Scarcely two decades have passed since the Nobel Prize in Chemistry was awarded for the discovery of polymer (semi)conductors, and organic electronic materials are now the key enablers of a large and diverse set of developing technologies. Energy devices, flexible and printed circuitry, optoelectronic and electro-mechanical actuators are just some of the recognized applications of organic semiconductors, with new fields emerging continuously, as in the case of organic bioelectronics and organic neuromorphic devices1–3. Interestingly, despite the extensive research conducted on this class of materials and devices, their use in microwave applications has been comparatively scarce, since organic electronics is generally considered “slow” with respect to other semiconductor technologies – although focused efforts are being dedicated to challenge this view4,5. In our recent work, we introduce a new device configuration where organic transistors actively tune a set of microwave metasurfaces and metamaterial-inspired resonant structures. These tuneable devices demonstrate the functionality of organic electronic materials in a yet unexplored – and possibly unexpected – operating regime.
The first realization of our devices was quite fortuitous. In 2018, after a graduate research experience in organic and printed electronics, I joined Silklab, the interdisciplinary research lab led by Prof. Fiorenzo Omenetto (Tufts University). As the name suggests, the laboratory deals primarily with biomaterials – silk in particular – and their applications within biodegradable/implantable electronics and bio-inspired photonic devices. Prior to my arrival, the group had developed different examples of passive microwave/THz metamaterial sensors6–10. In particular, just a few weeks before I joined the group, Prof. Omenetto and collaborators had just published a work demonstrating an innovative metamaterial-inspired dielectric sensor10. This device was specifically designed to be placed and operated from within the human oral cavity, with the goal of wirelessly monitoring and recognizing the intake of foods and liquids, in real-time. This sensor was based on a magnetic Split-Ring Resonator configuration (SRR), fabricated with biocompatible materials.
Shortly after my arrival, I become involved in continuing the project with the initial objective of devising a more scalable fabrication process for this type of sensors – possibly through inkjet printing/coating technologies. While tinkering with different SRR configurations and different printable conductive materials, I incidentally came to realize that the resonant behaviour of the metallic SRR structures could be completely quenched by drop-casting a thin-film of conductive polymer (PEDOT:PSS) on the split of the resonator. In that region, the polymer layer would act as a conductance in parallel to the capacitive element of the LC equivalent circuit (i.e. the split of the SRR), effectively shunting its contribution within the oscillator. In hindsight, this was clearly to be expected: PEDOT:PSS is largely used as an antistatic coating and for EMI shielding, and its conductivity is still reasonable even up to the THz range11,12. However, besides its favourable conductivity, PEDOT:PSS possesses an additional fundamental characteristic: its volumetric charge carrier density can be significantly modulated by means of electrostatic gating. This important property has been intensely exploited in organic electronic and bioelectronic devices, where PEDOT:PSS is the traditional active material in a class of devices called organic electrochemical transistors13.
The tuneable conductivity of PEDOT:PSS proved to be easily exploitable also in our SRRs: we fabricated a set of microwave resonators in which the split region was bridged by a thin-film of the conductive polymer, gated through a ionic medium by a lateral gate electrode. Just as in organic electrochemical transistors, a positive gate voltage results in cations being injected within the bulk of the negatively biased organic semiconductor, decreasing the charge carrier density throughout the volume of the polymer. This device configuration enables a fully reversible modulation mechanism, which requires very low voltages (<1 V), thanks to the optimal mixed ion-electron transporting ability of PEDOT:PSS. We demonstrated this tuning strategy on a number of different resonant configurations, including magnetic and electric resonators, frequency-tuneable SRRs, dual-band devices, and in a reconfigurable metasurface. All materials were deposited by means of inkjet printing – a mass-scalable and cost-effective fabrication technique – onto plastic and flexible substrates.
While our primary focus was to provide a first proof-of-concept of this new metadevice tuning strategy, we believe that our work now opens to a number of interesting scientific and technological questions concerning the potential of organic materials in microwave devices at large. What are the intrinsic limitations of our approach in terms of operating frequency and modulation bandwidth? How significant is the role of volumetric charge carrier modulation, typical of organic electrochemical transistors, in device operation? From a technological point of view, can this novel class of metadevices converge with other emerging technologies, such as organic bioelectronics and neuromorphics, to deliver a new generation of wireless bioelectronic platforms, and non-volatile programmable metasurfaces? These are just some of the questions raised by our recent work, but we hope many more will be soon explored.
I personally wish to thank Prof. Omenetto and the wonderful people at Silklab for their help and support during this exciting journey. We also acknowledge Prof. Alberto Salleo and his team, as well as the System Prototyping Facility at Stanford University, for their precious assistance in finalizing the research activities during the difficult circumstances of 2020.
- Guo, X. & Facchetti, A. The journey of conducting polymers from discovery to application. Nature Materials 19, 922–928 (2020).
- Ohayon, D. & Inal, S. Organic Bioelectronics: From Functional Materials to Next‐Generation Devices and Power Sources. Adv. Mater. 2001439 (2020). doi:10.1002/adma.202001439
- Van De Burgt, Y., Melianas, A., Keene, S. T., Malliaras, G. & Salleo, A. Organic electronics for neuromorphic computing. Nat. Electron. 1, 386–397 (2018).
- Zschieschang, U. et al. Roadmap to Gigahertz Organic Transistors. Adv. Funct. Mater. 30, 1903812 (2020).
- Perinot, A., Passarella, B., Giorgio, M. & Caironi, M. Walking the Route to GHz Solution‐Processed Organic Electronics: A HEROIC Exploration. Adv. Funct. Mater. 30, 1907641 (2020).
- Tao, H. et al. Silk-Based Conformal, Adhesive, Edible Food Sensors. Adv. Mater. 24, 1067–1072 (2012).
- Tao, H. et al. Metamaterial Silk Composites at Terahertz Frequencies. Adv. Mater. 22, 3527–3531 (2010).
- Tao, H. et al. Performance enhancement of terahertz metamaterials on ultrathin substrates for sensing applications. Appl. Phys. Lett. 97, 261909 (2010).
- Mannoor, M. S. et al. Graphene-based wireless bacteria detection on tooth enamel. Nat. Commun. 3, 763 (2012).
- Tseng, P., Napier, B., Garbarini, L., Kaplan, D. L. & Omenetto, F. G. Functional, RF-Trilayer Sensors for Tooth-Mounted, Wireless Monitoring of the Oral Cavity and Food Consumption. Adv. Mater. 30, 1703257 (2018).
- Yan, F., Parrott, E. P. J., Ung, B. S. Y. & Pickwell-Macpherson, E. Solvent doping of PEDOT/PSS: Effect on terahertz optoelectronic properties and utilization in terahertz devices. J. Phys. Chem. C 119, 6813–6818 (2015).
- Du, Y. et al. Dielectric Properties of DMSO-Doped-PEDOT:PSS at THz Frequencies. Phys. status solidi 255, 1700547 (2018).
- Rivnay, J. et al. Organic electrochemical transistors. Nat. Rev. Mater. 3, 17086 (2018).