Disordered systems, especially polymers, play an important role in shaping our modern society and are expected to be an even bigger part of the future. However, the complexities of these systems pose a challenge for scientists to characterize, given our current limited understanding. Elucidating how disordered systems respond to external stimuli thus becomes important.

https://www.nature.com/articles/s41563-024-01813-3

In this work, we revealed the tiny deformations and complex structural changes in water-swelled polymers in response to external voltages, uncovering the evolution of disordered polymer systems. Using a novel technique, X-ray photon correlation spectroscopy (XPCS), to monitor deformation with sub-second time resolution, we found that voltage application pathway significantly affects the polymer's stability and structural changes.

Unusual long-term asymmetry was observed between de-doping and re-doping. De-doping quickly reaches mesoscale equilibration due to moving structural distortions from coupled electronic/ionic motion (polarons). In contrast, re-doping shows a persistent non-equilibrium process with evolving mesoscale structures, driven by complex interactions among solvent, ions, electrons, and polymer networks, and significantly modified by the rapid generation of multi-charged species (e.g., bipolarons).

Coupling between charged species, mesoscale domain deformation, and external voltage is as always more complex than expected. Moreover, internal strain and structural hysteresis depend heavily on the sample’s cycling history.

This was a great team effort lead by grad student Ruiheng Wu, with Dilara Meli and Bryan Paulsen. Thanks to Chris Takacs for his leadership and mentorship, and to our Argonne collaborators Joe Strzalka, Suresh Narayanan and Qingteng Zhang.

New collaborations have created new opportunities for our synthetic biology sensors. The lab's efforts to build cell membrane-derived bioelectronic devices is led by Dr. Emily Schafer, with the aim of creating platforms with the same sensing mechanisms as living cells.

In this paper, now out in the Journal of the American Chemical Society, we teamed up with Dr. Stephen Sarles and colleagues from University of Tennessee to form a new class of polymer supported bilayers. These droplet polymer bilayers (DPBs) on PEDOT:PSS increase the reproducibility and dynamic range of the resulting membrane sensors. In fact, DPBs considerably outperform more standard supported lipid bilayers (SLBs) on PEDOT:PSS electronics.

Excitingly, droplet bilayers like DPBs enable integration of membrane sensors into electrode arrays, meaning that future work can test dozens of membranes in parallel. In this work, we show a proof-of-concept array design with a planar reference electrode to more simply characterize membrane sensing events in parallel.

The future vision of this work is to next incorporate complex transmembrane proteins for detection of biologically-relevant stimuli, such as ions, neurotransmitters, voltage, light, and more. We also hope that this membrane and device design can be used for understanding mechanisms of various drugs on membrane proteins and as a drug screening platform. We're proud to share this work and look forward to the next!

Anti-ambipolar transistors feature a drain current that moves from OFF to ON to OFF states with increasing gate bias. This property is intrinsic to some conjugated polymers. However, limited stable and tunable anti-ambipolar organic materials prevent the design of integrated, tunable, and multifunctional neuromorphic and logic-based systems. We offer a general approach for tuning anti-ambipolar characteristics through the design of a novel vertical OECT (vOECT) based on a p-n bilayer. This architecture allows reduction of device footprint and, by controlling the bilayer materials, tuning of the anti-ambipolarity characteristics. Our bilayer vertical architecture enables control of the device’s on and off threshold voltages, and peak position, by property selecting materials and thickness ratios. These anti-ambipolar bilayer vOECTs enable tunable threshold spiking neurons and logic gates, for bio-interfacing applications. To bring these concepts together, we used the logic gates to replicate the graded potentials processing of horizontal cells, while the tunable spiking circuits served to perform the spike encoding functions of retinal ganglion cells. This mimics the retinal pathway encoding wavelength and light intensity information, heralding future opportunities for customized and multifunctional neuromorphic circuitry.

https://www.nature.com/articles/s41467-024-50496-6

Our paper “Direct quantification of ion composition and mobility in organic mixed ionic-electronic conductors” was published in Science Advances.

Understanding ion behavior is important for OMIECs. While past methods were mostly indirect, our team introduced operando X-ray fluorescence (XRF) spectroscopy to directly monitor ion composition and mobility during OMIEC operation.

EG mixed PEDOT:PSS was used as the model system in this study. Thick films showed a similar composition to our previous ex situ results (ACS Appl. Mater. Interfaces 2023, 15, 25, 30553–30566), with only cations detected and their concentration modulating by 5-10% between dedoped and doped states. Notably, operando composition results may differ from ex situ data, and thin films revealed the presence of anions, highlighting the interfacial effect on film composition.

