejecting unwanted signals. Existing tunable filters are usually built on hard circuit boards and often need high control voltages or bulky magnets, making them poorly suited for thin, bendable products that must run on tiny batteries. Separate fixed filters for each band also take up space, which clashes with the push toward compact, body‑conforming electronics. The authors target two crowded and important bands—5G NR n79 around 4.4–5.0 GHz and Wi‑Fi 6E from 5.15–5.45 GHz—showing how one flexible, reconfigurable filter can cover both.

A Soft Transistor That Acts Like a Radio Switch

At the heart of the new filter is an organic electrochemical transistor, or OECT, built from a conducting polymer and a soft ion‑rich gel on a plastic sheet. In its natural state, the polymer is highly conductive, so radio signals flow easily between its metal contacts. When a small positive voltage is applied to a nearby gate contact, ions from the gel move into the polymer and chemically “turn down” its conductivity. This transforms the device from a low‑resistance path to an almost insulating gap, changing by more than a factor of one thousand. Because this switching relies on ion motion rather than large electric fields, the OECT can be driven with only about 1.3 volts—compatible with typical batteries and safe for skin‑mounted systems.

A Tiny Ring on Plastic That Can Shift Channels

The researchers pattern a ring‑shaped metal track on a thin PET plastic substrate, forming a microstrip resonator whose size determines which radio frequencies pass through. Four short openings in the ring are filled with OECT channels and grouped into top and bottom pairs. By deciding which pair is conductive or non‑conductive, the circuit effectively lengthens or shortens the electrical path around the ring, nudging the filter’s passband to higher or lower frequencies. Measurements show three clear operating states with center frequencies at roughly 5.15, 4.86, and 4.65 GHz—together spanning 4.37–5.45 GHz—while keeping signal loss between only 1.65 and 1.87 decibels and reflections close to 20 decibels, performance that rivals many rigid, high‑end filters.

Printed Like a Graphic, Bent Like a Bandage

Instead of using clean‑room microfabrication, the team relies on screen printing and simple solution processing. Silver ink is printed and heat‑treated to form smooth, highly conductive traces; the polymer channel is drop‑cast; and the ion gel is coated over the active area. These steps are compatible with large‑area and potentially roll‑to‑roll manufacturing. Careful testing shows that the printed silver films and polymer layers remain conductive and well‑adhered at the chosen processing temperatures. When the finished filter is bent to radii down to about 50 millimeters and cycled through many bends, its key radio characteristics—center frequency, signal loss, and return loss—change only slightly, indicating strong mechanical robustness.

What This Means for Everyday Devices

In plain terms, the authors have built a soft, low‑power "channel selector" that can live on a flexible plastic sheet without sacrificing the precision demanded by modern 5G and Wi‑Fi links. By combining large on/off switching range, modest signal loss, low drive voltage, and scalable printing, their OECT‑based design overcomes many drawbacks of older tunable technologies that relied on rigid parts, high voltages, or moving liquids. While further work is needed to speed up switching, harden the device against moisture, and integrate it with full radio front‑ends, this study shows a practical path toward wearable and conformal gadgets whose radio brains are just as flexible as their bodies.

Citation: Yang, W., Wu, L., Wei, J. et al. A low-voltage three-state flexible tunable bandpass filter using organic electrochemical transistors for 5G NR n79 and Wi-Fi 6E applications. npj Flex Electron 10, 43 (2026). https://doi.org/10.1038/s41528-026-00548-2

Keywords: flexible electronics, tunable RF filter, organic electrochemical transistor, 5G and Wi-Fi, wearable wireless devices