We also made new discoveries about ion transport: 1) The cation transport in the first cycle can be separated into an initial rapid electrowetting and a slower proton exchange stage; 2) operando XRF results show consistency with previous optical moving front results in 20% EG-PEDOT:PSS, but in 5% EG-PEDOT:PSS, the XRF moving front lags behind the optical moving front; the difference might be due to residue proton transport; 3) exploring different thicknesses in 5% and 20% EG-PEDOT:PSS, we observed faster moving fronts with thinner films; plotting ion mobility against inverse thickness offering insights into bulk mobility (intercept) and interfacial effects (slope).

This was a great team effort led by grad student Ruiheng Wu, with mentorship from Jonathan Rivnay and postdoc Xudong Ji. We also thank for the help from our previous members Bryan Paulsen and Josh Tropp, and to our Argonne collaborators Qing Ma.

Our review, ”Organic mixed conductors for electrochemical transistors,” was published in Matter.

https://www.sciencedirect.com/science/article/pii/S2590238523002199

The paper presents material design considerations for the next generation of organic mixed ionic-electronic conductors (OMIECs), which are semiconducting materials that enable critical components used in bioelectronics technologies such as sensors, stimulation elements and neuromorphic devices.

Recently-reported strategies used to develop high performance OMIECS are summarized summarized, and we discuss topics such as batch-to-batch variability, stability, OMIEC processing and alternative platforms.

The paper was written by former group postdoc and current Texas Tech faculty member Joshua Tropp and PhD candidate Dilara Meli.

Implantable cell therapies and tissue transplants require a reliable oxygen supply to function effectively. However, achieving sufficient oxygenation within the transplant host remains challenging due to limited vascularization. Previous methods for exogenous oxygenation were bulky and had limited oxygen production or regulation.

https://www.nature.com/articles/s41467-023-42697-2

In this highly interdisciplinary study co-led by Northwestern and Carnegie Melon Universities, we developed an electro-catalytic approach called “ecO₂” that enables bioelectronic control of oxygen generation in complex cellular environments. We used a nanostructured sputtered iridium oxide film (SIROF) as the catalyst for oxygen evolution at neutral pH. The ecO₂ platform exhibited lower oxygenation onset, selective oxygen production, and no toxic byproducts. Importantly, it sustained high cell loadings (>60k cells/mm³) in hypoxic conditions both in vitro and in vivo.

This work demonstrates that exogenous oxygen production devices can be integrated into bioelectronic platforms, enabling high cell densities in smaller devices with broad applicability. The ability to precisely control oxygen generation offers a significant advantage over older methods, potentially improving the success rate of cell-based treatments for a variety of diseases. The ecO₂ system holds promise for improving the viability and therapeutic functionality of transplanted cell-based therapies.

The Rivnay Group welcomes two new post-doctoral researchers, Yebin Lee and Priscila Cavassin, working on biochemical sensing and material characterization respectively. We look forward to fantastic science and mentorship from Yebin and Priscila. To learn more about them, please navigate to the “People” section, where you can read about all of our team members.

The Rivnay Group welcomes our new first-year PhD students, Victoria Kindratenko, Beliz Utebay, Rosalba Huerta and Junyi Liu, working on sensor robustness, lipid bilayer sensing, fundamentals and oxygenation projects, respectively. We have no doubt that their excitement, hard work and thoughtfulness will lead to important contributions to the field of bioelectronics that improve human health.

Check out their profiles in the “People” section for more on our new team members.

Dr. Rebecca Keate successfully defended her PhD thesis, titled “Designing conductive polymer biomaterials for regenerative engineering applications.”

In her work, Dr. Keate examines the mechanisms by which conductive polymers enhance or otherwise influence the repair of biological tissues, from the standpoint of properties such as mechanics, surface charge, conductivity and hydrophobicity, which all independently affect cellular phenomena. She demonstrates several conductive polymer systems that are suited for interrogation of the mechanisms that most significantly impact cell fate, and thus, overall tissue health.

Her work demonstrates, in-vitro and -vivo, how these conductive polymers may be applied to future tissue-regenerative applications.

Well done, Dr. Keate!

Our very own Dr. Ruiheng Wu successfully defended his PhD thesis, titled “Operando Characterization of Structure, Composition, and Charge Transport of OMIECs.”

Dr. Wu’s work on the fundamental chemistry, charge transport and structure of OMIECs lays the foundation for development of better conductive polymers that can improve bioelectronic medicine.

Read Ruiheng's work, including his most recent paper, “Quantitative composition and mesoscale ion distribution in p-type organic mixed ionic-electronic conductors,” under "Publications.”

Congratulations, Dr. Wu!

We enjoyed our bi-annual retreat to Michigan City, Indiana on a warm weekend in September. Board games, cooking and outdoor fun were matched with equally energetic discussions on our research priorities and personal goals. We returned to Evanston and Chicago, research efforts redoubled, with appreciation for how lucky we are to work together.

Rivnay Group postdoc Joshua Tropp has joined the Department of Chemistry and Biochemistry at Texas Tech University as an assistant professor. Professor Tropp is an exceptional mentor and scientist, and anyone interested in the areas of organic material design and synthesis, chemical sensing and/or biomaterial design for bioelectronics should check his website for open positions.

Congratulations, Professor Tropp, we miss you!

Ziplines, ropes courses, yard games, home-cooked food, and charades late into the night—a full day for the Rivnay Group on our quarterly retreat.

Members of the Rivnay lab, Jonathan Rivnay, Xudong Ji and Abhijith Surendran, have contributed to a new paper, Vertical organic electrochemical transistors for complementary circuits.

A transdisciplinary Northwestern University research team has developed a revolutionary transistor that is expected be ideal for lightweight, flexible, high-performance bioelectronics. 

“This exciting new type of transistor allows us to speak the language of both biological systems, which often communicate via ionic signaling, and electronic systems, which communicate with electrons,” Rivnay said. “The ability of the transistors to work very efficiently as ‘mixed conductors’ makes them attractive for bioelectronic diagnostics and therapies.”

Read more about the paper here.

Great group outings this summer:

In June the group went to Hearthstone and Terrace in Chicago for a Terrarium building workshop! Lots more greenery in lab and at home!

A last Harrah to summer: group Kayaking near Starved Rock, followed by a Cookout by the Illinois river.

Postdoc Josh Tropp was named ACS PMSE Future Faculty Fellow: https://pmsedivision.org/pmse-future-faculty/ He received this honor and gave a number of talks at the ACS Fall Meeting in Chicago.

In addition, Josh was named NU Postdoctoral Association’s Postdoc of the month for the month of August 2022: https://sites.northwestern.edu/nupa/postdoc-of-the-month-1/

Congratulations to Graduate Student Rebecca Keate, joint with Guillermo Ameer, on her Rising Stars lecture. See also her SQI interview here (https://sqi.northwestern.edu/news-and-events/news-and-announcements/2022/meet-the-researchers-rebecca-keate.html )

Congratulations to Jonathan Rivnay who was promoted to Full Professor with tenure in the Dept. of Biomedical Engineering, effective Sept 1, 2022.

Our paper “Sources and Mechanism of Degradation in p-Type Thiophene-Based Organic Electrochemical Transistors” was published in ACS Applied Electronic Materials.

https://pubs.acs.org/doi/full/10.1021/acsaelm.1c01171

Most work on OMIEC degradation focuses just on the polymer itself, and maybe the polymer in a particular electrolyte. While this is helpful, it doesn’t tell the whole story…what about device architecture and implementation? What about how you operate/bias the OECT?

If it’s pulsed in a diode-connected manner (G-D shorted) the degradation is much lower than when operated in a 3-terminal OECT or as an electrode (S-D shorted). It’s all about the combo of oxidative and reductive bias stress.

The reaction of dissolved oxygen at the buried Au/OMIEC interface of the drain electrode experiencing reductive potentials produces mobile reactive species that degrade the oxidized OMIEC in the device. This seems to be general across a number of thiophene p-type OMIECs.

This leads to design rules: we can avoid degradation by (1) removing oxygen (not practical for bioelectronics), (2) avoiding reductive potentials via device biasing scheme, (3) replacing Au electrodes with a noncatalytic alternative, or (4) passivating Au electrodes with self-assembled monolayers.

This was a great team effort lead by grad student Emily Schafer, with Ruiheng Wu, Dilara Meli, Josh Tropp and Bryan Paulsen, and of course, with materials from Iain McCulloch and team.

Graduate Student Reem Rashid defended her PhD Thesis Spring 2022. Congratualtions Doctor! She is the lab’s first PhD graduate.

Thanks to the committee: John Rogers, Neha Kamat (NU), and Simone Fabiano (Linkoping)